Study of the effect of gallium grading in Cu(In,Ga)Se 2

Abstract Bandgap engineering in Cu(In,Ga)Se2 (CIGS) based solar cells is an interesting tool to improve their performance. We fabricate and analyse various kinds of gallium gradings in CIGS absorbers starting with different kinds of linear graded absorbers deposited by co-evaporation, double graded absorbers by the three-stage process and finally multi graded absorbers by co-evaporation. Devices with linear grading show a strong influence of the slope of the grading on the open circuit voltage Voc and the short circuit current Isc. The three-stage process employed for double graded profiles allows variations only in a limited range not suitable for systematic grading studies. Finally, the multi graded profiles combine the positive impacts of the linear graded devices. We found strong indications that it is possible to prepare a smaller bandgap to improve the current and a larger bandgap to improve the voltage in the same device by appropriate grading. The best devices show active area efficiencies up to η=16.7 % and open circuit voltages up to V oc =691 mV at a minimum bandgap of E g =1.1 eV.