Formamidinium Lead Halide Perovskite Crystals with Unprecedented

Formamidinium Lead Halide Perovskite Crystals with Unprecedented Long Carrier Dynamics and Diffusion Length Ayan A. Zhumekenov,†,⊥ Makhsud I. Saidaminov,†,⊥ Md Azimul Haque,‡ Erkki Alarousu,† Smritakshi Phukan Sarmah,† Banavoth Murali,† Ibrahim Dursun,† Xiao-He Miao,§ Ahmed L. Abdelhady,†,∥ Tom Wu,‡ Omar F. Mohammed,*,† and Osman M. Bakr*,† †

Division of Physical Sciences and Engineering, Solar and Photovoltaics Engineering Center, ‡Materials Science and Engineering Program, and §Imaging and Characterization Core Lab, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Kingdom of Saudi Arabia ∥ Department of Chemistry, Faculty of Science, Mansoura University, Mansoura 35516, Egypt S Supporting Information *

ABSTRACT: State-of-the-art perovskite solar cells with record efficiencies were achieved by replacing methylammonium (MA) with formamidinium (FA) in perovskite polycrystalline films. However, these films suffer from severe structural disorder and high density of traps; thus, the intrinsic properties of FA-based perovskites remain obscured. Here we report the detailed optical and electrical properties of FAPbX3 (where X = Br− and I−) single crystals. FAPbX3 crystals exhibited markedly enhanced transport compared not just to FAPbX3 polycrystalline films but also, surprisingly, to MAPbX3 single crystals. Particularly, FAPbBr3 crystals displayed a 5-fold longer carrier lifetime and 10-fold lower dark carrier concentration than those of MAPbBr3 single crystals. We report long carrier diffusion lengthsmuch longer than previously thoughtof 6.6 μm for FAPbI3 and 19.0 μm for FAPbBr3 crystals, the latter being one of the longest reported values in perovskite materials. These findings are of great importance for future integrated applications of these perovskites. that for MAPbI3 is limited to hundreds of nm.18 Furthermore, Hanusch et al. showed that the carrier lifetime in polycrystalline thin films of FAPbBr3 is 200 ns, while that for MAPbBr3 is limited to 17 ns.15 Yet despite the significance of these studies, they are limited by the intrinsically disordered form of the materials being investigated. For instance, while utilizing the same deposition techniques, the quality of FAPbX3 and MAPbX3 films (crystallinity, morphology, grain sizes, and grain boundaries) may still differ considerably due to the different nature of the precursors. The perovskite community is well aware of the tremendous effect of small additives to perovskite film quality,14,19,20 which can strongly affect the carrier recombination and transport properties. At this point, a critical question arises: is this observed superiority of FAPbX3 over MAPbX3 due to the difference in film quality or to the nature of perovskites, that is, due to their intrinsic properties?

A

n era of unlimited, free, and clean energy sounds more realistic today than ever since the invention of solar cells based on an inexpensive1−3 and solutionprocessed1,4−6 class of semiconductors−hybrid organic− inorganic perovskites (ABX3, where A = CH3NH3+ (MA) or NH2CHNH2+ (FA), B = Pb2+ or Sn2+, and X = Cl−, Br−, or I−). MAPbX3 particularly showed phenomenal success in photovoltaics, inspiring researchers to utilize them for devising ultrasensitive photodetectors,7,8 ultralow-threshold lasers,9 and ultrabright light-emitting diodes.10,11 The state-of-the-art perovskite solar cells are, however, based on FAPbI3, a sister compound of MAPbI3. Initially, the perovskite community was inspired by the lower band gap of FAPbI3.12−14 Later the better stability of FAPbX3, an essential requirement for mass production and reliable performance, was demonstrated.14,15 Moreover, FAPbX3 polycrystalline films showed surprisingly enhanced carrier transport properties compared to MAPbX3 films.15,16 Recently, Rehman et al. showed that the carrier diffusion length of polycrystalline thin films of FAPbI3 is 3.1 μm,17 while © XXXX American Chemical Society

Received: March 3, 2016 Accepted: April 13, 2016

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DOI: 10.1021/acsenergylett.6b00002 ACS Energy Lett. 2016, 1, 32−37

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http://pubs.acs.org/journal/aelccp

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ACS Energy Letters

Figure 1. Powder XRD patterns of ground (A) FAPbBr3 and (B) FAPbI3 crystals. (Insets) Pictures of the corresponding single crystals.

obtained by converting the reflectance data using the Kubelka− Munk equation:24 f(R) = (1 − R)2/(2R). In contrast to perovskite polycrystalline films,15,17 absorption profiles for FAPbX3 single crystals appear flat with a clear band edge cutoff with no excitonic signature (Figure 2A and B), suggesting a minimal density of in-gap defect states. This observation is in line with flat absorption curves for other perovskite single crystals reported independently by other groups as well.25−28 The optical band gaps estimated from the corresponding Tauc plots show values of 2.15 and 1.41 eV for FAPbBr3 and FAPbI3 crystals, respectively, which is in good agreement with previous reports.21 The photoluminescence (PL) peaks of FAPbBr3 and FAPbI3 single crystals are located at 587 and 843 nm, respectively (Figure 2A and B). Noteworthy, the PL peak for FAPbX3 single crystals is remarkably red-shifted compared to that in the polycrystalline films located at ∼550 nm for FAPbBr3 and ∼810 nm for FAPbI3.12,14 This observed spectral shift is consistent with previous reports29−31 and is attributed to a high degree of order and low density of defects in single crystals.30,32 From photoelectron spectroscopy in air (PESA), we estimated the valence band maxima (VBM) of FAPbX3 crystals to be situated at −5.74 and at −5.63 eV from the vacuum level for FAPbBr3 and FAPbI3, respectively (Figure 2C and D). By combining these values with the corresponding optical band gaps, we deduce the conduction band minimum (CBM) to be at −3.59 eV for FAPbBr3 and −4.22 eV for FAPbI3 (insets of Figure 2C and D). 33 The band alignment of FAPbX3 demonstrated here is of interest for engineering optoelectronic devices, for example, to choose a suitable electron or hole transporting materials for solar cells.34 Furthermore, it is also critical for choosing an appropriate metal electrode to study the carrier transport properties of FAPbX3 single crystals. We then investigated the key semiconducting parameters such as trap density nt, carrier mobility μ, carrier lifetime τ, and carrier diffusion length LD. We estimated the FAPbX3 trap density and hole carrier mobility by employing the space-charge-limited current (SCLC) technique.35,36 Typically, the crystals were sandwiched between two gold electrodes to form hole-only devices, and then the dark current of these devices was measured under applied bias. Figure 3A and B displays the current−voltage characteristics for the hole-only devices, which clearly show three regions of behavior.

This fundamental concern can be addressed only through a study of both materials in their single-crystal form, which represents the highest degree of crystallinity unburdened by morphological effects and grain boundaries. Single crystals provide an ideal platform to uncover the limit of charge carrier dynamics. In this work, we grew FAPbBr3 and FAPbI3 single crystals by inverse temperature crystallization (ITC)21 and studied their charge carrier dynamics. The high quality of crystals and minimum structural defects enabled us to investigate the intrinsic properties of the materials. Along with the band alignment, we demonstrated the key recombination and charge transport parameters, such as carrier mobility, lifetime, and diffusion length. By comparing these merits in FAPbX3 with those in MAPbX3 single crystals, we revealed the major role of the organic cation in the carrier dynamics. These findings are essential for utilization of FAPbX3 in practical applications, such as in photodetectors, solar cells, or light-emitting diodes. We grew FAPbBr3 and FAPbI3 single crystals of 4−5 mm size (insets of Figure 1A and B) by the ITC technique.21 Powder X-ray diffraction (XRD) of the ground FAPbX3 crystals confirmed the single phases of FAPbBr 3 and α-FAPbI3 perovskites (Figure 1A and B). The space group (Pm-3m for both materials) and unit cell parameters (a = 5.9944 Å for FAPbBr3 and a = 6.3573 Å for α-FAPbI3) measured by singlecrystal XRD (Table S1) were found to be consistent with those from previous reports,15,22 indicating the single-crystalline nature of the crystals. FAPbI3 crystallizes in its black α-phase at high temperatures. 12,21 At room temperature, the black α-FAPbI 3 spontaneously transforms to a more stable yellow δFAPbI3.12,23 We noticed that the black-to-yellow transformation starts from the bulk of the crystals and not from the surface (Figure S1A,B). To investigate the stability of αFAPbI3, we ground a freshly grown black crystal and divided it into two parts. The first part was kept at room temperature. The second part was kept at 185 °C for 1 h to stabilize the black phase, as reported earlier.23 Then we monitored the change of the powder color with time. Figure S1C,D shows that the first part completely becomes yellow within 7 h, while the second part remains in the black phase for ∼7 days. Therefore, for subsequent studies, we used only crystals stabilized at 185 °C. Next, we studied optical properties of the synthesized FAPbX3 (X = Br− and I−) crystals. The absorption spectra were 33

DOI: 10.1021/acsenergylett.6b00002 ACS Energy Lett. 2016, 1, 32−37

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Figure 2. Steady-state absorption of (A) FAPbBr3 and (B) FAPbI3 single crystals. (Right insets) PL spectra; (left insets) corresponding Tauc plots determining the extrapolated optical band gaps. PESA measurements showing VBM at −5.74 eV for (C) FAPbBr3 and −5.63 eV for (D) FAPbI3. (Insets) Band alignments in the corresponding crystals. Energy levels are expressed from the vacuum, which is set at zero.

thin films (14 cm2 V−1 s−1 for FAPbBr3 and 27 cm2 V−1 s−1 for FAPbI3).17 This finding is consistent with previous reports on MAPbX3 single crystals, which also have larger mobility values compared to their polycrystalline film counterparts.29,30 The recently reported low mobility in FAPbI3 single crystals is likely related to the instability of the material and the quality of the metal−perovskite interface combined with the synthetic procedure.23 The free carrier density (nC) of the crystals can be estimated by relating the conductivity and mobility using the expression nC = σ/μe. A value of 1.5 × 109 cm−3 was obtained as the free carrier density for FAPbBr3 crystals, and 3.9 × 109 cm−3 was obtained for FAPbI3. We note that the conductivity and mobility of FAPbX3 crystals are comparable to values reported for MAPbX3 crystals.29,30 However, to our surprise, two other key semiconductor characteristics considerably differ from those of MAPbX3 crystals. First, FAPbX3 crystals showed 1 order of magnitude lower dark charge carrier concentration than MAPbX3 crystals.30 The low dark carrier concentration is critical for high-performance photodetectors and solar cells. Second, the trap density of FAPbBr3 crystals is ∼3-fold lower than that of MAPbBr3 crystals grown by the same technique.29 By contrast, FAPbI3 and MAPbI3 crystals have comparable trap densities.29 The trap state density in FAPbBr3 is markedly low, even for hybrid perovskites.26 We hypothesized that this low trap density should manifest in a longer carrier lifetime. To validate our hypothesis, we studied the recombination dynamics of FAPbBr3 single crystals.

The first region at low bias ( 3, blue lines), and a Child’s regime at high bias (I−V2, n = 2, green lines). (C) PL time-decay trace of FAPbBr3 crystals monitored at λ = 580 nm after 800 nm excitation, with biexponential fit revealing fast (τ1 ≈ 687 ns, pink curve) and slow (τ2 ≈ 2272 ns, blue curve) components. (Inset) Comparison of PL decay traces of FAPbBr3 and MAPbBr3 crystals under the same pump fluence. (D) HR-XRD rocking curves at the (001) plane of FAPbBr3 (red) and MAPbBr3 (black) single crystals.

We studied the recombination dynamics of photoexcited species in FAPbBr3 crystals by time-resolved PL spectroscopy. Detailed information regarding the experimental setup has been published elsewhere.39,40 To investigate the carrier dynamics of the bulk of crystal deeper, two-photon excitation was used.41 The PL time decay trace was monitored at 580 nm after 800 nm excitation with two different pump fluences (7 and 20 μJ/ cm2). In agreement with previous reports, the measured PL lifetime values were found to be inversely proportional to the fluence (Figure 3C and Figure S2).42−44 For a fluence of 7 μJ/cm2, the biexponential fit of the PL decay for FAPbBr3 crystals reveals two different carrier dynamics with characteristic time constants of 687 and 2272 ns (Figure 3C). We propose that these fast and slow time components are related to the different recombination mechanisms on the surface and in the bulk of the crystals, respectively.23,30 The PL measurements for MAPbBr3 single crystals carried out under the same pump fluence (7 μJ/cm2) resulted in significantly shorter carrier lifetimes (τ1 ≈ 46 ns and τ2 ≈ 415 ns for a fast and slow components; see the inset of Figure 3C). Interestingly, a similar trend was also observed in polycrystalline films of these two materials.15 Unfortunately, we were not able to perform time-resolved PL for FAPbI3 single crystals due to our PL detection limitations. Therefore, for future calculations, we used the values reported by Han et al. of 32 and 484 ns for a fast and slow components of the carrier lifetime, respectively.23

Combining the obtained values of carrier mobility and carrier lifetime, the diffusion length was estimated using the following relation

LD =

kBT × μτ e

(3)

where kB is Boltzman’s constant and T is temperature. The lifetimes corresponding to a slow transient give the bestcase diffusion length of 19.0 μm for FAPbBr3 and 6.6 μm for FAPbI3. The worst-case diffusion lengths, derived from the lifetimes of a fast transient, are 10.5 μm for FAPbBr3 and 1.7 μm for FAPbI3. Thus, FAPbX3 single crystals outperform polycrystalline film counterparts (1.3 μm for FAPbBr3 and 3.1 μm for FAPbI3),17 which is consistent with previous reports on other hybrid perovskite single crystals.29,30 The obtained diffusion length of 19 μm in FAPbBr3 crystals is ∼4 times longer than that in the MAPbBr3 analogue (4.3 μm29). On the other hand, FAPbI3 and MAPbI3 crystals have comparable diffusion lengths. It is worth noting that FAPbI3 and MAPbI3 adopt different crystal symmetries of cubic22 and tetragonal,12 respectively, while both FAPbBr3 and MAPbBr3 perovskites adopt the same cubic symmetry with minor differences in unit cell parameters.15 Therefore, the effect of the organic cation on the properties of these materials can be understood by comparing FAPbBr3 and MAPbBr3. 35

DOI: 10.1021/acsenergylett.6b00002 ACS Energy Lett. 2016, 1, 32−37

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ACS Energy Letters Notes

We performed high-resolution X-ray diffraction (HR XRD) measurements on FAPbBr3 and MAPbBr3 single crystals. The full width at half-maxima (fwhm) of rocking curves at the (001) diffraction plane of both materials are comparable with each other (0.050° for FAPbBr3 and 0.048° for MAPbBr3; see Figure 3D). Thus, we eliminate the effect of strains on their carrier recombination and transport properties. Hence, we suggest that the observed superior properties of FAPbBr3 over MAPbBr3 in either single crystals or polycrystalline thin films are primarily a consequence of the intrinsic properties of these materials. As we expected, the intrinsically lower trap density in FAPbBr3 resulted in a longer lifetime of photoexcited species. The observed effect of the organic cation on the carrier recombination and transport properties can also be explained through polar domains.45−47 The junctions between the domains are hypothesized to facilitate charge separation and reduce the recombination of carriers, serving as channels for their segregated transfer.45,46,48 Interestingly, although the dipole moment of FA is much lower than that of MA,45 theoretical simulations show that due to its size, the FA cation has more limited rotational freedom within the inorganic cage, likely inducing the formation of more efficient polar domains.45,49,50 In summary, we reported the optical and carrier transport properties of FAPbBr3 and FAPbI3 single crystals. Charge transport parameters such as carrier mobilities and trap densities were estimated. The values for both FAPbBr3 and FAPbI3 crystals were found to be superior to their polycrystalline thin film counterparts and MAPbX3 single crystals. In particular, we revealed that FAPbBr3 crystals exhibit 1 order of magnitude lower dark charge carrier concentration and 5-fold longer lifetime of photoexcited species than those of MAPbBr3 crystals. The optical properties of the FAPbX3 single crystals and the diffusion length, such as the 19 μm for FAPbBr3 single crystals, make them an ideal candidate not just for tandem solar cells but also for high open-circuit voltage single-junction solar cells. The remarkable properties of FAPbX3 single crystals reported here could motivate researchers to utilize them for high-performance solution-processed optoelectronic applications.

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The authors declare no competing financial interest.

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(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) Kim, H.-S.; Lee, C.-R.; Im, J.-H.; Lee, K.-B.; Moehl, T.; Marchioro, A.; Moon, S.-J.; Humphry-Baker, R.; Yum, J.-H.; Moser, J. E.; et al. Lead Iodide Perovskite Sensitized All-Solid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%. Sci. Rep. 2012, 2, 591. (3) Snaith, H. J. Perovskites: The Emergence of a New Era for LowCost, High-Efficiency Solar Cells. J. Phys. Chem. Lett. 2013, 4, 3623− 3630. (4) Burschka, J.; Pellet, N.; Moon, S.-J.; Humphry-Baker, R.; Gao, P.; Nazeeruddin, M. K.; Gratzel, M. Sequential Deposition as a Route to High-Performance Perovskite-Sensitized Solar Cells. Nature 2013, 499, 316−319. (5) Liu, D.; Kelly, T. L. Perovskite Solar Cells with a Planar Heterojunction Structure Prepared Using Room-Temperature Solution Processing Techniques. Nat. Photonics 2013, 8, 133−138. (6) Peng, W.; Wang, L.; Murali, B.; Ho, K.-T.; Bera, A.; Cho, N.; Kang, C.-F.; Burlakov, V. M.; Pan, J.; Sinatra, L.; et al. Solution-Grown Monocrystalline Hybrid Perovskite Films for Hole-Transporter-Free Solar Cells. Adv. Mater. 2016, DOI: 10.1002/adma.201506292. (7) Saidaminov, M. I.; Adinolfi, V.; Comin, R.; Abdelhady, A. L.; Peng, W.; Dursun, I.; Yuan, M.; Hoogland, S.; Sargent, E. H.; Bakr, O. M. Planar-Integrated Single-Crystalline Perovskite Photodetectors. Nat. Commun. 2015, 6, 8724. (8) Lian, Z.; Yan, Q.; Lv, Q.; Wang, Y.; Liu, L.; Zhang, L.; Pan, S.; Li, Q.; Wang, L.; Sun, J.-L. High-Performance Planar-Type Photodetector on (100) Facet of MAPbI3 Single Crystal. Sci. Rep. 2015, 5, 16563. (9) Zhu, H.; Fu, Y.; Meng, F.; Wu, X.; Gong, Z.; Ding, Q.; Gustafsson, M. V.; Trinh, M. T.; Jin, S.; Zhu, X. Y. Lead Halide Perovskite Nanowire Lasers with Low Lasing Thresholds and High Quality Factors. Nat. Mater. 2015, 14, 636−642. (10) Cho, H.; Jeong, S.-H.; Park, M.-H.; Kim, Y.-H.; Wolf, C.; Lee, C.-L.; Heo, J. H.; Sadhanala, A.; Myoung, N.; Yoo, S.; et al. Overcoming the Electroluminescence Efficiency Limitations of Perovskite Light-Emitting Diodes. Science 2015, 350, 1222−1225. (11) Tan, Z.-K.; Moghaddam, R. S.; Lai, M. L.; Docampo, P.; Higler, R.; Deschler, F.; Price, M.; Sadhanala, A.; Pazos, L. M.; Credgington, D.; et al. Bright Light-Emitting Diodes Based on Organometal Halide Perovskite. Nat. Nanotechnol. 2014, 9, 687−692. (12) Stoumpos, C. C.; Malliakas, C. D.; Kanatzidis, M. G. Semiconducting Tin and Lead Iodide Perovskites with Organic Cations: Phase Transitions, High Mobilities, and near-Infrared Photoluminescent Properties. Inorg. Chem. 2013, 52, 9019−9038. (13) Koh, T. M.; Fu, K.; Fang, Y.; Chen, S.; Sum, T. C.; Mathews, N.; Mhaisalkar, S. G.; Boix, P. P.; Baikie, T. Formamidinium-Containing Metal-Halide: An Alternative Material for near-IR Absorption Perovskite Solar Cells. J. Phys. Chem. C 2014, 118, 16458−16462. (14) Eperon, G. E.; Stranks, S. D.; Menelaou, C.; Johnston, M. B.; Herz, L. M.; Snaith, H. J. Formamidinium Lead Trihalide: A Broadly Tunable Perovskite for Efficient Planar Heterojunction Solar Cells. Energy Environ. Sci. 2014, 7, 982−988. (15) Hanusch, F. C.; Wiesenmayer, E.; Mankel, E.; Binek, A.; Angloher, P.; Fraunhofer, C.; Giesbrecht, N.; Feckl, J. M.; Jaegermann, W.; Johrendt, D.; et al. Efficient Planar Heterojunction Perovskite Solar Cells Based on Formamidinium Lead Bromide. J. Phys. Chem. Lett. 2014, 5, 2791−2795. (16) Pellet, N.; Gao, P.; Gregori, G.; Yang, T.-Y.; Nazeeruddin, M. K.; Maier, J.; Grätzel, M. Mixed-Organic-Cation Perovskite Photovoltaics for Enhanced Solar-Light Harvesting. Angew. Chem., Int. Ed. 2014, 53, 3151−3157. (17) Rehman, W.; Milot, R. L.; Eperon, G. E.; Wehrenfennig, C.; Boland, J. L.; Snaith, H. J.; Johnston, M. B.; Herz, L. M. Charge-

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.6b00002. Details of all experimental procedures. Results of the SC XRD and PL lifetime. Details of thermal stabilization of FAPbI3 crystals (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (O.F.M.). *E-mail: [email protected] (O.M.B.). Author Contributions ⊥

A.A.Z. and M.I.S. contributed equally to this work.

Funding

The authors acknowledge the support of King Abdullah University of Science and Technology (KAUST) and Saudi Arabia Basic Industries Corporation (SABIC). 36

DOI: 10.1021/acsenergylett.6b00002 ACS Energy Lett. 2016, 1, 32−37

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ACS Energy Letters Carrier Dynamics and Mobilities in Formamidinium Lead MixedHalide Perovskites. Adv. Mater. 2015, 27, 7938−7944. (18) 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. (19) Jeon, Y.-J.; Lee, S.; Kang, R.; Kim, J.-E.; Yeo, J.-S.; Lee, S.-H.; Kim, S.-S.; Yun, J.-M.; Kim, D.-Y. Planar Heterojunction Perovskite Solar Cells with Superior Reproducibility. Sci. Rep. 2014, 4, 6953. (20) Liang, P.-W.; Liao, C.-Y.; Chueh, C.-C.; Zuo, F.; Williams, S. T.; Xin, X.-K.; Lin, J.; Jen, A. K. Y. Additive Enhanced Crystallization of Solution-Processed Perovskite for Highly Efficient Planar-Heterojunction Solar Cells. Adv. Mater. 2014, 26, 3748−3754. (21) Saidaminov, M. I.; Abdelhady, A. L.; Maculan, G.; Bakr, O. M. Retrograde Solubility of Formamidinium and Methylammonium Lead Halide Perovskites Enabling Rapid Single Crystal Growth. Chem. Commun. 2015, 51, 17658−17661. (22) Weller, M. T.; Weber, O. J.; Frost, J. M.; Walsh, A. Cubic Perovskite Structure of Black Formamidinium Lead Iodide, α[HC(NH2)2]PbI3, at 298 K. J. Phys. Chem. Lett. 2015, 6, 3209−3212. (23) Han, Q.; Bae, S. H.; Sun, P.; Hsieh, Y. T.; Yang, Y. M.; Rim, Y. S.; Zhao, H.; Chen, Q.; Shi, W.; Li, G.; et al. Single Crystal Formamidinium Lead Iodide (FAPbI3): Insight into the Structural, Optical, and Electrical Properties. Adv. Mater. 2016, 28, 2253−2258. (24) Kubelka, P.; Munk, F. An Article on Optics of Paint Layers. Z. Technol. Phys. 1931, 12.593609 (25) Bi, Y.; Hutter, E. M.; Fang, Y.; Dong, Q.; Huang, J.; Savenije, T. J. Charge Carrier Lifetimes Exceeding 15 μs in Methylammonium Lead Iodide Single Crystals. J. Phys. Chem. Lett. 2016, 7, 923−928. (26) Adinolfi, V.; Yuan, M.; Comin, R.; Thibau, E. S.; Shi, D.; Saidaminov, M. I.; Kanjanaboos, P.; Kopilovic, D.; Hoogland, S.; Lu, Z.-H.; et al. The In-Gap Electronic State Spectrum of Methylammonium Lead Iodide Single-Crystal Perovskites. Adv. Mater. 2016, DOI: 10.1002/adma.201505162. (27) Stoumpos, C. C.; Malliakas, C. D.; Peters, J. A.; Liu, Z.; Sebastian, M.; Im, J.; Chasapis, T. C.; Wibowo, A. C.; Chung, D. Y.; Freeman, A. J.; et al. Crystal Growth of the Perovskite Semiconductor CsPbBr3: A New Material for High-Energy Radiation Detection. Cryst. Growth Des. 2013, 13, 2722−2727. (28) Yang, D.; Xie, C.; Sun, J.; Zhu, H.; Xu, X.; You, P.; Lau, S. P.; Yan, F.; Yu, S. F. Amplified Spontaneous Emission from Organic− Inorganic Hybrid Lead Iodide Perovskite Single Crystals under Direct Multiphoton Excitation. Adv. Opt. Mater. 2016, DOI: 10.1002/ adom.201600047. (29) Saidaminov, M. I.; Abdelhady, A. L.; Murali, B.; Alarousu, E.; Burlakov, V. M.; Peng, W.; Dursun, I.; Wang, L.; He, Y.; Maculan, G.; et al. High-Quality Bulk Hybrid Perovskite Single Crystals within Minutes by Inverse Temperature Crystallization. Nat. Commun. 2015, 6, 7586. (30) Shi, D.; Adinolfi, V.; Comin, R.; Yuan, M.; Alarousu, E.; Buin, A.; Chen, Y.; Hoogland, S.; Rothenberger, A.; Katsiev, K.; et al. Low Trap-State Density and Long Carrier Diffusion in Organolead Trihalide Perovskite Single Crystals. Science 2015, 347, 519−522. (31) Maculan, G.; Sheikh, A. D.; Abdelhady, A. L.; Saidaminov, M. I.; Haque, M. A.; Murali, B.; Alarousu, E.; Mohammed, O. F.; Wu, T.; Bakr, O. M. CH3NH3PbCl3 Single Crystals: Inverse Temperature Crystallization and Visible-Blind UV-Photodetector. J. Phys. Chem. Lett. 2015, 6, 3781−3786. (32) Grancini, G.; D’Innocenzo, V.; Dohner, E. R.; Martino, N.; Srimath Kandada, A. R.; Mosconi, E.; De Angelis, F.; Karunadasa, H. I.; Hoke, E. T.; Petrozza, A. CH3NH3PbI3 Perovskite Single Crystals: Surface Photophysics and Their Interaction with the Environment. Chem. Sci. 2015, 6, 7305−7310. (33) Abdelhady, A. L.; Saidaminov, M. I.; Murali, B.; Adinolfi, V.; Voznyy, O.; Katsiev, K.; Alarousu, E.; Comin, R.; Dursun, I.; Sinatra, L.; et al. Heterovalent Dopant Incorporation for Bandgap and Type Engineering of Perovskite Crystals. J. Phys. Chem. Lett. 2016, 7, 295− 301.

(34) Liu, D.; Yang, J.; Kelly, T. L. Compact Layer Free Perovskite Solar Cells with 13.5% Efficiency. J. Am. Chem. Soc. 2014, 136, 17116− 17122. (35) Rose, A. Space-Charge-Limited Currents in Solids. Phys. Rev. 1955, 97, 1538−1544. (36) Smith, R. W.; Rose, A. Space-Charge-Limited Currents in Single Crystals of Cadmium Sulfide. Phys. Rev. 1955, 97, 1531−1537. (37) Lampert, M. A. Simplified Theory of Space-Charge-Limited Currents in an Insulator with Traps. Phys. Rev. 1956, 103, 1648−1656. (38) Mott, N. F.; Gurney, R. W. Electronic Processes in Ionic Crystals; Clarendon Press, 1948. (39) Pan, J.; Sarmah, S. P.; Murali, B.; Dursun, I.; Peng, W.; Parida, M. R.; Liu, J.; Sinatra, L.; Alyami, N.; Zhao, C.; et al. Air-Stable Surface-Passivated Perovskite Quantum Dots for Ultra-Robust, Singleand Two-Photon-Induced Amplified Spontaneous Emission. J. Phys. Chem. Lett. 2015, 6, 5027−5033. (40) Mohammed, O. F.; Xiao, D.; Batista, V. S.; Nibbering, E. T. J. Excited-State Intramolecular Hydrogen Transfer (ESIHT) of 1,8Dihydroxy-9,10-Anthraquinone (DHAQ) Characterized by Ultrafast Electronic and Vibrational Spectroscopy and Computational Modeling. J. Phys. Chem. A 2014, 118, 3090−3099. (41) Walters, G.; Sutherland, B. R.; Hoogland, S.; Shi, D.; Comin, R.; Sellan, D. P.; Bakr, O. M.; Sargent, E. H. Two-Photon Absorption in Organometallic Bromide Perovskites. ACS Nano 2015, 9, 9340−9346. (42) 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, 034007. (43) 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. (44) Stewart, R. J.; Grieco, C.; Larsen, A. V.; Maier, J. J.; Asbury, J. B. Approaching Bulk Carrier Dynamics in Organo-Halide Perovskite Nanocrystalline Films by Surface Passivation. J. Phys. Chem. Lett. 2016, 7, 1148−1153. (45) Frost, J. M.; Butler, K. T.; Brivio, F.; Hendon, C. H.; van Schilfgaarde, M.; Walsh, A. Atomistic Origins of High-Performance in Hybrid Halide Perovskite Solar Cells. Nano Lett. 2014, 14, 2584− 2590. (46) Liu, S.; Zheng, F.; Koocher, N. Z.; Takenaka, H.; Wang, F.; Rappe, A. M. Ferroelectric Domain Wall Induced Band Gap Reduction and Charge Separation in Organometal Halide Perovskites. J. Phys. Chem. Lett. 2015, 6, 693−699. (47) Egger, D. A.; Rappe, A. M.; Kronik, L. Hybrid OrganicInorganic Perovskites on the Move. Acc. Chem. Res. 2016, 49, 573− 581. (48) Sherkar, T. S.; Jan Anton Koster, L. Can Ferroelectric Polarization Explain the High Performance of Hybrid Halide Perovskite Solar Cells? Phys. Chem. Chem. Phys. 2016, 18, 331−338. (49) Frost, J. M.; Butler, K. T.; Walsh, A. Molecular Ferroelectric Contributions to Anomalous Hysteresis in Hybrid Perovskite Solar Cells. APL Mater. 2014, 2, 081506. (50) Zhou, Y.; Huang, F.; Cheng, Y.-B.; Gray-Weale, A. Photovoltaic Performance and the Energy Landscape of CH3NH3PbI3. Phys. Chem. Chem. Phys. 2015, 17, 22604−22615.

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DOI: 10.1021/acsenergylett.6b00002 ACS Energy Lett. 2016, 1, 32−37