Time-resolved Photo-assisted Kelvin Probe Force Microscopy on Cu(In,Ga)Se2 Solar Cells

Photo-assisted Kelvin probe force microscopy (P-KFM), which is one application of atomic force microscopy, is very useful for examining local photovoltaic characteristics on solar cells, and improvement of a time resolution in P-KFM is desired for investigating carrier dynamics in time domain. In this study, we have introduced an intermittent bias application method and a pump-probe method into P-KFM to realize time-resolved measurements at μsec order and have applied it to Cu(In,Ga)Se2 [CIGS] solar cells with different Ga compositions. From the temporal waveforms of photovoltage and the temporal change in photovoltage distribution on the CIGS solar cell, we have found certain differences of rising and decay rates of photovoltage between the samples. Especially for the photovoltage rise, the photo-generated electrons in CIGS have to be injected into a surface n-type layer, and the slow rising rate of photovoltage means a decrease in the injection rate of the photo-generated electrons from CIGS. Therefore, we consider that the rising time constant becomes one indicator of the solar cell performance, and have found that a difference in rising time constant was consistent with a difference in conversion efficiency. From the time evolution images of photovoltage just after the onset of the light pulse, in addition, the spatial distribution of photovoltage related with the surface topography were recognized, which is attributable to the in-plane band diagram around the grain boundary in CIGS. Furthermore, dependence of the time evolution of photovoltage distribution on the incident photon energy was examined, from which an influence of V-shaped conduction band structure in CIGS on the transport of the photo-generated electrons was discussed.