Publication date: 1st April 2013
Recently, a new type of solid-state mesoscopic heterojunction solar cell has been reported, employing lead halide perovskites as the light absorbing material. This system, when deposited onto mesoporous TiO2 , was shown to provide remarkable solar conversion efficiencies, up to 9%, in conjunction with the well-known spiro-OMeTAD acting as the hole transport material.1
Remarkably, Lee et al. found that replacement of the mesoporous n-type TiO2 with insulating Al2O3 could provide excellent power conversion efficiency, up to 10.9%.2 The aluminum oxide having a large bandgap, electron injection from the perovskite to the metal oxide semiconductor is not energetically allowed, and the Al2O3 purely acts like a mesoporous scaffold for the perovskite/spiro-OMeTAD system.
The mechanistic processes in the device using mesoporous TiO2 are still under debate, as these hybrid organic inorganic lead halide perovskite cumulate both functions of light absorption and n-type conduction.
This work reports an optical spectroscopy study of solid-state methyl ammonium lead iodide (CH3NH3)PbI3 based cells. Femtosecond Transient Absorption Spectroscopy and Terahertz Spectroscopy have been applied to both TiO2 and Al2O3-based cells in order to unravel the processes of charge separation and recombination following light absorption in the device. The main investigations efforts were aimed at evidencing electron injection from the perovskite material to the TiO2 scaffold. This pathway appeared to be closely related to the preparation of the perovskite layer, as the morphology of such a layer can have a major effect on the contacts at the heterojunction. Hole injection into the spiro-OMeTAD after charge separation, a feature that can be clearly observed in the steady-state regime through Photo-Induced Absorption Spectroscopy measurements, was also investigated on short timescales.
1. Kim, H.-S. et al. Lead Iodide Perovskite Sensitized All-Solid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%. Sci. Rep. 2012, 2, 591, DOI:10.1038/srep00591. 2. Lee, M. M., Teuscher, J., Miyasaka, T., Murakami, T. N. & Snaith, H. J. Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites. Science 2012, 338, 643–647.
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