Influence of Perovskite Morphology on Slow and Fast Charge

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Letter pubs.acs.org/JPCL

Influence of Perovskite Morphology on Slow and Fast Charge Transport and Hysteresis in the Perovskite Solar Cells Nasim Mohammadian,† Ahmad Moshaii,*,† Amirhossein Alizadeh,‡ Saba Gharibzadeh,† and Raheleh Mohammadpour§ †

Department of Physics, Tarbiat Modares University, P.O. Box 14115-175, Tehran, Iran School of Electrical and Computer Engineering, Tarbiat Modares University, P.O. Box 14115-194, Tehran, Iran § Institute for Nanoscience and Nanotechnology, Sharif University of Technology, P.O. Box 14588-89694, Tehran, Iran ‡

S Supporting Information *

ABSTRACT: We have investigated the influence of perovskite morphology on slow and fast charge transport in the perovskite solar cells. Solar cells with different perovskite cuboid sizes (50−300 nm) have been fabricated using various methylammonium iodide concentrations. Both the low-frequency capacitance and hysteresis are maximum for the cell with the largest perovskite grains (300 nm). The low-frequency capacitance is about three orders of magnitude greater than the intermediate frequency capacitance, indicating the great role of ions on the slow responses and hysteresis. The measurement of opencircuit voltage decay indicates that for the large grains of 300 nm up to 70% of Voc remains across the cell, even after passing ∼40 s. Such a long time Voc decay demonstrates the large accumulation of the ions at the perovskite interfaces with electron and hole transport layers, which conduct slow redistribution of the charges after the light is turned off.

S

permittivity19,27,29 and band bending due to ion migration.30−32 Of course, the charge trapping and detrapping on the surfaces of perovskite grain boundaries were initially supposed to be the origin of hysteresis.20,33,34 However, the time scale of trapping processes was shown to be three orders of magnitude greater than the hysteresis time scale in PSCs.35 The ion migration in perovskite thin films has been calculated to be activated by the energies around 0.5 eV.31,32,36 The accumulation of opposite ions at the interfaces of the perovskite layer with electron transport material (ETM) and hole transport material (HTM) generally modifies the net electric field through the perovskite; consequently, this affects the movements of charges at the interfaces. The relatively slow movement of ions through the perovskite is supposed to be responsible for the J−V hysteresis.21,37,38 In this work, we investigate the effect of perovskite grain size on the slow movement of ions and corresponding J−V hysteresis in halide PSCs. Different PSCs with various perovskite grain sizes have been fabricated by the two-step spin coating in which the change of methylammonium iodide (MAI) concentration results in different roughness of the perovskite surface. The frequency dependency of the capacitance of the cells has been measured, and the results indicate that for the perovskite with larger grain sizes, more hysteresis is observed. From the impedance spectroscopy

olar cells based on perovskite organic−inorganic materials have proceeded significantly from the first report in 2009.1 Nowadays, the efficiency of the perovskite cells is comparable to that of the silicon technology by reaching 20.1.2,3 The high efficiency, relatively low cost and easiness of fabrication propose perovskite solar cells (PSCs) as a potential technology for future photovoltaic industry.4 However, there are some problems in stability5 and environmental compatibility of these cells6,7 restricting their widespread marketing. Photovoltaic performance of PSCs strongly depends on fabrication process of the perovskite layer by various methods, including sequential deposition,8 two-source thermal evaporation,9 two-step spin coating,10 and two-step combined spin and spray coating.11 Despite enormous progress achieved in the production of various perovskite compositions with high efficiencies,12,13 high stability,14,15 and more nature friendliness,16,17 the physics of photoelectrical properties of the perovskite structure have not yet clearly been specified. The existence of amazing phenomena such as slow electrical response under light irradiation18,19 and anomalous current− voltage hysteresis20−23 draws much attention to clarifying the physics of such anomalous behaviors. The J−V hysteresis has been reported in PSCs with various structural designs.20−23 Experiments show that the J−V response of a cell depends on several factors including the direction and the rate of voltage change, the range of the applied voltage, and the configuration of the perovskite layer.19−22,24−28 From previous publications, two main theories are deduced to describe the hysteresis origin, including the capacitive effect of very large photoinduced dielectric © 2016 American Chemical Society

Received: August 23, 2016 Accepted: November 2, 2016 Published: November 2, 2016 4614

DOI: 10.1021/acs.jpclett.6b01909 J. Phys. Chem. Lett. 2016, 7, 4614−4621

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The Journal of Physical Chemistry Letters analysis, it is shown that for the larger MAI concentrations, where the size of the perovskite grains is smaller, the accumulation of ions at the interface of perovskite with HTM/ETM surfaces becomes lower. This lower ions accumulation causes less low-frequency capacitance in addition to less J−V hysteresis for the cells. Also, the cells with smaller perovskite grains have much faster open-circuit voltage decays. This indicates that the ions are mostly trapped at the interfaces of large perovskite grains with ETM/HTM layers. The results presented here were conducted from 64 PSCs produced in four fabrication runs. In each run, four cells are fabricated for each four MAI concentrations. To provide a confirmation of the formation of perovskite structure for the cells with various MAI concentrations, XRD analysis has been performed and the results have been shown in Figure 1a−d. In the XRD patterns, the peaks at the angles of

Figure 2. Tilt-view field-emission scanning electron microscopy (FESEM) images from the deposited perovskite layer on TiO2/FTO substrates. The four images are corresponding to fabrication of perovskite by four different MAI concentrations including (a) 5, (b) 7, (c) 9, and (d) 11 mg/cm3. The perovskite cuboids have the average sizes of 300, 200, 100, and 50 nm in panels a−d, respectively. The scale bar in all images is 500 nm.

Figure 3. Cross-view FESEM images for the complete devices fabricated by the MAI concentrations of (a) 5 and (b) 11 mg/mL.

Figure 1. XRD patterns of the perovskite thin film produced by various MAI concentrations.

sizes. The similarity of the thickness of the perovskite layer is again confirmed here, which is ∼300 nm. The reduction of the grain size with increasing MAI concentration results in better smoothness of the perovskite surface and consequently better contact between the perovskite and ETM/HTM surfaces. The J−V analysis of the fabricated PSCs with different perovskite grain sizes is shown in Figure 4a−d for both forward and reverse bias scans. The results are obtained under one-sun illumination (AM 1.5 G) with the voltage scan rate of 50 mV/s. The best and average photovoltaic quantities of the fabricated cells for both forward and reverse bias scans are shown in Table 1. It is seen that the open-circuit voltage as well as the short circuit current both increase with reduction of the grain sizes. From the atomic force microscopy (AFM) images of various perovskite grain sizes shown in Figure S1 in Supporting Information, we see that the roughnesses of the perovskite layer increase for larger perovskite grains that are prepared by lower MAI concentrations. In fact, by reducing the grains sizes, the contact between the perovskite with less roughness and the ETM/HTM surfaces leads to better charge extraction, and this considerably reduces the recombination of charges at these interfaces. Consequently, more photogenerated charges can be transferred to the electrodes, and this appears in the higher values of open-circuit voltage and short-circuit current. Although in some previous publications,10,41 it was shown that better charge transport occurs for the perovskite with larger grains, there are some other reports in which no

14.7, 28.5, and 33.06° are assigned to the diffractions from the planes of (100), (200), and (210) of the perovskite structure, in agreement with the other reports.39,40 The XRD results clearly confirm the formation of perovskite structure in all fabricated cells. Figure 2a−d shows the tilt-view FESEM images of various perovskite thin films synthesized on the TiO2/FTO substrates. Very good coverage of the perovskite layer is seen for all perovskite grain sizes (different MAI concentrations). Also, no direct contact between the TiO2 and spiro-OMeTAD layers is observed in the images, which completely discards any shunt contact or pinhole structure in the perovskite layer. We see that the thickness of the perovskite layer for all four MAI concentrations is almost similar. According to the details of SEM images, the perovskite cuboid sizes corresponding to the MAI concentrations of 5, 7, 9, and 11 mg/mL are approximately 300, 200, 100, and 50 nm, respectively. In the following, we use these measured grain sizes for denoting the results of various MAI concentrations. For better study of the cell architecture, the cross-view FESEM images of the complete cells for the largest and the smallest perovskite grain sizes of 300 and 50 nm are shown in Figure 3a,b. Five distinct layers of FTO/TiO2/perovskite/spiroOMeTAD/gold are clearly identified in the images. Again, we see no shunt contact exists in the perovskite layer or in the spiro-OMeTAD layer for both the small and the large grain 4615

DOI: 10.1021/acs.jpclett.6b01909 J. Phys. Chem. Lett. 2016, 7, 4614−4621

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Figure 4. Diagrams of voltage−current variations for the fabricated perovskite solar cells with different perovskite grain sizes of 300, 200, 100, and 50 nm. The cartoon below shows the difference in ion migration in various perovskite grain sizes.

Table 1. Photovoltaic Parameters of the Perovskite Solar Cells with Various Perovskite Grain Sizes for Both Forward Scan (FS) and Reverse Scan (RS)a JSC (mA/cm2)

FF

PCE (%)

hysteresis index factor22

grain size distribution (nm)

sweep direction

best

ave.

best

ave.

best

ave.

best

ave.

best

ave.

300

FS RS FS RS FS RS FS RS

17.15 17.23 18.32 18.41 18.58 19.01 18.88 19.22

17.05 17.14 17.65 17.78 17.91 18.35 17.97 18.18

0.82 0.83 0.89 0.88 0.91 0.92 0.99 0.96

0.8 0.8 0.85 0.86 0.87 0.88 0.88 0.87

0.47 0.60 0.49 0.61 0.57 0.66 0.50 0.57

44 57 46 57 54 62 46 52

6.59 8.56 7.96 9.82 9.63 11.49 9.34 10.51

6.01 7.81 6.90 8.71 8.41 10.01 7.27 8.22

0.32

0.33

0.29

0.28

0.22

0.19

0.14

0.12

200 100 50 a

VOC (V)

All measured parameters are shown for the champion cells and that obtained from averaging of 16 cells with similar perovskite grain sizes.

perovskite the length of diffusion of the charges is relatively high (1 to 1.7 μm),42 the interfacial recombination at the interface of perovskite with ETM/HTM surfaces should be more important than the recombination due to bulk grains inside the perovskite. Therefore, when the interfacial contact for smaller grain sizes leads to better charge extraction, the current density decreases with increment of the cuboids’ size. Also, better charge extraction from smaller perovskite grains leads to more accumulations of carriers at the electrodes appearing as the higher open-circuit voltages. As a characteristic quantity for the hysteresis effect, we define the hysteresis index factor as the relative difference of the current densities for the forward and the reverse biases at the typical voltage of 0.8 VOC22

monotonic increment has been observed for the current density versus the grain size.22 Of course, the increment of charge transport for larger grains was reported to be accompanied by increment of the perovskite thickness.41 While, according to the SEM images of Figure 2a−d, almost all perovskite layers in our work have similar thicknesses. Basically, there are two important effects determining the variations of the current density versus the perovskite grain size. The first is the number of grain boundaries in front of the charge carriers during their passage inside the perovskite. For a higher MAI concentration, due to increment of the number of grains, the charges should pass through more boundaries, and this effectively increases the probability of recombination of electrons and holes. The second effect is the contact between the perovskite with ETM and HTM surfaces. For larger grain sizes, due to more roughness of the perovskite surface, the charge transport across the interface of perovskite layer and ETM/HTM surfaces decreases and more recombinations occur for the passage of charges through these interfaces. In fact, from the point of view of contact between the perovskite and ETM/ HTM surfaces, the charge extraction is better for smaller grain sizes, while, considering the higher number of grains inside the bulk perovskite with smaller grains, the recombination probability inside the perovskite is higher for smaller grains. These two effects mutually determine the trend of change of current density with the perovskite grain size. Because in the

hysteresis index factor =

jRS (0.8VOC) − jFS (0.8VOC) jRS (0.8VOC)

where JRS (0.8 VOC) and JFS (0.8 VOC) are the current densities in the reverse and forward scans, respectively. Table 1 shows the results of hysteresis index factor for various cells with different grain sizes. Also, in Figure S2a−d in the Supporting Information, we have compared the J−V curves and the hysteresis index factor for two different voltage scan rates of 5 and 50 mV/s. It is seen that the highest hysteresis is obtained for the cell with 300 nm grain size. By decreasing the size of 4616

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charge transport through the device.45 In general, in the measured Nyquist plots of IS analysis, two different regions exist including low and intermediate frequency responses of the device.45−47 The low-frequency region is mainly attributed to the IS results for the range of frequencies of 0.01−10.0 Hz. Also, the intermediate results correspond to the range of 10.0 Hz to 100 kHz. The high-frequency response that is attributed to the charge transportation in the electrodes as well as through ETM and HTM is too fast to be observed by the common impedance analysis, which is usually restricted to less than 1 MHz. In the perovskite devices, the low-frequency response is basically related to the charge transfer at the interfaces of the perovskite layer with ETM and HTM surfaces.46,48 However, the intermediate response is corresponding to the charge transfer inside the bulk perovskite.45 Figure 6a,b shows the Nyquist plots of impedance spectra for various perovskite grain sizes under dark and light conditions

perovskite grains, the hysteresis effect noticeably decreases. This phenomenon probably occurs because of greater accumulation of charges at the interface of large cuboids with HTM and ETM (see Figure 4 cartoon). We further specify this issue by impedance analysis in the following. For the study of the light absorption properties of various perovskite grain sizes, we measured the diffusive reflectance/ transmittance spectra of different perovskite layers, and the results have been shown in Figure 5a−c. We see that in the

Figure 5. Spectra of diffusive absorbance (a), transmittance (b), and reflectance (c) of the perovskite films with different grain sizes. (d) IPCE curves for the cells with various perovskite grains.

Figure 6. Nyquist plots of impedance spectra for various perovskite grain sizes under dark (a) and one sun illumination (b). The cell voltage is 0.7 V, and the frequencies range is from 10 mHz to 1 MHz. The insets in both graphs show details of the curves close to zero impedance.

range of 400−650 nm the absorbance of the perovskite layer increases with the cuboid size. This mainly goes back to more absorption of the incident light by the larger perovskite grains. Also, near-zero integrated sphere reflectance is seen for all of the four perovskite grain sizes in the range of 400−700 nm. Of course, the higher absorption does not guarantee for higher charge transport to the electrodes. In fact, the photogenerated charges should pass through the interfaces of the perovskite with ETM/HTM surfaces, where most of recombination occur there. From the results of Table 1, we see that the short-circuit current of the cells increases with reducing the cuboids size, which is in contrast with the more absorption of the larger cuboids. This indicates that the role of charge transfer at the interfaces is much more important than the change of absorbance of the perovskite by the grains size. In support of the role of charge transfer through the interfaces of perovskite with ETM/HTM, the results of induced photon to current efficiency (IPCE) are shown in Figure 5d. We see that the quantum efficiency for the cells with smaller grain sizes is higher in the range of wavelengths of 300−800 nm. This result coincides with better charge transport through the smaller perovskite grains. To investigate the physical reason for the J−V hysteresis, we measured impedance spectroscopy (IS) of the devices to further specify low-frequency movement of charges as the origin of hysteresis. Basically, impedance spectroscopy is a frequency characterization technique that allows decoupling of physical processes with different characteristic times.43,44 The interpretation of IS results is usually based on the appropriate RC model used to identify the important time scales of the

for a typical bias voltage of 0.7 V. In all impedance curves, the real part (Z′) of the left arc demonstrates resistance of the perovskite layer against recombination of electrons and holes.45,48 As much as this resistance becomes larger, the recombination rate inside the perovskite becomes smaller. It is seen that the resistance against recombination monotonically increases with decreasing the perovskite grain size. This means that the rate of recombination decreases with decreasing the perovskite grain size. This effect is seen for both dark and light conditions. This leads to better transport of electrons and holes through these interfaces, which results in less recombination. On the contrary, as shown in Figure 6b, for the light condition, due to much higher concentrations of photogenerated electrons and holes, the resistivity of the perovskite layer becomes considerably smaller. The low frequency responses of the IS results in Figure 6a,b (the right arcs) are denoting the impedance of the cells due to the slow charge transfer through the interfaces of perovskite with ETM/HTM. The slowly moving charges in this case are mostly ions, which are easily produced by the energy fluctuation on the order 0.1 to 0.8 eV.30 By applying an appropriate circuit model to fit it with the measured impedance data (Figure S3 in Supporting Information), it is possible to extract quantities relating to the low and intermediate frequency responses. Accordingly, Rrec and Cmf, which are attributed to the intermediate frequency responses 4617

DOI: 10.1021/acs.jpclett.6b01909 J. Phys. Chem. Lett. 2016, 7, 4614−4621

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The Journal of Physical Chemistry Letters (10 Hz to 100 kHz), can be obtained for various perovskite morphologies, as shown in Figure 7a,b. We see that the order of

Figure 8. Low-frequency [0.01−10 Hz] capacitance for various perovskite grains under different cell voltages of 0 to 0.8 V for the light illumination.

slowly moving charges (ions) inside the perovskite layer.27,28 The J−V hysteresis in the perovskite cells is shown to be generally originated from such low-frequency responses that are relating to the ion migrations.21,30,37 In Figure 8, we see that by applying enough bias voltages, the low-frequency capacitance considerably increases with the voltage. This increment is more considerable for larger perovskite grains. In fact, the existence of large grains leads to more accumulation of the low-frequency charges (ions) at the interface of perovskite with ETM/HTM. This means that the more roughness of the perovskite leads to more accumulation of ions at the interfaces, which finally conducts more J−V hysteresis effect. Comparing the intermediate frequency capacitance (Cmf in Figure 7) with the low-frequency capacitance (Clf in Figure 8) indicates that both the value and the increment of Clf with the voltage are much higher than those of Cmf. This shows that the movement of slowly moving charges (ions) provides much more capacitance for the device than that of the electron and hole movements. In Figure 9, the capacitance measured by the EIS analysis as a function of all measured frequencies have been shown for

Figure 7. Intermediate frequency [10 Hz to 100 kHz] capacitance (a) and recombination resistance (b) for various perovskite grain sizes at different cell voltages of 0 to 0.8 V under the light illumination. (c,d) RC time constant for the devices with different grain sizes under the light and dark conditions. The time constant is obtained from multiplying Rrec by Cmf.

Cmf is in the range of 10−6 F/cm2. Such order of the intermediate capacitance is mostly related to the dielectric response of the perovskite layer,28,49 and the chemical capacitance seems to be invisible in the electrochemical impedance spectroscopy (EIS) analysis.49 We see that Cmf is greater for larger perovskite grains. This means that the capability of the perovskite layer to accept or release charges decreases with reducing the grains size.49 The origin of this behavior goes back to the increment of light absorption of the perovskite layer with the grains size (Figure 5a). This higher absorption is accompanied by the creation of more electron−hole pairs, leading to more release of charges inside the perovskite layer, and therefore greater Cmf is obtained for the cell. In Figure 7b, we see that with reduction of the grains size Rrec increases. This means that the recombination rate is less for the perovskite with smaller grains. The reason is better passage of charges through the interfaces, and consequently less recombinations occurs. To investigate the carriers’ lifetime in the perovskite layer,50 the RC time constant has been calculated for the cells with different perovskite grains, and the results are presented in Figure 7c,d, too. The RC time constant is obtained from the EIS results through multiplying Rrec by Cmf. We see that the time constant for the cells with larger grains is smaller for both the light and dark conditions. This indicates that the accumulation of ions at the perovskite interfaces leads to reduction of the carriers’ lifetime. In fact, increasing the perovskite roughness causes less extraction of the charges. Also, for larger grains more accumulation of ions occurs, and this produces larger opposite internal electric field, which diminishes the charge extraction. We see that the RC time constant under the light condition is smaller than that of the dark due to more recombination occurring in the light case. Figure 8 shows the results of low-frequency capacitance (Clf) as a function of bias voltage for various perovskite grains. The low-frequency capacitance mostly relates to the movement of

Figure 9. Capacitance bode diagrams for various perovskite grain sizes under light condition at the cell voltage of 0.7 V.

various grain sizes and for the light condition. At low frequencies, the cell capacitance mostly relates to the movement of slowly moving charges. In this case, for the larger grains more accumulation of ions occurs at the interfaces, leading to higher low-frequency capacitance and also more J−V hysteresis effect. As another support for dependency of the hysteresis on the accumulation of ions at the perovskite interfaces with ETM/ HTM, we have provided the open-circuit voltage decay for various perovskite grain sizes in Figure 10. For the case of 50 nm grain size, after switching off the light, the Voc abruptly 4618

DOI: 10.1021/acs.jpclett.6b01909 J. Phys. Chem. Lett. 2016, 7, 4614−4621

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dissolving 462 mg PbI2 (Sigma-Aldrich) in 1 mL of dimethylformamide (DMF) (Sigma-Aldrich) under stirring at 70 °C for overnight. PbI2 solution was spin-coated for 5 s at 3000 rpm and 5 s at 7000 rpm. Then, the film was dried at 40 °C for 3 min and 100 °C for 5 min. After cooling to room temperature, 200 μL of various concentrations of 5, 7, 9, and 11 mg/mL of the CH3NH3I solution in 2-propanol (Sharif-Solar) were spin-coated for 30 s at 4000 rpm (at room temperature). After spin-coating the CH3NH3I, to produce a crystalline perovskite film on the samples, they were kept on the hot plate at 100 °C for 5 min. The hole-transporter layer was deposited by spin-coating (4000 rpm for 30 s) of 20 μL of chlorobenzene solution (V. Gene Pars Delta) that contained 72.3 mg spiro-OMeTAD (Sharif-Solar), 28.8 μL of tert-butylpyridine (tBP) (SigmaAldrich), and 17 μL of lithium bis(trifluoromethylsyfonyl)imide salt (Sigma-Aldrich). Finally, the counter electrode was deposited by thermal evaporation of 70 nm gold under a pressure of 5 × 10−5 Torr.

Figure 10. Open-circuit voltage decay for the champion cells of various perovskite grain sizes.

decays to zero. The fastness of this decay mostly relates to the fast movements of electrons and holes, which shortly diminishes the initial open-circuit voltage. In fact, for the small grain sizes, the contribution of the accumulated ions at the interfaces on the open-circuit voltage is small. Accordingly, the low-frequency capacitance and the hysteresis are both small for the small grains due to the smallness of the number of trapped ions at the perovskite interfaces. However, for the large grains, many ions exist at the interfaces of perovskite with ETM/HTM, and after switching off the light, these ions remain between the cuboids and consequently the decay of Voc takes a long time (>40 s). In particular, for the grain size of 300 nm, >70% of the Voc remains across the cell after turning off the light. This large residual Voc, which mostly relates to the ions at the interfaces, is the main origin of hysteresis. In conclusion, we investigated the effect of perovskite morphology on the low-frequency charge transport and J−V hysteresis in PSCs. Using different MAI concentrations, various PSCs with different perovskite grain sizes were fabricated by two-step spin coating. The results of J−V analysis indicate that the hysteresis is higher for the cell with larger perovskite grain sizes. From the IS analysis, it was shown that for smaller perovskite grains the accumulation of ions at the perovskite interfaces with HTM/ETM is smaller. This lower ion accumulation causes less low-frequency capacitance in addition to less J−V hysteresis for the cell. Also, the decay of opencircuit voltage for the cells with larger grains takes long times due to great contribution of the trapped ions on the Voc and on the hysteresis as well.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b01909. Device fabrication, solar cell characterization details, the J−V curves for two different scan rates of the bias voltage, the AFM images of various perovskite grain sizes, the impedance model for the cells, the series and shunt resistances of the devices, and the J−V results before and after EIS analysis. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been supported by Tarbiat Modares University (TMU). We thank Dr. Mahmoud Samadpour for the valuable discussions.

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EXPERIMENTAL MATERIAL AND METHODS Fluorine-doped tin oxide (FTO)-coated glasses (Sharif-Solar) were cleaned in an ultrasonic bath once in acetone (20 min) and subsequently twice in ethanol and deionized (DI) water (20 min each). To prevent shunting contact with the measurement pins, initially FTO was removed from the anode contact by etching with 2 M HCl (Merck) and zinc powder (V. Gene Pars Delta). Substrates were then cleaned as above again. A solution of titanium isopropoxide (V. Gene Pars Delta) in absolute ethanol (430 mM) (Merck) and 27 mM solution of HCl in ethanol were mixed together slowly. This solution was spin coated at 2000 rpm for 30 s to make a TiO2 blocking layer. The compact layer was then sintered at 100 °C for 30 min. After cooling to room temperature, the TiO2 paste containing TiO2 dyesol and ethanol with the weight ratio of 2 to 7 was spin-coated on the substrate at 4500 rpm for 30 s. By drying in 70 °C for 20 min, the film was then annealed at 100 °C for 30 min. For preparing the perovskite active layer by two-step spin coating method, PbI2 solution (1 M) was prepared by

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DOI: 10.1021/acs.jpclett.6b01909 J. Phys. Chem. Lett. 2016, 7, 4614−4621

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