Facile Face-Down Annealing Triggered Remarkable Texture

Jan 26, 2017 - Facile Face-Down Annealing Triggered Remarkable Texture Development in CH3NH3PbI3 Films for High-Performance Perovskite Solar Cells ...
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Facile Face-Down Annealing Triggered Remarkable Texture Development in CH3NH3PbI3 Films for High-Performance Perovskite Solar Cells Weidong Zhu,†,‡ Lei Kang,†,‡ Tao Yu,*,†,‡,§,∥ Bihu Lv,† Yangrunqian Wang,†,‡ Xingyu Chen,†,‡ Xiaoyong Wang,† Yong Zhou,†,‡ and Zhigang Zou†,‡,§,∥ †

National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, P. R. China Eco-Materials and Renewable Energy Research Center (ERERC) at Department of Physics, Nanjing University, Nanjing 210093, P. R. China § Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, P. R. China ∥ Jiangsu Key Laboratory for Nano Technology, Nanjing 210093, P. R. China ‡

S Supporting Information *

ABSTRACT: Herein, we demonstrate that the facile face-down annealing route which effectively confines the evaporation of residual solvent molecules in one-step deposited precursor films can controllably enable the formation of (110) textured CH3NH3PbI3 films consisting of high-crystallinity well-ordered micrometer-sized grains that span vertically the entire film thickness. Such microstructural features dramatically decrease nonradiative recombination sites as well as greatly improve the transport property of charge carries in the films compared with that of the nontextured ones obtained by the conventional annealing route. As a consequence, the planar-heterojunction perovskite solar cells with these textured CH3NH3PbI3 films exhibit significantly enhanced power conversion efficiency (PCE) along with small hysteresis and excellent stability. The champion cell yields impressive PCE boosting to 18.64% and a stabilized value of around 17.22%. Particularly, it can maintain 86% of its initial value after storage for 20 days in ambient conditions with relative humidity of 10−20%. Our work suggests a facile and effective route for further boosting the efficiency and stability of low-cost perovskite solar cells. KEYWORDS: CH3NH3PbI3, texture development, defects, face-down annealing, perovskite solar cells CH3NH3PbI3 films has been recognized as the most critical one that enables reliable cell performance.4,8,10 Currently, additivefree crystallization-engineered one-step routes, including fast deposition−crystallization procedures,11 solvent-engineering technology,12 and Lewis base adduct-based approachs13 have become more and more popular for solution processing of desired CH3NH3PbI3 films.1,14 For each of them, the recipe of timely drop-casting antisolvent such as toluene or diethyl ether during spinning the precursor solution that contains equimolar

1. INTRODUCTION Organolead trihalide perovskite (OTP) materials have recently emerged as promising candidates for photovoltaic devices that are known as perovskite solar cells.1−4 In just few years, the power conversion efficiency (PCE) of wet-processed singlejunction cells with the OTP polycrystalline films such as CH3NH3PbI3 has risen rapidly from 3.8% to over 20%.5,6 This unprecedented progress mainly relies on the inherently excellent optoelectronic features of CH3NH3PbI3 films1,4,7,8 as well as the fundamental research efforts focused on device architecture, interface engineering, and materials and deposition routes for each of the functional layers.3,4,8,9 Among all of the concerns, effective deposition of pinhole-free and uniform © 2017 American Chemical Society

Received: December 4, 2016 Accepted: January 26, 2017 Published: January 26, 2017 6104

DOI: 10.1021/acsami.6b15563 ACS Appl. Mater. Interfaces 2017, 9, 6104−6113

Research Article

ACS Applied Materials & Interfaces

Figure 1. Schematic of experimental procedures for conventional annealing and face-down annealing routes, respectively.

Figure 2. (a) XRD patterns of samples CA, FDA-60, FDA-40, and FDA-RT, respectively. (b) Corresponding histograms of XRD peak intensity ratios of (110) to (310) planes and (220) to (310) planes as well as calculated Lotgering factors. (c−f) Cross-sectional SEM images of samples CA, FDA-60, FDA-40, and FDA-RT. Left inset describes the stereoshape model proposed for CH3NH3PbI3 grains on TiO2/FTO substrate. The {110} facets are marked in brown for clarity.

CH3NH3I and PbI2 in N,N-dimethylformamide (DMF) or γbutyrolactone/dimethyl sulfoxide (DMSO) is required because it can speed the nucleation kinetics or decouple the nucleation and growth processes of CH3NH3PbI3 grains and thereby avoid the formation of pinholes in the films.1,14 Because of the excessively fast crystallization process, the resultant CH3NH3PbI3 films generally consist of small grains with broad size distribution in the range of ∼100−500 nm.12,15,16 Such a microstructured feature unavoidably leads to high density of crystal imperfections such as grain boundaries and intragranular defects in the films.15,17 They can seriously deteriorate the electrical and optical qualities of CH3NH3PbI3 films, in turn strictly limiting the PCE of cells.15,17−19 In addition, the anomalous photocurrent density−voltage (J−V) hysteresis in perovskite solar cells that makes accurate efficiency determination challenging also intrinsically links to such crystal imperfections.17,20 In attempting to circumvent these problems, some successful strategies have been exploited such as interface

engineering of cells,9,21 passivation of grain boundaries,17,22,23 and coarsening grains of CH3NH3PbI3 films.16,24,25 For polycrystalline films, equally to grain size, the orientation of the crystal axis in each grain is also one of the important microstructural features that dominates their physical properties.26 Specifically, the ones with aligned crystal axes are socalled textured films.26,27 They possess a single-crystal-like nature along the crystal axis, and hence, an enhancement of physical properties is expected.28 Ordinarily prepared polycrystalline films are composed of grains with random orientation. The methodology that is utilized to develop texture to enhance the functional properties of polycrystalline films is known as texture engineering.28,29 Specifically, the onestep deposited CH3NH3PbI3 polycrystalline films are similarly characterized with randomly oriented grains.10,30−32 Recent studies revealed their tremendous intragranular heterogeneities in photoluminescence (PL),33 cathodoluminescence,34 and cell performance parameters.35,36 Therefore, texture engineering is of particular importance to modify the electrical and optical 6105

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Figure 3. (a−h) Top-view SEM images of sample CA (a and e), FDA-60 (b and f), FDA-40 (c and g), and FDA-RT (d and h). (i−l) Topographical AFM images of sample CA (i), FDA-60 (j), FDA-40 (k), and FDA-RT (l). The insert is the corresponding histogram of height distribution, which was fit using the Gaussian distribution function.

properties of such CH3NH3PbI3 films and hence further improve the performance of ultimate cells. Nevertheless, stateof-the-art routes for texture engineering of CH3NH3PbI3 films inevitably include complex additives,6,17,31,37−39 sophisticated processing control,40−42 or special substrates.43−45 It is thus of interest to advance a more facile route to develop texture in one-step deposited CH3NH3PbI3 films. Herein, a facile face-down annealing strategy exempting the use of any additives or special substrates is proposed to develop texture in CH3NH3PbI3 films. It involves the deposition of CH3NH3PbI3 precursor films via Lewis base adduct-based onestep spin-coating method,13 followed by preheating the films face-up on hot plate at controlled temperatures and then turning them face-down during annealing. By this method, the (110) textured CH3NH3PbI3 films composed of highcrystallinity, well-ordered, micrometer-sized grains can be successfully obtained, and all of the grains vertically penetrate the entire film thickness. The planar-heterojunction perovskite solar cells based on these textured CH3NH3PbI3 films yield remarkably improved performance as well as small hysteresis and excellent stability, in contrast to the cells based on the nontextured ones obtained by conventional annealing.

CH3NH3PbI3 films were thus obtained. The corresponding samples with the preheating temperatures of RT, 40, and 60 °C were labeled as FDA-RT, FDA-40, and FDA-60, respectively. The CH3NH3PbI3 films prepared by the conventional annealing route were labeled as CA for comparison. It should be noted that, as revealed in Figure S1, some residual DMSO molecules exist in the precursor films. The face-down annealing route can effectively confine the evaporation of those DMSO molecules, so better control of nucleation and growth of CH3NH3PbI3 grains is expected.1,6,24,42 To identify the crystal structure of resultant CH3NH3PbI3 films, X-ray diffraction (XRD) measurements were carried out. As shown in Figure 2a, all of the XRD patterns can be indexed to pure phase CH3NH3PbI3 compound with tetragonal I4/mcm structure.12,25,46 However, the major diffraction peaks for samples FDA-60, FDA-40, and FDA-RT are clearly intensified in contrast to those of sample CA, which suggests the decreased density of crystal imperfections/structural defects in CH3NH3PbI3 films formed by face-down annealing.38,47,48 Moreover, the relative intensities of dominant diffraction peaks corresponding to (110), (220), and (310) lattice planes also vary significantly between the concerned samples. Figure 2b gives the peak intensity ratios of (110) to (310) planes and (220) to (310) planes. It can be seen that such ratios increase higher in sequence for samples FDA-60, FDA-40, and FDA-RT with respect to those of sample CA. This implies the possible formation of crystallographic (110) texture in CH3NH3PbI3 films formed by face-down annealing.10,26,27,40 Therefore, the texture degree of samples was estimated using the Lotgering factor ( f) which is defined as f = (p − p0)/(1 − p0),27 where p = ∑I(110)/∑I(hkl), I is the specific diffraction peak intensity, and p0 is the value of p for a randomly oriented sample. The value of f ranges from 0 for a nontextured sample to 1 for a completely textured one. Here, p0 is derived from the XRD data of two-step spin-coated CH3NH3PbI3 film, as given in Figure S2. Then, the value of f is calculated to be 0.02 for sample CA

2. RESULTS AND DISCUSSION The face-down annealing route for preparing CH3NH3PbI3 films is illustrated in Figure 1 and described in detail in the Experimental Section. Briefly, the CH3NH3PbI3 precursor films composed of the CH3NH3I·PbI2·DMSO complex were first coated on compact TiO2/FTO glass substrates by the Lewis base adduct-based one-step spin-coating method proposed by Park et al.13 Then, they were placed face-up on a hot plate and preheated in batches at temperatures of 23 (room temperature, RT), 40, and 60 °C for 30 s. Subsequently, all of the films were turned face-down and annealed at 100 °C for 10 min. After being removed from the hot plate, a series of blackish 6106

DOI: 10.1021/acsami.6b15563 ACS Appl. Mater. Interfaces 2017, 9, 6104−6113

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Figure 4. (a) UV−vis absorption spectra and (b) room-temperature transient PL spectra of samples CA, FDA-60, FDA-40, and FDA-RT.

values are estimated to be 0.28, 0.52, 0.92, and 1.10 μm for samples CA, FDA-60, FDA-40, and FDA-RT, respectively. To get a better view of grain morphology of the films, atomic force microscopy (AFM) was used. Figures 3i−l give topographical AFM images together with corresponding histograms of height distribution. The calculated root-mean-square (RMS) value is 9.3 nm for sample CA, while the values are 15.2, 32.5, and 42.1 nm FDA-60, FDA-40, and FDA-RT, respectively. This means that the CH3NH3PbI3 films formed by face-down annealing possess increased surface roughness. Even so, it is still relatively low compared with that of the sample prepared by the sequential deposition technique.50 In addition, a symmetric Gaussian distribution of heights can be observed for sample CA, signifying that the film surface has a continuously undulated profile with no specific preferred height.47 Yet, the height distribution becomes asymmetric with a dominant distribution coupled with an additional narrow distribution for samples FDA-60, FDA-40, and FDA-RT. As in a polycrystalline film, the lower regions represent the grain-boundary grooves, and the higher regions represent the topmost profile of grains.10,47 So, we can infer that the grain growth preferentially selects the certain plane in those films, agreeing with the texture development in them. All the above observations provide conclusive evidence that the facile face-down annealing route can successfully realize the (110) textured CH3NH3PbI3 film, and the texture degree, average grain size, and surface roughness increase in sequence for samples FDA-60, FDA-40, and FDA-RT. The formation of such textured CH3NH3PbI3 films is mainly attributed to the following two aspects. On the one hand, the DMSO molecules will be confined on the surface of the CH3NH3PbI3 film during the initial crystallization stage of the film. As proven in previous works,13,51 the O atom in DMSO interacts strongly with Pb atoms in CH3NH3PbI3.52 A strong interaction of DMSO with CH3NH3PbI3 surface atoms will effectively lower the surface energy of (110) crystal facets, which results in preferred growth of CH3NH3PbI3 grains along [110] direction, namely (110) texture in the CH3NH3PbI3 film. This situation is similar to the formation of the (110) oriented CH3NH3PbI3 film by DMSO solvent annealing.53,54 On the other hand, the nucleation sites for the CH3NH3PbI3 grains can be reduced due to the slow evaporation of DMSO molecules, which leads to the growth of large-sized grains in the CH3NH3PbI3 film.48 The above two aspects account for the formation of the textured CH3NH3PbI3 film with a large grain size. The optoelectronic properties of resultant CH3NH3PbI3 films were then investigated. Figure 4a gives their UV−vis

and 0.42, 0.78, and 0.85 for samples FDA-60, FDA-40, FDART, respectively, as displayed in Figure 2b. These results reveal explicitly that, unlike the one attained by conventional annealing, the CH3NH3PbI3 film is (110) textured by facedown annealing, and the texture degree increases monotonically with the decrease in preheating temperature. The texture development in corresponding samples can also be directly observed from their cross-sectional scanning electron microscope (SEM) images, as given in Figures 2c−f. As seen, although the films have similar thicknesses of ∼450 nm, the microstructural feature of grains differs remarkably. In detail, for sample CA, the grain size is small, and the grain orientation is completely random, revealing its nontextured nature. Further, three or more packed grains can be identified across the film thickness. For sample FDA-60, most of larger grains can fully span the film thickness and are vertically oriented to the substrate with few fine grains located at their triple junctions, suggesting that the texture begins to develop in the film. For samples FDA-40 and FDA-RT, all of the grains with much larger size can penetrate the entire film and are uniformly aligned along the direction perpendicular to substrate, which clearly reveals the texture development in the films. It has been verified that the {110} facet normal of the CH3NH3PbI3 film is preferentially along the direction perpendicular to TiO2/FTO substrate,32,45,48 mainly due to the better structural matching between rows of adjacent surface iodides and uncoordinated titanium atoms.30,49 Therefore, we can further confirm the development of (110) texture in CH3NH3PbI3 films prepared by face-down annealing. It is important to note that the face-down annealing strategy presented here does not require any additives or hightemperature thermal treatment; thus, the pure phase and proper stoichiometry of resultant films can be guaranteed inherently. Besides the amazing simplicity, this is another unique advantage of it compared with the state-of-the-art routes for texture engineering of CH3NH3PbI3 films.6,17,31,37−39 The surface morphology of prepared samples was studied by SEM. As shown in Figures 3a−d, apart from sample FDA-RT with few pinholes, all of the other samples possess full surface coverage and excellent uniformity, and the films compose of polygonal CH3NH3PbI3 grains with typical triple junction of grain boundaries. However, as revealed in Figures 3e−h, the CH3NH3PbI3 films formed by face-down annealing exhibit dramatically increased grain size compared with those by conventional annealing, in good agreement with the results observed from their cross-sectional SEM images. The statistical grain size distributions are presented in Figure S3. The average 6107

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Figure 5. (a) Simplified sketch of device configuration for the planar-heterojunction perovskite solar cell with layer stack of FTO/TiO2/ CH3NH3PbI3/spiro-MeOTAD/Ag. (b) Cross-sectional SEM image of typical cell. (c) Statistic PCEs of 25 cells with samples CA, FDA-60, FDA-40, and FDA-RT. (d) Typical J−V curves for the cells with samples CA and FDA-40, which were measured at FS (from −0.1 to 1.2 V) and RS (from 1.2 to −0.1 V) under simulated AM 1.5, 100 mW/cm2 solar irradiation. (e) Photocurrent density measured as a function of time for the cells with samples CA and FDA-40, which were biased at 0.730 and 0.889 V, respectively. (f) Evolution of PCE of sample CA and FDA-40-based devices with storage time. The cells were stored in ambient environment at 25 °C with a relative humidity of 10−20%.

absorption spectra. It can be seen that all of the films possess typical absorption character of tetragonal CH3NH3PbI3.5 Compared with sample CA, the absorption onset exhibits slight bathochromic shift for textured CH3NH3PbI3 films, especially samples FDA-40 and FDA-60. Moreover, the absorption intensity of those samples in the range of 550 nm to onset is also higher than that of sample CA. Such features are probably caused by the high crystallinity of grains25,38 and strong light scattering associated with a rough surface of the films.50 It should be noted that the absorption intensity of sample FDA-RT decreases significantly below 550 nm, which mainly results from the pinholes in the film wherein light just passes without being absorbed.11 To probe the recombination dynamics of photogenerated charge carriers in concerned samples, time-resolved PL measurements were conducted, as given in Figure 4b. Each of samples was deposited on blank glass substrate and photoexcited from the side of the glass substrate. The curves were fitted with a biexponential decay function. The fast component corresponds to trap-assisted recombination, while the slow component is attributed to recombination in the bulk of CH3NH3PbI3 grains.24,33,48 Taking the weighted average of the two components, the lifetimes of minority carriers are estimated to be 21, 46, 66, and 75 ns for samples CA, FDA, FDA-40, and FDA-RT, respectively. The prolonged lifetime indicates the decrease in

nonradiative recombination sites.7,33,48,55 Therefore, we can infer that the reduced nonradiative recombination sites can be achieved in the textured CH3NH3PbI3 film, and the density of them decreases in turn for samples FDA-60, FDA-40, and FDART. To probe the possible influence of texture development in the CH3NH3PbI3 film on device performance, a planarheterojunction perovskite solar cell was fabricated following the typical configuration, as schematically shown in Figure 5a, wherein the CH3NH3PbI3 film behaves as a light absorption layer; the FTO transparent electrode and Ag layer serve as bottom cathode and top anode, respectively, while the compact TiO2 layer and spiro-MeOTAD layer function as electron transport and hole transport, respectively.3 Figure 5b gives the cross-sectional SEM image of one typical cell with textured CH3NH3PbI3 film. To exclude the experimental errors during cell fabrication and PCE measurement, 25 cells for each type of sample are included. Figure 5c gives the statistic PCEs. Thus, the average PCE is derived to be 13.41% for the cells with sample CA, whereas the values are 15.81, 17.11, and 10.94% for the cells with samples FDA-60, FDA-40 and FDA-RT, respectively. Clearly, the textured CH3NH3PbI3 films including samples FDA-60 and FDA-40 can remarkably boost the PCE of corresponding cells. Note that the inferior PCE of the cells with sample FDA-RT is mainly caused by pinholes in the films, 6108

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the cell with sample FDA-40. To further distinguish this difference, the photocurrent densities of these two cells at the bias voltages of their respective maximum power points were recorded as a function of time, as presented in Figure 5e. We can see that, for the cell with sample FDA-40, the current density increases rapidly to the maximum value of 19.37 mA/ cm2 in about 10 s with stabilized PCE of around 17.22%, showing a small difference from that derived from the light J−V curve under RS. In contrast, it takes about 50 s to reach the maximum current density of 15.02 mA/cm2 for the cell with sample CA, yielding significantly reduced real PCE of 10.96%. Such results explicitly consolidate the suppressed hysteresis for the cell with sample FDA-40. Then, performance stability of the unencapsulated champion cells with samples CA and FDA-40 was evaluated in ambient environment at 25 °C with a relative humidity of 10−20%. As shown in Figure 5f, for the champion cell with sample CA, the PCE decreases to 50% of its initial value after being exposed for 8 days, and no performance can be detected from it after longer exposure. In contrast, the cell with sample FDA-40 exhibits much improved stability. It can maintain 86% of the initial performance after testing for 20 days. All in all, the above results demonstrate that the cells with textured CH3NH3PbI3 films, especially sample FDA-40, show dramatically improved PCE, suppressed hysteresis, and enhanced stability compared with those of the cells with nontextured CH3NH3PbI3 films, namely sample CA. To understand the much improved performance features for the cell with textured CH3NH3PbI3 film rather than a nontextured one, J−V curves of the above two champion cells were further analyzed based on the theory of the

which can result in the formation of low-resistance shutting paths.11,39 Thus, in terms of solar cell performance, the optimized preheating temperature is 40 °C for preparing textured CH3NH3PbI3 films, so the following discussions are mainly focused on sample FDA-40. To further identify the improved PCE, the light J−V curves for the champion cells with samples CA and FDA-40 are presented in Figure 5d, which were measured at both reverse scan (RS) and forward scan (FS). Table 1 summarizes the basic performance parameters, Table 1. Summary of Photovoltaic Parameters of the Champion Cells with Samples CA and FDA-40 Measured at FS and RS CA FDA-40

scan

Jsc (mA cm−2)

Voc (V)

FF

PCE (%)

FS RS FS RS

21.99 21.94 23.13 22.90

1.046 1.062 1.113 1.115

0.41 0.64 0.62 0.73

9.43 14.19 15.96 18.64

including short circuit current density (Jsc), open circuit voltage (Voc), fill factor (FF), and PCE together with J−V scan direction. Under RS, the champion cell with sample CA exhibits low PCE of 14.19% with Jsc of 21.99 mA/cm2, Voc of 1.062 V, and FF of 0.64. In contrast, all performance parameters are improved significantly for the champion cell with sample FDA40. The PCE jumps to 18.64% with corresponding Jsc, FF, and Voc increased to 22.90 mA/cm2, 1.062 V, and 0.73, respectively. Under FS, the increases in performance parameters are similarly observed for this cell. Furthermore, it appears that the notorious J−V hysteresis is also eliminated substantially for

Figure 6. Plots of (a) −dV/dJ vs (Jsc − J)−1 and (b) ln(Jsc − J) vs (V + RsJ) with the linear fittings for the champion cells with samples CA and FDA40. (c) J−V curves for the hole-only devices fitted with the Mott−Gurney law. The insert is the sketch of hole-only device configuration. (d) Impedance spectroscopy lifetime at different bias voltages for the champion cells with samples CA and FDA-40. 6109

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films, especially sample FDA-40, to the much improved transport property of charge carriers and greatly reduced nonradiative recombination sites in the textured film. Such desired characteristics mainly come from the unique microstructural features of the textured CH3NH3PbI3 film. As revealed in Figures 2 and 3, the textured CH3NH3PbI3 film is composed of micrometer-sized grains with excellent crystallinity and uniformly aligned orientation perpendicular to the substrate. On the one hand, the out of-plane order of grains means that the transport and transfer of charge carriers along the direction perpendicular to the substrate proceed in the single crystal grains and do so without crossing any grain boundaries.11,38,48 Such peculiarity gives rise to the excellent transport behavior of charge carriers in the textured CH3NH3PbI3 film. On the other hand, the large size and superior crystallinity of grains in the textured CH3NH3PbI3 film signifies the low density of crystal imperfections such as grain boundaries and intragranular defects. It is well-acknowledged that the crystal imperfections usually act as trap states and can cause serious nonradiative recombination of charge carriers in polycrystalline films.15,16,19,33 In the present case, the decreased nonradiative recombination sites in the textured CH3NH3PbI3 film can be thus ascribed to the reduced crystal imperfections. Further, the decrease in crystal imperfections can partially weaken the ion migration in polycrystalline CH3NH3PbI3 films,17,20 which explains well the suppressed J−V hysteresis in the cell with sample FDA-40. Furthermore, it is also beneficial to prevent the diffusion of water molecules from atmosphere into the film, ultimately boosting the stability of the cell with sample FDA-40.18,37,40 In short, the textured CH3NH3PbI3 film can effectively promote the PCE, suppress the J−V hysteresis, and improve the stability of planarheterojunction perovskite solar cells, which mainly correlate with the improved transport property of charge carriers and reduced nonradiative recombination sites as a result of uniformly aligned orientation, large size, and superior crystallinity of grains in the textured film.

heterojunction solar cell. In this manner, the series resistance (Rs) and saturated recombination current density (J0) can be derived from the intercepts of linear fitting of −dV/dJ vs (Jsc − J)−1 and ln(Jsc − J) vs (V + RsJ) plots,55,56 respectively, as given in Figures 6a and b. Rs is determined to be 4.07 and 1.82 Ω cm2 for the cells with samples CA and FDA-40, respectively, while the corresponding J0 values are 8.5 × 10−4 and 1.2 × 10−5 mA/ cm2, respectively. Thus, we can infer that the improved PCE for the cell with sample FDA-40 is mainly related to the much decreased Rs and J0. On the one hand, Rs represents the energy loss of charge carriers during the processes of transport and collection.55,56 Because the only difference consists in the light absorption layer for these two cells, the decrease in Rs may result from the improved transport behavior of charge carriers in sample FDA-40. This inference can be further verified by comparing the hole-only dark J−V curves of samples CA and FDA-40, as presented in Figure 6c. Compared with sample CA, the larger current densities at same bias voltages can be measured from sample FDA, indicating its better hole conductivity. Moreover, both of the films show ohmic behavior at low electric fields, followed by a nonlinear increase in current at the bias voltages that exceed the trap-filled limit (VTFL), demonstrating that all of the trap states are filled. The VTFL is related to trap density: VTFL = entL2/(2εε0),46 where e represents the elementary charge of the electron, L is the thickness of the CH3NH3PbI3 film, ε and ε0 are the relative dielectric constant of CH3NH3PbI3 (ε = 32) and vacuum permittivity, respectively, and nt is trap density. Thus, the hole trap density of sample CA is derived to be 2.83 × 10−16 cm−3, while the value decreases to be 1.69 × 10−16 cm−3 for sample FDA-40. When operating in the trap-free space charge limit current (SCLC) regime above 2.0 V, the dark current follows the Mott−Gurney law well: J = 9εε0V2/(8L3).46 Therefore, the hole mobility is estimated to be ∼23 and ∼49 cm2/(V s) for samples CA and FDA-40, respectively. The better hole conductivity as well as lower trap density and higher mobility suggest the more superior charge transport behavior in sample FDA-40. On the other hand, J0 represents the thermal emission rate of electrons from valence band to conduction band in the cell’s light absorbing layer, which is directly proportional to recombination rate of charge carriers.55 The decreased J0 for the cell with sample FDA-40 indicates the reduced recombination of charge carriers. Therefore, the charge carrier recombination lifetime under real cell working conditions was further quantified by impedance spectroscopy measurements as the product of recombination resistance and chemical capacitance.38,45 The Nyquist plots of measured impedance spectra for the cells with samples CA and FDA-40 are given in the inset of Figure 6d. A clear semicircle can be observed in intermediate frequency region, which reflects the charge transfer at FTO/ TiO2/CH3NH3PbI3/spiro-MeOTAD/Ag interfaces.25 Figure S4 gives the equivalent circuit that is used to fit the measured impedance spectra. The derived recombination lifetimes under various bias voltages are plotted in Figure 6d. The recombination lifetime decreases exponentially with the increase in bias voltage for both cells. Yet, the longer recombination lifetimes at each bias voltage can be observed for the cell with sample FDA-40, confirming the decreased charge recombination.38,45 This merit mainly originates from the reduced nonradiative recombination sites in sample FDA40, as proven by time-resolved PL results. On the basis of the above analysis, we can thus attribute the dramatically increased PCE of cells with textured CH3NH3PbI3

3. CONCLUSIONS In summary, we present a facile face-down annealing route to develop texture in one-step deposited CH3NH3PbI3 films. This annealing route, without use of any additives or special substrates, can controllably realize the (110) textured CH3NH3PbI3 films. Such films are composed of micrometersized grains with excellent crystallinity and uniformly aligned orientation perpendicular to the substrate. Further, all of the grains can vertically penetrate the entire film thickness. As a result, the planar-heterojunction perovskite solar cells with these textured CH3NH3PbI3 films show remarkably improved PCEs with the average of 17.11% and the highest up to 18.64%. The notorious J−V hysteresis is also alleviated substantially in the champion cell. More importantly, the champion cell can maintain 86% of its initial PCE after storage of 20 days in ambient conditions with relative humidity of 10−20%. Such desired performance features can be mainly attributed to the much reduced nonradiative recombination sites as well as greatly improved transport property of charge carriers in the textured CH 3NH 3PbI 3 films. This work represents an important step toward realizing high-quality CH3NH3PbI3 films for efficient optoelectronic devices, especially perovskite solar cells. Further, we envision that it can be generalized to other OTP films such as CH3NH3PbBr3, HC(NH2)2 PbI3, CsPbI3, and mixtures thereof. 6110

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4. EXPERIMENTAL SECTION Materials and Reagents. All chemicals are of analytical grade and used without further purification. PbI2 (99.999%), DMF (99.8%), DMSO (≥99.9%), anhydrous acetonitrile (99.8%), 4-tert-butylpyridine (TBP, 96%), bis(trifluoromethane)sulfonimide lithium salt (Li-TFSI, 99.95%), hydroiodic acid (no stabilizer, 57 wt % in H2O), and methylamine (33 wt % in absolute ethanol) were purchased from Sigma-Aldrich. Chlorobenzene (99.5%) was purchased from Aladdin Reagents. Absolute ethanol, diethyl ether, and acetone were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). SpiroMeOTAD (≥99%) was purchased from Shenzhen Feiming Science and Technology Co., Ltd. (China). CH3NH3I Synthesis. Thirty milliliters of hydroiodic acid was added dropwise to 25 mL of methylamine under N2 atmosphere with vigorous stirring at 0 °C for 2 h. The solvent was evaporated with a rotary evaporator at 70 °C, and the resultant product was washed twice with diethyl ether. Then, it was redissolved in ethanol at 70 °C, recrystallized by diethyl ether at room temperature, and dried at 60 °C in a vacuum oven for 24 h. Thus, the CH3NH3I powder was obtained. Preparation of Compact TiO2/FTO Glass Substrates. FTO glass slides with sheet resistance of 14 Ω/sq were cleaned by sonication in detergent, deionized water, acetone, and absolute alcohol for 30 min successively and dried by nitrogen stream. Finally, they were cleaned by UV-ozone treatment for 30 min. The compact TiO2 layer with the thickness of ∼100 nm was then coated on the FTO layer by spin-coating of TiO2 sol at 3000 rpm for 30 s, followed by annealing at 450 °C for 60 min in air. Thus, the compact TiO2/FTO glass substrates were attained. Solar Cell Fabrication. The CH3NH3PbI3 precursor film was first deposited on the TiO2/FTO glass substrates by the Lewis base adductbased one-step spin-coating method proposed by Park et al.13 Specifically, 50 μL of precursor solution with the premixed 160 mg of CH3NH3I, 461 mg of PbI2, 78 mg of DMSO, and 600 mg of DMF was dropped on the TiO2/FTO glass substrates, which was followed by spinning at 4500 rpm for 20 s. During the initial 6 s of the spin process, 500 μL of diethyl ether was added dropwise on the substrates. After spinning, the CH3NH3PbI3 precursor films were formed. These films were crystallized to tetragonal CH3NH3PbI3 by the face-down annealing as follows. First, the CH3NH3PbI3 precursor film coated TiO2/FTO glass substrates were placed face-up on a hot plate (the hot plate surface was polished with sandpaper repeatedly prior to use) and preheated in batches at the temperatures of RT, 40, and 60 °C for 30 s. Subsequently, all of the films were turned face-down and annealed at 100 °C for 10 min. After being removed from the hot plate, a series of blackish CH3NH3PbI3 films were thus obtained. To fulfill the solar cell fabrication, a 100 nm spiro-MeOTAD layer was coated on the CH3NH3PbI3 film by spinning 100 μL of the premixed solution of 72.5 mg of spiro-MeOTAD, 28 μL of TBP, 17.5 μL of Li-TFSI in acetonitrile, and 1 mL of chlorobenzene at 3000 rpm for 20 s. After being stored in a drybox for 24 h, the Ag top anode with a thickness of ∼100 nm was deposited by the thermal evaporation method. Thus, the complete solar cells were obtained. Cell performance was measured by the method described in our previous work.25

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Research Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Tao Yu: 0000-0003-1981-3469 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported primarily by the national Natural Science Foundation of China (Grant 61377051), the National Basic Research Program of China (Grant 2013CB632404), and the Natural Science Foundation and Key Research Project of Jiangsu Province (Grants BK20130053, BK20141233, and BE2015090).

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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/acsami.6b15563. Fourier transform infrared spectra of CH3NH3PbI3 precursor films and the crystallized CH3NH3PbI3 film, XRD pattern of the CH3NH3PbI3 film prepared by twostep spin-coating method, the statistical grain size for the concerned samples, and the equivalent circuit for Nyquist data fitting (PDF) 6111

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