Solar cell having doped buffer layer and method of fabricating the solar cell

A method includes: forming a buffer layer over an absorber layer of a photovoltaic device; and extrinsically doping the buffer layer after the forming step.

BACKGROUND

This disclosure related to fabrication of thin film photovoltaic cells. Solar cells are electrical devices for generation of electrical current from sunlight by the photovoltaic (PV) effect. Thin film solar cells have one or more layers of thin films of PV materials deposited on a substrate. The film thickness of the PV materials can be on the order of nanometers or micrometers.

Absorber layers absorb light for conversion into electrical current. Examples of thin film PV materials used as absorber layers in solar cells include copper indium gallium selenide (CIGS) and cadmium telluride. Solar cells also include front and back contact layers to assist in light trapping and photo-current extraction and to provide electrical contacts for the solar cell. The front contact typically comprises a transparent conductive oxide (TCO) layer. The TCO layer transmits light through to the absorber layer and conducts current in the plane of the TCO layer. Some solar cells include a buffer layer between the absorber layer and the TCO layer. The buffer layer can increase cell efficiency, increase open circuit voltage (Voc), short circuit current (Jsc) and fill factor (FF), compared to a similar solar cell without a buffer layer.

Suitable buffer layers, such as CdS and ZnS, have been used in high efficiency CIGS solar cells. The buffer layers function as n-type materials to form a p/n junction with p-type CIGS or CdTe. Typically, CdS and ZnS-based buffer layers are prepared using chemical bath deposition (CBD) due to its low cost, simple processing and scalability.

DETAILED DESCRIPTION

In a photovoltaic cell, the photo-generated electron-hole pair is dissociated within the depletion region where a built-in potential is formed by the p/n junction. In order to widen the depletion region and thus improve the junction quality, increased carrier concentration of the buffer layer is desired. To achieve this goal, the buffer layer can be modified by extrinsic doping (e.g., doping with boron). The modified buffer layer can offer a more favorable substrate surface for TCO formation, thus enhancing the adhesion between the buffer layer and the TCO layer.

If the buffer layer is deposited by CBD, the carrier concentration of the buffer layer can be altered by varying the processing condition of CBD, such as bath temperature, or ion ratio. Dosing dopant sources into the solution can increase the carrier concentration of the buffer layer. In other embodiments described herein, extrinsic doping of the buffer layer can be extended to non-CBD processed buffer layers. For example, the methods described below can be used to extrinsically dope buffer layers deposited by processes such as atomic layer deposition (ALD), sputtering, electro-deposition, evaporation and chemical vapor deposition (CVD).

This disclosure describes solar cells and methods of fabricating the solar cells, in which a buffer layer is first formed by any suitable method, and then the buffer layer is extrinsically doped. In some embodiments, the extrinsic doping is performed by annealing the buffer layer in a selected atmosphere in the same metal organic chemical vapor deposition (MOCVD) tool in which the front contact transparent conductive oxide (TCO) material is deposited. Thus, the buffer layer can be extrinsically doped regardless of whether the buffer layer is formed by CBD, ALD, sputtering, electro-deposition, evaporation, or CVD.

FIG. 1shows a solar panel100as it is configured after front contact formation, in accordance with some embodiments. The portion of the solar panel100shown inFIG. 1includes an interconnect structure172, which provides a series connection between two adjacent solar cells of the panel100. InFIG. 1, the width of the interconnect structure172is exaggerated relative to the width of the collection region170for clarity, but the collection region170is actually much wider than the interconnect structure172.

The solar cell100includes a solar cell substrate110, a back contact layer120, an absorber layer130, a buffer layer140and a front contact layer150.

Substrate110can include any suitable substrate material, such as glass. In some embodiments, substrate110includes a glass substrate, such as soda lime glass, or a flexible metal foil, or a polymer (e.g., a polyimide, polyethylene terephthalate (PET), polyethylene naphthalene (PEN)). Other embodiments include still other substrate materials.

Back contact layer120includes any suitable back contact material, such as metal. In some embodiments, back contact layer120can include molybdenum (Mo), platinum (Pt), gold (Au), silver (Ag), nickel (Ni), or copper (Cu). Other embodiments include still other back contact materials. In some embodiments, the back contact layer120is from about 50 nm to about 2 μm thick.

In some embodiments, absorber layer130includes any suitable absorber material, such as a p-type semiconductor. In some embodiments, the absorber layer130can include a chalcopyrite-based material comprising, for example, Cu(In,Ga)Se2(CIGS), cadmium telluride (CdTe), CuInSe2(CIS), CuGaSe2(CGS), Cu(In,Ga)Se2(CIGS), Cu(In,Ga)(Se,S)2(CIGSS), CdTe or amorphous silicon. Other embodiments include still other absorber materials. In some embodiments, the absorber layer140is from about 0.3 μm to about 8 μm thick.

The buffer layer140is deposited on the absorber130to form a p/n junction. The buffer layer140can include any suitable buffer material, such as n-type semiconductors. In some embodiments, buffer layer140can include cadmium sulphide (CdS), zinc sulphide (ZnS), zinc selenide (ZnSe), indium sulfide (In2S3), indium selenide (In2Se3), Zn1-xMgxO, (e.g., ZnO), or in combination. Other embodiments include still other buffer materials. In some embodiments, the buffer layer140is from about 1 nm to about 500 nm thick.

In some embodiments, the buffer layer140is extrinsically doped in a process described below. In some embodiments, the dopant is boron. In some embodiments, the dopant distribution within the buffer layer is controlled to be position dependent. For example, the dopant concentration can be varied between the top surface of the buffer layer (interface between the TCO and the buffer layer) and the bottom surface (interface between the buffer layer and the absorber layer).FIG. 6is a diagram of an example of a doping concentration profile for a modified buffer layer140as described herein. The dopant concentration is highest at the interface between the buffer layer140and the front contact layer150. The concentration falls off most quickly near the interface between the buffer layer140and the front contact layer150. The rate of change of the dopant concentration is smaller further away from the interface between the buffer layer140and the front contact layer150.

In some embodiments, front contact layer150includes an annealed transparent conductive oxide (TCO) layer of constant thickness of about 100 nm or greater. The terms “front contact” and “TCO layer” are used interchangeably herein; the former term referring to the function of the layer150, and the latter term referring to its composition. In some embodiments, the charge carrier density of the TCO layer150can be from about 1×1017cm−3to about 1×1021cm−3. The TCO material for the annealed TCO layer can include suitable front contact materials, such as metal oxides and metal oxide precursors. In some embodiments, the TCO material can include AZO, GZO, AGZO, BZO or the like) AZO: alumina doped ZnO; GZO: gallium doped ZnO; AGZO: alumina and gallium co-doped ZnO; BZO: boron doped ZnO. In other embodiments, the TCO material can be cadmium oxide (CdO), indium oxide (In2O3), tin dioxide (SnO2), tantalum pentoxide (Ta2O5), gallium indium oxide (GaInO3), (CdSb2O3), or indium oxide (ITO). The TCO material can also be doped with a suitable dopant.

In some embodiments, in the doped TCO layer150, SnO2can be doped with antimony, (Sb), flourine (F), arsenic (As), niobium (Nb) or tantalum (Ta). In some embodiments, ZnO can be doped with any of aluminum (Al), gallium (Ga), boron (B), indium (In), yttrium (Y), scandium (Sc), fluorine (F), vanadium (V), silicon (Si), germanium (Ge), titanium (Ti), zirconium (Zr), hafnium (Hf), magnesium (Mg), arsenic (As), or hydrogen (H). In other embodiments, SnO2can be doped with antimony (Sb), F, As, niobium (Nb), or tantalum (Ta). In other embodiments, In2O3can be doped with tin (Sn), Mo, Ta, tungsten (W), Zr, F, Ge, Nb, Hf, or Mg. In other embodiments, CdO can be doped with In or Sn. In other embodiments, GaInO3can be doped with Sn or Ge. In other embodiments, CdSb2O3can be doped with Y. In other embodiments, ITO can be doped with Sn. Other embodiments include still other TCO materials and corresponding dopants.

In some embodiments, a plurality of solar cells100are arranged adjacent to each other, with the front contact150of each solar cell conducting current to the next adjacent solar cell. Each solar cell100includes an interconnect structure172for conveying charge carriers from the front contact150of a solar cell to the back contact120of the next adjacent solar cell on the same panel. The layers120,130,140and150are provided in the collection regions170. The interconnect structure172also includes three lines, referred to as P1, P2, and P3. The P1scribe line extends through the back contact layer130and is filled with the absorber layer material. The P2scribe line extends through the buffer layer140and the absorber layer130, and contacts the back contact120of the next adjacent solar cell, thus connecting adjacent solar cells in series. The P3line extends through the front contact layer150, buffer layer140and absorber layer130. The P3line of the adjacent solar cell is immediately to the left of the collection region170of the solar cell100.

The P3line separates the front contacts150, buffer layers140and absorber layers130of adjacent solar cells, so that each front contact can transmit current through the P2scribe line to the back contact of the next adjacent solar cell without shorting between front adjacent contacts.

FIG. 2is a flow chart of a method of making the solar cell according to some embodiments.

At step10, the back contact120is formed over the solar cell substrate110. The back contact can deposited by PVD, for example sputtering, of a metal such as Mo, Cu or Ni over the substrate, or by CVD or ALD or other suitable techniques. Then the P1scribe line is formed through the back contact layer120. For example, the scribe line can be formed by mechanical scribing, or by a laser or other suitable scribing process. Each solar cell in the panel100has a respective P1scribe line.

At step12, an absorber layer130is formed over the back contact layer120. The absorber layer130can be deposited by PVD (e.g., sputtering), CVD, ALD, electro deposition or other suitable techniques. For example, a CIGS absorber layer can be formed by sputtering a metal film comprising copper, indium and gallium then applying a selenization process to the metal film.

At step14, the buffer layer140is formed over the absorber layer130by any suitable method. For example, the buffer layer140can be deposited by chemical deposition (e.g., chemical bath deposition, or CBD), PVD, ALD, sputtering, evaporation CVD or other suitable techniques. Following the buffer layer formation, the P2scribe line is formed, extending completely through the buffer layer140and absorber layer130.

At step16, the buffer layer140is extrinsically doped after the buffer layer forming step14. In some embodiments, the step16of extrinsically doping the buffer layer140includes annealing the buffer layer in an atmosphere comprising a dopant. In some embodiments, the atmosphere comprises at least one of the group consisting of boron (B), aluminum (Al), gallium (Ga), indium (In), or combinations thereof. In some embodiments, the atmosphere comprises B2H6.

In some embodiments, the annealing is performed at a temperature in a range from about 70° C. to about 400° C. For example, in some embodiments, the annealing temperature can be 68° C. or 72° C., or 395° C. or 405° C. In some embodiments, the annealing temperature can be about 170° C. For example, the temperature can be 167° C. or 173° C.

In some embodiments, the buffer layer is formed of CdS (or ZnS), the atmosphere comprises B2H6, and the annealing temperature and time are selected to achieve a dopant concentration ratio of dopant/Cd (or dopant/Zn) is in a range of 10−3to 9×10−1. The charge carrier concentration of undoped CdS is about 1016/cc. The concentration of doped CdS can range from about 1017/cc to 1018/cc. In some embodiments, at a temperature of about 170° C., a target doping concentration is achieved in about 10 minutes. For example, in some embodiments, the annealing time is 9 minutes or 11 minutes. In some embodiments, this dopant concentration can provide an increase in the energy of the optical bandgap of 0.05 eV. In other embodiments, lower annealing temperatures are used with longer annealing times.

In some embodiments, step16is performed in a chamber suitable for an MOCVD process.

At step18, the front contact layer150is formed over the buffer layer140, which is over the absorber layer130. This step includes depositing a front contact material (TCO) over the buffer layer140. In some embodiments, step16of extrinsically doping the buffer layer140and step18of forming the front contact layer150are both performed in the same tool. In some embodiments, the step18of depositing the front contact material comprises chemical vapor deposition (CVD), such as metal organic chemical vapor deposition (MOCVD). In other embodiments, the front contact material is deposited by low pressure chemical vapor deposition (LPCVD) or by plasma enhanced chemical vapor deposition (PECVD).

In some embodiments, steps16and18are both performed in the same chamber of a single processing tool. In other embodiments, steps16and18are performed in two different chambers within the same tool, and a robotic device moves the substrate110between the first chamber (where annealing is performed) and the second chamber (where the TCO material is deposited, without breaking the vacuum in the tool between steps16and18.

In some embodiments, the front contact150allows at least 50% incident photons to be transmitted through the front contact150.

FIG. 3is a flow chart of an example of the method ofFIG. 2according to some embodiments.

Steps10and12ofFIG. 3can be the same as steps10and12ofFIG. 2, and for brevity, descriptions thereof are not repeated.

At step24, according to some embodiments, the buffer layer140is formed by a non-CBD method, such as ALD, sputtering, electro-deposition, evaporation or CVD. Because the methods described herein include a separate, non-CBD step for doping, the methods described herein can, but are not required to, include formation of the buffer layer by CBD.

At step26, the buffer layer140is extrinsically doped by annealing within an MOCVD chamber in an atmosphere of B2H6. In some embodiments, the annealing is performed at a temperature of about 170° C. for about 10 minutes.

At step28, the front contact150is formed on the extrinsically doped buffer layer by MOCVD in the same chamber in which the annealing is performed.

FIG. 4is a Tauc plot for CdS films without and with extrinsic doping by the method shown inFIG. 3. The prepared CdS films were formed on bare glass and first transferred into the MOCVD chamber. The CdS film was then annealed at 170° C. under a B2H6atmosphere for 10 min.

The Tauc plot ofFIG. 4can be used to determine the optical bandgap in amorphous thin film materials such as CdS or other buffer layer materials. Data corresponding to the undoped CdS film are indicated by circles402, and data corresponding to the extrinsically doped CdS film are indicated by triangles404. The plot shows the quantity hv (the optical energy of the light) on the abscissa. The quantity (αhv)2(optical density) is plotted on the ordinate, where a is the absorption coefficient of the material.

Each of the Tauc plots inFIG. 4has a linear region which denotes the onset of absorption. Extrapolating this linear region to the abscissa indicates the energy of the optical band gap of the material. The plot shows extrapolation lines403and404for the undoped film and for the extrinsically doped film404, respectively. Thus, in the data plotted inFIG. 4, the optical bandgap energies are about 2.33 eV for the undoped CdS and 2.38 eV for the extrinsically doped CdS. From the plot of (αhv)2 vs. (hv), the shift of the band-gap toward higher energy is indicative of doping and suggests the incorporation of boron into the CdS films shown by the triangles404. That is, the doping process can be performed by annealing in an atmosphere containing the desired dopant for modifying the carrier concentration and optical bandgap of buffer layers. The amount of bandgap increase can be controlled by controlling the doping level.

FIG. 5is a schematic diagram of an MOCVD chamber500for extrinsically doping the buffer layer140shown inFIG. 1. The solar cell substrate is provided on a staging surface, such as a platen or conveyor504. In some embodiments, the chamber500has a plurality of nozzles for dispensing respective materials into the chamber. For example,FIG. 5shows a first nozzle506for dispensing the doping material (e.g., B2H6) into the MOCVD chamber500, and a second nozzle508for dispensing the TCO material (e.g., ITO) into the same MOCVD chamber500for forming the front contact layer150. There is no need to break vacuum between the doping and front contact forming steps. In other embodiments (not shown), the conveyor504(or a robotic device) transports the solar cell100between different stations in the same tool for doping and applying TCO material, respectively.

By increasing the optical bandgap of the buffer layers, the efficiency of the solar array (for absorbing incoming light) can be increased. The short circuit current Jsc can be increased correspondingly. The increase in carrier concentration also increases the conductivity of the buffer layer140. Thus, the overall series resistance of the solar cell can be reduced.

The methods described herein perform a separate extrinsic doping step on a buffer layer of solar cell, after depositing the buffer layer. In some embodiments, the extrinsic doping step anneals the buffer layer in an atmosphere containing the desired dopant. In some embodiments, the annealing is performed in the same MOCVD chamber used to perform subsequent TCO material deposition to form the front contact of the solar cell.

Increasing carrier concentration of the buffer layer140by extrinsic doping is beneficial for improving device performance of solar cells for a number of reasons. For example, the device resistance can be reduced by the enhanced conductivity of a doped buffer layer. Also, the increased carrier concentration of the doped buffer layer140contributes to the improvement in the open-circuit voltage Voc. Further, the interface recombination at the CdS/CIGS (buffer/absorber) interface can be reduced by increasing the carrier concentration of the buffer layer140, improving the junction quality. Meanwhile, the increased optical bandgap of the CdS films can improve the photocurrent. The extrinsic doping process can be performed within the same MOCVD tool which is used for TCO preparation, so there is no extra equipment or facilities cost. Because a single tool is used for doping the buffer layer and depositing the front contact TCO material, there is no delay from breaking vacuum or transferring the substrate between the tool used for doping and the tool used for TCO deposition.

In some embodiments, a method comprises: forming a buffer layer over an absorber layer of a photovoltaic device; and extrinsically doping the buffer layer after the forming step.

In some embodiments, a method comprises: forming a buffer layer over an absorber layer of a photovoltaic device by a process from the group consisting of atomic layer deposition, sputtering, evaporation or chemical vapor deposition; and extrinsically doping the buffer layer.

In some embodiments, a method comprises: forming a buffer layer over an absorber layer of a photovoltaic device; annealing the buffer layer in a chamber having an atmosphere containing a dopant after the forming step; and forming a front contact layer on the buffer layer in the same chamber where the annealing is performed.