The effect of nanocrystal reaction time on Cu2ZnSn(S,Se)4 solar cells from nanocrystal inks

• CZTSSe solar cells were made from nanocrystal inks with different reaction times.
• We find improved device efficiency with increasing reaction time.
• Longer reaction times have lower defect densities in the selenized devices.
• The crystalline domain size of selenized films is proportional to the starting NC size.

The sintering of nanocrystals to form a bulk film is a promising route for the production of low cost Cu2ZnSn(S,Se)4 solar cells. However, very little is known about how the synthesis of the nanocrystals impacts the properties of the sintered film or the performance of the resulting photovoltaic device. Here, we present a study in which devices were made from NC inks with different reaction times. Variation in reaction time produces inks with different average size, composition, and compositional heterogeneity. Accounting for the influence of composition, we find that longer NC reaction times result in improved current collection, larger quasi-Fermi level splitting, lower defect concentrations, and higher power conversion efficiency in the selenized devices. The improvements correlate well with the breadth of the x-ray diffraction peaks, suggesting that the longer reaction times result in larger crystalline domain sizes, and that grain boundaries may act as non-radiative recombination sites. Further, the crystalline domain size in the sintered film is proportional to the mean NC size in the ink, suggesting that some interparticle sintering occurs but that the ultimate grain size in the film is dependent on the starting NC size.