CIGSSe Absorber Research Articles

Solution‐Processed Bifacial Cu(In, Ga)(S, Se)2 Thin‐Film Solar Cells with a Frontside Power Efficiency of 9.73%

Cu(In, Ga)(S, Se)2 (CIGSSe) solar cell is one of the most promising thin‐film solar cells with a record power conversion efficiency (PCE) of 23.6%. Conventional CIGSSe thin‐film solar cells are usually fabricated on Mo‐coated soda‐lime glass substrate, thereby, the sunlight on the rear side cannot be utilized by the monofacial CIGSSe solar cells owing to the opacity of the Mo electrode. CIGSSe solar cells fabricated on transparent conductive oxide (TCO) electrode enable to fabricate bifacial solar cells, which can generate power on both sides of solar cells, so that bifacial solar cells have a high potential application in building‐integrated photovoltaics. However, the PCEs of CIGSSe solar cells fabricated on TCO substrates are significantly lower than those of ones on Mo substrates. In this article, bifacial CIGSSe solar cells on fluorine‐doped tin oxide (FTO) substrate are fabricated by using an ionic liquid‐assisted solution approach and utilizing a MoO3 thin film as an interface layer between FTO and CIGSSe absorber layer, yielding a front‐side PCE of 9.73% and a back‐side PCE of 1.77%.

Integration of rough RTP absorbers into CIGS-perovskite monolithic tandems by NiOx(:Cu)+SAM Hole-transporting Bi-layers

We investigate the performance of monolithic copper-indium-gallium-selenide (CIGS)/perovskite tandem solar cells with two different CIGS bottom device absorbers: Cu(In,Ga)Se2 or Cu(In,Ga)(S,Se)2 and with three different hole-transporting layers (HTLs): NiOx + SAM, NiOx:Cu + SAM and SAM alone. NiOx(:Cu) is (2 wt% copper-doped) nickel oxide and SAM is the MeO-2PACz ([2-(3,6-dimethoxy-9H-carbazol-9-yl)ethyl]phosphonic acid) self-assembled monolayer. The CIGSe is fabricated by physical-vapor deposition (PVD), has a Eg ∼ 1.06eV, and a σRMS,PVD ∼ 65 nm, while the CIGSSe is fabricated by rapid-thermal processing (RTP), has a Eg ∼ 1.01eV, and a σRMS,RTP ∼ 120 nm. While the current certified, 24.2%-efficient, world-record monolithic CIGSe-perovskite tandem solar cell has previously been achieved with SAM as a stand-alone HTL, this work investigates whether SAM can yield similarly high efficiencies also on industrially compatible, very rough RTP CIGSSe absorbers. We find that the devices with SAM as stand-alone HTL suffer from severe FF and Voc losses and that NiOx:Cu is needed to act as a shunt-quenching layer below that SAM, ensuring conformal coverage of the rough bottom sub-cell surface. Within this work the highest-achieved (in-house measured) PCEs for the RTP and PVD CIGS-based tandems are 21.6% and 23.2% respectively, on a cell area of 1.08 cm2, both of which are obtained with NiOx:Cu + SAM as an HTL.

Efficiency Improvement of Narrow Band Gap Cu(In,Ga)(S,Se)2 Solar Cell with Alkali Treatment via Aqueous Spray Pyrolysis Deposition

The chalcopyrite Cu(In,Ga)(S,Se)2 (CIGSSe) solar cell with a low band gap is a promising candidate for use as the bottom cell in high-efficiency tandem solar cells. In this study, we investigated narrow band gap CIGSSe solar cells, both with and without alkali treatment. The CIGSSe absorbers were fabricated using aqueous spray pyrolysis in an air environment, with the precursor solution prepared by dissolving constituent metal salts. We found that the power conversion efficiency (PCE) of the fabricated solar cell was significantly enhanced when rubidium postdeposition treatment (PDT) was applied to the CIGSSe absorber. The Rb-PDT facilitates defect passivation and a downshift of the valence band maximum of the CIGSSe absorber, thereby improving the power conversion efficiency and all device parameters. Due to these beneficial effects, a PCE of ∼15% was obtained with an energy band gap of less than 1.1 eV, making it suitable for use as the bottom cell in a highly efficient tandem solar cell.

Chemical and Electronic Structure at the Interface between a Sputter‐Deposited Zn(O,S) Buffer and a Cu(In,Ga)(S,Se)2Solar Cell Absorber

The chemical and electronic structure of the interface between a sputter‐deposited Zn(O,S) buffer layer and an industrial Cu(In,Ga)(S,Se)2(CIGSSe) absorber for thin‐film solar cells is investigated with X‐ray and UV photoelectron spectroscopy, inverse photoemission spectroscopy, and X‐ray emission spectroscopy. We find a CIGSSe absorber surface band gap of 1.61 (±0.14) eV, which is significantly increased as compared to the minimal value derived with bulk‐sensitive methods (≈1.1 eV). We find no indication for diffusion of absorber elements into the buffer layer. Surface‐ and bulk‐sensitive measurements of the buffer layer suggest the presence of S‐Zn and S‐O bonds in the Zn(O,S) layer. We find that the naturally existing downward band bending toward the CIGSSe absorber surface is increased by the formation of the interface, likely enhancing carrier separation under illumination. We also derive a flat conduction band alignment, in line with the reported high conversion efficiencies of corresponding large‐area solar cells.

High Efficiency Aqueous Solution Sprayed CIGSSe Solar Cells: Effects of Zr4+‐Alloyed In2S3 Buffer and K‐Alloyed CIGSSe Absorber

AbstractEven though solution‐processed Cu(In,Ga)(S,Se)2 (CIGSSe) solar cells have been developed for long time, their power conversion efficiency (PCE) is still much behind that achieved by vacuum‐based process, and some solvents used in solution‐based processes are not environmental friendly. High‐efficiency CIGSSe solar cells made by using water‐based spray deposition in an air environment are reported. Water is the most eco‐friendly and cheapest solvent compared to the other solvents used in solution‐based fabrication. In2S3‐based buffer (eco‐friendly Cd‐free buffer) and CdS buffer are tested as a buffer layer. For higher PCE with In2S3‐based buffer, a Zr4+‐alloyed In2S3 is used and Zr4+ alloying effects are investigated. It is found that Zr4+ alloying increases the electron concentration in the buffer and passivates defect states in the CIGSSe absorber, which enhance PCE significantly. On the other hand, it is found that K‐alloying into the CIGSSe absorber passivates defects in the CIGSSe absorber, which further increases PCE. Owing to the observed beneficial effects, highest PCE of 16.08% and 17.74% is achieved with Zr4+‐In2S3 buffer and CdS buffer, respectively. In addition, since fabricated solar cells are made with bandgap below 1.15 eV, they can be utilized as a bottom cell for high efficiency tandem cell.

Total band alignment in theoretical and experimental aspects for enhanced performance of flexible and Cd‐free Cu (In,Ga)(S,Se)2 solar cell fabricated by all‐dry process

AbstractTotal band alignment parameters, which are CBO, ΔEC‐TA, and ΔEC‐TB values, in the Cu (In,Ga)(S,Se)2 (CIGSSe) solar cells are theoretically and experimentally optimized. Conduction band minimum (EC) difference between the CIGSSe absorber and buffer is represented by CBO, and that between absorber and transparent conductive oxide (TCO) is denoted by ΔEC‐TA. The EC difference between buffer and TCO is represented by ΔEC‐TA. Based on the measurement of photoelectron yield spectroscopy, the actual CBO, ΔEC‐TA, and ΔEC‐TB values are disclosed. According to the numerical simulation, the useful overview information about the appropriate CBO, ΔEC‐TA, and ΔEC‐TB values is disclosed for the selections of the suitable buffer and TCO materials to reduce the carrier recombination, thus enhancing the photovoltaic performances of the CIGSSe solar cell. As a result, the sputtered Zn0.84Mg0.16O buffer and sputtered Zn0.88Mg0.12O:Al TCO layers are experimentally applied for the appropriate CBO of +0.11 eV, ΔEC‐TA of +0.16 eV, and ΔEC‐TB of +0.05 eV, thus giving rise to the flexible, Cd‐free, and all‐dry process CIGSSe solar cell on stainless steel substrate with the increased conversion efficiency up to 15.3% (and the highest one of 16.5%) as the eco‐friendly and low‐cost solar cell. It is ultimately suggested that the experimental results are well predicted by the overview numerical results, which are useful for the improvement of the photovoltaic performances through the total band alignment.

Efficiency improvement of ZnMgO/CIGSSe heterojunction solar cell by using double graded Cu2O ER-HTL and CeMgO2 HR-ETL materials

- The performance of the ZnMgO/CIGSSe heterojunction solar cell is addressed in this article. Different structures are designed using different materials to improve their parameters. The AFORS-HET simulation tool is used here to justify the photovoltaic results of the reference SLG/(Ni/Al)/ZnMgO:Al/ZnMgO/CIGSSe/Mo solar cell. Later, cuprous oxide (Cu2O) is added to construct a single graded solar cell as an electron reflected-hole transport layer (ER-HTL) between the back metal contact and the CIGSSe layer in the reference device. Furthermore, in the single-graded device, magnesium doped cerium oxide (CeMgO2) as a hole reflected-electron transport layer (HR-ETL) is put between the ZnMgO buffer and the CIGSSe absorber layer to make a double-graded proposed solar cell. The photogenerated recombination losses are get diminished by this double-graded proposed SLG/(Ni/Al)/ZnMgO:Al/ZnMgO/CeMgO2/CIGSSe/Cu2O/Mo solar cell and improved the photovoltaic parameters of the cell such as VOC = 858.3 mV, JSC = 40.18 mA/cm2, FF = 83.42% and η = 28.77%.

Fabrication of in situ alkali doped flexible CIGSSe solar cells by using aqueous spray deposition

Flexible Cu(In,Ga)(S,Se)2 (CIGSSe) solar cells were fabricated on stainless steel foil by using aqueous spray deposition method. Since stainless steel foil is used, external alkali doping is necessary to passivate defects in CIGSSe absorber. We investigated effects of (Na, K) co-doping and selenization temperature on solar cell performance. With co-doping of Na and K, defect states were passivated significantly and highest power conversion efficiency (PCE) was obtained. However, at higher K concentration of more than 0.5%, different growth mode of CIGSSe was observed and deep defect states were developed, decreasing PCE. In addition, selenization temperature effects were studied by varying selenization temperatures. With higher selenization temperature, defect passivation for the CIGSSe absorber was more apparent, which facilitated increase of open circuit voltage. However, short circuit current was observed to be decreased at higher selenization temperature of 570 °C. This work demonstrates in-situ alkali co-doping into CIGSSe absorber with simple and low-cost spray deposition in an air environment.

Study on Na-Ga-F compound in CIGSSe absorber prepared by CuGa-NaF target

The CuGa-NaF target route (CNTR) is meant to enhance the efficiency of Cu(In,Ga)(S,Se)2(CIGSSe) solar cells by reducing the amount of Ga that sinks into the absorber layer. However, researchers have yet to reach a consensus as to the role of Na-Ga-F compounds and its mechanism at the interface. This study sought to elucidate the mechanism by which Na-Ga-F compounds affect the behavior of Ga at the absorber surface. In experiments, the efficiency of devices fabricated using CNTR (13.25%) was higher than that of devices fabricated using the conventional soda lime glass route (CSGR) (11.39%). It appears that the addition of F to the CNTR results in the formation of Na-Ga-F compound, which acts as an anchor for the Ga, thereby reducing the likelihood of its sinking below the surface of the absorber layer. In addition, the uniform distribution of Na in the CIGSSe absorber under the CNTR (particularly at the Mo/CIGSSe interface), was also shown to increase Voc and device efficiency. Finally, the CNTR was shown to passivate the grain boundaries, inhibit the formation of secondary phases on the surface and ameliorate interface recombination.

Formation of Native Inx(O,S)y Buffer through Surface Oxidation of Cu(In,Ga)(S,Se)2 Absorber for Significantly Enhanced Conversion Efficiency of Flexible and Cd‐Free Solar Cell by All‐Dry Process

Herein, flexible, Cd‐free, and all‐dry process Cu(In,Ga)(S,Se)2 (CIGSSe) solar cells on stainless steel (SUS) substrates are developed with manufacturing CIGSSe absorbers fabricated in real production line with large sample size of 0.94 × 1.23 m2. Air‐annealing process under a temperature of 130 °C, 1 atmospheric pressure, and air humidity of about 50% (accelerated oxidizing process) is conducted on absorbers for their oxidized surface. It is demonstrated for the first time that oxidized surface of CIGSSe absorbers after air‐annealing process gives rise to considerable increases in photovoltaic performances of CIGSSe solar cells. This occurs because of formation of native Inx(O,S)y buffer near CIGSSe surface formed after air‐annealing process, which yields self‐forming heterojunction, acting as charge separation and hole‐blocking barrier with increased valence band offset of native Inx(O,S)y/CIGSSe to 1.4 eV, thereby avoiding sputtering damage on CIGSSe surface. Therefore, carrier recombination rates at the interface and depletion regions of the solar cell are reduced, implying improvement of interface and near‐surface qualities. Ultimately, conversion efficiency of 16.7% for flexible, Cd‐free, and all‐dry process CIGSSe solar cell on SUS substrate is attained after the air‐annealing process (130 °C and 6 h) with potential toward Lab‐to‐Fab transition.

Exfoliation methods for compositional and electronic characterization of interfacial Mo(Sex,Sy) in Cu(In,Ga)(Se,S)2 solar cells by X‐ray and UV photoelectron spectroscopy

Mo(SexSy) is a transition metal dichalcogenide typically applied as a back contact interlayer in Cu(In,Ga)(Se,S)2 (CIGSSe) solar cells. Band alignment at the buried Mo/CIGSSe junction mediated by Mo(SexSy) is important for current transport and enables quasi‐ohmic behavior between the CIGSSe absorber and the Mo back electrode. Furthermore, the S/(Se + S) ratio is a crucial parameter that determines the height of the valence band offset at the CIGSSe/Mo(SexSy) interface. Because the interlayer is formed during rapid thermal processing, an MoSe2 or MoS2 thin film grown on free substrate surfaces will not be representative for a realistic solar cell device. Thus, for fundamental thin‐film material analysis, as well as functional characterization and modeling, appropriate preparation and analytical techniques are required in order to prevent artifacts. In principal, the weak van der Waals forces between two‐dimensional stacked Mo(SexSy) sheets allow the implementation of exfoliation procedures to generate free Mo(SexSy) surfaces out of CIGSSe solar cell layer stacks. In this article, two different exfoliation‐based Mo(Sex,Sy) preparation methods are investigated and evaluated with respect to subsequent surface analytical characterization by X‐ray and ultraviolet photoelectron spectroscopy. A special focus is laid on an artifact‐free characterization of chemical and electronical properties of the exposed layers for a number of samples. In a first instance, the compositional Se/S and (Se + S)/Mo ratios at the surface are quantitatively analyzed on the basis of dedicated peak‐fitting routines. Artifacts from carbonaceous contamination due to different exfoliation glues can be prevented through a detailed comparative analysis of carbon 1s and KLL Auger peaks. Furthermore, a significant surface band bending is observed that can be reduced by low‐energy Ar ion in situ sputtering. A simple model for the sputter removal of a charged surface layer is presented, which allows to approximately calculate the absolute valence band maximum (VBM) positions required for band alignment and numerical device simulations. The presented exfoliation surface analysis methodology is important for the whole CIGS(Se) solar cell community and may be of general interest for emerging applications of further 2D transition metal dichalcogenides as well.

Mg Content Impact of a Sputtered Zn1–xMgxO:Al Transparent Electrode on Photovoltaic Performances of Flexible, Cd-Free, and All-Dry-Process Cu(In,Ga)(S,Se)2 Solar Cells

Development of flexible, Cd-free, and all-dry-process Cu(In,Ga)(S,Se)2 (CIGSSe) solar cells on stainless-steel (SUS) substrates has been conducted with sputtered Zn0.84Mg0.16O buffer and sputtered Zn1–xMgxO:Al transparent conductive oxide (TCO). The conduction band minimum (EC) difference between the CIGSSe absorber and Zn1–xMgxO:Al TCO layer, which is called ΔEC-TA, is examined as the promising parameter to enhance conversion efficiency (η). According to photoelectron yield spectroscopy, actual valence band maximum (EV) of Zn1–xMgxO:Al is almost constant from −7.54 to −7.57 eV under the increased Mg contents from 0 to 0.19, where the band gap (Eg) is changed from about 3.5 to 3.9 eV, respectively. Namely, Mg content variation in Zn1–xMgxO:Al primarily changes the EC value, thereby giving rise to the adjustment of ΔEC-TA. It is disclosed that ΔEC-TA plays an important role in increasing η. In theoretical results, optimized ΔEC-TA for high performances is in a range from +0.07 to +0.57 eV. In experimental results, the η values are clearly enhanced from 13.7% with a ΔEC-TA of −0.06 eV (for ZnO:Al TCO) to 16.1% (and the highest η of 16.5%) with the optimal ΔEC-TA from +0.16 eV to +0.22 eV (for Zn1–xMgxO:Al TCO) under an optimal Mg content from 0.12 to 0.16, well consistent with the theoretical results.

Interface Engineering for High-Efficiency Solution-Processed Cu(In,Ga)(S,Se)2 Solar Cells via a Novel Indium-Doped CdS Strategy.

Indium doping of cadmium sulfide (CdS) by chemical bath deposition (CBD) can be an efficient strategy to boost the CIGSSe efficiency. However, limited by the extremely low solubility of In2S3, it is difficult to increase the In doping contents and inhibit the band energy-level regulation for CdS through the traditional CBD process. In this work, we perform a novel CBD method to prepare an indium-doped CdS (In:CdS) buffer, in which the indium source is sequentially slowly added in the growing aqueous solution. In this process, the In ion concentration involved in the real-time deposition is significantly reduced. Thus, compact and uniform In:CdS with higher indium doping content is obtained. Indium doping can elevate the CdS conduction band edge and construct a more favorable spike band alignment with a CIGSSe absorber. Moreover, it introduces efficient carrier transport and reduced interface defect density. As a result, improved CIGSSe heterojunction quality is realized by utilizing In:CdS. Also, the solution-processed CIGSSe device with In:CdS as a buffer yields a high efficiency of 16.4%, with a high VOC of 670 mV and an FF of 75.3%.

Impact of UV-induced ozone and low-energy Ar+-ion cleaning on the chemical structure of Cu(In,Ga)(S,Se)2 absorber surfaces

Dry buffer layer deposition techniques for chalcopyrite (CIGSSe)-based thin-film solar cells lack the surface-cleaning characteristics of the commonly used CdS or Zn(O,S) wet-chemical bath deposition. A UV-induced ozone and/or a low-energy Ar+-ion treatment could provide dry CIGSSe surface cleaning steps. To study the impact of these treatments, the chemical surface structure of a CIGSSe absorber is investigated. For this purpose, a set of surface-sensitive spectroscopic methods, i.e., laboratory-based x-ray photoelectron spectroscopy and x-ray-excited Auger electron spectroscopy, is combined with synchrotron-based soft x-ray emission spectroscopy. After treatment times as short as 15 s, the UV-induced ozone treatment decreases the amount of carbon adsorbates at the CIGSSe surface significantly, while the oxygen content increases. This is accompanied by the oxidation of all absorber surface elements, i.e., indium, selenium, sulfur, and copper. Short (60 s) low-energy Ar+-ion treatments, in contrast, primarily remove oxygen from the surface. Longer treatment times also lead to a removal of carbon, while extremely long treatment times can also lead to additional (likely metallic) Cu phases at the absorber surface as well.

Boosting Solar Cell Performance via Centrally Localized Ag in Solution-Processed Cu(In,Ga)(S,Se)2 Thin Film Solar Cells

Fabrication of Cu(In,Ga)(S,Se)2 (CIGSSe) absorber films from environmentally friendly solutions under ambient air conditions for use in solar cells has shown promise for the low-cost mass production of CIGSSe solar cells. However, the limited power conversion efficiency (PCE) of these solar cells compared with their vacuum-processed counterparts has been a critical setback to their practical applications. This study aims to fabricate solution-processed CIGSSe solar cells with high PCEs by incorporation of Ag into the precursor layer of the CIGSSe absorber films. The results showed that Ag doping promoted grain growth by accelerating Se uptake irrespective of location within the CIGSSe film. Nevertheless, uniform Ag doping formed crevices that lowered the PCE of the cells, while centrally localizing the doped Ag prevented the formation of crevices, resulting in high PCEs up to 15.3%. Our results demonstrate that carefully doping Ag into a selected area of the precursor layer of CIGSSe films can realize solution-processed chalcopyrite solar cells with high PCE.

Effect of Alkali Treatment on Photovoltaic Performances of Cu(In,Ga)(S,Se)2 Solar Cells and Their Absorber Quality Analyzed by Urbach Energy and Carrier Recombination Rates

Cd-free Cu(In,Ga)(S,Se)2 (CIGSSe) solar cells are fabricated by all-dry process, where CIGSSe absorbers with and without alkali (K and Cs) treatments are used. Urbach energy (EU) is estimated in long-wavelength edge of external quantum efficiency. It is revealed that reduced EU through K and Cs treatments yields decreased open-circuit voltage deficit. Carrier recombination rates across devices are reduced through K and Cs treatments, corresponding to increased carrier lifetimes and decreased EU. Conversion efficiency of Cd-free CIGSSe solar cells with a structure of glass/Mo/CIGSSe/Zn0.8Mg0.2O/Zn0.9Mg0.1O:Al is enhanced from about 17.8% (none alkali) to 18.5% (with K) and to 20.3% (with Cs).

Sulfur-Alloying Effects on Cu(In,Ga)(S,Se)2 Solar Cell Fabricated Using Aqueous Spray Pyrolysis.

We fabricated Cu(In,Ga)(S,Se)2 (CIGSSe) solar cells using aqueous spray based deposition, which is inexpensive and covers a large area. To apply the sprayed film to a photoabsorber of a solar cell, post-sulfo-selenization was carried out. Through the sulfo-selenization process, we were able to fabricate various S-alloyed CIGSSe films from S/(S + Se) = 0 (S-0.0) to S/(S + Se) = 0.4 (S-0.4). CIGSSe solar cells were made with the S-alloyed CIGSSe absorbers. Power conversion efficiency of CIGSSe solar cell was found to be increased with S-alloying up to S-0.3, and the best efficiency of 10.89% was obtained with the S-0.3 CIGSSe absorber. Comparison study of S-alloyed CIGSSe solar cells showed that enhanced efficiency in S-0.3 solar cell is due to the increased open-circuit voltage and an improved fill factor, which is induced by S-alloying. In addition, admittance spectroscopy revealed that the defect density of the deep level was developed in the S-alloyed S-0.3 CIGSSe absorber. However, the defect density was observed to be rather reduced. Details of characterization and analysis results are discussed in this paper.

Cd-Free Cu(In,Ga)(Se,S)2 Thin-Film Solar Cell With Record Efficiency of 23.35%

In this article, the excellent properties of state-of-the-art Cd-free Cu(In,Ga)(Se,S)2 (CIGSSe) solar cells with Zn(O,S,OH) x /Zn0.8Mg0.2O double buffer layers, deposited by a combination of chemical bath deposition and atomic layer deposition techniques, are presented. By the replacement of conventional CdS buffer layers with this double buffer layer, the open-circuit voltage ( V oc) deficit of the devices could be significantly reduced, and V oc increased by approximately 15 mV. In addition, the fill factor and short-circuit current were also improved, increasing the device efficiency by approximately 0.5 absolute percent compared with devices with CdS buffers. The Cd-free double buffer layer improved the device efficiency regardless of the bandgap of the CIGSSe absorber. The minority carrier lifetime ( τ ) measured via time-resolved photoluminescence became longer, indicating that carrier recombination is mitigated using the double buffer layer. Based on the device parameters extracted by fitting the Suns– V oc characteristics to the double-diode model, the longer τ could be attributed to the decreased recombination rate in the space-charge region, rather than in the bulk and at the interface. The best performing cell was evaluated by a reliable third party, the National Institute of Advanced Industrial Science and Technology; this cell achieved a new world record efficiency of 23.35% for 1-cm2-sized thin-film polycrystalline solar cells. The device parameters of this cell are also discussed in this article.

Aging Effect of a Cu(In,Ga)(S,Se)2 Absorber on the Photovoltaic Performance of Its Cd‐Free Solar Cell Fabricated by an All‐Dry Process: Its Carrier Recombination Analysis

AbstractCd‐free Cu(In,Ga)(S,Se)2 (CIGSSe) solar cells are fabricated by an all‐dry process (a Cd‐free and all‐dry process CIGSSe solar cell) with aged CIGSSe thin film absorbers. The aged CIGSSe thin films are kept in a desiccator cabinet under partial pressure of oxygen of ≈200 Pa for aging time up to 10 months. It is reported for the first time that aged CIGSSe thin film with increased aging time results in significant enhancement of photovoltaic performance of Cd‐free and all‐dry process CIGSSe solar cells, regardless of the alkali treatment. Based on carrier recombination analysis, carrier recombination rates at the interface and in the depletion region of the Cd‐free and all‐dry process CIGSSe solar cells are reduced owing to avoidance of sputtering damage on CIGSSe absorber surface, which is consistent with the strong electron beam‐induced current signal near CIGSSe surface after the increased aging time. It is implied that the interface and near‐surface qualities are clearly improved through the increased aging time, which is attributable to the self‐forming of Inx(O,S)y near CIGSSe surface, which acts as a buffer layer. Ultimately, the 22.0%‐efficient Cd‐free CIGSSe solar cell fabricated by all‐dry process is achieved with the aged Cs‐treated CIGSSe absorber with the aging time of 10 months.

Urbach energy of Cu(In,Ga)Se2 and Cu(In,Ga)(S,Se)2 absorbers prepared by various methods: Indicator of their quality

Correlation between Urbach energy (EU) and open-circuit voltage deficit (VOC,def) of Cu(In,Ga)Se2 (CIGSe) and Cu(In,Ga)(S,Se)2 (CIGSSe) solar cells is examined, where the CIGSe and CIGSSe absorbers are deposited by different methods. The EU is directly defined from an exponential tail in a long-wavelength edge of an external quantum efficiency. It is shown that the EU is decreased with increased carrier lifetimes corelating to the VOC,def of the solar cells. Consequently, the EU demonstrates a close relation with the VOC,def, where the reduced EU decreases the VOC,def. In addition, conversion efficiency (η) values of the CIGSe solar cells grown by 3-stage and multi-layer precursor methods are significantly enhanced close to 20% with a reduction of the VOC,def, when the EU values of their CIGSe absorbers are reduced to approximately 20 meV. The η values are over 20% with the further decrease in the VOC,def, when the EU values are below 20 meV, mainly estimated from CIGSSe absorbers grown by selenization and sulfurization process. According to the result, the EU can be utilized as one of the intriguing indicators to evaluate the CIGSe absorber quality for the development of the solar cells and should be low (<20 meV), thereby leading to the improved photovoltaic performances.