Impact of Postsynthetic Surface Modification on Photoluminescence

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Impact of Postsynthetic Surface Modification on Photoluminescence Intermittency in Formamidinium Lead Bromide Perovskite Nanocrystals Naoki Yarita, Hirokazu Tahara, Masaki Saruyama, Tokuhisa Kawawaki, Ryota Sato, Toshiharu Teranishi, and Yoshihiko Kanemitsu J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b02840 • Publication Date (Web): 30 Nov 2017 Downloaded from http://pubs.acs.org on December 2, 2017

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The Journal of Physical Chemistry Letters

Impact of Postsynthetic Surface Modification on Photoluminescence Intermittency in Formamidinium Lead Bromide Perovskite Nanocrystals Naoki Yarita, Hirokazu Tahara, Masaki Saruyama, Tokuhisa Kawawaki, Ryota Sato, Toshiharu Teranishi, and Yoshihiko Kanemitsu*

Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *ORCID: 0000-0002-0788-131X

ACS Paragon Plus Environment

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ABSTRACT: We study the origin of photoluminescence (PL) intermittency in formamidinium lead bromide (FAPbBr3, FA=HC(NH2)2) nanocrystals and the impact of postsynthetic surface treatments on the PL intermittency. Single-dot spectroscopy revealed the existence of different individual nanocrystals exhibiting either a blinking (binary on-off switching) or flickering (gradual undulation) behavior of the PL intermittency. Although the PL lifetimes of blinking nanocrystals clearly correlate with the individual absorption cross-sections, those of flickering nanocrystals show no correlation with the absorption cross-sections. This indicates that flickering has an extrinsic origin, which is in contrast to blinking. We demonstrate that the postsynthetic surface treatment with sodium thiocyanate improves the PL quantum yields and completely suppresses the flickering, while it has no effect on the blinking behavior. We conclude that the blinking is caused by Auger recombination of charged excitons, and the flickering is due to a temporal drift of the exciton recombination rate induced by surface-trapped electrons.

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Lead halide perovskites are cost-effective and high-quality direct band-gap semiconductors with large absorption coefficients in the visible range and therefore have been studied intensively with respect to application as active layers in solar cells.1−3 These materials have excellent optical properties including strong bimolecular recombination of free carriers,4 extremely low defect/trap densities,4,5 long carrier diffusion lengths,6−8 and photon recycling,9,10 as well as the practical advantage of simple layer fabrication by straightforward solution processes that enable use in highly efficient solar cells. Enormous efforts have been made to reveal the properties of lead halide perovskite single crystals, thin films and devices. In addition to these structures, their nanocrystal (NC) counterparts have been receiving much attention since the successful synthesis of hybrid organic-inorganic halide perovskite MAPbX3 (MA = CH3NH3; X = Cl, Br, I) colloidal NCs with unique photoluminescence (PL) properties due to quantum confinement effects.11−12 Furthermore, all-inorganic halide perovskite CsPbX3 NCs have also been synthesized recently.13 These NCs exhibit remarkable optical properties such as a high PL quantum yield (PLQY) up to 90% and a tunable wavelength range covering the entire visible spectrum through halide exchange and size control.13,14 The implementation of perovskite NCs in optical devices including cavity lasers15 and light emitting diodes16 is expected to be important for future applications. A high PLQY and the long-term stability of PL are fundamental requirements for application of semiconductor NCs. Therefore, it is necessary to clarify the radiative and nonradiative carrier recombination processes in the NCs. Previous works on II-VI semiconductor NCs clearly showed that not only the existence of extrinsic nonradiative recombination centers but also the intrinsic nonradiative Auger recombination is able to quench the PLQY.17 Single-dot PL fluctuation, also known as PL intermittency, and the decrease in the PLQY of charged excitons


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(trions) and biexcitons in NCs are determined by nonradiative Auger recombination processes and surface trapping.18−24 Two types of PL intermittency have been observed; the blinking (binary on-off switching) or flickering (gradual undulation) behavior of the PL.25,26 Understanding of the physical origin of both is essential for control and application in devices. In-depth studies of the PL intermittency in II-VI semiconductor NCs revealed that the nonradiative Auger recombination and surface trapping can be suppressed by employing core/shell structures.27−29 However, in perovskite NCs, the detailed mechanisms of radiative and nonradiative recombination processes still remain unclear. Since MAPbX3 and CsPbX3 NCs are unstable in air and exhibit irreversible photoinduced degradation,30,31 more stable NCs are required to study the intrinsic radiative and nonradiative recombination processes. Recently, it has been shown that the formamidinium lead bromide perovskite FAPbBr3 (FA = HC(NH2)2) and its NCs exhibits relatively high stability in air at high temperatures32−34 and the surface trap density of perovskite NCs can be drastically reduced by postsynthetic surface treatment.35 In this work, we study the mechanisms of PL intermittency in single FAPbBr3 NCs using time- and energy-resolved single-dot spectroscopy. The PL intensity, the PL spectrum and the second-order photon correlation for 67 single FAPbBr3 NCs were collected, and we found that the temporal PL trace of each NC can be classified into either binary switching or gradually undulating PL, which we call binary blinking and continuous flickering, respectively. For blinking NCs, an abrupt switching between the highly emissive state with long PL lifetime and the poorly emissive state with short PL lifetime is observed. For flickering NCs, a gradual change is observed in the temporal traces of the PL intensity and lifetime. We found that the surface treatment with sodium thiocyanate (NaSCN) significantly improves the PLQYs and suppresses the flickering, while it has no effect on the blinking behavior. Our results evidence


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that blinking and flickering have different physical origins. The impact of the surface treatment on the carrier recombination processes is discussed. In our experiments, we used cubic FAPbBr3 NCs with 10.5 ± 1.3 nm edge length (Figure S1). The NCs were synthesized according to a method described in the literature.34 One half of the synthesized NCs were treated with NaSCN in a following process as outlined in the Experimental Methods. For single-dot spectroscopy, the FAPbBr3 NCs were diluted with polymethylmethacrylate (PMMA) in toluene and spin-coated on a cover glass. We performed both time-resolved PL spectrum measurements and time-tagged time-resolved PL (TTTR-PL) measurements on single FAPbBr3 NCs. The latter is a technique that allows simultaneous measurement of PL time decays and the second-order photon correlation function g(2).36 The NC film was excited by a laser pulse with photon energy of 2.81 eV. The excitation fluence was set to 1.7 µJ/cm2, which corresponds to an average absorption rate of 0.1 photons in one NC per pulse (the average absorption cross-section of the FAPbBr3 NCs was 3.4 × 10−14 cm2 at 2.81 eV; see Figure S2). Further experimental details are described in the Experimental Methods. We performed the abovementioned single-dot spectroscopy for 67 untreated single FAPbBr3 NCs, and found that their temporal PL fluctuation profiles can be classified into two types: blinking and flickering. The blinking is characterized by an abrupt change in the PL intensity, while the flickering exhibits a gradual, slowly undulated change in the PL intensity. Figure 1 displays the representative data of the TTTR-PL measurement on a blinking FAPbBr3 NC. Figure 1a shows the temporal change of the PL intensity that was recorded with a bin time of 25 ms. The switching between the high PL intensity state (on state; red area) and the low PL intensity state (gray state; blue area) can be clearly observed. The blinking was observed in 37 single NCs out of the measured 67 single NCs.


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To clarify the origin of the two discrete states, we analyzed the simultaneously measured PL decay curves. In Figure 1b, we show the two PL decay curves obtained by averaging the respective time-resolved data sets that belong to the two different intensity regions in Figure 1a. The red and blue circles correspond to the high-intensity and the low-intensity regions in Figure 1a, respectively. The PL decay curve of the on state can be fitted by a single exponential function with a long lifetime of 13 ns, which indicates that this state is due to emission from neutral excitons.24 On the other hand, an extremely fast decay component appears in the PL decay curve of the gray state. This fast decay component is attributed to the recombination of trions, similar to the case of the well-studied CdSe/CdS and CdSe/ZnS core/shell NCs.26,27,37,38 The gray state provides a PL intensity between the bright on state and the non-emissive state (the so-called off state) and the PL decay curve can be well fitted by a double exponential function. The fast lifetime is 170 ps and the slow lifetime is 8 ns, which is close to that of the neutral exciton. Based on the decay dynamics observed in previous studies,26,39 we consider that the slow component in the gray state is a result of trapping of hot electrons by surface traps, before they relax to the ground state and form an exciton. Figure 1c plots the second-order photon correlation function g(2), which reveals a clear photon antibunching (very small center peak intensity) and proves that the PL signal was emitted from a single NC. Figure 1d displays the fluorescence-lifetime-intensity-distribution (FLID). In the FLID analysis, the decay curve of each bin time is fitted by a single exponential function. To suppress the influence of the long decay component, the fitting time range was limited from 0 to 3 ns. A clear correlation between the PL intensity and lifetime is observed in Figure 1d. The separation between the data points in terms of both PL intensity and lifetime allows to assign the on state (denoted by X: exciton state) and the gray state (denoted by X*: trion state).


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Figure 1. Characteristic data of the TTTR-PL measurement for a blinking FAPbBr3 NC. (a) PL intensity time trace, (b) PL decay curves derived from the time-resolved data corresponding to the red and blue shaded areas in panel (a), (c) second-order photon correlation function g(2), and (d) FLID.

The remaining 30 single NCs exhibited a fundamentally different PL fluctuation behavior, i.e., the flickering, which is characterized by a gradually undulating fluctuation of PL intensity and lifetime. In Figure 2, we show the representative experimental data of the TTTR-PL measurement from a flickering NC. For a simple analysis, we introduced three intensity ranges, but we emphasize that there are no clear discrete levels except the uppermost red range, and the PL intensity slowly goes up and down. We verified that the classification into blinking and flickering is independent of the bin time (Figure S3,S4). Similar flickering has also been observed in individual conjugated polymers and other perovskite NCs.30,40 It is interesting to note that in the case of the II-VI semiconductor NCs, an intentionally applied voltage induces the PL


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flickering behavior, and therefore it has been associated with charging/discharging of NCs.25,26 On the other hand, the flickering observed here occurs without applying voltage. The experiments in the following clarify the physical origin of flickering. Figure 2b shows three PL decay curves corresponding to the three intensity regions in Figure 2a. The PL decay curve in the high-intensity region (red) is single exponential and has a long lifetime of 14 ns, which indicates that this “on” state is due to emission from neutral excitons. On the other hand, the PL decay curves of the intermediate PL intensity region (green) and low PL intensity region (blue) require a double exponential function for accurate fitting. The lifetimes of the two decay components are tau τ1 = 3.3 ns and τ2 = 11 ns for the intermediate PL intensity region, and τ1 = 1.6 ns and τ2 = 9 ns for the low PL intensity region. The value of τ2 is similar to the on-state lifetime obtained from the high intensity state (red data). However, in both regions, the fast lifetime τ1 is significantly longer than that of the NC shown in Figure 1b and τ1 gradually decreases with decreasing PL intensity. This gradual fluctuation of the PL intensity and the PL lifetime cannot be explained with a signal from multiple NCs since a clear photon antibunching was observed (Figure 2c), which is evidence for single NC emission. The FLID displayed in Figure 2d shows a broad and continuous distribution of the PL intensity and lifetime and a clear positive correlation between them. The completely different FLIDs for blinking NCs and flickering NCs suggest a different physical origin.


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Figure 2. Characteristic data of the TTTR-PL measurement for a flickering FAPbBr3 NC. (a) PL intensity time trace, (b) PL decay curves derived from the time-resolved data corresponding to the red, green and blue shaded areas in panel (a), (c) second-order photon correlation function g(2), and (d) FLID.

In order to clarify the physical reasons for the blinking and flickering, we measured the temporal evolution of the PL spectra from the single FAPbBr3 NCs. The data evidenced that the time traces of the PL spectra can also be classified into two types according to the PL fluctuation behavior of the NC (i.e., blinking and flickering; see Figure S5,S6). In order to analyze the temporal evolution of the spectra, we used an accumulation time of 40 ms and simultaneously measured PL intensity and PL peak energy for each PL spectrum. The representative correlation data for a blinking NC is summarized in Figure 3a. We can see that the PL peak energy redshifts when the PL intensity is low (below 200 cts/bin). It is known that the trion emission energy is lower than that of the exciton by the binding energy of an excess carrier.38 Therefore, the redshift


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of the emission peak observed in Figure 3a supports our assignment that the gray state corresponds to the trion. From Figure 3a, the emission energies of trions and excitons were evaluated to be 2.367 eV and 2.350 eV, respectively. These values yield a binding energy of 17 meV for trions, similar to the previous report.41 Figure 3b displays the correlation between the PL intensity and PL peak energy for a flickering NC. Compared to the case of a blinking NC shown in Figure 3a, the PL intensity and PL peak energy has a significantly broader distribution. All measured flickering NCs obey this tendency (Figure S7). The broader distribution of the PL peak energy for flickering NCs indicates that the upper or lower energy level for recombination slightly shifts with time. Note that the trion emission is not observed for the flickering NCs. Therefore, we consider that the trion is not responsible for flickering.

Figure 3. We measured the temporal evolution of the PL spectra of single FAPbBr3 NCs for 40 s with a bin time of 40 ms. Correlation between PL intensity and PL peak energy for (a) blinking and (b) flickering NCs.


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To obtain a deeper understanding of blinking and flickering mechanisms, we summarize the time-integrated PL intensity, the fast lifetime, the PL peak energy, and the exciton lifetime as a function of the absorption cross-section σ for all measured NCs. The evaluation method of σ for the individual single NCs is described in the Experimental Methods. Since it is very difficult to directly measure the absorption cross-section of single NCs, the absorption cross-section is usually evaluated using PL or photothermal spectroscopy.37,42 Because we used the PL from single NCs under assumption of a constant PLQY, the evaluated absorption cross-section σ has an uncertainty due to the variation of the PLQY in each single NC. However, we confirmed that the average value evaluated from the single-dot spectroscopy 3.0 × 10−14 cm2 matched the absorption cross-section of 3.4 × 10−14 cm2 that was directly obtained by transient absorption spectroscopy on ensemble NCs (see Figure S2). Thus, we consider that the uncertainty due to the PLQY variation is not dominant and that the absorption cross-section of each NC is evaluated from the single-dot spectroscopy with sufficient accuracy. In Figure 4a, the integrated PL intensities (integration time 100 s) of the 67 untreated single NCs are plotted against σ. The integrated PL intensity for a flickering NC tends to be lower than the intensity for a blinking NC. Thus, the suppression of flickering contributes more significantly to the enhancement of the PLQYs of single NCs than the suppression of blinking. In Figure 4c, the fast lifetimes obtained from the PL decays in the low intensity region are plotted against σ. For the case of blinking NCs (red circles), the fast lifetime (߬௑ ∗ ) is proportional to the 1.72th power of σ (broken line in Figure 4c). This superlinear dependence is similar to that observed for the trion in II-VI semiconductor NCs.43 Thus, we consider that our observed power dependence reflects the stronger suppression of the trion’s Auger recombination for larger NCs as a result of the weaker wave function overlap of electron and hole.


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The PL peak energy and the exciton lifetime of single NCs also have a clear dependence on σ (Figure S8) due to a quantum confinement effect. With a decrease of the NC size, the PL peak energy is blueshifted. Moreover, the exciton lifetime becomes shorter in smaller NCs because of enhancement of an overlap between the wave functions of electron and hole. On the other hand, in the case of flickering NCs (Figure 4c; blue triangles), the fast lifetime shows no clear dependence on σ. This fact indicates that the flickering strongly depends on extrinsic factors such as surface states or surrounding environment of the NCs, rather than on the size of the NCs. We consider that surface trap states play an important role in the PL flickering processes. Since a postsynthetic surface-treatment with NaSCN is able to repair a lead-rich surface of a CsPbBr3 NC and simultaneously improves its PLQY,35 we prepared FAPbBr3 NCs using the postsynthetic NaSCN surface treatment technique (see Experimental Methods). The presence of SCN ligands on surface-treated NCs was confirmed from Fourier-transform infrared (FT-IR) spectra (Figure S9). A peak at 2060-2070 cm-1, which corresponds to the C≡N bond of the thiocyanate,35 is clearly observed only for treated NCs. From this result, we consider that the surface treatment with NaSCN is also effective in the case of the FAPbBr3 NCs studied here. In Figure 4b,d, the integrated PL intensities and the fast lifetimes obtained from 70 surfacetreated NCs are plotted as a function of σ. Although about half of the NCs exhibited the flickering before treatment, almost all of the treated NCs exhibit blinking behavior (Figure S10). Therefore, the origin of the flickering can be attributed to the lead-rich surface of the NCs, which acts as an electron trap.35 In the case of II-VI semiconductor NCs, it has been demonstrated that an intentionally applied voltage induces PL flickering in NCs.25,26 Our result suggests that even without applying voltage, the charge carriers trapped at surface defects can induce significantly strong electric fields in the NC and possibly cause the flickering. This is the physical


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interpretation of the flickering behavior in the FAPbBr3 NCs, and we consider that it also applies to other materials systems.

Figure 4. (a,b) Time-integrated PL intensity and (c,d) lifetime of the fast component in the PL decay curve for the low PL intensity region plotted for untreated (a,c) and treated (b,d) FAPbBr3 single NCs. Red circles and blue triangles represent blinking and flickering NCs, respectively.

We note that the surface treatment has a minimal effect on blinking NCs. In Figure S11, the histograms of the “on” time fraction of blinking NCs are compared between treated and untreated samples. The average fraction of the “on” time for the treated NCs is 83.1%, which is almost the same as the 80.9% of the untreated NCs. This result implies that the surface condition of the FAPbBr3 NCs is not responsible for the blinking.


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From the data displayed in Figure 4a,b, we find that the averaged integrated PL intensities for the 67 untreated NCs and the 70 treated NCs are 4.6×106 cts/100s and 5.7×106 cts/100s, respectively. Therefore, the surface treatment with NaSCN results in the enhancement of the PLQY of the FAPbBr3 NCs by 26%, which is comparable to the reported values for the treated CsPbBr3 NCs.35 This fact implies that the enhancement of the PLQY by the treatment is attributed to the suppression of the flickering. In conclusion, we clarified two different mechanisms of PL intermittency, blinking and flickering, in single FAPbBr3 NCs. In both cases, the PL decay curve is single-exponential in the high intensity region and an additional fast decay component arises in the low intensity region. In the case of blinking NCs, the fast lifetime is due to the trion Auger process, evidenced through the dependence of the fast lifetime on the absorption cross-section. On the other hand, in the case of flickering NCs, there is no correlation between the fast lifetime and the absorption crosssection. We found that the surface treatment with NaSCN enhances the PLQY and completely suppresses the flickering, while it has no effect on the blinking behavior. This experimental result proves that surface states play a dominant role only for flickering. We conclude that the origin of the flickering is a slow temporal drift of the recombination rate of excitons induced by electrons that are localized at surface defects, and thus can occur without any charging/discharging of the NC.

EXPERIMENTAL METHODS Nanocrystal Synthesis. Synthesis of oleylammonium bromide. A 47 % HBr aqueous solution (8.56 mL, TCI) was slowly added to the oleylamine (12.5 mL, Acros Organics) / ethanol (100 mL) solution at 0 °C under


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nitrogen atmosphere. The solution was stirred at 0 °C for 12 hours. After evaporating the solvent, the white precipitate was washed with diethylether three times, and dried under vacuum for 12 hours.

Synthesis of FAPbBr3 nanocrystals. The formamidinium lead bromide (FAPbBr3) nanocrystals (NCs) were prepared according to the method described in the literature.34 The procedure is briefly outlined in the following. The Pb acetate (38 mg, Wako), the FA acetate (39 mg, TCI), dried 1-octadecene (4 mL, Sigma-Aldrich), and dried oleic acid (1 mL, TCI) were added in a three-neck flask and dried under vacuum at 50 °C for 30 minutes. Then the solution was heated to 130 °C under nitrogen atmosphere. 10 seconds after a mixture of oleylammonium bromide (105 mg) and toluene (1 mL) was injected, the solution was cooled in a water bath. Toluene (5 mL) and acetonitrile (2.5 mL) were added to the product solution at room temperature, and the mixture was centrifuged at 8900 rpm for 5 minutes. The supernatant was discarded and the precipitate was dispersed in hexane. Finally, the hexane solution was centrifuged at 8900 rpm for 5 minutes, and the precipitate was discarded to remove the large FAPbBr3 NCs.

Single-dot spectroscopy. The samples were prepared by spin-coating a diluted solution of FAPbBr3 NCs in a polymethyl methacrylate (PMMA)/toluene solution onto a cover glass. We performed the second-order photon correlation studies on individual FAPbBr3 NCs using a versatile home-built confocal microscope system with a Hanbury Brown-Twiss interferometry setup. As excitation source, a supercontinuum light source (WhiteLase, Fianiuum) was combined with a tunable bandpass filter (SuperChrome, Fianiuum) to obtain the excitation beam with bandwidth of 20 nm and center


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wavelength at 441 nm. The emitted light from the sample was collected by an oil immersion objective for the visible range (100x, NA = 1.3). All measurements were performed under ambient conditions.

Time-tagged time-resolved photoluminescence (TTTR-PL) measurement. The emitted light from the sample was divided with a beam splitter (Transmission 50%) and measured with a pair of identical silicon avalanche photodiodes (APDs) (ID Quantique) to obtain the second-order photon correlation function g(2). The signal from each APD was recorded on a separate timecorrelated single-photon counting board (Becker-Hickl) in the time-tag mode, which enables the extraction of the photoluminescence (PL) decays. The time resolution of our setup was ~240 ps. The repetition rate of the excitation pulse was set to 5 MHz.

Time-resolved PL spectrum measurement. For this measurement, the emitted light from the sample was guided towards a monochromator and detected with a liquid-nitrogen-cooled chargecoupled device (CCD). The repetition rate was set to 40 MHz to improve the signal-to-noise ratio of the PL spectrum.

Evaluation of the absorption cross-section for the individual single NCs. The absorption crosssection of a single NC is estimated by the following equation:37

ߪ=

‫ܫ‬௑ ‫݆ ∙ ܨ‬ୣ୶ ∙ ߟ௑ ∙ ߦ

(1)

where IX is the exciton PL count rate, F the pump pulse repetition rate (5 MHz), jex the excitation photon fluence (3.8×1012 photons/cm2), ηX the PLQY of the excitons, and ξ is the detection


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efficiency of our system (approximately 7%).24,37 For FAPbBr3 NCs, ηX has been reported to be 85%.34 Surface treatment by sodium thiocyanate. The method reported by Koscher et al.35 was employed for the surface treatment of the FAPbBr3 NCs with sodium thiocyanate (NaSCN) and is briefly outlined in the following. The sodium thiocyanate used in this work was purchased from Sigma-Aldrich and is usually stored in a nitrogen filled glove box. For the surface treatment, excess sodium thiocyanate was added into a hexane solution containing the FAPbBr3 NCs and stirred for 20 minutes. The crude suspension was filtered with a polytetrafluoroethylene (PTFE) syringe filter to remove the residue.

AUTHOR INFORMATION Corresponding Author *[email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENT The authors thank T. Aharen for helpful discussions and experimental help. Part of this work was supported by JST-CREST (JPMJCR16N3). ASSOCIATED CONTENT Supporting Information. PL and absorption spectra of NC ensemble, NC size distribution, transmission electron microscope image, average absorption cross-section of NC ensemble, binning time dependence of the PL intensity fluctuation, PL spectrum time trace, PL peak energy


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distribution, PL energies and exciton lifetimes of single NCs, FT-IR spectra, the PL blinking of surface-treated NCs. (PDF)

REFERENCES (1) Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc. 2009, 131, 6050−6051. (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. (3) Kim, H.-S.; Lee, C.-R.; Im, J.-H.; Lee, K.-B.; Moehl, T.; Marchioro, A.; Moon, S.-J.; Lead Iodide Perovskite Sensitized All-Solid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%. Sci. Rep. 2012, 2, 591. (4) Yamada, Y.; Nakamura, T.; Endo, M.; Wakamiya, A.; Kanemitsu, Y. Photocarrier Recombination Dynamics in Perovskite CH3NH3PbI3 for Solar Cell Applications. J. Am. Chem. Soc. 2014, 136, 11610−11613. (5) Stranks, S. D.; Burlakov, V. M.; Leijtens, T.; Ball, J. M.; Goriely, A.; Snaith, H. J. Recombination Kinetics in Organic-Inorganic Perovskites: Excitons, Free Charge, and Subgap States. Phys. Rev. Appl. 2014, 2, 2034007. (6) 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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