Insight into the Role of Guanidinium and Cesium in Triple Cation Lead Halide Perovskites

The overall impact of the partial replacement (5–15%) of methylammonium (MA) in the MAPbI₃ perovskite by cesium or guanidinium (Gua) cations to fabricate thin films of triple cation Cs _x Gua _y MA_1–x–y PbI₃ perovskites is studied. The structural changes are investigated by using X‐ray diffraction measurements revealing shrinkage or expansion of the unit cell upon Cs or Gua incorporation, respectively. The optoelectronic properties are characterized with photoluminescence (PL) time‐resolved spectroscopy and the space charge limited current (SCLC) method. Shorter PL time constants are obtained for the samples with only Cs, while longer PL decays are measured for the perovskites containing additional Gua cation. The SCLC measurements reveal a larger density of trap states in the Cs _x MA_1–x PbI₃ perovskites compared to the MAPbI₃ material. The PSCs fabricated with the different mixed cation Cs _x Gua _y MA_1–x–y PbI₃ perovskites reveal a good correlation with the measured optoelectronic properties. The power conversion efficiency (PCE) improves from an average value of 18.6% for the MAPbI₃ to a value of 20.0% for the Cs_0.05Gua_0.05MA_0.90PbI₃ perovskite with a champion cell delivering 21.2%. On the opposite, the PCE decreases to a value of 17.3% for the double cation perovskite with Cs.