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 MAPbI(3) perovskite by cesium or guanidinium (Gua) cations to fabricate thin films of triple cation Cs(x)Gua(y)MA(1-x-y)PbI(3) 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)PbI(3) perovskites compared to the MAPbI(3) material. The PSCs fabricated with the different mixed cation Cs(x)Gua(y)MA(1-x-y)PbI(3) 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 MAPbI(3) to a value of 20.0% for the Cs(0.05)Gua(0.05)MA(0.90)PbI(3) 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.