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 (FeS
₂
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 FeS
₂
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 (E
_u
) estimation, we found that the FeS
₂
NCs synthesized via HT method had a higher E
_u
= 2.20 meV as compared to the HI method (E
_u
= 0.77meV), which indicates more surface defect states or sulfur vacancies (V
_s
) in the hydrothermally grown FeS
₂
NCs. Similar variations in the charge carrier decay lifetime were also observed from their time resolved photoluminescence spectra. Hydrothermally synthesized FeS
₂
NCs showed triply exponential decay function, where as FeS
₂
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 FeS
₂
NCs synthesized via HI method (2.3 × 10
¹⁷
cm
⁻³
) as compared to the HT method (1.8 × 10
¹⁶
cm
⁻³
), where as both showed p-type conductivity. Effects of defects on photovoltaic solar cell characteristics of the synthesized FeS
₂
NCs as a photo-absorber material were investigated by fabricating hybrid devices with P3HT:PC
₇₁
BM polymer blend. Devices doped with HI grown FeS
₂
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 FeS
₂
NCs synthesized through different routes, which is crucial to understand the performance limitations of the material in question.