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 Voc 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 Jsc of 23.55 mA cm-2, Voc of 1.11 V, and FF of 0.78 have been achieved, with stable photovoltaic performance for 900 h in air.