Surface thermal stability of iron pyrite nanocrystals: Role of capping ligands

a r t i c l e i n f o Keywords: Iron pyrite FeS 2 nanocrystals Thin films Ligand exchange Surface stability Photovoltaic Iron pyrite (FeS 2) is a promising photovoltaic absorber material with a high natural abundance and low cost, but surface defects and low photoresponse inhibit sunlight energy conversion. The surface stability of pyrite FeS 2 nanocrystals synthesized in oleylamine (OLA) with trioctylphosphine oxide (TOPO) as an additional capping ligand was investigated using Fourier transform infrared spectroscopy, Raman spectroscopy and X-ray diffrac-tion. Tunable laser exposure during Raman spectroscopy measurement was developed for convenient and systematic evaluation of the stability of FeS 2 nanocrystals. The surface stability of 100–200 nm diameter cubic nanocrystals with long-chain (OLA, TOPO) or small-molecule (pyridine) capping ligands was evaluated after high-intensity laser exposure as well as after thermal annealing in air and N 2. While increasing surface coverage with OLA and TOPO capping ligands provided additional protection against oxidation, FeS 2 nanocrystals capped with pyridine showed good stability at temperatures up to 200 °C in air and 400 °C in N 2. These results provide greater understanding of the processing of nanocrystal-based iron pyrite thin films for photovoltaic applications. Due to its high natural abundance, nontoxicity, low cost and the high absorption coefficient of 1–5 × 10 5 cm −1 [1,2], an attractive application of iron pyrite (FeS 2) is as a photovoltaic (PV) absorber material. The estimated highest attainable efficiency of pyrite PV energy conversion is as high as that of single crystal silicon solar cells [3]. This prospectus is further promoted with the recent development of highly crystalline, phase-pure iron pyrite nanocrystal (NC) inks that enable large through-put and low cost fabrication of solar panels using established solution-based techniques [4–11]. Despite the huge promise iron pyrite holds, this material has not yet been developed into efficient PV devices. The highest pyrite PV device efficiency has been obtained in a photoelectrochemical cell with a reported short circuit current (I sc) of 42 mA/cm 2 , open circuit voltage (V oc) of 187 mV, and fill factor (FF) of 50%, yielding an efficiency of 2.8% [2,12]. The limiting factor for high efficiency is the high dark current that leads to the very small V oc , less than 20% of its bandgap (0.95 eV) [2,12–16]. Natural bulk pyrite and synthetic thin films generally have common traits of high carrier concentrations in excess of …