Charge carrier dynamics in organic-inorganic triiodide and mixed halide perovskites: planar versus mesostructured films

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

Oral, Eline M. Hutter, presentation 077
Publication date: 5th February 2015

The efficiencies of solar cells with organic-inorganic metal halide perovskites (OMHPs) as light absorber material have rapidly increased in the past years to over 20%. Different parameters, e.g. the morphology, structure and composition, have been varied in these OMHPs to optimize their photovoltaic performance. However, a solid understanding of the relationship between these parameters and the photophysical properties is still lacking. In this work, we study the kinetics of charge carriers in films of OMHPs of variable compositions and structures. By using the electrodeless time-resolved microwave conductivity technique (TRMC), the charge carrier mobilities and lifetimes are obtained from the time-dependent change in photoconductance. Planar thin films of CH₃NH₃PbI₃ (pIPer) and CH₃NH₃PbI_3-xCl_x (pClPer) arespin-coated on glass from precursor solutions. In addition, mesoporous alumina scaffolds are infiltrated with perovskite yielding mesostructured films of CH₃NH₃PbI₃ (mIPer) and CH₃NH₃PbI_3-xCl_x (mClPer). The scaffold limits the crystal domain sizes to a few tens of nanometers, whereas the crystal domains can be micrometer-sized in pClPer. For lightintensities ranging from 10⁹ to 10¹² photons/cm², we find maximum effective mobilities of around 25 cm²/Vs for pIPer as well as pClPer. In both mesostructured perovskites, however, these values do not exceed 15 cm²/Vs. The TRMC traces of pClPerrecorded at different light intensities show that charge carriers undergo (radiative) second order electron hole recombination, which results in charge carrier lifetimes of many microseconds at incident light intensities of 10⁹ photons/cm². In contrast, for both mesostructured samples and pIPer the intensity-dependent TRMC traces are characterized by first order kinetics and show half lifetimes close to 100 ns. We interpret these observations with the model recently proposed by Stranks et al.¹, in which photo-excited conduction band electrons are rapidly quenched via intra-band gap states. For the pClPer sample, however, these non-radiative processes are much slower and hence the majority of the conduction band electrons undergo second order recombination to the groundstate under solar operating conditions. We believe that our results contribute to a better understanding of the recombination pathways of photo-excited electrons in OMHPs with different compositions and structures. Our results also suggest that for planar perovskite solar cells, the perovskites prepared by spin-coating from the mixed chloride/iodide precursors are a better choice than their triiodide counterparts in view of the higher charge carrier mobilities and longer lifetimes.
Figure 1: Time-resolved photoconductance traces of spin-coated films of MAPbI3-xClx (planar: red, in alumina mesostructure: green) and MAPbI3 (planar: orange, in alumina mesostructure: blue) recorded at 5 nJ/cm2 (λ = 600 nm, I0=1.5E10 photons/cm2), corresponding to a charge carrier density in the order of 10E14 cm-3.
(1) Stranks, S. D.; Burlakov, V. M.; Leijtens, T.; Ball, J. M.; Goriely, A.; Snaith, H. J. Phys. Rev. Appl. 2014, 2, 034007.

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