Repairing Defects of Halide Perovskite Films To Enhance Photovoltaic

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Repairing Defects of Halide Perovskite Film to Enhance Photovoltaic Performance Mengru Wang, Bo Li, Jifeng Yuan, Fei Huang, Guozhong Cao, and Jianjun Tian ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b12760 • Publication Date (Web): 09 Oct 2018 Downloaded from http://pubs.acs.org on October 9, 2018

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ACS Applied Materials & Interfaces

Repairing Defects of Halide Perovskite Film to Enhance Photovoltaic Performance Mengru Wanga, Bo Lia, Jifeng Yuana, Fei Huanga, Guozhong Caob, Jianjun Tiana* a Institute for Advanced Materials and Technology, University of Science and Technology Beijing, Beijing, 100083, China. b Department of Materials and Engineering, University of Washington, Seattle, WA, 98195-2120, USA. *Email:[email protected] Abstract On account of the low-temperature solution fabrication, the much high defects density at the interfaces and grain boundaries of halide perovskite film is recognized as one of the big obstacles toward high efficiency solar cells. Here, the time-resolved photoluminescence (TRPL) with incident light exciting from the upper surface and bottom of halide perovskite film, respectively, showed the very different results, verifying the much more surface trap states in the film. To eliminate the defects and enhance the photovoltaic properties of perovskite solar cells (PSCs), we designed a facile and effective method to repair the defects of the perovskite film using formamidinium iodine (FAI) solution. The dissociative FA+ and I- ions could compensate for the loss of volatile organic cations, but also fill the Ivacancies of halide perovskites. After repairing defects with proper concentration of FAI solution, the TRPL curves obtained by light exciting from the different sides of the perovskite film nearly overlap together, indicating the reduction of surface traps. As a result, both the total carrier lifetime and charge extractions were improved by removing the nonradiative channels (surface traps), which universally enhanced the power conversion efficiency (PCE) and stability of the planar heterojunction structural PSCs. Keywords: perovskite, defects, time-resolved photoluminescence, radiative recombination, formamidinium iodine

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Introduction The mixed metal halide perovskite materials have been extensively researched for almost a decade due to excellent properties, such as high absorption coefficient, low exciton binding energy and remarkable photoluminescence output.1-7 Consequently, the power conversion efficiency (PCE) of perovskite solar cells (PSCs) has surged up to 23.3% in 2018 from 3% in 2009,8-10 which gets close to the efficiency of other types of PSCs.11 In addition, such perovskites allow low-cost solution processing and show long charge carrier diffusion length for the simple planar PSCs architecture.12-14 Despite rapid progress and sufficient merits, PSCs are still facing many challenges to further improve the performance and realize the commercial application.4, 15-16 In particular, a large amount of defects at the interfaces and grain boundaries of the perovskite film is recognized as one of the barriers toward high efficiency devices.17-19 Relevant works have shown that the surface defects within the perovskite films prepared by solution process are mainly point defects, such as vacancies, interstitials and substitutions.20-23 These defects located at the surface or grain boundary (GB) are most likely due to the nature of perovskite materials themselves, such as low formation energy and instability.22, 24-25 In particular, the organic components (MA) can easily evaporate from the surface during annealing process, especially for the anti-solvent washing fabrication method.26 As a result, the formation of MA vacancies increases the trap densities in the perovskite films. These trap sites would be the main cause for poor devices performance. For example, the deep-level defects, such as vacancy trap VPb and interstitial trap MAi, are the main non-radiative recombination center in perovskite layers.15,

19, 27

Consequently, the energy disorder induced by traps results in the significantly decrease of the open circuit voltage (Voc) of devices.15, 28-29. These defects are also responsible for low short-circuit current density (Jsc) and poor fill factor (FF).30 Moreover, some research suggests that the recombination on perovskite surface is much more serious than that in


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bulk which would reduce the overall carrier lifetime.18 Besides, these defects can induce the ion migration and associated hysteresis as well as accelerate the permeation of moisture and oxygen into the perovskite films.27, 31-32 Therefore, the surface defects must be carefully regulated to achieve high performance PSCs. Various methods have been tried to reduce the defects in perovskite film, which including post-treatment of the perovskite surface or regulation of the solution fabrication. For example, Grätzel and co-workers employed adamantane and 1-adamantylamine to modify the surface of the perovskite films. By filling the A-cation vacancies, the resulted PSCs show enhanced Voc and working stability.33 Huang et al. passivated the surface defects of the perovskite films with Phenyl-C61-butyric acidmethyl ester (PCBM) to react with under-coordinated Pb atoms or with quaternary ammonium halide anions to fill the A-cation vacancies and I-anion vacancies. As a result, the photocurrent hysteresis was significantly decreased.34-35 Some other works focused on regulating the perovskite crystal growth to reduce grain boundaries. For example, our group has demonstrated that the monolayer-like perovskite film with micrometer-sized grains would be obtained by adding the methylammonium chloride additive into perovskite precursor solutions. The performance of PSCs was significantly improved owing to low trap density which resulting from decreased grain boundaries and enhanced crystallination.36-40 However, due to the easy evaporation of organic composition such as MA under thermal annealing, the decomposition of the perovskite would occur and thus cause severe charge recombination even for the monolayer-like perovskite film. Thus, the post-treatment passivation of surface trap states will be viewed as a promising means of eliminating the defects of the perovskite films. Herein, we identified the presence of a large amount of surface traps on the perovskite film by using the time-resolved photoluminescence (TRPL). A 450nm incident excitation light was applied to excite the perovskite film either from the glass side or from the perovskite side. The TPRL results are quite different and indicate that there are much more



defects on the perovskite surface. Therefore, we presented an effective post-treatment method to repair the defect sites at the surface of the perovskite films. A formamidinium iodine (FAI) solution was spin-coated on the perovskite surface and followed with a post annealing process. FA+ and I− ions can diffuse into the vacancies which was formed due to the loss of organic cations and halogen ions. After passivated with 5 mg/mL FAI solution, the TRPL curves excited from the different sides of the perovskite film nearly overlap together, indicating the reduction of surface defects. Due to the significantly reduced nonradiative recombination, the passivated perovskite films show increased PL intensities, extended PL lifetimes, and improved charge-transfer in the perovskite/hole transporter layer (HTL).Thus, with FAI solution repairing process, the PCE of PSCs increased by 14%. Moreover, the thermal stability of the perovskite films at a relatively high temperature (100 °C) is also significantly enhanced compared with the pristine perovskite films.


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Results and discussion

Figure 1 Schematic illustration of the filling of I- and MA+ vacancies in the perovskite films by FAI post-treatment process. In this study, the MAPb(I95%Br5%)3 films were prepared by anti-solvent deposition method. FAI/IPA solution was spin-coated on the perovskite film followed with annealing at 100 C for 15 minutes. The detailed process information is described in the Experiment Section. For FAI post-treatment process, the dissociative FA+ and I- ions will fill the cationic MA+ vacancies and anionic I- vacancies at the perovskite surface, respectively. Figure 1 illustrates the FAI passivating process of the perovskite surface to eliminate the defects, in which FA+ is expected to fill MA+ vacancies by occupying cubo-octahedral sites loss on the film surfaces and the I- is expected to fill I- vacancies as well as decrease the undercoordinated Pb atoms.



Figure 2 TRPL decay of the perovskite films with 450 nm excitation light. (a) Scheme of the incident excitation light either irradiate from the glass side or from the perovskite side. The TRPL decay curves of the samples (b) without (control sample) and (c), (d), (e), (f) with 1mg/mL, 5 mg/mL, 10 mg/mL, 15 mg/mL FAI treatment, respectively. TRPL decay measurements were performed to study the trap states on the surfaces of control sample and passivated samples as shown Figure 2. Table S1 summarizes the decay lifetimes by fitting to a bi-exponential function. Figure 2a illustrates the PL measurement process. The 450 nm incident excitation light excites either from the glass side or from the perovskite side. Figure 2b shows the TRPL results of pristine perovskite obtained by incident light exciting from the glass side and from the perovskite surface, respectively, are quite different. For the TRPL curve measured from the perovskite surface side, the presence of fast and slow components is clearly identified, which are correlated with the non-radiative recombination and radiative recombination, respectively.41 We can find that the non-radiative recombination component of the perovskite surface is about seven times more than that of the perovskite bottom. According to literatures, non-radiative recombination is due to the existence of deep-level defects.15, 19, 27 This result suggests that there are many deep-level traps on the perovskite surface. After the pristine perovskite


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films were repaired by FAI solution, the proportion of the non-radiative recombination decreases. As shown in Figure 2 (b, c, d), with the increase of the FAI concentration, the TRPL curves measured from two sides are getting closer with the decrease of the nonradiative component ratio, while the average carrier lifetime increases accordingly. When the FAI content is 5 mg/mL, the two curves nearly overlap together. Thus, FAI can repair the deep-level defects located at the perovskite surface. However, the TRPL curves measured from two sides of the perovskite film begin to separate and the non-radiative recombination proportion increases accordingly when further increasing the concentration of FAI solution to 10 mg/mL, as shown in Figure 2(e, f). It implies that the excessive FAI may be non-ideal for repairing the perovskite surface traps. Hence, the post treatment of the perovskite film surface with suitable concentration of FAI solution is an effective way to repair the surface traps and enhance carrier lifetime.

Figure 3 Steady-state PL spectra (black squares) and steady-state PL quenching (red dots) of the perovskite films excited with 450 nm incident excitation light from the perovskite side of perovskite/glass and Spiro-oMeTAD/perovskite/glass structures, respectively. (a) Control sample. (b), (c), (d), (e) The samples post-treated with 1mg/mL, 5 mg/mL, 10 mg/mL, 15 mg/mL FAI solutions, respectively. (f) Comparison of Steady-state PL peak intensity statistics for five samples.



Figure 3 exhibits the emission property of the perovskite films and carrier transfer performance between perovskite absorber and HTL using steady-state PL. The radiation luminescence intensity is significantly increased with increasing the FAI solution concentration. The PL intensity reaches the maximum value when the FAI solution concentration is 5 mg/mL. Further increasing the FAI solution concentration, the PL intensity begins to decrease. It indicates that the repairing process with 5 mg/mL FAI solution could achieve the best effect with significantly reduced non-radiative recombination and enhanced radiative recombination. It should be noted that a shoulder peak at around 790 nm is observed, which may be derived from the shallow level traps within the band gap due to low boiling point of organic components.28, 34 We also studied the charge extraction and hole transport behavior within the perovskite films and the perovskite/Spiro-OMeTAD interface. The steady-state PL quenching experiment was conducted with the structure of Spiro-OMeTAD/perovskite/glass structure (Figure 3). Comparing the steady-state PL and PL quenching spectra, the sample passivated with 5 mg/mL FAI solution shows the most efficient PL quenching. The efficient PL quenching suggests that the charge carrier extraction across the interface between Spiro-oMeTAD and perovskite layer is more effective. However, when the concentration of FAI solution reaches 10 mg/mL or 15 mg/mL, the PL quenching is less efficient, which suggest that holes cannot be efficiently transferred into Spiro-oMeTAD. The possible reason is that the excessive FAI would serve as the hole barrier due to the electrical insulation of FAI material.


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Figure 4 (a) SEM top-view images; Scalebar: 1 μm, (b) UV-vis light absorption spectra, (c) XRD spectra, and (d) The FA content in perovskite film (x in FAxMA(1-x)) calculated by XRD and UV-vis absorption of the perovskite films post treated with different FAI content. (e, f) SKPM images (5x5 μm2) of the control sample and 5 mg/mL FAI treated sample. We further assessed the effects of the FAI repairing process on the performance of perovskite films by using the scanning electronic microscope (SEM), UV-vis light absorption spectra and X-ray diffraction (XRD) spectra. The morphologies of the perovskite films post-treated with different concentration of FAI are shown in Figure 4a. For the control sample, the perovskite crystallite sizes varies from 160 nm to 400 nm and the average size is about 224 nm. And the average grain size of the perovskite films shows slightly increase along with increasing FAI solution concentration (see the statistical averaged values in Figure S1). The growth of crystals is possibly due to the secondary growth of perovskite with the assistance of FAI.42 However, it can be clearly seen that there is inhomogeneous distribution of FAI residual on the surface of perovskite after 15 mg/mL



FAI solution treatment (see Figure S2), which may be the reason for the less efficient charge carrier transfer between perovskite and Spiro-oMeTAD as shown in Figure 3. Figure 4b shows the light absorption spectra of the different perovskite films. The absorption in the short wavelength region increases after the FAI solution treatment, indicating the surface of the film becomes more smooth.38 With increasing the concentration of FAI solution, continuous red shifts of the absorption edges are observed at about 770nm, as shown in the zoomed light absorption spectra. The gradual red shift may be derived from FA cations incorporated into MA-perovskite lattice due to the wider light absorption range of FAPbX3 than that of MAPbX3. XRD patterns were performed to analyze the change of the crystallinity as shown in Figure 4c. Clearly, the peak intensity of the perovskite crystal increases with increasing FAI solution concentration. And the full width at half maximum (FWHM) narrows down accordingly. The statistic of XRD patterns are exhibited in Table S2. All these indicate the improvement in crystallinity after the films were repaired by FAI solution. According to the XRD, the lattice parameters calculated from the XRD peaks are 6.264 Å, 6.276 Å and 6.294 Å, for perovskite film fabricated by 1mg/ml, 5mg/ml and 10mg/ml FAI solution treatment, respectively. This phenomenon also indicates the insertion of FA+ into MA-perovskite lattice because the ionic radius of FA+ (1.9-2.2 Å) is larger than that of MA+(1.8 Å). The FA content in the perovskite film can be determined from peak shift at (110) of perovskite film in XRD pattern.43-44 As shown in Figure 4d, the ratio of FA/MA compositions in the perovskite film treated with 1mg/ml, 5mg/ml and 10mg/ml FA are FA0.05MA0.95, FA0.17MA0.83 and FA0.28MA0.72, respectively. We also calculated the FA content by the changes of bandgap as shown in Figure 4d. The bandgap was obtained by Tauc plot of FAxMA(1-x)PbX3 in Figure S3.The calculation values of FA content are consistent with that of XRD. Therefore, we propose that FA+ could insert into perovskite lattice to fill the MA+ vacancies as shown in Figure 1. The insertion of FA+ also narrows the bandgap, increases the grain size and improves the grain crystallinity. While the excessive FAI could make the perovskite films inhomogeneous and limit the


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extraction of charge carriers, which will damage device performance. Scanning kelvin probe microscopy (SKPM) was carried out to evaluate the surface electronic properties. For simplicity, only the control film and 5 mg/mL FAI passivated perovskite film are compared in Figure 4(d, e), respectively. In Figure 4d, blight or dark stripes unevenly distribute on the surface of the control film with the surface potential ranging from -41 mV to 42 mV. This inhomogeneous distribution is not conducive to the selective collection of carriers, resulting in the performance degradation of the devices. In contrast, the perovskite film post-treated with FAI solution exhibits a homogenous potential distribution varies only from -35 mV to 36 mV. The difference of surface potential between two samples is most likely due to the more local charge accumulation or deficiency on the surface of control film than on the FAI passivated film surface. The uneven distribution of surface charge would impede the charge transport and could also serve as trap-assisted recombination centers.26, 45-46 Figure S4 shows the Electron probe microanalysis (EPMA) images, which are performed to determine the type of charge accumulation by comparing the elements distribution on the perovskite surface. For the control sample, the proportion of I in high concentration area is higher than that in the FAI passivated sample, which means the distribution of I is relatively unevenly. This phenomenon for Pb is not so serious. The inhomogeneous distribution of I leads to irregular I-poor and I-rich zones. So, we propose that the FAI treatment could eliminate the inhomogeneous distribution of local charges to decrease the recombination centers. We also assessed the thermal stability of the perovskite films. Figure S5 shows the changes in light absorption and the photos comparison of the perovskite films of the control sample and 5 mg/mL FAI passivated sample, which were held on a hot plate at 100 °C in air condition with humidity 50 % for different times. Compared with the control film， the passivated perovskite film with 5 mg/mL FAI solution could still retain the dark black color and higher light absorption after heating for 900 min. Although degradation is still exist for the passivated film, the decomposition rate is much slower than that of the control



sample. Therefore, FAI treatment of the perovskite film could also improve its thermal stability.

Figure 5 (a) Cross-sectional SEM image of PSCs. (b) I-V characteristic of the electron-only devices. (c) Open-circuit voltage of PSCs as a function of light density. (d) Photovoltaic metrics of PSCs with different FAI solution treatment, the box charts with same color correspond to the same FAI solution concentration. To assess the superiorities of the FAI passivated perovskite films, we assembled devices using different perovskite films. Figure 5a shows a cross-sectional SEM image of PSCs. The structure of PSCs is FTO/SnO2/perovskite/Spiro-oMeTAD/Ag and the


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thickness of perovskite film is around 330 nm. We adopted electron-only devices to evaluate the charge-transport properties of the perovskite film. The structure of the electron-only device is FTO/SnO2/perovskite/PCBM/Ag. And the current–voltage (I-V) characteristics were measured and analyzed using the space charge limited current (SCLC) method in Figure 5b. Each I-V curve has three different regions. Under lower voltage, there is a linear ohmic regime (IαV). As the voltage increases, the trap-filled limit regime (TFL IαVn>3) and Child’s regime (IαV2) appear one after another. And VTFL are used to mark the transition from the ohmic stage into the TFL stage.37, 47 According to equation: VTFL = entrapL2 2ε0ε

, the trap density (ntraps) is proportional to the VTFL.38 Thus，the trap density could be

roughly estimated by comparing the values of VTFL. In the Child’s regime, according to 9

V2

equation:JD = 8ε0εμ𝐿3, the charge carrier mobility can be roughly calculated by JD/V2.48-49 According to Figure 5b, the passivation process could reduce defects and enhance the carrier transport capacity due to the decreased VTFL and increased current density at the same voltage in the measured Child’s regime. The larger VTFL value and lower mobility of the perovskite film treated with 10 mg/mL FAI solution is due to the suppression of carrier transport characteristics resulting from electrical insulting FAI residual at the perovskite surface, which has been verified in stable PL quenching spectra (Figure 3) and SEM (Figure 4). To reveal the interface charge recombination in PSCs, Figure 5c exhibits the dependence of Voc on the light intensity. The diode ideality factor, n, was commonly used to compare the density of traps and recombination mechanism50. Its value is defined as: 𝑛 𝑞

𝑑𝑉𝑜𝑐

= 𝑘𝑇.𝑑𝑙𝑛(ψ). Here, the fitted values of n are all located between 1 and 2, that indicates Shockley–Read–Hall trap-assisted recombination in our devices.40 For 1 mg/mL and 5 mg/mL FAI post-treatment perovskite samples, the values of n are 1.58 and 1.59, respectively. Both of them are lower than that of the control sample (1.77). However, the value of n rises up to 1.82 when the FAI solution concentration is 10 mg/mL, which is



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likely due to the introduction of traps by excessive FAI. These results demonstrate that the interface charge recombination in PSCs repaired with suitable FAI concentration is less serious than that of control devices. The possible reason is due to the reduction of interface defects between the perovskite layer and HTL. Figure 5d summarizes the photovoltaic metrics of PSCs assembled using the perovskite films with different FAI solution treatment. Several batches of PSCs were fabricated. The average value of Voc increases with increasing the FAI solution concentration and reaches a maximum value of 1.106 ± 0.0126 V at 5 mg/mL FAI. Further increasing the FAI solution concentration to 10 mg/mL, the Voc value shows a significantly decrease. This is due to that the excessive electrical insulating FAI can form a barrier to hinder the effective charge transfer. The JSC increases slightly as FAI solution concentration increased, and there is a significantly increase at 10 mg/mL.

We believe

that the increased JSC is derived from improved light capturing likely due to broaden light absorption range with FAI solution treatment. The average value of FF increases from 71.25 ± 2.35% (without FAI) to 74 ± 1.72% (5 mg/mL FAI), achieving a maximum value of 75.6 %. Then the average FF decreases slightly when further increasing the FAI solution concentration. Consequently, the average PCE increases from 14.65 ± 1.06% for control devices to 16.47 ± 0.81% for the devices using 5 mg/mL FAI solution passivation, and achieving a maximum value of 17.82%. Higher concentrations of FAI (10 mg/mL) leads to reduced PCE, primarily due to the reduced VOC and FF. The J-V curve of PSC fabricated by the perovskite film treated with 15 mg/mL FAI solution is showed in Figure S6. As expected, the VOC and FF are remarkably reduced due to the electrical insulation of excessive FAI.


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Figure 6 Performance comparison between control sample and the sample modified with 5 mg/mL FAI solution. (a) J-V characteristics. (b) IPCE and integrated current density. (c) The steady-state output at a given constant bias. Bias of the control sample is 0.85 V and bias of modified sample is 0.87 V. Note the stabilized photocurrent density (red) and PCE (black). (d) The statistical data of PCE (30 PSCs from different batches). The current density-voltage (J-V) curves of champion PSCs fabricated by the perovskite films without and with 5 mg/mL FAI solution treatment are shown in Figure 6a. The control cell prepared by perovskite film without post-treatment shows a PCE of 15.68% with a Voc of 1.09 V, a Jsc of 19.34 mA/cm2 and a FF of 74.1% under 1 sunlight illumination. In contrast, the perovskite device after FAI treatment exhibits significant improvement in Voc and Jsc. Thus, an enhanced PCE of 17.82% with a Voc of 1.13 V, a Jsc of 21.34 mA/cm2 and FF of 74.3% was obtained. Figure 6b presents the incident photon-to-current conversion efficiency (IPCE) of the devices. The IPCE curve of the passivated device is higher than that of the control device throughout the whole wavelength range. A small redshift of the light harvesting edge is observed after FAI solution treatment, which may stem



from the narrowed bandgap after FA+ filling into MA-lattice. Correspondingly, the integrated photocurrent increases from 18.98 mA/cm2 to 20.30 mA/cm2. Figure 6c shows the steady photocurrent and PCE of the best device measured at the maximum power output point. The values of steady photocurrent and PCE are close to the values shown in Figure 6(a, b). Compared with the device with FAI solution passivation, due to the more traps located in the perovskite film of the control device, its photocurrent and PCE show a slightly decrease with time prolonging. Figure S7 shows different scan direction of the JV curves of the two PSCs. The hysteresis are less than 2%. And the efficiencies of the PSCs from different batches are counted in Figure 6d. The small variance between the maximum PCE (17.82%) and average PCE (16.47%) for the FAI passivated samples indicates its better repeatability compared to control samples. We also measured the stability of PSCs of the unencapsulated control sample and passivated sample under ambient conditions with a humidity of 45-50%. As shown in Figure S8, the control samples degraded dramatically and retained its initial PCE of 60% after 400 h. However, the device stability was substantially improved when post-treated with FAI solution. It may result from the reduced concentration of defects in perovskite layer.27,

31-32

The slow decrease of PCE mainly

origins in the diffusion of Ag into perovskite. The damage of the metal electrode to perovskite are commonly reported in the literatures.37, 51 Therefore, this repairing surface defects process could improve the stability of the perovskite devices. Conclusion A large amount of defects within the halide perovskite films inhibit the performance improvement of solar cells. The deep-level defects induced by the I vacancies and undercoordinate Pb atoms could lead to a lower Voc. The shallow-level defects induced by the organic vacancies could result in the poor device stability. A FAI solution post-treatment method was introduced to repair the defects and improve the performance of MAPb(I95%Br5%)3 perovskite films and devices in this work. The dissociative ions of FA+


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and I- filled the cationic MA+ and anionic I- vacancies in the perovskite surface, and eliminated the under-coordinated Pb atoms. According to TRPL and steady-state PL results, this process remarkably reduced the trap density, prolonged the carrier lifetime and thus enhanced the carrier charge transfer capacity, which therefore enhances the Voc and PCE of devices. As a result, the planar-structured PSC with a facile 5 mg/mL FAI solution treatment presented the champion PCE of 17.82 % with Voc of 1.13V. Compared with the PCE of 14-15% for the PSC without treatment, there is almost 14% enhancement. Furthermore, FAI solution passivation can efficiently increase the thermal stability of the perovskite film at high temperature (100 °C) compared to pristine perovskite films. The removal of non-radiative channels through surface repairing treatments helps us to build up an effective way to obtain high performance devices. Supporting Information Experimental detail of the manufacturing procedures, additional supplementary figures and/or tables about material/ device characterization, device statistics.

Notes The authors declare no competing financial interests. Acknowledgements This work was supported by the National Science Foundation of China (51774034, 51772026 and 51611130063), Beijing Natural Science Foundation (2182039), Fundamental Research Funds for the Central Universities (FRF-TP-17-083A1).

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Repairing Defects of Halide Perovskite Films To Enhance Photovoltaic

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