CuIn(Ga)Se2 based thin film solar cells with electrodeposited absorber on flexible steel foils

In this work we studied the electrical properties of CuIn(Ga)Se₂ solar cells electrodeposited on steel substrate with chromium as diffusion barrier and molybdenum back electrode. The open circuit voltage (V_OC=435 mV) of the samples lies approx. 80 mV below the value commonly observed for devices based on neat CuInSe₂ [1]. From current-voltage measurements (I-V) under variable temperature and light intensity we found that the activation energy E_a of the recombination current J₀ equals the absorber band gap E_g. Hence, interface recombination via defects at the heterojunction seems to be less likely to explain the V_OC loss. Another explanation for this performance limit could be the presence of deep recombination centers in the volume of the absorber layer caused, e.g., by iron diffusion from the substrate. Admittance (AS) and deep level transient spectroscopy (DLTS) revealed the presence of such deep states close to mid-gap position which may facilitate Shockley-Read-Hall (SRH) recombination. To verify or falsify that the observed recombination centers originate from iron diffusion we performed the same experiments on CuIn(Ga)Se₂ solar cells prepared on Ti substrates. In contrast to our expectations we found the same mid-gap states in both sample configurations which point towards an origin being independent of the substrate.