WO2018021966A2 - Pile solaire à support chaud et son procédé de formation - Google Patents

Pile solaire à support chaud et son procédé de formation Download PDF

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WO2018021966A2
WO2018021966A2 PCT/SG2017/050365 SG2017050365W WO2018021966A2 WO 2018021966 A2 WO2018021966 A2 WO 2018021966A2 SG 2017050365 W SG2017050365 W SG 2017050365W WO 2018021966 A2 WO2018021966 A2 WO 2018021966A2
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hot
carrier
ncs
nanocrystals
energy
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PCT/SG2017/050365
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WO2018021966A3 (fr
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Tze Chien Sum
Mingjie LI
Subodh Gautam Mhaisalkar
Nripan Mathews
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Nanyang Technological University
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Priority to CN201780037453.3A priority Critical patent/CN109804479B/zh
Publication of WO2018021966A2 publication Critical patent/WO2018021966A2/fr
Publication of WO2018021966A3 publication Critical patent/WO2018021966A3/fr

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    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
    • H10K30/00—Organic devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation
    • H10K30/20—Organic devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation comprising organic-organic junctions, e.g. donor-acceptor junctions
    • H10K30/211—Organic devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation comprising organic-organic junctions, e.g. donor-acceptor junctions comprising multiple junctions, e.g. double heterojunctions
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
    • H10K85/00—Organic materials used in the body or electrodes of devices covered by this subclass
    • H10K85/50—Organic perovskites; Hybrid organic-inorganic perovskites [HOIP], e.g. CH3NH3PbI3
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
    • H10K30/00—Organic devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation
    • H10K30/50—Photovoltaic [PV] devices
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/50—Photovoltaic [PV] energy
    • Y02E10/549—Organic PV cells

Definitions

  • Various aspects of this disclosure relate to hot-carrier solar cells. Various aspects of this disclosure relate to methods of forming hot-carrier solar cells.
  • Various embodiments may provide a hot-carrier solar cell.
  • the solar cell may include a nanocrystal containing layer containing or including one or more nanocrystals, each of the one or more nanocrystals including a halide perovskite material.
  • the hot-carrier solar cell may also include a first electrode in contact with a first side of the nanocrystal containing layer.
  • the hot-carrier solar cell may further include a second electrode in contact with a second side of the nanocrystal containing layer opposite to the first side.
  • the nanocrystal containing layer may have a thickness of less than 100 nm.
  • Various embodiments may provide a method to form a hot-carrier solar cell.
  • the method may include providing or forming the nanocrystal containing layer including one or more nanocrystals (as described herein), each of the one or more nanocrystals including a halide perovskite material.
  • the method may also include forming a first electrode so that the first electrode is in contact with a first side of the nanocrystal containing layer.
  • the method may further include forming a second electrode so that the second electrode is in contact with a second side of the nanocrystal containing layer opposite to the first side.
  • the nanocrystal containing layer may have a thickness of less than 100 nm.
  • FIG. 1 shows a general illustration of a nanocrystal according to various embodiments.
  • FIG. 2 shows a general illustration of a hot-carrier solar cell according to various embodiments.
  • FIG. 3 is a schematic showing a method of forming a nanocrystal according to various embodiments.
  • FIG. 4 is a schematic showing a method of forming a hot-carrier solar cell according to various embodiments.
  • FIG. 5 is a schematic showing hot-carrier cooling with (a) intraband Auger-type energy transfer, (b) phonon-bottleneck effect and (c) interband Auger processes in semiconductor nanocrystals.
  • FIG. 6 shows representative transmission electron microscopy (TEM) images of methylammonium lead bromide perovskite (MAPbBr 3 ) nanocrystals (NCs) with relatively (a) small, (c) medium and (e) large sizes according to various embodiments and the respective size histograms (b, d, f) on their right.
  • the size distribution may be modeled with a Gaussian distribution.
  • FIG. 7 shows (a) top-view and (b) side-view scanning electron microscopy (SEM) images of methylammonium lead bromide (MAPbBr 3 ) bulk-film.
  • FIG. 8 shows (a) a plot of photoluminescence (PL) intensity (in arbitrary units or a.u.) as a function of wavelength (in nanometers or nm) illustrating photoluminescence (PL) spectra of methylammonium lead bromide perovskite (MAPbBr 3 ) nanocrystals (NCs) according to various embodiments dispersed in toluene and bulk film counterpart, (b) a plot of absorbance (in arbitrary units or a.u.) as a function of wavelength (in nanometers or nm) illustrating ultraviolet-visible (UV-vis) absorption spectra of methylammonium lead bromide perovskite (MAPbBr 3 ) nanocrystals (NCs) according to various embodiments dispersed in toluene and bulk film counterpart, (c) a plot of energy of Is exciton, E ls (in electron volts or eV) as a function of average nanocrystal radius
  • FIG. 9A shows pseudo color transient absorption (TA) plot (upper panel, time (in picoseconds or ps) as a function of energy (in electron volts or eV)) and normalized transient absorption (TA) spectra (lower panel, normalized transmittance change ⁇ / ⁇ as a function of energy (in electron volts or eV)) for medium methylammonium lead bromide perovskite nanocrystals (MAPbBr 3 NCs) (radii -4-5 nm) according to various embodiments in solution at low pump fluence (left panel) with initially average generated electron-hole pair per nanocrystal, ⁇ Vo> -0.1 (average carrier density per nanocrystal volume, n 0avg ⁇ 2.6 x 10 17 cm ⁇ 3 ) and high pump fluence (right panel) with ⁇ Vo> -2.5 (n 0avg ⁇ 6.5x 10 18 cm “3 ).
  • TA medium methylammonium lead bromide
  • FIG. 9B shows pseudo color representation (upper panel, time (in picoseconds or ps) and normalized transient absorption (TA) spectra (lower panel, normalized transmittance change ⁇ / ⁇ as a function of energy (in electron volts or eV)) for MAPbBr 3 bulk-film at low pump fluence (left panel) with initially generated carrier density no -2.1 x 10 17 cm 3 and high pump fluence (right panel) with no -1.5 x 10 19 cm 3 .
  • TA transient absorption
  • FIG. 10 shows plots of time (in picoseconds or ps) as a function of energy (electron volts or eV) showing pseudocolor transient absorption (TA) spectra (time in picoseconds or ps as a function of energy in electron volts or eV) for (a) small and (b) large sized methylammonium lead bromide perovskite nanocrystals (MAPbBr 3 NCs) according to various embodiments in solution at low pump fluence (left panel) with initially average generated electron-hole pair per nanocrystal ⁇ No> -0.1 and high pump fluence (right panel) with ⁇ Vo> -2.5 following 3.1 eV photoexcitation.
  • TA pseudocolor transient absorption
  • FIG. 11 illustrates (a) a plot of normalized transmittance change ⁇ / ⁇ as a function of energy (in electron volts or eV) showing normalized TA spectra at different short time delays of medium sized methylammonium lead bromide perovskite nanocrystals (MAPbBr 3 NCs) according to various embodiments in toluene with average generated electron-hole pair per nanocrystal, ⁇ No> ⁇ 0.1 (following 3.1 eV photoexcitation) and (b) the un-normalized transient absorption (TA) spectra of (a).
  • MAPbBr 3 NCs medium sized methylammonium lead bromide perovskite nanocrystals
  • FIG. 14 shows a table comparing properties of methylammonium lead bromide perovskite nanocrystals (MAPbBr 3 NCs) according to various embodiments, methylammonium lead bromide perovskite bulk films, and other materials reported in literature.
  • MAPbBr 3 NCs methylammonium lead bromide perovskite nanocrystals
  • FIG. 15 show plots of carrier temperatures (in Kelvins or K) as a function of time delay (picoseconds or ps) for three different sized methylammonium lead bromide perovskite nanocrystals (MAPbBr 3 NCs) according to various embodiments and bulk-film (a) at low pump fluence (corresponding to ⁇ No> -0.1 in NCs and no -2.1 x 10 17 cm 3 in bulk-film) and (b) at high pump fluence (corresponding to ⁇ Vo> -2.5 in NCs and no -1.5 x 10 19 cm 3 in bulk- film) following 3.1 eV photoexcitation.
  • MAPbBr 3 NCs methylammonium lead bromide perovskite nanocrystals
  • FIG. 16 illustrates plots of normalized transmittance change ⁇ / ⁇ as a function of time (picoseconds or ps) showing normalized photobleaching dynamics probed at the band edges of methylammonium lead bromide (a) bulk-film, (b) small nanocrystals (NCs) according to various embodiments, (c) medium nanocrystals (NCs) according to various embodiments, and (d) large nanocrystals (NCs) according to various embodiments, in solution with high and low pump fluence respectively, as well as (e) comparison of transient absorption (TA) dynamics for medium nanocrystals (NCs) according to various embodiments in solution and spin-coated nanocrystals (NCs) film according to various embodiments, and (f) pump- fluence-dependent bleaching dynamics probed at the band edge of small methylammonium lead bromide nanocrystals (NCs) according to various embodiments in solution.
  • TA transient absorption
  • FIG. 17 shows (a) a plot of energy loss rate (electron volts per picosecond eV ps "1 ) as a function of carrier temperature (in Kelvins or K) illustrating energy loss rate of hot-carriers as a function of carrier temperature T for methylammonium lead bromide perovskite nanocrystals (MAPbBr 3 NCs) according to various embodiments with ⁇ No> -0.1 and methylammonium lead bromide perovskite (MAPbBr 3 ) bulk-film with no -2.1 x 10 17 cm “3 , (b) a plot of normalized transmittance change ⁇ / ⁇ as a function of time (in picoseconds or ps) illustrating normalized bleaching dynamics probed at the band-edge for colloidal methylammonium lead bromide perovskite nanocrystals (MAPbBr 3 NCs) according to various embodiments and bulk-film at low carrier density, and (c) a plot of rise time
  • FIG. 18 is a plot of Raman intensity (in arbitrary units or a.u.) as a function of wavenumbers (in per centimeter or cm "1 ) showing room temperature Raman spectrum of as-prepared methylammonium lead bromide perovskite nanocrystals (MAPbBr 3 NCs) according to various embodiments drop-cast on glass substrate.
  • MAPbBr 3 NCs as-prepared methylammonium lead bromide perovskite nanocrystals
  • FIG. 19 shows (a) a plot of normalized transmittance change ⁇ / ⁇ as a function of time (in picoseconds or ps) illustrating normalized bleaching dynamics probed at the band edge for colloidal CdSe NCs with different diameters (shown in legend) at low pump fluence, and (b) plots of time (in picoseconds or ps) as a function of energy (in electron volts or eV) illustrating low pump fluence with initially generated ⁇ No> -0.1 (left) and high pump fluence with ⁇ No> -2.5 (right).
  • the phtotoexcitation energy is 3.1 eV.
  • FIG. 20 shows (a) a plot of energy loss rate (in electron volts per picosecond or eV ps "1 ) as a function of carrier temperature (in Kelvins or K) illustrating energy loss rate vs carrier temperature T c for methylammonium lead bromide perovskite nanocrystals (MAPbBr 3 NCs) according to various embodiments with ⁇ Vo> -2.5 and MAPbBr 3 bulk-film with no -1.5 x 10 19 cm “3 , (b) a plot of lifetime (in picoseconds or ps) as a function of nanocrystal volume (in cubic nanometer or nm 3 ) illustrating Auger recombination lifetimes and hot-carrier cooling time vs perovskite nanocrystal (NC) volume according to various embodiments, and (c) a plot of normalized hot carrier concentration ti ot as a function of time (in picoseconds or ps) showing normalized hot-carrier
  • FIG. 21 is a plot of energy loss rate (electron volts per picosecond or eV ps "1 ) as a function of carrier temperature (in Kelvins or K) showing energy loss rate of hot-carriers as a function of carrier temperature T c for methylammonium lead bromide (MAPbBr 3 ) bulk-film at low and high carrier densities.
  • eV ps "1 energy loss rate
  • FIG. 22 shows plots of photoelectron intensity (counts per second or cts/s) as a function of energy (in electron volts or eV) showing the ultraviolet photoelectron spectroscopy (UPS) spectrum of (a) 1,2-ethanedithiol (EDT)-treated and (b) post-annealed EDT-treated methylammonium lead bromide (MAPbBr 3 ) nanocrystals (NCs) films according to various embodiments, and (c) 7-diphenyl-l, 10-phenanthroline (Bphen) film on indium tin oxide (ITO) substrates.
  • UPS ultraviolet photoelectron spectroscopy
  • FIG. 23A is a flat-band energy diagram (vertical axis in electron volts or eV) to illustrate hot- electron extraction from perovskite nanocrystals according to various embodiments to 7- diphenyl- l,10-phenanthroline (Bphen) with competing hot-electron cooling pathways.
  • FIG. 23B shows an atomic force microscopy (AFM) image of 1,2-ethanedithiol (EDT)- treated nanocrystal (NC) film according to various embodiments.
  • AFM atomic force microscopy
  • FIG. 23C is a scanning electron microscopy (SEM) image of 1,2-ethanedithiol (EDT)-treated nanocrystal (NC)/ 7-diphenyl-l, 10-phenanthroline (Bphen) bilayer according to various embodiments.
  • SEM scanning electron microscopy
  • FIG. 23D is a plot of normalized transmittance change ( ⁇ / ⁇ ) as a function of energy (in electron volts or eV) showing normalized transient absorption (TA) spectra for about 35 nm thick of 1,2-ethanedithiol (EDT)-treated nanocrystal (NC) film with (continuous lines) / without (dashed lines) 7-diphenyl- l,10-phenanthroline (Bphen) according to various embodiments following 3.1 eV photoexcitation with ⁇ No> around 0.1.
  • TA transient absorption
  • FIG. 23E is a plot of hot-carrier temperature (in Kelvins or K) as a function of delay time (in picoseconds or ps) for 1,2-ethanedithiol (EDT)-treated nanocrystal (NC) film and 1,2- ethanedithiol (EDT)-treated nanocrystal (NC) film /7-diphenyl-l, 10-phenanthroline (Bphen) bilayer according to various embodiments at different pump fluences.
  • K hot-carrier temperature
  • FIG. 23F is a plot of extraction efficiency ?/hot (in percent or %) as a function of hot-electron excess energy (in electron volts or eV) showing pump energy dependence of the hot-electron extraction efficiencies in about 35 nm-thick 1,2-ethanedithiol (EDT)-treated nanocrystal (NC)/ 7-diphenyl- l,10-phenanthroline (Bphen) bilayer according to various embodiments.
  • EDT 1,2-ethanedithiol
  • NC nanocrystal
  • Bphen 7-diphenyl- l,10-phenanthroline
  • 23G is a plot of extraction efficiency rjhot (in percent or %) as a function of thickness (in nanometres or nm) showing perovskite film thickness dependence of the hot electron extraction efficiencies upon 3.1 eV pump energy excitation for 1,2-ethanedithiol (EDT)- treated nanocrystal (NC)/ 7-diphenyl- l,10-phenanthroline (Bphen) bilayer according to various embodiments and bulk-film/7-diphenyl- l, 10-phenanthroline (Bphen) bilayer.
  • EDT 1,2-ethanedithiol
  • NC nanocrystal
  • Bphen 7-diphenyl- l,10-phenanthroline bilayer
  • Bphen bulk-film/7-diphenyl- l, 10-phenanthroline
  • FIG. 24 shows (a) plot of transmittance (in arbitrary units or a.u.) as a function of wavenumber (in per centimeter or cm "1 ) showing the attenuated total reflection- fourier transform infrared (ATR-FTIR) spectra of as-prepared methylammonium lead bromide nanocrystals (MAPbBr 3 NCs), 1,2-ethanedithiol (EDT)-treated nanocrystals (EDT-NCs) and 70 °C annealed 1,2-ethanedithiol nanocrystals (Ann-EDT-NCs) according to various embodiments, and plots of photoemission intensities (in arbitrary units or a.u.) as a function of binding energy (in electron volts or eV) showing sulfur (S) 2p X-ray photoelectron spectroscopy (XPS) spectra of (b) non-annealed and (c) 70 °C post-annealed 1,2- ethan
  • FIG. 25 shows (a) atomic form microscopy (AFM) image of un-treated medium methylammonium lead bromide nanocrystals (MAPbBr 3 NCs) film according to various embodiments, and (b) representative transmission electron microscopy (TEM) image of 1,2- ethanedithiol -treated methylammonium lead bromide nanocrystals (EDT-treated MAPbBr 3 NCs) according to various embodiments.
  • AFM atomic form microscopy
  • MAPbBr 3 NCs un-treated medium methylammonium lead bromide nanocrystals
  • TEM transmission electron microscopy
  • FIG. 26 shows pseudocolor transient absorption (TA) spectra for (a) medium methylammonium lead bromide nanocrystals (MAPbBr 3 NCs) film according to various embodiments, (b) 1,2-ethanedithiol -treated nanocrystals (EDT-treated NCs) film according to various embodiments, and (c) on 1,2-ethanedithiol-treated nanocrystals film/7-diphenyl- 1,10-phenanthroline (EDT-treated NCs film/Bphen) bilayer according to various embodiments at low pump fluence (left panel) with initially generated ⁇ No> -0.1 and high pump fluence (right panel) with ⁇ Vo> -2.5.
  • TA pseudocolor transient absorption
  • FIG. 27 shows energy diagrams (y axis: energy in electron volts or eV) showing flat-band energy level alignment as determined from the ultraviolet photoelectron spectroscopy (UPS) and ultraviolet-visible (UV-VIS) spectroscopy measurements for non-annealed, annealed 1,2- ethanedithiol -nanocrystals (EDT-NCs) films and 7-diphenyl-l, 10-phenanthroline (Bphen) according to various embodiments - illustrated for the case of hot-electron extractions.
  • UPS ultraviolet photoelectron spectroscopy
  • UV-VIS ultraviolet-visible
  • FIG. 28 shows (a) a plot of absorbance (in arbitrary units or a.u.) as a function of wavelength (in nanometers or nm) showing linear absorption spectra of Bphen film on glass; (b) plot of normalized negative transmittance change - ⁇ / ⁇ as a function of wavelength (in nanometer or nm) showing negative transient absorbance spectra of 7-diphenyl- l,10-phenanthroline (Bphen) (300 nm pump with intensity of 20 ⁇ cm "2 , 400 nm pump with intensity of 40 ⁇ cm "2 ), perovskites nanocrystals (NCs) (400 nm pump with intensity of 15 ⁇ cm-2) according to various embodiments and 1,2-ethanedithiol nanocrystals / 7-diphenyl-l ,10-phenanthroline (EDT-NCs/Bphen) (400 nm pump with intensity of 15 ⁇ cm-2) according to various embodiments at 2 ps after excitation
  • FIG. 29 are plots of normalized transmittance change ⁇ / ⁇ as a function of time (in picoseconds or ps) showing normalized band-edge bleaching dynamics of 1,2-ethanedithiol- treated nanocrystals (EDT-NCs) film according to various embodiments and 1,2- ethanedithiol-treated nanocrystals / 7-diphenyl- l,10-phenanthroline (EDT-NCs/Bphen) bilayer according to various embodiments under (a) low ( ⁇ No> ⁇ 0. l) and (b) high ( ⁇ No> -2.5) pump fluence with 3.1 eV photoexcitation.
  • EDT-NCs 1,2-ethanedithiol- treated nanocrystals
  • Bphen 7-diphenyl- l,10-phenanthroline
  • FIG. 30A is a plot of normalized transmittance change ⁇ / ⁇ as a function of energy (electron volts or eV) showing normalized transient absorption spectra for annealed 1,2-ethanedithiol treated (EDT-treated) medium methylammonium lead bromide nanocrystals (MAPbBr 3 NCs) film with (continuous lines) and without (dashed lines) 7-diphenyl- l, 10-phenanthroline (Bphen) extraction layers according to various embodiments at low fluence with ⁇ No> - 0.1.
  • FIG. 30B is a plot of carrier temperature (in Kelvins or K) as a function of time (in picoseconds or ps) showing extracted hot-carrier temperature as a function of delay time for two samples according to various embodiments.
  • FIG. 30C is a plot of normalized transmittance change ⁇ / ⁇ as a function of energy (in electron volts or eV) showing normalized transient absorption (TA) spectra for methylammonium lead bromide (MAPbBr 3 ) bulk-film (-240 nm thick) with (lines) and without (dashes) 7-diphenyl- l,10-phenanthroline (Bphen) extraction layers at low pump fluence with 2x 10 17 cm "3 .
  • TA transient absorption
  • FIG. 30D shows a plot of carrier temperature (in Kelvins or K) as a function of time delay (in picoseconds or ps) for two samples according to various embodiments.
  • FIG. 31 shows cross-sectional scanning electron microscopy (SEM) images of 1,2- ethanedithiol-treated nanocrystals (EDT-NCs film) with different thickness according to various embodiments.
  • Embodiments described in the context of one of the methods or nanocrystals/solar cells/devices is analogously valid for the other methods or nanocrystals/solar cells/devices.
  • embodiments described in the context of a method are analogously valid for nanocrystals/solar cell/device, and vice versa.
  • the word “over” used with regards to a deposited material formed “over” a side or surface may be used herein to mean that the deposited material may be formed “directly on”, e.g. in direct contact with, the implied side or surface.
  • the word “over” used with regards to a deposited material formed “over” a side or surface may also be used herein to mean that the deposited material may be formed "indirectly on” the implied side or surface with one or more additional layers being arranged between the implied side or surface and the deposited material.
  • a first layer “over” a second layer may refer to the first layer directly on the second layer, or that the first layer and the second layer are separated by one or more intervening layers.
  • the articles “a”, “an” and “the” as used with regard to a feature or element include a reference to one or more of the features or elements.
  • FIG. 1 shows a general illustration of a nanocrystal 100 according to various embodiments.
  • the nanocrystal 100 may include a halide perovskite material.
  • the nanocrystal may have a diameter having a value selected from a range of 1 nm to 100 nm, e.g. of 4 nm to 14 nm or 4 nm to 13 nm.
  • a radius of the nanocrystal may be any one value selected from 0.5 nm to 50 nm, e.g. from 2 nm to 7 nm.
  • the expressions "from X to Y" or "a range of X to Y” may refer to a range including the values of X and Y, in addition to all values between X and Y.
  • Various embodiments may slow down the hot-carrier cooling processes through the phonon bottleneck effect or interband Auger process (which is also known as Auger heating).
  • Various embodiments may be employed in solar cells, which may overcome the SQ-limit by harvesting the excess energy from above-bandgap photons, thus improving efficiency.
  • NCs inorganic semiconductor nanocrystals
  • hot- carrier harvesting in these inorganic semiconductor nanocrystals are compromised by highly competitive relaxation pathways (e.g. , intraband Auger process, defects) that overwhelm their phonon bottlenecks.
  • Kilina et al. Quantum Zero Effect Rationalizes The Phonon Bottleneck In Semiconductor Quantum Dots", Physical Review Letters 110, 180404, p. 1 - 6, 2013
  • Kilina et al. Quantum Zero Effect Rationalizes The Phonon Bottleneck In Semiconductor Quantum Dots
  • Kilina et al. (“Quantum Zero Effect Rationalizes The Phonon Bottleneck In Semiconductor Quantum Dots", Physical Review Letters 110, 180404, p. 1 - 6, 2013) reports that the effect of phonon bottleneck in cadmium selenide (CdSe) quantum dots remains elusive due to Auger processes and structural defects.
  • CdSe cadmium selenide
  • colloidal halide perovskite NCs may transcend these limitations.
  • the halide perovskite NCs may exhibit ⁇ 2 orders longer hot- carrier cooling times and ⁇ 4 times higher hot-carrier temperatures than their bulk-film counterparts.
  • hot-carrier cooling mediated by a phonon bottleneck may surprisingly be slower in smaller NCs (in contrast to conventional NCs in which cooling time decreases as size decreases).
  • Auger heating may dominate hot-carrier cooling, which is slower in larger NCs (hitherto unobserved in conventional NCs).
  • the inventors demonstrate efficient room temperature hot-electrons extraction (up to -83%) by an energy- selective electron extraction layer from surface-treated perovskite NCs thin films within 1 picosecond (ps). These insights may allow fresh approaches for extremely thin absorber and/or concentrator-type hot-carrier solar cells.
  • the halide perovskite material may be represented by the general formula AMX 3 , where A may be a monopositive organic or inorganic cation (e.g. an organic group or organic cation or a metal cation or element), or a mixture of organic and/or inorganic cations, M may be a divalent metal cation or element, and X may be a halogen anion or element.
  • A may be a monopositive organic or inorganic cation (e.g. an organic group or organic cation or a metal cation or element), or a mixture of organic and/or inorganic cations
  • M may be a divalent metal cation or element
  • X may be a halogen anion or element.
  • MA may refer to methylammonium (CH 3 NH 3 )
  • the divalent cation may be Pb 2+ , Sn 2+ .
  • M may be lead (Pb) or tin (Sn).
  • the halide perovskite material may include one or more halide anions selected from a group consisting of ⁇ , CI " and Br " .
  • X 3 may be I 3 , Cl 3 , Br 3 , or a combination thereof (e.g. Cl 2 Br).
  • the halide perovskite material may include an organic ammonium cation.
  • the organic ammonium cation A may be selected from a group consisting of an ammonium cation, a hydroxylammonium cation, a methylammonium cation (MA + ), a hydrazinium cation, an azetidinium cation, a formamidinium cation (FA + ), an imidazolium cation, a dimethylammonium cation, an ethylammonium cation, a phenethylammonium cation, a guanidinium cation, and combinations thereof.
  • the organic ammonium cation may be a cation with formula CnEhn+i NH 3 + where 2 ⁇ n ⁇ 20.
  • A may be CnEhn+i NH 3 .
  • the halide perovskite material may include a metal cation such as cesium ion (Cs + ).
  • the nanocrystal 100 may exhibit a hot-carrier cooling lifetime of any value of at least 0.5 ps, e.g. from 0.5 ps to 40 ps.
  • the hot-carrier cooling lifetime may be defined as the time interval from pulse excitation till the cooling of hot-carriers to 600K.
  • FIG. 2 shows a general illustration of a hot-carrier solar cell 200 according to various embodiments.
  • the solar cell 200 may include a nanocrystal containing layer 202 containing or including one or more nanocrystals (as described herein), each of the one or more nanocrystals including a halide perovskite material.
  • the hot-carrier solar cell 200 may also include a first electrode 204 in contact with a first side of the nanocrystal containing layer 202.
  • the hot-carrier solar cell 200 may further include a second electrode 206 in contact with a second side of the nanocrystal containing layer opposite to the first side.
  • the nanocrystal containing layer 202 may have a thickness of less than 100 nm.
  • the solar cell 200 may include a nanocrystal containing layer 202 which contains one or more nanocrystals.
  • the layer 202 may be sandwiched by electrodes 204, 206.
  • the nanocrystal containing layer 202 may also be referred to as the absorbing layer or hot-carrier absorber.
  • the thickness of the layer 202 may be less than the hot-carrier diffusion length so that hot carriers can be extracted by the electrodes 204, 206 before cooling.
  • the hot-carrier solar cell 200 may receive incoming light (from the sun) and may be configured to generate electrical energy based on the solar energy from the incoming light.
  • the hot-carrier solar cell 200 may further include an optical arrangement configured to direct solar energy (from the sun) to the nanocrystal containing layer 202.
  • the hot-carrier solar cell 200 may be a concentrator hot carrier solar cell.
  • the optical arrangement may include one or more optical elements to direct solar energy to the nanocrystal containing layer 202.
  • the one or more optical elements may be or may include optical lenses and/or mirrors. Hot-carrier cooling may be become slower with increasing photoexcited charge carrier density at higher pump fluence.
  • the hot-carrier cooling lifetime may exceed 30 ps (compared to 1.5 ps in bulk film), which may be due to Auger-heating effect in the quantum confined system.
  • These features may be favorable for application of concentrator solar cells which are operated at higher power density by focusing light to a spot of the photovoltaic cell.
  • the hot-carrier lifetime in perovskite NCs may be longer at high pump fluence.
  • These features may be favorable for application of concentrator hot carrier solar cells, which may be operated at higher illumination (about or exceeding 1000 suns) using hot-carrier absorbers.
  • the hot-carrier solar cell 200 may be a single-junction solar cell. In various alternate embodiments, the hot-carrier solar cell 200 may be a multi- junction solar cell.
  • the first electrode 204 may be or may include a hot- electron extraction layer.
  • the first electrode 204 may be or may include a n-type layer.
  • the n-type layer or hot-electron extraction layer may include any one material selected from a group consisting of titanium oxide, zinc oxide, phenyl-C61 -butyric acid methyl ester (PCBM), 4,7-diphenyl-l,10-phenanthroline (Bphen), poly(9-vinylcarbazole) (PVK), 2-(4- biphenylyl)-5-phenyl- 1 ,3 ,4-oxadiazole (PBD), 2,2',2"-( 1 ,3 ,5-benzinetriyl)-tris( 1 -phenyl- 1 -H- benzimidazole) (TPBI), poly(9,9-dioctylfluorene) (F8), and bathocuproine (BCP).
  • PCBM phenyl-C61 -butyric acid methyl ester
  • Bphen 4,7-dip
  • the first electrode 204 may be an energy selective contact configured to allow electrons having excess energies at or above a predetermined value to pass through, and further configured to reflect electrons having excess energies below the predetermined value back to the nanocrystal containing layer 202.
  • excess energies may refer to energy of the electrons beyond the conduction band minimum of the nanocrystal containing layer 202.
  • the predetermined value of excess energy may be any value selected from a range of around 0.1 eV to 2 eV.
  • the second electrode 206 may be or may include a hot- hole extraction layer.
  • the second electrode 206 may be or may include a p-type layer.
  • the second electrode 206 may be an energy selective contact configured to allow holes having excess energies at or above a predetermined value to pass through, and further configured to reflect holes having excess energies below the predetermined value back to the nanocrystal containing layer 202.
  • excess energies may refer to energy of the holes beyond the valence band maximum of the nanocrystal containing layer 202.
  • the predetermined value of excess energy may be any value selected from a range from around 0.1 eV to 2 eV.
  • the second electrode 206 may include a molecular semiconductor material.
  • the p-type layer or hot-hole extraction layer may include any one material selected from a group consisting of 2,2' ,7,7'- tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9'-spirobifluorene (spiro-OMeTAD), poly(3- hexylthiophene-2,5-diyl) (P3HT), poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT: PSS), and poly(9,9-dioctyl-fluorene-co-N-(4-butylphenyl)diphenylamine) (TFB).
  • spiro-OMeTAD poly(3- hexylthiophene-2,5-diyl)
  • P3HT poly(3,4-ethylenedioxythiophene) polystyrene sulfonate
  • the one or more nanocrystals exhibit a hot-carrier cooling lifetime of at least 0.5 ps, e.g. above 30 ps.
  • a radius of each of the one or more nanocrystals is any one value selected from 0.5 nm to 50 nm, e.g. 2 nm to 7 nm.
  • the halide perovskite material may be an organic- inorganic halide perovskite material, such as MAPbI 3 , MAPbBr 3 , MAPbBr 2 I, FAPbI 3 , FAi- yCs y PbI 3 , or Cs x (MAi-yFAy)i-xPb(Ii-zBr z ) 3 (where each of x, y or z may be any value selected from a range of 0 to 1).
  • the halide perovskite material may be an inorganic halide perovskite material such as CsSnI 3 or CsPbI 3 .
  • Extraction of hot-carriers may be required to be very fast to limit energy loss; where the competition is between extraction rate and cooling rate rather than recombination rate.
  • the one or more nanocrystals may be treated with 1,2- ethanedithiol (EDT).
  • Bphen may be selected as the hot-electron extraction material because this molecular semiconductor has a high electron mobility and a higher lowest unoccupied molecular orbital (LUMO) than the conduction band minimum (CBM) of the EDT-treated NC's.
  • EDT treatment may be used to substitute the long and highly insulating oleic acid ligands that is present on the as- prepared NC surfaces with thiolate for more effective electronic coupling with Bphen and within NCs films.
  • FIG. 3 is a schematic 300 showing a method of forming a nanocrystal according to various embodiments.
  • the method may include, in 302, using a solution-based process to form the nanocrystal including a halide perovskite material.
  • the nanocrystal may have a diameter having a value selected from a range of 1 nm to 100 nm, e.g. of 4 nm to 14 nm.
  • the solution-based process may include mixing a plurality of precursors with a solvent to form a precursor solution.
  • the plurality of precursors may include an organic ammonium halide.
  • the solution-based process may further include adding one or more ligands and/or one or more surfactants to the precursor solution.
  • methylammonium bromide (MABr, with MA representing methyammonium) may be mixed with lead bromide (PbBr 2 ) in a solvent of dimethyformamide (DMF) to form an initial precursor solution.
  • Oleyamine (OAm) and oleic acid (OAc) may be added to the DMF solvent to form a final precursor solution for forming methylammonium lead bromide perovskite nanocrystals.
  • the solution-based process may further include heating a further solvent.
  • the further solvent may be heated to a predetermined temperature, e.g. 60 °C.
  • the solution based process may additionally include mixing the precursor solution with the heated further solvent under stirring to form the nanocrystal.
  • the further solvent may be toluene.
  • FIG. 4 is a schematic 400 showing a method of forming a hot-carrier solar cell according to various embodiments.
  • the method may include, in 402, providing or forming the nanocrystal containing layer including one or more nanocrystals (as described herein), each of the one or more nanocrystals including a halide perovskite material.
  • the method may also include, in 404, forming a first electrode so that the first electrode is in contact with a first side of the nanocrystal containing layer.
  • the method may further include, in 406, forming a second electrode so that the second electrode is in contact with a second side of the nanocrystal containing layer opposite to the first side.
  • the nanocrystal containing layer may have a thickness of less than 100 nm.
  • the solar cell may include a nanocrystal containing layer containing one or more nanocrystals as described herein and electrodes.
  • the method steps shown in FIG. 4 may not necessarily be in sequence.
  • the first electrode may be formed, before forming the nanocrystal containing layer.
  • the method may further include forming an optical arrangement configured to direct solar energy to the nanocrystal containing layer.
  • the optical arrangement may include one or more optical elements configured to direct solar energy to the nanocrystal containing layer.
  • the one or more optical elements may be or may include optical lenses and/or mirrors.
  • the first electrode 204 may be or may include a hot- electron extraction layer.
  • the first electrode 204 may be or may include a n-type layer.
  • the n-type layer or hot-electron extraction layer may include any one material selected from a group consisting of titanium oxide, zinc oxide, phenyl-C61 -butyric acid methyl ester (PCBM), 4,7-diphenyl-l,10-phenanthroline (Bphen), poly(9-vinylcarbazole) (PVK), 2-(4- biphenylyl)-5-phenyl-l,3,4-oxadiazole (PBD), 2, 2',2"-(l,3,5-benzinetriyl)-tris(l-phenyl-l-H- benzimidazole) (TPBI), poly(9,9-dioctylfluorene) (F8), and bathocuproine (BCP).
  • PCBM phenyl-C61 -butyric acid methyl ester
  • Bphen 4,7-diphenyl-l,10-
  • the first electrode 204 may be an energy selective contact configured to allow electrons having excess energies at or above a predetermined value to pass through, and further configured to reflect electrons having excess energies below the predetermined value back to the nanocrystal containing layer 202.
  • excess energies may refer to energy of the electrons beyond the conduction band minimum of the nanocrystal containing layer 202.
  • the predetermined value of excess energy may be any value selected from a range of around 0.1 to 2 eV.
  • the second electrode 206 may be or may include a hot- hole extraction layer.
  • the second electrode 206 may be or may include a p-type layer.
  • the second electrode 206 may be an energy selective contact configured to allow holes having excess energies at or above a predetermined value to pass through, and further configured to reflect holes having excess energies below the predetermined value back to the nanocrystal containing layer 202.
  • excess energies may refer to energy of the holes beyond the valence band maximum of the nanocrystal containing layer 202.
  • the predetermined value of excess energy may be any value selected from a range of around 0.1 to 2 eV.
  • the second electrode 206 may include a molecular semiconductor material.
  • the p-type layer or hot-hole extraction layer may include any one material selected from a group consisting of 2,2',7,7'- tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9'-spirobifluorene (spiro-OMeTAD), poly(3- hexylthiophene-2,5-diyl) (P3HT), poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT: PSS), and poly(9,9-dioctyl-fluorene-co-N-(4-butylphenyl)diphenylamine) (TFB).
  • spiro-OMeTAD poly(3- hexylthiophene-2,5-diyl)
  • P3HT poly(3,4-ethylenedioxythiophene) polystyrene sulfonate
  • the one or more nanocrystals exhibit a hot-carrier cooling lifetime of at least 0.5 ps, e.g. above 30 ps.
  • a radius of each of the one or more nanocrystals is any one value selected from 0.5 nm to 50 nm, e.g. 2 nm to 7 nm.
  • the halide perovskite material may be an organic- inorganic halide perovskite material, such as MAPbI 3 , MAPbBr 3 , MAPbBr 2 I, FAPbI 3 , FAi- yCs y PbI 3 , or Cs x (MAi-yFAy)i-xPb(Ii-zBr z ) 3 (where each of x, y or z may be any value selected from a range from 0 to 1).
  • the halide perovskite material may be an inorganic halide perovskite material such as CsSnI 3 or CsPbI 3 .
  • MAPbBr 3 methylammonium lead bromide perovskite nanocrystals
  • the hot-carrier cooling may be dramatically slower than the perovskite bulk-films.
  • the hot-carrier cooling time may be increased to 30 ps (or above) in MAPbBr 3 NCs which is ⁇ 2 orders slower than their bulk-film counterparts under comparable photoexcitation conditions.
  • the hot-carrier temperature of the MAPbBr 3 NCs is 4 times larger than their bulk-film counterparts under comparable photoexcitation conditions.
  • Controlling hot-carrier cooling dynamics may be challenging but critical for improving the performance of many semiconductor photonic and electronic devices.
  • the hot-carrier cooling time/rate may depend both on the volume and carrier densities.
  • the hot-carrier cooling in the NCs may be modulated by varying the size of NCs.
  • the hot-carrier cooling rate may be lower in smaller sized NCs due to confinement induced phonon bottleneck effect.
  • MAPbBr 3 perovskites NCs
  • other perovskites NCs with substitution of organic component and/or the metal element such as MAPbI 3 , MAPbBr x Ii- x (x is the ratio of Br/(Br + I), which is determined by the contents of Br and I in the precursors during synthesis), CsSnI 3 , CsPbI 3 , FAPbI 3 .
  • NCs may allow a wide choice of hot-carrier absorbers with different bandgaps. Moreover, after the surface chemical treatment, the hot electrons from NCs thin film may be efficiently injected (up to -83%) into electron extraction layers within ⁇ 1 ps. These insights may enable fresh approaches for hot-carrier and concentrator-type perovskite NC photovoltaic s.
  • Various embodiments may relate to the fabrication of the low temperature solution processed organic-inorganic perovskite nanocrystals, the observation of slow hot-carrier cooling, and/or the potential application of these nanocrystals for hot-carrier solar cells and concentrator hot-carrier solar cells.
  • concentrator hot-carrier solar cell may use focusing lenses or curved mirrors to focus the sunlight by a factor of between 300 to 1000 times onto a small cell area.
  • a concentrator cell may therefore operate at much higher light-generated current densities than the normal hot- carrier solar cell. The higher injection densities may induce the higher hot-carrier temperature and longer hot-carrier lifetimes, which may further increase the efficiencies of hot-carrier solar cells.
  • the nanocrystals may be fabricated using a low temperature solution processed approach in atmosphere.
  • traditional Si-based solar cells are usually produced at elevated temperatures and using high vacuum growth techniques that require significant infrastructural investments.
  • Hot-carrier cooling may be critical in many photonic and electronic devices.
  • a solution processable material may have much greater versatility than traditional material for integration with existing silicon based technologies. It can be applied to a much wider range of device designs and substrates by simply spin-coating, dip-coating or drop-casting.
  • Solution processed organic-inorganic perovskite nanocrystals may provide simple and inexpensive alternatives of material for potential photovoltaic applications as compared to traditionally silicon thin film produced with expensive gas-phase methods.
  • the low temperature of processing may also enable integration of these materials into flexible substrates.
  • the perovskites NCs may have much slower hot-carrier cooling as compared with that of current perovskites thin films used as absorbers in solar-cells which permit the efficient hot-carrier extraction. These features may be beneficial for the achievement of hot- carrier solar cell.
  • the hot-carrier cooling time/rate may also be tuned by modifying NC size.
  • Various embodiments may find wide use in the application area of photovoltaics as absorption materials, such as NCs- sensitized nanocrystalline T1O2 solar cells, concentrator solar cells, NCs-conducting polymer blend solar cells, p-i-n array solar cells and/or concentrator solar cells.
  • FIG. 5 is a schematic 500 showing hot-carrier cooling with Auger processes in semiconductor nanocrystals.
  • hot-carrier cooling is made possible via intraband Auger- type energy transfer.
  • a hot electron (dot) may be cooled by Auger-type energy transfer to densely spaced hole states (e.g. , CdSe NCs), then the hot holes (circle) can relax rapidly via a cascade of single phonon emissions (arrows);
  • (c) shows, hot-carrier re-excitation by interband Auger-recombination of carriers at band edges, also called Auger-heating.
  • FIG. 6 shows representative transmission electron microscopy (TEM) images of methylammonium lead bromide perovskite (MAPbBr 3 ) nanocrystals (NCs) with relatively (a) small, (c) medium and (e) large sizes according to various embodiments and the respective size histograms (b, d, f) on their right.
  • the size distribution may be modeled with a Gaussian distribution.
  • FIG. 7 shows (a) top-view and (b) side-view scanning electron microscopy (SEM) images of methylammonium lead bromide (MAPbBr 3 ) bulk-film.
  • SEM scanning electron microscopy
  • FIG. 8 shows (a) a plot of photoluminescence (PL) intensity (in arbitrary units or a.u.) as a function of wavelength (in nanometers or nm) illustrating photoluminescence (PL) spectra of methylammonium lead bromide perovskite (MAPbBr 3 ) nanocrystals (NCs) according to various embodiments dispersed in toluene and bulk film counterpart, (b) a plot of absorbance (in arbitrary units or a.u.) as a function of wavelength (in nanometers or nm) illustrating ultraviolet-visible (UV-vis) absorption spectra of methylammonium lead bromide perovskite (MAPbBr 3 ) nanocrystals (NCs) according to various embodiments dispersed in toluene and bulk film counterpart, (c) a plot of energy of Is exciton, E ls (in electron volts or eV) as a function of average nanoc
  • results reveal that the weakly confined MAPbBr 3 NCs (FIG. 8) are very promising hot-carrier absorber materials as they possess much higher hot-carrier temperatures and longer cooling times (as compared to typical perovskite bulk-films under comparable photoexcitation conditions). This may be attributed to their intrinsic phonon bottleneck and Auger-heating effects at low and high carrier densities, respectively. Importantly, the hot- carriers may be efficiently extracted from MAPbBr 3 NC thin films at room temperature by using a molecular semiconductor as an energy selective contact. [0079] FIG.
  • FIG. 9A shows pseudo color transient absorption (TA) plot (upper panel, time (in picoseconds or ps) as a function of energy (in electron volts or eV)) and normalized transient absorption (TA) spectra (lower panel, normalized transmittance change ⁇ / ⁇ as a function of energy (in electron volts or eV)) for medium methylammonium lead bromide perovskite nanocrystals (MAPbBr 3 NCs) (radii -4-5 nm) according to various embodiments in solution at low pump fluence (left panel) with initially average generated electron-hole pair per nanocrystal, ⁇ Vo> -0.1 (average carrier density per nanocrystal volume, n 0avg ⁇ 2.6 x 10 17 cm ⁇ 3 ) and high pump fluence (right panel) with ⁇ No> -2.5 (n 0avg ⁇ 6.5x 10 18 cm ⁇ 3 ).
  • TA medium methylammonium lead bromide
  • FIG. 9B shows pseudo color representation (upper panel, time (in picoseconds or ps) and normalized transient absorption (TA) spectra (lower panel, normalized transmittance change ⁇ / ⁇ as a function of energy (in electron volts or eV)) for MAPbBr 3 bulk-film at low pump fluence (left panel) with initially generated carrier density no -2.1 x 10 17 cm 3 and high pump fluence (right panel) with no -1.5 x 10 19 cm 3 .
  • TA transient absorption
  • FIGS. 9A-B show a comparison of the pseudo color TA plots and TA spectra of the medium MAPbBr 3 NCs (radius -4.5 nm) against MAPbBr 3 bulk-film at low and high pump fluence, respectively.
  • the plots/spectra display a prominent photo-bleaching (PB) peak with a high energy tail near the bandgap because of the state- filling effects. Similar results were also observed in the small and large NCs.
  • PB photo-bleaching
  • FIG. 10 shows plots of time (in picoseconds or ps) as a function of energy (electron volts or eV) showing pseudocolor transient absorption (TA) spectra (time in picoseconds or ps as a function of energy in electron volts or eV) for (a) small and (b) large sized methylammonium lead bromide perovskite nanocrystals (MAPbBr 3 NCs) according to various embodiments in solution at low pump fluence (left panel) with initially average generated electron-hole pair per nanocrystal ⁇ No> -0.1 and high pump fluence (right panel) with ⁇ Vo> -2.5 following 3.1 eV photoexcitation.
  • TA pseudocolor transient absorption
  • the high energy tails of the PB peak originate from the rapid distribution of initial non-equilibrium carriers into a Fermi-Dirac distribution via elastic scatterings (including electron-hole scattering at low pump fluence and carrier-carrier scattering at high pump fluence) that can be characterized by a carrier temperature T .
  • T can thus be extracted by fitting the high-energy tail of the TA spectra with a simple Maxwell- Boltzmann function of exp(Ef - E/KB 7 ), where KB is the Boltzmann's constant and Ef is the quasi-Fermi energy.
  • the discrete energy levels can be approximately treated as a continuum in the case where thermal energy k ⁇ T » energy level spacing AE.
  • the perovskites NCs may be in the weak confinement regime with energy levels that are more closely spaced.
  • the TA spectra may be collected from an ensemble of NCs whose size distribution may cause inhomogeneous broadening, (i.e., overlapping TA spectrum from single NC). All these properties rightly lead to a continuous TA spectrum from the NCs ensemble to resemble that of the bulk materials.
  • the high- energy tail of the NCs' TA spectra may also be described by a Maxwell-Boltzmann distribution. The representative fits of the high-energy tails and non-normalized TA spectra are presented in FIG. 11.
  • FIG. 11 illustrates (a) a plot of normalized transmittance change ⁇ / ⁇ as a function of energy (in electron volts or eV) showing normalized TA spectra at different short time delays of medium sized methylammonium lead bromide perovskite nanocrystals (MAPbBr 3 NCs) according to various embodiments in toluene with average generated electron-hole pair per nanocrystal, ⁇ Vo> ⁇ 0.1 (following 3.1 eV photoexcitation) and (b) the un-normalized transient absorption (TA) spectra of (a).
  • the solid black lines in (a) fit to the high-energy tails using the Maxwell-Boltzmann distribution function.
  • the analysis and interpretation of the TA spectra of halide perovskites are well- documented in the literature.
  • the TA spectra of MAPbBr 3 perovskites (bulk and nanocrystals) are similar with that of previous studies.
  • the positive TA peak (at -2.3 eV as shown in FIG. 11) arose from ground states bleaching (GSB) due to the state-filling of the carriers at the band edge.
  • GSB ground states bleaching
  • the first slope i.e. , the steeper one closer to the GSB peak
  • the second gentler slope of the high energy tail e.g. , start at -2.5 eV in FIG.
  • the negative part (photoinduced absorption) of this high energy side is caused by the photoinduced change of the imaginary part of the refractive index; while the negative part of the low energy side of the GSB is attributed to bandgap renormalization.
  • Hot-carrier temperature T is extracted by fitting the high-energy tail of the TA spectra with a Maxwell-Boltzmann function.
  • the Fermi-Dirac distribution function can be approximately described with an exponential - i.e. , a Maxwell- Boltzmann distribution:
  • PL
  • ⁇ of NCs can be obtained by fitting the data with equation of 1— e ⁇ Ja (solid lines). The fitted ⁇ are 8.5+0.5xl0 "15 , 3.2+0.2xl0 "14 and 6.8+0.3xl0 "14 cm 2 from small to large NCs, respectively.
  • the maximum T at the excitation onset with ⁇ No> -0.1 may be around 1700 K, which is about four times higher than that for the bulk-film sample with comparable carrier densities. The smaller T in the latter may be attributed to arise from the ultrafast cooling of hot-carriers, which had occurred on a timescale much shorter than the temporal resolution of the TA measurements.
  • hot-carrier cooling times due to several factors: (i) the pump energy (i.e., carriers' excess energy - typically, higher excess energies lead to longer hot carrier lifetimes); (ii) the initial hot-carrier densities (i.e., higher carrier densities usually lead to longer hot carrier lifetimes); and/or (iii) the energy loss rate at a specific hot-carrier temperature (i.e., generally, lower hot carrier temperatures yield smaller energy loss rates).
  • pump energy i.e., carriers' excess energy - typically, higher excess energies lead to longer hot carrier lifetimes
  • initial hot-carrier densities i.e., higher carrier densities usually lead to longer hot carrier lifetimes
  • the energy loss rate at a specific hot-carrier temperature i.e., generally, lower hot carrier temperatures yield smaller energy loss rates.
  • the hot-carrier cooling lifetime as described herein may be defined as the time interval from pulse excitation till the cooling of hot-carriers to 600 K. This temperature is used as the benchmark because previous theoretical calculations have shown that for T > 600 K, there may still be an appreciable hot-carrier conversion efficiency (i.e., > 40%) over a wide range of absorber bandgaps.
  • the hot-carrier cooling lifetime may be defined as the time interval from pulse excitation till the cooling of hot- carriers to 600K. 600K may be used as the benchmark because previous theoretical calculations have shown that for T > 600 K, there may still be an appreciable hot-carrier conversion efficiency (i.e., > 40%) over a wide range of absorber bandgaps.
  • TA refers to "Transient Absorption”
  • TRPL refers to "time-resolved photoluminescence".
  • FIG. 15 show plots of carrier temperatures (in Kelvins or K) as a function of time delay (picoseconds or ps) for three different sized methylammonium lead bromide perovskite nanocrystals (MAPbBr 3 NCs) according to various embodiments and bulk-film (a) at low pump fluence (corresponding to ⁇ No> -0.1 in NCs and no -2.1 x 10 17 cm 3 in bulk-film) and (b) at high pump fluence (corresponding to ⁇ Vo> -2.5 in NCs and no -1.5 x 10 19 cm 3 in bulk- film) following 3.1 eV photoexcitation.
  • MAPbBr 3 NCs methylammonium lead bromide perovskite nanocrystals
  • the lifetime of large NCs may be around 40x longer than that for the bulk-film sample, where the bulk film was excited at almost one order higher carrier density of 1.5 x 10 19 cm "3 .
  • the hot-carrier cooling lifetimes can be as long as -32 ps (FIG. 15) for large NCs with ⁇ N 0 > - 2.5 (or n 0 av g of - 3.5 x 10 18 cm "3 ).
  • the hot-carrier cooling lifetimes of the MAPbBr 3 NCs may be much longer than those reported for other semiconductor bulk and nano materials.
  • the reported cooling lifetime is -2 ps with carrier densities of -6.0 x 10 18 cm “ 3 and excess energies of 1.7 eV; and for CdSe nanorods, the reported cooling lifetime is -0.8 ps with carrier densities of -5.5 x 10 18 cm “3 and excess energies of 1.1 eV.
  • the MAPbBr 3 NCs may compare very favorably with much longer lifetimes of 18 ps, excited with much lower excess energies of -0.7 eV at comparable carrier densities of 6.5 x 10 18 cm "3 .
  • FIG. 16 illustrates plots of normalized transmittance change ⁇ / ⁇ as a function of time (picoseconds or ps) showing normalized photobleaching dynamics probed at the band edges of methylammonium lead bromide (a) bulk-film, (b) small nanocrystals (NCs) according to various embodiments, (c) medium nanocrystals (NCs) according to various embodiments, and (d) large nanocrystals (NCs) according to various embodiments, in solution with high and low pump fluence respectively, as well as (e) comparison of transient absorption (TA) dynamics for medium nanocrystals (NCs) according to various embodiments in solution and spin-coated nanocrystals (NCs) film according to various embodiments, and (f) pump-fluence-dependent bleaching dynamics probed at the band edge of small methylammonium lead bromide nanocrystals (NCs) according to various embodiments in solution.
  • the inset of (f) in FIG. 16 shows the extracted Auger
  • the hot-carrier relaxation mechanism at low pump excitation may therefore be representative of the material's intrinsic properties and may not be influenced by extrinsic effects such as the multi-particle Auger-recombination.
  • FIG. 17 shows (a) a plot of energy loss rate (electron volts per picosecond eV ps "1 ) as a function of carrier temperature (in Kelvins or K) illustrating energy loss rate of hot-carriers as a function of carrier temperature T for methylammonium lead bromide perovskite nanocrystals (MAPbBr 3 NCs) according to various embodiments with ⁇ No> -0.1 and methylammonium lead bromide perovskite (MAPbBr 3 ) bulk-film with no -2.1 x 10 17 cm “3 , (b) a plot of normalized transmittance change ⁇ / ⁇ as a function of time (in picoseconds or ps) illustrating normalized bleaching dynamics probed at the band-edge for colloidal methylammonium lead bromide perovskite nanocrystals (MAPbBr 3 NCs) according
  • FIG. 18 is a plot of Raman intensity (in arbitrary units or a.u.) as a function of wavenumbers (in per centimeter or cm "1 ) showing room temperature Raman spectrum of as-prepared methylammonium lead bromide perovskite nanocrystals (MAPbBr 3 NCs) according to various embodiments drop-cast on glass substrate. The peaks originate from LO phonons. From the Raman measurement shown in FIG.
  • the available phonon modes for hot-carrier cooling in MAPbBr 3 NCs are located at around -150 cm “1 (assigned to the stretching of the Pb-Br bonds) and 300 cm “1 (which could be from the second-order of 150 cm “1 and/or the torsional mode of MA cations), respectively.
  • Solid lines in FIG. 17(a) represent the numerical fits with LO-phonon model.
  • the arrow indicates the maximum T obtained for the bulk-film.
  • the inset in FIG. 17(a) shows the representative TEM images of small (S), medium (M) and large (L) perovskites NCs.
  • Solid lines in FIG. 17(b) are the single exponential fits.
  • Inset in FIG. 17(b) shows a schematic of the phonon bottleneck induced slow hot-carrier cooling in symmetric conduction and valence bands with discrete energy levels.
  • the energy loss rates per carrier J t may slowly decrease within the range of 0.6-0.3 eV ps "1 until T c reaches -700 K (FIG. 17(a)), below which J r plunges by several orders of magnitude until the T approaches the lattice temperature.
  • Such cooling trend is similar to that for the bulk-film sample as well as in other bulk inorganic semiconductors and nanostructures.
  • the initial rapid cooling i.e., higher cooling rate
  • the initial J t for small NCs is smaller than the large NCs by a factor -2 (indicating a weaker carrier-phonon interaction in the former).
  • the subsequent slower cooling of the hot-carriers closing to the band-edges is determined by the thermal equilibration between longitudinal optical phonons (LO phonons) and acoustic phonons.
  • the energy loss rate was fitted by using a LO-phonon interaction model (see below under LO-phonon model), the fitted TLO (characteristic LO-phonon decay time) increases with reducing NCs dimensionality (see FIG. 17(a)), which may provide direct evidence of the reduction in the optical phonon relaxation by the quantum confinement. This is a characteristic of the phonon bottleneck effect, which thus retards the hot-carriers cooling.
  • the NCs are in the weak confinement regime with confinement energy around -15- 60 meV, several early theoretical papers had shown that even in this weak confinement regime where the level spacing is only a few meV, the carrier relaxation mediated by phonon interactions can still be dramatically hindered.
  • the band-edge bleach buildup approach was also used to elucidate the hot-carrier cooling properties.
  • an alternative method is to probe the intraband relaxation of the photoexcited carriers high above the band-edge. This can be achieved through monitoring the buildup of the band-edge bleach as the recombination of the band- edge carriers ( ⁇ ns) is much slower than its intraband relaxation process (from several to tens of ps).
  • This latter approach is commonly used for investigating the hot-carrier dynamics in strongly confined quantum colloidal semiconductor NCs given the overlapping PB bands from the discrete energy levels make resolving their hot-carrier distribution extremely challenging.
  • the latter approach may be applied for a fair comparison of the hot-carrier cooling of perovskites NCs with that of conventional inorganic semiconductor NCs (e.g., CdSe NCs).
  • FIG. 17(b) shows the normalized TA spectra of the perovskite samples probed at their band-edge PB peaks following photoexcitation with similar excess energies at low carrier densities.
  • Each buildup process is fitted with a single-exponential growth function to yield a rise time (T r i se ).
  • the rise of the band-edge bleach occurs at sub-ps timescale that becomes slower with decreasing NC size, consistent with the smaller J r and slower hot-carrier temperature decay (FIG. 15) for smaller perovskites NCs.
  • the trend exhibited by the perovskite NCs is completely opposite to that for CdSe NCs (spanning the strong to weak quantum confinement regimes - FIG. 17(c) and FIG. 19).
  • FIG. 19 shows (a) a plot of normalized transmittance change ⁇ / ⁇ as a function of time (in picoseconds or ps) illustrating normalized bleaching dynamics probed at the band edge for colloidal CdSe NCs with different diameters (shown in legend) at low pump fluence, and (b) plots of time (in picoseconds or ps) as a function of energy (in electron volts or eV) illustrating low pump fluence with initially generated ⁇ No> -0.1 (left) and high pump fluence with ⁇ No> -2.5 (right).
  • the phtotoexcitation energy is 3.1 eV.
  • Solid lines in FIG. 19 (a) are the single exponential growth fitting curves.
  • the inset in FIG. 19(a) schematically shows the hot-carrier cooling process via Auger-type energy transfer.
  • the perovskite NCs rise times may also be much longer.
  • the faster hot-carrier cooling with decreasing CdSe NCs is consistent with previous reports, which is attributed to an Auger-type energy transfer from the hot electrons to the dense hole states. Results clearly show that such Auger-transfer mechanism present in conventional inorganic semiconductor NCs may be naturally suppressed in perovskites NCs.
  • FIG. 20 shows (a) a plot of energy loss rate (in electron volts per picosecond or eV ps -1 ) as a function of carrier temperature (in Kelvins or K) illustrating energy loss rate vs carrier temperature T for methylammonium lead bromide perovskite nanocrystals (MAPbBr 3 NCs) according to various embodiments with ⁇ No> -2.5 and MAPbBr 3 bulk-film with no -1.5 x 10 19 cm “3 , (b) a plot of lifetime (in picoseconds or ps) as a function of nanocrystal volume (in cubic nanometer or nm 3 ) illustrating Auger recombination lifetimes and hot-carrier cooling time vs perovskite nanocrystal (NC) volume according to various embodiments, and (c) a plot of normalized hot carrier concentration Hhot as a function of
  • the solid line in FIG. 20(a) represents the LO-phonon model at low carrier densities.
  • the dashed lines in FIG. 20(b) are guides to the eye showing the scaling of the lifetimes with the square root of nanocrystal (NC) volume, while the inset illustrates the hot-carrier re-excitation by Auger-recombination of carriers at band-edge (also known as Auger-heating), and the error bars represent standard errors.
  • Solid lines in FIG. 20(c) are bi-exponential decay fits. Photoexcitation energy for FIG. 20 (a) - (c) is 3.1 eV. The bulk film is about 240 nm thick.
  • FIG. 20(a) shows contrasting trends of energy loss rates vs carrier temperature between the three-different sized NCs (at ⁇ No> -2.5) and the bulk-film sample (at no - 1.5xl0 19 cm "3 ).
  • the initial hot-carrier cooling governed by the carrier- LO-phonon interactions may be nearly independent of carrier densities. This can be concluded from: (i) the almost identical initial fast decay of T at different carrier densities (FIG. 12), and (ii) the similar initial energy loss rate at high carrier temperatures for both low and high carrier densities (FIG. 17(a) and FIG. 20(a)).
  • the elongation of hot-carrier lifetime FIG.
  • Equation 21 is a plot of energy loss rate (electron volts per picosecond or eV ps "1 ) as a function of carrier temperature (in Kelvins or K) showing energy loss rate of hot-carriers as a function of carrier temperature T for methylammonium lead bromide (MAPbBr 3 ) bulk-film at low and high carrier densities.
  • Solid lines represent the fits numerically fitted with Equation (3) (shown below under section on "LO-phonon model”).
  • the fitted LO-phonon lifetime TLO and acoustic temperature T d are 150 + 20, 280 + 20 fs, and 305 + 10 and 350 + 10 K with low and high carrier densities, respectively.
  • FIG. 20(c) shows that the calculated concentration ( «hot (t)) of hot-carriers for different sized NCs relaxes bi-exponentially with a fast decay occurring within 1 ps and a slower decay of several tens of ps - similar to the behavior of the hot-carrier temperatures (FIG. 12 and FIG. 15).
  • the fast decay may be attributed to the carrier-LO-phonon interactions.
  • the fitted slow decay lifetimes of nhot (t) are well-matched with the 1/3 relation of their Auger lifetime TAug (i.e., Thot ⁇ TAug/3 - FIG. 20(b), see also below under "Auger-heating model" section).
  • the slower decay lifetime for the small NCs is fitted to be -12 ps, which is very close to 1/3 of its ⁇ 3 ⁇ 4 of 38 ps.
  • the excellent agreement between the experimental data and our simple model that includes Auger effects may strongly substantiate the dominant Auger heating contribution in further retarding the hot- carrier cooling at high carrier densities.
  • Auger induced hot-carrier cooling lifetime may be sublinearly dependent on the NC volume (FIG. 20(b)).
  • Auger heating causes a slowdown of the hot- carrier cooling rate favorable for hot-carrier extraction, it should also be noted that Auger effects may conversely reduce the carrier densities. It may therefore be necessary to balance the hot-carrier lifetime and carrier losses in the application of concentrator-type hot-carrier solar cells at high pump fluence.
  • FIG. 22 shows plots of photoelectron intensity (counts per second or cts/s) as a function of energy (in electron volts or eV) showing the ultraviolet photoelectron spectroscopy (UPS) spectrum of (a) 1,2-ethanedithiol (EDT)-treated and (b) post-annealed EDT-treated methylammonium lead bromide (MAPbBr 3 ) nanocrystals (NCs) films according to various embodiments, and (c) 7-diphenyl-l,10-phenanthroline (Bphen) film on indium tin oxide (ITO) substrates.
  • UPS ultraviolet photoelectron spectroscopy
  • the valence band maximum may be determined by linear extrapolation of the leading edge of the valence band to the background intensity, which is 1.9 + 0.1, 2.3 + 0.1 and 2.9 + 0.1 eV for FIG. 22 (a) -(c), respectively.
  • Bphen may be selected as the hot-electron extraction material because this molecular semiconductor has a high electron mobility and possess a higher lowest unoccupied molecular orbital (LUMO) than the conduction band minimum (CBM) of our EDT-treated NCs (see FIG. 22 for UPS measurements), implying only hot-carriers with sufficient excess energies above band-edge can be injected into Bphen (see FIG. 23A).
  • LUMO lowest unoccupied molecular orbital
  • CBM conduction band minimum
  • FIG. 23 A is a flat-band energy diagram (vertical axis in electron volts or eV) to illustrate hot-electron extraction from perovskite nanocrystals according to various embodiments to 7-diphenyl-l,10-phenanthroline (Bphen) with competing hot-electron cooling pathways.
  • Conduction band minimum (CBM) or LUMO levels
  • VBM valence band minimum
  • HOMO highest occupied molecular orbital
  • FIG. 23B shows an atomic force microscopy (AFM) image of 1,2-ethanedithiol (EDT)-treated nanocrystal (NC) film according to various embodiments.
  • FIG. 23C is a scanning electron microscopy (SEM) image of 1,2-ethanedithiol (EDT)-treated nanocrystal (NC)/ 7-diphenyl- l, 10-phenanthroline (Bphen) bilayer according to various embodiments.
  • the scale bar is 100 nm.
  • FIG. 23D is a plot of normalized transmittance change ( ⁇ / ⁇ ) as a function of energy (in electron volts or eV) showing normalized transient absorption (TA) spectra for about 35 nm thick of 1,2-ethanedithiol (EDT)-treated nanocrystal (NC) film with (continuous lines) / without (dashed lines) 7-diphenyl- l,10-phenanthroline (Bphen) according to various embodiments following 3.1 eV photoexcitation with ⁇ No> around 0.1.
  • Inset of FIG. 23D shows the un-normalized transient absorption (TA) spectra at 0.8 ps.
  • 23E is a plot of hot-carrier temperature (in Kelvins or K) as a function of delay time (in picoseconds or ps) for 1,2-ethanedithiol (EDT)-treated nanocrystal (NC) film and 1,2-ethanedithiol (EDT)- treated nanocrystal (NC) film /7-diphenyl-l, 10-phenanthroline (Bphen) bilayer according to various embodiments at different pump fluences. Dotted arrows show the decrease of the initial hot-carrier temperatures after adding the Bphen layer, indicating effective hot-electron extraction.
  • K hot-carrier temperature
  • FIG. 23F is a plot of extraction efficiency rjhot (in percent or %) as a function of hot-electron excess energy (in electron volts or eV) showing pump energy dependence of the hot-electron extraction efficiencies in about 35 nm-thick 1,2-ethanedithiol (EDT)-treated nanocrystal (NC)/ 7-diphenyl-l, 10-phenanthroline (Bphen) bilayer according to various embodiments.
  • EDT 1,2-ethanedithiol
  • NC nanocrystal
  • Bphen 10-phenanthroline
  • 23G is a plot of extraction efficiency rjhot (in percent or %) as a function of thickness (in nanometres or nm) showing perovskite film thickness dependence of the hot electron extraction efficiencies upon 3.1 eV pump energy excitation for 1,2- ethanedithiol (EDT)-treated nanocrystal (NC)/ 7-diphenyl-l, 10-phenanthroline (Bphen) bilayer according to various embodiments and bulk-film/7-diphenyl- l, 10-phenanthroline (Bphen) bilayer.
  • Inset shows the un-normalized TA spectra at 0.8 ps following 3.1 eV photoexcitation with ⁇ No> around 0.1 for about 140 nm-thick EDT-NCs film with/without Bphen.
  • Error bars on the x axis represent the uncertainties in the determination of excess energies in FIG. 23F and sample thickness in FIG. 23G and on the y axis represent uncertainties in the determination of extraction efficiencies.
  • FIG. 24 shows (a) plot of transmittance (in arbitrary units or a.u.) as a function of wavenumber (in per centimeter or cm "1 ) showing the attenuated total reflection- fourier transform infrared (ATR-FTIR) spectra of as-prepared methylammonium lead bromide nanocrystals (MAPbBr 3 NCs), 1,2-ethanedithiol (EDT)-treated nanocrystals (EDT-NCs) and 70 °C annealed 1,2-ethanedithiol nanocrystals (Ann-EDT-NCs) according to various embodiments, and plots of photoemission intensities (in arbitrary units or a.u.) as a function of binding energy (in electron volts or eV) showing sulfur (S) 2p X-ray photoelectron spectroscopy (XPS) spectra of (b) non-annealed and (c) 70 °C post-annealed 1,2-
  • Bphen possesses a narrow electron bandwidth, which may allow it to approximate the energy selective contact required in hot-carrier solar cells.
  • EDT treatment may be used to substitute the long and highly insulating oleic acid and oleylamine ligands that is present on the as-prepared NC surfaces with thiolate (see ATR-FTIR and XPS measurements in FIG. 24 and section on FTIR and XPS analysis on ligand exchange) for more effective electronic coupling with Bphen and within NCs films (evident from the closer NCs packing after treatment as shown in TEM images in FIG. 25).
  • FIG. 25 shows (a) atomic form microscopy (AFM) image of un-treated medium methylammonium lead bromide nanocrystals (MAPbBr 3 NCs) film according to various embodiments, and (b) representative transmission electron microscopy (TEM) image of 1,2-ethanedithiol-treated methylammonium lead bromide nanocrystals (EDT-treated MAPbBr 3 NCs) according to various embodiments.
  • AFM atomic form microscopy
  • MAPbBr 3 NCs un-treated medium methylammonium lead bromide nanocrystals
  • TEM transmission electron microscopy
  • TA pseudocolor transient absorption
  • the high energy tails may be reduced for EDT-NCs/Bphen following 3.1 eV photoexcitation.
  • Hot-electron extraction from spin-coated EDT-NCs thin film (see AFM and SEM images in FIGS. 23B-C) by Bphen may be validated by the clear reduction of the high energy tails of TA spectra for the EDT-NCs/Bphen bilayers that occurs instantaneously (see FIG. 23 D, pseudo color TA spectra in FIG. 26 and section on "Effects of photocharged NCs and trions in NCs films on hot-carriers").
  • FIG. 27 shows energy diagrams (y axis: energy in electron- volts or eV) showing flat-band energy level alignment as determined from the ultraviolet photoelectron spectroscopy (UPS) and ultraviolet- visible (UV-VIS) spectroscopy measurements for non-annealed, annealed 1,2-ethanedithiol -nanocrystals (EDT-NCs) films and 7-diphenyl-l,10-phenanthroline (Bphen) according to various embodiments - illustrated for the case of hot-electron extractions.
  • Excess energy of extracted hot-electrons may be determined from the band offset between the conduction band minimum of NCs and the LUMO of Bphen.
  • hot-electrons may be likely to be injected into Bphen through electron diffusion inside the NC and hopping at NC-interfaces.
  • the driving force of hot- electron transfer may be the energy difference between the hot-carrier energy and the LUMO energy with respect to the Fermi energy as shown in FIG. 23A.
  • a large density of states is typical for organic molecules.
  • the highly efficient hot-carrier transfer may thus be attributed to the high density of acceptor states in LUMO levels of Bphen together with the strong electronic coupling between Bphen and NCs.
  • NIR near-infrared
  • PIA photoinduced absorption
  • the efficiency of hot-electron extraction may be estimated based on the percentage reduction of the band-edge photo-bleaching intensities at ⁇ 0.8 ps after adding Bphen (as when the hot-electrons are relaxed to the band-edges, the reduced band-edge bleaching intensity can then be attributed to the extraction of hot-carriers).
  • Calculated hot for -35 nm thick EDT-NCs/Bphen bilayer is -72 % and -58 % at ⁇ N 0 > -0.1 and 2.5 pump intensities, respectively.
  • the reduced multi-hot-electron injection efficiency at higher pump fluence may be due to the increased back-electron transfer from Bphen to the NCs with the estimated back-electron transfer time of -80 ps (FIG. 29 and section under "Estimation of back-electron transfer time").
  • FIG. 29 are plots of normalized transmittance change ⁇ / ⁇ as a function of time (in picoseconds or ps) showing normalized band-edge bleaching dynamics of 1,2- ethanedithiol-treated nanocrystals (EDT-NCs) film according to various embodiments and 1,2-ethanedithiol-treated nanocrystals / 7-diphenyl-l,10-phenanthroline (EDT-NCs/Bphen) bilayer according to various embodiments under (a) low ( ⁇ Vo> ⁇ 0.1) and (b) high ( ⁇ No> -2.5) pump fluence with 3.1 eV photoexcitation.
  • FIG. 30A is a plot of normalized transmittance change ⁇ / ⁇ as a function of energy (electronvolts or eV) showing normalized transient absorption spectra for annealed 1,2-ethanedithiol treated (EDT-treated) medium methylammonium lead bromide nanocrystals (MAPbBr 3 NCs) film with (continuous lines) and without (dashed lines) 7-diphenyl-l,10-phenanthroline (Bphen) extraction layers according to various embodiments at low fluence with ⁇ No> - 0.1.
  • Inset of FIG. 30A shows the un-normalized TA spectra at 0.8 ps. rjhot is determined to be -83 %.
  • FIG. 30B is a plot of carrier temperature (in Kelvins or K) as a function of time (in picoseconds or ps) showing extracted hot-carrier temperature as a function of delay time for two samples according to various embodiments.
  • FIG. 30C is a plot of normalized transmittance change ⁇ / ⁇ as a function of energy (in electron volts or eV) showing normalized transient absorption (TA) spectra for methylammonium lead bromide (MAPbBr 3 ) bulk-film (-240 nm thick) with (lines) and without (dashes) 7-diphenyl-l,10-phenanthroline (Bphen) extraction layers at low pump fluence with 2x 10 17 cm "3 .
  • TA transient absorption
  • FIG. 30C shows the un-normalized TA spectra at 0.8 ps. ⁇ hot is determined to be -16 %.
  • FIG. 30D shows a plot of carrier temperature (in Kelvins or K) as a function of time delay (in picoseconds or ps) for two samples according to various embodiments.
  • the photoexcitation energy is 3.1eV.
  • FIG. 31 shows cross-sectional scanning electron microscopy (SEM) images of 1,2-ethanedithiol- treated nanocrystals (EDT-NCs film) with different thickness according to various embodiments. Scale bar in FIG. 31 (a) - (d) is 100 nm.
  • the rjhot of bulk- film/Bphen with thickness of -240 nm is -16 % with changing of T only from -450 to 380 K under similar photoexcitation conditions (FIGS. 30C-D). Even when the bulk- film thickness is reduced to -40 nm, rjhot may be still much smaller than EDT-NCs film.
  • colloidal MAPbBr 3 NCs may exhibit approximately 2 orders slower hot-carrier cooling times and about 4 times larger hot-carrier temperatures as compared to perovskites bulk-films under similar photoexcitation conditions.
  • hot-carrier cooling in NCs may be mediated by the phonon bottleneck effect, which is surprisingly slower in smaller NCs (contrasting with conventional NCs).
  • This finding contravenes the conventional understanding in traditional colloidal semiconductor nanocrystals that intraband Auger effects is more dominant with decreasing dimensionality, resulting in the breach of the phonon bottleneck.
  • Auger heating dominates the hot-carrier cooling rate, which may be slower in larger NCs (previously unobserved in conventional NCs).
  • the augmented slow hot-carrier cooling in these colloidal perovskite nanocrystals may enable efficient hot-carrier extraction. It is demonstrated that the hot electrons with up to -0.6 eV excess energy can be efficiently injected (up to -83%) from surface-treated MAPbBr 3 NCs films into electron extraction layers with an injection time of ⁇ 0.2 ps.
  • Hot-carrier properties in perovskites NCs may enable fresh opportunities for extremely thin absorber (ETA) and concentrator-type hot-carrier solar cells.
  • ETA-solar cells may be conceptually close to dye-sensitized heterojunctions.
  • the molecular dye may be replaced by an extremely thin ( ⁇ tens of nm) semiconductor absorber layer.
  • nano structuring the electrodes e.g., using highly porous T1O2 scaffold, ZnO nanowire arrays etc.
  • the effective area covered by the thin absorber can be increased by several orders of magnitude due to the surface enlargement and multiple scattering.
  • the ETA layer may be extremely beneficial for hot-carrier extractions owing to the shorter transport path length for hot-carriers.
  • the illumination power in the concentrator- type solar-cells can be increased to 1000 suns, much larger than the 1-sun intensity in typical cells, the Auger-heating induced slower hot-carrier cooling in perovskite NCs may also be applicable.
  • LRP ligand-assisted re-precipitation
  • OAm oleylamine
  • OAc oleic acid
  • the precipitation of MAPbBr 3 NCs was re-dissolved in toluene solution for further studies.
  • the precipitate was separated by using centrifuge speed of 12000 rpm, 8000 rpm and 4000 rpm, respectively.
  • the mean diameters are ⁇ 4.9, 8.9 and 11.6 nm for small, medium and large sized NCs, respectively (FIG. 6).
  • a solution containing 0.6 M MAPbBr3 in DMF was spin-coated (5000 rpm, 12 s) on quartz substrates. During spin-coating, few drops of toluene were added to the film at 3 s after the beginning of spinning. The film was then dried in room temperature for 30 minutes and annealed at 70 °C for 5 minutes. All the film deposition and annealing was done in N 2 - filled glove box. The grain size of bulk-film is larger than ⁇ 1 ⁇ and the thickness is around 240 nm (FIG. 7).
  • MAPbBr 3 NCs film and 1,2-ethanedithiol (EDT)-treated NCs were grown by a layer-by-layer spin-coating processing method. All the spin-coating steps were set at 1000 rpm and spin-time was fixed for 30 s.
  • NCs in toluene (10 mg ml "1 ) was spin-coated on glass substrates for two layers.
  • each layer of EDT-treated NCs film consisted of three steps: (1) spin-coating of NCs solution on top of substrate; (2) cover the NCs film with 0.2 M EDT solution in 2- Propanol and wait for 30 s and then spin-coat; (3) dropping of anhydrous toluene on film and followed by spin-coating to clean the remaining long chained ligands. The above process was repeated for 2-10 times to obtain the NCs-film with different thickness. For post-annealed samples, the annealing was performed at 70 °C for 5 mins. All processing was performed in a N 2 -filled glove box.
  • Bphen 4,7-diphenyl-l,10-phenanthroline (bathophenanthroline, or Bphen) was deposited through a thermal evaporation method under a pressure of 10 "6 torr. Bphen was deposited on spin-coated non-annealed or annealed perovskites NCs films at a rate of 0.1-0.2 nm s "1 .
  • CdSe nanocrystals dispersed in toluene were purchased from Sigma-Aldrich Co. LLC.
  • Transient absorption (TA) measurements in the time range of fs-ns were performed using a Helios spectrometer (Ultrafast Systems, LLC).
  • the pump pulse was either generated from an optical parametric amplifier (Coherent OPerA SoloTM or Light Conversion TOPAS- CTM) that was pumped by a 1-kHz regenerative amplifier (i.e., Coherent LibraTM (50 fs, 1 KHz, 800 nm) or Coherent LegendTM (150 fs, 1 KHz, 800 nm)) or by frequency doubling the 800-nm fundamental regenerative amplifier output with a BBO crystal to obtain 400 nm pulses.
  • Coherent OPerA SoloTM or Light Conversion TOPAS- CTM was pumped by a 1-kHz regenerative amplifier (i.e., Coherent LibraTM (50 fs, 1 KHz, 800 nm) or Coherent LegendTM (150 fs, 1 KHz, 800 nm)
  • the white light continuum probe beam (in the range from 400 nm-1500 nm) was generated by focusing a small portion ( ⁇ 10 uJ) of the regenerative amplifier's fundamental 800 nm laser pulses into either a 2-mm sapphire crystal (for visible range) or a 1 cm sapphire crystal (for NIR range).
  • the probe beam was collected using a CMOS sensor for UV-VIS region and InGaAs diode array sensor for NIR region.
  • the samples were kept in a N2-filled chamber at room temperature during measurements.
  • the pump beam excited the samples from the side of Bphen based on the sample structure of Bphen/perovskite/glass substrate.
  • NCs were determined by transmission electron microscopy (TEM, JEOL JEM-2010).
  • TEM transmission electron microscopy
  • AFM Asylum Research MFP-3D
  • SEM scanning electron microscopy
  • Ultraviolet photoelectron spectroscopy was used to investigate the interfacial energy level alignment of the valence occupied states.
  • the spectra collection was performed with the same instrument as that in XPS.
  • Photoelectrons were collected at surface normal using CAE mode with 2.00 eV pass energy with the samples biased at -10 V.
  • X-ray photoelectron spectroscopy (XPS) was performed to analyze the composition of samples. Samples were transferred to an ultra-high vacuum (UHV) analysis chamber from the glove box through an air-tight sample transfer containment. The pressure of the UHV chamber was held under lxlO "9 torr.
  • the crystal structures were analyzed by powder X-ray diffraction (XRD, Bruker D8 Advance).
  • the absorption spectra were recorded using a UV-VIS spectrometer (SHEVIADZU UV-3600 UV-VIS-NIR Spectrophotometer) with an integrating sphere (ISR- 3100).
  • FTIR spectra of the all samples were measured by a Frontier FT-IR/NIR spectrometer (PerkinElmer, Waltham, MA, USA) equipped with a universal attenuated total reflection (ATR) sampling accessory (PerkinElmer, Waltham, MA, USA).
  • Raman spectra were recorded with a WITec Raman microscope (WITec GmbH, Ulm, Germany) using a 633 nm HeNe laser as the excitation source.
  • the pump energy i.e., carriers' excess energy - typically, higher the excess energies lead to longer hot carrier lifetimes
  • the measured hot-carrier lifetime could be limited by the time-resolution of the experimental techniques used, thereby yielding artificially longer lifetimes that are limited by the system temporal response rather than its intrinsic hot-carrier lifetime.
  • measurement of the hot-carrier lifetime by the time-resolved photoluminescence (TRPL) technique using a streak camera or time related single photon counting (TCSPC) system may be constrained by the system resolution of these equipment (i.e., -10 ps for most streak cameras, as high as ⁇ 1 ps for Hamamatsu systems and typically -50 ps for TCSPC systems).
  • the TA or fluorescence upconversion PL techniques have much higher system temporal response of ⁇ 150 fs, which would identify more authentic hot carrier lifetimes of the material. Hence, due care must be taken for a fair comparison of the reported values in the literature.
  • the hot-carrier cooling lifetime in FIG. 14 is defined as the time interval from pulse excitation until the cooling of hot-carriers reach 600 K (for point (iii) above). This temperature is used as the benchmark because previous theoretical calculations have shown that for T > 600 K, there may still be an appreciable hot-carrier conversion efficiency (i.e., > 40%) over a wide range of absorber bandgaps).
  • T lo is the characteristic LO-phonon decay time
  • ⁇ ⁇ is the acoustic phonon temperature
  • hm 0 is the phonon energy (-42 meV)
  • NUJ(T) is the LO-phonon occupation number at temperature T.
  • T D for the MAPbBr 3 NCs (-310 K) and bulk- film (-305 K)
  • TLO is -340 fs, 220 fs and 180 fs for small, medium and large NCs, respectively, in contrast to a fast T lo of - 150 fs for the bulk-film.
  • Auger-heating Model [00161] Auger-heating Model [00162] Auger decay lifetimes of MAPbBr 3 NCs are extracted from the pump fluence dependent band-edge photobleaching dynamics (FIG. 16(f)), which exhibit a sublinear dependence on the NC volume (VNC) as ⁇ 3 ⁇ 4 ⁇ V(VNC ) (FIG. 20(a)). This behavior agrees with recent observations of biexciton Auger recombination in weakly confined CsPbBr 3 NCs, but contrasts with the linear dependence of ⁇ 3 ⁇ 4 on NC size for strongly confined systems. The sublinear dependence can therefore be attributed to the weaker confinement in our perovskites NCs.
  • Equation (4) therefore predicts that the hot-carrier population decays bi-exponentially, with one of its lifetime corresponding to TAug/3.
  • FIG. 20(c) shows the normalized calculated hot-carrier densities as a function of decay time at different pump fluences.
  • the hot-carrier diffusion length in MAPbBr 3 can be estimated as follows. Firstly, the carrier's diffusion coefficient depends on the defect density of the fabricated material.
  • the reported electron diffusion coefficient D may range from ⁇ 1 cmV 1 for polycrystalline perovskite thin films to 5 - 8 cmV 1 for bulk MAPbBr 3 at room temperature (-300 K).
  • the hot-carrier diffusion length may be obtained by Xhot ) ⁇ 16 nm.
  • the high pump fluence at hot-carrier lifetime of -32 ps yields a diffusion length of L ⁇ 90 nm.
  • higher concentration of hot-carriers in perovskites closest to the Bphen may thus be more easily injected into Bphen.
  • NCs-film given that some Bphen molecules could penetrate into the upper layer of the NCs film, and the hot-carriers undergoing rapid hopping, the extracted -70% hot-carrier transfer efficiency for -35 nm thick NCs-film at low pump fluence may therefore be reasonable.
  • the hot-carrier diffusion length may be -10 nm. Therefore, the -15% transfer efficiency for bulk-film may also be reasonable.
  • MAPbBr3 NCs reveals two sets of S 2p doublets, with the 2p 3/2 peak position at binding energies -162.5 eV and -164.2 eV (-162.7 eV and -164.3 eV for post-annealed) arising from bound thiolate and unbounded thiol of EDT to the surface of the NC respectively (FIG. 24).
  • the ratio of bound-to-unbound thiol groups in the NC without post- annealing is -1.04, which increases to -1.47 with post-annealing of the NC at 70 °C.
  • post-annealing treatment further increases the electronic coupling of EDT-NCs with Bphen.
  • FIG. 16(e) shows the comparison of photobleaching dynamics at the band-edges between medium NCs in solution and spin-coated film. From the exponential fitting (solid curves), the lifetime changes from ⁇ 4.5 to ⁇ 3 ns at low pump fluence, the acceleration may be due to the existence of the photocharged NCs. At high pump fluence, another fast decay with lifetime of ⁇ 290 ps except for the Auger recombination may emerge in the spin-coated NCs films, which can be attributed to trions (photocharged excitons). However, they only induced the broadening in the lower energy side of GSB (see the appearance of a bleaching tail at lower energy side for NCs-films in FIG. 26 as compared with FIG.
  • trions 10 for NCs in solution in the pseudo color TA spectra.
  • the reduced energy of trions may be due to the exciton-exciton interactions.
  • the trions in the NCs-film may not affect the dynamics of hot- carriers located at the higher energy side of the GSB.
  • the MAPbBr 3 NCs films demonstrate similar hot-carrier cooling dynamics with/without the EDT-treatment. Without the EDT-treatment, the NCs demonstrate similar hot-carrier cooling dynamics with/without the Bphen extraction layer. Furthermore, we did not find any obvious change in the hot-carrier properties in the NCs films that underwent the exact same processing in the thermal evaporator except no Bphen layer was actually deposited.
  • the NCs are selectively excited with 400 nm light.
  • a weak PIA band similar to pristine Bphen may be observed (FIG. 28(b) and FIG. 28(c)).
  • any further increase of pump power may lead to the degradation of the perovskite.
  • the relaxation of PIA may possess one fast decay lifetime (-70 ps for pristine Bphen, and -25 ps for NCs/Bphen) and one slow decay lifetime (-1 ns for pristine Bphen, and -0.5 ns for NCs/Bphen) (FIG. 28(d)
  • the fast decay may be due to the carrier trapping to defects for pristine Bphen and additional electron back- transfer to the NCs for NCs/Bphen.
  • the slow decay may be due to the recombination of radical anions/excitons in the Bphen and with holes in NCs in the NCs/Bphen hybrids.

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Abstract

Divers modes de réalisation concernent une pile solaire à support chaud. La pile solaire peut comprendre une couche contenant des nano-cristaux, contenant ou comprenant un ou plusieurs nano-cristaux, chacun du ou des nano-cristaux comprenant un matériau de pérovskite à halogénure. La pile solaire à support chaud peut également comprendre une première électrode en contact avec un premier côté de la couche contenant des nano-cristaux. La pile solaire à support chaud peut en outre comprendre une seconde électrode en contact avec un second côté de la couche contenant des nano-cristaux opposé au premier côté. La couche contenant des nano-cristaux peut avoir une épaisseur de moins de 100 nm.
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WO2020084283A1 (fr) * 2018-10-22 2020-04-30 Oxford University Innovation Limited Procédé de production d'une couche avec système de solvant mixte
CN114203918A (zh) * 2021-12-09 2022-03-18 西北工业大学 一种基于PVK/ZnO异质结构的新型光电忆阻器
WO2023213707A1 (fr) 2022-05-06 2023-11-09 Institut Photovoltaique d'Ile de France Cellules solaires multi-jonctions a porteurs chauds

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CN109065722A (zh) * 2018-07-12 2018-12-21 西南大学 一种基于热载流子的太阳能电池及其制备方法
WO2020084283A1 (fr) * 2018-10-22 2020-04-30 Oxford University Innovation Limited Procédé de production d'une couche avec système de solvant mixte
US11943993B2 (en) 2018-10-22 2024-03-26 Oxford University Innovation Limited Process for producing a layer with mixed solvent system
CN114203918A (zh) * 2021-12-09 2022-03-18 西北工业大学 一种基于PVK/ZnO异质结构的新型光电忆阻器
CN114203918B (zh) * 2021-12-09 2023-09-12 西北工业大学 一种基于PVK/ZnO异质结构的新型光电忆阻器
WO2023213707A1 (fr) 2022-05-06 2023-11-09 Institut Photovoltaique d'Ile de France Cellules solaires multi-jonctions a porteurs chauds
FR3135349A1 (fr) 2022-05-06 2023-11-10 Institut Photovoltaique d'Ile de France Cellules solaires multi-jonctions à porteurs chauds

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