WO2017037693A1 - Dispositif photovoltaïque à hétérojonction - Google Patents

Dispositif photovoltaïque à hétérojonction Download PDF

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Publication number
WO2017037693A1
WO2017037693A1 PCT/IL2016/050919 IL2016050919W WO2017037693A1 WO 2017037693 A1 WO2017037693 A1 WO 2017037693A1 IL 2016050919 W IL2016050919 W IL 2016050919W WO 2017037693 A1 WO2017037693 A1 WO 2017037693A1
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Prior art keywords
photovoltaic device
absorbing structure
light absorbing
partial buffer
charge
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PCT/IL2016/050919
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English (en)
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Nir Tessler
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Technion Research & Development Foundation Limited
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Publication of WO2017037693A1 publication Critical patent/WO2017037693A1/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
    • H10K30/30—Organic devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation comprising bulk heterojunctions, e.g. interpenetrating networks of donor and acceptor material domains
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
    • H10K2101/00—Properties of the organic materials covered by group H10K85/00
    • H10K2101/30—Highest occupied molecular orbital [HOMO], lowest unoccupied molecular orbital [LUMO] or Fermi energy values
    • 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

  • the present invention is in the field of photovoltaic devices and is particularly relevant for second or third generation photovoltaic devices, such as bulk heteroj unction or thin film photovoltaic device structure.
  • Photovoltaic devices are used for conversion of solar radiation, or generally electromagnetic radiation, into electric radiation.
  • various types of photovoltaic devices have been produced, enabling collection of electromagnetic radiation of selected spectra and generation of charge carriers within the photovoltaic device.
  • Different types of photovoltaic device utilize variation of electronic properties to utilize absorption of electromagnetic radiation to cause charge separation within the device, and to collect the generated free charges.
  • 2 nd or 3 rd generation devices aimed at improving efficiency of energy conversion.
  • thin film solar cells as CIGS, CdTe, CdTe/CdS, a-Si, thin-film Si, Thin GaAs
  • organic solar cells heterojunction, bulk-heteroj unction organic, organic-inorganic
  • dye sensitized liquid or solid film
  • nano or micro crystal based cells CdSe, PbS, PbSe, InAs, InP, Si.
  • photovoltaic devices utilize internal electric fields, or potential differences, to promote the charge separation due to radiation absorption.
  • Heterojunction based photovoltaic devices utilize interface between materials having different electronic properties in order to increase this charge separation. More specifically, photon absorption in the device generates electron-hole pairs or excitons, which are to be split to allow separate collection of the charge carrier by corresponding electrodes. Splitting of the electron-hole pair is typically promoted by potential difference due to variation in work functions between the cathode (negative charge carriers collection electrode) and anode (positive charge carriers collection electrode).
  • Various device configurations and appropriate material selection have been used to optimize efficiency of such photovoltaic devices in harvesting of input electromagnetic radiation.
  • the tandem cell may include a first subcell comprising a first photoactive region and a second subcell comprising a second photoactive region.
  • the first and second photoactive regions are designed to minimize spectral overlap and maximize photocurrent.
  • the device may further include an interconnecting layer, disposed between the first subcell and the second subcell, that is at least substantially transparent.
  • photovoltaic devices As indicated above, the use of electromagnetic radiation, or specifically solar radiation, for generation and collection of free charge carriers provides clean and re- usable energy source.
  • a significant portion of photovoltaic devices used are formed with a light absorbing structure that do not rely on P-N junctions but utilize non-doped material(s) as the active region of the light absorbing structure.
  • Such photovoltaic devices provide energy conversion utilizing charge selective electrode connections, by electron/hole blocking layer or suitable selection of electrode material based on electron/hole Fermi level and free states.
  • BHJ-PV devices Bulk Heteroj unction (BHJ) photovoltaic (PV) device, generally referred to herein as BHJ-PV devices.
  • BHJ-PV devices Bulk Heteroj unction (BHJ) photovoltaic (PV) device, generally referred to herein as BHJ-PV devices.
  • BHJ-PV devices Bulk Heteroj unction (BHJ) photovoltaic (PV) device, generally referred to herein as BHJ-PV devices.
  • BHJ-PV devices may typically utilize one or more organic materials (polymers or small molecules) selected in accordance with energy gap and values of energy levels thereof, as well as based on electron/hole transport and states properties.
  • the inventors have found that typical BHJ-PV are limited by loss of open circuit voltage that is found to be lower with respect to energy gap of the light absorbing structure. More specifically, the inventors have also found that such energy loss is often accompanied by loss of current.
  • the technique of the present invention provides a novel configuration of photovoltaic devices, providing improved efficiency. More specifically, according to the inventors' understanding, such energy loss is at least partially due to pinning (or positioning) of the electrodes' energy levels with respect to those of the light absorbing structure.
  • the present invention provides a novel configuration of the photovoltaic device providing modified energy band structure at vicinity of the charge collection electrodes to thereby allow charge collection while increasing open circuit voltage closer to the level of the energy gap of the light absorbing structure.
  • the light absorbing structure of the photovoltaic device comprises at least one partial buffer region interfacing with at least one of the electrodes, i.e. the light absorbing structure may include an anode partial buffer region interfacing with the anode or a cathode partial buffer layer interfacing with the cathode or both partial buffer regions.
  • the at least one partial buffer region may be optically active or not, and is configured to modify the energy bands of the light absorbing structure to optimize energy of charge collection from the device.
  • the partial buffer layer may include a layered structure providing gradual change in energy levels or the LUMO and/or HOMO states, or include one or more additional materials diluting concentration of the light absorbing material(s) of the light absorbing structure to thereby gradually reduce density of states for charge carriers.
  • the material composition of the at least one partial buffer layer may be selected to provide enhanced charge mobility (e.g. ten times higher or more with respect to the mobility of light bulk material of the light absorbing structure) in addition to varying the energy levels, to avoid increase in resistance to charge transport and collection.
  • a photovoltaic device comprising a light absorbing structure having a general energy gap level and being located between first and second electrodes configured for collection of positive and negative charge carriers respectively, the light absorbing structure being configured to provide at least one of the following: (a) valence band energy levels being reduced by 5%-45%, or between 10%-45% or between 15% and 50% of the energy gap within a range of 10-lOOnm from an interface of the absorbing structure and the first electrode defining a first partial buffer layer within said range; and (b) conduction band energy levels being raised by 5%-45%, or between 10%-45% or between 15% and 50% of the energy gap within a range of 10-lOOnm from an interface of the absorbing structure and the second electrode defining a second partial buffer layer within said range.
  • the light absorbing structure may comprise at least two materials comprising at least one charge donor and at least one charge acceptor.
  • said light absorbing structure may be configured with increased charge mobility within a range of 10-lOOnm from an interface of the absorbing structure with at least one of the first and second electrodes.
  • the increase in charge mobility may be an increase of between 10 to 1000 fold, and may be provided by dilution of the charge donor/acceptor material and/or by proper material selection within the above defined range.
  • the charge mobility in the bulk of the light absorbing structure may be about 10 "4 cm 2 /(V- s) where the charge mobility at the range of 10-lOOnm from the interface with the first and/or second electrodes may be 10 " -10 "2 cm 2 /(V- s).
  • the light absorbing structure may comprises a first layered structure of the first partial buffer layer, having thickness of 10-lOOnm located at an interface between said light absorbing structure the first electrode, said layered structure is configured to gradually reduce valence band energy level by 5%-45%, or between 10%-45% or between 15% and 50% with respect to said energy gap.
  • the first layered structure may comprise one or more of the following small molecules, polymers, nanoparticles, nanocrystals, metal oxides, amorphous semiconductors, polycrystalline semiconductors, and monocrystalline semiconductors.
  • the first layered structure may comprise one or more material selected from the following: Spiro-MeO-TAD, Spiro- MeO-TPD, Spiro-TTB and Spiro-TAD, poly-TPD, PFB, TFB, MoOx, Ge, Si. CdSe, InAs, PbS, PbSe, CdTe, and CIGS.
  • the light absorbing structure may comprise a second layered structure of the second partial buffer layer, having thickness of 10-lOOnm located at an interface between said light absorbing structure the second electrode, said layered structure is configured to gradually increase conduction band energy level by 5% -45%, or between 10%-45% or between 15% and 50% with respect to said energy gap.
  • the second layered structure may comprise one or more of the following materials: PCBM, bisPCBM, ICBA, C60(OCH3)4-PCBM, Perylene diimide (PDI) based molecules or polymers, Naphthalene diimide (NDI) based molecules or polymers, TiOx, ZnO, Ge, Si. CdSe, In As, PbS, PbSe, CdTe, and CIGS.
  • the materials of the first and/or second layered structures are preferably selected to be chemically and structurally compatible with material or materials of the light absorbing structure.
  • the first and second electrodes may be configured with increase variation in energy level appropriate for charge collection thereby providing effective gap between the electrodes' Fermi level corresponding with positive and negative charge collection being within a range of the general energy gap of the light absorbing structure and up to 150% of said general energy gap of said light absorbing structure, thereby providing increase efficiency in energy harvesting.
  • the effective gap between the electrodes' Fermi levels may be increased to be about 105%- 110% of the general energy gap of the light absorbing structure, or about 120%, or about 130% or 140% or 150%.
  • the first and/or second partial buffer layer may modify energy states at the interface with the electrodes by increasing/deceasing energy of interface states by up to 50% with respect to energy level of corresponding states at the center of the light absorbing structure.
  • the energy gap variation at the vicinity of said at least first and second electrodes may be graded variation, being in the form of one or more step variations.
  • the energy gap variation may be formed by 1, 2 or 3 step variations or more, providing said graded variation in energy gap.
  • the different steps may correspond to multilayer structure of the corresponding region utilizing selected different materials, or using proper dilution ratios of the charge donor/acceptor materials.
  • each step variation provides energy gap variation of 0.2eV or less, or 0. leV or less.
  • a photovoltaic device comprising at least one light absorbing structure comprising one or more materials and being located between first and second electrode elements configured for collection of positive and negative charge carriers respectively; said light absorbing structure further comprising at least one of the following: first partial buffer region located between bulk of said light absorbing structure and the first electrode, and second partial buffer region located between bulk of said light absorbing structure and the second electrode; said at least one of the first and second partial buffer regions are configured to provide increased energy gap for charge carriers transfer towards at least one of the first and second electrodes.
  • Said one or more materials of the light absorbing structure may comprise at least charge donor material and charge acceptor material.
  • the light absorbing structure may comprise one or more organic or polymeric materials.
  • Said increased energy gap for charge carrier may correspond to increased energy level of conduction band or decreased energy level of the valance band at said at least one of the first and second partial buffer regions.
  • the first partial buffer region may comprise material composition selected to provide dilution of the charge donor material to thereby reduce density of states for positive charge carriers within said first partial buffer region.
  • the second partial buffer region may comprise material composition selected to provide dilution of the charge acceptor material to thereby reduce density of states for the negative charge carriers within said partial buffer region.
  • At least one of the first and second partial buffer regions may comprise at least one additional material selected to dilute charge carriers density within the partial buffer region.
  • Said at least one of the first and second partial buffer regions may comprise at least one material selected to provide increased charge mobility within said at least first and second partial buffer regions.
  • the photovoltaic device may be configured as a thin film photovoltaic device.
  • Said one or more partial buffer region may be configured with thickness of a few tens of nanometers.
  • the one or more partial buffer region is configured with thickness of between lOnm and 200nm, or between lOnm and lOOnm.
  • the photovoltaic device may further comprise one or more thin dipole layers located between said one or more partial buffer region and the corresponding electrode.
  • the thin dipole layer may generally be configured to align, or modify, work function of the electrode with conduction or valence bands of the respective buffer layer to thereby optimize efficiency of charge collection.
  • the thin dipole layer may be configured to provide alignment of the electrode's Fermi level with respect to energy level of interface states of the partial buffer region, or modify the Fermi level of the electrodes to position it slightly within the energy gap (as modified by the partial buffer region).
  • an electronic device comprising a light absorbing structure located between a first and second electrodes, the light absorbing structure comprises at least one partial buffer region located in at least one interface with said first and second electrodes, wherein, said at least one partial buffer region comprises material composition selected to reduce density of free charge carriers to thereby optimize potential difference in charge collection from the device.
  • the at least one partial buffer region may comprise material composition selected to reduce concentration of at least one of charge donor and charge acceptor to thereby reduce said density of free charge carriers.
  • Figs. 1A and IB illustrate schematically two examples of photovoltaic devices as generally known in the art
  • Fig. 2 is a schematic illustration of a photovoltaic device according to some embodiments of the present invention.
  • Figs. 3A and 3B show simulated voltage-current (V-I) curves for photovoltaic cells of the conventional configuration and configured according to the present invention
  • Fig. 3 A shows V-I curve for photovoltaic cell according to the invention with no modifications to the electrodes
  • Fig. 3B shows V-I curve for photovoltaic device according to the present invention utilizing electrode alignment;
  • Figs. 4A to 4D illustrate band diagram simulation results for photovoltaic device having general energy gap of 1.36eV with conventional configuration (Fig. 4A), and configuration according to the present invention using one graded variation of 0. leV (Fig. 4B), 0.2eV (Fig. 4C) and 0.3eV (Fig. 4D).
  • Figs. 5A and 5C illustrate examples of photovoltaic devices according to embodiments of the present invention, utilizing one step configuration (Fig. 5A) and one and two sided two step configurations (Figs. 5B and 5C respectively);
  • Fig. 6 illustrates a part of a photovoltaic device according to some additional embodiments of the present invention utilizing three step configuration
  • Fig. 7 show simulated voltage-current (V-I) curves for photovoltaic cells of the conventional configuration, having linear variation in energy gap and having 1, 2, and 3 step variation in energy gap at the interface with the electrodes according to the present invention
  • Figs. 8A to 8D show simulated voltage-current results (Fig. 8A) for exemplary photovoltaic device configurations with conventional energy gap (Fig. 8B) and according to the present invention (Figs. 8C and 8D);and
  • Figs. 9A and 9B illustrate thin film photovoltaic devices according to some embodiments of the present invention utilizing one step or continuous variation in energy gap (Fig. 9A) and several (Three) steps variation in energy gap (Fig. 9B).
  • FIG. 1A and IB schematically illustrating two examples of photovoltaic device 100 configurations as known in the art in its basic structure.
  • the photovoltaic devices 100 are illustrated by energy band diagram, in open circuit and under impinging radiation, to simplify description of the operational principles thereof.
  • the device 100 shown in Fig. 1A includes a light absorbing structure 10, which contains one or more materials providing separated valence 12 and conduction 14 bands having an energy gap ⁇ between them.
  • the light absorbing structure 10 includes two or more materials to provide a heteroj unction and assist in charge separation.
  • the light absorbing structure 10 is located in electrical contact with two electrode units configured for collection of charge carriers such that a first electrode, cathode 16, is configured for collection of negative charge carriers (e.g. electrons) and a second electrode, anode 18, is configured for collection of positive charge carriers (e.g. holes).
  • the collected charges may propagate within corresponding electric transmission arrangement 22 to a load 20, or to further electric assembly.
  • Selective collection of charge carriers, i.e. collection of electrons by the cathode 16 and of holes by the anode 18, may be provided by proper selection of the electrodes' materials as having suitable transport properties and Fermi levels being close to the conduction band 14 or the valence band 12 of the light absorbing structure 10.
  • Some 5 configurations also utilize a charge blocking layer, as illustrated in Fig.
  • At least one charge blocking layer 24 or 26 is located between the light absorbing structure 10 and the cathode 16 and/or anode 18.
  • Such charge blocking layer is generally a thin layer having electronic properties allowing transfer of electrons (holes) and blocking transfer of holes (electrons).
  • the hole blocking layer is generally a thin layer having electronic properties allowing transfer of electrons (holes) and blocking transfer of holes (electrons).
  • the hole blocking layer is generally a thin layer having electronic properties allowing transfer of electrons (holes) and blocking transfer of holes (electrons).
  • the hole blocking layer is generally a thin layer having electronic properties allowing transfer of electrons (holes) and blocking transfer of holes (electrons).
  • the 10 24 is configured of materials selected to have valence band states having lower energy with respect to the valence band states 12 of the light absorbing structure 10, thus preventing transmission of holes through the layer.
  • the electron blocking layer 26 is configured of materials selected to have conduction band states with energy higher with respect to those of the light absorbing structure 10 to thereby prevent transmission
  • some photovoltaic cell configurations utilize a blocking layer configured for blocking excitons from getting too close to the electrodes 16 or 18.
  • the blocking layers e.g. layers 24 and/or 26 in Fig. IB
  • the blocking layers are configured with material composition having energy gap that is higher with respect to energy gap ⁇ of the light absorbing structure 10.
  • charge carrier in order to be transmitted
  • the layer should generally be thinner than about lOnm. Utilizing such a thin layer, defects that unavoidably occur in the blocking layer, provide sufficient transport properties to allow transmission of charge carriers and collection thereof by the electrodes 16 and 18.
  • the intrinsic properties of the contact may indicate that the electron (hole) Fermi level of the cathode (anode) is to be aligned or located above (below) the conduction (valence) band; however, it is known in the art that once the electrodes are brought in contact and interface with the material of the light absorbing structure 10 the interface interactions will effectively position the electrode within the gap of the light absorbing structure.
  • the technique of the present invention utilizes band shaping of the light absorbing structure 10, and more specifically, of region between the main light absorbing structure 10 and the electrodes interfacing therewith.
  • the shaping of the energy band structure allow optimizations of energy harvesting and provides for greater energetic efficiency to photovoltaic devices.
  • the central region of the light absorbing structure 10 is where most of the impinging radiation is expected to be absorbed, however, in some configurations, the external regions of the light absorbing structure, defined herein as partial buffer regions exemplified in the figure as regions 10a and 10b, may take part in radiation absorption and thus are considered herein as part of the light absorbing structure.
  • the technique of the present invention utilizes band shaping of the light absorbing structure 10 to thereby allow selection of electrodes such that the resulting energetic difference between collected positive and negative charge carriers is as similar as possible to the general energy gap ⁇ of the absorbing structure and in some cases the technique of the invention may also enable the potential difference to exceed the general energy gap ⁇ .
  • Fig. 2 illustrates, in form of energy bands structure, a photovoltaic structure 1000 according to some embodiments of the invention. As shown, the light absorbing structure 10 is configured to form gradient in energy levels of the conduction 14 and/or valence 12 bands approaching the interface with the electrodes 16 and 18.
  • the gradient 12a and 14a in energy band provides for increased energy gap at regions approaching the electrodes and thus enables the use of cathode 16 and anode 18 selected to have proper Fermi levels providing higher potential difference between collected charges.
  • the technique of the invention utilized shifting of the valance band (HOMO states) or conduction band (LUMO states) in accordance with the corresponding electrode to thereby increase efficiency of energy collection of electrons (holes), which generally not providing barrier for transmission of hole (electrons).
  • non-doped materials may include one or more materials having a charge donor and charge acceptor (which may be different materials or the same material) and configured to absorb impinging radiation of one or more predetermined wavelength ranges.
  • the absorbed radiation causes generation of electron- hole pairs, as excitons, on the material and thus allow for charge separation and collection thereof by the cathode 16 and anode 18.
  • photovoltaic devices may utilize organic (polymers and/or small molecules) light absorbing structure, e.g.
  • the conduction and valence energy bands may be better described as Low Unoccupied Molecular Orbitals (LUMO) and Highest Occupied Molecular Orbitals (HOMO) states as known in the art.
  • LUMO Low Unoccupied Molecular Orbitals
  • HOMO Highest Occupied Molecular Orbitals
  • the technique of the present invention utilizes representation of the generation recombination process at the electrodes (cathode 16 or anode 18) interface with the light absorbing structure 10 based on Boltzmann statistics that relate the Fermi level of the electrodes to the charge density at the interface as follows:
  • n e ,h the free charge (electron or hole) density
  • / is the generated current (e.g. photocurrent)
  • ⁇ is the charge recombination time
  • N e ,h is the electron or hole density of states
  • E V is the corresponding energy of the conduction (LUMO) states or the Valence (HOMO) states
  • ⁇ 3 ⁇ 43 ⁇ 43 ⁇ 4 is the energy of Fermi levels in the cathode 16 or anode 18
  • kT is the Boltzmann constant and the temperature.
  • the resulting voltage (potential difference) provided by a photovoltaic device may be increased by increasing the electron Fermi energy Ep e of the cathode 16 and/or reducing the Fermi energy Ep h of the anode 18.
  • the Fermi energy of the cathodes without raising the density n e,h of charges at the interface, either the density of states N e,h need to be reduced, or the conduction/valence band energy need to be increased accordingly.
  • the technique of the present invention thus provides a novel configuration of a photovoltaic device configured to be operable with electrode connection having potential difference AEb 17 between the corresponding Fermi levels thereof, being within a range of 0.4eV or less, and preferable below 0.25eV (and more preferably below 0.2eV) with respect to the relevant energy band of the light absorbing structure 10. This allows maximizing the efficiency of the photovoltaic device 1000 by reducing energy loss in charge collection.
  • the energy variation AEb 17 of the partial buffer region(s) defines the change with respect to the level of the energy band at the central region of the light absorbing structure and the relation between the actual, modified energy at the partial buffer region and the Fermi level of the corresponding electrode (16 or 18) may be any relation, and in some cases the electrode' s Fermi level may be higher (for the cathode 16, or lower for the anode 18) from the level at the interface of the partial buffer region 10a and 10b.
  • the photovoltaic device 1000 as exemplified in Fig. 2 is configured with at least one partial buffer region/layer 10a located near the interface of the light absorbing structure 10 and the cathode 16 and/or anode 18.
  • the photovoltaic device may include both cathode partial buffer region 10a and anode partial buffer region 10b as exemplified in Fig. 2. It should however be understood that in some configuration, a single anode or cathode partial buffer region may be used, possibly providing limited increase in efficiency.
  • the partial buffer regions 10a and/or 10b are configured with varying material composition selected to provide a gradient in energy of the LUMO (conduction) 14a or HOMO (valence) 12a states such as to effectively increase the energy gap of the light absorbing structure 10 approaching the cathode 16 or anode 18.
  • the partial buffer regions have thickness that is typically greater than lOnm and is preferably between lOnm and lOOnm.
  • the use of the partial buffer regions 10a and/or 10b enables selection of the cathode 16 and anode 18 such that the corresponding Fermi levels thereof not pinned to the LUMO and HOMO states of the light absorbing structure 10 thereby significantly reducing loss of energy in charge collection. Additionally, the gradient in energy increase as well as defects in the partial buffer region 10a and 10b allow for charge carriers to be transmitted therethrough and be collected by the electrodes.
  • Figs. 3A and 3B showing simulation results of current voltage curves in photovoltaic devices as known in the art (and as exemplified in Fig. 1A) Gl and modified photovoltaic devices G2 according to the present invention.
  • curve G2 relates to modified band structure as shown in Fig. 2, without any changes in the electrodes.
  • the cathode 16 and anode 18 are configured, or result, with similar Fermi level as in the device 100 of Fig. 1.
  • This configuration indeed reduces efficiency as the modified band structure acts as barrier to charge collection.
  • 3B relates to a photovoltaic device modified according to the present invention and utilizing shifting of the Fermi level of the electrodes to provide increased potential difference between collected electrons and collected holes.
  • This configuration enhances the open-circuit voltage, and may also be used to enhance short-circuit current, and thus provides for minimizing loss of energy in charge collection.
  • the simulated voltage-current curves shown in Figs. 3A and 3B are based on P3HT:ICBA absorbing structure having energy gap of 1.36eV.
  • Figs. 4A to 4D illustrating band diagram simulation results for photovoltaic device having general energy gap of 1.36eV with conventional configuration (Fig. 4A), and configuration according to the present invention using graded variation of O.leV (Fig. 4B), 0.2eV (Fig. 4C) and 0.3eV (Fig. 4D).
  • elements of the simulated band structure are marked in Fig. 4B in accordance with the notation of Fig. 2.
  • the open circuit voltage achievable by the conventional device is typically about 0.83V out of the general energy gap of 1.36eV.
  • Increase of the energy gap by O. leV at the interface with the charge collection electrodes according to the present invention may increase the achievable open circuit energy to about 0.92V.
  • Figs 4C and 4D show band structure using increments of 0.2 and 0.3eV in band gap, providing resulting open circuit voltage of 0.98V and 1.02V.
  • the energy band modification within the partial buffer regions may be provided by various techniques.
  • the partial buffer regions 10a or 10b may include additional materials mixed together with the light absorbing materials of the structure 10, and in varying concentrations. This is to provide gradual reduce in the density of states for electrons approaching the cathode 16, or of holes approaching the anode 18.
  • the partial buffer layers may be configures as layered structures having two or more layers of material selected to provide the desired shaping of the energy band.
  • Figs. 5A and 5C illustrating energy band structures of three exemplary configurations of photovoltaic devices 1000 according to embodiments of the present invention.
  • Fig. 5A exemplifies a single-layer structure of the partial buffer regions 10a and 10b providing a one-step variation in energy band Lhl and Hel respectively.
  • the partial buffer regions as exemplified provides an increase in energy of the conduction band by about O.leV (up to an unavoidable measurement error).
  • Fig. 5B exemplifies a bi-layer structure of the partial buffer regions 10a and 10b providing step like variation in energy band Lhl and Lh2 and Hel and He2 respectively.
  • the bi-layer structure provide increase in energy band by two steps of about O.leV each step.
  • the LUMO states are raised in partial buffer region 10a (approaching the cathode) and the HOMO states are lowered in partial buffer region 10b (approaching the anode).
  • the example of Fig. 5C also illustrates that the HOMO states Hhl and Hh2 of partial buffer region 10a are also lowered, in addition to the raise in energy of the corresponding LUMO states.
  • the variations in energy levels of the HOMO and LUMO states as exemplified in Figs. 5A to 5C may be between 0.5eV to 0.3eV for each partial buffer region 10a or 10b.
  • the inventors have found that the variation in energy levels may preferably be about O.leV or lower for each step to provide enhanced efficiency in operation of the photovoltaic device 1000, as will be described in more details further below. It should be understood that disorder or intermixing of thin layers of the partial buffer layers 10a or 10b at the interface may create effective grading of energy levels thereof, allowing for larger energy steps between the materials forming the partial buffer layer.
  • the partial buffer regions 10a or 10b may generally include two or more layers configured with material compositions selected to provide the gradual variation in energy of the HOMO and/or LUMO states to provide a gradual increase in energy gap at the vicinity of the electrodes.
  • the partial buffer regions 10a is shown including two layers having LUMO states Lhl and Lh2 gradually increasing the energy of states approaching the cathode 16.
  • the anode partial buffer regions 10b is shown including two layers having HOMO state Hel and He2 providing similar, but opposite effect to the HOMO states approaching the anode 18. It should be understood, and is described above with reference to Figs. 3A and 3B that the Fermi states of the cathode and anode are preferably aligned with respect to the modified HOMO and LUMO states to allow collection of charges at energy difference as close as possible to the general energy gap of the light absorbing structure.
  • the potential difference between the anode and cathode is preferably not smaller than 80% of the energy gap ⁇ , and is preferably not smaller than 90% of ⁇ , and more preferably between 95% and 120% of ⁇ .
  • the potential difference of collected charges may be 95% to 100% of the energy gap, and may also be greater than 100% and possibly up to 110% or 120% of the energy gap.
  • the partial buffer regions 10a and/or 10b may generally include two or more, and preferably three or four, layers providing the desired modification in energy band structure.
  • the different layers may typically be configured with material composition selected in accordance with relative HOMO and LUMO (or valence and conduction bands) energies to provide step like modulation. This is to reduce charge accumulation and additional interfaces phenomena, resulting from the sharp variations if HOMO and LUMO energy levels.
  • the cathode partial buffer region 10a may for example be configured of the following materials:
  • anode partial buffer region 10b may for example be configured with the following materials:
  • the electrodes may be configured of any suitable electrically conducting material including metal, metal oxide, or highly doped semiconductor (incl. doped small molecule or polymer).
  • Additional materials may be used to form the partial buffer region(s), selected in accordance with material of the light absorbing structure 10.
  • the material of the partial buffer layer are selected to comply with chemical potential (energy band) as well as physical structure (e.g. lattice constants) with the main material of the light absorbing structure 10 providing the general energy gap for light absorption.
  • Such material may be for example selected form: Spiro-MeO-TAD, Spiro-MeO-TPD, Spiro- TTB and Spiro-TAD, poly-TPD, PFB, TFB, MoOx, Ge, Si.
  • PDI Perylene diimide
  • NDI Naphthalene diimide
  • a thin dipole layer may be used to provide an electrostatic shift and vary the relative energy of the Fermi level of the corresponding electrode.
  • Fig. 6 illustrating a portion of the photovoltaic device 1000 including a thin dipole layer 30 located at the interface between the light absorbing structure 10, or the corresponding partial buffer region 10a, and the cathode 16. Also illustrated in Fig.
  • the dipole layer 10a is graded configuration of the partial buffer layer 10a in the form of three steps of energy gap variation including steps Lhl, Lh2 and Lh3, each providing small increase in energy of the conduction band (LUMO states), but may symmetrically be graded variation of the valance band (HOMO states) at the hole collecting electrode interface.
  • the dipole layer is used to bring the work function of the electrode in line with the desired energy band of the light absorbing structure 10. This tuning is provided by selection of the dipole directions to increase or decrease the work function, and dipole density to provide the desired shift.
  • the dipole layer 30 is a very thin layer, having thickens of a few nanometers, to reduce its effect on charge transport other than alignment of the electrode work function. It should be noted that although exemplified here at the cathode interface, such thin dipole layer 30 may be used at the anode interface as well and may differ in dipole orientation and/or density to desirably position the anode work function.
  • Fig. 7 showing voltage-current curves for conventional photovoltaic structure SI, and PV of the invention having continuous linear variation in energy band GO, variation in 3 steps of O. leV each Gl, variation in two steps of 0.15eV each G2, and variation in one step of 0.3eV G3.
  • maximal increase in efficiency is provides by the continuous linear variation GO.
  • an almost similar increase in efficiency can be achieved using bang gap variation using 3 step configuration where, each step provides an increase in band-gap of 0. leV, where a single step increase of 0.3eV provides poor efficiency.
  • Figs. 8A to 8D show a comparison of voltage-current curves for several configurations of the device utilizing variation in energy levels at the partial buffer region of 0.3eV and 0.5eV as compared to similar structure with no partial buffer layer, while assuming pinning of the electrodes' Fermi levels (17) at 0.4eV.
  • Fig. 8A shows simulated voltage-current curves
  • Fig. 8B illustrates band structure of the conventional photovoltaic device
  • Figs. 8C and 8D illustrate energy levels variations of 0.3eV and 0.5eV respectively.
  • Fig. 8A show curves PI corresponding to the conventional structure exemplified in Fig.
  • curve P2 corresponding to graded variation of 0.3eV at the partial buffer layer, as exemplified in Fig. 8C, with charge mobility of ⁇ 10 ⁇ 4 cm 2 /(V- s) along the structure; curve P3 corresponding to a device using grades variation of energy level of 0.3eV where the partial buffer layer is also configured with material selected to provides charge mobility of ⁇ 10 ⁇ 3 cm 2 /(V- s); and curves P4 and P5 corresponding to the structure as exemplified in Fig. 8D, providing variation of 0.5eV in energy gap at the partial buffer layer as well as partial buffer layer configured to provide charge mobility of ⁇ 10 "3 and ⁇ 10 "2 cm 2 /(V- s) respectively.
  • the partial buffer layers may also enhance energetic efficiency of the light absorbing structure utilizing increase in charge mobility by proper material selection.
  • photovoltaic devices may be configured a thin film photovoltaic devices.
  • Layer structure of such thin film photovoltaic device is exemplified in Figs. 9A and 9B exemplifying two configurations of thin film photovoltaic devices.
  • the light absorbing structure 10 is typically located between the anode 18 and cathode 16, where at least one of the anode 18 or cathode 16 may preferably be configured as optically transparent electrode to allow light input to reach the light absorbing structure 10.
  • the light absorbing structure 10 includes one or two partial buffer regions at the interfaces with the cathode 16 or anode. Two such partial buffer regions are shown, 10a and 10b.
  • the partial buffer regions are generally configured as described above, to provide desired modification to the energy states of the light absorbing structure to thereby increase energy of the LUMO states approaching the cathode 16 and/or decrease the energy of the HOMO states approaching the anode 18.
  • the partial buffer regions may be relatively thick, and specifically the partial buffer regions 10a and 10b may be of a few tens of nanometers in thickness. In this connection it should be noted that a few tens of nanometers may by any thickness between lOnm and 200nm.
  • the partial buffer layers may be between lOnm and lOOnm, and in some configurations the partial buffer layers may be between 15nm and 45nm, or 20nm or 30nm.
  • the two partial buffer layers may be equal in thickness or not, as well as being equal or not in the number of layers therein or increase/decrease in energy of the corresponding states.
  • Fig. 9B exemplify an additional configuration of the photovoltaic device 10, in which the partial buffer regions 10a and 10b are each configured as multi-layer partial buffer region including layers 51, 52 and 53 for region 10b and layers 54, 55 and 56 of region 10a.
  • the multilayer structure of the partial buffer regions provides for multi-step variation in the energy gap, or in the energy level of the corresponding HOMO and LUMO states as described above. It should be noted that although Fig.
  • the partial buffer layers 10a and/or 10b may be configured as multilayered structures to enable variations of no more than 0. leV for each step of the multilayer structure, while resulting in total variation that is greater than O.leV, e.g. total variation between 0.3eV and up to 0.5eV.
  • the technique of the present invention provides a novel photovoltaic device configuration allowing higher efficiency in energy harvesting from input radiation.
  • the photovoltaic device utilizes one or more partial buffer regions located at interface between light absorbing structure thereof and anode and/or cathode used for charge collection.
  • the use of the one or more partial buffer regions allows alignment of the electrodes' work functions (or Fermi levels) enabling the use of the entire band gap energy difference of the light absorbing structure and maintaining potential difference in collected charges.

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  • Physics & Mathematics (AREA)
  • Electromagnetism (AREA)
  • Photovoltaic Devices (AREA)

Abstract

L'invention porte sur un dispositif photovoltaïque comprenant une structure d'absorption de lumière ayant un niveau général de bande interdite de matériau absorbant la lumière et située entre des première et seconde électrodes conçues pour recueillir des porteurs de charges positives et négatives provenant d'elles, respectivement. La structure d'absorption de lumière est conçue pour procurer : des niveaux d'énergie de bande de valence qui sont réduits de 10 % à 50 % de la bande interdite dans une plage de 10 à 100 nm d'une interface de la structure d'absorption et de la première électrode; et/ou des niveaux d'énergie de bande de conduction qui sont augmentés de 10 % à 50 % de la bande interdite dans une plage de 10 à 100 nm d'une interface de la structure absorbante et de la seconde électrode.
PCT/IL2016/050919 2015-09-01 2016-08-24 Dispositif photovoltaïque à hétérojonction WO2017037693A1 (fr)

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Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20080230120A1 (en) * 2006-02-13 2008-09-25 Solexant Corp. Photovoltaic device with nanostructured layers
US20100258164A1 (en) * 2007-08-31 2010-10-14 Toyota Jidosha Kabushiki Kaisha Photovoltaic force device
US20110297217A1 (en) * 2010-06-07 2011-12-08 The Governing Council Of The University Of Toronto Photovoltaic devices with multiple junctions separated by a graded recombination layer
US20130255758A1 (en) * 2006-07-14 2013-10-03 Barry Rand Architectures and criteria for the design of high efficiency organic photovoltaic cells

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20080230120A1 (en) * 2006-02-13 2008-09-25 Solexant Corp. Photovoltaic device with nanostructured layers
US20130255758A1 (en) * 2006-07-14 2013-10-03 Barry Rand Architectures and criteria for the design of high efficiency organic photovoltaic cells
US20100258164A1 (en) * 2007-08-31 2010-10-14 Toyota Jidosha Kabushiki Kaisha Photovoltaic force device
US20110297217A1 (en) * 2010-06-07 2011-12-08 The Governing Council Of The University Of Toronto Photovoltaic devices with multiple junctions separated by a graded recombination layer

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