Photovoltaic Material and Use of it in a Photovoltaic Device

Abstract

The present invention relates to a photovoltaic material and a photovoltaic device comprising the photoactive material arranged between a hole transport layer and an electron acceptor layer. The present invention also relates to the use of the photovoltaic material.

Claims

1. A photovoltaic material comprising: a cubic nanocrystalline material with the following phases: XYS.sub.2 and XYS.sub.2-zV.sub.z where 2>z>0, wherein X is selected from Ag or Cu; Y is selected from Bi or Sb, and V is selected from a halogen; in the cubic lattice the cations, X and Y alternate in space and the anionic sites are occupied by sulfur atoms, or V.

2. A photovoltaic device comprising: a substrate; a photovoltaic material comprising a cubic nanocrystalline material with the following phases: XYS2 and XYS2-zVz where 2>z>0, wherein X is selected from Ag or Cu; Y is selected from Bi or Sb, and V is selected from a halogen; in the cubic lattice the cations, X and Y alternate in space and the anionic sites are occupied by sulfur atoms, or V; arranged between: a hole transport layer whose conduction band is lower than that of the cubic nanocrystalline material and whose valence band is lower than that of the cubic nanocrystalline material; and an electron acceptor n-type layer whose conduction band is higher, with respect to vacuum, than that the conduction band of the cubic nanocrystalline material and whose valence band is higher than the valence band of the cubic nanocrystalline material; and metal contacts that are placed on top.

3. The photovoltaic device of claim 2 wherein the substrate is selected from: a glass or plastic substrate coated with a transparent conductive oxide.

4. The photovoltaic device of claim 3 wherein the transparent conductive oxide is selected from: ITO (indium doped Tin Oxide), FTO (Fluorine doped Tin Oxide) or AZO (Aluminum doped Zinc Oxide).

5. The photovoltaic device of claim 2 in which the electron acceptor has a thickness from 50 nm to 500 nm.

6. The photovoltaic device of claim 2 wherein the n-type electron acceptor is selected from: ZnO, indium-doped ZnO, aluminum doped ZnO or TiO.sub.2 or combinations thereof.

7. The photovoltaic device of claim 2 with a photoactive layer thickness that is from 10 nm to 500 nm.

8. The photovoltaic device of claim 2 in which the hole transport layer is selected from: an organic layer of Spiro-O-MeTAD, PTAA, PEDOT or a conjugated polymer selected from P3HT or PTB7, or a inorganic oxide selected from NiO, MoO.sub.3, WO.sub.3 or SnO.sub.2.

9. The photovoltaic device of claim 2 wherein the metal contacts are selected from: MoO.sub.3, Au, Ag, Mo, Al or Mo.

10. A solar cell comprising a photovoltaic material comprising a cubic nanocrystalline material with the following phases: XYS2 and XYS2-zVz where 2>z>0, wherein X is selected from Ag or Cu; Y is selected from Bi or Sb, and V is selected from a halogen; in the cubic lattice the cations, X and Y alternate in space and the anionic sites are occupied by sulfur atoms, or V.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] FIG. 1A shows absorbance of the nanocrystals in solution for different reaction temperatures at a given molar concentration of the cations.

[0033] FIG. 1B shows different concentrations of the cationic precursor during synthesis for a fixed temperature of 75 C.

[0034] FIG. 2A shows a corresponding XRD pattern that demonstrates the cubic crystal structure of the nanocrystalline material and shows the bandgap modifications of the nanocrystals using a different concentration of Ag ions for a synthesis performed at 75 C.

[0035] FIG. 2B shows a corresponding XRD pattern that demonstrates the cubic crystal structure of the nanocrystalline material and shows the bandgap modifications of the nanocrystals using a different concentration of Ag ions for a synthesis performed at 75 C.

[0036] FIG. 2C shows a corresponding XRD pattern that demonstrates the cubic crystal structure of the nanocrystalline material and shows the bandgap modifications of the nanocrystals using a different concentration of Ag ions for a synthesis performed at 75 C.

[0037] FIG. 2D shows a corresponding XRD pattern that demonstrates the cubic crystal structure of the nanocrystalline material and shows the bandgap modifications of the nanocrystals using a different concentration of Ag ions for a synthesis performed at 75 C.

[0038] FIG. 3 demonstrates the current voltage characteristic of the reported devices under dark and solar light illumination conditions.

[0039] FIG. 4 illustrates the external quantum efficiency spectrum of the solar cell that follows the absorption of the AgBiS.sub.2 semiconductor.

DETAILED DESCRIPTION

[0040] As mentioned above, an aspect of the present invention relates to a photovoltaic device comprising:

[0041] A substrate;

[0042] the photoactive material described above arranged between:

[0043] a hole transport layer whose conduction band is lower than that of the cubic nanocrystalline material and whose valence band is lower than that of the cubic nanocrystalline material and;

[0044] an electron acceptor n-type layer whose conduction band is higher, with respect to vacuum, than that the conduction band of the cubic nanocrystalline material and whose valence band is higher than the valence band of the cubic nanocrystalline material; and

[0045] metal contacts that are placed on top.

[0046] In a preferred embodiment the substrate is selected from: a glass or plastic substrate coated with a transparent conductive oxide. In a more preferred embodiment, the transparent conductive oxide is selected from: ITO (indium doped Tin Oxide), FTO (Fluorine doped Tin Oxide) or AZO (Aluminum doped Zinc Oxide).

[0047] In a preferred embodiment the n-type electron acceptor is selected from: ZnO, TiO.sub.2, aluminum doped ZnO, or indium-doped ZnO or combinations thereof.

[0048] The hole transport layer is selected from: inorganic oxide selected from NiO, MoO.sub.3, WO.sub.3 or SnO.sub.2 or an organic hole transport layer such as SpiroMeOTAD (N.sup.2,N.sup.2,N.sup.2,N.sup.2,N.sup.7,N.sup.7,N.sup.7,N.sup.7-octakis(4-methoxyphenyl)-9,9-spirobi[9H-fluorene]-2,2,7,7-t etramine), PEDOT (Poly(3,4-ethylenedioxythiophene)) and its derivatives, PTAA (Poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine]) or other conjugated polymers like P3HT (Poly(3-hexylthiophene-2,5-diyl) or PTB7 (Poly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b]dithiophene-2,6-diyl} {3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl}.

[0049] In a preferred embodiment the electron acceptor has a thickness between 5 nm and 500 nm.

[0050] In a preferred embodiment the photoactive layer thickness is between 10 nm and 500 nm.

[0051] In a preferred embodiment the metal contacts are selected from: MoO.sub.3, Au, Ag, Mo, Al or Mo.

[0052] To fabricate the devices, we spin-cast our nanocrystals from solution onto a ZnO layer that covers the ITO coated glass substrate. The nanocrystals then undergo a processing step in which the original oleic acid molecules are removed from the surface and are replaced with other crosslinking molecules such as ethanedithiol, benzenedithiol or alternative thiol-functionalized head groups. The ligand exchange is performed via replacement of oleic acid using halide-containing molecules such as tetra-butyl-ammonium-halide (halide=bromide, iodide, chloride). In this case cationic sites are passivated from the halide atoms forming a quaternary nanocrystal system of AgBiS2-xYx (Y=I, Br, Cl), in which charge neutrality is achieved with the inclusion of the halide atoms. Metal electrodes are then deposited on top.

[0053] Alternatively, a photovoltaic device can be made in the reverse order. In this case, the hole transport layer is deposited onto the substrate. The nanocrystals are then deposited as described above. An electron acceptor layer is then deposited and metal contacts are deposited on top.

EXAMPLES

[0054] The following examples are provided for illustrative means, and are not meant to be limiting of the present invention.

Example 1

Development of the Photovoltaic Device

[0055] Synthesis of the Nanocrystals

[0056] An example of implementing this photoactive material is via the use of colloidal chemistry for the synthesis of nanocrystals. For a typical synthesis of AgBiS.sub.2 nanocrystals, 1 mmol of Bi(OAc).sub.3, 1 mmol of Ag(OAc) and 17 mmol of oleic acid (OA) were pumped overnight at 100 C. to form the bismuth and silver oleates and remove oxygen and moisture.

[0057] After this time the reaction atmosphere was switched to Ar and the temperature changed to 75 C., 100 C., 135 C.

[0058] When the mixture reached the fixed temperature 1 mmol of HMS (Hexamethyldisilathiane) dissolved in 5 ml of ODE (1-Octadecene) were quickly injected to the flask.

[0059] The nanocrystals were then isolated and purified with a solvent anti-solvent sequential process and were finally dissolved in toluene or other non-polar solvents.

[0060] The effect on the growth temperature of the reaction on the absorption spectrum is shown in FIG. 1A.

[0061] To tailor the relative stoichiometry of the cations in the nanocrystals and thereby their bandgap, the Ag precursor concentration can be adjusted accordingly.

[0062] FIG. 1B shows the bandgap modifications of the nanocrystals using different concentration of Ag ions for a synthesis performed at 75 C.

[0063] FIG. 2A-2B show XRD plots for different Ag molar concentrations in the reaction for a temperature of 75 C. These plots verify the phase purity of the material and are in accordance with the cubic phase of AgBiS.sub.2 semiconductors.

[0064] Fabrication of the Photovoltaic Device

[0065] ITO covered glass substrates were cleaned thoroughly by ultrasonicating in water, acetone and 2-propanol and dried using a nitrogen gun prior to device preparation.

[0066] A ZnO electron transport layer was grown using a sol-gel method. Briefly, 0.5 g of zinc acetate dihydrate was dissolved in 5 mL of methoxyethanol and 0.142 mL of ethanolamine. The solution was spin-cast onto the ITO-coated glass substrates at 3000 rpm and annealed at 200 C. for 30 minutes. This process is done twice.

[0067] Solutions of 20 g/L of AgBiS.sub.2 nanocrystals in toluene were prepared by adding toluene to the original highly concentrated solution. The desired film thickness was obtained via repetition of the layer-by-layer process. One drop of nanocrystals was dispensed on top of a spinning (2000 rpm) substrate, followed by 3 drops of 1,2-ethanedithiol (EDT) 2% vol. in acetonitrile or 10 drops of tetrabuthylammonium iodide (TBAI) 1 g/L in methanol. The films are rinsed with either acetonitrile or methanol and toluene after the completion of each layer.

[0068] A 5 mg/mL PTB7 solution in dichlorobenzene was prepared by stirring overnight at 50 C. A PTB7 layer was spincoated onto the AgBiS.sub.2 films at 2000 rpm spin velocity for 1 minute. Metal deposition was carried out on a thermal evaporator system with a base pressure lower than 210.sup.6 mbar.

[0069] A thin layer of molybdenum oxide (3 nm, 0.1 s.sup.1) followed by Ag (150 nm, 2.5 s.sup.1) was deposited as the top electrical contact.

[0070] The devices demonstrate typically power conversion efficiencies of 5% and the current voltage characteristics are shown in FIG. 3.

[0071] The spectral EQE shown in FIG. 4 illustrates the panchromatic solar harnessing of this device determined by the bandgap of the AgBiS.sub.2 semiconductor.