Ion exchange-prepared NaSbSe2 nanocrystals: electronic structure and photovoltaic properties of a new solar absorber material
Abstract
We report the calculated electronic structure, syntheses and photovoltaic properties of a new ternary solar absorber material NaSbSe2. NaSbSe2 nanocrystals (NCs) have been prepared from a Na–Sb–S precursor by the solution-based Se2− anion exchange reaction. The Na–Sb–S precursor was grown on a TiO2 electrode using the successive ionic layer adsorption and reaction (SILAR) method. X-ray diffraction shows that the synthesized NaSbSe2 NCs have the same crystal structure as the NaSbS2 precursor with the diffraction angles significantly down-shifted. Energy-dispersive X-ray spectroscopy confirms the complete anion exchange and formation of the NaSbSe2 phase. First principles calculations show that the ordered NaSbSe2 structure resulting from the ion exchange synthesis is important for the performance. The NaSbSe2 NCs have an average size of ∼17 nm and a near-optimal optical band gap Eg of 1.48 eV that is lower than the NaSbS2 precursor. Liquid-junction NaSbSe2 quantum dot-sensitized solar cells (QDSSCs) were fabricated from the synthesized NaSbSe2 NCs for the first time. The best cell, prepared using the Au counter electrode and the polysulfide electrolyte, yielded an efficiency of 2.22%, a short current density of 1.31 mA cm−2, an open-circuit voltage of 0.30 V and a fill factor of 56.4% under the reduced light intensity of 10% sun. The external quantum efficiency (EQE) spectrum covers the spectral range of 300–900 nm with a maximum EQE of 75% at λ = 500 nm. The near-optimal Eg suggests that NaSbSe2 could be a potential material for solar cells. In addition, the ion exchange method can be extended to the preparation of many new metal selenide-based solar materials from their corresponding sulfides. These materials may show improved characteristics compared to samples with more disorder.