Variability of Cu2ZnSnS4 nanoparticle hot injection synthesis and modifications by thin film annealing

Abstract

As a quaternary semiconductor with a direct energy bandgap of around 1.4 eV, Cu₂ZnSnS₄ is a promising candidate for absorber layers in next generation thin-film solar PV devices. It has the advantage of being based on low cost earth-abundant elements. Solution based synthesis approaches show the greatest potential for scaling up manufacture. Cu₂ZnSnS₄ devices are currently limited in efficiency because of a large open circuit voltage deficit, arising predominantly from high concentrations of point defects and charge compensation defect complexes. To drive device efficiency robust, reliable and reproducible synthesis protocols are required. We have produced a series of Cu₂ZnSnS₄ thin films by spin coating nanoparticle ink suspensions fabricated under nominally identical conditions to investigate the inherent variability in hot injection synthesis of Cu₂ZnSnS₄ nanoparticles by fabricating 11 batches using the same initial conditions. We use two different chemical routes to extratct nanoparticles from solution after synthesis. We find that the lattice constants of the nanocrystalline material do not change significantly. The relative concentration of the constituent elements varies with S having the largest anion variation of ±3.8% as compared to metal cation variations of Zn ±2.4%, Cu ±1.8%, and Sn ±1.4% with Zn having the largest cation variation. We compare data from energy dispersive X-ray (EDX) and inductively coupled plasma mass spectroscopy (ICPMS) chemical analysis methods and find that the ICPMS analysis has a consistently smaller standard deviation, an average of 0.1 lower, as this technique samples a large volume of material. We observe variation in the kesterite tetragonal lattice constants a and c, and energy bandgap E_g across the different samples, although there is no systematic change in the chemical composition. The average bandgap of as-synthesised films is 1.14 eV. We find that annealing in a sulphur rich environment has no systematic impact on the Cu/(Zn + Sn) cation ratio and leads to a decrease of −0.4 in the Zn/Sn ratio. At higher annealing temperatures, 500–600 °C, the bandgap shows a linear increase of +0.15 eV accompanied by the formation of abnormal grains and an increase in the size of the crystalline scattering domain τ, determined from the X-ray spectra, from 30–100 nm. The most dramatic changes occur in the first 0.5 hours of annealing. These findings will help in the design of fabrication strategies for higher efficiency Cu₂ZnSnS₄ photovoltaic devices.