Incorporated guanidinium expands the CH₃NH₃PbI₃ lattice and enhances photovoltaic performance

Guanidinium (GA) has been widely used as an additive in solar cells for enhanced performance. However, the size of the guanidinium cation is too large to be incorporated in the cage of the perovskite structure. Instead, GA forms a variety of structures with lead iodide, where its role in the perovskite crystal as well as solar cell devices is unclear. In this study, we demonstrate that GA can be incorporated into the structure of MAPbI₃ as (GA)_x(MA)₁−_xPbI₃. From single-crystal X-ray crystallographic refinement, we observe lattice expansion and Pb−I bond elongation with GA incorporation similar to exerting “negative pressure”, which weakens orbital overlap and widens the band gap from 1.49 to 1.53 eV. We find that the highest percentage of GA that can be incorporated into the 3D MAPbI₃ structure is 5.26%, as confirmed by nuclear magnetic resonance. The alloyed (GA)_x(MA)₁−_xPbI₃ exhibits increased PL lifetimes from 154.4 to 266.3 ns after GA incorporation while the V_oc of (GA)_x(MA)₁−_xPbI₃ devices enlarges from 1.05 to 1.11 V. High efficiencies in solar cell devices up to 20.38% with a J_sc of 23.55 mA cm⁻², V_oc of 1.11 V, and FF of 0.78 have been achieved, with stable photovoltaic performance for 900 h in air.