Absorber composition: A critical parameter for the effectiveness of heat treatments in chalcopyrite solar cells

Post-device heat treatment (HT) in chalcopyrite [Cu (In,Ga)(S,Se)(2)] solar cells is known to improve the performance of the devices. However, this HT is only beneficial for devices made with absorbers grown under Cu-poor conditions but not under Cu excess. We present a systematic study to understand the effects of HT on CuInSe(2)and CuInS(2)solar cells. The study is performed for CuInSe(2)solar cells grown under Cu-rich and Cu-poor chemical potential prepared with both CdS and Zn(O,S) buffer layers. In addition, we also study Cu-rich CuInS(2)solar cells prepared with the suitable Zn(O,S) buffer layer. For Cu-poor selenide device, low-temperature HT leads to passivation of bulk, whereas in Cu-rich devices, no such passivation was observed. The Cu-rich devices are hampered by a large shunt. The HT decreases shunt resistance in Cu-rich selenides, whereas it increases shunt resistance in Cu-rich sulfides. The origin of these changes in device performance was investigated with capacitance-voltage measurement, which shows the considerable decrease in carrier concentration with HT in Cu-poor CuInSe2, and temperature-dependent current-voltage measurements show the presence of barrier for minority carriers. Together with numerical simulations, these findings support a highly doped interfacial p(+)layer device model in Cu-rich selenide absorbers and explain the discrepancy between Cu-poor and Cu-rich device performance. Our findings provide insights into how the same treatment can have a completely different effect on the device depending on the composition of the absorber.