Guanidium-assisted crystallization engineering for highly efficient CsPbI3 solar cells

Iodine vacancies and uncoordinated iodide ions of CsPbI3 films are mainly responsible for nonradiative recombination. Here, we report a composition-engineering passivation method that through guanidium (GA(+)) and I- forms strong hydrogen bonds to passivate iodine vacancies and reduce defects. Both experimental and theoretical results confirmed strong chemical interactions between GA(+) and uncoordinated I- in the GA(x)Cs(1-x)PbI(3) bulk or at the grain boundary. Moreover, GA(+) doping could slow down the crystallization speed of perovskite films during the deposition process. As a result, we observed GA(+) modified films with much lower defect density, larger grain size, and better carrier extraction and transportation. Upon GA(+) passivation, the power conversion efficiency (PCE) is boosted from 18.01% to 19.05%, with open-circuit voltage (V-OC) enhancement from 1.08 V to 1.14 V.