Proceedings of 6th International Conference on Hybrid and Organic Photovoltaics (HOPV14)
Publication date: 1st March 2014
Hybrid organic-inorganic lead halide perovskite semiconductors have recently been successfully applied as light absorbers in novel solid-state solar cells displaying a power conversion efficiency of 16% and an unprecedented open circuit voltage of 1.1 Volt. The detailed working mechanism of such photovoltaic system has remained unclear, as cells based on nanocrystalline titanium dioxide framework and devices deprived of electron acceptor material were both shown to be effective. We investigated the dynamics of photoinduced charge transfer processes at the surface of the perovskite semiconductor by time-resolved techniques. Ultrafast transient absorption spectroscopy and time-resolved terahertz photoconductivity measurements provided important insights into the charge transfer processes taking place in TiO2 and Al2O3 mesoporous films impregnated with CH3NH3PbI3 perovskite and the organic hole-transporting material (HTM) spiro-MeOTAD.
Transient absorption of photogenerated charge carriers in the perovskite was monitored in the near infrared. Results showed that the decay of the carrier population due to recombination is markedly slowed down upon infiltration of the hole-transporting material, which is consistent with primary hole-injection from the photoexcited perovskite into the HTM. Evidence for ultrafast electron injection from CH3NH3PbI3 into the TiO2 film was found as well. In photovoltaic systems based on a TiO2 | perovskite | HTM architecture, we showed then that primary charge separation occurs at both junctions with the electron-transporting oxide and the HTM simultaneously, with ultrafast electron- and hole-injection taking place from the photoexcited light-absorbing semiconductor within similar timescales.
Time-resolved THz experiments support results obtained by transient NIR absorption spectroscopy. The photoconductivity of the active layer arises almost exclusively from carriers in the perovskite material. The effect of the excitation wavelength upon the transient THz absorption amplitude and spectrum allowed evidencing the formation of excitons, polarons and bipolarons during the first picoseconds following pulsed excitation of the perovskite, on a time-scale slower than interfacial charge transfer. We thus conclude that the exceptionally large carrier diffusion lengths in the semiconductor (100 nm-1µm) allow photogenerated electrons and holes to reach the selective contacts separately (TiO2 and the HTM, respectively), where they are readily injected, before they could associate in the form of excitons and eventually recombine. This unique charge separation mechanism unraveled here by use of ultrafast laser spectroscopy makes perovskite hybrid solar cells a new type of photovoltaic converter of its own and a new realm of scientific investigation and technological development.
[1] Kim, H.-S.; Marchioro, A. 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] Marchioro, A. et al. Photoinduced Processes in Lead Iodide Perovskite Solid-State Solar Cells. Proc. SPIE 2013, 8811, 881108. [3] Marchioro, A. et al. Unravelling the Mechanism of Interfacial Photoinduced Charge Transfer in Lead Iodide Perovskite Solar Cells. Nat. Phot. 2014, 8, DOI:10.138/nphoton.2013.374.
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