Defects in Phase-Pure Iron Pyrite Nanocubes: Implications for Photovoltaics

Here, we report a detailed study on the defect state formation and their effects on various optoelectronic properties of phase pure iron pyrite nanocubes (FeS2 NCs) synthesized through two nonexpensive and low-temperature chemical routes; hot injection (HI) and hydrothermal (HT) methods. The structural analysis confirmed the phase purity of the crystalline FeS2 NCs synthesized from both the methods. Different synthetic routes resulted in a variation in the crystallite size and indirect optical band gap of the NCs. Through Urbach energy (Eu) estimation, we found that the FeS2 NCs synthesized via HT method had a higher Eu = 2.20 meV as compared to the HI method (Eu = 0.77meV), which indicates more surface defect states or sulfur vacancies (Vs) in the hydrothermally grown FeS2 NCs. Similar variations in the charge carrier decay lifetime were also observed from their time resolved photoluminescence spectra. Hydrothermally synthesized FeS2 NCs showed triply exponential decay function, where as FeS2 NCs grown from HI method showed doubly exponential carrier decay, indicating the presence of different electronic states for luminescence. The estimated carrier density is found to be higher for FeS2 NCs synthesized via HI method (2.3 × 1017 cm−3) as compared to the HT method (1.8 × 1016 cm−3), where as both showed p-type conductivity. Effects of defects on photovoltaic solar cell characteristics of the synthesized FeS2 NCs as a photo-absorber material were investigated by fabricating hybrid devices with P3HT:PC71BM polymer blend. Devices doped with HI grown FeS2 NCs exhibited ∼ 50% enhanced power conversion efficiency as compared to the HT grown NCs. This study provides a direct correlation of defect formation and tunable optoelectronic properties of FeS2 NCs synthesized through different routes, which is crucial to understand the performance limitations of the material in question.