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

Goddard, William A.

doi:10.1063/1.4893985

Title: 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

Journal Article · Tue Sep 02 00:00:00 EDT 2014 · Journal of Chemical Physics

DOI:https://doi.org/10.1063/1.4893985· OSTI ID:1467647

Goddard, William A. ^[1]

California Inst. of Technology (CalTech), Pasadena, CA (United States)

The laboratory performance of CIGS (Cu(In,Ga)Se₂) 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 CuInSe2/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 that Cu vacancies at the interface play the key role in enabling the tunability of CBO. We predict that Na 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.

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Research Organization:: California Institute of Technology (CalTech), Pasadena, CA (United States)

Sponsoring Organization:: USDOE Office of Science (SC), Basic Energy Sciences (BES)

Grant/Contract Number:: SC0004993

OSTI ID:: 1467647

Journal Information:: Journal of Chemical Physics, Vol. 141, Issue 9; ISSN 0021-9606

Publisher:: American Institute of Physics (AIP)Copyright Statement

Country of Publication:: United States

Language:: English

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Cited by: 22 works

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Title: 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