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In this study, we investigated the effect of the stacking order of metal precursors on the formation of volume defects, such as blisters and nanopores, in CZTSSe thin-film solar cells. We fabricated CZTSSe thin films using three types of metal-precursor combinations, namely, Zn/Cu/Sn/Mo, Cu/Zn/Sn/Mo, and Sn/Cu/Zn/Mo, and studied the blister formation. The blister-formation mechanism was based on the delamination model, taking into consideration the compressive stress and adhesion properties. A compressive stress could be induced during the preferential formation of a ZnSSe shell. Under this stress, the adhesion between the ZnSSe film and the Mo substrate could be maintained by the surface tension of a metallic liquid phase with good wettability, or by the functioning of ZnSSe pillars as anchors, depending on the type of metal precursor used. Additionally, the nanopore formation near the back-contact side was found to be induced by the columnar microstructure of the metal precursor with the Cu/Zn/Mo stacking order and its dezincification. Based on the two volume-defect-formation mechanisms proposed herein, further development of volume-defect-formation suppression technology is expected to be made.

In this study, to control the formation of non-uniformly distributed large voids and Cu–Sn alloy agglomeration, which leads to local compositional misfit and secondary phase formation, a SnS compound precursor was applied instead of metal Sn to avoid compositional non-uniformity. Using a Cu/Zn/SnS stacked precursor, a temperature tracking experiment was conducted to confirm the formation controllability of the void and the secondary phase. According to the results of this temperature-profile tracking experiment, it was confirmed that the large void was successfully controlled; however, an additional ZnSSe secondary phase layer was formed in the middle of the CZTSSe upper layer and small voids were distributed relatively uniformly in the bottom CZTSSe layer. An efficiency of approximately 8% was obtained when the Cu/Zn/SnS stacked precursor was used. The origins of the low short-circuit current and fill factor are posited to be caused by the increase of the energy bandgap of the CZTSSe layer due to the SnS precursor, the thin top CZTSSe layer (around 600 nm) of the triple CZTSSe layer, and the diffusion length extension of the minor carriers caused by bypassing the ZnSSe phase.

Currently, many researchers are focused on promising new energy technologies such as Cu 2 ZnSnS 4 -based solar cells to replace fossil fuel sources. Investigations into a variety of methods have been done to analyze the interfacial problems and improve the cell interfacial properties via a variety of methods. In particular, In order to improve the electrical performance of Cu 2 ZnSnSe 4 (CZTSe) based solar cells, the Cd partial electrolyte (Cd PE) treatment that is an effective method in CIGS solar cells prior to applying their CdS buffer layer to their absorber layer has been studied. In our study, we investigated the Cd PE treatment time, the Cd PE bath temperature, and the correlation between Cd PE treatment and CdS thickness. The optimum Cd PE absorber treatment was 7 min at 70 °C, with a CdS layer of 35 nm thick deposited cells. This combination increased photo-carrier collection in the short wavelength range and reduced absorber-buffer interface recombination. The efficiency of a 35 nm buffer layer sample without PE treatment was 8.90%. The efficiency of another 35 nm buffer layer sample was 10.38% (Voc: 441 mV, Jsc: 38.15 mA/cm2, and FF: 61.58%) after Cd PE treatment due to the increase of Jsc, FF and Rs despite the deficit of Voc. [ABSTRACT FROM AUTHOR]

Recently, highly efficient CZTS solar cells using pure metal precursors have been reported, and our group created a cell with 12.6% efficiency, which is equivalent to the long-lasting world record of IBM. In this study, we report a new secondary phase formation mechanism in the back contact interface. Previously, CZTSSe decomposition with Mo has been proposed to explain the secondary phase and void formation in the Mo-back contact region. In our sulfo-selenization system, the formation of voids and secondary phases is well explained by the unique wetting properties of Mo and the liquid metal above the peritectic reaction (η-Cu6Sn5→ ε-Cu3Sn + liquid Sn) temperature. Good wetting between the liquid Sn and the Mo substrate was observed because of strong metallic bonding between the liquid metal and Mo layer. Thus, some ε-Cu3Sn and liquid Sn likely remained on the Mo layer during the sulfo-selenization process, and Cu–SSe and Cu–Sn–SSe phases formed on the Mo side. When bare soda lime glass (SLG) was used as a substrate, nonwetting adhesion was observed because of weak van der Walls interactions between the liquid metal and substrate. The Cu–Sn alloy did not remain on the SLG surface, and Cu–SSe and Cu–Sn–SSe phases were not observed after the final sulfo-selenization process. Additionally, Mo/SLG substrates coated with a thin Al2O3layer (1–5 nm) were used to control secondary phase formation by changing the wetting properties between Mo and the liquid metal. A 1 nm Al2O3layer was enough to control secondary phase formation at the CZTSSe/Mo and void/Mo interfaces, and a 2 nm Al2O3layer was enough to perfectly control secondary phase formation at the Mo interface and Mo–SSe formation.

In recent years, Cu2ZnSn(S1-xSex)4(CZTSSe) prepared by a two-step process using metal precursors has been reported to exhibit a relatively high power conversion efficiency, and a high efficiency of 12.5% by two-step process contained via sputtering method was recently confirmed by our group. In this study, we proposed formation mechanisms for the CZTSSe double layer, voids and ZnSSe layer, which were observed in the CZTSSe using metal precursor. Due to the persistent dezincification from the metal precursors and preferential reaction between the Zn and chalcogens such as S and Se, almost all Zn is consumed to form the ZnSSe layer; as a result, large voids are produced first under the ZnSSe layer. Cu2Se and SnSe are grown on the ZnSSe layer via migration of the Cu and Sn through the grain boundaries of the ZnSSe layer. Thus, additional small voids are expected to form due to the mass transfer of Cu and Sn. Because of the preferentially formed ZnSSe layer and the chalcogenation of Cu and Sn after the mass transfer, a CZTSSe double layer can be formed, and ZnSSe can exist between these CZTSSe layers. Finally, we propose a method based on the formation mechanism to control the voids and secondary phases, which affect the fill factor and output current.

Cu2ZnSn(S,Se)S4(CZTSSe) thin films have attracted attention as low-cost absorber materials for solar cells; however, further studies are required to develop flexible solar cells from this material and to achieve a high power conversion efficiency. Toward this objective, this work investigated eight types of precursors applied on flexible Mo foil substrates, some of which also contained a layer of NaF. Secondary phases, defects, and defect clusters were different in the various samples, and the surface electrical characteristics of the CZTSSe absorber layer varied accordingly. In contrast to those in the CIGS-based cells, defects and defect clusters generated in the CZTSSe absorber layer caused an upward band bending-like band structure to form at the grain boundaries (GBs), thereby forming an intra-grain (IG) current path. By improving carrier separation, a flexible CZTSSe thin-film solar cell was developed on a Mo foil substrate with a power conversion efficiency of 7.04%. Thus, the efficiency of CZTSSe thin-film solar cells could be increased through carrier separation measures that enabled the collection of holes toward the GBs and of electrons toward IGs.

We investigated limitation factors of high efficiency Cu2ZnSn(S,Se)4(CZTSSe) solar cells, where the CZTSSe absorbers were made by using sulfo-selenization process. CZTSSe absorbers with two S/(S+Se) ratios, ~ 0.12 (Se-rich) and ~ 0.22 (S-increased), were prepared by varying the sulfo-selenization temperature. The Se-rich CZTSSe solar cells were found to have larger conduction band offset (CBO) between the absorber and the buffer, which was reflected in the kinked J-V curves at low temperatures. Considering that the larger CBO prevents electron transport from absorber to buffer and resultantly reduces short circuit current and fill factor, it could be possible limitation factor of the high efficiency solar cell. Contrary to Se-rich solar cells, S-increased solar cells showed reduced CBO and no kinked J-V curve. However, deep defects were found to be generated, which induced defect centers of charge recombination both at interface and in bulk of the absorber. The larger CBO in Se-rich CZTSSe solar cell and deep defects in S-increased CZTSSe solar cell are observed even in ~ 12% efficiency solar cells. Thus, we believe that these possible limitation factors should be resolved to achieve high efficiency kesterite CZTSSe solar cell above 12%.

To commercialize Cu2ZnSn(S,Se)4(CZTSSe) thin-film solar cells, it is necessary to improve their efficiency and to develop the technological ability to produce large-area modules. Defect formation due to the secondary phase is considered to be one of the main reasons for decreased CZTSSe thin-film solar-cell efficiency. This study explores the potential capabilities of large-area thin-film solar cells by controlling the defect formation using various CZTSSe precursor designs, and by improving the characteristic uniformity within the thin-film solar cells. Alloying the precursor as a stack of discrete layers can result in lateral segregation of elements into stable-phase islands, yielding a non-uniform composition on small length scales. It is found that the application of an indiscrete layer by minimizing the precursor-layer thickness allows avoiding Zn rich inhomogeneities in the absorber that would favor formation of detrimental ZnS-ZnSe secondary phases and deep defects. Among the various precursor layers designed by considering the reaction mechanism under annealing, a sample with 15 precursor layers is found to exhibit a shallow electron-acceptor energy level, high photovoltaic conversion efficiency, and uniform characteristics over the corresponding thin-film solar cell. Based on such improvements in both the efficiency and characteristic distribution, it is expected that the commercialization of CZTSSe thin-film solar cells can be advanced.

In this study, we investigate methods of improving the efficiency of Cu 2 ZnSn(S,Se) 4 (CZTS)-based solar cells by comparing Cu(In,Ga)Se 2 (CIGSe)- and CZTS-based absorber layers. In particular, the CZTS-based absorber exhibits lower current characteristics than the CIGSe absorber layer in terms of the band gap alignment and electron-hole recombination at the CdS-absorber interface. Moreover, we demonstrate that defects are one of the causes of the voltage loss. In order to improve the efficiency of CZTS-based solar cells, it is important to control the band gap alignment at the CdS-absorber layer interface and to suppress the formation of secondary phases inside the absorber. [ABSTRACT FROM AUTHOR]

Anti-reflective (AR) layers play an important role in boosting the amount of light entering a device and reducing reflection losses in a device, thereby enhancing the power conversion efficiency of solar cells. We have coated an AR layer on the surface of a dye-sensitized solar cell device by using an electron beam evaporation system and investigated the effects of the AR layer by measuring photovoltaic performance. The AR layer is found to increases the Jsc and η of the solar cell.

We fabricated thin film transistors (TFTs) using HfInZnO thin films as active channel layers. The thin films of HfInZnO were deposited by co-sputtering from HfO2and InZnO targets. The HfInZnO TFTs were investigated according to the radio-frequency power applied to the HfO2target. The transistor on and off currents were greatly influenced by the composition of Hf atoms suppressing the formation of oxygen vacancies. The electrical characteristics of the TFTs show a field-effect mobility of 3.53cm2V−1s−1, a threshold voltage of 1.28 V, an on/off ratio of 1.4×10−7, and a subthreshold swing of 95 mV/dec.