Publication date: 18th December 2016
The opto-electronic properties of hybrid perovskites can be easily tailored by changing their components. Specifically, mixing the common short organic cation (methyl ammonium (MA)) with a larger one (e.g. butyl ammonium (BA)) creates multilayered 2D perovskites: (BA)2(MA)n-1PbnI3n+1. These materials, also known as Ruddlesden-Popper phases,have proven to make highly efficient, solution-processed and stable LEDs (EQE = 8.8%) and photovoltaic solar cells (PCE = 12.5%). We have studied 2D (BA)2(MA)n-1PbnI3n+1 Ruddlesden-Popper hybrid perovskites using two distinct TRMC techniques with different excitation sources: high-energy electron pulse and laser photo-excitation. Our combined experimental results show a clear increase of the mobility, probability of exciton dissociation and lifetime of charges with the thickness of the [(MA)n-1PbnI3n+1]2- slabs. The increase in mobility is consistent with DFT calculations that show a decrease of the effective mass of holes. The larger exciton dissociation yield and longer lifetime of charges are explained by a decrease of the Coulombic interactions and exciton binding energy. We estimated the binding energies of these materials combining the temperature trend of the charge mobility (PR-TRMC) with the photo-conductivity TRMC. The obtained temperature trend of the yield of exciton dissociation was analyzed in the framework of the Saha equation to show that the exciton binding energies range between ~80 meV and ~370 meV depending on the thickness of the [(MA)n-1PbnI3n+1]2- slabs. This finding was confirmed by temperature dependent photo-luminescence measurements that show the presence of bound excitons at low temperature. These results demonstrate that the opto-electronic properties of these 2D materials are highly tunable for specific applications.
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