BACKGROUND OF THE INVENTION

Photovoltaic cells are optoelectronic devices that convert light energy into electrical energy. Typical photovoltaic cells include a substrate layer for mounting the cell and two ohmic contacts or electrode layers for passing current to an external electrical circuit. The cell also includes an active semiconductor junction, usually comprising two or three semiconductor layers in series. The two layer type of semiconductor cell is composed of an n-type layer and a p-type layer, and the three layer type includes an intrinsic (i-type) layer positioned between the n-type layer and the p-type layer for absorption of light radiation. The photovoltaic cells operate by having readily excitable electrons that can be energized by solar energy to higher energy levels, thereby creating positively charged holes and negatively charged electrons at the interface of various semiconductor layers. The creation of these positive and negative charge carriers applies a net voltage across the two electrode layers in the photovoltaic cell, establishing a current of electricity.

Photovoltaic cells have been produced using a variety of materials for the various functional layers of the cell. In particular, semiconductor layers of alloys using cadmium, tellurium, sulfur, indium, gallium, and even iron are known in the art. Thin film solar cells use a very thin absorbing layer (such as CdTe, CIGS, or a:Si) compared to crystalline silicon solar cells. CdTe represents the most eco-efficient photovoltaic technology, with low green house gas emission and the smallest energy payback time. CdTe solar cell technology is one of the well-established approaches for high efficiency, low cost, and stable thin films photovoltaics (PV). With a direct band gap of 1.45 eV, CdTe is an ideal absorber material for single junction solar cells, and a power conversion efficiency (PCE) of 22.1% has been achieved in small area devices.

The ohmic contacts of photovoltaic cells are often configured as front and back contacts. The front electrical contact is a transparent or semi-transparent layer that is electrically conductive and permits light energy to pass through to the semiconductor layers below. The back contact is also electrically conductive but is not necessarily a transparent layer. Back contacts are known to include materials containing copper, gold, zinc, aluminum, and graphite, for example. These materials, however, may be adversely reactive with adjacent semiconductor layers, may pose manufacturing or durability concerns, or may be expensive. Furthermore, as the work function of commonly available metal is smaller than the work function of CdTe, a Schottky junction (rectifying barrier) is generally formed at the CdTe/metal interface. The diode of opposite nature formed at the interface has a tendency to block the photo-generated holes from reaching the back contact metal, resulting severe performance degradation in the devices. An increase in carrier density of the CdTe film is generally necessary to minimize rectifying behavior. Therefore, there is a need for a suitable hole transport layer in CdTe solar cells, to minimize or eliminate the back barrier effects.

SUMMARY OF THE INVENTION

In a first aspect, provided herein is an optoelectronic device comprising a front electrode, at least one semiconductor layer on the front electrode, a back electrode on the semiconductor layer, and a hole transport layer disposed between the semiconductor layer and the back electrode, where the hole transport layer comprises a perovskite.

In certain embodiments, the perovskite has the general formula ABX3, where A is an alkaline earth element, a rare earth element, or an organic compound; B is a transition metal, a Group 13 metal, a Group 14 metal, or a Group 15 metal; and X is a halogen. In particular embodiments, the halogen is selected from the group consisting of iodine, bromine, and chlorine. In particular embodiments, the perovskite has the formula CH3NH3PbX3, and X is selected from the group consisting of I, Br, Cl, and F. In particular embodiments, the perovskite is CH3NH3PbI3.

In certain embodiments, the perovskite has the general formula ABZyX3-y, where A is an alkaline earth element, a rare earth element, or an organic compound; B is a transition metal, a Group 13 metal, a Group 14 metal, or a Group 15 metal; X and Z are each different halogens, and y ranges from 0 to 1. In particular embodiments, the perovskite comprises MAPbBryI3-y, wherein y ranges from 0 to 1.

In certain embodiments, the perovskite comprises MAPb(Z1-xYx)3, wherein x ranges from 0 to 1, Z is a first halogen, and Y is a second halogen. In particular embodiments, Z is iodine and Y is bromine. In particular embodiments, x is selected from the group consisting of 0.1, 0.3, 0.5, 0.7, and 0.9.

In certain embodiments, the optoelectronic device is a photovoltaic device. In particular embodiments, the semiconductor layer includes one of CdTe, copper indium zinc gallium di-selenide (CIGS), copper zinc tin sulfide (CZTS), copper zinc tin sulfur selenium alloy (CZTSSe), copper antimony sulfide, or tin sulfide. In particular embodiments, the optoelectronic device further includes a second semiconductor layer that forms a photovoltaic heterojunction with the at least one semiconductor layer. In particular embodiments, the at least one semiconductor layer comprises CdTe, and the second semiconductor layer comprises CdS. In certain embodiments, the back electrode comprises Au, Cu, Sb, Hg, Bi-telluride, or graphene.

Also provided is a photovoltaic device comprising a support, a transparent conductor (TC) layer on the support, a photovoltaic heterojunction on the TC layer, where the photovoltaic heterojunction comprises a CdTe layer, a hole transport layer comprising a perovskite material on the CdTe layer, and a back contact on the hole transport layer. In certain embodiments, the perovskite has the general formula ABX3, where A is an alkaline earth element, a rare earth element, or an organic compound; B is a transition metal, a Group 13 metal, a Group 14 metal, or a Group 15 metal; and X is a halogen. In certain embodiments, the perovskite has the formula CH3NH3PbX3, and X is selected from the group consisting of I, Br, and Cl. In particular embodiments, the perovskite is CH3NH3PbI3. In certain embodiments, the perovskite has the general formula ABZyX3-y, where A is an alkaline earth element, a rare earth element, or an organic compound; B is a transition metal, a Group 13 metal, a Group 14 metal, or a Group 15 metal; X and Z are each different halogens; and y ranges from 0 to 1. In particular embodiments, the perovskite comprises MAPbBryI3-y, wherein y ranges from 0 to 1.

Also provided is a photovoltaic device comprising a support, a front electrode on the support, an electron transport layer on the front electrode, where the electron transport layer comprises CdTe, an absorber layer on the electron transport layer, where the absorber layer comprises a perovskite, and a back contact on the absorber layer. In certain embodiments, the perovskite has the formula CH3NH3PbX3, and X is selected from the group consisting of I, Br, and Cl. In particular embodiments, the perovskite is CH3NH3PbI3. In certain embodiments, the perovskite has the general formula ABZyX3-y, where A is an alkaline earth element, a rare earth element, or an organic compound; B is a transition metal, a Group 13 metal, a Group 14 metal, or a Group 15 metal; X and Z are each different halogens; and y ranges from 0 to 1. In particular embodiments, the perovskite comprises MAPbBryI3-y, wherein y ranges from 0 to 1.

Also provided is a layer stack comprising a support, a front contact layer on the support, a photovoltaic heterojunction on the front contact layer, where the photovoltaic heterojunction comprises a CdTe layer, and a layer comprising a cadmium perovskite directly on the CdTe layer. In certain embodiments, the cadmium perovskite comprises MA2CdX4perovskite, wherein MA is methylammonium and X is selected from the group consisting of I, Br, Cl, and F.

Also provided is a method of forming a diode, the method comprising contacting a semiconductor layer comprising CdTe with an organo halide solution for a period of time to create a Te-rich region of the semiconductor layer or a layer of Te on the semiconductor layer, and forming an electrical contact on the Te-rich region or on the layer of Te to produce a diode. In certain embodiments, the organo halide solution comprises an organic cation capable of forming a soluble structure with cadmium in solution, and a halide anion selected from the group consisting of fluoride, chloride, bromide, and iodide. In certain embodiments, the organo halide solution comprises methylammonium iodide (MAI), methylammonium chloride (MACl), methylammonium bromide (MABr), or methylammonium fluoride (MAF). In certain embodiments, the organo halide solution comprises formamidinium iodide (FAI), formamidinium chloride (FACl), formamidinium bromide (FABr), or formamidinium fluoride (FAF). In certain embodiments, the organo halide solution comprises a solvent selected from the group consisting of anhydrous dimethylformamide (DMF), dimethyl sulfoxide (DMSO), anhydrous isopropanol (IPA), and mixtures thereof. In certain embodiments, the organo halide solution comprises an organo halide at a concentration ranging from about 10 mM to about 1 M. In certain embodiments, the organo halide solution comprises an organo halide at a concentration ranging from about 125 mM to about 500 mM. In certain embodiments, the period of time ranges from about 20 seconds to about 60 seconds. In certain embodiments, the organo halide solution forms an organo halide thin film layer on the CdTe, and the method further comprises rinsing the semiconductor layer with a solvent to remove the organo halide solution. In particular embodiments, the organo halide thin film layer comprises a methylammonium iodide (MAI) thin film or a formamidinium iodide (FAI) thin film. In particular embodiments, the solvent comprises anhydrous isopropanol.

Also provided is a method of making a photovoltaic device, the method comprising treating a semiconductor layer comprising CdTe by contacting the semiconductor layer with an organo halide solution for a period of time to form a Te-rich region of the semiconductor layer or a layer of Te on the semiconductor layer, where the semiconductor layer is on a layer stack comprising a front contact, forming a hole transport layer on the semiconductor layer or the layer of Te, where the hole transport layer comprises a perovskite, and forming a back contact on the hole transport layer. In certain embodiments, the period of time ranges from about 20 seconds to about 60 seconds. In certain embodiments, the halide perovskite precursor solution comprises methylammonium iodide (MAI) or formamidinium iodide (FAI).

Also provided is a method of making a photovoltaic device, the method comprising depositing a CdTe layer on a layer stack comprising a front contact, depositing an absorber layer on the CdTe layer, where the absorber layer comprises a perovskite, and forming a back contact on the absorber layer to make a photovoltaic device. In certain embodiments, the perovskite has the general formula ABX3, where A is an alkaline earth element, a rare earth element, or an organic compound; B is a transition metal, a Group 13 metal, a Group 14 metal, or a Group 15 metal; and X is a halogen. In certain embodiments, the perovskite has the formula CH3NH3PbX3, and X is selected from the group consisting of I, Br, Cl, and F. In particular embodiments, the perovskite is CH3NH3PbI3. In certain embodiments, the perovskite has the general formula ABZyX3-y, where A is an alkaline earth element, a rare earth element, or an organic compound; B is a transition metal, a Group 13 metal, a Group 14 metal, or a Group 15 metal; X and Z are each different halogens; and y ranges from 0 to 1. In particular embodiments, the perovskite comprises MAPbBryI3-y, where y ranges from 0 to 1.

Further provided is a photovoltaic device comprising a first photovoltaic device comprising a first photovoltaic sub-cell disposed on a second photovoltaic sub-cell, wherein the first photovoltaic sub-cell includes a photovoltaic heterojunction comprising a CdTe layer, and a hole transport layer comprising a perovskite on the CdTe layer; and the second photovoltaic sub-cell includes a photovoltaic heterojunction comprising a CdXTe layer, wherein X is Zn, Mg, or another metal.

Further provided is a photovoltaic device comprising a first photovoltaic device comprising a first photovoltaic sub-cell disposed on a second photovoltaic sub-cell, wherein the first photovoltaic sub-cell includes an electron transport layer comprising CdTe, and an absorber comprising a perovskite; and the second photovoltaic sub-cell includes a photovoltaic heterojunction comprising a CdXTe layer, wherein X is Zn, Mg, or another metal.

