Shift Happens. How Halide Ion Defects Influence Photoinduced

Letter

Shift Happens. How Halide Ion Defects Influence Photoinduced Segregation in Mixed Halide Perovskites Seog Joon Yoon, Ken Kuno, and Prashant V. Kamat ACS Energy Lett., Just Accepted Manuscript • Publication Date (Web): 01 Jun 2017 Downloaded from http://pubs.acs.org on June 1, 2017

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ACS Energy Letters

Shift Happens. How Halide Ion Defects Influence Photoinduced Segregation in Mixed Halide Perovskites Seog Joon Yoon,1,2 Masaru Kuno,2 and Prashant V. Kamat1,2*

Radiation Laboratory and Department of Chemistry and Biochemistry University of Notre Dame, Notre Dame Indiana, 46556, United States

*Corresponding author: [email protected] 1

Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana, 46556, United States

2

Department of Chemistry and Biochemistry, University of Notre Dame

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Abstract Minimizing photoinduced segregation in mixed halide lead perovskites is important for achieving stable photovoltaic performance. The shift in the absorption and the rate of formation of iodide- and bromiderich regions following visible excitation of mixed halide lead perovskites is found to strongly depend on the halide ion concentration. Slower formation and recovery rates observed in halide deficient films indicate the involvement of defect sites in influencing halide phase segregation. At higher halide concentrations (in stoichiometric excess) segregation effects become less prominent as evidenced by faster recovery kinetics. These results suggest that light-induced compositional segregation can be minimized in mixed halide perovskite films by using excess halide ions. The findings from this study further reflect the importance of halide ion post treatment of perovskite films to improve their solar cell performance.

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3 Recent advances in achieving high power conversion efficiencies in metal halide lead perovskitebased solar cells have made it necessary to unravel their intriguing properties.1-3 While the latest studies shed light on photoinduced processes such as excited state decay pathways,4 radiative and non-radiative electron-hole recombination,5-7 and charge carrier diffusion,8-10 there remain many unanswered questions. One such topic of interest is the halide ion mobility and its influence on solar cell operation.1114 Garcia-Belmonte and Bisquert have recently analyzed capacitive and noncapacitive currents in perovskite solar cells and have shown that reversible ionic and electronic charge accumulation results in electrode polarization.15 Simply treating lead iodide perovskite films with a halide rich compounds (e.g., monovalent cation halide or 1% bromide) can decrease disorder present within the film and improve photovoltaic performance.16-19 Such improvements result in increased emission lifetimes suggesting passivation of surface defects.18 However, the details regarding how exactly excess halide ions influence excited state dynamics are lacking. Mixed halide lead perovskites are useful materials for their composition-dependent and tunable bandgaps (Eg).17, 20-21 It is possible to tune Eg values between 1.55 eV and 2.43 eV by varying the halide composition of lead iodide/bromide [MAPbBrxI3-x (x=0 to 3), MA=CH3NH3+] perovskites.22-24 Solid-state nuclear magnetic resonance studies that probe 207Pb have highlighted the complex structure of mixed halide perovskites and its influence on emission properties.25 In addition to photovoltaic applications, mixed halide perovskites exhibit interesting excited state properties.2, 26-29 One such response involves light-induced halide ion migration, which leads to phase segregation and growth of I-rich and Br-rich domains.30-33 This segregation effect introduces characteristic shifts in both absorption and emission spectra. Although segregation effects are reversible under short term illumination conditions, concerns have been raised regarding the influence of halide ion mobility on the long term stability and open-circuit voltage of hybrid perovskite-based photovoltaic devices.34 Several experimental as well as computational modeling studies have now attempted to establish halide ion mobility and the reasons behind light-induced halide phase segregation.12-14, 35 In a previous study, we have tracked these photosegregation effects at different excitation intensities using transient differential absorption (TDA) spectroscopy.32 Despite the broad absorption of the mixed lead halide as well as that of segregated I-rich and Br-rich phases, we observed emission to arise from the lowest bandgap region of nucleated I-rich phases. Grain boundaries also influence ion migration.36 It has been posited that the ionic nature of the lead halide perovskite facilitates free carrier-related deformation of the underlying lattice through strong electron–phonon coupling. This, in turn, facilitates ion migration3738 and ultimately leads to demixing of the mixed halide solid solution.39 Thus, photoinduced halide phase segregation remains an intriguing electromechanical property of perovskite materials. These interesting aspects of mixed halide perovskites demand a deeper understanding of factors that dictate photoinduced halide anion mobilities and their ultimate influence on perovskite optical and electronic properties. Spectral shifts in the absorption of mixed halide perovskite films offer a convenient way to probe the kinetics of halide ion migration. Consequently, steady state and TDA measurements of halide-deficient and halide-rich perovskite films are presented here to develop insight into the kinetics of light-induced halide ion migration and segregation.


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4 Influence of halide ion composition on segregation effect Mixed halide films, containing equimolar amounts of Br- and I- but having different Pb:halide ratios, were prepared by premixing known amounts of PbI2, MAPbBr2 and MAPbI2 solutions in N,Ndimethylformamide. Films were cast on glass slides by spin-coating followed by annealing at 100 °C (see Supporting Information (SI) for details). Films with four different Pb:halide premixing ratios were prepared, having the following compositions: 1:2.4, 1:2.7, 1:3 and 1:5. The film thickness as measured from cross section SEM measurements was in the range of 60-80 nm. Figure 1 shows absorption spectra of these films recorded before and after 20 minutes of irradiation with a 405 nm continuous-wave (CW) diode laser (25 mW/cm2). Except for the 1:5 sample, all specimens exhibit an absorption peak between 620-630 nm (Figure 1 A-C). This corresponds to effective bandgaps of Eg ~ 2 eV. According to the estimate by Noh et al40, a 2 eV bandgap corresponds to a mixed halide composition of 60% bromide and 40% iodide. As previously discussed in the literature,41 the Pb2+/Brcomplexation constant is greater than that for Pb2+/I-. For the same reason, films prepared with excess halide ions (e.g. 1:5) behave in a similar manner to pure MAPbBr3. The film with Pb:halide ratio of 1:2.4 do not show any major changes in the absorption (Figure 1 A) when subjected to irradiation with 405 nm light. However the films with Pb: halide ratio 1:2.7 and 1:3 show bleaching (decrease in absorbance) at the band edge and small increases in their absorption at wavelength both shorter and longer relative to the band edge absorption maximum Figure 1 B, C). These changes reflect a decrease of the parent mixed halide composition through partial halide segregation. This segregation is represented by the formation of I-rich and Br-rich regions within the film (Reaction 1) nMAPbBrxI3-x+ hν → (n – 2m)MAPbBrxI3-x + mMAPbBrx-yI3-x+y+ mMAPbBrx+yI3-x-y .

(1)

Interestingly the majority of the change seen in the absorption recovers when the films are stored in dark (spectra c in Figures 1 B-D). As pointed out in our earlier study the recovery is process is rather slow and depends on the conditions employed for photoirradiation. Mechanisms rationalizing such halide ion phase segregation following photoirradiation vary in origin and remain a subject of current debate.30-33, 39 The following discussion sheds new insights into the underlying mechanism of light-induced halide phase segregation.


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Figure 1. Absorption spectra of mixed halide perovskite films containing Pb:halide compositions of (A) 1:2.4, (B) 1:2.7, (C) 1:3, and (D) 1:5. Absorption spectra were recorded (a) before and (b) after 20 minutes of irradiation with a 405 nm diode laser (25 mW/cm2). (c) Recovery in dark as probed through absorption spectra recorded 2 hours after stopping the 400 nm illumination. Films with a 1:5 Pb:halide ratio show a blue shifted band edge peak (at 465 nm) as compared to films with a 1:3 ratio (~630 nm). Such a blue shift indicates the preferred association of Pb2+ with Br-. As shown earlier,41 Pb2+/Br- complexation is favored over Pb2+/I- when excess halide ions (equimolar Br- and I-) are present. In this regard, a detailed complexation study has shown that the Pb2+/Br- complexation constant is seven times larger than that of Pb2+/I-.41 Upon continuous-wave irradiation with 405 nm light, the 1:5 absorption peak shifts from 465 to 435 nm. This 30 nm blue shift corresponds to further growth of bromide-rich domains within the film. Concurrently, there is increased absorption in the red due to a resulting iodide-rich phase. Thus, even in mixed halide films dominated by bromide-rich phases, lightinduced phase segregation occurs. In a separate experiment, involving a stoichiometric film (Pb:halide = 1:3) but where the halide content is predominantly bromide (Br:I =3.5:1), 405 nm irradiation also causes a similar shift in the absorption peak from 565 nm to 540 nm (See Figure S1, SI).


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Figure 2. TDA spectra of mixed halide perovskite films with Pb:halide compositions of (A) 1:2.4, (B) 1:2.7, (C) 1:3, and (D) 1:5. TDA spectra were recorded in a femtosecond transient spectrometer (1 ps delay) with films continuously subjected to 405 nm CW diode laser irradiation (25 mW/cm2). Spectral changes reflect the timescale over which halide ions migrate during photoirradiation. The center schematic shows the excitation configuration employed for these measurements. Tracking the light-induced halide phase segregation through transient absorption spectroscopy In a previous communication, we have shown that photoinduced halide segregation in MAPbBrxI3−x films can be tracked by recording transient difference absorption (TDA) spectra in a femtosecond transient differential absorption spectrometer while the film is subjected to continuous irradiation from a separate CW diode laser (405 nm).32 In the current study, prior to CW irradiation, TDA spectra of all four samples were recorded to establish baseline responses. These spectra show characteristic bleaches at corresponding band edge excitonic absorption wavelengths as a result of photoinduced charge separation (Figure S2). Photogenerated charges subsequently recombine, leading to a bleach recovery, which occurs over the course of a few nanoseconds. Bimolecular charge recombination, contributing to this bleaching recovery, has been discussed in our previous study.5-6 As can be seen from Figure S2, the bleach maximum for each composition is characteristic of the mixed halide composition and the bleach wavelength matches that of absorption peaks seen in Figure 1 (panels A-D). No significant shift in the bleach position (Figure S2) is seen during recovery confirming that the pulsed laser excitation (387 nm) in the TDA measurement does not induce halide phase segregation or other


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7 compositional changes. It is therefore apparent that TDA measurements offer a convenient way to track photoinduced changes in mixed halide perovskite films. Mixed halide specimens were then subjected to continuous 405 nm diode laser irradiation while monitoring changes to their absorption using the femtosecond TDA spectrometer. TDA spectra of each sample, recorded immediately after 387 nm laser pulse excitation but at different macroscopic 405 nm irradiation. The transient spectra at ∆t387 nm = 1 ps (probe delay of 1 ps following 387 nm excitation) shown in Figure 2 show the changes occurring in the excited state during 405 nm irradiation. For 1:2.4 films, we observe small changes to their transient spectra even at the longest 405 nm illumination times. This suggests that light-induced halide phase segregation is suppressed in 1:2.4 films (Figure 2A). For 1:2.7 and stoichiometric 1:3 film (Figure 2 B and C, we observe a decrease of the parent bleach at 630 nm with increasing irradiation time and growth of a new bleach in the 700 nm region. An effective 70 nm red shift is therefore observed as the 1:3 film undergoes photoinduced halide phase segregation. As discussed earlier,32 growth of the ~700 nm bleach represents the formation of I-rich regions within the mixed halide film. A different scenario emerges in the 1:5 excess halide case. Here the band edge effectively red shifts from 500 nm to 535 nm and is accompanied by growth of an induced absorption around 480 nm. The relatively small 35 nm band edge shift as compared to the ~70 nm red shift in 1:3 films indicates that halide phase segregation becomes less prominent in films in the presence of excess bromide ions. This is likely due to the predominant complexation of Pb2+ with Br- 41, which, in turn, leads to Br-rich films that are less susceptible to further phase segregation. The excited state dynamics of photosegregated films with different Pb:halide ratios have also been studied. TDA spectra were recorded after 20 minutes of continuous 405 nm irradiation (Figure 3). In halide-deficient 1:2.4 samples (Figure 3A), the transient behavior is similar to that seen before any 405 nm irradiation (Figure S2A). This behavior is consistent with the fact that these films do not undergo halide ion segregation. In 1:2.7 (Figure 3B) and 1:3 (Figure 3C) samples, the primary bleach is red-shifted relative to the bleach seen prior to photoirradiation (see Figure S2 to compare transient absorption spectra before photoirradiation). Furthermore, the appearance of a bleach in the 650-700 nm region clearly shows I-rich phase excited state dynamics –specifically band filling state.5 In this regard, although the 387 nm pump in the TDA experiment excites both bromide- and iodide-rich regions of the film, photogenerated holes and electrons migrate and eventually recombine within the lower band gap iodide-rich phase. The predominance of emission in the red region (~760 nm) confirms charge carrier accumulation and recombination at iodide rich domains.32 As shown for gradient CsPb(BrxI1-x)3 films, electron/hole migration occurs on a ~1 ps timescale.42 Finally, in 1:5 sample (Figure 3D), only a small red-shift of the parent bleach is observed as phase segregation appears limited in these films.


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Figure 3. TDA spectra of mixed halide lead perovskite films containing Pb:halide compositions of (A) 1:2.4, (B) 1:2.7, (C) 1:3, and (D) 1:5 following 20 minutes of CW photoirradiation (405 nm, 25 mW/cm2). Kinetics of photoinduced phase segregation Both the bleaching of the mixed halide absorption and the appearance of an I-rich absorption in the red make it convenient to track the kinetics of halide ion segregation via temporally-resolved difference absorption (TDA) spectroscopy. Steady state absorption spectra (Figure 1) and TDA measurements (Figure 2) were therefore further analyzed to record absorption changes at the bleach maximum with respect to 405 nm irradiation time (Figure 4A). In all cases, TDA spectra were acquired with a 1 ps delay (∆t387 nm=1 ps). Corresponding steady state absorption changes recorded during the experiment are shown in Figure S3. The emission spectra recorded for these four films (before and after photoirradiation) are shown in Figure S4 (Supporting Information). The rate of phase segregation as measured from first order, exponential growth, kinetic fits of the TDA data are summarized in Table 1. Halide deficient 1:2.4 films show negligible absorption changes and hence are not included in the analysis. The data reveals that the slowest segregation rate is observed for 1:2.7 films (k=0.33 ×10-2 s-1). By contrast, 1:5 films exhibit the fastest segregation rates (=2.15 ×10-2 s-1). These results therefore suggest that increasing the halide ion concentration in films increases the rate of phase segregation.


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9 To probe the role of excess halide ions in the segregation, we spin-coated an additional layer of methyl ammonium halide (referred to as MAX with X=I- + Br-) on top of a 1:2.4 film. The MAX solution consists of equimolar amounts of I- and Br-. Following MAX deposition, 1:2.4 films are observed to exhibit fast (k=1.89 ×10-2 s-1) phase segregation under illumination. Recall that native 1:2.4 films do not exhibit any noticeable segregation under 405 nm irradiation (Figures 1A and 2A). Changes in halide segregation kinetics as seen by monitoring the bleach feature at 630 nm before and after MAX treatment are shown in Figure 4B. Identical experimental conditions as in Figure 4A were employed. The faster growth kinetics seen in MAXtreated 1:2.4 film mirrors that of 1:5 films (Figure 4A). It may be interesting to note that similar halide ion post treatment is employed to passivate halide ion defects in hybrid perovskite active layers so as to improve solar cell efficiencies.16, 18-19, 43

Figure 4. (A) Halide phase segregation rate as monitored from TDA changes at the bleach maximum of films with Pb:halide ratio of (a) 1:2.4, (b) 1:2.7, (c) 1:3, and (d) 1:5 during 405 nm CW irradiation. (B) Effect of MAX on the phase separation kinetics of a halide deficient 1:2.4 film during 405 nm CW irradiation (a) before and (b) after post treatment. Table 1. Photoinduced Phase Segregation and Dark Recovery Kinetics MAPbBrxI3-x films Pb : halide ratio

Segregation rate constant (10-2s-1)

Recovery rate constant (10-3s-1)

1:2.4 (20% halide deficient)

NA

NA

1:2.7 (10% halide deficient)

0.33

0.06

1:3 (stoichiometric ratio)

0.40

1.89

1:5 (excess halide)

2.15

37.60

1:2.4 MAX-treated (X=I+Br-)

1.89

0.60


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10 Recovery kinetics As reported earlier, halide segregation is reversible at low irradiation intensities.30, 32-33 In the present case, mixed halide perovskite films with different halide compositions completely recover in the dark as demonstrated through steady state absorption spectra in Figure 1. Following 20 minutes of 405 nm irradiation (25 mW/cm2), the recovery kinetics of each of the four films was monitored. Figure 5 shows resulting kinetics from TDA data. Corresponding first order recovery rate constants, as determined from exponential fits, are summarized in Table 1. Trends in the dark recovery kinetics are similar to that seen for photoinduced phase segregation except with slower kinetics. Films with Pb:halide ratios of 1:5 show the fastest recovery kinetics. By contrast,

Figure 5. (A) Dark recovery kinetics of (a) 1:2.7, (b) 1:3, and (c) 1:5 mixed halide perovskite films following 20 minutes of 405 nm irradiation. (B) Effect of MAX treatment on the dark recovery kinetics of a halide deficient 1:2.4 film following 405 nm CW irradiation (a) before and (b) after post MAX treatment. 1:2.7 samples recover the slowest. In this regard, whereas 1:5 and 1:3 films fully recover in less than an hour, 1:2.7 films only exhibit a ~40% recovery during the same period of time. Furthermore, as seen for photoinduced halide segregation, it is apparent that the halide ion concentration in the film dictates the dark recovery kinetics. Interestingly, for 1:2.4 MAX-treated films, the recovery is quick and completed in about two hours. The ability to modulate I- and Br- segregation kinetics in mixed halide perovskite films therefore offers potentially new insights into the movement of halide ions in these films. Understanding the role of halide ions in photoinduced phase segregation Halide ion mobility in metal halide perovskites is known to play an important role in establishing the photovoltaic performance of solar cells.20 The movement of halide ions within the perovskite crystallites can induce destabilizing effects in mixed halide lead perovskites under long term photoirradiation. Based on theoretical and experimental studies, several models have been developed to explain the movement of halide ions under photoirradiation. They include (i) differences in dark and photoirradiated formation energies,14 (ii) the existence of two potential energy minima in the excited state which facilitates phase


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11 segregation33, (iii) polaron-assisted halide phase segregation39 and (iv) defect site-driven movement of halide ions.13, 35 While these models explain segregation effects under specific experimental conditions, a complete understanding of this process, including complete recovery under dark conditions, remains elusive. In our previous study, we analyzed excitation intensity-induced phase segregation to probe the ability of charge carriers to diffuse long distances and to induce emission at I-rich regions of phase-separated films.32 It has also been proposed that photogenerated polarons localize in iodide-rich regions of MAPb(I1−xBrx)3 thin films, inducing local lattice strain.39 This, in turn, promotes iodide migration and results in iodide-rich domain growth. The dependence of the segregation rate on the halide ion concentration seen in this study highlights the important role played by halide ions in dictating phase segregation kinetics.

Scheme 1. Schematic representation of halide vacancies in the Pb-I plane of mixed halide perovskite films which influences halide ion migration and which ultimately leads to I-rich and Br-rich domains (marked by dashed circles). In their computational modeling, Eames et al.13 have considered the role of vacancies to rationalize ion migration in lead halide perovskites. Among estimated activation energies, I− (0.58 eV) was found to have a favorable energy for ion diffusion as compared to Pb2+ (2.31 eV) and MA+ (0.84 eV). Scheme 1 shows an extended model proposed by Eames et al.13, which depicts halide ion migration in halide-deficient perovskites, leading to iodide- and bromide-rich domains. The migration mechanism entails a series of small steps between adjacent vacancies to ultimately induce local phase segregation. With increasing halide ion deficiency (e.g. 1:2.7), the timescale needed to attain a stabilized segregated state increases. This is because migrating halide ions undergo more steps to associate with a Pb center. Even though the effective vacancy-to-vacancy spacing decreases, the overall scarcity of iodide and bromide anions in the film increases the overall distance required for a given halide ion to reach an existing iodide-rich or bromide-rich domain within the lattice. In the limit of highly halide deficient films (e.g. 1:2.4), phase segregation kinetics are slowed down to such an extent that phase segregation appears completely suppressed. In contrast, excess halide ions decrease halide migration timescales since there is increased availability of halide ions in the lattice. Consequently, despite decreasing vacancy availability for transport, the effective distance separating a given halide species from an existing iodide-rich or bromide-rich domain within the lattice decreases. Changes in kinetics are evidenced by comparing the


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12 observed segregation time scales of halide-deficient (1:2.7) films (ksegregation=0.33×10-2 s-1) versus those of halide-rich (1:5) films (ksegregation=2.15×10-2 s-1). Likewise, treating halide-deficient perovskites (1:2.4) with MAX promotes faster halide segregation kinetics (ksegregation=1.89×10-2 s-1). The difference in Br- and I- affinity towards Pb is another important aspect that must be taken into account when evaluating halide phase segregation. This is especially the case when excess halide ions are present. Earlier studies have confirmed the higher complexation affinity of Pb2+ and Br- relative to Pb2+ and I-.41 We therefore see predominant MAPbBr3 formation when concentrations of Br- and Iexceed that which is stoichiometric. Accordingly, we see a blue shift in the absorption of the 1:5 mixed halide perovskite film (Figure 1D), in contrast to the absorption of stoichiometric 1:3 films (Figure 1C). In TDA measurements (Figure 2D) the band edge bleach of 1:5 films red shifts ~35 nm, indicating a relatively small enrichment of iodide-rich phases. This behavior distinguishes itself from the prominent formation of I-rich phases in 1:2.7 (Figure 2B) and 1:3 (Figure 2C) films wherein both exhibit larger ~70 nm red shifts and growth of a distinct bleach around 700 nm. Halide concentration-dependent photosegregation thus highlights two key processes responsible for the selective migration of bromide and halide ions: (i) vacancy-influenced halide ion migration and (ii) the differing affinity of Pb2+ towards Br− and I−. Individual contributions from these two factors vary depending upon the actual Pb:halide ratio employed. While factors such as lattice distortion arising from polaron localization could facilitate halide ion migration, vacancies play a major role in achieving stable iodide- and bromide-rich phases following photoirradiation. Although mixed halide perovskite solar cell instabilities due to halide ion migration are of concern, there appears to be a silver lining here. By employing excess halide ions within films, halide ion segregation effects can be minimized. Efforts are therefore underway to establish factors responsible for dark recovery following halide ion segregation. ASSOCIATED CONTENT Supporting Information. Experimental methods including film preparation and various characterization results (UV-Visible absorption, photoluminescence spectroscopy, femtosecond transient absorption spectroscopy) are presented in the Supporting Information. AUTHOR INFORMATION Corresponding Author *Address correspondence to this author: [email protected] twitter: @kamatlabND Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We would like to thank Weixin Huang for his assistance in SEM measurements and Sergiu Draguta for his assistance in emission measurements. P.V.K. and S.J.Y acknowledge the support of the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy, through Award DE-FC02-04ER15533 for carrying out steady-state absorption and transient


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13 absorption measurements, and M.K. acknowledges the support of the Division of Materials Sciences and Engineering Office of Basic Energy Sciences of the U.S. Department of Energy through Award DESC0014334 for carrying out the analysis and discussion of results. This is contribution number NDRL No. 5172 from the Notre Dame Radiation Laboratory. References: 1. Bisquert, J.; Qi, Y.; Ma, T.; Yan, Y., Advances and Obstacles on Perovskite Solar Cell Research from Material Properties to Photovoltaic Function. ACS Energy Lett. 2017, 2, 520-523. 2. Manser, J. S.; Christians, J. A.; Kamat, P. V., Intriguing Optoelectronic Properties of Metal Halide Perovskites. Chem. Rev. 2016, 116, 12956–13008. 3. Giustino, F.; Snaith, H. J., Toward Lead-Free Perovskite Solar Cells. ACS Energy Lett. 2016, 1, 1233-1240. 4. Guo, Z.; Manser, J. S.; Wan, Y.; Kamat, P. V.; Huang, L., Spatial and Temporal Imaging of LongRange Charge Transport in Perovskite Thin Films by Ultrafast Microscopy. Nat Commun 2015, 6, Article No. 7471 5. Manser, J. S.; Kamat, P. V., Band Filling with Charge Carriers in Organometal Halide Perovskites. Nature Photonics 2014, 8, 737–743. 6. Stamplecoskie, K. G.; Manser, J. S.; Kamat, P. V., Dual Nature of the Excited State in OrganicInorganic Lead Halide Perovskites. Energy & Environ. Sci. 2015, 8, 208 - 215. 7. Johnston, M. B.; Herz, L. M., Hybrid Perovskites for Photovoltaics: Charge-Carrier Recombination, Diffusion, and Radiative Efficiencies. Acc. Chem. Res. 2016, 49, 146-154. 8. Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J. P.; Leijtens, T.; Herz, L. M.; Petrozza, A.; Snaith, H. J., Electron-Hole Diffusion Lengths Exceeding 1 Micrometer in an Organometal Trihalide Perovskite Absorber. Science 2013, 342, 341-344. 9. Deschler, F., et al., High Photoluminescence Efficiency and Optically Pumped Lasing in SolutionProcessed Mixed Halide Perovskite Semiconductors. J. Phys. Chem. Lett. 2014, 5, 1421-1426. 10. Zhumekenov, A. A., et al., Formamidinium Lead Halide Perovskite Crystals with Unprecedented Long Carrier Dynamics and Diffusion Length. ACS Energy Lett. 2016, 1, 32-37. 11. Colella, S., et al., MAPbI3-xClx Mixed Halide Perovskite for Hybrid Solar Cells: The Role of Chloride as Dopant on the Transport and Structural Properties. Chem. Mater. 2013, 25, 4613-4618. 12. Mosconi, E.; De Angelis, F., Mobile Ions in Organohalide Perovskites: Interplay of Electronic Structure and Dynamics. ACS Energy Lett. 2016, 182-188. 13. Eames, C.; Frost, J. M.; Barnes, P. R. F.; O'Regan, B. C.; Walsh, A.; Islam, M. S., Ionic Transport in Hybrid Lead Iodide Perovskite Solar Cells. Nat Commun 2015, 6, Article No. 8497 14. Brivio, F.; Caetano, C.; Walsh, A., Thermodynamic Origin of Photoinstability in the CH3NH3Pb(I1– xBrx)3 Hybrid Halide Perovskite Alloy. J. Phys. Chem. Lett. 2016, 7, 1083-1087. 15. Garcia-Belmonte, G.; Bisquert, J., Distinction between Capacitive and Noncapacitive Hysteretic Currents in Operation and Degradation of Perovskite Solar Cells. ACS Energy Lett. 2016, 1, 683-688. 16. Abdi-Jalebi, M., et al., Impact of Monovalent Cation Halide Additives on the Structural and Optoelectronic Properties of CH3NH3PbI3 Perovskite. Adv. Energy Mater. 2016, 6, Article No. 1502472. 17. Jesper Jacobsson, T.; Correa-Baena, J.-P.; Pazoki, M.; Saliba, M.; Schenk, K.; Gratzel, M.; Hagfeldt, A., Exploration of the Compositional Space for Mixed Lead Halogen Perovskites for High Efficiency Solar Cells. Energy & Environ. Sci. 2016, 9, 1706-1724. 18. Zhang, H.; Mao, J.; He, H. X.; Zhang, D.; Zhu, H. L.; Xie, F. X.; Wong, K. S.; Gratzel, M.; Choy, W. C. H., A Smooth CH3NH3PbI3 Film via a New Approach for Forming the PbI2 Nanostructure Together with Strategically High CH3NH3I Concentration for High Efficient Planar-Heterojunction Solar Cells. Adv. Energy Mater. 2015, 5, Article No. 1501354.


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14 19. Dar, M. I.; Abdi-Jalebi, M.; Arora, N.; Moehl, T.; Gratzel, M.; Nazeeruddin, M. K., Understanding the Impact of Bromide on the Photovoltaic Performance of CH3NH3PbI3 Solar Cells. Adv. Mater. 2015, 27, 7221-+. 20. McMeekin, D. P., et al., A Mixed Cation Lead Mixed Halide Perovskite Absorber for Tandem Solar Cells. Science 2016, 351, 151-155. 21. Klein-Kedem, N.; Cahen, D.; Hodes, G., Effects of Light and Electron Beam Irradiation on Halide Perovskites and Their Solar Cells. Acc. Chem. Res. 2016, 49, 347-354. 22. Sutter-Fella, C. M.; Li, Y.; Amani, M.; Ager, J. W.; Toma, F. M.; Yablonovitch, E.; Sharp, I. D.; Javey, A., High Photoluminescence Quantum Yield in Band Gap Tunable Bromide Containing Mixed Halide Perovskites. Nano Lett. 2016, 16, 800-806. 23. Sadhanala, A., et al., Blue-Green Color Tunable Solution Processable Organolead Chloride– Bromide Mixed Halide Perovskites for Optoelectronic Applications. Nano Lett. 2015, 15, 6095-6101. 24. Protesescu, L.; Yakunin, S.; Bodnarchuk, M. I.; Krieg, F.; Caputo, R.; Hendon, C. H.; Yang, R. X.; Walsh, A.; Kovalenko, M. V., Nanocrystals of Cesium Lead Halide Perovskites (CsPbX3, X = Cl, Br, and I): Novel Optoelectronic Materials Showing Bright Emission with Wide Color Gamut. Nano Lett. 2015, 15, 3692-3696. 25. Rosales, B. A.; Hanrahan, M. P.; Boote, B. W.; Rossini, A. J.; Smith, E. A.; Vela, J., Lead Halide Perovskites: Challenges and Opportunities in Advanced Synthesis and Spectroscopy. ACS Energy Lett. 2017, 2, doi: 10.1021/acsenergylett.1026b00674. 26. Christians, J. A.; Manser, J. S.; Kamat, P. V., Multifaceted Excited State of CH3NH3PbI3. Charge Separation, Recombination, and Trapping. J. Phys. Chem. Lett. 2015, 6, 2086-2095. 27. Dar, M. I.; Jacopin, G.; Hezam, M.; Arora, N.; Zakeeruddin, S. M.; Deveaud, B.; Nazeeruddin, M. K.; Gratzel, M., Asymmetric Cathodoluminescence Emission in CH3NH3PbI3-xBrx Perovskite Single Crystals. ACS Photonics 2016, 3, 947-952. 28. Pathak, S., et al., Perovskite Crystals for Tunable White Light Emission. Chem. Mater. 2015, 27, 8066-8075. 29. Dursun, I., et al., Perovskite Nanocrystals as a Color Converter for Visible Light Communication. ACS Photonics 2016, 3, 1150-1156. 30. Hoke, E. T.; Slotcavage, D. J.; Dohner, E. R.; Bowring, A. R.; Karunadasa, H. I.; McGehee, M. D., Reversible Photoinduced Trap Formation in Mixed-Halide Hybrid Perovskites for Photovoltaics. Chem. Sci. 2015, 6, 613-617. 31. Hentz, O.; Zhao, Z.; Gradečak, S., Impacts of Ion Segregation on Local Optical Properties in Mixed Halide Perovskite Films. Nano Lett. 2016, 16, 1485-1490. 32. Yoon, S. J.; Draguta, S.; Manser, J. S.; Sharia, O.; Schneider, W. F.; Kuno, M.; Kamat, P. V., Tracking Iodide and Bromide Ion Segregation in Mixed Halide Lead Perovskites during Photoirradiation. ACS Energy Lett. 2016, 290-296. 33. Slotcavage, D. J.; Karunadasa, H. I.; McGehee, M. D., Light-Induced Phase Segregation in HalidePerovskite Absorbers. ACS Energy Lett. 2016, 1199-1205. 34. Jeong, J., et al., Ternary Halide Perovskites for Highly Efficient Solution-Processed Hybrid Solar Cells. ACS Energy Lett. 2016, 1, 712-718. 35. Barker, A. J., et al., Defect-Assisted Photoinduced Halide Segregation in Mixed-Halide Perovskite Thin Films. ACS Energy Lett. 2017, 2, 1416–1424. 36. Yun, J. S., et al., Critical Role of Grain Boundaries for Ion Migration in Formamidinium and Methylammonium Lead Halide Perovskite Solar Cells. Adv. Energy Mater. 2016, 6, 1600330-n/a. 37. Wright, A. D.; Verdi, C.; Milot, R. L.; Eperon, G. E.; Pérez-Osorio, M. A.; Snaith, H. J.; Giustino, F.; Johnston, M. B.; Herz, L. M., Electron–Phonon Coupling in Hybrid Lead Halide Perovskites. Nat. Commun. 2016, 7, Article No. 11755.


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15 38. Neukirch, A. J., et al., Polaron Stabilization by Cooperative Lattice Distortion and Cation Rotations in Hybrid Perovskite Materials. Nano Lett. 2016, 16, 3809-3816. 39. Bischak, C. G.; Hetherington, C. L.; Wu, H.; Aloni, S.; Ogletree, D. F.; Limmer, D. T.; Ginsberg, N. S., Origin of Reversible Photoinduced Phase Separation in Hybrid Perovskites. Nano Lett. 2017, 17, 10281033. 40. Noh, J. H.; Im, S. H.; Heo, J. H.; Mandal, T. N.; Seok, S. I., Chemical Management for Colorful, Efficient, and Stable Inorganic–Organic Hybrid Nanostructured Solar Cells. Nano Lett. 2013, 13, 17641769. 41. Yoon, S. J.; Stamplecoskie, K. G.; Kamat, P. V., How Lead Halide Complex Chemistry Dictates the Composition of Mixed Halide Perovskites. J. Phys. Chem. Lett. 2016, 7, 1368-1373. 42. Hoffman, J. B.; Schleper, A. L.; Kamat, P. V., Transformation of Sintered CsPbBr3 Nanocrystals to Cubic CsPbI3 and Gradient CsPbBrxI3–x through Halide Exchange. J. Am. Chem. Soc. 2016, 138, 8603–8611. 43. Jacobsson, T. J.; Correa-Baen, J. P.; Pazoki, M.; Saliba, M.; Schenk, K.; Gratzel, M.; Hagfeldt, A., Exploration of the compositional space for mixed lead halogen perovskites for high efficiency solar cells. Energy & Environ. Sci. 2016, 9, 1706-1724.


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