Vacancy doping and charge transport in Bi2S3 nanoparticle films for photovoltaic applications

Abstract

Native point defect doping via thermal treatment is an easy and promising method to tune the electrical transport properties of semiconductors made for renewable-energy conversion. In this study, we investigate the vacancy doping of the lowly toxic semiconductor $\mathrm{B}{\mathrm{i}}_{2}{\mathrm{S}}_{3}$ using electrical conductivity as well as thermoelectric power measurements. We enhance the electrical conductivity of bismuth sulfide nanoparticle layers by more than four orders of magnitude by a stepwise thermal treatment in a moderate temperature range (300--480 K). Via thermoelectric power measurements we attribute this enhancement to an increase in charge-carrier mobility by two orders of magnitude and to an increase in charge-carrier density by more than two orders of magnitude. We find that the energetic position of the electron-doping sulfur vacancies of bismuth sulfide nanoparticles is significantly shallower than previously reported for bulk material. Subsequently, we implement $\mathrm{B}{\mathrm{i}}_{2}{\mathrm{S}}_{3}$ nanoparticles doped with sulfur vacancies by thermal annealing in photovoltaic devices using P3HT as an electron donor molecule. We find that annealing up to 383 K yields the best compromise between improving charge-carrier transport and increasing defect densities.