Photoelectric characterization of Cu(In,Ga)S2 solar cells obtained from rapid thermal processing at different temperatures

CuInS2-based solar cells have a strong potential of achieving high efficiencies due to their ideal band gap of 1.5 eV. A further increase in the efficiency is expected from doping the absorber film with gallium due to enlargement of the band gap (Eg) and correspondingly the open-circuit voltage (VOC). We investigated Cu(In,Ga)S2 solar cells obtained from stacked metal layers sputtered from In and (Cu,Ga) targets followed by rapid thermal processing (RTP) in sulfur vapor. Depending on the actual RTP temperature profile, the films might exhibit CuInS2/CuGaS2 (top/bottom) segregation, which is rather detrimental for a large VOC. We found that only precursors sulfurized at sufficiently high temperatures exhibit the desired interdiffusion of the segregated CuInS2/CuGaS2 layers resulting in an increased VOC. Moreover, temperature dependent current–voltage profiling (suns-VOC-analysis) yielded strong indications for improved current collection and reduced losses for devices with proper interdiffusion of the CuInS2/CuGaS2 layers. A more fundamental question is related to the variation and formation of defect states in differently processed absorber films. The studied samples were thus further investigated by means of admittance spectroscopy, which allowed us to confirm the presence of three individual defect states in both absorber configurations. Two defects exhibit activation energies, which remain unchanged upon varying the RTP temperature whereas a third state exhibits significantly increased activation energy in devices showing interdiffusion of CuInS2/CuGaS2 layers. According to the characteristic shift of the conduction band edge upon Ga-doping we conclude that the latter defect level corresponds with the minority carriers in the p-type absorbers.