Enhanced Open-Circuit Voltage of Cs-Containing FAPbI3 Perovskite

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Article

Enhanced Open Circuit Voltage of Cs-containing FAPbI Perovskite Solar Cells by a Formation of Seed Layer through Vapor Assisted Solution Process 3

Jing Chen, Jia Xu, Chenxu Zhao, Bing Zhang, Xiaolong Liu, Songyuan Dai, and Jianxi Yao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05610 • Publication Date (Web): 12 Jan 2019 Downloaded from http://pubs.acs.org on January 13, 2019

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ACS Sustainable Chemistry & Engineering

Enhanced Open Circuit Voltage of Cs-containing FAPbI3 Perovskite Solar Cells by a Formation of Seed Layer through Vapor Assisted Solution Process Jing Chena,b, Jia Xub,c, Chenxu Zhaoa,b, Bing Zhangb,c, Xiaolong Liua,b, Songyuan Dai,a,c Jianxi Yaoa,b* aBeijing

Key Laboratory of Energy Safety and Clean Utilization, North China Electric

Power University, Beijing 102206, China. bState

Key Laboratory of Alternate Electrical Power System With Renewable Energy

Sources, North China Electric Power University, Beijing 102206, China cBeijing

Key Laboratory of Novel Film Solar Cell, North China Electric Power University,

Beijing 102206, China.

Abstract The quality of the perovskite films contribute significantly to the photovoltaic performance of perovskite solar cells. An improved perovskite seeding growth process has been employed to fabricate Cs-containing FAPbI3 perovskite films in vapor assisted solution process by regulating the nucleation and crystallization. Compared to the film without seeding growth process, the as-prepared perovskite film showed a larger grain size, a better crystallinity, a longer carrier lifetime and a lower defect density. The improved photovoltaic performance was demonstrated by the enhancement of open circuit voltage (Voc). The champion power conversion efficiency was reached 16.17% with a remarkable Voc of 0.99 V, a current density of 22.55 mA/cm2, and a fill factor of 71.84%. Moreover, the device performance was more stable after twenty days than the controlled one under ~30 % humidity in air. Keywords: vapor assisted solution process, improved perovskite seeding process, enhanced open circuit voltage 1 ACS Paragon Plus Environment

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Introduction Hybrid organic-inorganic perovskite solar cells (PSCs) have attracted intensively research interest in the last few years due to their high power conversion efficiency (PCE) and low cost1. With their superior long carrier diffusion length, high absorption coefficients and high carrier mobility, the PSCs have displayed a PCE as high as 23.7%.2 The typical perovskite structure is represented by the general formula ABX3, where A+ is an organic cation ((CH3NH3+ (MA+) and CH3(NH2)2+(FA+)) or an inorganic cation (Cs+ and Rb+), B2+ is a divalent metal cation (Pb2+ and Sn2+) and X- is a halide anion (Cl-, Br- and I-).3-6 Until now, most of reported PSCs were based on CH3NH3PbI3 (MAPbI3) system, which have attracted intensively interest for the photovoltaic applications due to the appropriate band gap of 1.50 eV7-8. However, the reversible phase transition from tetragonal to cubic structure of MAPbI3 hampered the stability of PSCs seriously, which was one of the major obstacles for its commercialization9-11. Moreover, there were some important concerns with respect to the degradation upon contact with moisture, as well as thermal stability. Recently, HC(NH2)2PbI3 (FAPbI3) has widely been suggested as an alternative to MAPbI3 due to its broader optical absorption range derived from the narrower band gap of 1.48 eV, longer charge diffusion length and superior intrinsic thermal stability11-13. However, the black phase α-FAPbI3 would transform to yellow phase δ-FAPbI3 at room temperature in the ambient condition, which hindered its stable photovoltaic performance for practical application12-15. To stabilize the α-FAPbI3, the mixed-cation approach has been demonstrated to be an effective strategy. A small amount of MA was already sufficient to induce a preferable crystallization into the photoactive layer of FA-based perovskite resulting in a more structurally stable composition than pure FAPbI3.16-17 However, these (FA,MA)PbI3 were found to exhibit limited thermal stability due to the volatile of MA.18 Alternatively, FAPbI3 can be stabilized in the required black α-phase by the partial substitution of FA with 2 ACS Paragon Plus Environment

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cesium cation due to entropic gains and the small internal energy input.19-23. Stable perovskite phase with enhanced photovoltaic performance has been realized by this replacement.22-23 Cs-containing FAPbI3-based PSCs also showed enhanced moisture stability

24-25.

Therefore,

Cs-containing FAPbI3-based solar cells stabilized the photovoltaic performance effectively. To achieve high efficient PSCs, it was significant to improve not only their stability but also carrier transport properties which was closely related to the high quality of the perovskite film with a lowered carrier recombination and reduced defect states.21, 24-30 Some reports have demonstrated that rapid nucleation and slow crystallization was an effective way to realize this goal.30-40 Huang et al employed N-methylimidazole (NMI) as an additive in FAI solution. In the presence of NMI, perovskite crystallization has effectively been slowed down resulting in a much more uniform morphology with enlarged grain size. The reduced grain boundary has efficiently decreased the recombination centers. The PCE has been improved from 12.69 % of pure FAPbI3 to 15.38 % of Cs0.15FA0.85PbI3.20 Besides, Zhao Y et al devised a perovskite seeding growth (PSG) method by incorporating cesium into the PbI2 precursor solution to form a number of perovskite seeds, which acted as the nucleation centers to modulate the perovskite crystallization beneficially. In the PSG process, perovskite growth commenced on the nucleation centers immediately when the alkylammonium halide salts were deposited on PbI2 film. Therefore, rapid nucleation was very favorable for achieving large grain size, low trap density and high performance devices.41 Besides, it was known that the rapid intercalating reaction rate of lead source and ammonium salt in vapor process can be slowed down effectively compared with the traditional solution method. And the undesired structural transformation during solution process can be avoided, which would lead to the optimization of the perovskite surface morphology.17,

42-45

Moreover, it has been reported due to the FAI volatility during the

annealing process based on solution process the components of obtained Cs-containing 3 ACS Paragon Plus Environment


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FAPbI3 films was difficult to control precisely, which usually resulted in the non-stoichiometric composition. In vapor deposition the lead source coated substrates was exposed in the saturated FAI vapor atmosphere during the whole reaction process. The sufficient FAI vapor could effectively avoid the volatility of FA+ and ensured the accurate stoichiometry. To our best knowledge, there was only one report about the preparation of Cs-containing FAPbI3-based PSCs based on vapor deposition. Luo et al reported that Cs0.15FA0.85PbI3 based PSC with a PCE of 14.45% and Voc of 0.86 V. The Voc in their study was far below the Voc of the Cs-containing FAPbI3 solar cells prepared by solution process. The complexity of Voc increase was associated with the trap densities46, thus improving the quality of perovskite films could be regarded as an effective strategy to enhance the Voc of CsxFA1-xPbI3 system. In this study, the combination of PSG process and VASP called the improve PSG was employed to solve the existing problem of low Voc in Cs-containing FAPbI3 PSCs fabricated by vapor assisted solution process (VASP). In the improved PSG process, a formamidinium iodide (FAI) layer spin-coated on lead source coated substrate, acting as nucleation centers by forming perovskite nucleation seeds in the subsequent reaction. XRD results have demonstrated that these nucleation centers could regulate the crystallization in the growth process. Compared to the controlled one, the perovskite films prepared by the improved PSG process showed enlarged grain size, improved crystallinity, prolonged carrier lifetime and lowered defect density. And the photovoltaic performance has been improved mainly presented by the enhancement of open circuit voltage (Voc). The optimal device exhibited a PCE of 16.17 % with a Voc of 0.99 V, a short circuit current (Jsc) of 22.55 mA/cm2 and a fill factor (FF) of 71.84 %. Moreover, the device performance was more stable by twenty days than the controlled one under ~30 % humidity in air. Experimental Section 4 ACS Paragon Plus Environment

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Materials FAI, spiro-OMeTAD, Li-TFSI and FK209-cobalt(III)-TFSI were purchased from Borun Chemicals. PbI2, CsI, chlorobenzene, Titanium (IV) isopropoxide and 1,2-dichlorobenzene were purchased from Acros. C60, DMF and DMSO were purchased from Alfa Aesar. Anhydrous isopropanol was purchased from J&K Co., tBP was purchased from Sigma-Aldrich. All reagents were used without further purification. Fluorine-doped tin oxide (sheet resistance 15 Ω/square) layer coated on the glass substrates were used for the PSCs. Device Fabrication The PSCs based on FA1-xCsxPbI3 (x=0, 0.05, 0.1 and 0.15) with different Cs concentration have been prepared by VASP. It was found that the best condition was x=0.1 (FA0.9Cs0.1PbI3) (Figure S1). Therefore, the FA0.9Cs0.1PbI3 perovskite material was used for the investigation in the following discussion. The devices structure was FTO/compact-TiO2/C60/FA0.9Cs0.1PbI3/spiro-OMeTAD/Au. The controlled device preparation process was the same as our previous study.17,

45

The

improved PSG process in vapor deposition was as following. After FTO/TiO2/C60 cooling to room temperature, FAI with different concentrations (2.5 mg/ml, 5 mg/ml and 7.5 mg/ml) dissolved in IPA were spin-coated onto FTO/TiO2/C60 at 3000rpm for 30s. Then the lead source (PbI2 and 0.1M CsI) was spin-coated onto FAI layer and annealed at 170 °C for 15 min in nitrogen-filled glovebox, whose purpose was to form perovskite seeds. Then lead source coated substrates were put in a covered petri dish with FAI powder spreading around uniformly. The subsequent preparation process was the same as conventional VASP as described above. The perovskite films on these pre-deposited substrates were labelled as FAI-2.5, FAI-5 and FAI-7.5, respectively. While the sample untreated by FAI, was labelled as controlled in following discussions. Then, the hole transport layer (spiro-OMeTAD) was spin-coated on the sample at 4000rpm for 20 s. The spiro-OMeTAD solution was composed 5 ACS Paragon Plus Environment


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of a 1 mL spiro-OMeTAD/chlorobenzene solution (72.3 mg / mL), 28.8 μL tBP, 17.5 μL Li-TFSI/acetonitrile solution (520 mg / mL), and 8 μL FK209-cobalt(III)-TFSI/acetonitrile solution (300 mg / mL). Finally, Au was thermally evaporated on the top of PSCs to form the back electrode (~60 nm). Characterization Keithley 2400 was used to measure the current density-voltage (J-V) curves with a sunlight simulator under AM 1.5 illumination. The effective area of PSC was 0.09 cm2. The detailed characterizations of scanning electron microscopy (SEM), X-ray diffraction (XRD) characteristics, X-ray photoemission spectra (XPS), steady-state photoluminescence (PL) spectra,

time-resolved

photoluminescence

(TR-PL)

spectra

and

the

incident

photon-to-electron conversion efficiency (IPCE) referred to experimental sections of previous reports.45 Impedance spectroscopy (IS) were carried out under AM 1.5 illumination, using a 10 mV perturbation with 0~0.8 V applied voltage. A Zahner electrochemical workstation was used as a frequency response analyzer, and impedance measurements were performed in 10 mHz ~ 100 kHz. Voc decay of devices was measured on the period of 2 s measured using Zahner

electrochemical

workstation.

Electron-only

devices

(FTO/c-TiO2/FA0.9Cs0.1PbI3/PCBM/Ag) were fabricated to calculate the electron mobility of the devices. The mobility was derived from the J-V curves by the Mott-Gurney equation. The trap state density was determined by the trap-filled limit voltage. Confocal laser scanning microscope (CLSM) images were measured by Ni-C2-SIM, Nikon, Japan. The selective laser (561 nm) was used for excitation. This measurement was carried out in Technical Institute of Physics and Chemistry CAS. Results and Discussions The process for fabarication of perovskite films with improved PSG was illustrated in Figure1a. Firstly, the perovskite seeds were formed by the reaction of lead source (1M PbI2 6 ACS Paragon Plus Environment

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and 0.1M CsI) and the pre-deposited FAI layer after annealing. The as-prepared seeds act as nucleation centers in subsequent vapor reaction process. Then the PbI2 and the pre-generated perovskite seeds react with FAI vapor, forming FA0.9Cs0.1PbI3 perovskite film. The XRD patterns of lead sourece (1M PbI2 and 0.1M CsI) layer without and with FAI were shown in Figure 1b and 1c. No characteristic peak of perovskite in the XRD patterns of the controlled film. The characteristic peak attributed to the FA0.9Cs0.1PbI3 perovksite phase at 14.06° was found with 30 mg/ml FAI pre-deposited (Figure 1c). Due to the characteristic peak was not clearly seen in XRD patterns when the FAI concentration lowered than 30 mg/ml (Figure S2), the films preapared with 30 mg/ml of FAI was carried out. The XRD results indicated that the pre-deposited FAI layer reacted with lead source forming the perovskite seeds in PbI2 film. CLSM was also used to identify the perovksite seeds in lead source film as shown in Figure S3. In order to observe the small amount of perovskite seeds, the single channel laser with 561 nm excitation wavelength was used. The emission light between 570 nm and 1000 nm was attributed to the perovskite, in red color. The light field image of lead source without laser excitation was a reference. CLSM images overlaying on top of the light field image of the samples without and with pre-deposited by FAI (30mg/ml) indicated that the red color distributed densely of FAI-5 film comparing to that of controlled film. Therefore, the red emissions originating from the perovksite hinted the existence of perovksite seeds in lead source deposited by FAI. SEM and XRD were carried out to investigate the morphology and crystallinity of FA0.9Cs0.1PbI3 films prepared by the improved PSG process through vapor deposition. The average grain size of the controlled, FAI-2.5, FAI-5 and FAI-7.5 films was ~ 780 nm, 800nm, 900 nm and 600 nm respectively (Figure 2a-2d). The surface texture of the perovskite films was examined by AFM (Figure S4). The root-mean-square (RMS) roughness were 32.6 nm, 33.1 nm, 28.2 nm and 35.3nm for the controlled, FAI-2.5, FAI-5 and FAI-7.5 films 7 ACS Paragon Plus Environment


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respectively. Comparing to other films the FAI-5 film was more homogeneous. Besides, the crystallinity of different perovskite films was investigated by XRD. The characteristic diffraction peaks of FA0.9Cs0.1PbI3 marked by black solid diamonds in Figure 2e. Corresponding enlarged XRD patterns at 14.06° of all the films was shown in Figure 2f. It can be seen that peak positions at 14.06° of the perovskite films were no shifting, indicating that all final films were composed of FA0.9Cs0.1PbI3. Moreover, the full width at half maximum (FWHM) derived from (101) main peaks of FAI-2.5 and FAI-5 film decreased (Figure 2f). The FWHM values of controlled one, FAI-2.5, FAI-5 and FAI-7.5 films were 0.139, 0.128, 0.126 and 0.130, respectively. FAI-7.5 perovskite film became poor with smaller grain size, rougher surface and lower crystallinity. Thus, it can be concluded that a proper FAI concentration was favorable for improving the quality of perovskite film. The reason might be that much more nucleation centers in high concentration of FAI could lead smaller grain size and coarse surface. And the inferior quality of the perovskite film would destroy the transportation property; the corresponding photovoltaic performance would be deteriorated. To study the exact impact of the improved PSG process on the photovoltaic performance of FA0.9Cs0.1PbI3 solar cells, twenty cells were fabricated. As shown in Figure 3, the statistical results of devices photovoltaic parameters based on controlled, FAI-2.5, FAI-5 and FAI-7.5 films were counted. The photovoltaic performance parameters of the optimal devices under each condition were summarized in Table 1 and corresponding J-V curves were shown in Figure S5. The corresponding median value and the standard deviation of Jsc, Voc, FF and PCE insert in each figure. From the statistical calculation of the photovoltaic parameters, it can be seen that the devices based on FAI-2.5, FAI-5 and FAI-7.5 films showed a maximum (average) PCE of 15.21% (14.53%), 16.17% (15.42%) and 15.05% (13.78%), respectively. The photovoltaic performances enhance by the improved PSG process, and the device based 8 ACS Paragon Plus Environment

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on FAI-5 has the optimal efficiency. Moreover, another representative feature of PSCs was hysteresis effect. The hysteresis index (HI) was used to quantify the hysteresis effect of the device which was defined by equation 1.47-49: 𝑂𝐶

𝐻𝐼 = 𝑂𝐶

∫𝑆𝐶 (𝐽𝑅𝑆(𝑉) ― 𝐽𝐹𝑆(𝑉))𝑑𝑉

(1)

𝑂𝐶

∫𝑆𝐶 𝐽𝑅𝑆(𝑉)𝑑𝑉 𝑂𝐶

∫𝑆𝐶 𝐽𝑅𝑆(𝑉)𝑑𝑉 and ∫𝑆𝐶 𝐽𝐹𝑆(𝑉)𝑑𝑉 represent the area of J-V curves under reverse and forward scanning respectively. SC and OC were abbreviations of short circuit and open circuit. In the equation 1, the integration of all bias voltages reflected all the changes in hysteresis. For the controlled devices, the calculated HI was 0.062. And the HIs of the FAI-2.5, FAI-5 and the FAI-7.5 devices were 0.043, 0.014 and 0.027 respectively. Fabricated by the improved PSG process, the HIs of devices has been decreased and the FAI-5 device has the minimized HI, indicating that the improved PSG process could reduce device hysteresis effectively. Therefore, in all cases, the performances of the devices fabricated by the improved PSG process were significantly better than that of controlled devices which obtained a maximum (average) PCE of 14.23 % (13.56%). Especially, the Voc increased from 0.89 V of device based on controlled film to 0.99 V of device based on FAI-5 film. And the change tendency of FF was consistent with Voc. From the statistical results, it can be concluded that the improvement of PCE was mostly attributed to the enhanced Voc and FF, while Jsc stay constant at ~22 mA/cm2. Therefore, for PSCs of Cs containing FAPbI3, the problem of low Voc in VASP has been improved by the improved PSG porcess.46 The FAI-5 perovskite film exhibited optimal photovoltaic performance. Thus, the comparative investigations were carried out between the controlled and FAI-5 samples in the following discussions. The growth kinetics of the controlled film and FAI-5 perovskite film were shown in Figure 4a and Figure 4b respectively. The XRD patterns evolution with the reaction time were assigned by 0 min, 3 min, 6 min, 9 min, 12 min, 15 min, 20 min, 25 min 9 ACS Paragon Plus Environment


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and 30 min. For the controlled one, the characteristic peak (2θ=14.06°), which was attributed to the (101) crystallographic plane of FA0.9Cs0.1PbI3 of perovskite phase, appeared at 9 min. While the characteristic peak (2θ=14.06°) of FA0.9Cs0.1PbI3 appeared prominently at 6 min for the FAI-5 film. The subsequent reaction was the continuation of the lead source and FAI gas molecules until the complete formation of FA0.9Cs0.1PbI3. It can be concluded that FAI assisted the FA0.9Cs0.1PbI3 perovskite crystallization effectively with perovskite seeds layer. The corresponding morphology evolution without and with perovskite seeds in the complete reaction process were shown in Figure S6 and S7. Before the reaction beginning, some bright rods spreading on the surface uniformly can be seen at 0 min. After reacting with FAI gas molecules, a small amount of darker crystal grains were observed in FAI-5 film at 6 min, but not on the controlled film. As the reaction going on, the darker grains in each image in FAI-5 film were relatively more than those in controlled film. Together with the results of XRD, it can be concluded that the darker grains in the morphology were the perovskite grains. To prove the quality of perovskite film prepared by PSG process was superiority than the controlled one in the whole reaction process, the evolution of relative intensity of (101) peak (2θ=14.06°) to the substrate peak (2θ=37.87°) was calculated (Figure 4c). With the reaction proceeding, the relative intensity of (101) peak increased gradually. And the relative intensity of FAI-5 was higher than that of the controlled one, indicating that the crystallinity of FAI-5 perovskite film was better than that of controlled one. It was expected that the PSCs based on FAI-5 film have improved photovoltaic property. Electrical impedance spectroscopy (EIS) provided the insight into the charge transportation process and recombination mechanism of PSCs.50-51 The EIS measurements of PSCs prepared without and with FAI were carried out at different bias voltage regions (0 ~ 0.8V) under AM 1.5 illumination. Figure 5a showed the Nyquist plots of PSCs based on controlled and FAI-5 film at bias voltage of 0.8 V. The Nyquist plots have two diacritical 10 ACS Paragon Plus Environment

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characteristic arcs. The high-frequency semicircle was attributed to contact resistance (RCT) at the contact layer/perovskite layer; the low-frequency semicircle was caused by the recombination resistance (Rrec) between the perovskite layer and the electron transport layer. As known that, the recombination rate of photo-generated carriers was inversely related to the Rrec.52-54 As the contact layer and the final component of perovskite film (FA0.9Cs0.1PbI3) was same in our investigation, the recombination process in perovskite layer was the main involved factor. The obtained parameters Rrec and Rs by fitting the Nyquist plots with equivalent circuit diagram (Figure S8) were shown in Figure 5b. Rs was the series resistance of the external circuit and electrode. With the increase of applied voltage, the Rs decreased slightly. The average Rs for controlled and FAI-5 devices were 21.41 Ωcm2 and 17.53 Ωcm2. Lower Rs resulting in higher FF consisted with the statistic results in Figure 3c. The devices based on FAI-5 film fabricated by the improved PSG process have the largest Rrec, indicating that recombination rate of FAI-5 film was lowest. Therefore, fabricated by the improved PSG process the interface contact between the perovskite layer and the electron transport layer improved effectively. Steady-state PL spectra was executed to explore the charge separation and recombination dynamics in perovskite layer. As shown in Figure 6a, the absorption onset of controlled and FAI-5 perovskite films almost no shifts (~ 810 nm), indicating the negligible contribution to potential changes of device performance. The PL emission peak position of the controlled film slightly red shifted comparing to that of the FAI-5 perovskite film, which might be caused by some deep level defects.55-56 These deep level defects could capture the electrons and quench the intrinsic emission, which was also reported by Huang et al.55 Moreover, the steady-state PL intensity of FAI-5 perovskite was an order of magnitude higher than that of controlled sample. It can be concluded that perovskite film fabricated by the improved PSG process had a relatively low non-radiative recombination loss.22 Corresponding TRPL spectra 11 ACS Paragon Plus Environment


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were also conducted (Figure 6b). A double exponential function was used to fit the measured TRPL decay curves. The decay curves were divided two decay components, including a fast decay component (τ1) and a slow decay component (τ2). And τ1 was ascribed to the interface recombination and τ2 was related to the recombination process happened in perovskite layer. The parameters obtained from the fitting curves with double exponential function were summarized in Table 2. Comparing to τ2=15.17 ns of the controlled film, the FAI-5 film had a longer carrier lifetime τ2=32.99 ns. The reduced recombination rate in perovskite layer demonstrated by EIS might be one of the main reasons for the increasing carrier lifetime. Besides, the controlled film has average charge carrier lifetime (τavg) of 11.50 ns, while for FAI-5 perovskite film τavg prolonged to 28.52 ns. The recombination in perovskite film was reduced and the transport property of perovskite film was also enhanced fabricated by the improved PSG process. Besides, the Voc decay was another reliable strategy for analyzing the electron recombination process of devices. Based on our previous reports,17,45 by fitting the Voc decay (Figure S9) the derived electron lifetime (τn ) of the injected carrier against the voltage of devices were plotted in Figure 6c. The device based on FAI-5 film was provided with a much longer τn than that of controlled sample. The τn were 0.022 s and 0.87 s of PSCs at 0.45 V based on controlled and FAI-5 respectively. The improved PSG process was beneficial to the increasing of the electron lifetime and reducing the carrier recombination rate in perovskite layer. For quantification analysis, the space chare limited current (SCLC) method was used to study the defect states. The dark J-V curve was measured with an electron-injecting device. Figure 6d and 6e showed the dark J-V curves of devices based on controlled film and FAI-5 film, respectively. The voltage (VTFL), which marked the transition from the ohmic region (n=1) to the trap-filled limit (TFL) region (n>3), was used to calculate the trap state density by equation 2,57-58 12 ACS Paragon Plus Environment

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𝑛𝑡𝑟𝑎𝑝 =

2𝜀0𝜀𝑟𝑉𝑇𝐹𝐿

2

𝑒𝐿

(2)

Where e is the elementary charge, εr and ε0 are the relative dielectric constant of FA0.9Cs0.1PbI3 film and vacuum permittivity, respectively, L is the thickness of the perovskite films. The densities of trap states decreased from 6.11×1015 cm-3 of the controlled film to 4.43×1015 cm-3 of FAI-5 film. The densities of trap states in the FAI-2.5 and FAI-7.5 films were shown in Figure S10. The density of trap state was significantly lowered by improved PSG process, resulting in higher FF and Voc of the corresponding devices. Furthermore, the intrinsic mobility was estimated using SCLC (Figure 6f).50, 58-59 The average mobility were 0.122cm2 V−1 s−1, 0.198 cm2 V−1 s−1, 0.291 cm2 V−1 s−1 and 0.183 cm2 V−1 s−1, respectively. All the electron mobility of as-perovskite films by improved PSG process was improved, and the FAI-5 perovskite film has the largest electron mobility, which might be attributed to that shallower trap state helps carrier transport. The rapid nucleation and crystal growth of FA0.9Cs0.1PbI3 film can improve the transportation effectively. Meanwhile, the enhanced Voc can be attributed to the carrier lifetime associated with the reduced trap densities. In addition, stability of the devices based on controlled and FAI-5 films was also explored. The J-V curve of the best-forming device and the corresponding device structure illustrated in Figure 7a, showing an integrated current density of 21.14mA/cm2 (Figure 7b) which was close to the short circuit current density (Jsc) listed in Table 1. The devices based on FAI-5 have a relatively stable current and stable output power, which was measured at the maximum power point of 0.79 V. Similar stable PCE of 16.17% (Figure 7c). Furthermore, the operational stability was also measured under varied humidity stored in air for twenty days. Ten PSCs based on each condition were prepared with and without the improved PSG method. The devices were stored in ~30 % humidity in air for twenty days (Figure 7d). And the normalized PCE decayed to 42 % of the controlled devices, the devices prepared with the improved PSG decreased to 67 %. It can be concluded that the FAI layer has advantages for 13 ACS Paragon Plus Environment


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improving the stability of device which might relate to the high quality perovskite film with lowered defects. High crystallized perovskite film with low defects could effectively reduce the possibility of undesired water penetration and decrease the perovskite decomposition. Conclusion In summary, we have designed an improved PSG process to enhance the quality of FA0.9Cs0.1PbI3 perovskite films based on VASP. The pre-deposited FAI induces perovskite seeds, which is favorable for rapid nucleation of perovskite films. Comparing with the controlled perovskite film, perovskite film prepared by the improved PSG has larger grain size, better crystallinity, longer carrier lifetime and lower density of defects. And the photovoltaic performance is improved, which is mainly manifested by the enhancement of Voc. The best-performing PSC possessed a PCE of 16.17% with a Voc of 0.99 V, a Jsc of 22.55 mA/cm2 and a FF of 71.84%. Moreover, the performance of the device was stable for at least twenty days under ~30 % humidity in air. ASSOCIATED CONTENT Supporting Information Histograms of PCEs of CsxFA1-xPbI3 with different Cs concentrations, XRD pattern of lead source spin-coated with different FAI concentrations, CLSM image of the controlled sample and FAI-30 sample, AFM images and J-V curves of controlled, FAI-2.5, FAI-5 and FAI-7.5 sample, SEM images of perovskite growth process without and with perovskite seed, equivalent circuit for the Nyquist plots, Voc decay curves of devices based on controlled film and FAI-5 film. AUTHOR INFORMATION Corresponding Author * Corresponding Authors: Email: [email protected]. Notes 14 ACS Paragon Plus Environment

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The authors declare no competing financial interest. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 51772095), the Fundamental Research Funds for the Central Universities (No. 2018MS040 and 2018ZD07) References 1.

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Figures and Captions

Figue 1 (a) Preparation of perovskite films using an improved perovskite seeding growth based on vapor assisted solution process. XRD patterns of lead source film without (b) and with (c) pre-deposited by FAI. The inset figures in (b) and (c) are the enlarged XRD spectra in the range 12.8° - 14.8°.


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Figure 2 SEM images of perovskite films (a) without and with (b) 2.5mg/ml, (c) 5mg/ml, (d) 7.5mg/ml pre-deposited by FAI, respectively. (e) The corresponding XRD patterns from 2θ = 10° - 50°. The black “*” represents the substrate FTO/TiO2, and the black diamond “♦” indicates characteristic peaks of FA0.9Cs0.1PbI3. (f) The enlarged XRD patterns for all the samples illustrated in (e) with 2θ range from 13° to 15°. The diffraction peaks correspond to the (101) lattice plane for FA0.9Cs0.1PbI3. (g) The full width at half maximum (FWHM) derived from (101) main peaks in (f).



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Figure 3 Statistics of (a) Jsc, (b) Voc, (c) FF and (d) PCE values of PSCs based on perovskite films without and with pre-deposited by different concentration of FAI. The median value and the standard deviation of Jsc, Voc, FF and PCE were inserted in the corresponding figures respectively. 20 samples of each kind of device-set are measured.


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Figure 4 XRD patterns of the samples during the reaction process of the lead source without (a) and with (b) pre-deposited FAI. The evolution of relative intensity of perovskite (101) peak (2θ = 14.06°) and substrate peak (2θ = 37.9°) with the reaction time (c).



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Figure 5 (a) The Nyquist plots of devices based on perovskite films prepared without and with per-deposited by FAI. (b) The fitting parameter Rrec and Rs derived from the fitting model in Figure S6.


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Figure 6 (a) Steady-state PL and (b) TRPL spectra of the perovksite films prepared without and with FAI. (c) The relationship between extracted lifetime of the injected carriers in the devices and Voc of the PSCs based on control and FAI-5. Dark J-V measurements of the electron-only devices displaying VTFL bend point behavior for perovskite films prepared without (d) and with (e) per-deposited by FAI. (f) Electron mobilities of devices without and with per-deposited by FAI.



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Figure 7 (a) J-V cureves of the best-performaning device based on FAI-5 under reverse and forwarod scanning, respectively. The inset is the corresponding structure of the device. (b) IPCE and integrated J plots from the same PSC. (c) Stable PCE and J at maximum power point of the same PSC. (d) Normalized PCE of devices based on controlled film and FAI-5 film in twenty days under ~30% humidity in air.


Page 31 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Tables Table 1 The performances of the optimal devices under reverse and forward scanning in Figure 3. Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

reverse

0.89

22.88

69.87

14.23

forward

0.87

22.84

65.28

12.90

reverse

0.96

22.81

69.53

15.21

forward

0.95

22.68

63.83

13.82

reverse

0.99

22.55

71.84

16.17

forward

0.99

22.45

66.41

14.95

reverse

0.93

22.61

71.82

15.05

forward

0.93

22.62

65.38

13.81

samples controlled FAI-2.5 FAI-5 FAI-7.5

PCEave (%)

HI

13.56±0.44

0.062

14.53±0.56

0.058

15.42±0.56

0.014

13.78±0.54

0.027

Table 2 Time parameters obtained from the fitted TR-PL decay curves in Figure 6b.

samples

τ1

τ2

τave

Value (ns)

Rel. (%)

Value (ns)

Rel. (%)

Value (ns)

Controlled

1.412

79.59

15.17

20.41

11.50

FAI-5

1.857

74.56

32.99

25.44

28.52



Page 32 of 32

Toc graphic

Synopsis: The improved perovskite seeding process in vapor assisted solution process enhanced the PSCs efficiency, especially for Voc increasement.


Enhanced Open-Circuit Voltage of Cs-Containing FAPbI3 Perovskite

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