Intrinsic point defect tolerance in selenium for indoor and tandem photovoltaics†

Abstract

Selenium has reemerged as a promising absorber material for tandem and indoor photovoltaic (PV) devices due to its elemental simplicity, unique structural features, and wide band gap. However, despite rapid recent improvements, record Se solar cells only reach a third of their achievable efficiencies at the radiative limit, primarily due to a low open-circuit voltage relative to the band gap. The origins of this voltage deficit, along with the high doping densities often reported for trigonal selenium (t-Se), remain unclear. Here, we explore the point defect chemistry of t-Se combining first-principles calculations with experimental studies of thin-films from state-of-the-art PV devices. Our findings reveal a remarkable ability of the helical t-Se chains to reconstruct and form low-energy amphoteric defects, particularly in the case of self-vacancies and hydrogen, pnictogen, and halogen impurities. While chalcogen impurities and self-interstitials also form low-energy defects, these are electrically neutral. We also find that both intrinsic and extrinsic point defects do not contribute significantly to doping, either due to electrical inactivity (chalcogens) or self-compensation (hydrogen, halogens, pnictogens). Finally, we show that intrinsic point defects do not form detrimental non-radiative recombination centres and propose that PV performance is instead limited by other factors. These findings highlight the potential of Se as a defect-tolerant absorber, while optimising interfaces and extended structural imperfections is key to unlocking its full performance potential.