Aspects for the optimization of CIGSe growth at low temperatures for application in thin film solar cells on polyimide foil

Cu(In,Ga)Se₂ (CIGSe) thin film solar cells are the most efficient thin film photovoltaic technology available. Deposited onto the appropriate substrate they are potentially flexible, very light, robust and low cost. Due to their excellent radiation hardness and potentially high specific power, they have also attracted interest for use in space applications. Highest quality CIGSe absorber layers are usually grown at temperatures well above 500°C. So far only metal foils are a suitable choice as flexible substrate material in this temperature range. However, as those are conductive, the use of monolithic integration for solar cell interconnection requires an electrically insulating barrier between substrate and solar cell back contact. A non-conductive alternative to metal is polyimide foil. Commercially available polyimide foils are only tolerant to temperatures of up to around 400°C. It is therefore necessary to identify and understand the influence of main process parameters in order to achieve growth of high quality absorber material at these low temperatures. Former work has already highlighted that the amount of sodium present during film growth is a key parameter regarding optimum growth results. The work that is presented here summarizes previous work and investigates the complex relationship between the growth temperature and the effect of Na on the compositional, structural and electronic properties of CIGSe thin films.