Difference of conduction band minimum (E C ) between transparent conductive oxide (TCO) and absorber, named ΔE C-TA , in thin-film solar cell is investigated for high cell performance using device simulator. According to the simulation, the optimized ΔE C-TA value is different, depending on the carrier density in buffer layer, N D-B . With ΔE C-TA above 0.6 eV for both N D-B s of 1.0 ' 10 13 and 1.0 ' 10 18 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 ΔE C-TA s below %0.2 and %0.4 eV for N D-B s of 1.0 ' 10 13 and 1.0 ' 10 18 cm %3 , the solar cells demonstrate double diode characteristics, thereby decreasing cell efficiency. Eventually, the optimized ΔE C-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 N D-B s of 1.0 ' 10 13 and 1.0 ' 10 18 cm %3 , respectively.

The effect of the band-gap profile on the performance of Cu 2 ZnSn(S x ,Se 1%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.

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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.

The 2‐µm‐thick Cu(In,Ga)Se2 (CIGS) absorber is divided into two areas, low defect density layer (front side) and high defect density layer (back side). In this paper, the optimized thickness of the low defect density layer and the high defect density layer is estimated for high cell performance. The optimization is performed by using device simulator. As a result, the high defect density layer (back side) has no influence on cell performance in the case of CIGS with high optical‐absorption coefficient owing to almost no photocarriers, which are generated in the high defect density layer. In addition, the necessary thickness for absorbing all light to obtain high conversion efficiency is considered. CIGS with graded band profile can be thinner than that with flat band profile for high cell performance owing to the aid of back surface field. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)