Influence of Defects and Dopants on the Photovoltaic Performance of Bi₂S₃

Bi₂S₃ attracted intensive attention recently as light-absorber, sensitizer or electron acceptor materials in various solar cells. Using first-principles calculations, we find that the photovoltaic efficiency of Bi₂S₃ solar cells is limited by its intrinsic point defects, i.e., both S vacancy and S interstitial can have high concentration and produce deep defect levels in the bandgap. Unexpectedly most of the intrinsic defects in Bi₂S₃, including even the S interstitial, act as donor defects, explaining the observed n-type conductivity and also causing the high p-type conductivity impossible thermodynamically. The doping in Bi₂S₃ by a series of extrinsic elements is studied, showing that most of dopant elements such as Cu, Br and Cl make the material even more n-type and only the Pb doping makes it weakly p-type. Based on this, we propose that the surface region of n-type Bi₂S₃ nanocrystals in p-PbS/n-Bi₂S₃ nano-heterojunction solar cells may be type-inverted into p-type due to the Pb doping, with a buried p-n junction formed in the Bi₂S₃ nanocrystals, which provides a new explanation to the observed longer carrier lifetime and higher efficiency.