Photovoltage Decay Processes in Cu(In,Ga)Se2 Solar Cells Studied by Photo-assisted Kelvin Probe Force Microscopy

Hyeondeuk Yong1, Yu Nakajima1, Takashi Minemoto3, Takuji Takahashi1,2 1Institute of Industrial Science, The University of Tokyo, Tokyo, Japan /2Institute for Nano Quantum Information Electronics, The University of Tokyo, Tokyo, Japan /3Department of Photonics, Ritsumeikan University, Shiga, Japan

Photovoltaic measurements by the photo-assisted Kelvin probe force microscopy (P-KFM) which we originally proposed were performed on Cu(In,Ga)Se2 [CIGS] solar cells with different Ga contents in order to investigate recombination process of photo-carriers. From the temporally-averaged photovoltage measured as a function of the modulation frequency of the incident light, we evaluated a time constant  tau for photovoltage decay, being considered to represent a time constant required for the carrier transport across the potential barriers at the hetero-interfaces in the cell structure, and a contribution ratio r of fast carrier recombination process in the entire recombination processes. The results indicate that the sample with high Ga content (50 %) [Sample D] exhibited long tau and large r compared with the sample with low Ga content (23 %) [Sample A]. This elongation of tau in Sample D is interpreted as a deduction that the electron transport from the surface ZnO layer to the CIGS layer is slightly disturbed due to the broadening of the CIGS bandgap. Since, on the other hand, the increase of r means that the fast carrier recombination in the CIGS layer is enhanced, which may degrade the solar cell performance, large r observed in Sample D is very consistent with a fact that the conversion efficiency of Sample D (= 9.1 %) is worse than that of Sample A (= 14.7%). From the similar measurements performed on other samples for further investigation of the dependence of r on the Ga content, we found that the value of r gradually increases as an increase of Ga content. These results are attributable to another deduction that the electron and hole separation near a grain boundary is more weakened due to lowering of built-in electric field in the sample with higher Ga content. This work was partly supported by JSPS and MEXT.