Photogenerated polarons in organo lead halide perovskites

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Oral, Giampiero Ruani, presentation 140
Publication date: 5th February 2015

The high photovoltaic energy conversion efficiency, exceeding 15%^{1, 2}, achieved by using organo lead halide perovskites (CH₃NH₃PbI_3-xCl_x; x≥0) as active layer in solid state dye-like solar cells have attracted an increasing interest on these materials both in improving device performances and in understanding the reasons of their behaviour. The large diffusion length for both electrons and holes observed in these materials³ are probably the base in understanding the high energy conversion efficiency of photovoltaic devices based on these materials. With this in mind it is important to understand which kind of interactions are ruling the transport of the charged carriers in organo lead halide perovskites. The vibrational response of these systems instead can provide very useful information on their dynamics⁴. Moreover, the variation of this response when carriers are photogenerated (IR photoinduced absorption - PIA) can also provide information on possible interactions of mobile carriers with the lattice⁵. Here we show that even at room temperature steady state PIA (exciting at 488 nm) is showing the bleachings of IR modes and the appearance of a new one in the Far-IR indicating that photogenerated carriers are long lived and locally induce a deformation of the lattice changing the symmetry and relaxing the infrared absorption selection rules. This result suggests a strong electron phonon interaction in organo lead halide perovskites; this finding is consistent with the observation of polarons signature in these materials by dielectric relaxation⁶ and time resolve photoluminescence measurements⁷.
IR photoinduced absorption in CH3NH3PbI3 exciting at 488nm.
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