Hydrogen-enhanced carrier collection enabling wide-bandgap Cd-free Cu2ZnSnS4 solar cells with 11.4% certified efficiency

Wide-bandgap kesterite Cu2ZnSnS4 offers an economically viable, sustainably sourced and environmentally friendly material for both single-junction and tandem photovoltaic applications. Nevertheless, since 2018 the record efficiency of such solar cells has stagnated at 11%, largely due to carriers recombining before they are collected. Here we demonstrate enhanced carrier collection in devices annealed in a hydrogen-containing atmosphere. We find that hydrogen is incorporated mainly in n-type layers and on the absorber surface. Furthermore, we show that the hydrogen treatment triggers the out-diffusion of oxygen and sodium from the absorber bulk to the surface, favourably diminishing the acceptor concentration at the surface and increasing the p-type doping in the bulk. Consequently, Fermi-level pinning is relieved and carrier transport in the absorber is facilitated. We achieve a certified efficiency of 11.4% in Cd-free devices. Although hydrogenation already plays a major role in silicon photovoltaics, our findings can further advance its application in emerging photovoltaic technologies. The power conversion efficiency of wide-bandgap pure-sulfide kesterite is low. Wang et al. propose annealing in a hydrogen-containing atmosphere and report the underlying mechanism that enables a certified efficiency of 11.4%.