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 E_g 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 E_g 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.