High resolution characterization of grain boundaries in Cu2ZnSnSe4 solar cells synthesized by nanoparticle selenization

We report a novel fabrication and multiple high resolution characterizations of Cu2ZnSnSe4 solar cells. The fabrication is based on nanoparticle precursor production by liquid-phase pulsed laser ablation (LP-PLA), electrophoretic deposition of precursor thin film under ambient condition, and selenization. Columnar grain boundaries are studied using spatially mapped Raman spectroscopy and scanning probe microscopy for their compositional and electrical properties. We observe high electrical conductivity near columnar grain boundaries, and propose poor Cu composition as the cause of the enhancement of the collection of minority carriers. We also study horizontal grain boundaries using cross-sectional scanning transmission electron microscopy (STEM). Combining with the device I-V and quantum efficiency, we suggest that the horizontal grain boundaries act as barriers to the transportation of minority carriers. CZTSe cells with efficiencies of 4.77% and 2.20% are compared.