JP6251742B2 - Quantum dot sensitized solar cell - Google Patents

Description

  The present invention relates to a solar cell. In particular, the present invention relates to quantum dot sensitized solar cells.

Photoelectrochemical cells (PEC) based on mesoporous nano-sized crystalline TiO ₂ films (TiO ₂ films) sensitized with organic or organometallic dyes are a low-cost alternative to conventional solid-state photovoltaic power generation As a possibility, it has been actively studied over the past 20 years. There has been great progress in optimizing the components of dye-sensitized solar cells (DSSCs), with currently reported efficiencies exceeding up to 11%. As part of the search for new approaches to further efficiency improvements over the past few years, several research groups have sensitized dyes to semiconductor nanosize crystal quanta of materials such as InP, CdS, CdSe, CdTe, PbS and InAs. A study of PEC replaced with a dot (NQD; nanocrystalline quantum dot) was reported. In these studies, semiconductor NQDs function as efficient sensitizers over a wide spectral range from the visible to the mid-infrared, maintaining the attractiveness of low manufacturing costs, while maintaining the attractiveness of optical properties and simpler NQD sizes. It has been demonstrated that advantages such as being able to adjust the electronic structure can be provided.

Recent studies have demonstrated two different approaches to sensitization of TiO ₂ using narrow gap semiconductors. In one approach, the semiconductor NQD is generated in situ on the surface of the TiO ₂ film using chemical solution deposition (CBD) or sequential ionic layer adsorption reaction (SILAR) or sequential ionic layer adsorption and reaction. Is done. The advantages of the in situ deposition approach are simplicity, the direct contact of NQD with TiO ₂ and the ease of producing TiO ₂ films with high surface coverage of sensitized NQD. However, the in situ approach has some limitations such as lack of control over the chemical composition, crystallinity, size and surface properties of NQD, which can hinder the effective use of the benefits of NQD.

Peng X. G. Et al., Nature 404, p. 59 (2000) Peng Z. A. Et al., Journal of American Chemical Society, 123, p. 183 (2001) O'Reagan et al., Nature, 353, pp. 737-740, October 24, 1991. Murray et al., “Synthesis and Characterization of Nearly Monodisperse of CdE (E = S, Se, Te) Semiconductor Nanosize Crystallites (E = S, Se, Te) Semiconductor Nc ), Journal of American Chemical Society, 1993, 115, p. 8706-8715 Argazzi et al., “Enhanced Spectral Sensitivity from Ruthenium (II) Polypyridyl Based Photovoltaic Organics (33). Volume, p. 5741-5749 Jimenez et al., “Improving the Performance of Colloidal Quantum-Dot-Sensitized Solar Cells,” Nanotechnology, Vol. Wang et al., “Enhance the Performance of Dye-Sensitized Solar Cells by Co-Grafting Amphiphile. Sensitizer and Hexadecylmalonic Acid on TiO2 Nanocrystals ", Journal of Physical Chemistry B, 2003, 107, p. 14336-14341 Li, et al., Method (Journal of American Chemical Society 2011, 133 (5), pp. 1176-1179).

An alternative approach is based on a two-step process, in which NQDs are first separated by organic ligand layers such as tri-n-octylphosphine oxide (TOPO), aliphatic amines, or acids using established colloidal synthesis methods. The TiO ₂ film is then sensitized by exposure to a solution of NQD. The advantage of this approach is that the chemical, structural and electronic properties of the NQD are better controlled compared to the in situ approach. By exposing the "naked" TiO ₂ film or a bifunctional organic linker (i.e. organic molecules comprising functional groups for chemical attachment to TiO ₂ and NQD surface) TiO ₂ film functionalized with a solution of NQD Several groups have demonstrated that TiO ₂ films are effectively sensitized and that device performance is better without the linker than with the linker. In PEC performance studies, several parameters such as NQD size and counter electrode material have been evaluated, but NQD organic surface passivation has been rarely studied.

The present invention further includes a substrate, a metal oxide film on the substrate, and a nano-sized crystal quantum dot on the metal oxide film, wherein the ligand is attached to the quantum dot, the ligand having a chemical formula RNH ₂ An embodiment of an article comprising a quantum dot is provided. In some embodiments, the ligand is butylamine, such as n-butylamine or t-butylamine.

  The present invention further includes a substrate; a metal oxide film on the substrate; and a quantum dot on the metal oxide film, the ligand attached to the quantum dot, wherein the ligand is tri-n-octylphosphine oxide, 1- Also provided are articles comprising quantum dots, which are primary amines having a size smaller than that of dodecanethiol, oleylamine, or oleic acid / oleate.

The present invention comprises a conductive substrate and, including the photoanode comprising the nano-sized crystalline film of a conductive metal oxide on a substrate, including an embodiment of a photoelectrochemical batteries (PEC). The nano-sized crystal film has a pore structure defined inside, and further has a nano-sized crystal quantum dot (NQD) formed in advance in the pore structure. The preformed NQD has an organic passivating ligand that is a primary amine attached to the NQD. The PEC also includes a counter electrode and an electrolyte that contacts both the photoanode and the counter electrode.

  To achieve the object, the article of the present invention comprises a substrate, a metal oxide film on the substrate, and a quantum dot on the metal oxide film, the quantum dot further comprising a t-butylamine ligand attached to the quantum dot. Including dots.

In order to achieve the object, the PEC of the present invention is a conductive substrate and a metal oxide nano-sized crystal film on the conductive substrate, and has a pore structure defined therein, and is formed in advance. A nano-sized crystal quantum dot (NQD) in the pore structure, the preformed NQD having a nano-sized crystal film having a t-butylamine ligand attached to the NQD; An electrode; and an electrolyte in contact with both the photoanode and the counter electrode.

To achieve the object, the PEC of the present invention is a conductive substrate and a nano-sized crystalline TiO2 film on the conductive substrate, having a pore structure defined therein, a light absorbing layer and Including a light scattering layer, as well as a plurality of preformed nano-sized crystal cation-exchanged CuInSe _x S _2-x quantum dots (NQDs) on a TiO ₂ film, where the cation is cadmium and 0 <x <2. , NQD includes a photoanode including a TiO 2 film further comprising a t-butylamine ligand attached to the NQD; a counter electrode; and an electrolyte in contact with both the photoanode and the counter electrode.

N-Butylamine (BA) or tri-n-octylphosphine oxide (TOPO) passivation was used, deposited on TiO ₂ film (film thickness about 5 × 10 ⁻⁶ m) and suspended in hexane solution. It is the figure which showed the absorption spectrum of CdSe NQD (r about 2.15nm). The TiO ₂ film was exposed to a 3.0 × 10 ⁻⁶ M NQD hexane solution for 48 hours to prepare an NQD / TiO ₂ film. Also shown is the absorption spectrum of the raw TiO ₂ film. Light-harvesting efficiency (LHE) determined in the two membrane experiments shown in FIG. 1a was measured using an organometallic chromophore known as N3 dye [cis-di (thiocyanate) -bis (2,2 ′ -Bipyridyl-4,4'-dicarboxylate) ruthenium (II), Ru (dcbpy) ₂ (NCS) ₂ ] compared to a TiO ₂ film of the same thickness. The dotted line represents the measurement error for membranes prepared independently by the same procedure. A TiO ₂ film sensitized with N3 dye was prepared by exposing the TiO ₂ film to an ethanol solution of 0.3 M dye for 48 hours. It is the figure which showed the molar extinction coefficient of CdSe NQD (TOPO) of various sizes compared with the molar extinction coefficient of N3 dye. FIG. 2 is a diagram showing LHE calculated for the same series of CdSe NQD (TOPO) as in FIG. 1c, assuming that the sized surface coverage is the same as the N3 dye indicated by the dashed line. The dotted line represents LHE calculated for CdSe NQD with BA as the passivating ligand. N-Butylamine (BA) -capped (square) quantum dot-sensitized solar cells and tri-n-octylphosphine oxide (TOPO) -capped (triangular) quantum dot-sensitized solar using an aqueous 1M Na ₂ S electrolyte It is the figure which showed the dependence with respect to the light irradiation intensity | strength of the short circuit current measured using the battery. The straight line (solid line: BA, dotted line: TOPO) is a straight line fitted to the first measurement result at the lowest light irradiation intensity from the origin. The area of the device was 0.2209 cm ² . Comparison of photoelectric conversion efficiency (IPCE) of CdSe NQD / TiO ₂ solar cells using NQD with n-butylamine (BA) or tri-n-octylphosphine oxide (TOPO) as a cap ligand FIG. The electrolyte is an electrolyte in a 1M Li ₂ S aqueous solution. It is the figure which showed the dependence of IQE (internal quantum efficiency, internal quantum efficiency) calculated as IQE = (IPCE /% TFOT) /% LHE . Figure 3 shows the dependence of IPCE on various device preparation conditions. A significant change in the path length from the single layer TiO ₂ film to the double layer film increases the absorption spectrum at all wavelengths. 1 is a schematic diagram of an exemplary quantum dot sensitized solar cell. FIG. In view showing a photoluminescence intensity obtained by normalizing the _CuInSe 1.4 _S 0.6 NQD before (# 19) and after (# 19-Cd-tBA) of the cap with the cation exchange and t- butylamine cadmium is there. Cadmium exchanged CuInSe after capping with various ligands or mercaptopropionic acid linker (MPA) including t-butylamine (tBA), n-butylamine (nBA), s-butylamine (sBA), pyridine (py) _. of ₄ S _0.6 NQD, a diagram showing a comparison of the photoluminescence intensity normalized. CuInSe _1.4 S _0.6 NQD capped with various ligands including t-butylamine (tBA), n-butylamine (nBA), s-butylamine (sBA), mercaptopropionic acid (MPA), and pyridine (Py) it is a graph of current density for the quantum dot / TiO ₂ solar cell voltage was used. It is a graph of 1-T (T = transmittance) with respect to incident photon energy of the solar cell of FIG. FIG. 4 is a graph of current versus voltage for PEC containing t-butylamine capped Cd exchanged CuInSe _1.4 S _0.6 NQD and polysulfide electrolyte containing 75% methanol / 25% H ₂ O. Data were obtained under the following conditions: temperature 30-5.0 ° C. to 30 + 5.0 ° C .; device area 0.2223 cm ² ; irradiance 1000.0 W / m ² ; spectrum: ASTM G173 global. ^{_{J sc = 17.565mA / cm 2,}} V oc = 0.5402V, FF = 0.5410, PCE = 5.13%. It is a graph of the quantum efficiency with respect to the wavelength of PEC of FIG. Data were obtained under the following conditions: temperature 25.0-2 ° C. to 25.0 + 2 ° C .; device area 0.2200 cm ² ; zero voltage bias; light bias = 1.00 mA / 0.22 cm ² .

In the present specification, embodiments of the photoelectrochemical cell, in particular, discloses an embodiment of a photoelectric chemical batteries including quantum dots includes a primary amine ligand.

  “Nano-sized crystal quantum dots” include nano-sized crystal particles of all shapes and sizes. Nano-sized crystal quantum dots preferably have, but are not limited to, at least one dimension that is less than about 100 nanometers. The rod may be of any length. “Nanosize crystals”, “nanorods” and “nanosize particles” can be used interchangeably herein and are used interchangeably. In some embodiments of the invention, the nanosized crystal particles may have two or more dimensions that are less than about 100 nanometers. The nano-sized crystal may be a nucleus type, a nucleus / shell type, or may have a more complicated structure. For example, some branched nanosized crystal particles according to some embodiments of the invention can have arms having an aspect ratio greater than about 1. In other embodiments, the arm may have an aspect ratio greater than about 5 and may have an aspect ratio greater than about 10. The arm width may be less than about 200 nanometers, less than 100 nanometers, or even less than 50 nanometers in some embodiments. For example, in the example of a tetrapod having a nucleus and four arms, the nucleus has a diameter of about 3 to about 4 nanometers, and each arm has a diameter of about 4 to about 50 nanometers, 100 nanometers, 200 nanometers, 500 nanometers. It can have a length of nanometers or even greater than about 1000 nanometers. Of course, the tetrapods and other nanosized crystal particles described herein may have other suitable dimensions. In embodiments of the present invention, the nano-sized crystal particles may be monocrystalline or polycrystalline. The present invention also contemplates the use of CdSe and CdTe nanorods formed according to the processes described in the literature, with aspect ratios greater than 20, as high as 50, and lengths greater than 100 nm. Peng X. G. Et al., Nature 404, p. 59 (2000) and Peng Z. A. Et al., Journal of American Chemical Society, 123, p. 183 (2001).

The nanosize crystal quantum dots disclosed herein are generally referred to as colloidal nanosize crystal quantum dots. These colloidal nano-sized crystal quantum dots may be made of a single material or may include an inner core and an outer shell. The outer shell includes an inorganic material. In one embodiment, the outer shell may be basically made of an inorganic material. The shape of the colloidal nano-sized crystal quantum dots may be spherical, rod-shaped, disk-shaped, and combinations thereof, and may or may not have facets. In one embodiment, the colloidal nano-sized crystal quantum dot may be a binary semiconductor material nucleus, for example, M may be cadmium, zinc, indium, lead, or alloys thereof, and X is sulfur, selenium, tellurium, nitrogen, Contains a nucleus of formula MX, which is phosphorus, arsenic, antimony or mixtures thereof. In another embodiment, the colloidal nanosized crystal quantum dot may be a core of a ternary semiconductor material, e.g., M ₁ and M ₂ may be cadmium, zinc, indium, copper, tin and mixtures or alloys thereof, and X is Contains a nucleus of formula M ₁ M ₂ X, which is sulfur, selenium, tellurium, nitrogen, phosphorus, arsenic, antimony or mixtures thereof. In another embodiment, the nuclei of colloidal nanosized crystal quantum dots can be, for example, M ₁ , M ₂ and M ₃ can be cadmium, zinc, indium and X is sulfur, selenium, tellurium, nitrogen, phosphorus, arsenic. , Quaternary semiconductor material of formula M ₁ M ₂ M ₃ X, which is antimony or a mixture thereof. Non-limiting examples of suitable nuclear materials include cadmium sulfide (CdS), cadmium selenide (CdSe), cadmium telluride (CdTe), lead sulfide (PbS), lead selenide (PbSe), lead selenide sulfide (PbSe _x S _1-x ), zinc sulfide (ZnS), zinc selenide (ZnSe), zinc telluride (ZnTe), indium arsenide (InAs), indium nitride (InN), indium phosphide (InP), antimony Included are mixtures of such materials, such as indium halide (InSb), zinc cadmium selenide (ZnCdSe ₂ ), or any other semiconductor or similar material. The nuclear material is preferably selected from the group consisting of InP, InAs, InSb, CdS, CdSe, CdTe and combinations thereof, more preferably the nuclear material is CdSe. In another embodiment, the core of the colloidal nanosized crystal quantum dot may be a ternary semiconductor material, e.g. M ₁ and M ₂ may be cadmium, zinc, indium, copper, tin and mixtures or alloys thereof and X is Contains a nucleus of formula M ₁ M ₂ X, which is sulfur, selenium, tellurium, nitrogen, phosphorus, arsenic, antimony or mixtures thereof. Examples of other core materials include copper indium sulfide (CuInS ₂ ), copper indium selenide sulfide (CuInSe _x S _2-x ), copper zinc tin selenide sulfide (CuZn _y Sn _1-z Se _x S _2-x And combinations thereof, wherein 0 <x <2, such as 1 ≦ x <2, or 1 ≦ x ≦ 1.5, and 0 <z <1, such as z = 0.5.

In some embodiments, the quantum dots further comprise a cation exchanged outer shell or outer layer. For example, quantum dots, Cd, Zn, Sn, Ag , Au, Hg, Cu, In or cations partially by the metal _{M 4} is selected from combinations thereof, substantially completely or completely replaced An outer cation exchange layer. In some embodiments, the outer cation exchange layer may be an outer monolayer. As used herein, the term “outer monolayer” refers to a single atomic thickness layer of surface cations and anions surrounding the nuclei of quantum dots. In some embodiments, M ₄ is Cd or Zn, and the cation concentration of the quantum dot comprises about 5-40% M ₄ , such as about 5-20% or 5-10% M ₄ .

  The core material and / or outer layer is selected by its nature having a surface suitable for binding of primary amine ligands.

  Some embodiments use a relatively short ligand on the quantum dot. Such ligands can include at least one of allylamine, propylamine, butylamine, pentylamine, hexylamine, heptylamine, octylamine, aniline, and benzylamine. Butylamine (eg, n-butylamine, s-butylamine, t-butylamine) is a preferred amine.

The metal oxide includes a transition metal. The metal oxide may be a mixed metal oxide. The metal oxide can include a dopant. Examples of suitable metal oxides include titanium oxide (TiO ₂ ), tin oxide (SnO ₂ ), zinc oxide (ZnO), tungsten oxide (WO ₃ ), niobium oxide (Nb ₂ O ₅ ), tantalum oxide ( Ta ₂ O ₅ ), zirconium oxide (ZrO ₂ ), barium titanate (BaTiO ₃ ), strontium titanate (SrTiO ₃ ), zinc titanate (ZnTiO ₃ ), copper titanate (CuTiO ₃ ), and combinations thereof However, it is not limited to this. The structure of the metal oxide film may be a thin film, a nanotube, or a nanorod, but is not limited thereto. The metal oxide may be a nanosize crystal.

  A photoelectrochemical cell (PEC) includes a typical device architecture of the known art. Examples of PEC devices are described, for example, in O'Reagan et al., Nature, 353, pp. 737-740, Oct. 24, 1991, the contents of which are incorporated herein by reference. Incorporated into.

The electrolyte of the solar cell of the present invention is generally an aqueous solution of sulfides such as lithium sulfide (Li ₂ S), sodium sulfide (Na ₂ S), potassium sulfide, rubidium sulfide, cesium sulfide, and combinations thereof. In some embodiments, the aqueous electrolyte is lithium sulfide, sodium sulfide, or sodium sulfide / sulfur, such as 1M Na ₂ S, 1M S.

In some embodiments, the NQDs used herein are described by Murray et al., “Synthesis and Characterization of Nearly Monodispersed CdE (E = S, Se, Te) Semiconductor Nanosize Crystallites. Characteristic of Nearly Monodisperse CdE (E = S, Se, Te) Semiconductor Nanocrystallines), Journal of American Chemical, Vol. 8706-8715 is synthesized and purified according to standard literature procedures, and such references are incorporated herein by reference. A CdSe NQD / TiO ₂ composite film was prepared by directly depositing NQD on a nano-sized crystalline TiO ₂ film (TiO ₂ film) newly prepared from hexane or toluene solution.

Optical studies of NQD / TiO ₂ films showed that the amount of NQD deposited in the TiO ₂ film was significantly affected by the type of NQD surface passivation (see FIGS. 1a and 1b). FIG. 1a compares the absorption spectra of a CdSe NQD / TiO ₂ film prepared using TOPO-capped NQD and a film prepared using n-butylamine (nBA) capped NQD. NBA-capped CdSe NQD (FIGS. 1a, 1b NQD (BA)) was obtained from the same batch of TOPO-capped CdSe NQD (NQD (TOPO)) from continuous precipitation with MeOH at high temperature and NQD n-butylamine. Prepared by medium dissolution (see Methods section for details). FIG. 1 a also includes the absorption spectrum of the same NQD in hexane solution and the absorption spectrum of the TiO ₂ film. Comparison of spectral features indicates that the electronic structure of NQD does not change significantly due to changes in surface passivation or adsorption of NQD onto TiO ₂ films. However, for NQD (BA), significantly higher optical densities were consistently observed with NQD / TiO ₂ films. This is consistent with the results of light collection efficiency (LHE) measurements summarized in FIG. 1b, where NQD (BA) shows a clear improvement in LHE. FIG. 1b shows an organometallic chromophore known as N3 dye [cis-di (thiocyanate) -bis (2,2′-bipyridyl-4,4′-dicarboxylate) ruthenium (II), Ru (dcbpy) ₂ Also included is the LHE of the TiO ₂ film sensitized with (NCS) ₂ ]. Analysis of LHE values of N3 / TiO ₂ , NQD (TOPO) / TiO ₂ , and NQD (BA) / TiO ₂ provides important insights into the impact of NQD surface passivation on the optical properties of NQD / TiO ₂ films Is obtained.

As a first step in the analysis, the experimentally determined molar extinction coefficient of N3 dye was compared with the estimated molar extinction coefficient of CdSe NQD. As is known, the absorption cross section depending on the size of CdSe NQD at 400 nm is empirical relationship σ ₀ (cm ² ) = (n _CdSe / n _solvent ) 1.6 × 10 ⁻¹⁶ [R (nm)] ³ where σ ₀ is the absorption cross section, n _CdSe and n _solvent are the refractive indices of CdSe NQD (assuming 2.5) and solvent (n _hexane = 1.354), R Is the radius of the NQD. The radius of the NQD can be estimated from the absorption spectrum of the NQD solution using an empirical relationship between the size of the NQD and the NQD band gap, which is typically the peak of the lowest energy electronic transition (1 s). The N _A as Avogadro's number, using the relationship _{ε = σ 0 N A /(1000×2.303)=σ 0} × 2.61 × 10 20, σ 0 molar extinction coefficient calculated values (cm ²⁾ Converted to ε (M ⁻¹ cm ⁻¹ ). Comparison of the calculated ε for multiple sizes of CdSe NQD with the molar extinction coefficient of the N3 dye shows that NQD is a significantly better absorber than the dye on a molar basis (FIG. 1c). This feature makes NQD a very attractive alternative to molecular dyes as sensitizers in PEC. However, since NQD is typically much larger than molecular dyes, the amount of NQD adsorbed per TiO ₂ surface area unit can be significantly less than the amount of dye. Therefore, from a practical point of view, a comparison of LHE in composites with similar chromophore surface coverage is more useful.

  In the past, Argazzi et al., “Enhanced Spectral Sensitivity from Ruthenium (II) Polyvalently Based Photovoltaic Intrinsic (inventive Spectral Sensitivity) 33, p. As shown by 5741-5749, when the scattering and reflectance is small compared to the absorption loss, LHE is directly related to the molar extinction coefficient of the chromophore, as shown in Equation 1.

  (1)

式（１）において、εはモル吸光係数であり、Γはｍｏｌ／ｃｍ^２での発色団表面被覆率である。図１ｄに破線で、Ｎ３色素について計算されたＬＨＥを示している。計算では、Ｎ３色素のＬＨＥの計算値（５３５ｎｍ）を図１ｂに示す実験で観察されたＬＨＥ値（５３５ｎｍ）とマッチさせるように、表面被覆率の値を調節した（実験で観察されたＬＨＥが高いＴｉＯ_２吸収のために高エネルギー側で広がり、部分的に歪められていることに注意されたい）。同じ条件下でＮＱＤにより達成可能な最大ＬＨＥを推定するために、ＮＱＤ表面被覆率を、Γ_ＮＱＤ＝Γ_Ｎ３（Ｓ_Ｎ３／Ｓ_ＮＱＤ）の関係を用いて調整した。Ｓ_Ｎ３およびＳ_ＮＱＤはそれぞれ、Ｎ３色素およびＮＱＤの断面積である。それぞれの値をＳ＝π×ｒ^２として計算し、ｒ_Ｎ３は０．５８ｎｍとし、ｒ_ＮＱＤはＮＱＤの半径にリガンドの長さ（ＴＯＰＯでは１．１ｎｍ、ｎＢＡでは０．４ｎｍと推定）を加えたものとした。この計算では、キャップリガンドは「非貫通性」である、すなわちＮＱＤの表面間の距離は、リガンド長の二倍に等しいものとみなす。ＴＯＰＯでキャップした複数のサイズのＮＱＤについての計算結果を、図１ｄに実線で示している。また、ｎＢＡでキャップした２．１５ｎｍの粒径を有するＮＱＤについての計算結果も示している（点線）。

In equation (1), ε is the molar extinction coefficient, and Γ is the chromophore surface coverage at mol / cm ² . The dashed line in FIG. 1d shows the LHE calculated for the N3 dye. In the calculation, so as to match the N3 Calculated LHE dye (535 nm) and has been LHE values observed in the experiment shown in FIG. 1 b (535 nm), was adjusted to a value of surface coverage (observed experimentally LHE (Note that it is spread and partially distorted on the high energy side due to high TiO ₂ absorption). In order to estimate the maximum LHE achievable with NQD under the same conditions, the NQD surface coverage was adjusted using the relationship Γ _NQD = Γ _N3 (S _N3 / S _NQD ). S _N3 and S _NQD are the cross-sectional areas of the N3 dye and NQD, respectively. Each value is calculated as S = π × r ² , r _N3 is 0.58 nm, and r _NQD is the radius of _NQD plus the length of the ligand (estimated 1.1 nm for TOPO, 0.4 nm for nBA) It was assumed. In this calculation, the cap ligand is considered “non-penetrating”, ie, the distance between the surfaces of the NQD is equal to twice the ligand length. The calculation results for NQDs of a plurality of sizes capped with TOPO are shown by solid lines in FIG. Also shown are the calculation results for NQD having a particle size of 2.15 nm capped with nBA (dotted line).

Several observations can be obtained from the calculation results of LHE in FIG. 1d and comparison of this calculation result with the experimental LHE shown in FIG. 1b. First, after size adjustment, NQD has a significantly higher molar extinction coefficient than N3 dye, but NQD is an absorber that is significantly better than molecular dye at least in the vicinity of the band edge. Absent. Second, both theoretical analysis (FIG. 1d) and experiment (FIG. 1b) show that decreasing the length of the NQD cap ligand can significantly improve the LHE of NQD / TiO ₂ films. Finally, the high NQD LHE observed in the experiment suggests that NQD effectively coats the TiO ₂ surface.

Effect of surface passivation on short circuit current and mass transport in CdSe NQD / TiO ₂ PEC. FIG. 2 shows the short circuit current (I _sc ) observed in the experiment with respect to the irradiation light intensity for two CdSe NQD / TiO ₂ PECs prepared under the same conditions that differ only in the type of NQD cap layer. The first device group used TOPO-capped NQD, and the second device group replaced the TOPO cap layer with nBA prior to device fabrication (see method section for details). In both device groups, a nearly linear increase in _Isc was observed with increasing irradiation intensity, which is expected by Equation 2.

  (2)

式中、

Where

である。

It is.

In Equation 2, I ₀ is the incident light intensity at the wavelength λ,% T (λ) (substrate) is the transmittance of the substrate at the incident wavelength, φ _inj is the electron injection rate, and φ _coll Is the charge collection efficiency including contribution from electron transport in TiO ₂ films and hole transport mediated by redox couples between sensitizer and counter electrode.

Although the overall trend of _Isc dependence on light intensity is similar for the two devices, there are some notable differences. The most obvious difference is the difference in absolute value of I _sc at all irradiation intensities, with significantly higher I _sc being observed in NQD (BA). Consistent with the results shown in Formula 2 and _1b, the part of the improvement in _{I sc,} NQD (TOPO) / increase in LHE of the TiO ₂ film and NQD compared (BA) / TiO ₂ film is caused it is conceivable that. More improvement of _{I sc} by better penetration of the NQD to the large pore size of the TiO ₂ film, the past Jimenez (Gimenez) et al., "Improvement of the performance of the colloidal quantum dot-sensitized solar cells (Improving the Performance of Colloidal Quantum-Dot-Sensitized Solar Cells ", Nanotechnology 2009, 20, 295204. However, the replacement of TOPO to nBA results in an approximately 40% improvement in LHE at the 1s peak (FIG. 1b), but the improvement in _Isc is approximately fourfold (FIG. 2). This indicates that there is a factor contributing to the improvement of _{I sc} in the device NQD (BA) based Besides LHE. Without being bound to this explanation, the improvement of _{I sc} in NQD (BA) device is associated with improved charge collection efficiency, the use of shorter BA ligand, through the pores of NQD / TiO ₂ film It is believed that better diffusion of the electrolyte as well as better access of S ²⁻ to the NQD surface will be possible. This idea is the first experimental data point observed at the axis origin and the lowest illumination intensity observed in the experimental values of _Isc indicated by white squares and white triangles for NQD (BA) and NQD (TOPO), respectively. This is supported by the deviation from the line drawn between In the case of NQD (BA), the deviation between the experimental point and the straight line is very small, indicating that there is no effect of mass transport limitations on charge collection efficiency even at high light intensity. However, for NQD (TOPO) based devices, the experimental value of short circuit current clearly deviates from the linear plot at high light intensity, suggesting an increase in mass transport limits, which are due to electrolyte diffusion and NQD surface. It was thought to be due to limited accessibility.

  Effect of the degree of adsorption of electrolyte and CdSe NQD on IPCE. In agreement with the measurement result of the short circuit current in FIG. 2, it was found that the measured value of IPCE was significantly smaller at NQD (TOPO) than at NQD (BA). In NQD (BA), it was found that the IPCE increases with the concentration of the NQD solution, which is due to the improvement of LHE.

Determination of the internal quantum efficiency (IQE) of CdSe NQD / TiO ₂ PEC. IQE is an important feature of PEC and indicates how efficiently absorbed photons (not incident) are converted to current in the external circuit. The IQ of the PEC can be estimated from the IPCE and LHE determined by the experiment according to Equation 3 after considering the loss due to light absorption by the FTO substrate.

  (3)

３×１０^−６ＭのＮＱＤ溶液および電解質として１Ｍの水性Ｌｉ_２Ｓを用いて調製したＮＱＤ（ＢＡ）／ＴｉＯ_２およびＮＱＤ（ＴＯＰＯ）／ＴｉＯ_２デバイスのＩＱＥ分析の結果が、図３ｂに示される。ＩＰＣＥの測定はＰＥＣデバイスで行われ、ＬＨＥの測定は同一条件下で電解質の非存在下で調製されたＴｉＯ_２／ＮＱＤ膜で行われたことに注意されたい。分析の結果は、ＮＱＤ（ＢＡ）／ＴｉＯ_２のＩＱＥがＮＱＤ（ＴＯＰＯ）／ＴｉＯ_２より高いことを示す。式３により示唆されるように、ＩＱＥの結果は、ＮＱＤ（ＴＯＰＯ）／ＴｉＯ_２よりもＮＱＤ（ＢＡ）／ＴｉＯ_２を用いたほうが電子注入および電荷収集効率のいずれも高いことを示す。

The results of IQE analysis of NQD (BA) / TiO ₂ and NQD (TOPO) / TiO ₂ devices prepared using 3 × 10 ⁻⁶ M NQD solution and 1M aqueous Li ₂ S as electrolyte are shown in FIG. 3b. It is. Note that IPCE measurements were made with PEC devices, and LHE measurements were made with TiO ₂ / NQD films prepared in the absence of electrolyte under the same conditions. The results of the analysis show that NQD (BA) / TiO ₂ of IQE is higher than NQD (TOPO) / TiO _2. As suggested by Equation 3, IQE results indicate that both NQD (BA) / TiO ₂ have higher electron injection and charge collection efficiencies than NQD (TOPO) / TiO ₂ .

Effect of TiO ₂ film structure on IPCE. To further improve the IPCE of NQD PEC, a lower light absorbing layer (in contact with FTO) (about 5 micrometers (10 ⁻⁶ m) for 20 nm particles) and an upper light scattering layer (about 5 × 10 5 for 400 nm particles). A series of devices were manufactured using a two-layer TiO ₂ film structure consisting of ^-6 m). This type of structure is commonly used to improve DSSC LHE. The results of a two-layer IPCE study compared to various configurations of single layer devices using the same size NQD (1s is 590 nm; r is about 2.3 nm) is shown in FIG. 3c. The results clearly show an improvement in the IPCE of the bilayer device at all wavelengths above 450 nm, which is due to the increased path length of incident light caused by scattering.

  Synthesis and purification of CdSe NQD. TOPO-capped NQDs were synthesized and purified according to the Murray standard literature procedure described above. All synthesis and purification steps were performed under an argon atmosphere and the product was stored in a glove box filled with argon until use.

  Ligand exchange of CdSe NQD. All manipulations were performed in a glove box under argon. CdSe NQD growth solution (1 g) was dissolved in 1.5 mL hexane at 35 ° C. To this solution, 8-10 mL of MeOH was added to precipitate NQD. The solution was centrifuged and decanted and the decanted NQD was dissolved in 0.5 mL n-butylamine. This solution was heated at 55 ° C. for 40-60 minutes, poured into a centrifuge tube and precipitated with 5 mL of MeOH. The solution was centrifuged and decanted, and the precipitate was redissolved in 1.2 mL n-butylamine. The solution was heated again at 55 ° C. for 15-30 minutes and then precipitated with 4 mL of MeOH. The last step was repeated once more and the resulting NQD was dissolved in 0.2 mL n-butylamine + 2 mL toluene and stored in this mixture for future use.

Preparation of CdSe NQD / TiO ₂ film. Wang et al., “Enhance the performance of Dye by co-grafting amphiphilic sensitizers and hexadecylmalonic acid on TiO 2 nanocrystals, incorporated herein by reference. -Sensitized Solar Cells by Co-Grafting Amphiphilic Sensitizer and Hexadecylmalonic Acid on TiO ₂ Nanocyclics, Journal of Physical Chemistry, Vol. 107, Journal of Physic. A nanosize crystalline TiO ₂ film was prepared using the procedure 14336-14341. In optical measurement, a film was deposited on a glass slide (Marathon Glass) having a thickness of 1 mm, and in the device, a film was deposited on 1 mm of fluorine-doped tin oxide glass (F-SnO ₂ glass). After deposition, the film was sintered at 500 ° C. to remove organics. The film thickness was determined by step shape measurement using an Alpha Step 500 (TENCOR INSTRUMENTS) surface shape measuring device. Under an argon atmosphere, by exposing the TiO ₂ newly sintered toluene solution of CdSe NQD capped hexane solution or n- butylamine CdSe NQD capped with TOPO, was prepared NQD / TiO ₂ film. As is evident from absorption and LHE measurements, deposition of TOPO-capped NQD from toluene solution on TiO ₂ is more than deposition of NQD (TOPO) from hexane or NQD (BA) from toluene. The efficiency was found to be significantly lower. Unless otherwise stated in the text, a typical exposure time was 48 hours. The NQD / TiO ₂ film was washed twice with a suitable solvent and dried under argon. The dried membrane was stored in a glove box under an argon atmosphere in the dark until use.

Manufacture of CdSe NQD PEC. NQD-based solar cells were fabricated using a two-electrode sandwich cell configuration similar to the standard DSSC arrangement. Platinum-coated F-SnO ₂ glass was used as the counter electrode (CE). The two electrodes (NQD / TiO ₂ film and CE on F-SnO ₂ glass) were separated by Surlyn spacers (thickness 40-50 × 10 ⁻⁶ m, Du Pont), and the polymer frame was heated and sealed. The battery was filled with electrolyte (aqueous 1M Na ₂ S or Li ₂ S) utilizing capillary action.

Characterization of PEC devices. IPCE measurements were performed using a QE / IPCE measurement kit equipped with a 150 W Xe lamp (# 6253 NEWPORT) light source and an ORIEL CORNERSTONE # 260 1/4 m monochromator. The light intensity was adjusted with a series of neutral density filters and monitored with a NEWPORT optical power meter 1830C power meter equipped with a calibrated Si power meter, NEWPORT model 818UV. The photocurrent generated by the device was measured using a KEITHLEY6517A electrometer. Communication between the devices and the computer was facilitated via the GPIB interface, and device control and data processing were performed using software written locally in LABVIEW. Current voltage (IV) measurements were made using a KEITHLLY 2400 source meter as part of a model SC01 solar cell characterization system (software and hardware) built by PV MEASURMENTS. A class ABA solar simulator (AM1.5), similarly constructed by PV MEASURMENTS, was calibrated using NEWPORT certified single crystal Si solar cells and then used to irradiate PEC during IV measurements. Before the measurement, the voltage was swept from -0.1 V to 0.6 V at 0.01 V / step with the holding time at each point being 1 second. In order to prevent irradiation with scattered light, a square black shield (0.2209 cm ² ) was attached to the solar cell.

The optical properties of CdSe NQD / TiO ₂ composite films and their application in PEC are being investigated. The results indicated that a reduction in the size of the NQD surface cap ligand can result in a significant improvement in the LHE of the composite film by more efficient coating of the TiO ₂ surface. Similarly, the use of shorter n-butylamine capped ligands results in a significant improvement in PEC performance compared to devices using NQD capped with tri-n-octylphosphine oxide. The improvement in IPCE is attributed to improvements in both charge injection and charge collection efficiency in devices using n-butylamine capped NQDs.

Synthesis of CuInSe _x S _2-x NQD. CuInSe _x S _2-x NQD was synthesized according to the method of Li et al. (Journal of American Chemical Society 2011, 133 (5), p. 1176-11179). Briefly, 1 mmol CuI and 1 mmol indium acetate were added to 5 ml 1-dodecanthiol (DDT) and 1 ml oleylamine and degassed at 100 ° C. for 30 minutes in a 50 ml flask. The solution was heated to 130 ° C. until it became yellow and transparent, and then degassed again at 100 ° C. for 30 minutes. The temperature was then set to 230 ° C., and when 220 ° C. was reached, 2M TOP-Se was slowly injected into the flask using a syringe pump and heating was continued during NQD nucleation and growth. After some time, typically 10-30 minutes later, the reaction was cooled, dissolved in chloroform, and washed with NQD by precipitation with methanol. After washing, the QD was stored in 5 ml octane. The result of the synthesis typically yields a chemical yield of NQD greater than 90% (relative to the copper and indium precursors).

Preparation of cation exchanged CuInSe _x S _2-x NQD. For cation exchange with Cd, a 0.5 M cadmium oleate stock solution was prepared with 3: 1 oleic acid: Cd dissolved in octadecene (ODE). Depending on the degree of cation exchange desired, washed NQD (about 50 mg / ml) in 4 ml octane solution was added to 4 ml 0.5 M cadmium oleate solution and set at 50 ° C. Cation exchange was performed for 10 minutes.

Licap of CuInSe _x S _2-x NQD with ligand: t-butylamine (tBA), n-butylamine (nBA), s-butylamine (sBA), mercaptopropionic acid (MPA), or pyridine (py) (FIGS. 6-10) ). NQD was washed twice as follows. NQD was dissolved in chloroform, and acetone was added to precipitate NQD. NQD was centrifuged and redissolved in chloroform. Methanol was added to precipitate NQD. Precipitated NQD was collected by centrifugation. Precipitated NQD was dissolved in the ligand (concentration about 0.05 g / ml), and then precipitated by adding methanol (methanol: ligand about 3: 1), and the NQD was recapped and centrifuged. The supernatant was discarded and NQD was again dissolved in the same amount of ligand. The solution was sonicated for several minutes. Methanol was added to precipitate the capped NQD and the NQD was collected by centrifugation. The capped NQD was dissolved in octane (approximately 0.05 g / ml) and the solution was centrifuged at very high rpm (eg, 20,000 rpm for 30 minutes) to remove aggregates formed during recapping. . All the precipitate was discarded. The NQD-octane supernatant was diluted with octane to an absorbance of about 0.2 (about 0.01 g / ml) with a 1S absorption peak (typically 850 nm) to obtain an NQD-sensitized mesoporous mp ₂ membrane. Used for preparation. When the ligand is MPA, the precipitated NQD is dissolved in a mixture of 1: 1 chloroform and MPA at about 0.05 g / ml, and methanol is added (methanol: (chloroform + MPA) is about 3: 1). The NQD was recapped and centrifuged. The supernatant was discarded and NQD was dissolved in the same total amount of MPA and methanol (1: 1). The solution was sonicated for several minutes. NQD capped with MPA was added by adding chloroform, and NQD was collected by centrifugation. MPA-capped NQD was dissolved in methanol (about 0.05-0.1 g / ml), and the solution was centrifuged to remove aggregates formed during recapping and partially recapped QD was removed. .

Preparation of CuInSe _x S _2-x NQD PEC: 10 × 10 ⁻⁶ to 15 × 10 ⁻⁶ m thick mesoporous (mp) TiO ₂ film on fluorinated tin oxide coated glass was diluted with diluted QD octane solution (ligand In the case of MPA, sensitization was performed by immersing in QD methanol solution for 24 hours. The membrane was then rinsed with chloroform or octane. A counter electrode was manufactured by thermally depositing 100 nm of Cu on a fluorinated tin oxide (FTO) coated glass substrate to form Cu _y S / FTO where 0.5 <= y <= 2.0. Thereafter, the electrode was immersed in a polysulfide electrolyte (aqueous 1M Na ₂ S, 1M S). Devices were completed using Surlyn spacers (DuPont) from 40 × 10 ⁻⁶ to 50 × 10 ⁻⁶ m and sealed by heating the polymer frame. The cell was filled with polysulfide electrolyte (aqueous 1M Na ₂ S, 1M S).

FIG. 4 shows an example of PEC. The solar cell 10 includes a porous metal oxide film 20 on a conductive substrate 30 (for example, fluorinated tin oxide-coated glass). The quantum dots 40 are attached to the surface of the metal oxide film 20. Solar cell 10 further includes an electrolyte 50 and a counter electrode 60. In some embodiments, the metal oxide film 20 includes a light absorbing layer that contacts the conductive substrate 30 (approximately 5 micrometers (10 ⁻⁶ m) for 20 nm particles) and a light scattering layer that contacts the electrolyte 50. And a two-layer structure including (about 5 × 10 ⁻⁶ m for 400 nm particles). In such a configuration, the quantum dots 40 are disposed on the light scattering layer. In some embodiments, the light absorbing layer includes particles having a diameter of 1-50 nm, such as a diameter of 10-50 nm, and the light scattering layer includes particles having a diameter of 100-500 nm, such as a diameter of 300-500 nm. The light absorption layer has a thickness of 1 to 30 × 10 ⁻⁶ m, and the light scattering layer only needs to have a thickness of 1 to 10 × 10 ⁻⁶ m. In one example, the light absorbing layer includes 20 nm particles and has a thickness of 10 × 10 ⁻⁶ m, and the light scattering layer includes 400 nm particles and has a thickness of 5 × 10 ⁻⁶ m.

Absorption and photoluminescence (PL) spectroscopy. A visible ultraviolet absorption spectrum was obtained with an Agilent 8453 photodiode array spectrometer. Static visible PL was measured using Horiba-Yvon Fluoromax 4 with excitation at 500 nm. Using the same Horiba-Yvon Fluoromax 4, time-resolved PL was measured with pulsed LED excitation at 455 nm. Near IR PL measurement was performed using a diffraction grating monochromator and a liquid N ₂ cooled InSb detector under excitation of a 808 nm diode laser. The excitation was mechanically chopped and the signal was amplified with a lock-in amplifier.

Effect of cation exchange and cap. FIG. 5 compares the normalized PL intensity of CuInSe _1.4 S _0.6 NQD before and after partial cation exchange with cadmium cation and recapping with t-butylamine at 50 ° C. Partial cation exchange typically results in NQDs having a cation composition containing 5-10% Cd cations. As shown in FIG. 5, cation exchange and subsequent recap with t-butylamine resulted in a significant increase in PL intensity.

FIG. 6 shows cadmium cation exchange and after capping with t-butylamine (tBA), n-butylamine (nBA), s-butylamine (sBA), or pyridine (py) ligand, or cadmium exchange and mercaptopropionic acid (MPA). It is a comparison of PL intensity after attachment of a linker. The results demonstrate that tBA produces superior results compared to sBA and nBA.

^１Ｊ_ｓｃ＝短絡電流密度
^２Ｖ_ｏｃ＝開路電圧
^３Ｔ＝透過率
^４効率＝電力変換効果、すなわち太陽電池の電気出力対入射エネルギーの比
オレイン酸カドミウムで、５０℃でカチオン交換した後、ｔ‐ブチルアミンでリキャップしたＣｕＩｎＳｅ_１．４Ｓ_０．６ＮＱＤによりＰＥＣを構築した。電池は、接触ポイントが銀塗料で被覆されたＦＴＯ‐ガラス基板を含む光陽極と、３０ｎｍ細孔を有する２０ｎｍのＴｉＯ_２粒子の１０×１０^−６ｍの層と、４００ｎｍのＴｉＯ_２粒子の５×１０^−６ｍの層とを有した。光陽極を、（オクタン中）量子ドット溶液に３６時間浸漬した。対電極は、接触ポイントが銀ペーストで被覆されたＦＴＯ‐ガラス上のＣｕ_ｙＳとし、熱蒸着機により厚さ１００ｎｍの銅膜を堆積し、電極を電解質に浸漬した。電解質は、２５％Ｈ_２Ｏ、７５％メタノール中のＮａ_２Ｓ：Ｓ（１：１）飽和液とした。 FIG. 7 is a graph of current density versus voltage for a solar cell constructed using cadmium exchanged CuInSe _1.4 S _0.6 NQD capped with tBA, nBA, sBA, MPA or pyridine. FIG. 8 is a graph of 1-T (T is transmittance) of the sensitizing film with respect to incident photon energy of the solar cell of FIG. 1-T is a measure of the amount of NQD supported in the sensitized mp-TiO ₂ film. The battery was characterized and the results are shown in Table 1 below.

¹ J _sc = short circuit current density
² V _oc = open circuit voltage
³ T = Transmissivity
⁴ Efficiency = Power conversion effect, that is, the ratio of solar cell electrical output to incident energy. PEC was constructed with CuInSe _1.4 S _0.6 NQD capped with cadmium oleate at 50 ° C and capped with t-butylamine. did. The battery consists of a photoanode comprising an FTO-glass substrate coated with silver paint at the contact point, a 10 × 10 ⁻⁶ m layer of 20 nm TiO ₂ particles with 30 nm pores, and 5 of 400 nm TiO ₂ particles. And a layer of × 10 ⁻⁶ m. The photoanode was immersed in a quantum dot solution (in octane) for 36 hours. The counter electrode was Cu _y S on FTO-glass coated with silver paste at the contact point, a copper film having a thickness of 100 nm was deposited by a thermal evaporation machine, and the electrode was immersed in an electrolyte. The electrolyte was a Na ₂ S: S (1: 1) saturated solution in 25% H ₂ O, 75% methanol.

Certification data was obtained from the National Renewable Energy Laboratory (NREL). FIG. 9 is a graph of current against voltage of PEC. FIG. 10 is a graph of external quantum efficiency (optical bias = 0.02 cm ² to 1.00 mA) versus wavelength. The short circuit current (I _sc ) of the PEC is 3.9047 mA, the short circuit current density (J _sc ) is 17.565 mA / cm ² , the open circuit voltage (V _oc ) is 0.5402 V, the fill factor is 0.5410, and the efficiency is 5. 13%, Imax is _{3.2325mA, V max} is _{0.3530V, P max} was 1.1410MW.

  All documents cited in the detailed description of the invention are incorporated herein by reference in their entirety, and the citation of any document should not be construed as an admission that it is prior art to the present invention. In the event that any meaning or definition of a term herein contradicts any meaning or definition of the same term in a document incorporated by reference, the meaning or definition given to that term herein shall prevail .

  While specific embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various other changes and modifications can be made without departing from the spirit and scope of the invention. Accordingly, all such changes and modifications within the scope of this invention are intended to be covered by the appended claims.

Claims (9)

A conductive substrate;
A nano-sized crystalline TiO ₂ film of the conductive substrate has a picture made pore structure therein, and a light absorbing layer and the light scattering layer, and the TiO ₂ film,
A plurality of preformed nano-sized crystal cation-exchanged CuInSe _x S _2-x quantum dots (NQDs) on the TiO ₂ film, wherein the cation is cadmium, 0 <x <2, The NQD further comprises a t-butylamine ligand attached to the NQD, and a photoanode comprising NQD ;
With a counter electrode;
An electrolyte in contact with both the photoanode and the counter electrode;
A photoelectrochemical cell (PEC) . The photoelectrochemical cell according to claim 1 , wherein the conductive substrate is fluorine-doped tin oxide on glass. The electrolyte includes an alkali metal sulfide, photoelectrochemical cell according to claim 1 or 2. The light-absorbing layer is in contact with the conductive substrate, photoelectrochemical cell according to any one of claims 1 to 3. The photoelectrochemical cell according to claim 1, wherein 1 ≦ x ≦ 1.5. The photoelectrochemical cell according to any one of claims 1 to 5, wherein the NQD has a cation composition including 5 to 10% of cadmium cation. The conductive substrate is fluorinated tin oxide coated glass;
The counter electrode is Cu _y S on fluorinated tin oxide-coated glass, and 0.5 ≦ y ≦ 2.0.
The photoelectrochemical cell according to claim 1 . The electrolyte 1 25% _H 2 O, 75% methanol Na ₂ S and S: up to saturation at a rate of 1 which is a liquid material obtained by dissolving, photoelectrochemical cell according to claim 1. The light absorbing layer comprises TiO ₂ particles having an average diameter of 10 to 50 nm, and is in contact with the conductive substrate;
The light scattering layer includes TiO ₂ particles having an average diameter of 300 to 500 nm,
The photoelectrochemical cell according to any one of claims 1 to 8 .

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