Improved Crystallization of Perovskite Films by Optimized Solvent

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Improved Crystallization of Perovskite Films by Optimized Solvent Annealing for High Efficiency Solar Cell Jiang Liu,*,† Cheng Gao,†,‡ Xulin He,† Qinyan Ye,† Liangqi Ouyang,§ Daming Zhuang,§ Cheng Liao,*,† Jun Mei,† and Woonming Lau† †

Chengdu Green Energy and Green Manufacturing Technology R&D Centre, Chengdu Development Center of Science and Technology, China Academy of Engineering Physics, Chengdu 610207, China ‡ School of Optoelectronic Information, University of Electronic Science and Technology, Chengdu 610054, China § School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China S Supporting Information *

ABSTRACT: Organic−inorganic halide perovskite-based thin film solar cells show excellent light-to-power conversion efficiency. The high performance for the devices requires the preparation of well-crystallized perovskite absorbers. In this paper, we used the postannealing process to treat the perovskite films under different solvent vapors and observed that the solvent vapors have a strong effect on the film growth. A model regarding the perovskite film growth was proposed as well. Intensive characterizations including scanning electron microscopy, electrochemical impedance spectroscopy, and admittance spectroscopy allowed us to attribute the improved performance to reduced recombination loss and defect density. Solar cell based on the DMSO-treated films delivered a power conversion efficiency of over 13% with negligible photocurrent hysteresis. KEYWORDS: organic lead iodide, solvent annealing, crystallization, film growth, solar cell, PCBM

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INTRODUCTION Organic−inorganic halide perovskites have received considerable attention in recent years owing to their excellent optical and electronic properties including high absorption coefficient, low exciton binding energy,1−3 ambipolar charge transport,4 and long electron−hole diffusion length.5,6 Methylammonium lead iodide (CH3NH3PbI3, MAPbI3) is the most commonly used perovskite photovoltaic material and has a direct bandgap of 1.55 eV, which makes it a good light absorber in photovoltaic devices.7,8 By substituting bromine for iodide, the band gap can be varied from 1.55−2.3 eV.8 Owing to these merits, the application for this kind of perovskite materials in organic lasing and light-emitting devices has also been explored extensively.9,10 Since the first report of perovskite materials in photovoltaic device was published in 2009,11 impressive progress in improving the photovoltaic performance has been made.12−17 Besides their high conversion efficiency, one of the particular advantages of the perovskite materials is that they can be solution processed into thin films at low temperature, which could benefit the fabrication of low-cost solar cells.18−20 Two types of device architectures are usually used for constructing perovskite solar cells. It was initially assumed that a mesostructured (or nanostructured) matrix may be necessary for efficient charge transport and collection in perovskite solar cell, but many studies21,22 further confirmed that planar device structure is also very suitable for perovskite solar cell due to © XXXX American Chemical Society

their long charge diffusion lengths. In the case of planar device structure, the common TiO2-based device always suffers from severe hysteresis of photocurrent,23−25 whereas the inverted pi-n planar device with n-type PCBM ((6,6)-phenyl-C61-butyric acid methyl ester) layer on the top of perovskite film shows negligible photocurrent hysteresis.26,27 The performance of planar device is expected to depend strongly on film crystallinity and morphology. A great deal of effort has been put to control the morphology of the resulting film by adjusting the annealing condition28,29 using additives30−32 and mixed solvents,13 etc. Recently, Xiao et al.33 and Jeon et al.13 independently reported a nonpolar washing process involving the use of nonpolar solvent during the spin-coating of perovskite film. The nonpolar solvent could induce fast crystallization of perovskite film and allow us to obtain an extremely flat surface with 100% coverage, which is important for achieving high-performance devices. However, in our past work,34 the perovskite film by the method always demonstrated a relatively low crystallinity with grain size in the range of 100− 300 nm, which is lower than the film thickness. The low crystallinity could influence the improvement of device performance according to the empirical observation. Therefore, Received: July 27, 2015 Accepted: October 13, 2015

A

DOI: 10.1021/acsami.5b06780 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 1. Schematic diagram of the annealing treatment of perovskite films under solvent vapor.

Figure 2. Surface (top row) and cross-sectional (bottom row) SEM images of perovskite films on ITO-coated substrate after annealing under different atmospheric conditions: (a, f) N2, (b, g) H2O, (c, h) GBL, (d, i) DMF, (e, j) DMSO. was spin coated onto the substrates at 5000 rpm for 55 s. During the spin-coating, 50 μL of anhydrous chlorobenzene (Sigma-Aldrich) was quickly dropped in the center of the substrates after six or seven seconds. The spin-coated films were transferred onto a hot plate for drying at 100 °C for 2 min and then annealed under different atmospheres at 100 °C for 30 min. For the N2-treated sample, the annealing treatment was carried out under a dry nitrogen atmosphere in a glovebox (Innovative Technology Inc.). For the solvent-treated samples, 10 μL of solvent was dropped into a small ceramic crucible, and then a glass Petri dish was used to cover the samples and crucible. Solar Cell Fabrication. Solar cells with ITO/PEDOT:PSS/ perovskite/PCBM/Ag planar device structure were prepared. The ITO glass substrates (1.5 cm × 1.5 cm) were ultrasonically cleaned with detergent, deionized water, acetone, and ethanol, sequentially, and then were blow-dried in nitrogen. Before deposition of other films, the ITO substrates were treated with ultraviolet-ozone for 10 min. After that, PEDOT:PSS solution (Clevious Al 4083) was spin-coated on ITO-coated substrates at 3000 rpm for 45 s, and subsequently the PEDOT:PSS films were annealed at 150 °C for 20 min. After perovskite films were deposited, 20 mg/mL of PCBM (FEM. Inc.) solution in chlorobenzene was then spin-coated at 1000 rpm for 45 s. Finally, Ag top contact was deposited by evaporation through an aperture mask under a base pressure of 7 × 10−6 Torr. The active device area is 0.12 cm2. Measurement and Characterization. Scanning electron microscope (SEM) images were obtained using Hitachi S5200. X-ray diffraction (XRD) analysis was performed on a D/max-RB diffractometer (Rigaku) using Cu Ka radiation at a scan rate of 6°/ min. Steady-state photoluminescence (PL) spectra were measured using an excitation wavelength of 430 nm in an Edinburgh FLS 920 spectrophotometer. Atomic force microscopy (AFM) measurements were performed using Asylum Research MFP-3D. Current−voltage measurements were carried out using Keithley 2400 at room

it is desired to develop some treatment approaches to increase the perovskite grain size for further advancement. Recently, You et al.35 reported that the perovskite film annealed in ambient air demonstrated improved grain size and carrier mobility compared with that in nitrogen, which is regarded as an example of solvent annealing process.36 Inspired by their work, here we adopted several solvents for annealing treatment of perovskite films, and the perovskite films annealed under different solvent vapor were systematically investigated in terms of its crystallization behavior as well as the corresponding photovoltaic performance. It is found that the improved crystallinity after annealing under those solvent has strong effect on the charge collection and resulted in increased short circuit current density (Jsc) and power conversion efficiency (PCE).

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EXPERIMENTAL SECTION

Perovskite Film Fabrication. Methylammonium iodide (CH3NH3I, MAI) was synthesized according to the reported procedure37 by reacting the hydroiodic acid (10 mL, 57 wt % in water, Sigma-Aldrich) and methylamine (24 mL, 33 wt % in absolute ethanol, Sigma-Aldrich) at 0 °C with stirring for 60 min. Raw CH3NH3I was obtained by removing the solvent at 50 °C on a rotary evaporator. The material was then washed in diethyl ether and filtered several times. The precipitate was then dried in vacuum oven overnight at 50 °C and then kept in nitrogen-filled glovebox. The perovskite precursor solution was prepared by mixing 1.2 mmol of PbI2 (99.99%, Alfa Aesar) and 1.2 mmol of CH3NH3I in 1 mL of anhydrous N,N-dimethylformamide (DMF, Sigma-Aldrich). To prepare perovskite films, 50 μL of perovskite precursor solutions B

DOI: 10.1021/acsami.5b06780 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 3. Growth model for annealing treatment of perovskite films under solvent vapor.

Figure 4. (a) XRD and (b) normalized PL spectra of perovskite films with different annealing processes on glass substrates. temperature under AM 1.5G illuminations (1000 W/m2) from a solar simulator, which was calibrated using a standard silicon solar cell device. Incident photon-current conversion efficiency (IPCE) spectra were measured from 300−850 nm using a Xe source. The signal was recorded using a Model SR830 DSP Lock-In Amplifier (Stanford Research Systems) and calibrated using a standard silicon detector. Impedance spectra (IS) were measured using an electrochemical workstation (Zahner, Zennium) with a 20 mV rms amplitude over the frequency range of 1 Hz to 1 MHz under low light intensity (0.1 sun). Frequency-dependent capacitances were measured in the parallel equivalent circuit mode at zero bias. The density of the defect is given by the equation: Nt (Eω) = −Ud/qω dC/dω ω/kT, where Ud denotes the built-in voltage in the junction, ω is the angular frequency, q is the elementary charge, k is the Boltzmann’s constant, and T is the temperature. The demarcation energy Eω is given by the equation Eω = kT ln(ν/ω), where ν is the attempt-to-escape frequency. Only the defects below the demarcation energy can follow the applied ac voltage, and the charging and discharging of the defect contribute to the capacitance.

the planimetric method involving the counts of the number of grains within a given area. Some voids were often found inside the film. It may occur during the stage of nonpolar washing where the perovskite species precipitate from precursor solution and shrink leaving isolated voids behind. In addition, it is noticed that the site of the voids is mainly positioned at the bottom of the film, which could be attributed to the slower nonpolar induced precipitation rate at that region. The film could still be said to be continuous and free of pinhole-free. After the annealing treatment under GBL and DMSO, the voids disappear, and the crystallinity improves obviously. The surface morphology of the perovskite films was also characterized by the AFM measurement, shown in Figure S1. The root−mean−square (rms) roughness values were calculated to be 10.73 nm, 11.49 nm, 21.53 nm, 14.58 nm, and 25.75 nm for that under N2, H2O, DMF, GBL, and DMSO, respectively. It could be found that the film growth results in increase of surface roughness. Despite that, all those rms values were relatively low, and the films still retains a uniform, pinhole-free surface (Figure S2), which is critical for efficient planar devices. No obvious grain growth was found for the film treated with H2O. It is known that H2O can degrade the perovskite into MAI and PbI2 and dissolve MAI species with PbI2 residue. That may indicate only MAI species could migrate during the annealing process, which inhibited the film growth. Another important factor to consider is that these processes take place at the substrate temperatures of 100 °C, which equals to the boiling point of H2O under normal atmosphere. This may mean that most of H2O is hard to reach the surface of perovskite film, which thus makes the case almost the same as that under N2. GBL, DMF, and DMSO are the most commonly used polar solvents for perovskite precursor solution. During the annealing treatment, these polar solvents vapors could erode the perovskite film surface and thus shape the film morphology. The variation in morphology and crystallinity would be expected because there are some differences for the three solvents in coordinating capability and vapor pressure. The vapor pressure at room temperature (20 °C) is 1.5, 2.7,

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RESULTS AND DISCUSSION Figure 1 illustrates the preparation procedure of perovskite films in this study. First, the pristine perovskite films were prepared based on the study of Xiao et al.38 in which nonpolar solvent chlorobenzene was dripped on the substrate during spinning. After transferred onto the hot plate, the perovskite films turn dark brown quickly and display mirror-like appearance. Then, the perovskite films were subjected to postannealing treatment. A small crucible was put on the center of hot plate, and the samples are placed around the crucible. After 10 μL of solvent was dripped into the small crucible, the samples and crucible were covered by a glass Petri dish rapidly. To investigate the influence of different annealing atmospheres on perovskite films, five atmospheric conditions including N2, H2O, DMF, γ-butyrolactone (GBL), and dimethyl sulfoxide (DMSO) were employed. Figure 2 shows the SEM micrographs of the perovskite films after annealing under different solvent vapor. The perovskite film annealed under N2 exhibits an extremely flat surface with fine crystallites. The surface grain size was estimated to be approximately 160 nm according to C

DOI: 10.1021/acsami.5b06780 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 5. (a) J−V curves under both (open square) forward and (solid square) reverse scans, and (b) IPCE curves of the perovskite solar cells with different annealing processes.

Table 1. Photovoltaic Parameters of Devices Annealed under Different Atmospheric Conditions. Average PCE Values Were Obtained from 5−8 Cells for Each Type of Devices solvent N2 H2O GBL DMF DMSO

scan direction

Voc (V)

Jsc (mA/cm2)

FF (%)

PCEbest (%)

forward reverse forward reverse forward reverse forward reverse forward reverse

0.93 0.91 0.95 0.94 0.92 0.92 0.91 0.91 0.93 0.93

16.9 17.1 18.7 18.6 20.9 20.8 20.2 20.3 20.9 20.9

0.53 0.55 0.51 0.51 0.64 0.65 0.62 0.64 0.68 0.69

8.34 8.55 8.99 8.85 12.29 12.47 11.29 11.89 13.21 13.59

PCEaverage (%) 7.39 7.59 7.50 7.62 11.50 11.32 10.47 10.65 11.89 12.04

± ± ± ± ± ± ± ± ± ±

0.62 0.60 1.40 1.14 0.74 0.86 0.82 0.70 1.43 1.27

cell refinement, which is consistent with the reported results. The two main peaks located at 14.1° and 28.4° can be indexed to the (110) and (220) planes. For the films after exposure to the GBL, DMF, and DMSO vapor, several peaks at 23.5°, 24.5°, and 31.9° diffracted by the (211), (202), and (310) planes become stronger. That could be explained that the annealing treatments under GBL, DMF, or DMSO induce a strong recrystallization process and lead the grains to rearrange into a more favorable packing arrangement. In addition, for the film treated under H2O, a peak at 12.7° related to PbI2 (001) is also obviously observed, which indicates the partial decomposition of perovskite film after exposure to H2O. The steady PL for the perovskite film on glass substrate was measured, as shown in Figure 4, panel b. An excitation light of 430 nm falls on the sample from the air side, and the PL signal was also collected from the air side. Four samples among those exhibit a PL peak at 785 nm, which is in agreement with data reported elsewhere. Nevertheless, the film treated under DMSO has a PL peak at 775 nm, which is blue-shifted relative to others. Generally, the PL emission is related to the recombination channel concerning the bandgap and trap state. The blueshift phenomenon may imply the fully grown film under DMSO vapor has a relatively lower trap density around the band-edge than others, which is expected to reduce recombination loss and hence improve photocurrent collection. To probe the effect of annealing treatment on device performance, perovskite solar cells from the perovskite film annealed under different atmospheric conditions were fabricated. Because hysteresis phenomenon often exists in perovskite devices depending on the quality of the device and the architecture, to better characterize the device performance, the forward and reverse current density−voltage (J−V) curves

and 0.42 mmHg for GBL, DMF, and DMSO, respectively, which is consistent with the trend of grain size of perovskite films. Since DMSO has a low volatility due to its relatively high boiling point (189 °C) and low saturated vapor pressure,39 it is very likely that the DMSO vapor environment could be maintained in a long time and thus results in remarkable film growth. On the basis of detailed investigation of the growth of perovskite films, a model shown schematically in Figure 3 was proposed. As the films were placed in a closed space, the solvent vapor molecular could condense on the surface of perovskite films, and simultaneously the high substrate temperature could lead to its re-evaporation, in which a simple dynamic near-equilibrium may exist. It is well-known that those polar solvent can dissolve the perovskite film. With the films on hot plate and limited solvent vapor pressure, the evaporated solvent molecule would not cause wide-range dissolution and impose irreparable damages, but probably lead to the formation of liquid or quasi-liquid phase on the surface and void area of perovskite film. The liquid phase could play a role of binder, which bonds the adjacent grains and remelts a significant fraction of the grain boundary. The atoms will go into the liquid phase across the liquid−solid interface and then recrystallize in areas of lower chemical potential where grains are not close or in contact. The process can lead to the film densification in a way similar to liquid phase sintering,40 as evidenced by the comparison between the N2-treated sample and solvent-treated samples. To check the structural variation, we measured the XRD patterns of perovskite films, as shown in Figure 4, panel a. That indicates the prepared films have a tetragonal crystal structure and a lattice parameters of a = b = 8.87 Å and c = 12.65 Å after D

DOI: 10.1021/acsami.5b06780 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 6. (a) Representative Nyquist plots measured at 200 mV applied bias for perovskite solar cells annealed under N2, H2O, and DMSO. (b) Applied bias dependences of recombination resistance extracted from the fitting of the arc at lower frequencies.

Figure 7. (a) Frequency dependences of capacitance for perovskite solar cells with different annealing processes. (b) Density of states (DOS) distribution derived from admittance spectroscopy.

The Jsc value increases from 16.9 mA/cm2 for N2-treated device to 18.7 mA/cm2 for H2O-treated device and to 20.9 mA/cm2 for DMSO-treated device. The trend is in accordance with the SEM observation in Figure 2, indicating the improved crystallization results in efficient charge collection. To further investigate this issue, incident photon-current conversion efficiency (IPCE) measurements have been performed. Figure 5, panel b shows the representative IPCE curves for those devices. The curves start increasing rapidly at 800 nm, which is related to the optical absorption of the perovskite absorber. For the N2-treated device, a relatively lower IPCE intensity was observed, thus contributing to lower Jsc value. For the GBL-, DMF-, and DMSO-treated samples, the IPCE curves show a broad plateau of >75% in the wavelength range of 420−750 nm and thus yield a high Jsc value. The calculated photogenerated current density values by integrating the IPCE curves with the AM 1.5G spectrum fall in the range of 19.0−19.7 mA/cm2 for the three devices, which is close to the measured Jsc values. The DMSO-treated device has the highest PCE among those, exhibiting a Jsc of 20.9 mA/cm2, FF of 0.68, Voc of 0.93 V, and PCE of 13.21% under forward scans, and a Jsc of 20.9 mA/cm2, FF of 0.69, Voc of 0.93 V, and PCE of 13.59% under reverse scans. To gain insight into the charge transport properties of the solar cells, electrochemical impedance spectroscopy (EIS) was performed. The obtained spectra show two semicircles and were fitted following a simplified circuit model (Figure 6b, inset). According to the previous report,43 the lower frequency arc of the impedance spectra is commonly attributed to the recombination resistance (Rrec), and Rrec is inversely related to

were recorded in this study. Figure 5 shows the J−V curves for the best performing devices measured under forward and reverse scans. The detailed photovoltaic parameters are also summarized in Table 1. As expected, the devices show negligible photocurrent hysteresis, which is already a noteworthy feature derived from the PCBM passivation of perovskite trap states for the inverted perovskite/PCBM planar device. The solar cell annealed under N2 has an average efficiency of 7.39% measured by forward scan, while the device annealed under H2O show the value to be 7.50%. The XRD results in Figure 4, panel a show that there is a pronounced presence of PbI2 in the film annealed under H2O; nevertheless, the corresponding solar cells still demonstrated a slightly better performance than that under N2. The results could, to some extent, confirm that the impurity phase PbI2 in perovskite film would not cause deadly consequence. In addition, the role of PbI2 has been discussed by some researchers recently.41,42 Their works showed that a small amount of PbI2 in perovskite films could act as passivation layer and lead to longer recombination time.42 In this work, the increase in the opencircuit voltage (Voc) and Jsc could be attributed to the combined effect of PbI2 passivation and improved crystallinity. It can be also associated with the aging process of perovskite solar cells in which the amount of PbI2 increased gradually from the decomposition of perovskite layer. In the beginning stage of degradation, the decrease in performance may arise mainly from the reduced amount of perovskite absorber, which lowers the utilization efficiency of incident light. However, when the amount of impurity phase PbI2 reaches to a certain degree, the performance would decay rapidly. E

DOI: 10.1021/acsami.5b06780 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

method could be extended to more perovskite films prepared by other techniques.

the recombination rate. Figure 6, panel a shows the representative Nyquist plots of the three solar cells annealed under N2, H2O, and DMSO. Obviously, the plot for the DMSO-treated device shows a broader spectrum than others, indicating a low recombination loss and, consequently, contributing to high PCE. The Rs value evaluated from the high frequency intercept on the x-axis falls in the range of 11− 13 Ω and shows little variation for the devices, indicating annealing treatment under different atmospheric condition has no strong effect on the contact resistance. The recombination resistance Rrec extracted under different bias was shown in Figure 6, panel b. The Rrec decreases exponentially with increasing applied bias, which is related to the recombination mechanism. The slopes for the curves in Figure 6, panel b are nearly the same. This may imply that there is not a significant difference in types of the recombination mechanisms for those devices. Nevertheless, the Rrec for DMSO-treated sample is substantial higher than others in a wide bias voltage range, indicating a lower carrier recombination loss intensity. It could be associated with the grain boundary density in perovskite layer. Generally, the grain boundaries can act as charge carrier trapping centers. The DSMO-treated sample demonstrates a large grain size comparable to its thickness and thus results in a very low recombination loss. To provide more information on the electrical defect characteristics in our perovskite solar cell, we measured the frequency-dependent capacitance by following the procedure described in the literature.44−46 The admittance spectroscopy has been proposed as a tool to determine the density of the defect states (Nt) in metal-oxide-semiconductor (MIS), Schottky junctions, and solar cells.44−46 As seen in Figure 7, the capacitance increases as the frequency decreases, and this is related to the defect emission. The defect distribution (Figure 7b) can be obtained by calculating the derivative of the capacitance with respect to the frequency, which shows a taillike defect distribution for all samples. It is necessary to point out that the energetic position may not be completely accurate due to a lack of corrections of some parameters such as attempt-to-escape frequency. The purpose is to analyze the trend in the perovskite devices with different annealing condition. The major difference in Figure 7, panel b appears in the energy range of >0.5 eV, in which the GBL- and DMSOtreated samples show a relatively lower value. The reduced density of defect states would lower the charge trapping centers and thus increase the FF and PCE.

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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.5b06780. AFM height images and SEM images of perovskite films after annealing under different annealing processes, representative Nyquist plots of perovskite solar cells (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We wish to thank Professor Xiaowei Guo for asking student Cheng Gao for help with thin-film preparation and EIS measurements. This work is supported by the National Nature Science Foundation of China (51202227), the Science and Technology Development Foundation of China Academy of Engineering Physics (2014A0302015 and 2014B0302054), the Synergistic Innovation Joint Foundation of CAEP-SCU (XTCX2014008), Sichuan International Cooperation Research Project (No. 2014HH0068), and the Fundamental Research Funds for the Central Universities of China (No. 2672012ZYGX2012J065).

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REFERENCES

(1) D’Innocenzo, V.; Grancini, G.; Alcocer, M. J.; Kandada, A. R.; Stranks, S. D.; Lee, M. M.; Lanzani, G.; Snaith, H. J.; Petrozza, A. Excitons Versus Free Charges in Organo-Lead Tri-Halide Perovskites. Nat. Commun. 2014, 5, 3586. (2) Manser, J. S.; Kamat, P. V. Band Filling with Free Charge Carriers in Organometal Halide Perovskites. Nat. Photonics 2014, 8, 737−743. (3) Lin, Q.; Armin, A.; Nagiri, R. C. R.; Burn, P. L.; Meredith, P. Electro-Optics of Perovskite Solar Cells. Nat. Photonics 2014, 9, 106− 112. (4) Laban, W. A.; Etgar, L. Depleted Hole Conductor-Free Lead Halide Iodide Heterojunction Solar Cells. Energy Environ. Sci. 2013, 6, 3249−3253. (5) Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J.; Leijtens, T.; Herz, L. M.; Petrozza, A.; Snaith, H. J. Electron-Hole Diffusion Lengths Exceeding 1 Micrometer in an Organometal Trihalide Perovskite Absorber. Science 2013, 342, 341− 344. (6) Xing, G.; Mathews, N.; Sun, S.; Lim, S. S.; Lam, Y. M.; Gratzel, M.; Mhaisalkar, S.; Sum, T. C. Long-Range Balanced Electron- and Hole-Transport Lengths in Organic-Inorganic CH3NH3PbI3. Science 2013, 342, 344−347. (7) Gratzel, M. The Light and Shade of Perovskite Solar Cells. Nat. Mater. 2014, 13, 838−842. (8) Noh, J. H.; Im, S. H.; Heo, J. H.; Mandal, T. N.; Seok, S. I. Chemical Management for Colorful, Efficient, and Stable InorganicOrganic Hybrid Nanostructured Solar Cells. Nano Lett. 2013, 13, 1764−1769. (9) Tan, Z. K.; Moghaddam, R. S.; Lai, M. L.; Docampo, P.; Higler, R.; Deschler, F.; Price, M.; Sadhanala, A.; Pazos, L. M.; Credgington, D.; Hanusch, F.; Bein, T.; Snaith, H. J.; Friend, R. H. Bright LightEmitting Diodes Based on Organometal Halide Perovskite. Nat. Nanotechnol. 2014, 9, 687−692.

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CONCLUSION In summary, we investigated the effect of annealing treatment of perovskite films under different atmospheric conditions on the crystallinity of perovskite films and photovoltaic performance of the solar cells. A model was derived to explain the role of solvent vapor in improving film crystallinity. The vapor could condense on the surface and void area of perovskite film and lead to the elimination of the grain boundaries and appropriate rearrangement of perovskite film. Our results demonstrated the importance of the fully grown perovskite films in collecting photogenerated charge carrier and reducing recombination loss. The DMSO-treated perovskite film exhibits good crystallinity with grain size exceeding its film thickness. With improved crystallization, the perovskite/PCBM planar solar cell exhibits a power conversion efficiency of over 13% under AM 1.5 G, 100 mW/cm2 condition. We believed the post annealing treatment F

DOI: 10.1021/acsami.5b06780 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

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DOI: 10.1021/acsami.5b06780 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsami.5b06780 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX