Insight into the role of guanidinium and cesium in triple cation lead halide perovskites

Abstract

The overall impact of the partial replacement (5–15%) of methylammonium (MA)in the MAPbI3perovskite by cesium or guanidinium (Gua) cations to fabricatethinfilms of triple cation CsxGuayMA1–x–yPbI3perovskites is studied.The structural changes are investigated by using X-ray diffraction measurementsrevealing shrinkage or expansion of the unit cell upon Cs or Gua incorporation,respectively. The optoelectronic properties are characterized with photolumi-nescence (PL) time-resolved spectroscopy and the space charge limited current(SCLC) method. Shorter PL time constants are obtained for the samples with onlyCs, while longer PL decays are measured for the perovskites containing additionalGua cation. The SCLC measurements reveal a larger density of trap states in theCsxMA1–xPbI3perovskites compared to the MAPbI3material. The PSCs fabricatedwith the different mixed cation CsxGuayMA1–x–yPbI3perovskites reveal a goodcorrelation with the measured optoelectronic properties. The power conversionefficiency (PCE) improves from an average value of 18.6% for the MAPbI3to avalue of 20.0% for the Cs0.05Gua0.05MA0.90PbI3perovskite with a champion celldelivering 21.2%. On the opposite, the PCE decreases to a value of 17.3% for thedouble cation perovskite with Cs.