Impact of MAPbBr3 Additives on Crystallinity and Charge Carrier Dynamics of Mixed Perovskite Films

Organic-metal halide perovskite is one of the most promising candidates for the next generation solar cell material owing to its superior photo-electric properties and relatively simple fabrication process. In particular, FAPbI3 turned out to be very suitable for efficient solar cell due to its low band gap energy (~1.5 eV). However, FAPbI3 shows the crystal phase instability between unfavourable yellow trigonal δ-phase and perovskite-type black cubic α-phase at room temperature. To stabilize it, various additives such as MACl, Cs and MAPbBr3 have been proposed and studied. Recently, there was a report that 0.8 mol% of MAPbBr3 additive stabilizes the crystal phase of FAPbI3, and subsequently, a photo conversion efficieny (PCE) of 25.2% was achieved [1]. However, the influence of MAPbBr3 additives on charge carrier dynamics and its correlation with crystallinity is still unrevealed. In the present work, (FAPbI3)1-x(MAPbBr3)x films with different MAPbBr3 contents of x = 0, 0.5, 0.8, 1, 2.5, 10 and 20 mol% are fabricated on quartz substrates using spin coating method. From the measured linear absorption spectra of the mixed perovskite films, we determine the composition-dependent Urbach energy which quantitatively represents the crystallinity (Fig 1.a). This result suggests that two opposite impacts of MAPbBr3 additive, which are the crystal phase stabilization and the structural disordering, compete each other and the crystallinity is enhanced around 0.8 mol% content. More homogeneous 2D PL mapping image and disappearance of vibration peaks correspond to δ-phase in the Raman spectrum at 0.8 mol% content support our suggestion.