Excitons and free carriers in organometal perovskites

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

Oral, Michele Cadelano, presentation 129
Publication date: 5th February 2015

Organometal trihalide perovskites are a novel class of hybrid solution-processed materials that show features typical both of organic and inorganic semiconductors. Due to their nature, it is not clear whether their optoelectronic properties are mediated by bound electron-hole states (i.e. excitons) as in organic materials, or by free carriers as in low-gap inorganic semiconductors. Here, we presentoptical measurements with the aim to provide a deeper understanding of the photophysics of methylammonium lead trihalide perovskites. We show that the linear absorption coefficient for photon energies just above the absorption edge can be accurately described in the framework of the Elliot’s theory of Wannier excitons, with a band-to-band energy gap of 1.647 eV and an exciton binding energy of 25 meV. These results indicate that most of primary excitations generated by solar light absorption are free carriers. Excitons can be in fact generated only in a narrow band close the exciton resonance, as it happens in low-gap inorganic semiconductors. A charge-neutral exciton gas could be formed at later times, following thermalization. However, studying the emission properties by ultrafast photoluminescence spectroscopy at room temperature, we find that optical pumping always leads to a thermalized population of free carriers, with a negligible exciton density for all optical intensities of interest, i.e. from light intensities much smaller than those typical of solar illumination to those typical to obtain light amplification. Evidence of trap states, common in all solution-processed semiconductors, is found from steady-state photoluminescence spectroscopy^[1]. Nevertheless, the cross-section for carrier capture by traps is found to be low, justifying the achievement of light amplification at relatively low pumping rates^[2] and the observation of efficient charge extraction in photovoltaic devices^[3].
Excitons and free carriers at 300K in a MAPbI3 perovskite film. (a) Absorption spectrum. A model that takes into account both the excitonic and band-to-band transitions fits the experimental data with a satisfactory agreement. (b) Photoluminescence intensity estimated at time t=0 (PL0) following sub-picosecond pulsed excitation, as a function of the injected carrier density. PL0 scales quadratically with the injected carrier density up to 10^18 cm^(-3), as expected for bimolecular recombination from an electron-hole plasma.
[1] Saba, M. et al. Correlated electron–hole plasma in organometal perovskites. Nat. Commun. 2014, 5:5049, doi: 10.1038/ncomms6049. [2] Xing, G. et al. Low-temperature solution-processed wavelength-tunable perovskites for lasing. Nat. Mater. 2014, 13, 476-480. [3] Wehrenfennig, C., Eperon, G. E., Johnston, M. B., Snaith, H. J. & Herz, L. M. High charge carrier mobilities and lifetimes in organolead trihalide perovskites. Adv. Mater. 2014, 26, 1584-1589.

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