Thiazole induced surface passivation and recrystallization of

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Thiazole induced surface passivation and recrystallization of CH3NH3PbI3 films for perovskite solar cells with ultrahigh fill factors Hongbin Zhang, Hui Chen, Constantinos C. Stoumpos, Jing Ren, Qinzhi Hou, Xin Li, Jiaqi Li, Hongcai He, Hong Lin, Jinshu Wang, Feng Hao, and Mercouri G. Kanatzidis ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b16124 • Publication Date (Web): 20 Nov 2018 Downloaded from http://pubs.acs.org on November 20, 2018

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ACS Applied Materials & Interfaces

Thiazole induced surface passivation and recrystallization of CH3NH3PbI3 films for perovskite solar cells with ultrahigh fill factors Hongbin Zhang,a Hui Chen,a Constantinos C. Stoumpos,b Jing Ren,a Qinzhi Hou,a Xin Li,c Jiaqi Li,a Hongcai He,a Hong Lin,c Jinshu Wanga,d*, Feng Haoa*, Mercouri G. Kanatzidisb*

a. School of Materials and Energy, University of Electronic Science and Technology of China, Chengdu 610054, China. b. Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States c. State Key Laboratory of New Ceramics & Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China d. School of Materials Science and Engineering, Beijing University of Technology, Beijing 100124, China. *Corresponding authors: Email address: [email protected] Email address: [email protected] Email address: [email protected]

Abstract: The quality of perovskite films is a crucial factor governing the photovoltaic performance of perovskite solar cells (PSCs). However, perovskite films fabricated by the conventional one-step spin coating procedure are far from ideal due to uncontrollable crystal growth. Herein, we report a facile recrystallization procedure using thiazole additive coupled with vapor annealing to simultaneously modulate the perovskite crystal growth and suppress the surface defects. High quality perovskite films with no pin holes, high crystallinity, large grain size and low roughness were obtained. Moreover, using the space charge limited current (SCLC) method we observe that the defect density of the as-prepared perovskite films with 1 ACS Paragon Plus Environment

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thiazole additive was decreased by 40% when compared with the film without thiazole. The lower defect density of these perovskite films enables the achievement of a final power conversion efficiency (PCE) of 18% as well as an exceptionally high fill factor of 0.82, which corresponds a 25% enhancement compared with the control device. Our results reveal a novel and facile path to modulating the perovskite crystal growth and simultaneously suppressing the film defect density and increasing efficiency in perovskite photovoltaics and related optoelectronic applications.

Keywords: Film quality, spin coating, crystal growth, defect density, vapor annealing.

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1. Introduction Hybrid organic–inorganic metal halide perovskites are considered one of the most promising semiconductors for future photovoltaic applications. More recently, the solar-toelectric power conversion efficiencies (PCEs) of perovskite photovoltaic (PV) devices have hit the extraordinarily high 23.3% mark, close to the PCE of commercialized crystalline silicon solar cells and thin film solar cells such as CdTe.1-10 The perovskites have the formula ABX3, where the A is monovalent cation (typically methylammonium, formamidium, or Cs+); the B is a divalent metal cation (such as Pb2+, Sn2+, or Ge2+ ) and X is a halogen (such as F−, Cl−, Br−, or I−).11 Numerous studies have revealed that the quality of the perovskite films, including the film coverage, grain size, trap states density are limiting factors in the photovoltaic performance of resultant perovskite solar cells (PSCs).12 A high-quality perovskite film with smooth surface, full coverage, uniform grain size and negligible trap states density is crucial to obtain sufficient and wide light absorption, and the shunting loss will also be decreased. On the other side, high-speed charge-transport channels rely on highquality perovskite films, which can reduce recombination loss.13-17 However, the benchmark CH3NH3PbI3 (MAPbI3) films prepared by the broadly practiced one-step spin-coating procedure are usually defective due to the uncontrollable crystal growth in this procedure, undesirably resulting in pin hole, uneven grain size and high surface roughness of the asprepared perovskite films. Recent work demonstrates that MAPbI3 perovskite films after antisolvent precipitation are far from perfect on the surface. They have low crystallinity, associated with incomplete phase conversion and perovskite amorphization and exhibit numerous defects and notable chemical composition inhomogeneity.18 Because of this, new methodologies allowing delicate control over the film formation and crystallization should be developed. This will improve charge generation, diffusion, and collection across the grain boundaries and interfaces so that the overall solar cell performance can be ultimately optimized. To date, tremendous efforts have been made to improve the 3 ACS Paragon Plus Environment


quality of perovskite films, such as solvent annealing,19 trap passivation,17 methylamine induced defect-healing20 and additive engineering.21 Among these methods, additive engineering is the most widely used to modulate the microstructure, crystallinity and morphology of perovskite films, due to its simplicity and versatility. Indeed, most of the reported additives could be classified into several categories such as polymers,22 ionic liquids,23 organic molecules,24 inorganic or ammonium salts,14, 25 etc. Jiang et al. employed ptype semiconductors (J71) as additives to dope the perovskite layer, fabricating a uniform film and increasing the solar cell efficiency to as high as 19.19%.26 By doping a new type of ionic liquid, 1-ethylpyridinium chloride, Wan et al. successfully controlled the morphological growth of MAPbI3 during the one-step deposition method, resulting in a high-quality film with continuous and dense morphology.27 Fei at el. introduced thiourea into the MAPbI3 precursor growing a compact micro-sized and monolithically grained perovskite films, significantly improved the performance of perovskite solar cell with a PCE of 18.46%.28 Zhu at el. added thiadiazole derivative in perovskite precursor, suppressed the bulk defect and obtained a PCE of 19.04% with negligible hysteresis and excellent stability.29 Zuo at el. improved the crystallinity and morphology of the MAPbI3 layer using NH4Cl as additive and obtained a fill factor record of 80.11%.30 Although applying these classes of additives to perovskite precursor have indeed improved the morphology of perovskite films, the mechanism of morphological improvement and the relationship between the structure and the device photovoltaic performance are rarely discussed. Among the photovoltaic parameters in PSCs, the fill factor (FF) is quite sensitive to the active layer microstructure, carrier mobility, and the bulk and interfacial charge recombination. The limiting causes for FF are believed to be mainly due to trap states and departure from diode ideality, and field-dependent competition between charge extraction and non-geminate recombination playing an important role. Accordingly, FF in a PSC correlates tightly with the trap state density in the perovskite films and the resultant interfacial charge recombination. Most of the studies to date showed a 4 ACS Paragon Plus Environment

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FF less than 0.8,31-33 with little discussion devote to issues of interfacial carrier recombination and the perovskite film quality in terms of the trap density. Herein, we report the use of thiazole as additive in a perovskite precursor solution, successfully modulate the micro-environment for nucleation and growth of perovskite crystal in one step coating procedure. Compared with other organic molecules explored, sulfur containing molecules, such as thiourea,28 have their chemical affinity to Pb2+ which can help the formation of stronger and more covalent bonds at the interfaces and thus passivate dangling bonds. Precursor solutions used in film formation have different species of Pb2+ ions depending on what complexes they form with the iodide and solvent molecules. We hypothesized that because thiazole presents two functional groups, a sulfur as well as a nitrogen donor atoms it is better suited to interacting with the different types of solvated Pb2+ atoms in solution than other additives having only one type of functional group. Employing the aromatic thiazole molecule as an additive is advantageous for obtaining purer perovskite films, since thiazole’s high vapor pressure allows for its facile removal during the thermal annealing step. Complementary to thermal annealing, a small sealed space near the film surface with thiazole and DMF vapor was created during the annealing process to induce further surface recrystallization. This simple additive-vapor co-assisted method effectively provide homogenous nucleation sites for the perovskite film growth, gain bigger crystalline grain and smooth surface. The perovskite film with thiazole has been proven to have fewer trap states, which would reduce recombination of charge carriers and facilitate carrier transfer. After optimizing the additive concentration, the champion cells exhibited a PCE as high as 18% under simulated one sun illumination, which was 25% higher the control devices in absence of the thiazole additive. Moreover, the devices with thiazole additive showed remarkably high mean fill factor of 0.78 in a batch of 50 devices and a champion FF as high as 0.82, which is among the highest reported value in PSCs. 2. Results and discussion 5 ACS Paragon Plus Environment


Figure 1a. shows the fabrication process of MAPbI3 films with controllable morphology by adding thiazole as additive in the MAI/PbI2/DMF solution coupled with the thiazole/DMF vapor assisted annealing process. First, the precursor solution with added thiazole was spun on NiOx substrates to form a uniform film. Then, the film was immediately transferred to a hot plate for annealing at 100 oC. Meanwhile, a substrate-size-like glass bell jar was used to cover the film to create a small sealed space, where the thiazole and DMF vapor could accumulate on the film surface to induce surface recrystallization. Control films without the thiazole additive and the vapor annealing were made by anti-solvent method. X-ray diffraction (XRD) measurements were used to investigate the impact of the additive on MAPbI3 crystal growth, crystallinity and phase purity of the perovskite films. As shown in Figure 1b-c, the Bragg peaks at 14.08, 28.48, 31.98 and 40.48 deg can be related to (110), (220), (310) and (224) planes of the MAPbI3 perovskite crystal.34 The phase purity of the perovskite films can be presumed to be high since no additional Bragg peak was observed in the XRD patterns. Notably, the observed increase of the (110) and (220) Bragg peak intensity indicates that the crystallinity was enhanced by using our thiazole additive strategy, which is ascribed to S-donor and N-donor atoms of thiazole that can suitably interact with Pb2+ based species in solution forming a coordination intermediates to control the growth of perovskite crystals.35-36 The scheme of the reaction between thiazole and CH3NH3PbI3 is shown in Figure S1. Figure 2a-b, d-e shows the top-view SEM images of perovskite with and without thiazole additive. It can be clearly found that the film with thiazole as additive presents a more flat and uniform surface morphology and an enlarged grain size compared to the one without thiazole additive. The average grain size of the perovskite films was further evaluated by Image-pro from the SEM image, as shown in Figure 2c, f the average grain size increased from ~120 nm of the control film to ~240 nm with thiazole additive and thiazole/DMF vapor annealing process. The average grain size enlargement minimizes the grain boundary energy, which is 6 ACS Paragon Plus Environment

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beneficial for charge carrier transport in the perovskite films. Employing thiazole as a weakly coordinating molecule and controller of morphology suppresses the disordered crystal growth and provides well distributed nucleation sites, resulting in homogeneous film morphology. The film roughness was further probed with Atomic Force Microscope (AFM). Figure 2g-h shows the AFM morphology of the as-deposited perovskite films. The root mean square roughness values for the perovskite with and without thiazole are ~9.3 nm and ~20 nm, respectively. A cross-sectional SEM image of an optimized device is presented in Figure 2i. It can be easily observed that the entire perovskite film is composed of a homogeneous, well crystallized perovskite layer in the perovskite film with thiazole additive. A comparison of cross-sectional SEM for devices fabricated with and without thiazole is shown in Figure S2. Assuming grain boundaries are detrimental, their substantial decrease in the perovskite film with thiazole additive is expected to be beneficial for carrier transport in the resultant photovoltaic devices. Infrared (IR) spectroscopy was carried out to investigate the interaction between thiazole and perovskite in the precursor. The thiazole molecular can serve as Lewis base since the N and S atoms both contain lone pair electrons, which can react with PbI2 to form a relatively stable intermediate PbI2·CH3NH3I·thiazole.35-36 As shown in Figure S3, the XRD pattern of the intermediate PbI2·CH3NH3I·thiazole is different from the intermediate PbI2·CH3NH3I, especially for the peak of PbI2, indicating the interaction between thiazole and PbI2·CH3NH3I. Figure 3a-b shows the IR spectrum of thiazole and “PbI2·CH3NH3I·thiazole” intermediate. The stretching vibration of C=N appears at 1650 cm−1

35

for pure thiazole and is blue shifted

to 1625 cm−1 for “PbI2·CH3NH3I·thiazole” intermediate. The stretching vibration of S=C appears at 739 cm−1 29, 35-36 for pure thiazole and shows distinct shift to lower wavenumber of 708 cm−1 for PbI2·CH3NH3I·thiazole. This difference in the C=N and S=C vibration stretching shift might originate from the donor electron ability and reaction with PbI2. The blue shifting of C=N and S=C vibration indicates that thiazole interacts with perovskite 7 ACS Paragon Plus Environment


forming a coordination complex at the interfaces. To further certify the coordination between thiazole and perovskite, we have carried out the X-ray photoelectron spectroscopy (XPS) of PbI2·CH3NH3I·thiazole. As shown in Figure 3c, the binding energy of Pb core electrons in 4f7/2 and 4f5/2 was slightly shifted to higher ionization potentials for PbI2·CH3NH3I·thiazole, which experimentally demonstrates the formation of covalent bonds between thiazole and the Pb atoms.37 These results are consistent with the IR spectroscopy results well. Therefore, combined with the above XRD, IR and XPS characterization, we believe the interaction between the S and N atoms on the thiazole molecule and the Pb2+ atoms provides an effective means to control the nucleation and further growth of perovskite crystal. The photovoltaic performance of our perovskite solar cells, with the device structure glass/FTO/NiOx/MAPbI3/[6,6]-Phenyl-C61-butyric acid methyl ester (PCBM)/PEI/Ag, fabricated with and without thiazole additive are compared in Figure 4a-e. For the thiazole based devices, the champion device of optimized thiazole concentration exhibited a PCE of 17.98% (Figure 4a) from the reverse scan, featuring a Jsc of 21.04 mA cm-2, increased Voc of 1043 mV and a remarkable FF of 0.82. This represents a remarkable improvement over the control devices (highest PCE of 14.26%, 14.34% for forward and reverse scan respectively, the full cell parameters are shown in Table 1. The J-V curves with forward and reverse scan is shown in Figure S4). It should be specially pointed out that the FF achieved in our devices is among the highest values reported for PSCs. The FF, a most difficult parameter to optimize in the perovskite solar cells, is often sensitive to a series of parasitic loss mechanisms, which have mostly have an impact on Jsc or Voc. After the correction of parasitic resistances, carrier recombination primarily have an impact on FF. It reduces carrier lifetime, leading to the lower extractable current from the device. Our device with a FF as high as 0.82 underscores the substantial advantage of the perovskite film deposited with thiazole additive and thiazole/DMF vapor annealing process.


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The J-V curves of cells fabricated with different thiazole content in the precursor solution are shown in Figure S5. Figure 4d-f presents histograms of Jsc, FF and PCEs respectively, measured from 50 devices, for control devices and for devices with thiazole additive. All three photovoltaic parameters of the thiazole-adding devices yield higher average values than those of control devices. These result directly support that adding thiazole yield films which have lower defect density, which is expected to produce longer carriers diffusion lengths and better carrier charge collection at the interface with PCBM. The average device efficiency was about 16%, which illustrates the excellent reproducibility of our method. In order to eliminate instrumental error of PCE measurement, the incident photon-to-currents efficiency (IPCE) measurement was carried out to further confirm the difference of JSC between the two devices. As shown in Figure 4b, the champion devices with thiazole on the basis of wavelength shows a wide plateau of over 80% between 450 and 750 nm, yielding an integrated JSC value of 20.25 mA cm-2 that is consistent with the measured J−V curves, thus confirming the validity of the device performance. We also examined the steady-state efficiencies at the maximum power point voltage (Vmpp) for both control and thiazole-adding cells. As shown in Figure 4c, the device yields a stable photocurrent density of 20.57 mA·cm2,

consequently resulting in a stable efficiency of 17.53%, which matches exactly the PCE

obtained by reverse scan of the PSCs. The stability of perovskite solar cells of un-encapsulated devices based on MAPbI3 with thiazole was tested by aging the device at the maximum power point under constant illumination (100 mW·cm2) at room temperature in ambient air (RH, 50–60%). As shown in Figure S6, after 60 minutes of exposure, the PCE of the thiazole based device retained 70%, while the control device retained less than 60%. This result indicates the device with thiazole have promising stability against light and humidity, which might be ascribed to the improved perovskite crystallinity and ameliorated film morphology. 9 ACS Paragon Plus Environment


In order to investigate the dependence of the photophysical properties on the crystalline quality of perovskite films, we carried out a variety of optical measurements on the bare perovskite film derived from using thiazole as additive. For these measurements, the perovskite films were spun on glass substrates. Figure 5a shows the ultraviolet-visible absorption spectra of the two films with the same thickness. Both samples showed very similar absorption profiles but we discern that the thiazole derived MAPbI3 film shows a slightly higher absorption intensity over the entire region below 780 nm. This result suggests a higher phase purity and crystallinity for the MAPbI3 film with thiazole additive, which confirms with the XRD spectra. The photoluminescence (PL) characterization was carried out to understand the process of the charge separation and recombination dynamics in perovskite film. Figure 5b shows the steady-state PL spectra of thiazole derived MAPbI3 and the control sample. The steady-state PL intensity of thiazole derived perovskite film is an order of magnitude higher than that of the control sample, indicating a lower nonradiative recombination loss compared to the control sample.38 For further verification, the corresponding time-resolved PL (TRPL) spectra of these perovskite films were also measured. A fast decay and a slow decay process were observed by fitting the PL lifetime constants of TRPL with biexponential decay according to equation (1).39

𝑓(𝑥) = 𝐴𝑖∑𝑒 ―𝑡/𝜏𝑖 + 𝐵

(1)

where Ai is the decay amplitude, τi is decay time, and B is a constant for the base-line offset, respectively. Generally, τ1 is the fast decay component, attributed to surface recombination. τ2 is the long decay component, which attributed to recombination in the bulk.40 The perovskite films with thiazole presented a longer lifetime (τ2, 23.07 ns) than the pristine perovskite films (τ2, 8.13 ns), indicating enhanced crystalline quality of additive-vapor co-assisted perovskite film with thiazole additive. 10 ACS Paragon Plus Environment

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Figure S7 shows electrochemical impedance spectra (EIS) of the PSCs measured at 0.9 V forward bias under AM 1.5G illumination. The simplified equivalent circuit (inset of Figure S7) is used to analyze the recombination and transportation in the PSCs.41 Generally, Rs is corresponding to the series resistance between two electrodes, mainly contributed to the sheet resistance of FTO glass. Rrec is corresponding to the recombination in PSCs, the value of Rrec in the device is the inverse of the recombination. RHTM is related to the diffusion of holes into the hole transport layers.42 In Figure S7, two distinct semicircles can be seen, a small semicircle at the high frequency region and a large semicircle in the low frequency region, which are related to the interfacial charge transfer resistance (RHTM) and the charge recombination resistance (Rrec) respectively.43 Both devices, with and without adding thiazole, exhibit similar semicircle at high frequency, indicating that they have comparable interfacial charge transport ability. Nevertheless, the large semicircle at low frequency of the two devices are distinctly different. The Rrec value of the solar cell with adding thiazole was 64.31 Ω cm2, which is significantly larger than that of the device without adding thiazole (43.65 Ω cm2), indicating that adding thiazole along with the DMF vapor assisted surface recrystallization can efficiently reduce trap state of the surface, and further retard the charge recombination increasing carrier life time. The increased carrier lifetime is a crucial factor for higher performance of the PSCs. To estimate the number density of defects, electron-only devices have been fabricated by sandwiching the perovskite films between silver (Ag) and fluorine-doped tin oxide (FTO). We characterized the space-charge-limited current (SCLC) of perovskite films under different bias voltages. Figure 5d shows the current−voltage curves of the perovskite without and with adding thiazole. There is a sharp rise region in the two curves, which correspond to the trapfilled limit. The trap-filled bias voltage (VTFL) can assesse the trap-state density according to the equation (2):20 𝑁𝑑𝑒𝑓𝑒𝑐𝑡𝑠 = 2𝜀𝜀0𝑉𝑇𝐹𝐿/𝑒𝐿2

(2) 11 ACS Paragon Plus Environment


where ε is the static dielectric constants of MAPbI3 (taken as 70 from previous reports 44), ε0 is the vacuum permittivity, 𝑒 is the elementary charge, and L is the thickness of the deposited perovskite film. The trap density are calculated to be ~3.59×1016 cm-3 and ~2.1×1016 cm-3 for the perovskite without and with adding thiazole, respectively. The trap density is suppressed, which is consistent with the TRPL and EIS measurements. Therefore, the perovskite film with thiazole has reduced recombination of charge carriers and facilitates carrier transfer, which correlates well with the ultrahigh fill factors obtained for the devices with thiazole additive. 3. Conclusion The addition of the bifunctional aromatic molecule of thiazole in the precursor DMF solutions of PbI2 and MAI effectively modulates the micro-environment for nucleation and growth of perovskite crystal films when using the one-step solution process. Meanwhile a simple thiazole/DMF vapor assisted annealing method induces improved surface film recrystallization. Combining these two simple steps, homogenous nucleation sites and surface recrystallization we can obtain perovskite films with 97% larger grain size, smooth surface, as well as reduced trap states. Consequently, these high quality perovskite films enable a PCE of almost 18% for thiazole-adding devices with ultrahigh fill factor of 0.82. This bifunctional aromatic molecule strategy is likely to be extendable to the superior perovskite solar cells with mixed cations and halides and provide a facile route to further improvements in perovskite crystal film growth with lower defects, which is highly desired for the realization of ever higher efficiency perovskite photovoltaics and other related optoelectronic applications. 4. Experimental section Fabrication of PSCs. The fluorine-doped tin oxide (FTO) glass sheets were cleaned sequentially by ultrasonic cleaning in DI water, isopropanol, acetone, and absolute ethanol for 20 min, respectively. Then, nitrogen flow was used to make the glass sheets dry, and the substrates were treated by 12 ACS Paragon Plus Environment

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UV-ozone for 30 min before the spin coating of NiOx. The ethylene glycol solution containing 0.5 M nickel acetate and 0.5 M DEA was spun on the substrates by hot spin coating (both the solution and substrates were preheated to 120 ℃ before spinning) at 4000 rmp for 60 s, followed by annealing on the hotplate at 300 °C for 60 min in the ambient atmosphere. As for the perovskite layer, the mixture precursor solution containing CH3NH3I, PbI2 and thiazole (Sigma Aldrich) with molar ratio of 2:2:1 in anhydrous N, N-dimethylformamide (Sigma Aldrich) at a final concentration of 1.35 M was spun on NiOx layer at 5000 rmp for 30s. Then, the perovskite-to-be films were promptly transferred on a heating plate at 100 °C and simultaneously covered a substrate-size-like bell jar for 15 min. The 20 mg/mL PCBM chlorobenzene solution was spun cast onto the perovskite layer followed by spun casting of PEI in IPA with a concentration of 1.5 mg/mL. Finally, a ~100 nm-thick Ag film was thermally evaporated over the substrate through a shadow mask at a base pressure of 1×10-4 Pa to complete the devices. The active area of all the devices was 0.09 cm2. Characterization The crystalline structure of perovskite films were characterized by X-ray diffraction (XRD) (Bruker D8 Advance diffractometer, Germany) with Cu Kα1, λ = 1.5406 Å, 40 kV, 40 mA. A scanning electron microscope (Merlin, Zeiss, Germany) was used to characterize the morphology of samples. An atomic force microscopy (AFM) unit in noncontact mode (Nanonavi SPA400, SEIKO, Japan) was carried out to investigate the surface morphologies. FTIR were measured using Thermo Scientific Nicolet 6700 FTIR spectrometer (Nicolet 6700, Thermo Scientific, USA). X-ray photoelectron spectroscopy (XPS) was conducted using Axis Ultra DLD (Britain, Kratos) with a monochromated Al-Kα source (1486.6 eV) for excitation. Ultraviolet-visible absorption spectra were recorded on a Lambda 950 spectrophotometer in the 300-800 nm wavelength range at room temperature (PerkinElmer, USA). Steady-state photoluminescence (PL) emission spectra and time resolved photoluminescence (TRPL) were measured using a fluorescence spectrometer instrument (FLS920, Edinburgh Instruments, 13 ACS Paragon Plus Environment


Livingston, UK). Electrochemical impedance spectra (EIS) was measured by using the CHI600E system (CHI600E, CH Instruments Ins, USA). The ZSimpWin software was used to analyze the impedance data. The sunlight was simulated by a solar simulator (91192,Oriel, USA, calibrated with a standard crystalline silicon solar cell). The digital source meter (2400, Keithley Instruments, USA) under AM 1.5G illumination (100 mW·cm−2) was used to measure the photocurrent−voltage (J−V) characteristics of PSCs. The QEX10 solar cell quantum efficiency measurement system (QEX10, PV measurements, USA) was carried out to characterize the incident photon to current efficiency spectra (IPCE). A calibration silicon solar cell was used before the measurement.

Supporting Information: Schematic reaction process, SEM, XRD, J-V curves, impedance spectra and device stability of the solar cells.

Acknowledgements This work was financially supported by the National Natural Science Foundation of China NSFC (51702038) and the Recruitment Program for Young Professionals. At Northwestern this work was supported in part by the LEAP Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences under Award DE-SC0001059.

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Figure 1. (a) Schematic view of the one step spin-coating crystal growth process of perovskite thin film with thiazole additive and the thiazole/DMF vapor assisted annealing. (b)


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XRD pattern of MAPbI3 perovskite fabricated with and w/o thiazole additive. (c) the enlargement of the (110) and (220) peaks of perovskite films in (b), respectively.

Figure 2. Top-view SEM images in two different scales of MAPbI3 films: (a), (b) fabricated without thiazole; (d), (e) fabricated with thiazole additive. (c) and (f) Histogram of grain size for MAPbI3 films w/o and with thiazole. (g-h) AFM images of MAPbI3 films. (i) Crosssectional

SEM

image

of

the

perovskite

solar

glass/FTO/NiOx/MAPbI3/PCBM/Ag.


cell

with

the

structure


Figure 3. (a) Fourier transform infrared spectroscopy (FTIR) for bare thiazole and MAI·PbI2·thiazole. (b) The amplify regions for C=N and C=S stretching respectively. (c) Xray photoelectron spectroscopy (XPS) spectra of Pb atom.

Figure 4. Device performance. (a) J-V curves of the perovskite solar cells fabricated with and without thiazole. (b) IPCE curves of devices. (c) steady-state photocurrent and output power at the maximum power point for the corresponding devices. (d-f) Histogram of solar cell parameters for 50 devices, (d) Jsc, (e) FF, (f) PCE.


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Figure 5. (a) UV-vis absorption spectra of as-deposited thin perovskite film. (b) Normalized steady-state photoluminescence spectra and (c) time-resolved photoluminescent decay of MAPbI3 perovskite films fabricated on glass substrate via one step spin-coating. (d) Spacecharge-limited current vs voltage of devices with the structure of FTO/MAPbI3/PCBM/Ag.

Table 1. Photovoltaic performance of the best PSCs fabricated with and without thiazole added in the precursor solution.

Champion cell w/o thiazole with thiazole

Scanning direction Forward Reverse Forward Reverse

Jsc(mA/cm2)

Voc(V)

FF

PCE(%)

19.02 19.04 21.03 21.04

1.033 1.032 1.044 1.043

0.73 0.73 0.80 0.82

14.26 14.34 17.50 17.98



TOC


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Thiazole induced surface passivation and recrystallization of

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