Predicted roles of defects on band offsets and energetics at CIGS (Cu(In,Ga)Se2/CdS) solar cell interfaces and implications for improving performance

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.

Effect of Alkali Metal Atom Doping on the CuInSe2-Based Solar Cell Absorber

“…Band gaps for the cubic Alk 2 Se compounds are close to the band gaps for AlkInSe 2 . If present at the absorber surface, while probably harmless for the optical properties, the AM compounds can have significant effects on the electronic characteristics of the device by introducing barriers that impede charge carrier flow or by modifying the band alignment . On the other hand, they may be beneficial in passivating the CuInSe 2 surface or grain boundaries to prevent Cd in-diffusion or Cu out-diffusion.…”

Deep surface Cu depletion induced by K in high‐efficiency Cu(In,Ga)Se2 solar cell absorbers

“…Referring to theoretical calculations, both surface Cu vacancies and alkali impurities are expected to have a large and favorable influence on the conduction band offset and the valence band offset between the CIGS absorber and the CdS buffer layer. 23 The stronger band bending would repel holes and shift the pn-junction deeper into the CIGS absorber, lowering the recombination rate of the charge carriers at the CIGS/buffer interface. 43 In our experiments, we can correlate the highest solar cell efficiencies with the combination of the K-rich glass and the addition of PDT (with a thin CdS layer).…”

“…Such a Cu-depletion would increase the local band bending and thus create a higher potential barrier for holes to recombine at the interface. 2,23 It is however not clear yet if these different explanations about the passivation of the CIGS/buffer layer interface depend on the alkaline incorporation strategy. 24,25 Our study has been designed to measure the consequences of the different strategies to add alkalis on the local chemical composition of the absorber.…”

Binding and energetics of oxygen at the CuInSe2 chalcopyrite and the CuInSe2/CdS interface

“…CIS/CdS interfaces as well as interfaces of CdS with other chalcopyrite compounds were studied in the past using supercell approaches and employing hybrid-functional calculations [19,[34][35][36]. In these studies the interfacial energies and the band offsets between CdS and the chalcopyrites were determined, as well as the effect of Na and K impurities to the magnitude of the offsets for certain cases [35].…”

“…CIS/CdS interfaces as well as interfaces of CdS with other chalcopyrite compounds were studied in the past using supercell approaches and employing hybrid-functional calculations [19,[34][35][36]. In these studies the interfacial energies and the band offsets between CdS and the chalcopyrites were determined, as well as the effect of Na and K impurities to the magnitude of the offsets for certain cases [35]. In the present study the pristine CIS/CdS interfaces were also created geometrically by means of a periodic supercell approach, with the lattices of the constituent materials oriented and joined initially along their mutual nonpolar (110) planes.…”