Charge carrier transport in Cu(In,Ga)Se2 thin-film solar-cells studied by electron beam induced current and temperature and illumination dependent current voltage analyses

This work contributes to the understanding of generation dependent charge-carrier transport properties in Cu(In,Ga)Se2 (CIGSe)/ CdS/ ZnO solar cells and a consistent model for the electronic band diagram of the heterojunction region of the device is developed. Cross section electron-beam induced current (EBIC) and temperature and illumination dependent current voltage (IV) measurements are performed on CIGSe solar cells with varying absorber layer compositions and CdS thickness. For a better understanding of possibilities and limitations of EBIC measurements applied on CIGSe solar cells, detailed numerical simulations of cross section EBIC profiles for varying electron beam and solar cell parameters are performed and compared to profiles obtained from an analytical description. Especially the effects of high injection conditions are considered. Even though the collection function of the solar cell is not independent of the generation function of the electron beam, the local electron diffusion length in CIGSe can still be extracted. Grain specific values ranging from (480 ± 70) nm to (2.3 ± 0.2) μm are determined for a CuInSe2 absorber layer and a value of (2.8 ± 0.3) μm for CIGSe with a Ga-content of 0.3. There are several models discussed in literature to explain generation dependent charge carrier transport, all assuming a high acceptor density either located in the CIGSe layer close to the CIGSe/CdS interface (p+ layer), within the CdS layer or at the CdS/ZnO interface. In all models, a change in charge carrier collection properties is caused by a generation dependent occupation probability of the acceptor type defect state and the resulting potential distribution throughout the device. Numerical simulations of EBIC and IV data are performed with parameters according to these models. The model that explains the experimental data best is that of a p+ layer at the CIGSe/CdS interface and acceptor type defect states at the CdS/ZnO interface. The p+ layer leads to generation dependent transport in EBIC at room temperature, while defect states at the CdS/ZnO interface cause a significant reduction of the photocurrent in the red light illuminated IV characteristics at low temperatures. Electron beam irradiation of the heterojunction region alters EBIC profiles temporarily, and there is enhanced relaxation of this effect in air. In addition, the occurrence of generation dependent chargecarrier transport is found to correlate with the presence of an ordered defect compound (ODC) in the absorber layer. Thus, the p+ layer occurs in connection with an ordered defect compound (ODC) layer with a lowered valence band maximum as compared to that of the CIGSe layer and a high net p-type doping density (10 cm−3), possibly due to the passivation of donor type defect states by oxygen incorporation. Furthermore, shallow donor type defect states at the p+ layer/CdS interface of some grains of the absorber layer possibly caused by Cd on Cu site defects explain the fact that charge-carrier transport is grain specific, i.e. inhomogeneous, in EBIC experiments.