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FULL PAPER Perovskite Photovoltaics

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Pseudohalide-Induced Recrystallization Engineering for CH3NH3PbI3 Film and Its Application in Highly Efficient Inverted Planar Heterojunction Perovskite Solar Cells Hua Dong, Zhaoxin Wu,* Jun Xi, Xiaobao Xu, Lijian Zuo, Ting Lei, Xingang Zhao, Lijun Zhang, Xun Hou, and Alex K.-Y. Jen*

High crystallinity and compactness of the active layer is essential for metal-halide perovskite solar cells. Here, a simple pseudohalide-induced film retreatment technology is developed as the passivation for preformed perovskite film. It is found that the retreatment process yields a controllable decomposition-to-recrystallization evolution of the perovskite film. Corresponding, it remarkably enlarges the grain size of the film in all directions, as well as improving the crystallinity and hindering the trap density. Meanwhile, owing to an intermediate catalytic effect of the pseudohalide compound (NH4SCN), no crystal orientation changing and no impurity introduction in the modified film. By integrating the modified perovskite film into the planar heterojunction solar cells, a champion power conversion efficiency of 19.44% with a stabilized output efficiency of 19.02% under 1 sun illumination is obtained, exhibiting a negligible current density–voltage hysteresis. Moreover, such a facile and low-temperature film retreatment approach guarantees the application in flexible devices, showing a best power conversion efficiency of 17.04%.

Dr. H. Dong, Prof. Z. Wu, Dr. J. Xi, Dr. T. Lei, Prof. X. Hou Key Laboratory for Physical Electronics and Devices of the Ministry of Education & Shaanxi Key Lab of Information Photonic Technique School of Electronic and Information Engineering Xi’an Jiaotong University Xi’an 710049, China E-mail: [email protected] Dr. H. Dong, Dr. X. Xu, Dr. L. Zuo, Prof. A. K.-Y. Jen Department of Materials Science and Engineering University of Washington Seattle, WA 98195, USA E-mail: [email protected] Prof. Z. Wu Collaborative Innovation Center of Extreme Optics Shanxi University Taiyuan 030006, China Dr. X. Zhao, Prof. L. Zhang Key Laboratory of Automobile Materials of MOE and College of Materials Science and Engineer Jilin University Changchun 130012, China Prof. A. K.-Y. Jen Department of Physics & Materials Science City University of Hong Kong Kowloon 999077, Hong Kong

DOI: 10.1002/adfm.201704836

Adv. Funct. Mater. 2017, 1704836

1. Introduction

Metal halide perovskite-based solar cells have attracted great attention and progressed rapidly since 2009.[1–4] Based on the intriguing optoelectronic properties, such as tunable bandgap, large charge carrier mobility, and long carrier lifetime,[5–7] the power conversion efficiency (PCE) of perovskite solar cells has rapidly increased from 3.8% to 22.1% in less than 8 years.[1,8] It is recognized that a delicate control on the film quality of the perovskite is crucial for achieving both the excellent photovoltaic performance and desirable stability for the hybrid perovskite solar cells.[9–12] Considering the practical application in conventional and flexible planar heterojunction perovskite solar cells, low-temperature solution-processed method for preparing perovskite film is the one of the most broadly applied techniques due to the ease and scalability of fabrication. However, such a quick crystallization process would easily generate the relatively incontrollable morphology with small grain-size distribution, which may limit the device performance. Concomitantly, excessive grain boundaries (GBs) in the substandard film could induce the carriers recombination and ion migration in working device.[13,14] In addition, the perovskite crystal along the grain boundaries preferentially decomposed even under a much lower temperature compared to that in the inner of the films, leading to the poor stability.[15,16] To date, various film modification attempts have been developed to promote the homogeneity and compactness of the perovskite films, such as solvent retreatment,[17–19] gas vaporing treatment,[20,21] and organic additives assistance.[10,22] As for a chemical additive approach, the incorporation of the lead pseudohalide (Pb(SCN)2) in perovskite precursor could adjust the crystal nucleation–growth process and passivation of GBs, effectively enhancing the humidity resistance and the quality of the perovskite film.[23–27] However, views of the lead pseudohalide consisting in the perovskite films are still debatable, including the deduction of participation in final formation of the rebuilt perovskite crystal[23,28] or just working as the intermedium.[25,24] In addition, some studies also shows the fact that the addition of the Pb(SCN)2 could influence the sufficient

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reaction between the PbI2 and methylamine iodide, and thus it could easily lead to overmuch residual of PbI2 in the modified perovskite films, hindering the further improvement of performance. Although Kim et al.[28] reported that the adjustment of the stoichiometric composition in precursor could restrict the residual of the PbI2, the limitation of controllability still exists. To avoid complex component design of the precursor as well as to achieve better film homogeneity of the modified film, it is meaningful to explore the essential characteristic of the metalfree pseudohalide effecting and more appropriate film modification approach. Different from the reported additive-assisted methods, a lead-free pseudohalide compound “ammonium thiocyanate (NH4SCN)” was employed as the retreatment for the as-prepared perovskite film in our experiment. By utilizing this typical pseudohalide compound for a reprocessing, the pristine perovs kite film can undergo a progressive breaking-to-reconstruction process: 1 extracting partial organic cations from the pristine film; 2 restructuring the framework; and 3 recrystallizing of the perovskite film. Here, the pseudohalide SCN ion plays the role of the intermediate catalyst for the secondary formation of the perovskite film, which could ensure the compositional purity of the final polycrystal as well as the original film. As a result, such a convenient retreatment approach could modify the morphology of the perovskite film, that is, effectively eliminating

the grain boundaries and increasing the grain scale. Meanwhile, higher crystallinity of the final films with less trap states density was achieved. Based on the low temperature planar p–i–n heterojunction devices, a highest efficiency of 19.44%/19.04% was achieved in reverse and forward scan, respectively. Furthermore, this scalable engineering was successfully applied in flexible devices due to the simply and low-temperature characteristic, and a considerable performance of 17.04% was obtained. It is hoped that this technique could be an alternative approach to effectively improve the quality of hybrid perovskite films to satisfy the requirements for photovoltaic devices.

2. Results and Discussion 2.1. Properties of the Retreatment Perovskite Film The retreatment procedure is a sequential dipping–annealing method, illustrated in Figure 1a. Here, the preformed CH3NH3PbI3 film was fabricated through an antisolvent fastcrystallization method, which was reported in our previous report.[29] In our study, after applying a short treatment engineering (10 s dipping and 10 min annealing) by ammonium thiocyanate, both the uniformity and the crystallization of the perovskite film was improved obviously. The cross-section and

Figure 1. a) Schematic diagram of the retreatment approach for perovskite film; top-view and cross-section SEM images of the preformed CH3NH3PbI3 films b,c) and retreatment CH3NH3PbI3 films d,e); XRD patterns f) and absorption/PL spectrum g) of preformed and retreatment CH3NH3PbI3 films.


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top-view scanning electron microscope (SEM) images of the CH3NH3PbI3 films were shown in Figure 1b–e, before and after interfacial modification. It can be seen that the preformed CH3NH3PbI3 films were polycrystalline with a small average grain size due to the fast crystallization process. Surprisingly, after modification, the average grain size of CH3NH3PbI3 films enlarged obviously with several folds, accompanied with the less GBs on top surface. Furthermore, as for the modified film, vertical grain boundaries formed throughout the cross section. Such a renewed GBs distribution could guarantee the effective transport for photogenerated carriers when going through the active layer and reaching the corresponding charge carrier extraction interfaces, suppressing the recombination. Figure 1f shows the X-ray diffraction (XRD) patterns of the preformed and modified perovskite films. Compared to the original sample, the X-ray diffraction peaks (110) of CH3NH3PbI3 in modified film became stronger and sharper (the full width at half maximum (FWHM) of (110) peak of the CH3NH3PbI3 is 0.283°, which is lower than preformed one (0.358°)), which indicated the enhanced crystallinity with less low-dimensional

defects. The UV–vis absorption and photoluminescence (PL) spectra are also measured and shown in Figure 1g. Interestingly, it can be seen that a related flat absorbance profiles of the retreatment film can be observed in the high energy region (350–450 nm), which means the reduced chemical defects of the multiiodide plumbate.[30–32] Meanwhile, it is noticed that the PL peak shape of perovskite films is slightly narrow after modification, accompanied by the blueshift of the PL peak. The result can be explained by the optimized quality and less intraband traps of modified film which restricted the charge recombination.[16]

2.2. Mechanism and Optimization of the Recrystallization Process Interesting, both the step-by-step morphological evolution and composition reaction process were observed during this sequential posttreatment procedure. For comparison, preformed film, film after dipping, and film after annealing were named as

Figure 2. a) Top-view SEM image of the intermediate-state CH3NH3PbI3 film; b) XRD patterns of the three states of the CH3NH3PbI3 film; c) FTIR spectrum of the three states of CH3NH3PbI3 film and NH4SCN in IPA solution; d) schematic diagram of the recrystallization process: morphological evaluation and structure dynamic process of the CH3NH3PbI3 film.


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initial state, intermediate state, and final state, respectively. For the intermediate state, SEM image in Figure 2a shows that the CH3NH3PbI3 film partially dissolved, replaced by the mesoporous framework with the precipitation of the compound. XRD result indicated that the diffraction peak intensity of CH3NH3PbI3 in intermediate-state film decreased obviously compared to the initial state (shown in Figure 2b), accomplishing with the appearance of the PbI2 impurity peak. While for the final state, the peak of the PbI2 almost disappeared, and a stronger and sharper peak of CH3NH3PbI3 was observed again. Hence ,the appearance-to-disappearance evolution of the PbI2 clarifies a possible decomposition-to-recrystallization process for the CH3NH3PbI3 film. Actually, considering the different radius of SCN group and I ion (rSCN = 0.215 nm, rI = 0.220 nm),[33,34] if the SCN ion participated in the formation of the final perovskite, the lower shift in XRD peak position of the (110) plane should be observed.[35,36] However, no significant shift of the diffraction peak ratio in XRD results, demonstrating no crystal orientation changing for the initial and final film. The existing form of the SCN groups during the retreatment process is further confirmed by SEMbased X-ray energy-dispersive spectroscopy (EDS) analysis and the Fourier transform infrared (FTIR) spectroscopy spectra. As shown in Figure S1 (Supporting Information), S element can be observed in the EDS spectrum taken from the dippingstate perovskite film. while there is no trace in final perovskite film. The FTIR spectra of the NH4SCN in IPA and perovskite films with different states are shown in Figure 2c. The distinguishable peak related to SCN groups is clearly observed in the spectrum taken from the intermediate-state perovskite film. However, this peak is absent in the final-state perovskite film, suggesting that SCN groups do not exist in the lattice of the modified perovskite film. Related analysis revealed the medium catalytic feature of the pseudohalide compound, which avoided the introduction of the impurity as well as ensured the componential consistency for the perovskite film. As the induced origin for retreatment procedure, the specific effect of the pseudohalide compound was further confirmed by the FTIR spectrum. It is worth noting that the stretching vibration of free SCN groups in IPA solution appears at 2165 cm−1, while it shifts to a higher wave number at 2141 cm−1 when observed in intermediate-state perovskite film. The redshift phenomenon is indicative of the combination between the SCN groups and other chemical groups.[37] It is known that in CH3NH3PbI3 crystal, CH3 NH3+ group was bound around the PbI6 octahedron through the hydrogen bond between CH3 NH3+ and I−. While as a typical pseudohalide, SCN− is assumed to break the hydrogen bond between CH3 NH3+ and I− in initial CH3NH3PbI3, and new bond was formed consequently between SCN− and CH3 NH3+ groups. By using the first-principles density functional calculations, we performed the activation energy calculations for the reactions CH3 NH3+ + I− → CH3 NH3 I (1) CH3 NH3+ + [SCN − ] → CH3 NH3 [SCN] (2) The results give the zero activation energy for both of two reactions. This is expected since there is only the electrostatic


Coulomb interaction between CH3 NH3+ and I−/SCN−, no energy barrier is needed to make the reactions occur. Furthermore, we also calculated the enthalpy of formation for the two reactions (both are exothermic). We find that CH3NH3[SCN] has a larger enthalpy of formation by ≈3.0 KJ mol−1 than CH3NH3I. This means the reaction (2) is more thermodynamically prone to occur. This result can be also deduced from the experimental electronegativity data, that is the SCN has a larger electronegativity than the I,[38,39] so the ionic bonding between CH3 NH3+ and SCN− should be stronger than that between CH3 NH3+ and I−. These results indicate that the SCN− anion is more apt to form ionic bonding with the CH3 NH3+ cation than the I− anion and support our proposed intermediate state with the temporary displacement of I− by SCN− in the posttreatment approach. Based on the systematic analysis we studied, the mechanism of the typical pseudohalide-induced recrystallization process can be explained as Figure 2d, and the possible reaction dynamic process was speculated as following equations CH3 NH3PbI3 + SCN − → PbI64 − + CH3 NH3+ ⋅ SCN −

( dipping ) (3)

CH3 NH3+ ⋅ SCN − + NH+4 → CH3 NH3+ + HSCN ↑ +NH3 ↑

(annealing )

(4) PbI64 − + CH3 NH+3 → CH3 NH3PbI3

(annealing ) (5)

When the initial perovskite film contact with the NH4SCN, due to the extraction effect of the SCN−, partial decomposition of the CH3NH3PbI3 crystal occurred. PbI2 and the related intermediate compound CH3 NH3+ ⋅ SCN − were formed around the undissolved CH3NH3PbI3 (shown in Equation (3)). While experiencing the following annealing process, NH3 and thiocyanic acid (HSCN) formed first and volatilized quickly owing to the rather low thermal stability, with the remaining CH3 NH3+ in the film (shown in Equation (4)). Subsequently, new CH3NH3PbI3 crystal could form and grow around the undissolved perov skite architecture (shown in Equation (5)). Here, the undissolved polycrystal could play the role of the framework for the recrystallization process, which may impede the nucleation but facilitate the growth of the new CH3NH3PbI3 crystal. More importantly, during the annealing step, the connection between the CH3 NH3+ compound and the undecomposed preformed perovskite grains could also be beneficial for the modification of the morphology of the film and passivation of the GBs, which further restrained the intrinsic defect of the film.[20,21] Therefore, a final perovskite film with improved morphology as well as high crystalline could be achieved. To further demonstrate the recrystallization effect induced by SCN ion, rather than solvent or amine cation, we designed two other types of retreatment as comparison: (i) preformed film with a pure IPA retreatment (10 s dipping and 10 min annealing) and (ii) preformed film with NH4I in IPA (30 × 10−3 m mL−1 same as NH4SCN) solution retreatment (10 s dipping and 10 min annealing). Here, SEM images and XRD data of initial and final-state films were checked and shown in Figure S2 (Supporting Information). It was found that both the grain size and the morphology have no obvious evolution, and there is also no peak shift or intensity

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change in two types of modified films. This comparison further confirmed our assumption, that is, the recrystallization effect in retreatment process should be attributed to the pseudohalide anion. Since in the extraction process intermediate state is the key issue during the recrystallization process, a control reaction time between the pseudohalide compound and the pristine perovskite film is the important for the quality of the final perovskite film. Here, we designed a series of dipping reaction time (5, 10, 20, and 60 s) to optimize the approach, and the morphological and structural characteristics of the modified films were studied in detail. Figure 3a–e is the top-view SEM images of various final-state perovskite films for comparison. Compared with the pristine sample, modified films with a gradually enlarged grain size can be observed, accompanied by the increase of the dipping time. However, when suffering from overlong treatment (> 20 s), precipitates appeared on both the grain boundaries and the surface of the perovskite film, with an increase in tendency. To further analyze the component of the final produce, XRD of the various final films was measured and shown in Figure 3f. First, the intensity of the diffraction peak of CH3NH3PbI3 increased as the treatment time increased, without the apparent presence of the PbI2 peak. Whereas, decreased intensity peak of the CH3NH3PbI3 was observed when undergoing a longer reaction (20 and 60 s), and the diffraction peak of PbI2 became obviously. Accordingly, it can be inferred that the precipitate in the modified films should be the PbI2. Absorption spectrum of the perovskite films also indicated a decrease in tendency of intensity with an overtime treatment (shown in Figure 3g); here, the lower absorption intensity of the perovskite films was attributed to the poor films with excess PbI2. Why was an unsatisfactory modified film with poor morphology and crystalline Figure 3. Top-view SEM images of the initial-state and final-state CH NH PbI films with 3 3 3 obtained when undergoing an overtime dip- different reaction time: a) initial, b) 5 s, c) 10 s, d) 20 s, e) 60 s; XRD measurements f) and ping treatment? On account of the recrystalli- absorption spectrum g) of the initial-state and final-state CH3NH3PbI3 films with different zation mechanism, we assumed if the pristine reaction time. film suffered a long dipping, excess organic cation group (CH3 NH3+ ) could be extracted. light-colored evolution of the perovskite film was observed as the Meanwhile, the generated intermediate organic compounds time goes by. Eventually, a yellow film (shown in Figure S3a,b, (CH3 NH3+ ⋅ SCN − ) could partially dissolve in IPA solution as the Supporting Information) was obtained, which cannot transfer to time goes by. Considering this, such an overtime reaction would black color no matter how long it was annealed again. For both strike the ratio between the organic halide cation and metal intermediate and final product, XRD analysis shows that there halide for recrystallization, thus the precipitated PbI2 could is only diffusion peak of the PbI2 in perovskite film (shown in not transform to the perovskite sufficiently. In order to further verify the ion-extracted inference, a 10 min dipping experiment Figure S3c, Supporting Information), which is corresponding for the preformed film was designed. As expected, a gradually with the speculation of precipitate in final-state films. So, the


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time-dependent reaction should be exactly controlled to obtain the optimal quality of films detrimental for the device efficiency. In fact, Yang et al. reported that the introduction of NH4SCN in precursor could modify the HC(NH2)PbI3 (FAPbI3) films.[40] By utilizing the NH4SCN, the additive-toprecursor method was also investigated in our experiment as comparison. Here, the SEM images of preformed and NH4SCN-additive (0, 5, and 10 wt%) perovskite films were shown in Figure S4 (Supporting Information). It can be seen that although enlarged grain size was obtained in modified films, the morphology shows an incomplete coverage with too many pinholes. This unsatisfactory result may be attributed to the fast crystallization process by antisolvent one-step solution method, and the chemical heterogeneity effect of the precursor leads to the nonuniform nucleation-to-growth of the perovskite film. As comparison, the retreatment approach is more controllable and effective.

2.3. Discussion In view of the simplification and the low-temperature process of the posttreatment technology, a p–i–n heterojunction device configuration can be designed as ITO/PTAA (F4TCNQ)/CH3NH3PbI3/PCBM/BCP/Ag, shown in Figure 4a. By employing the initial and modified perovskite films with different reaction extent (5, 10, 20, and 60 s dipping), a series of devices were prepared. The current density–voltage (J–V) curves of representative devices under simulated 1 sun illumination (100 mW cm−2) were shown in Figure 4b. Results shows that the optimal dipping time for modifying the perovskite film was found to be 10 s when applied in the device, which exhibits a remarkably enhanced PCE of 18.67% compared to a 16.01% PCE of reference device with preformed film. However, as the reaction time increased beyond 10 s, the performances of devices gradually decreased. To study the reproducibility of the modification approach, a batch of 50 devices for each structure were prepared. The statistical distributions of PCE for the devices are depicted in Figure S5 in the Supporting Information, and the statistic suggests the considerable reproducibility of this modifying method. The champion-performing device with modified perovskite film exhibits a PCE of 19.44 (19.04)% with a VOC of 1.103 (1.098) V, a JSC of 22.55 (22.54) mA cm−2, and an FF of 78.2 (76.8)% measured under reverse (forward) voltage scan, which have a negligible photocurrent hysteresis (Figure 4c). The incident photon-to-electron conversion efficiency (IPCE) spectrum under a standard AM 1.5 G in Figure 5d demonstrated the high quantum conversion efficiency throughout the entire wavelength range. Integrating the product of the AM 1.5 G photon flux with the external quantum efficiency (EQE) spectrum yield predicted a JSC of 22.01 mA cm−2, in agreement with measured value from the J–V scanning (shown in Figure 4d). Stabilized power output measurement under working conditions is an effective method to estimate the efficiency of perovskite devices.[41] Figure 4e shows the steady photocurrent and PCE measured at the maximum power output point (0.92 V) of the best device. Here, the stable power output is consistent with the value obtained from J–V


curve, and both the parameters are very stable under 1 sun illumination for 400 s in ambient environment. In order to further investigate the origin of the efficiency enhancement for devices with modified perovskite films, that is, correlation between the enhanced PCE and improved morphology and better crystallinity, we employed thermal admittance spectroscopy (TAS) to measure the trap density of states (tDOS) of the devices with the initial/modified perovskite films. TAS is an established and effective technique and the energetic profile of tDOS can be derived through impedance spectroscopy analysis (shown in Supporting Information).[16,42] As shown in Figure 4f, the tDOS of the device based on modified film decreased nearly —one to two orders of magnitude compared with the standard device, from 1017 –1018 m−3 eV−1 to 1016 –1017 m−3 eV−1. The dramatic decrease of tDOS is consistent with the restriction of the charge recombination, which can be explained by the enhanced crystallinity and less GBs. This result also agrees with the reduced chemical defects of multiiodide plumbate in absorption spectrum. The photogenerated carrier lifetime of the different perovskite films was also characterized by time-resolved photoluminescence (TRPL) measurement. Figure 4g shows the TRPL decay of the initial and best modified (10 s dipping) CH3NH3PbI3 films on glass substrate. Here, the biexponential decay model was utilized as follows to fit the PL decay curves  t   t  I ( t ) = A1 exp  −  + A2 exp  −  (6)  τ1   τ2  Results show that the average carrier lifetime remarkably increased from 61.9 to 108.4 ns. Such a longer carrier lifetime exhibits the inhibition of the photogenerated charge carrier recombination, associated to the decreased trap density. Considering the simple, low-temperature and effective properties, this modification technology could also be applied in the flexible p–i–n solar cells. Here, the flexible devices were fabricated on ITO/polyethylene naphthalate (PEN) substrates. Employing the best modified perovskite film (10 s dipping time), The J–V curves of the champion-performing flexible device are shown in Figure S6 (Supporting Information), showing a PCE of 17.04 (16.09)%, a VOC of 1.083 (1.037) V, a JSC of 22.14 (22.07) mA cm−2, and an FF of 0.72 (0.70) derived from reverse and forward scans as shown in Figure S4 (Supporting Information). It implies such a considerable performance exhibits the potential for the flexible application.

3. Conclusion In this article, we demonstrated a scalable and convenient pseudohalide-induced recrystallization engineering for improving the quality of the perovskite film. Such a retreatment approach could contribute to the recast of both the morphology and crystalline quality of the perovskite film. Results show that the enlarged grain sizes and higher crystallinity of modified perovskite film were obtained, with lower trap density. Based on the inverted planar heterojunction devices, a champion device with the PCE of 19.44% was obtained, and the considerable application on flexible was also achieved due to its low-temperature

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Figure 4. a) Schematic diagram of the planar heterjunction device; b) J–V curves of the representative devices with the initial/modified perovskite films; c) J–V curves of the champion device with forward and reverse scanning; d) EQE and integrating current density; e) steady photocurrent and PCE measured at the maximum power output point (0.92 V); f) trap density of states of the devices with the pristine/modified perovskite films; g) PL lifetime of the pristine/modified perovskite films.

required process. We hope that this simple and facial film modification approach could provide a universal way for highperformance perovskite device fabrication.

4. Experimental Section Materials and Preformed Perovskite Film Preparation: NH4SCN (99.99% trace metal basis) was purchased from Sigma-Aldrich; PbI2


(99.999 wt%) was purchased from Alfa Aesar and used as received. CH3NH3I was prepared similar to a previously published method, in brief: 24 mL methylamine solution (33 wt% in ethanol, Sigma-Aldrich) and 10 mL hydriodic acid (57 wt% in water, Sigma-Aldrich) were diluted by 100 mL ethanol in a 250 mL round bottom flask by constant stirring at 0 °C for 2 h. The precipitate of CH3NH3I was gained by rotary evaporation at 40 °C and washed with dry diethyl ether until the solid became white. The final product was dried at 60 °C in a vacuum oven for 24 h. Phenyl-C61-butyric acid methyl ester (PC61BM) (99.5%) was

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purchased from Solenne and used as received. The BCP was purchased from Nichem (Taiwan). Preformed Perovskite Films Preparation and Postprocessing Approach: For the preformed perovskite films, 45 wt% CH3NH3I and PbI2 (1:1) were prepared in dimethylformamide (DMF) solution, then the precursor was spin coated on the substrate at 5000 rpm for 60 s with an 800 L h−1 dry nitrogen gas flow blowing over the film 5 s before spin coating till the end of it. During the spinning, few drops of chlorobenzene were quickly added to the film at the 5th second after the beginning of spin coating for a CH3NH3PbI3 in DMF solution. NH4SCN was dissolved in isopropyl alcohol (IPA) in a concentration of 30 × 10−3 m mL−1. Then preformed perovskite films were dipped in solution for a series of time (5, 10, 20, and 60 s) at room temperature. After dipping process, the samples were annealed at 90 °C for 10 min and washed by IPA followed by N2 blow gun drying. Device Completion: Indium tin oxide (ITO)-coated glass (20 Ω square−1) and ITO-PEN substrate were sequentially cleaned with detergent, deionized water, acetone for 15 min before drying. After exposure with a 5 min UV ozone plasma, 0.5 wt% poly(triaryl amine) (PTAA) solution doped with 1 wt% F4-TCNQ was spin coated onto the ITO substrates at 5000 rpm for 30 s, and the samples were then annealed at 110 °C. The initial and modified perovskite films were prepared as above mentioned. 20 mg mL−1 PC61BM in chlorobenzene was then spin coated on perovskite film at 3000 rpm respectively for 30 s and was annealed at 80 °C for 10 min to form a 30 nm electron transport layer. 5 nm bathocuproine (BCP) and 140 nm Ag was thermally evaporated onto the PCBM layer to eventually consummate the device. Material and Device Characterization: A field emission SEM (Quanta 250, FEI, USA) was used to investigate the morphology and crystallinity. The crystalline structure on ITO substrate was performed by an X-ray diffractometer (D/MAX-2400, Rigaku, Japan) with Cu Kα radiation. The absorption and PL spectra were obtained by UV–vis spectrophotometer (HITACHI U-3010, Japan) and fluorescence spectrometer (Fluoromax-4 spectrofluorometer) respectively. The timeresolved PL spectra were recorded by a 100 ps time resolution using a time-correlated single photon counting system (FLS920 spectrometer) (excited by picosecond pulsed Light-emitting diode (LEDs), pulse duration: