Dynamic Growth of Pinhole-Free Conformal CH3NH3PbI3 Film for

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Dynamic growth of pinhole-free conformal CH3NH3PbI3 film for perovskite solar cells Bo Li, Jianjun Tian, Lixue Guo, Chengbin Fei, Ting Shen, Xuanhui Qu, and Guozhong Cao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b11679 • Publication Date (Web): 28 Jan 2016 Downloaded from http://pubs.acs.org on January 31, 2016

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ACS Applied Materials & Interfaces

Dynamic growth of pinhole-free conformal CH3NH3PbI3 film for perovskite solar cells Bo Li1, Jianjun Tian1*, Lixue Guo2, Chengbin Fei2, Ting Shen1, Xuanhui Qu1, Guozhong Cao2, 3* 1 Institute of Advanced Materials and Technology, University of Science and Technology Beijing, 100083, China. 2 Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, 100083, China 3 Department of Materials and Engineering, University of Washington, Seattle, WA 98195-2120, USA. *Corresponding authors: [email protected]; [email protected] ABSTRACT Two-step dipping (TSD) is one of the popular low temperature solution methods to prepare organic-inorganic halide perovskite (CH3NH3PbI3) films for solar cells. However, pinholes in perovskite films fabricated by static growth method (SGM) results in low power conversion efficiency (PCE) in the resulting solar cells. In this work, the static dipping process is changed into dynamic dipping process by 1

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controlled stirring PbI2 substrates in CH3NH3I isopropanol solution. The dynamic growth method (DGM) produces more nuclei and decreases the pinholes during the nucleation and growth of perovskite crystals. The compact perovskite films with free pinholes are obtained by DGM, which present that the big perovskite particles with size of 350 nm are surrounded by small perovskite particles with size of 50 nm. The surface coverage of the perovskite film is up to nearly 100%. Such high quality perovskite film not only eliminated pinholes resulting in reduced charge recombination of the solar cells, but also improves the light harvesting efficiency. As a result, the PCE of the perovskite solar cells is increased from 11% for SGM to 13% for DGM. KEYWORDS CH3NH3PbI3 film, two step dipping process, dynamic growth, surface coverage, perovskite solar cells. 1. INTRODUCTION Organic-inorganic halide perovskite solar cells (PSCs) have attached much attention in the last several years, intensive research has resulted in a rapid progress and reached the certified power conversion efficiency (PCE) over 20%.1 Hybrid perovskites, such as (C4H9NH3)2PbI4, CH3NH3PbI3 and CH3NH3SnI3, have been synthesized using inexpensively low temperature solution processes for several decades.2 They have been widely used in organic light-emitting diodes (OLEDs), owing to high electron mobility, unique magnetic and optical properties.3, 4 As one of the excellent light absorbing materials, the bandgap of CH3NH3PbI3 is 1.5eV measured by ultraviolet-visible absorption spectroscopy (UV-Vis) and ultraviolet 2


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photoelectron spectroscopy (UPS),5, 6 which is very close to the Shockley–Queisser limit optimal band gap (1.34 eV) for solar cells.7 The incident photon conversion efficiency (IPCE) of PCSs can reach 90% in the wavelength range from 450~550 nm and has an excellent response from 550~850nm,8, 9, 10 providing an intense and broad absorption to visible light. In addition, CH3NH3PbI3 possesses excellent electrons and holes transport properties.8,

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However, the hybrid perovskite materials were

seldom successfully applied in solar cells, until Tsutomu Miyasaka and coworkers first reported photoelectrochemical cells that using CH3NH3PbBr3 and CH3NH3PbI3 as visible-light sensitizers in 2009.13 In spite of the intensive research efforts, fabrication methods of perovskite films for PSCs have remained unchanged, such as one step spin coating (OSC) and two step dipping (TSD),

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such fabrication methods often lead to poor quality of perovskite

films.15, 16, 17, 18 Although dual-source vapor deposition can achieve extremely uniform perovskite films and good photovoltaic performance,19 the fabrication process needs specific instruments and is energy intensive. There are still many efforts to focus on the improvement of the low temperature solution process including OSC and TSD. Adjusting the precursor solution concentration and composition is a simple and effective method to improve morphology of perovskite film. Li et al studied the different perovskite capping layer by controlling the concentration of PbI2 solution in TSD,20 the highest of PCE up to 10.3% was obtained. Lm et al found that the size of CH3NH3PbI3 crystals was controlled by the CH3NH3I solution concentration using two-step spin-coating procedure.9 Choosing additives or solvents can control the growth process of perovskite crystal. Zhao et al introduced a method to improve the crystallization process of perovskite by adding CH3NH3Cl to the one step deposition precursor.21 Jeon et al developt a solvent-engineering technology to form uniform and 3



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dense perovskite layer. As a result, a certified 16.2% PCE was obtained.17 The temperature is a key for the growth kinetics of perovskite crystals. Nie et al reported a hot-casting one step technique to fabricate millimeter-scale crystalline grains, the PCE reached 18% with 2% variability in the overall PCE.22 In brief, with the development of the fabrication methods for perovskite film, controlling perovskite morphology and crystallites becomes essential process for reproducibility and high efficient PSCs.20, 23, 24, 25

To the best of our knowledge, pioneer researches don’t pay much attention to control the growth process in TSD to improve the quality of the perovskite film. It is likely to be owed to the fast reaction between PbI2 and CH3NH3I, which is very difficult to control during TSD process. As for TSD, PbI2 is coated on the mesoporous TiO2 scaffolds by spin coating and then the scaffolds are immersed in the precursor solution for a period of time to form perovskites. The whole process of nucleation and growth of perovskites is under static conditions, which facilitates the formation of pinholes in the perovskite capping layer.25 In order to reduce the pinholes, we changed the second process in TSD from static into dynamic as depicted in Scheme 1. After the stepⅠof spin coating PbI2, the substrates coated PbI2 film were fixed on the fixture and then are placed in CH3NH3I isopropanol (IPA) solution, thus, a variety of stirring process was obtained by controlling direct current motor. It is exciting to obtain a compact CH3NH3PbI3 films by the device. Unlike usual TSD (the static growth method, referred to as SGM), we have found that dynamic growth method (DGM) had a notable influence on the nucleation and growth of perovskite crystals. As a result, the surface coverage of perovskite film reached nearly 100%. The contact of the bareness of mesoporous TiO2 layer and hole transporting layer (spiro-MeOTAD) 4


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was significantly avoided. Besides, the formation mechanism of pinhole-free perovskite film was also discussed in this paper. Due to high quality perovskite film, the PCE of PSCs was increased from 11% to 13%.

Scheme 1. Schematic of perovskite samples fabricated by SGM and DGM.

2. EXPERIMENTAL SECTION 2.1. Materials. CH3NH3I was prepared by the reported method.6 Methylamine in methanol (33 wt%) was mixed with the hydroiodic acid (HI, 57 wt%) in a molar ratio 1.2:1 at 0℃. The HI was added dropwise while stirring. After stirring for 2 h, the solvent was removed by the rotary evaporator at 50℃. Then the obtained powder was washed by diethyl ether, and recrystallized in methanol. Finally, the white powder was dried at 60℃for 24 h in vacuum oven. Mesoporous TiO2 paste was prepared by using a commercial TiO2 paste (Dyesol 18NRT, Dyesol) diluted in ethanol (1:3.5, weight ratio). PbI2 solution was prepared by dissolving 462 mg PbI2 (99%, Aldrich) in 1ml N,N-dimethylformamide (DMF, 99%, Sigma-Aldrich) under stirring at 70 ℃ 5



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overnight. The hole-transporting layer precursor was prepared by dissolving 72.3 mg Spiro-MeOTAD in 1ml chlorobenzene, then 28.8 µl of 4-tert-butylpyridine and 17.5

µl of lithium bis(trifluoromethanesulfonyl)imide salt (LiTFSI) solution (520 mg LiTFSI in 1ml acetonitrile) were added. 2.2. Solar Cell Fabrication. Firstly, the FTO-coated glass substrates (14 Ω/sq.) were etched by zinc powder and 2.5 M HCI, and then cleaned by ultrasonic bath using acetone, ethanol, and deionized water for 15min, respectively. After drying at 70℃ for 1 h, the substrates were treated in oxygen plasma for 15 min. A 30-40 nm thick compact TiO2 layer was deposited by DC magnet sputtering at 100 w for 10 min, using the TiO2 target, and then annealing at 500℃ for 30 min. A mesoporous TiO2 film was deposited on the compact TiO2 layer by spin coating at 5000 rpm for 30 s. After drying at 70℃ for 15 min, the substrates were annealed at 500℃ for 30 min. Without any post-treated, 1M PbI2 solution was spin coating on the mesoporous TiO2 at 6500 rpm for 20 s, the PbI2 solution was kept at 70℃ while spin coating and the substrates were pre-heated at 70℃ before spin coating. After spin coating, the PbI2 films were dried at 70℃ for 30 min. After cooling to room temperature, for SGM, the PbI2 films were directly dipped in 8mg/ml CH3NH3I IPA solution for 30 s; For DGM, the PbI2 films were fixed on the fixture, after pre-wetting in IPA, the films were stirred at 180 rpm, 270 rpm, 360 rpm and 450 rpm for 40 s in 8 mg/ml CH3NH3I IPA solution. Then rinsed with IPA and dried at 70℃ for 30 min. When the samples were cooled down, a hole-transporting layer was deposited via spin coating at 4000 rpm for 30 s. Devices were allowed to oxide for overnight in a desiccator. Finally, 50 nm of gold layer was deposited on the spiro-MeOTAD film by thermal evaporation.

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2.3. Characterization. The crystal structure was analyzed by X-ray diffraction (XRD) with monochromatic Cu Kα radiation. The Scanning electron microscopy (SEM) measurements were performed using a cold field emission scanning electron microscope (SU8020, Hitachi Co.). The absorption spectra was recorded using a Shi-madzu (UV-3600) UV-VIS-NIR spectrophotometer ranging from 300 nm to 900 nm. The current-voltage characteristics were measured using an electrochemical workstation (Zahner, Zennium) under AM 1.5 simulated sunlight (100 mW/cm2) provided by a solar simulator (Crowntech, SOL02 series, from open circuit to short circuit with a scan rate of 50 mV/s. The incident light intensity was calibrated with a NREL-calibrated Si solar cell (Newport, Stratford Inc., 91150V). The incident photon conversion efficiency (IPCE) was measured using a ZAHNER CIMPS (Germany, Zahner Company). The electrochemical impedance spectroscopy (EIS) measurements were also recorded by electrochemical workstation (Zahner, Zennium) under dark ranged from 100 mHz to 1 MHz, the data was fitted using Zview software. 3. RESULTS AND DISCUSSION Scanning electron microscopy (SEM) and X-ray diffraction (XRD) have been carried out for the investigation of the nucleation and growth of perovskites. Figure 1 (a) shows the SEM images of perovskite film prepared by static growth method (SGM) for different dipping time: 5 s, 10 s, 15 s and 20 s. When the mesoporous TiO2 scaffolds loaded with PbI2 are dipped into CH3NH3I IPA solution, the color of samples transform from yellow to light brown rapidly as shown in the insets of Figure 1 (a), indicating that the reaction of PbI2 convert to CH3NH3PbI3 is very fast. According to XRD patterns as shown in Figure 1 (c), nearly 90% of PbI2 have been consumed after dipping for 5 s. When the reaction time is up to 20 s, the substrate is 7



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fully covered by perovskite particles, and the color of the substrate becomes dark brown. The perovskite crystals present conformal structure and their average size is around 400 nm. However, as shown in Figure 2 (a), there are a lots of pinholes in the perovskites film. For this reason, some bare TiO2 nanoparticles can directly contact spiro-MeOTAD, which boosts the surface charge recombination so as to deteriorate the performance of the solar cells.

Figure 1. Surface SEM images of perovskite samples fabricated by SGM (a) and DGM (b) at different reaction time. Insets: Photographs of the samples. XRD patterns of perovskite samples fabricated by SGM (c) and DGM (d) at different reaction time. Histogram of counts for perovskite particles fabricated (e) by SGM and DGM at different reaction time in 2.4×2.4 µm2 area. Figure 1 (b) shows SEM images of perovskites prepared by dynamic growth method (DGM) for different stirring time. It can be seen that the coverage of perovskite film on the substrates is better than that of the perovskite film prepared by SGM. The big perovskite particles with size 350 nm are surrounded by a great number of small perovskite particles with size of 50 nm. It demonstrates that the stirring method will produce more crystal nuclei. As shown in the insets of Figure 1 8


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(b), the color of the samples becomes dark as stirring time increases. However, at the same reaction time, the color of the samples is lighter than that of samples prepared by SGM. It deduces that the remnant PbI2 in the DGM sample is more than that of SGM sample, because the reaction between PbI2 and CH3NH3I for DMG is delayed by pre-wetting the PbI2 substrates in IPA.10 As shown in Figure 1 (c, d), the diffraction peak at 12.65° shows the remnant PbI2 in both of perovskite samples. Usually, there is a little of the remnant PbI2 in the final perovskite layer as other reports.26, 27 The statistics of perovskite particles number at different reaction time of 5 s, 10 s 15 s and 20 s in 2.4×2.4 µm2 area of SEM images are illustrated in Figure 1 (e). It can be seen that, the number of perovskite particles change little after reaction time for 5 s, and the number of perovskite particles of DGM sample is nearly three times more than that of SMG sample. This result can prove that DMG method produces more crystal nuclei than SGM.

Figure 2. Surface SEM images of perovskite samples fabricated by SGM (a) and DGM (b) after reacting 30s. Cross-sectional SEM image of PSCs fabricated by SGM (c) and DGM (d). Figure 2 displays SEM images of perovskite film and cross section of PSC 9



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fabricated by SGM and DGM in precursor solution for 30 s, respectively. As shown in Figure 2 (a), the TiO2 substrate and Spiro-MeOTAD layer are not fully separated by perovskite capping layer. So some bare TiO2 nanoparticles contract directly with Spiro-MeOTAD layer, which is harmful for the photovoltaic performance of solar cells.23, 25 As shown in Figure 2 (b), the big perovskite particles (~ 350 nm) are surrounded by many small particles (~50 nm), the surface coverage of the perovskite capping layer can reach 100%. Figure 2 (c, d) shows the cross section of the perovskite solar cell, which is constituted by Au, Spiro-MeOTAD, perovskite capping layer, mesoporous TiO2 with perovskite, compact TiO2 layer and FTO from top to bottom. The roughness of the perovskite films prepared by SGM and DGM is different, which affects the thickness of Spiro-MeOTAD layer. The thickness of Spiro-MeOTAD layers is ~160 nm for SGM and ~180 nm for DGM as shown in Figure 2, respectively. Previous studies showed that there is not obvious changes for properties of PSCs when the thickness of Spiro-MeOTAD layer is round 100~200 nm.28 The thickness difference of Spiro-MeOTAD layers for SGM and DGM is no more than 50 nm, which has little effect on the properties of PSCs. As shown in Figure 2 (d), the compact perovskites capping layer avoid the possible shunting path between TiO2 and Spiro-MeOTAD. Previous reports also showed that the compact perovskite capping layer improved the photovoltaic performance of the PSCs.25, 29 As for DGM, stirring velocity has a dramatically effect on the coverage of perovskite film. Figure 3 displays the surface SEM of perovskite film fabricated by DGM at different stirring velocity. It can be seen that the average size of perovskite particles decreases as stirring velocity increases. When the stirring velocity is lower than 180 rpm, the growth mechanism of perovskite crystals is similar to that of SGM with pre-wetting.30 In low stirring velocity, the residual IPA liquid is still coated on 10


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the surface of PbI2 to hinder the reaction between PbI2 and CH3NH3I. As shown in Figure 3 (a), the perovskite film is consisted of large perovskite particles. There are a lot of pinholes among the large perovskite particles. With increasing the stirring velocity, the residual IPA liquid of the PbI2 surface decreases. The collision rate between PbI2 and CH3NH3I molecules increases accordingly. So the residual PbI2 is easy to contact CH3NH3I molecules and form small perovskite particles. Then, a significant amount of small perovskite particles are deposited into the gaps of perovskite capping layer. However, as shown in Figure 3 (d), the coverage of perovskite film is reduced with further increasing the stirring velocity. The reason is that the small perovskite particles may be swept away by the CH3NH3I IPA solution at the relatively high stirring velocity.

Figure 3. Surface SEM images of perovskite samples fabricated by DGM under different stirring velocity at: 180 rpm (a); 270 rpm (b); 360 rpm (c) and 450 rpm (d). Figure 4 shows the schematic variation of CH3NH3I solution concentration during SGM process (a) and DGM process (b). The initial concentration of CH3NH3I IPA solution is C0, while the concentration of the solution at the interfaces of PbI2/CH3NH3I is Cx. In SGM process, the reaction speed of “PbI2 and CH3NH3I” is 11



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very fast, while the diffusion rate of CH3NH3I is relatively slow. CH3NH3I constantly react with PbI2 at the interfaces, so Cx tends to decrease during the perovskite crystals nucleation and growth process. In addition, Cx shows big difference at different interfaces due to the heterogeneous nucleation in TSD. When Cx is high, the perovskite crystals grow quickly. When Cx is very low, the nucleation and growth of perovskite crystals is suppressed. So the pinholes are always formed in perovskite film during SGM process. As for DGM, the stirring process can eliminate the concentration gradient of Cx. Cx remains relatively high at the interfaces of PbI2/CH3NH3I. So the residual PbI2 is preferred to form many new perovskite crystals rather than to be swallowed by large perovskite particles. As a result, the coverage of perovskite film is improved by numerous small perovskite particles.

Figure 4. Schematic of the variation of CH3NH3I solution concentration during SGM process (a) and DGM process (b). The champion cell of J-V characteristics based SGM and DGM under the illumination of 1 sun (AM 1.5, 100 mW/cm2) is presented in Figure 5, the photovoltaic parameters are listed in Table 1. It can be seen that short circuit current density (Jsc), open circuit voltage (Voc), fill factor (FF), and PCE of cells fabricated by SGM are 19.53 mA/cm2, 0.95 V, 61.37 % and 11.43 %, respectively. All of Jsc, Voc, FF and PCE of the solar cells are improved by DGM, they are 20.42 mA/cm2, 0.96 V, 12


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66.83% and 13.08%, respectively. The enhanced photovoltaic performance of DGM is mainly attributed to the increase of Jsc and FF. The increase of Jsc may be caused by the increase number of perovskite particles in per unit area capping layer. The increase of FF is mainly depended on the decrease of charge recombination due to the decrease of the contact between bare TiO2 and Spiro-MeOTAD layer.

Figure 5. J-V characteristics of champion solar cell fabricated by SGM (black) and DGM (red). (From open circuit to short circuit with a scan rate of 50 mV/s.) Table 1 Main photovoltaic parameters for champion cell obtained by SGM and DGM, respectively. sample

Voc (V)

Jsc (mA/cm2)

PCE (%)

FF (%)

SGM DGM

0.95 0.96

19.53 20.42

11.43 13.08

61.37 66.82

Figure 6 shows the UV-vis absorption spectra, light harvesting efficiency (LHE) spectra and IPCE of samples fabricated by SGM and DGM, respectively. As shown in Figure 6 (a), the UV-Vis absorption of FTO/ mesoporous TiO2/CH3NH3PbI3 samples fabricated by DGM is lower than that of SGM sample in the short wavelength range from 320-520 nm. This result was confirmed by repeating measurement for many 13



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times. We do our best to seek the reasons of this result. It is pity that we didn’t find. For this study, the main differences of DGM and SGM samples are the coverage and the size of perovskite particles. Benefit from the improvement of coverage of perovskite capping layer, the absorption is enhanced in the long wavelength range (>520 nm) for DGM sample. The results show that DGM has not superiority on light absorption in comparison with SGM. In order to further compare the light harvesting, LHEs of samples are calculated by the results of UV-vis and reflection spectra as follows equation:31, 32 LHE= (1-R) (1-10(-A))

(1)

Where R and A represent reflectance and absorption, respectively. It can be seen that, profiting from the high absorption coefficient,10 both SGM and DMG samples can keep very high LHE (>90%) in wavelength range from 340 nm to 570 nm. The LHE dropped sharply from 550 nm to 750 nm is attributed to the thickness of perovskite active layer that is about 550 nm (include mesoporous TiO2). When the wavelength exceeds 550 nm, LHE of DGM sample is much higher than that of SGM sample. In the whole wavelength region, integral area of LHE curve for DGM is much larger than that for SGM. High integral area corresponds to high light harvesting efficiency. Figure 6 (c) shows the IPCE spectra, which are consistent with UV-Vis spectra and LHE curves. DGM sample shows high inject photon conversion efficiency at the long wavelength range from 520nm to 800nm. Current density calculated by IPCE spectra data is 18.77 mA/cm2 and 20.05 mA/cm2 for SGM and DGM, respectively. The results show that the increase of Jsc for DGM sample is mainly derived from the increase of light harvesting efficiency.

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Figure 6. The UV-vis absorption spectra (a) and LHE (b) of FTO/ mesoporous TiO2/CH3NH3PbI3 samples fabricated by SGM (black) and DGM (red). IPCE (c) of PSCs fabricated by SGM (black) and DGM (red). (Current density calculated by IPCE spectra data is 18.77 mA/cm2 and 20.05 mA/cm2 for SGM and DGM, respectively.) The electrochemical impedance spectroscopy (EIS) has been carried out to investigate the recombination resistance (Rrec). The two semicircles are fitted by a simplified equivalent circuit, the first semicircles in high frequency region is related to the charge transport at Au/HTM interface, the semicircles at low frequency region 15



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is related to the recombination at TiO2/perovskite/HTM interface.33, 34, 35 As shown in Figure 7 (a), it is clarity that the solar cell fabricated by DGM has higher Rrec than SGM in the whole frequency range at 0.7 V bias voltage under dark condition. As shown in Figure 7 (b), the Rrec decreases evidently as bias voltage increases. However, the Rrec of DGM sample is still higher than that of SGM at various bias voltages. The difference between SGM and DGM samples is derived from the coverage rate of perovskite layer. The higher Rrec for DGM sample in comparion with SGM is attributed to the less surface charge recombination. The main reason is that the contact between bare TiO2 and Sipro-MeOTAD layer is cut off by the compact perovskite capping layer. The result is also consistent with the improved FF shown in J-V characteristics.

Figure 7. Nyquist plots and equivalent circuit (a) of PSCs fabricated by SGM (black) and DMG (red) at 0.7 V bias voltage under dark; Recombination resistance (b) for PSCs fabricated by SGM (black) and DMG (red) at various bias voltages. The statistic data of Voc, Jsc, PCE and FF of 15 cells fabricated by SGM and DGM, repectively, are shown in Figure 8. For the cells fabricated by SGM, the current density shows discrete. The high Jsc can reach to 19.46 mA/cm2, but the low Jsc is no more than 16 mA/cm2. This reason is attributed to the imperfect perovskite capping layer. When the capping layer has overmuch pinholes, the Jsc is very low. If the pinholes in capping layer are reduced, Jsc shows high value. For DGM, the current 16


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density is mainly concentrated on 19 mA/cm2. The variation of Jsc is from 18.16 mA/cm2 to 20.13 mA/cm2. The photovoltaic parameters of the solar cells are summarized in Table 2. It exhibits that the improvement of the cells performance is mainly caused by the increase of the current density and fill factor. The open voltages of different cells are nearly the same. The average efficiency of the solar cells is 10.69% for SGM and 12.13% for DGM, respectively.

Figure 8. Statistic data of Voc (a), Jsc (b), PCE (c)and FF (d) of 15 cells fabricated by SGM and DGM. Table 2 Statistic average values of main photovoltaic parameters for 15 cells fabricated by SGM and DGM, respectively. sample

Voc (V)

Jsc (mA/cm2)

PCE (%)

FF (%)

SGM DGM

0.92 0.92

17.87 19.04

10.64 12.17

64.52 69.27

4. CONCLUSION For static growth method (SGM), many pinholes in the perovskites film results in 17



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the contact of bare TiO2 nanoparticles and hole transporting layer (spiro-MeOTAD), which increases the surface charge recombination so as to deteriorate the performance of the perovskite solar cells (PSCs). Here, we developed a controllable dynamic growth method (DGM) for the fabrication of the pinhole-free perovskite film. The surface coverage of perovskite film prepared by DGM reached nearly 100%, so that the contact of the bare TiO2 nanoparticles and spiro-MeOTAD was cut off. In addition, light harvesting efficiency (LHE) was also improved by the pinhole-free perovskite film. LHE of DGM sample is up to 71% at light wavelength of 700 nm, which much higher than that of SGM sample (59%). Therefore, both short current density (Jsc) and fill factor (FF) of the PSCs were increased by the pinhole-free perovskite film. As a result, the power conversion efficiency of the perovskite solar cells is increased from 11% for SGM to 13% for DGM. ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (51374029), Program for New Century Excellent Talents in University (NCET-13-0668), Fundamental Research Funds for the Central Universities (FRF-TP-14-008C1). This work was also supported by the "thousands talents" program for pioneer researcher and his innovation team, China.

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Table of Contents Graphic:

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Dynamic Growth of Pinhole-Free Conformal CH3NH3PbI3 Film for

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