Thermally Activated Second-Order Recombination Hints toward

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Thermally Activated Second-order Recombination Hints Toward Indirect Recombination in Fully Inorganic CsPbI3 Perovskites Eline M. Hutter, and Tom J. Savenije ACS Energy Lett., Just Accepted Manuscript • Publication Date (Web): 18 Jul 2018 Downloaded from http://pubs.acs.org on July 18, 2018

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ACS Energy Letters

Thermally Activated Second-Order Recombination Hints Toward Indirect Recombination in Fully Inorganic CsPbI3 Perovskites Eline M. Hutter* and Tom J. Savenije*

Department of Chemical Engineering, Delft University of Technology, van der Maasweg 9, 2629 HZ Delft, the Netherlands

*e-mail: [email protected] *e-mail: [email protected]

keywords: perovskite, CsPbI3, lifetime, mobility, vapour deposition, TRMC

Abstract The relationship between the dipole moment of the methylammonium cation and the optoelectronic properties of lead halide perovskites remains debated. In this work, we show that both the temperature-dependent charge carrier mobility and recombination kinetics are identical for methylammonium- and cesium lead iodide, indicating that the role of the monovalent cation is subordinate to the lead iodide framework. From the observation that for both perovskites the electronhole recombination is thermally activated, we speculate that the bandgap is slightly indirect.

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Application of lead halide perovskites with the general structure ABX3 in solar cell devices yields efficiencies close to 23%.1 This is attributed to the combination of high absorption coefficients, sufficiently high charge carrier mobilities and long lifetimes, enabling efficient collection of excess charges. Previous theoretical work on methylammonium lead iodide or bromide (MAPbX3) has shown that these long lifetimes could be related to the presence of the Rashba effect. Strong spin-orbit coupling in combination with locally polarized domains, induced by collective orientation of the methylammonium dipoles, leads to a splitting of the conduction band (CB) in k-space, resulting in a slightly indirect bandgap.2,3 Importantly, optical excitation of charges still predominantly occurs via direct transitions, while recombination involves an indirect transition.4 Using a variety of experimental tools, groups claim to have observed this splitting in methylammonium-based perovskites.5,6 Previously, we have shown that the second-order band-to-band recombination in methylammonium lead iodide (MAPbI3) is thermally activated, which is in line with this indirect nature of the bandgap.7 However, it has remained debated whether these effects are related to the dipole moment of the organic cation. That is, recent theoretical work has suggested that the role of the monovalent cation is actually subordinate to the dynamic distortion of the octahedral inorganic cage, which breaks the centro-symmetry.8 The resulting dynamic local electric fields interact with the Pb electrons and yield a splitting of the CB. In this work, we investigate the effect of the monovalent cation on the temperature-dependence of the second-order recombination. To this end, we have recorded the time-resolved microwave conductance at different temperatures, making a direct comparison between MAPbI3 and cesium lead iodide (CsPbI3), in which the monovalent cation is symmetric. Interestingly, we observe that both the temperature-dependent mobility and recombination kinetics in CsPbI3 follow the same trend as in MAPbI3. Importantly, in both cases, the second-order recombination of free charges is thermally activated, consistent with an indirect recombination pathway.7

The CsPbI3 films are prepared using physical vapour deposition (PVD), yielding samples with superior charge carrier lifetimes and photovoltaic performance over spin-coated CsPbI3.9,10 The films were heated to 300 °C and rapidly cooled to room temperature to obtain metastable orthorhombic black-phase CsPbI3.11 Using the Time-Resolved Microwave Conductivity (TRMC) technique, we have observed that the room-temperature mobility and second-order recombination rate constants in these vapour-deposited CsPbI3 films are similar to fully optimized MAPbI3 layers,10 which enables us to compare the opto-electronic properties of these materials as function of temperature. Figure 1 shows intensity-normalized TRMC traces recorded at 293 K, 260 K and 220 K, for laser intensities yielding charge carrier densities in the regime between 1015 and 1017 cm-3. From the maximum signal height, we deduce that the room temperature mobility is around 25 cm2/(Vs), consistent with previous work.10 Interestingly, on decreasing the temperature, the mobility in CsPbI3 gradually increases, following a similar temperature-dependent trend as observed for MAPbI3 (see


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ACS Energy Letters

Figure 2a) given by ߤ ∝ ܶ ିଵ.ହ.7,12 This shows that the charge carrier mobility in lead iodide perovskites is mainly limited by phonon scattering,7 independent of the monovalent cation. Remarkably, not only the mobility but also the lifetime of mobile charges follows the same temperature-dependence as that of MAPbI3. That is, for a range of excitation densities in between 5 x 1015 and 1017 cm-3 the lifetime is enhanced on lowering the temperature. Since this corresponds to a regime in which the lifetime decreases with increasing initial charge carrier densities, we can conclude that higher order recombination is retarded with decreasing temperatures. Below 5 x 1015 cm-3, the lifetime is not dependent on the excitation density, indicating a first-order process such as trap-assisted recombination. To quantify this, we fitted the experimental data at 220 K < T < 300 K using our previously described kinetic model (see also Figure S2),13 as shown by the dashed lines in Figure 1. The kinetic parameters are listed in Table S1. We note that at and below 180 K, we could not obtain satisfactorily fits for CsPbI3, which might be due to processes unaccounted for in our model, such as formation of excitons or third order (Auger) recombination.



Figure 1: Time-Resolved Microwave Conductivity traces for vapour-deposited black-phase CsPbI3, recorded at 293 K (a), 260 K (b) and 220 K (c) for initial charge carrier densities of 5 x 1014 to 1017 cm-3. The modelled traces are added as black dashed lines.

In Figure 2b, the second-order recombination rate (k2) is plotted for CsPbI3 together with values previously found for MAPbI3.7 Importantly, although the absolute values are different, in both cases the k2 at 220 K is less than half its room temperature value. This is in contrast with the temperaturedependent trend in recombination rate typically found in direct semiconductors and previously reported for MAPbI3 at high fluence (>1017 cm-3),12,14,15 which is several orders of magnitude higher than the charge densities in the present work. Although these previous reports14,15 suggest that direct recombination dominates at higher charge densities, our results show that in both CsPbI3 and MAPbI3, second-order recombination is actually a thermally activated process for charge densities ranging from 1015 to 1017 cm-3. Previously, we hypothesized that this was due to the conduction band minimum being slightly shifted in k-space from the valence band maximum,7 resulting in an indirect bandgap from which recombination is momentum-forbidden. In addition, the high absorption coefficients10 indicate the presence of a direct transition. The origin of the thermally enhanced recombination rates may therefore be two-fold: (1) thermal energy releases electrons from the CBM to a state from which direct recombination is possible and (2) the electrons decay from the CBM to the VBM via indirect recombination on interacting with a phonon, see inset in Figure 2b. The present observations suggest that the indirect recombination pathway of conduction band electrons is not unique to MA-based perovskites and hence, not related to (collective) orientations of methylammonium dipoles.

Figure 2: Effective mobility ϕ∑µ (a) and k2 (b) as function of temperature for CsPbI3 (open circles, left axis) and MAPbI3 (closed circles, right axis, data from Ref. 7). The dotted lines between the open circles are added to guide the eye.

We note that the increased lifetime observed at lowering the temperature is not related to reabsorption of emitted photons. That is, the external photoluminescence quantum efficiency (PLQE) in CsPbI3 is


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ACS Energy Letters

far below 1%.10 Even in the case of very poor output coupling and a ten-fold increase in PLQE on lowering the temperature (see Figure S1), still more than 95% of the second-order recombination is non-radiative and therefore cannot be reabsorbed. Therefore, photon reabsorption can be excluded as the origin of the charge carrier lifetime elongation with lower temperatures. Additional scenarios to explain the thermally enhanced recombination include charge immobilization into shallow traps or the formation of large polarons. However, since both of these do not satisfactorily explain the experimental temperature-dependent mobility following ܶ ିଵ.ହ ,7,16 these are in our opinion unlikely to dominate the charge carrier recombination properties. Instead, the combination of increasing mobility and retarded second-order recombination on decreasing the temperature suggests an indirect recombination pathway for mobile charges. The low PLQE indicates that the second-order recombination between mobile charges is mainly non-radiative, which could mean that indirect recombination does not result in the emission of a photon. Most importantly, our present results show that both the temperature-dependent mobility and recombination mechanism are fully dominated by the lead iodide framework instead of the dipole moment of the organic cation.

Acknowledgments Sanjana Chandrashekar is acknowledged for sample preparation.

Supporting Information Temperature-dependent absorption and photoluminescence, details about the kinetic model and fitting parameters and experimental methods are shown in the Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI:

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Thermally Activated Second-Order Recombination Hints toward

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