Daisuke Hironiwa

Simulation of optimum band-gap grading profile of Cu2ZnSn(S,Se)4 solar cells with different optical and defect properties

The effect of the band-gap profile on the performance of Cu2ZnSn(Sx,Se1−x)4 (CZTSSe) solar cells was investigated using a solar cell capacitance simulator (SCAPS) device simulation program. The band gap of CZTSSe is tunable from 1.0 to 1.5 eV by changing the S/(S + Se) ratio. Currently, the evolution of the electron affinity (χ) of CZTSSe at various band gaps has not been clarified yet, although two models with different χ values at various band gaps of CZTSSe have been proposed. We simulated solar cell performance using these two models and the differential rates of efficiency were compared between them. As a result, we were able to design the optimum band-gap profile using both models. Meanwhile, the characteristics of a solar cell with various optical absorption coefficients and defect densities of the CZTSSe absorber were simulated. The superiority of the graded band-gap profile was demonstrated by comparing the cell performances with and without a grading profile structure.

Sputtered (Zn,Mg)O buffer layer for band offset control in Cu2ZnSn(S,Se)4 solar cells

We fabricated Cu2ZnSn(S,Se)4 (CZTSSe) solar cells with (Zn,Mg)O buffer layers as an alternative to the CdS buffer layer for the improvement of cell performance, where the (Zn,Mg)O layers are deposited by sputtering. However, the solar cell efficiency decreased with the (Zn,Mg)O layer as compared with the CdS layer. Photoluminescence measurements indicated that the damage near the surface of the CZTSSe absorber was induced by the sputtering. To suppress the damage, a 10-nm-thick CdS layer was deposited on the absorber before sputtering. As a result, the efficiency achieved with the (Zn,Mg)O layer was the same as that with the CdS layer. To further improve the efficiency of the cell with the (Zn,Mg)O layer, it is necessary to eliminate sputtering damage. In addition, the conduction band offset of the (Zn,Mg)O/CZTSSe interface is controllable by varying the Mg content. Therefore, the (Zn,Mg)O buffer layer can be suitable against a large-band-gap CZTSSe absorber.

Optimum bandgap profile analysis of Cu(In,Ga)Se2 solar cells with various defect densities by SCAPS

The bandgap of a Cu(In,Ga)Se2 (CIGS) absorbing layer is varied from 1.0 to 1.7 eV by changing the composition ratio of gallium (Ga), realizing an optimum design for solar cell absorbers. In this study, the effects of a graded bandgap profile on the cell performance of a CIGS solar cell are investigated using a device simulator. Moreover, optimum bandgap profiles with various defect densities are simulated. In the case of low defect densities, when the lowest bandgap, Egmin, is inside the space-charge region (SCR), the double-graded structure is effective for achieving high efficiency. However, when Egmin is outside the SCR, the negative gradient from Egmin to the CIGS surface acts as a barrier that impedes the collection of photogenerated electrons, thereby increasing the recombination rate and decreasing cell efficiency. In the case of high defect densities, to decrease the recombination current and improve the efficiency, a more positive gradient from the back contact to the surface is needed.

Influence of conduction band minimum difference between transparent conductive oxide and absorber on photovoltaic performance of thin-film solar cell

Difference of conduction band minimum (EC) between transparent conductive oxide (TCO) and absorber, named ΔEC-TA, in thin-film solar cell is investigated for high cell performance using device simulator. According to the simulation, the optimized ΔEC-TA value is different, depending on the carrier density in buffer layer, ND-B. With ΔEC-TA above 0.6 eV for both ND-Bs of 1.0 × 1013 and 1.0 × 1018 cm−3, the spike is formed at the TCO/buffer interface, thus decreasing cell performances, especially short-circuit current density owing to impeding photo-generated carriers to TCO. On the other hand, with ΔEC-TAs below −0.2 and −0.4 eV for ND-Bs of 1.0 × 1013 and 1.0 × 1018 cm−3, the solar cells demonstrate double diode characteristics, thereby decreasing cell efficiency. Eventually, the optimized ΔEC-TA values for high cell performance are proposed to be in the ranges from −0.2 to 0.6 eV and from −0.4 to 0.6 eV for ND-Bs of 1.0 × 1013 and 1.0 × 1018 cm−3, respectively.

Evaluation of junction quality of buffer-free Zn(O,S):Al/Cu(In,Ga)Se2 thin-film solar cells

Cu(In,Ga)Se2 (CIGS) solar cells without buffer layers, namely, buffer-free solar cells, and those with conventional CdS buffer layers were fabricated. Zn(O,S):Al (AZOS) was used as a transparent conductive oxide layer. The AZOS/CIGS interface in the buffer-free solar cells and the AZOS/CdS/CIGS interface in the CdS-buffered solar cells were investigated by bright-field scanning transmission electron microscopy, high-angle annular dark-field scanning transmission electron microscopy, and fast Fourier transform. It was found that the open-circuit voltage and fill factor of the buffer-free solar cells are lower than those of the CdS-buffered solar cells, which is attributed to the nonepitaxial growth of AZOS on the CIGS layer, forming defects and deteriorating the junction quality of the buffer-free solar cells.

Phosphorous Gettering on Spherical Si Solar Cells Fabricated by Dropping Method

Si spheres for spherical Si solar cells are produced by a dropping method. In the current state, the Si spheres are generally multicrystalline. In the dropping method, some impurities, mainly iron, which act as minority carrier recombination centers can be introduced in the Si spheres. To remove the impurities, phosphorous diffusion gettering (P-gettering) has been performed on spherical Si solar cells at 925 ?C for 40 min. The effect of P-gettering was evaluated by solar cell performance and external quantum efficiency (EQE). The increase in the EQE of a long wavelength region was confirmed, which indicates that the deleterious impurities in a bulk region (not only the surface region) would be effectively reduced. Also, a minority carrier diffusion length estimated by the surface photovoltage method increased after P-gettering. Consequently, the efficiency of the spherical Si solar cell was improved. These results confirmed that P-gettering is effective to improve the solar cell performance of spherical Si solar cells.

Evaluation of sputtering damage in Cu2ZnSn(S,Se)4 solar cells with CdS and (Cd,Zn)S buffer layers by photoluminescence measurement

Cu2ZnSn(S,Se)4 (CZTSSe) solar cells are fabricated with CdS and (Cd,Zn)S buffer layers of different thicknesses to investigate sputtering damage on the absorber surfaces during ZnO:Al/ZnO layer deposition. In this work, the sputtering damage is scrutinized by photoluminescence (PL) measurement. The damage (i.e., non-radiative recombination centers) near the absorber surfaces investigated on the basis of PL peak intensity decreases with increasing thickness of the buffer layer. Furthermore, the intensity in the solar cells with the CdS buffer is higher than that with the (Cd,Zn)S buffer layer, suggesting that the CdS buffer layer demonstrates better capability of preventing sputtering damage near the CZTSSe surface than the (Cd,Zn)S buffer. PL ratio defined as the ratio of the PL peak intensity after sputtering to the PL peak intensity before sputtering is utilized to quantify sputtering damage. The solar cell performance increases with increasing PL ratio up to 0.5, followed by saturation at a ratio higher than 0.5. Taken together, PL ratio is proposed as a tool for monitoring sputtering damage for improving cell performance.

Post annealing effect on buffer-free CuInS2 solar cells with transparent conducting Zn1−xMgxO:Al films

CuInS2 solar cells without buffer layers, i.e., buffer-free cells, were fabricated. The typical structure of the cells is ZnO:Al/CdS/CuInS2/Mo/glass and thus includes CdS as a buffer layer. Eliminating the buffer layer is effective in preventing short-wavelength light absorption loss caused by CdS and simplifying the structure and process. However, the elimination of CdS from the structure results in the formation of a ZnO:Al/CuInS2 junction, leading to the mismatching of a conduction band offset (CBO). In this work, instead of ZnO:Al, we used a (Zn,Mg)O:Al layer prepared by co-sputtering ZnO:Al and MgO:Al for CBO matching. Also, the junction quality deteriorated by sputtering damage during (Zn,Mg)O:Al deposition was recovered by post annealing after cell fabrication. The low open-circuit voltage of as-fabricated cells of 0.25 V was improved to 0.52 V by annealing at 250 °C for 10 min, leading to an improvement in efficiency from 1.9 to 5.5%. The results indicate the possibility of realizing buffer-free CIS solar cells.

Simple fabrication of back contact heterojunction solar cells by plasma ion implantation

A back-contact amorphous-silicon (a-Si)/crystalline silicon (c-Si) heterojunction is one of the most promising structures for high-efficiency solar cells. However, the patterning of back-contact electrodes causes the increase in fabrication cost. Thus, to simplify the fabrication of back-contact cells, we attempted to form p-a-Si/i-a-Si/c-Si and n-a-Si/i-a-Si/c-Si regions by the conversion of a patterned area of p-a-Si/i-a-Si/c-Si to n-a-Si/i-a-Si/c-Si by plasma ion implantation. It is revealed that the conversion of the conduction type can be realized by the plasma ion implantation of phosphorus (P) atoms into p-a-Si/i-a-Si/c-Si regions, and also that the quality of passivation can be kept sufficiently high, the same as that before ion implantation, when the samples are annealed at around 250 °C and also when the energy and dose of ion implantation are appropriately chosen for fitting to a-Si layer thickness and bulk c-Si carrier density.

Theoretical Analysis of Optimum Bandgap Profile of Cu(In,Ga)Se 2 Solar Cells with Optical and Defect Properties

The effects of graded bandgap profile on the cell performance of Cu(In,Ga)Se2 solar cell were investigated by device simulator. Optimum bandgap profiles with various defect densities were simulated. In addition, the superiority of the graded bandgap profile was indicated by the comparison with CIGS solar cells without grading.