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Published September 2014 | Published
Journal Article Open

Predicted roles of defects on band offsets and energetics at CIGS (Cu(In,Ga)Se_2/CdS) solar cell interfaces and implications for improving performance


The laboratory performance of CIGS (Cu(In,Ga)Se_2) based solar cells (20.8% efficiency) makes them promising candidate photovoltaic devices. However, there remains little understanding of how defects at the CIGS/CdS interface affect the band offsets and interfacial energies, and hence the performance of manufactured devices. To determine these relationships, we use density functional theory with the B3PW91 hybrid functional that we validate to provide very accurate descriptions of the band gaps and band offsets. This confirms the weak dependence of band offsets on surface orientation observed experimentally. We predict that the conduction band offset (CBO) of perfect CuInSe_2/CdS interface is large, 0.79 eV, which would dramatically degrade performance. Moreover we show that band gap widening induced by Ga adjusts only the valence band offset, and we find that Cd impurities do not significantly affect the CBO. Thus we show thatCu vacancies at the interface play the key role in enabling the tunability of CBO. We predict thatNa further improves the CBO through electrostatically elevating the valence levels to decrease the CBO, explaining the observed essential role of Na for high performance. Moreover we find that K leads to a dramatic decrease in the CBO to 0.05 eV, much better than Na. We suggest that the efficiency of CIGS devices might be improved substantially by tuning the ratio of Na to K, with the improved phase stability of Na balancing phase instability from K. All these defects reduce interfacial stability slightly, but not significantly.

Additional Information

© 2014 AIP Publishing LLC. Received 30 June 2014; accepted 14 August 2014; published online 2 September 2014. We are grateful to Dr. Jamil Tahir-Kheli and Dr. Ravishankar Sundararaman for helpful discussions. This work was initiated with support from Dow Solar and completed with support from by the Joint Center for Artificial Photosynthesis (JCAP), a DOE Energy Innovation Hub, supported through the Office of Science of the U.S. Department of Energy under Award No. DE-SC0004993.

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August 20, 2023
October 17, 2023