Strong Photocurrent Amplification in Perovskite Solar Cells with a

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Letter pubs.acs.org/JPCL

Strong Photocurrent Amplification in Perovskite Solar Cells with a Porous TiO2 Blocking Layer under Reverse Bias Thomas Moehl,*,† Jeong Hyeok Im,†,‡ Yong Hui Lee,† Konrad Domanski,† Fabrizio Giordano,† Shaik M. Zakeeruddin,† M. Ibrahim Dar,† Leo-Philipp Heiniger,† Mohammad Khaja Nazeeruddin,† Nam-Gyu Park,‡ and Michael Graẗ zel† Laboratory of Photonics and Interfaces, Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne (EPFL), Station 6, CH-1015 Lausanne, Switzerland ‡ School of Chemical Engineering and Department of Energy Science, Sungkyunkwan University (SKKU), Suwon 440-746, Korea †

S Supporting Information *

ABSTRACT: We investigate two different types of TiO2 blocking layer (BL) deposition techniques commonly used in solid-state methylammonium lead triiodide perovskite (MaPbI3)-based solar cells. Although these BLs lead to similar photovoltaic device performance, their structure and blocking capability is actually very different. In one case, the “blocking” layer is porous, allowing an intimate contact of the perovskite with the fluorine-doped tin-dioxide (FTO)-covered glass substrate serving as transparent electron collector. This interface between the perovskite and the FTO shows rectifying behavior. Reverse biasing of such a solar cell allows the determination of the valence-band position of the MaPbI3 and the theoretical maximum attainable photovoltage. We show that under reverse bias strong photocurrent amplification is observed, permitting the cell to work as a high-gain photodetector at low voltage. Without BL, the solar-cell performance decreased, but the photocurrent amplification increased. At 1 V reverse bias, the photocurrent amplification is above a factor of 10 for AM 1.5 solar light and over 100 for lower light intensities. SECTION: Energy Conversion and Storage; Energy and Charge Transport

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electrons injected into the PA with the oxidized redox species. Several studies have been performed to quantify the role of BLs in DSCs,9−12 produced by various deposition methods such as spray pyrolysis,13−15 atomic layer deposition,8 sol−gel,10,16 spin coating,17 DC-magnetron sputtering,18,19 or electrochemical deposition.20 In many cases, cracks and pinholes can appear in the BL under the heat treatment applied subsequent to its deposition due to further crystallization and thermal stress (especially after the deposition and sintering of the superimposed mesoporous TiO2 or Al2O3 scaffold). In particular, thin layers deposited at room temperature tend to show cracks after sintering them with the mesoporous scaffold on top. The formation of such cracks and pinholes leads to a reduced holeblocking capability of such layers. In this study, we compare the properties of two commonly used types of BLs one made by spray pyrolysis, which leads to good coverage and blocking action and the other by spin coating, as reported by Im et al.21 We show that the latter is highly porous, therefore enabling an intimate contact between the FTO and the perovskite. The BLs were tested in complete solar cell devices with two different TiO2 nanoparticles as

ethylammonium lead halide perovskite-based solar cells have recently attracted great attention due to their ease of preparation and high power conversion efficiencies (PCEs).1−4 From the first application of perovskites in dyesensitized solar cells employing liquid electrolytes5,6 to the implementation into solid state organic−inorganic devices, the PCEs of perovskite-based solar cells have steeply increased.7 Even though most of the device types presented in the literature are solution-processed, high photovoltages, close to 1.2 V, have been reached with the band gap of the methylammonium lead triiodide being ∼1.55 eV. For a solution deposition technique, this high photovoltage is exceptional, fueling hopes for a cheap alternative to silicon solar cells. Traditionally, one of the important components of highefficiency perovskite solar cells is a high-quality compact TiO2 hole blocking layer (BL) acting as electron selective contact at the photoanode (PA). Normally, it is assumed that the layer should be 10−100 nm thick, electrically conductive, covering the substrate in a conformal fashion, and without pinholes. Any contact between the hole transport material (HTM) and the FTO should be avoided because it will electrically shunt the device, leading to a loss of PCE. BLs play a similar role in, for example, DSCs employing cobalt-based redox electrolytes8 and also in iodine-based DSCs for low-light applications to prevent the recombination of © 2014 American Chemical Society

Received: September 25, 2014 Accepted: October 23, 2014 Published: October 23, 2014 3931

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contrast, the BL made by spin coating is ∼40 nm thick, forming a porous structure on top of the FTO (Figure 1b). The samples for the SEM analysis with the spin-coated BLs were heated for 30 min at 550 °C, as is a common practice for preparing a mesoscopic PA for perovskite-based PV devices. We note that the spin coating normally results in the formation of amorphous TiOx films, which on heating crystallize, resulting in the formation of pores and cracks. From the top view shown in Figure 1c, one can indeed see the porous nature of the spincoated BL. (For additional SEM pictures of the spin-coated BL, see also Figure SI 2 in the Supporting Information.) From the morphology of the BLs, one would expect a low blocking capability for the spin-coated samples. For clarification, we performed CV measurements in aqueous electrolyte containing [Fe(CN)6]3−/[Fe(CN)6]4− as redox probe to assess the blocking effect of the BL toward charge transfer across the FTO (TiO2)/electrolyte interface. Because the redox probe cannot access the FTO through the compact BL, one can monitor in this fashion the presence of uncovered FTO. Thus, the electrochemical response gives a quantitative indication of the FTO fraction exposed to the electrolyte.11 Figure 2 presents the CVs of the different electrodes. Clearly, the charge transfer at the FTO is suppressed after the

mesoporous scaffold: the commonly used Dyesol 18 NRT as well as with the rutile particles as produced by Im et al.21 Interestingly, even the solar-cell devices made with the porous BL reached high PCEs. We find that when devices employing a porous TiO2 BL are polarized in reverse bias, holes are injected in the hole conductor via the pervoskite. The onset potential for this hole injection current reveals the energetic position of the valence band of the MaPbI3. We show that this enables the in situ determination of the maximum attainable photovoltage in devices with such architecture. Strikingly high photocurrents exceeding 100 mA/cm2 can be drawn in this reverse-bias regime, and the device shows high photocurrent amplification. The properties of the MaPbI3 with its wide absorption spectra, high absorption coefficient, facile, and low-cost production make this material not only a valuable candidate for low-cost high efficiency solar cells but also for low-cost photodetectors showing high gain at low reverse-bias voltage. Results and Discussion. Two types of TiO2 BLs were prepared: one by spray pyrolysis of titanium diisopropoxide bis(acetylacetonate) (TAA) and the other by spin coating of a TiO2 precursor solution (also containing TAA) onto FTO. The BLs were then analyzed with scanning electron microscopy (SEM), cyclic voltammetry (CV), and as a part of a complete photodiode. Figure 1 shows SEM pictures of the different BLs. From Figure 1a, the BL made by spray pyrolysis is ∼20 nm thick, exhibiting a compact morphology with small dots on its top. (See also Figure SI 1 in the Supporting Information.) In

Figure 2. Cyclic voltammogram of a bare FTO substrate (black) and an FTO with a spin-coated (blue) and a spray-coated (red) BL on top. Green represents a complete photoanode with Dyesol 18 NRT particles as mesoporous scaffold and magenta represents a complete PA with the rutile particles. (See the text.) All measurements were performed in aqueous 0.5 M KCl (pH 2.5) solution with 5 mM [Fe(CN)6]3−/[Fe(CN)6]4− as the probing redox system (scan velocity: 50 mV/s; potential vs Ag/AgCl (sat.); inset shows the same data on a logarithmic current scale).

deposition of the sprayed TiO2 layer. (See pure FTO in black and the spray pyrolysis-covered FTO in red.) In contrast, when using the spin-coated BL (blue in Figure 2), the current flow is substantial, indicating that this sample exposes a large fraction of the FTO area to the electrolyte. The peak-to-peak distance of the oxidation and reduction peak is increased from the FTO to the spin-coated BL, that is, from 160 to 230 mV. (See also Table SI 1 in the Supporting Information.) This indicates significant but rather small changes in the kinetics of the reaction. From the ratio of the redox peak currents of the FTO and the FTO covered with the spin-coated BL, we infer that more than 50% of the FTO electrode with the spin-coated BL remains uncovered. The low coverage confirms the porous

Figure 1. SEM pictures of (a) spray-pyrolized BL on FTO; (b) spincoated BL on FTO; and (c) top view of the spin-coated BL. (The porous structure of the spin-coated BL can be recognized by the dark spots.) 3932

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spongelike structure that is evident from the SEM pictures of this BL. During the fabrication process of a PA, several additional treatments are applied, which include deposition of the mesoporous TiO2 scaffold, followed by heating at 550 °C and, in particular, a TiCl4 treatment. (See the Experimental Section in the Supporting Information for more details.) The deposition of the mesoporous TiO2 (and the TiCl4 treatment) may enhance the blocking capability of the PA, while the heating steps would normally reduce it due to the formation of more cracks in the BL.11 Therefore, we also conducted CV measurements on regular PAs with the two different particles for the mesoporous scaffold. These PAs have passed all of the normal production steps prior to the infiltration with the PbI2 precursor solution (Figure 2, green and magenta lines). This additional treatment reduces the peak current marginally but increases clearly the peak-to-peak current separation between the redox peaks. (See Table SI 1 in the Supporting Information.) However, still at least 40% of the FTO remains uncovered. The change in the peak-to-peak distance can be due to two reasons: (i) a diffusion limitation of the redox couple inside the pores or (ii) the formation of a thin blocking TiO2 layer during the TiCl4 treatment on the FTO. To gain a deeper inside into the different electrode behaviors, electrochemical impedance measurements (of the same samples as used for the CV) were conducted in a potential range from 0.1 to 0.6 V. The resulting impedance spectra were fitted with Randles circuit (except the one with the sprayed BL). Typical Nyquist plots at 288 mV are presented in Figure SI 3 in the Supporting Information. The results from the impedance fitting are presented in Figure SI 4 in the Supporting Information. The charge-transfer resistance mirrors clearly the shape of the DC current response during the impedance measurement. Even though the lowest applied frequency in the impedance measurement is 0.1 Hz, the determination of the Warburg diffusion resistance is difficult. One reason can be (as stated by Kavan et al.11) that the FTO is a “macro” electrode in contrast with the covered FTO samples. There, depending on the size of the pin holes and their distance to each other different diffusion profiles will develop, which might even show microelectrode behavior. Nevertheless, the change in current seems to be mainly related to the change in charge-transfer resistance, as visible in Figure SI 4b in the Supporting Information, implying a partial coverage of the exposed FTO by an ultrathin layer of TiO2. Overall, the PAs clearly do not possess a valid BL, showing that vast areas of the FTO still remain accessible for the charge-transfer reactions directly with the perovskite. We subsequently tested the different BLs embedded in a complete photovoltaic device. The porous morphology of the spin-coated BLs is expected to induce a poor performance mainly due to increased dark current shunting the JV characteristic. However, as seen in Figure 3 and Table 1, this is not the case indicating that the lack of the complete BL does not promote the shunting under forward bias. These findings suggest that under these conditions the perovskite itself or PbI2 prevents the HTM from reaching the FTO by filling up the voids inside the porous TiO2 BL. This impairs recombination of the holes from the HTM with the electrons in the FTO conferring a rectifying character to the junction. To finally give further proof of our observations, we fabricated solar cell devices without any kind of BL and with the rutile TiO2 particles. Also, these devices did not show a

Figure 3. JV characteristics of the investigated devices. Spray_1 and Spray_2 are the devices with the sprayed BL, and Spin_1 and Spin_2 are the devices with the spin-coated BL. The two device types both employ 18 NRT anatase particles (particle size: 20 nm) for the mesoporous scaffold. The devices Spin_Rutile_1 and Spin_Rutile_2 employ the spin-coated BL with a mesoscopic scaffold made of 40 nm rutile TiO2 particles. (For details, see the Experimental Section in the Supporting Information.)

shunt, although their PCE is lower compared with the devices with a BL. (See the JV curve in Figure SI 5 in the Supporting Information with a Jsc of 14.36 mA/cm2, a Voc of 1.030 V, and an FF of 0.67; these devices still yield PCEs of 9.77%.) Because the porous BL (or similarly in the case without any BL) places the perovskite in direct contact with the FTO, this facilitates hole injection into the valence band (VB) of the perovskite under reverse bias. The process is similar to hole transfer from the FTO to the dye in DSCs under reverse bias.22 Polarizing a DSC in reverse bias leads to a conduction of holes (over the dye molecules adsorbed at the FTO and TiO2 surface) as soon as the HOMO level of the dye is above the Fermi level (EF) in the FTO. Figure 4 shows the comparison of the dark and photocurrent behavior of the different device types made with the 18 NRT paste. The effect of increasing dark and photocurrent under reverse bias is observed only in the case of the samples with a spin-coated BL. This verifies our conclusions from the SEM and CV analysis, providing additional proof of the direct contact between the FTO and the perovskite. The energetics of the devices with and without blocking BL are presented in Scheme 1a and b. Scheme 1a represents the situation with a compact, sprayed TiO2 BL. Under reverse bias, the hole transfer is hindered because the VB of the perovskite is not accessible from the FTO without a direct contact. Instead, the charge carriers would have to be channeled through the VB of the compact TiO2. In the case of a porous BL (Scheme 1b), there exists a contact between the perovskite and the FTO. This enables a direct charge transfer, resulting in current flowing under reverse bias. (See Figure 4 and Figure SI 5 in the Supporting Information.) The lack of shunting in conjunction with an increased current under reverse bias implies that the perovskite itself must act as a BL, preventing the direct contact between FTO and HTM. From these observations, neglecting band bending and surface traps, one can derive a simple energy diagram of such perovskite devices, which allows us to determine the offset of the VB (edge) position of the perovskite with regards to the Fermi level (EF) of the hole conductor in the absence of a bias (see Scheme 1c). It is normally assumed that at 0 V bias the EF is determined by the Fermi level inside the highly doped HTM. 3933

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Table 1. Comparison of the Photovoltaic Metrics for Perovskite Devices Using Two Different BLs and Two Different TiO2 Mesoporous Scaffolds Measured at AM 1.5 Solar Radiation spray_1 spray_2 spin_1 spin_2 spin_rutile_1 spin_rutile_2

Jsc (mA/cm2)

Voc (V)

FF

PCE %

BL type

TiO2 type

20.8 20.7 20.6 20.8 21.5 20.1

0.963 0.950 0.954 0.953 1.066 1.055

0.70 0.72 0.73 0.74 0.75 0.74

14.15 14.18 14.36 14.60 17.14 16.41

spray spray spin spin spin spin

18 NRT 18 NRT 18 NRT 18 NRT rutile rutile

Figure 4. (a) Plots of typical dark current behavior (in logarithmic scale) of devices made under similar conditions but with different BLs against applied bias (blue with spin-coated BL and red with spray-pyrolized BL). Inset shows the extrapolation (in green) for the dark current of a device with the spin-coated BL. (b) Typical behavior of the dark- and photocurrent in forward and reverse bias. In blue, the current response of the devices for the spin-coated BL is shown, and in red, that for the spray-pyrolized BL is shown. In green, the extrapolation of the photocurrent in reverse bias is shown. (Illumination was by a white light LED array with the intensity of about one sun. For the scan velocity, please see the Experimental Section in the Supporting Information.)

Scheme 1a

a

(a) Contact between the FTO, the sprayed BL, and the perovskite (P). No charge transfer is possible under such conditions. (b) Contact between the FTO, the porous spin-coated BL (which is infiltrated with the perovskite), and the perovskite. Charge transfer is enabled at this interface in reverse bias. (c) Band diagram of the investigated system.

Schulz et al. measured recently 5 eV as the position of the HOMO of the Spiro MeOTAD,23 although in literature also other values can be found, e.g., 5.2 eV).24,25 The VB edge position of the perovskite has been determined to be between 5.4 and 5.5 eV with the bandgap of the material being ∼1.55 eV.1,5,23,24,26−28 By extrapolating the dark current under reverse bias linearly to the x axis, one can determine the onset of the reverse bias current flow. (This is similar to the extrapolation of the UV−Vis absorption edge of a chromophore for the

determination of its HOMO−LUMO gap or in the case of a pigment the optical band gap from a Tauc plot.29,30) We performed similar extrapolation of the dark current and photocurrent data for four devices, yielding values of 355 (±29 mV) and 433 mV (±20 mV) respectively. The thusdetermined VB position of 5.4 to 5.5 eV for the perovskite is in close agreement with the value by Schulz et al. determined by photoemission spectroscopy.23 3934

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These results enable the determination of the maximum attainable photovoltage Vmax for such a solar-cell device structure (when neglecting any other loss processes). It would be between 1.1 and 1.2 V, under the assumption of a band gap of 1.55 eV (Vmax being equal to Eg − (EF‑0V − VBPerovskite); see Scheme 1c). Nevertheless, one should keep in mind the approximate character of this estimation because there is a variation in the reported values in literature for the band-gap of the perovskite and for the HOMO level of the HTM. One may also argue that this model is too simple to completely explain the situation at the different interfaces. It is, of course, a reduced picture of the actual situation in such devices. However, even under the condition of a space-charge layer formation at the perovskite/HTM interface, the observed current flow under reverse bias will start to be turned on as soon as the EF reaches the VB edge of the perovskite. Therefore, it is justified to use such a simplistic approach for the determination of Vmax. When investigating the current response in reverse bias in more detail, one observes a striking increase in the photocurrent. Note in Figure 4 the difference between the dark and photocurrent in reverse bias when passing the energetic position of the VB of the perovskite for the sample with the spin-coated BL. While in the dark the current is ∼1 mA/cm2 under 0.9 V reverse bias, it reaches ∼100 mA/cm2 under illumination (with the Jsc being ∼16.6 mA/cm2). The detailed analysis of this strong photo effect is beyond the scope of this publication, but it is most probably related to the strong increase in the conductivity of the perovskite under illumination. The amplification of the measured photocurrent signal shows a gain of six times at 0.9 V reverse bias. Such an increase in current under reverse bias going from dark conditions to illuminated conditions enables the application of the MaPbI3 as a photodetector material. To the best of our knowledge, this is the first time that it has been shown that this absorber material can be used as a high-gain, low-voltagea more favorable design because the interface area photodetector. There have recently been reports showing that the MaPbI3 can be used as a sensitizer for photodetectors based on TiO2 or transparent conduction oxides, although not quantifying the gain.31,32 For such applications, the device without any BL should have a more favorable design because the interface area between the FTO and the perovskite is increased. This should lead to a further amplification of the photocurrent, and indeed (see Figure SI 5 in the Supporting Information) at a reverse bias of 1 V the current measured is 200 mA/cm2 and more, which represents a 14-fold increase in the actual photocurrent at Jsc. With this device, we performed a testing of the light intensity dependence of the photocurrent amplification. At a constant bias applied (1 V in reverse), the results show that at lower light intensities the amplification is even higher (Figure 5). At 6% sun light intensity, the amplification factor (AF, defined here as follows: AF = (Jmeasured − Jdark)/Jsc with the Jmeasured and Jdark at the potential of interest for the AF and JSC being the current density of this light intensity at 0 V) shows a 150-fold increase in the actual Jsc. The latter measurement shows the great potential of the MaPbI3 for applications in photodetectors with high gain at low voltage. Conclusions. We show that perovskite solar cells also reach high PCE with a porous BL, enabling a direct contact between MaPbI3 and the FTO. More important for the high PCE is that the HTM does not get into contact with the substrate of the

Figure 5. Steady-state current at 1 V reverse bias for different light intensities. Inset shows the amplification factor at 1 V reverse bias plotted against different light intensities.

PA. This can be achieved either by employing a compact and therefore highly blocking BL, like in the case of the compact spray pyrolized TiO2 layer, or by depositing a compact absorber layer (of MaPbI3), which prevents the contact between the FTO and the HTM. The latter is accomplished by employing a porous BL, which, by its sponge-like character, facilitates the imbibition of the PbI2 precursor solution and the formation of a compact perovskite film. Furthermore, we show that even a porous BL is not essential. Although the efficiencies of devices without any BL showed lower PCE, no shunting was observed. By achieving the intimate contact of the perovskite and the FTO PA substrate, we determined in situ the VB position of the perovskite and could therefore estimate the maximum attainable voltage of such solar cell device design as 1.1 to 1.2 V. The direct contact of the FTO with the absorber material shows a high photocurrent amplification at low reverse voltages of up to 14 times at 1 sun light intensity. At lower light intensities, the amplification of the actual photocurrent signal increased to over 100 times, showing the great potential of this absorber material for low-voltage, high-gain photodetectors.

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ASSOCIATED CONTENT

* Supporting Information S

Experimental section, SEMs of spray-pyrolized and spin-coated BL, Nyquist plots, impedance results, and JV curve of a device without any BL. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: Thomas.Moehl@epfl.ch. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research leading to these results has received funding from the European Union Seventh Framework Programme [FP7/ 2007-2013] under grant agreement no. 604032 of the MESO project, ENERGY.2012.10.2.1, grant agreement no: 308997 of the NANOMATCELL, and the Advanced Research Grant (ARG 247404) funded under the “Mesolight” project. Financial support from the Swiss National Science Foundation in the form of the Romanian-Swiss Research Programme (RSRP) is 3935

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(17) Hayakawa, A.; Yoshikawa, O.; Fujieda, T.; Uehara, K.; Yoshikawa, S. High Performance Polythiophene/Fullerene BulkHeterojunction Solar Cell with a TiOx Hole Blocking Layer. Appl. Phys. Lett. 2007, 90 (16), 163517. (18) Braga, A.; Baratto, C.; Colombi, P.; Bontempi, E.; Salvinelli, G.; Drera, G.; Sangaletti, L. An Ultrathin TiO2 Blocking Layer on Cd Stannate as Highly Efficient Front Contact for Dye-Sensitized Solar Cells. Phys. Chem. Chem. Phys. 2013, 15, 16812−16818. (19) Waita, S. M.; Aduda, B. O.; Mwabora, J. M.; Niklasson, G. A.; Granqvist, C. G.; Boschloo, G. Electrochemical Characterization of TiO2 Blocking Layers prepared by Reactive DC Magnetron Sputtering. J. Electroanal. Chem. 2009, 637 (1−2), 79−83. (20) Kavan, L.; Oregan, B.; Kay, A.; Gratzel, M. Preparation of Tio2 (Anatase) Films on Electrodes by Anodic Oxidative Hydrolysis of TiCl3. J. Electroanal. Chem. 1993, 346 (1−2), 291−307. (21) Im, J.-H.; Jang, I.-H.; Pellet, N.; Grätzel, M.; Park, N.-G. Growth of CH3NH3PbI3 Cuboids with controlled Size for High-Efficiency Perovskite Solar Cells. Nat. Nanotechnol. 2014, DOI: 10.1038/ nnano.2014.181. (22) Mastroianni, S.; Lembo, A.; Brown, T. M.; Reale, A.; Di Carlo, A. Electrochemistry in Reverse Biased Dye Solar Cells and Dye/ Electrolyte Degradation Mechanisms. ChemPhysChem 2012, 13 (12), 2964−2975. (23) Schulz, P.; Edri, E.; Kirmayer, S.; Hodes, G.; Cahen, D.; Kahn, A. Interface Energetics in Organo-Metal Halide Perovskite-Based Photovoltaic Cells. Energy Environ. Sci. 2014, 7 (4), 1377−1381. (24) Polander, L. E.; Pahner, P.; Schwarze, M.; Saalfrank, M.; Koerner, C.; Leo, K. Hole-transport Material Variation in fully Vacuum Deposited Perovskite Solar Cells. APL Mater. 2014, 2 (8), 081503. (25) Jeon, N. J.; Lee, H. G.; Kim, Y. C.; Seo, J.; Noh, J. H.; Lee, J.; Seok, S. I. o-Methoxy Substituents in Spiro-OMeTAD for Efficient Inorganic-Organic Hybrid Perovskite Solar Cells. J. Am. Chem. Soc. 2014, 136 (22), 7837−7840. (26) 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 (4), 1764−1769. (27) Zhao, Y.; Zhu, K. Charge Transport and Recombination in Perovskite (CH3NH3)PbI3 Sensitized TiO2 Solar Cells. J. Phys. Chem. Lett. 2013, 4 (17), 2880−2884. (28) 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 (15), 9019−9038. (29) Tauc, J. Optical Properties and Electronic Structure of Amorphous Ge and Si. Mater. Res. Bull. 1968, 3 (1), 37−46. (30) Tauc, J.; Grigorovici, R.; Vancu, A. Optical Properties and Electronic Structure of Amorphous Germanium. Phys. Status Solidi B 1966, 15 (2), 627−637. (31) Hu, X.; Zhang, X.; Liang, L.; Bao, J.; Li, S.; Yang, W.; Xie, Y. High-Performance Flexible Broadband Photodetector Based on Organolead Halide Perovskite. Adv. Funct. Mater. 2014, DOI: 10.1002/adfm.201402020. (32) Xia, H.-R.; Li, J.; Sun, W.-T.; Peng, L.-M. Organohalide Lead Perovskite Based Photodetectors with Much Enhanced Performance. Chem. Commun. 2014, DOI: 10.1039/C4CC05960C.

gratefully acknowledged. J.-H.I. and N.-G.P. are grateful for financial support by the National Research Foundation of Korea (NRF) grants funded by the Ministry of Science, ICT & Future Planning (MSIP) of Korea under contract nos. NRF2010-0014992, NRF-2012M1A2A2671721, and NRF2012M3A6A7054861 (Global Frontier R&D Program on Center for Multiscale Energy System). This publication was made possible by partial support of NPRP grant N°6-175-2-070 from the Qatar National Research Fund.

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dx.doi.org/10.1021/jz502039k | J. Phys. Chem. Lett. 2014, 5, 3931−3936