Band alignment at the i-ZnO/CdS interface in Cu(In,Ga)(S,Se)2 thin-film solar cells

Abstract

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 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.

Soft X‐Ray and Electron Spectroscopy: A Unique “Tool Chest” to Characterize the Chemical and Electronic Properties of Surfaces and Interfaces

“…Hence, the surface band-gap energy derived after the sputter treatment is expected to be a good approximation of the CuInSe 2 band gap directly at the CdS/CuInSe 2 interface. This is consistent with earlier investigations, in which metallic species (both Cu and In) are found after prolonged sputter-treatment of the CuInSe 2 surface [45][46][47][48]. This is consistent with earlier investigations, in which metallic species (both Cu and In) are found after prolonged sputter-treatment of the CuInSe 2 surface [45][46][47][48].…”

Impact of bulk properties and local secondary phases on the Cu2(Zn,Sn)Se4 solar cells open-circuit voltage

“…The CBO of CdS and undoped ZnO is taken to be À0.2 eV, between the values reported by Refs. [29,30]. The interface recombination velocity at the CZTSe/CdS interface in the baseline case is set at a low value, S ¼100 cm/s, unless otherwise stated.…”

Efficiency limitations of chalcopyrite and kesterite solar cells

“…The conduction band offset ∆Ec between CdS and ZnO has been measured by at least 2 groups. According to [36]. We conclude that based on published data, a small negative conduction band offset may occur and indeed is often assumed in energy band diagrams of CuIn0.75Ga0.25Se2/CdS/ZnO solar cells [1,10,29,37].…”