Method of absorber surface repairing by solution process

Methods and systems for repairing oxidation of CIGS surfaces during manufacture of a CIGS solar cell are generally disclosed. Oxidation of an absorber reduces the photoluminescence intensity of the CIGS surface. The absorber is immersed in a reduction tank having a reducing reagent therein. The reducing reagent reverses the oxidation of the CIGS absorber, increasing the interface quality and corresponding photoluminescence intensity. After reversing the oxidation, a buffer layer is deposited on the CIGS absorber to prevent further surface oxidation.

BACKGROUND

This disclosure relates to thin film photovoltaic solar cells.

In a copper/gallium/selenium (“CIGS”) thin film solar cell, the thin film stack generally comprises a substrate, a molybdenum (“Mo”) thin film layer as a back contact layer (a.k.a., back electrode), and a CIGS thin film layer as the absorber layer. The structure can further include a buffer layer of CdS, for example. The buffer layer is followed by a front contact (a.k.a., front electrode layer or transparent conductive oxide, TCO, layer). Such conventional structure is formed by first depositing the molybdenum thin film layer over the substrate. The CIGS thin film absorber layer is formed by deposition of a Cu/In/Ga (CIG) intermetallic precursor layer on the Mo thin film layer and followed by selenization and optionally sulfurization of the CIG precursor in a furnace, thus, converting the CIG precursor layer into the final CIGS layer.

DETAILED DESCRIPTION

When a solar cell is fabricated, the CIGS absorber surface can gradually trap oxygen, for example, by forming Se—O bonds, causing the CIGS layer to oxidize when the absorber leaves a vacuum chamber. The surface oxidation behavior can damage the interface quality of the CIGS absorber.

The present disclosure generally provides methods and systems for repairing oxidation of CIGS surfaces during manufacture of a CIGS solar cell. Generally, when a CIGS absorber is exposed to atmosphere, oxidation of the CIGS surface occurs and reduces the interface quality of the absorber. The reduced interface quality can be measured by a reduced photoluminescence (PL) intensity. According to some embodiments disclosed herein, the absorber is immersed in a reducing reagent to restore the PL intensity of the absorber. The reducing agent reverses the oxidation of the CIGS surface and increases the PL intensity of the absorber. After restoring the CIGS surface, a buffer layer can be deposited on the absorber to prevent further surface oxidation.

FIG. 9is a cross-sectional view of a solar cell according to some embodiments. The portion of the solar panel100shown inFIG. 9includes an interconnect structure172, which provides a series connection between two adjacent solar cells of the panel100. InFIG. 9, 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), or Zn1-xMgxO, (e.g., ZnO). 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, 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 1×1018cm−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. 1Aillustrates a CIGS absorber102aterminating in a plurality of cations104, which include copper, indium and gallium ions, prior to deposition of the buffer layer140. When the CIGS absorber102ais exposed to the atmosphere, oxygen bonds with the copper, iridium, and gallium cations104of the CIGS absorber102asurface. The CIGS absorber102aexperiences surface oxidation106. The resulting oxidized surface is shown as the oxidized CIGS absorber102binFIG. 1B. The bonded oxygen reduces the interface quality of the absorber102b.

FIG. 1Cillustrates a CIGS surface152aterminating in a plurality of selenium anions154. When the CIGS surface152ais exposed to surface oxidation, oxygen bonds with the selenium of the CIGS absorber. The resulting oxidized absorber is shown inFIG. 1D. The oxidized selenium reduces the interface quality of the absorber152b.

In the absorbers shown inFIGS. 1B and 1D, the reduced interface quality of an absorber surface102b,152bresults in a reduced photoluminescence (PL) intensity. Reduced photoluminescence is an indication of reduced solar cell efficiency. The emission is caused by a band-to-band transition, and indicates the minority carrier diffusion length. A long minority carrier diffusion length causes brighter luminescence and higher photon conversion efficiency.

FIG. 2is a chart160illustrating the change in PL intensity162of a CIGS absorber due to surface oxidation. In the illustrated embodiment, the PL intensity is normalized such that the PL intensity of a fresh absorber is 100% PL intensity and oxidized absorbers are shown as a percentage of the fresh absorber PL intensity. When an absorber102ais exposed to air, surface oxidation occurs and the amount of oxidation will rapidly increase with exposure time. Absorber surface quality can be detected by a PL system. For example, in one embodiment, laser illumination is applied to the absorber surface. The laser illumination excites the absorber surface's electron holes/pairs. When a hole and/or electron recombines, a photon is released. A stronger PL intensity corresponds to better surface quality and is related to better efficiency. As shown inFIG. 2, PL intensity162of a CIGS surface rapidly degrades within the first few hours of exposure. The PL intensity162continues to degrade as the exposure time increases. The PL intensity162degradation is a result of surface oxidation of the CIGS absorber.

FIG. 3is a schematic diagram illustrating a process for restoring an oxidized absorber surface. In one embodiment, a CIGS absorber204is formed. The absorber204may be formed by any suitable method such as, for example, a vacuum method, a co-evaporation process, a two-step process, a solution process, and/or any other suitable absorber formation process. After formation, the absorber204can be exposed to atmosphere (or air), for example, by removing the absorber204from a vacuum used for absorber formation. Exposure of the absorber204to air causes surface oxidation of the absorber204, which reduces the interface quality and corresponding PL intensity of the absorber204. In order to restore interface quality of the absorber204, the absorber204is immersed in a reduction tank206having a reduction reagent therein. A controller210is configured to control the amount of time the absorber204is immersed in the reduction tank206. In various embodiments, the controller210can be configured to control the amount of time the absorber204is immersed based on one or more parameters of the reduction tank206, such as, for example, the reduction agent in the tank206, the temperature of the reduction agent, the pH of the reduction agent, the concentration of the reduction agent, and/or one or more additional parameters.

The reduction reagent composition is selected to reverse the surface oxidation of the absorber204. In other words, the reduction reagent deoxidizes the absorber204. For example, in some embodiments, a reduction reagent comprises a material configured to absorb one or more electrons through a redox process, stripping oxygen from the surface of the absorber204. Reduction reagents may include, for example, phosphorous (P) or sulfur (S) based reagents. Example reduction reagents include, but are not limited to, sodium sulfite (Na2SO3), sodium dithionite (Na2S2O4), sodium metabisulfite (Na2S2O5), sodium phosphinate (NaH2PO2), Thioreau (H2NCSNH2), to name just a few. Those skilled in the art will recognize that other suitable reduction reagents are not listed and are included within the scope of the disclosure.

When the absorber204is immersed in the reduction reagent, oxidation of the CIGS absorber204is reversed. Reversal of the surface oxidation produces a corresponding increase in PL intensity of the absorber204.FIG. 4is a chart300illustrating the change in PL intensity302of an absorber immersed in a Thioreau (TU)-based reagent solution. As shown inFIG. 4, immersion of the absorber204increases the PL intensity302of the absorber204. Starting at a time0, the absorber204has a PL intensity substantially equal to zero, that is, the absorber204is almost entirely oxidized. Immersion of the absorber204increases the PL intensity302, with longer immersion times producing greater PL intensity302increases. For example, in the illustrated embodiment, after about 25 minutes, the PL intensity302of the absorber is substantially equal to a PL intensity of a fresh absorber, or about 3×105.

In some embodiments, one or more parameters of the reduction reagent may be adjusted to effect the rate of change in the PL intensity. For example, in various embodiments, the temperature, pH, concentration, immersion time, and/or other parameters of a reduction reagent may affect the rate of change in the PL intensity.FIG. 5is a histogram400illustrating various pH values for a TU-based reagent solution. For each pH value, the solution is a TU-based solution at 25° C., having a 1M concentration. Change in PL intensity is shown for an oxidized absorber immersed for one minute in each of the solutions. A first set of PL intensity values402a-402ecorrespond to the PL intensity of oxidized absorbers prior to treatment. The oxidized absorbers are exposed to the TU-based reagent solution and the resulting PL intensities404a-404eafter one minute of immersion are shown. As shown inFIG. 5, generally acidic, or lower, pH values produce better restorative results than higher pH values. A pH value of about 2 produces a PL intensity404bsubstantially equal to a fresh absorber for the TU-based reagent solution. Although a TU-based reagent solution was used, it will be recognized that one or more properties, such as temperature, pH, concentration, immersion time, and/or additional properties can be adjusted for additional and/or alternative reagents to optimize the PL intensity response.

In one example, the reduction tank has a reduction agent temperature from 25° C. to 90° C., the concentration of the reduction agent is in a range from 0.05M to 5M, the solvent is one of the group consisting of deionized water, ethanol, or isopropanol, the immersion time is in a range from one minute to 30 minutes, and the pH of the reduction agent is in a range from 1 to 7. In some embodiments, the pH of the reduction agent is in a range from 1 to 5. In some embodiments, the pH of the reduction agent is in a range from 2 to 3.

FIG. 6illustrates an embodiment of an oxidized absorber500having a first side502arestored according to various embodiments of the disclosure. The first side502ais treated with a reduction reagent and a second side502bis left untreated. During testing, the first side502ashowed significant improvement in PL intensity over the second, untreated side502b. Table 1 shows a change in panel factors following an absorber reduction treatment as disclosed herein. As can be seen in Table 1, the performance in all measured categories increases after the absorber reduction immersion treatment. In Table 1, Jsc is short-circuit current, Voc is open-circuit voltage, FF is fill factor, Rs is series resistance, and Rsh is shunt resistance.

FIG. 7illustrates an EL image600of the absorber500ofFIG. 6. The EL image600illustrates the photo response of the treated side502aand the untreated side502bof the absorber500. As shown inFIG. 7, the untreated side502bis darker, indicating a lower PL intensity and greater oxidation. The treated side502ais brighter and illustrates the increased PL intensity.

FIG. 8illustrates one embodiment of a method700for forming a CIGS solar cell according to some embodiments of the disclosure. At step701, the substrate is cleaned. In some embodiments, substrate110is cleaned by using detergent or chemical in either brushing tool or ultrasonic cleaning tool.

At step702, back electrode layer120is then formed on a substrate110by sputtering, atomic layer deposition (ALD), chemical vapor deposition (CVD), or other suitable techniques.

At step704, the P1patterned scribe lines are next formed in bottom electrode layer120to expose the top surface of substrate110as shown. Any suitable scribing method can be used such as, without limitation, mechanical scribing with a stylus or laser scribing.

At step706, the p-type doped CIGS semiconductor light absorber layer130is next formed on top of bottom electrode layer120. The absorber layer130material further fills the P1scribe line and contacts the exposed top surface of substrate110to interconnect layer130to the substrate. Absorber layer130formed of CIGS can be formed by any suitable vacuum or non-vacuum process. Such processes include, without limitation, selenization, sulfurization after selenization (“SAS”), evaporation, co-evaporation, sputtering, electrodeposition, chemical vapor deposition, or ink spraying, a two-step process, a solution process, and/or any other suitable process. In some embodiments, the CIGS absorber is formed in a vacuum chamber.

After formation, the CIGS absorber can be removed from the vacuum chamber and can be exposed to oxygen. Oxidation of the CIGS absorber may occur, for example, during storage and/or transportation of the solar cell. For example, in some embodiments, the solar cell is formed in a batch process with one or more additional solar cells. The solar cell may be stored for a period time, such as, for example, several hours, while additional solar cells are formed. Once all of the solar cells in the batch are formed, oxidation of previously formed solar cells may be reduced.

The interface quality and corresponding PL intensity of the absorber is reduced by the surface oxidation. In a third step707, the solar cell is immersed in a reduction reagent. The reduction reagent is configured to deoxidize the CIGS absorber. The reduction reagent can comprise any suitable reduction reagent, such as, for example, a phosphorous or sulfur based reagent. Examples of suitable reagents include, but are not limited to, sodium sulfite, sodium dithionite, sodium metabisulfite, sodium phosphinate, and Thiourea. The solar cell is immersed for a predetermined time period sufficient to restore the CIGS absorber to a PL intensity substantially equal to the PL intensity of the CIGS absorber prior to the CIGS absorber being removed from the vacuum chamber. For example, the PL intensity can recover to 95% of its original value. Immersion time can vary depending on the pH of the solution. In some embodiments, the immersion time is about 9 minutes (e.g., from 7 minutes to 11 minutes) at 25° C. in a 1 Mol solution of Thiourea with pH of 1. In some embodiments the immersion time is about 25 minutes (e.g., from 23 minutes to 27 minutes) in a 1 Mol solution of Thiourea with pH of 7.

In step708, a barrier layer is applied to the CIGS layer to prevent further oxidation. The buffer layer140can be deposited by chemical deposition (e.g., chemical bath deposition), PVD, ALD, or other suitable techniques. In some embodiments, the buffer layer140is applied within the same chamber and/or tool used for the reduction reagent immersion to prevent additional oxidation of the CIGS layer prior to application of the barrier layer. In some embodiments, the solar cell is transferred to a different chamber for application of the barrier layer. During transfer, some oxidation of the CIGS layer may occur. However, as shown inFIG. 2, oxidation of the CIGS layer does not begin immediately after exposure to air. Therefore, transferring the solar wafer from the reduction reagent immersion to a device for barrier layer deposition does not significantly oxidize the CIGS layer.

At step710, the P2scribe line if formed through the buffer layer and absorber layer, exposing the back to expose the top surface of the bottom electrode120within the open P2scribe line or channel. Any suitable method can be used to cut the P2scribe line, including without limitation mechanical (e.g. cutting stylus) or laser scribing.

At step712, the front contact150is formed directly on the buffer layer140, filling the P2scribe line. In some embodiments, the step of forming the front contact150can include sputtering a layer of i-ZnO or AZO. In other embodiments, the step of forming the front contact150can include metal organic CVD (MOCVD) application of a layer of BZO.

At step714, after front contact layer formation, the P3scribe lines are next cut through the front contact150, buffer layer140and absorber layer130to expose the back contact120of the adjacent solar cell.

At step716, a combination of ethylene vinyl acetate (EVA) and butyl are applied to seal the solar cell100. The EVA and butyl encapsulant is applied directly onto the top electrode layer150in some embodiments. The EVA/butyl act as a suitable light transmitting encapsulant.

At step718, heat and pressure are applied to laminate the EVA/butyl film to the front contact150.

At step720, additional back end of line processes can be performed. This can include laminating a top cover glass onto solar cell structure to protect the top electrode layer150.

At step722, suitable further back end processes can then be completed, which can include forming front conductive grid contacts and one or more anti-reflective coatings (not shown) above top electrode150. The grid contacts protrude upwards through and beyond the top surface of any anti-reflective coatings for connection to external circuits. The solar cell fabrication process produces a finished and complete thin film solar module.

The disclosure generally provides methods and systems for repairing oxidation of CIGS absorber surfaces during manufacture of a CIGS solar cell. Generally, when a CIGS absorber is exposed to atmosphere, oxidation of the CIGS surface occurs and reduces the interface quality of the absorber. The reduced interface quality can be measured by a reduced photoluminescence (PL) intensity. According to embodiments disclosed herein, the absorber is immersed in a reduction reagent to restore the PL intensity of the absorber. The reducing agent reverses the oxidation of the CIGS surface and increases the PL intensity of the absorber. After restoring the CIGS surface, a buffer layer is deposited on the absorber to prevent further surface oxidation.

In some embodiments, a method for manufacturing a photovoltaic device is disclosed. The method comprises the steps of forming a copper/iridium/gallium/selenium (CIGS) absorber structure on a substrate, immersing the CIGS absorber structure in a reduction reagent, and forming a buffer layer on the CIGS layer.

In additional embodiments, a method of restoring a copper/iridium/gallium/selenium (CIGS) absorber is disclosed. The method comprises the steps of preparing a reduction reagent tank comprising a reduction reagent and immersing an oxidized CIGS absorber in the reduction reagent for a predetermined time period.

In additional embodiments, a system for restoring a CIGS layer is disclosed. The system comprises a reduction tank having a reduction agent therein and a buffer layer depositor. The reduction tank is configured to receive an oxidized copper/iridium/gallium/selenium (CIGS) absorber therein. The buffer layer depositor is configured to deposit a buffer layer on the CIGS absorber.