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We have revealed that the insertion of a Cu(In,Ga)3Se5 layer at the CdS/Cu(In,Ga)Se2 interface is effective for improving the efficiency of Cu(In,Ga)Se2 solar cells fabricated at low temperatures and with single-graded band profiles. Although the double-graded profile is usually adopted for Cu(In,Ga)Se2 solar cells grown at around 600 °C, the single-graded profile is more suitable for low-temperature-grown solar cells owing to the higher controllability of band profiling. We also analyzed the role of the Cu(In,Ga)3Se5 layer by transmission electron microscopy energy-dispersive X-ray spectrometry (TEM–EDX) and scanning electron microscopy electron-beam-induced current (SEM-EBIC), and found that Cd diffuses to the Cu(In,Ga)3Se5 layer, resulting in a shift of the pn-junction electrical interface from the highly defective structural interface. Finally, by applying this new band profile, VOC and FF increased, resulting in an efficiency improvement from 13.4 to 14.5% at a substrate temperature of 450 °C.

Flexible solar cells with a Cu(In,Ga)Se 2 (CIGS) absorber layer were fabricated on a polyimide thin film using a lift-off process. Polyimide-coated soda-lime glass (SLG) was used as a substrate for fabricating CIGS solar cells before the lift-off process conducted to make the cells flexible. A conversion efficiency of 13.4% was achieved by low temperature deposition; this value is comparable to that obtained by direct deposition on a rigid glass substrate even without an external Na source. The final conversion efficiency after the lift-off process was 12.7% with some area correction due to the partial peeling-off between CIGS and Mo. Open-circuit voltage and fill factor did not change before and after the lift-off process, suggesting that the lift-off process did not give any physical damage.

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We have developed a new evaluation method for electrical properties of Cu(In,Ga)Se 2 (CIGS) grown on a Mo-coated soda-lime glass (SLG). The method consists of the peel-off process and the AC Hall measurement, which enables us to evaluate CIGS films grown on the Mo electrode. It was found, from the measurement, that the hole concentration of CIGS grown on a Mo-coated SLG was approximately two orders of magnitude higher than that on a SLG, suggesting the Na-doping effect. Furthermore, the hole mobility of 0.47 cm 2 /(V&s) was simultaneously measured, even though the film was deposited on the Mo electrode.