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Control of valence band offset of Cu(In,Ga)Se2 solar cells with single-graded band profile

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.