10.3% Efficient Green Cd-Free Cu2 ZnSnS4 Solar Cells Enabled by Liquid-Phase Promoted Grain Growth.

Small grain size and near-horizontal grain boundaries are known to be detrimental to the carrier collection efficiency and device performance of pure-sulfide Cu2 ZnSnS4 (CZTS) solar cells. However, forming large grains spanning the absorber layer while maintaining high electronic quality is challenging particularly for pure sulfide CZTS. Herein, a liquid-phase-assisted grain growth (LGG) model that enables the formation of large grains spanning across the CZTS absorber without compromising the electronic quality is demonstrated. By introducing a Ge-alloyed CZTS nanoparticle layer at the bottom of the sputtered precursor, a Cu-rich and Sn-rich liquid phase forms at the high temperature sulfurization stage, which can effectively remove the detrimental near-horizontal grain boundaries and promote grain growth, thus greatly improving the carrier collection efficiency and reducing nonradiative recombination. The remaining liquid phase layer at the rear interface shows a high work function, acting as an effective hole transport layer. The modified morphology greatly increases the short-circuit current density and fill factor, enabling 10.3% efficient green Cd-free CZTS devices. This work unlocks a grain growth mechanism, advancing the morphology control of sulfide-based kesterite solar cells.

Photo-degradation of high efficiency fullerene-free polymer solar cells.

Polymer solar cells are a promising technology for the commercialization of low cost, large scale organic solar cells. With the evolution of high efficiency (>13%) non-fullerene polymer solar cells, the stability of the cells has become a crucial parameter to be considered. Among the several degradation mechanisms of polymer solar cells, burn-in photo-degradation is relatively less studied. Herein, we present the first systematic study of photo-degradation of novel PBDB-T:ITIC fullerene-free polymer solar cells. The thermally treated and as-prepared PBDB-T:ITIC solar cells were exposed to continuous 1 sun illumination for 5 hours. The aged devices exhibited rapid losses in the short-circuit current density and fill factor. The severe short-circuit current and fill factor burn in losses were attributed to trap mediated charge recombination, as evidenced by an increase in Urbach energy for aged devices.