High Performance of Perovskite Solar Cells via Catalytic Treatment in

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High Performance of Perovskite Solar Cells via Catalytic Treatment in Two-step Process: the Case of Solvent Engineering Wenzhe Li, Jiandong Fan, Jiangwei Li, Guangda Niu, Yaohua Mai, and Liduo Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09532 • Publication Date (Web): 14 Oct 2016 Downloaded from http://pubs.acs.org on October 16, 2016

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ACS Applied Materials & Interfaces

High Performance of Perovskite Solar Cells via Catalytic Treatment in Two-step Process: the Case of Solvent Engineering Wenzhe Li, † Jiandong Fan,‡* Jiangwei Li, † Guangda Niu, † Yaohua Mai, ‡ Liduo Wang†* † Key Lab of Organic Optoelectronics & Molecular Engineering of Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084, China ‡ Institute of New Energy Technology, College of Information and Technology, Jinan University, Guangzhou, 510632, China KEYWORDS: Perovskite solar cells, Thin films, Thermal stability, Solvent engineering, Twostep process

ABSTRACT

Currently, the potential mechanism of the solvent-assisted crystallization for mixed cations perovskite thin film (FAxMA1-xPbI3) prepared via two-step solution-process still remains obscure. Here, we clarified the molecular-competing-reacted process of NH2CH=NH2I (FAI) and CH3NH3I (MAI) with PbI2(DMSO)x complex in dimethyl sulfoxide (DMSO) and diethyl ether (DE) catalytic solvent system in the sequential two-step solution-process. The microscopic

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dynamics was characterized via the characterizations of in-situ photoluminescence (PL) spectra. In addition, we found that the thermal stability of the perovskite films suffered from the residual solvent with high boiling point, e.g. DMSO. The further DE treatment could promote the volatility process of DMSO and accelerate the crystallization process of perovskite films. The highest PCE over 19% with slight hysteresis effect was eventually obtained with a reproducible FA0.88MA0.12PbI3 solar cell, which displayed a constant power output within 100 s upon light soaking and stable PCE output within 30 days in the thermal stability test.

1. INTRODUCTION The organometallic halide perovskites have emerged as a promising photovoltaic material that exhibited high power conversion efficiency (PCE).1-5 The unprecedented rise in the efficiency from 3.8% to 22.1% was achieved in only 5 years, the rising rate in such short period extremely outperformed many proven commercial photovoltaic technologies, e.g. Si-, GaAs- and CdTebased solar cells.2,3,5-9 As light-absorber film, the highly crystallized perovskite thin film without defects appears to be the prerequisite that gives rise to satisfied photovoltaic performance. However, the presented deposition methods by either solution-processed technique and/or vacuum evaporation deposition still suffer from the inferior quality of perovskite thin films that usually lead to a poor stability of the film upon light soaking and/or with thermal treatment,10,11 which appears to be the obstacle for the successful commercialization of this technology. Recently, solvent engineering was proven to be capable of controlling the grain size and enhancing the quality of perovskite thin film.1,12,13 Seok, S. I, et al. reported that dimethyl sulfoxide (DMSO) can be used as a cosolvent in the gamma-butyrolactone (GBL) solution containing CH3NH3I (MAI) and PbI2 in one-step spin coating process,14 which enabled to


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improve the dissolubility of precursor and form dense perovskite films. Later on, they further optimized the techniques by means of a direct intramolecular exchange of DMSO molecules intercalated in PbI2 with NH2CH=NH2I (FAI) in a sequential two-step coating process.15 They further elucidated that DMSO has strong coordination ability with PbI2 by which the strong interaction between DMSO and PbI2 can retard the crystallization of PbI2. Very recently, we found that the controlled ratio of DMF and DMSO in the complex−precursor system can form a series PbI2(DMSO)x (0≤x≤1.86) complexes. The formed PbI2−DMSO complexes with tunable x value in PbI2(DMSO)x in turn effectively control the grain morphology, density, and roughness of perovskite thin film.16 Likewise, in the widely used cations system (FAI/MAI) of perovskite materials, the larger ion radius of FA+ displays a superiority in comparison to MA+ with respect to the crystallization kinetics: (i) It enables the perovskite to own higher lattice energy and thermal stability;17,18 (ii) The energy barrier of infixing FA+ into PbI2 is larger, which facilitates the process of DMSO-assisted solvent engineering.19 Again, the inferior thermal stability is regarded as an intractable subject in such perovskite solar cell.20-22 In previous reports, we have carefully studied the stability of perovskite thin films in various environments, e.g. different electrolytes23, oxygen-water environment22, 24, ultraviolet light25-27 and 4-tert-butylpyridine (TBP) additives28. It is well known that the perovskite materials can easily absorb many polar organic molecules, e.g. DMSO 29, DMF30 and CH3NH2 31 et al. These residual molecules in perovskite films can significantly open up the crystal structure, which facilitated ion migration and lattice distortion caused by accumulated charges, thus reducing the thermal stability of perovskite thin films.32 The post-annealing process at high temperature with long time was able to remove the residual molecules at the expense of losing counter ions at the surface, which gave rise to the defect formation of Pb(0) and thereby reduced


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the device performance.17 Seok et al employed toluene to drip on the film during spin-coating in order to stabilize the formation of a crystalline in one-step process.14 Instead of toluene, NamGyu Park et al found that diethyl ether (DE) was a more proper solvent that kept the nominal ratio of MAI/PbI2/DMSO = 1:1:1 with respect to the removal of DMF by DE.33. Aside from the removal of DMF by DE treatment, the supplementary function of DE in those systems may play a critical role in essentially enhancing the thermal stability of solar cell. Extending this issue to the widely used recipe that usually gives rise to solar cells with the state-of-the art power conversion efficiency (PCE), the solvent engineering is supposed to be an effective approach in realizing an efficient solar cell device with highly thermal stability by means of washing off the superfluous solvent. Despite the DMSO has been proven to be effective in well controlling the morphology of the perovskite layer, the fundamental mechanism behind superior performance related to the intramolecular exchange and/or dynamical growth process of perovskite crystal in the presence of DMSO still remains obscure, e.g., (i) How the formed PbI2−DMSO complexes react with the mixture of FAI and MAI precursor in a two-step sequential deposition; (ii) What is the intermediate phase formed during the solvent treatment step that plays a critical role in drastically lowering the energy barrier for perovskite formation; (iii) The react priority and ratio of PbI2-based complexes with FAI and MAI with regarding to their different sizes, are still pending; (iv) How to remove the remnant solvent that involved in the preparation process of thin films. In this study, we detailed the intramolecular exchange process in the system of mixed cation species (FAI/MAI) with PbI2(DMSO)x complexes. The molecular configurations of intermediate phase as well as its potential mechanisms in controlling the dynamics growth process of


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perovskite thin film were judiciously explored. On the basis of it, the highly thermal stable perovskite solar cell exceeding 19% via catalytic treatment of solvent, i.e., DMSO, DE, was obtained. 2. EXPERIMENTAL SECTION 2.1 Substrate Preparation. Devices were fabricated on fluorine-doped tin oxide (FTO) coated glass (YINGKOU OPV TCH NEW ENERGY CO.,LTD, OPV-FTO22-7). Initially FTO was etched with 2 mol/L HCl solution and zinc powder. Substrates were then cleaned sequentially in 2% HellmanexTM detergent, deionized water, acetone, ethanol, isopropanol (IPA) and UV exposure. The TiO2 compact layers were deposited on FTO glass using atomic layer deposition (Beneq TFS 200) as we mentioned previously elsewhere.16 The nanocrystalline TiO2 paste (18NRT from Dyesol Company; diluted to w/w 14.3%) was deposited on the pre-treated FTO substrate at 6000 rpm for 30 s, followed by heating at 500°C for 1 h. The thickness of the annealed TiO2 ﬁlms was nearly 100 nm, as determined by scanning electron microscopy (SEM; JEOL JSM-7401F). 2.2 Perovskite solar cell fabrication. The 150 mg of PbI2 (Aldrich, 99.9985%) in 220 uL DMF (Aldrich, 99.9%) and 20 uL DMSO (Aldrich, 99.9%) mixed precursor solution was magnetically stirred for 1h and the mixture was then spin coated on the as-prepared TiO2 film at 3000 rpm for 30 s in a glove box. Afterward, the as-prepared PbI2-based thin film was placed in glovebox for 30 min prior to use. A mixed solution including 20 mg CH3NH3I (MAI, Dyesol) and 25 mg NH=CHNH3I (FAI, p-OLED) was dissolved in 1 mL IPA. The obtained MAI-FAI mixed precursor solution was spinning coated on the as-prepared thin film with PbI2 complex at 5000 rpm for 30s. 200 uL diethyl ether (DE) was dynamically dropwise on the thin film when the spinning rate started to decrease. It took around


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30 min until the color of thin film remained stable. Afterward, the obtained thin film was annealed at 150ºC for around 7-10 min until its color remained stable. Note that the post thermal treatment enable the color of thin film changed to reddish black. A hole-transporting layer (HTL) was then deposited via spin-coating a 0.8 M solution of 2,2′,7,7′-tetrakis- (N,N-di-pmethoxyphenylamine) 9,9′-spirobifluorene (spiro-MeOTAD) in chlorobenzene, with additives of lithium bis(trifluoromethanesulfonyl)imide and 4-tert-butylpyridine. Spin-coating was carried out at 2000 rpm for 45 s. Devices were then left overnight in air, RH≈30%. Finally, 70 nm gold electrodes were thermally evaporated under vacuum of ≈10−6 Torr, at a rate of ≈0.02 nm/s. The Au attached area was fixed at 0.16 cm2. 2.3 Materials Characterizations. X-ray diffraction (XRD) patterns were obtained with Smart LAB instruments CuKα beam (λ = 1.54 Å). UV-vis absorption spectra were employed to assess the absorption properties of perovskite sensitized TiO2 thin film with a Hitachi U-3010 spectroscope. The morphology of the film was tested with scanning electron microscopy (SEM; JEOL JSM-7401F). X-ray photoelectron spectroscopy (XPS) was measured with a PHI 5300 ESCA Perkin-Elmer spectrometer. All spectra were shifted to account for sample charging using inorganic carbon at 284.60 eV as a reference. The topographical maps were measured under ambient air with an atomic probe force microscope (Seiko instrument SPA 400). The photoluminescence (PL) was carried out with an in-situ instrument (FluoroMax-3，Jobin Yvon) (Fig. 2a). In particular, a mesoporous ZrO2 thin film with the size of 15×20 mm2 was obtained on glass by a screen printing technique, the thickness of the ZrO2 thin film was controlled to be 1µm. Prior to the PL characterization, 1 µL PbI2 precursor solution was dropped on the mesoporous ZrO2 thin film until it thoroughly diffused on the film. The PL intensity at 762 nm as the function of time was


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acquired in the mode of “Time Base Acquisition”. We started the PL measurement upon fixing only the PbI2-DMSO thin film. Once the PL intensity was stable, we quickly added the MAI/FAI precursor solution into the container for the in-situ PL characterization. The Fourier transform infrared (FTIR) spectrum was measured with a Perkin-Elmer Spectrum GX FTIR spectrometer. The density functional theory (DFT) was calculated by the method of B3LYP, which is based on the set of 6-31g** OPT FREQ. 2.4 Solar Cell Characterizations. The current density–voltage (J-V) curves were measured with a 2400 Series SourceMeter (Keithley Instruments) under simulated AM 1.5 sunlight at an equivalent to 100 mW·cm−2 irradiance generated by an thermo oriel 91192-1000 simulator, with the intensity calibrated with an VLSI standards incorporated PN 91150V Si reference cell. The mismatch factor was calculated to be less than 1%. The solar cells were masked with a metal aperture to define the active area, typically 0.09 cm2. The J–V curves measured at a scan rate of 0.30 Vs-1 from open circuit voltage to short circuit current density, under simulated sunlight irradiance and in the dark. The as-prepared solar cells were stored at 85ºC in dark with a humidity of 20-30% for the characterization of thermal stability. The specific PCE as the function of time was obtained in order to clarify the PCE evolution of solar cells with thermal treatment in 30 days. 3. RESULTS AND DISCUSSION 3.1 Components and phase controlling via DMSO treatment In the present study, we used the mixture of FAI and MAI as the cations (A+) in general perovskite structure (ABX3). It is well known that the photo-electric and photovoltaic performances of corresponding solar cell are extremely sensitive to the ratio of FAI/MAI in precursor solution.15,34 We firstly explored the influence of the ratio of FAI/MAI in our system


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on the perovskite thin film as well as the performance of solar cell. Here, we increased the MAI amount from 0 to 30 mg/mL isopropanol (IPA) solution in the meantime decreasing the FAI amount from 45 to 0 mg, we called them M0F45, M15F25, M15F30, M15F35, M20F20, M20F25, M20F30, M25F10, M25F15, M25F20, M30F0, respectively. As shown in Figure S1, we were able to tune the bandgaps of perovskite thin film by means of varying the ratio of FAI/MAI. Particularly, the absorbance edge can shift from 780 nm to 835 nm when the ratio of FAI/MAI changed from M30F0 to M0F45. Such infrared shift was consistent with the previous report,7,19,34 which was supposed to enhance the optical absorbance and thereby the photocurrent density in a solar cell. We further studied the effect of the ratio of FAI/MAI on the photovoltaic performance of corresponding solar cell (Figure S2). After balanced the tradeoff between the intensity of its intrinsic optical absorbance and the bandgap shift-induced absorbance, it clearly turns out that the cell exhibited the best overall PCE in the case of M20F25. The different ratio from the previous report likely originated from the discrepancy of composition engineer and preparation method. 35 Here, the optimized ratio was fixed in the following study unless otherwise specified. Figure 1a displays the phase evolution of as-formed FAxMA1-xPbI3 as the function of reaction time/soaking time of PbI2(DMSO)x complex in M20F25 precursor solution. Here, we defined that the time between dropping the M20F25 solution and starting the spinning process as the reaction time/soaking time. In the beginning stage (0s: spinning immediately after dropping precursor solution), a trace amount of PbI2 and α-FAxMA1-xPbI3 were found on the substrate. With the reaction time going on (3s), the corresponding peaks were strengthened especially for the “black” α-FAxMA1-xPbI3 phase that is beneficial to the solar cells.36 Extending the reaction time to 5s, a pure phase of α-FAxMA1-xPbI3 was obtained while the PbI2 phase was completely disappeared. Further to 10s, aside from α-FAxMA1-xPbI3, a little amount of “yellow” δ-FAPbI3 phase was also


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formed. When the reaction time extended to over 30s, the intensity of α-FAxMA1-xPbI3 phase was weakened, a two-dimensional FA3PbI5 phase was attained, which was usually considered to devastate the performance of perovskite solar cells.36,

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These results proposed that excess

FAI/MAI would result in the formation of pernicious perovskite phase. e.g., δ-FAPbI3 phase and two-dimensional FA3PbI5 phase, whereas the incomplete reaction occurs once the FAI/MAI is deficient. The controlled quantity of FAI/MAI involved in the precursor appears to be essential for perovskite solar cell to achieve a high PCE,19,38 which is able to be accurately adjusted by means of controlling the reaction time in our system. In our previous study, DMSO was demonstrated to be capable of coordinating with PbI2 by coordinate covalent bond. An intramolecular exchange of DMSO with MAI enabled the complexes to form an ultra-flat and dense thin film of MAPbI3.16 In the following study, we aim to clarify the intermediate phase as well as the detailed process of intramolecular exchange in the system of PbI2(DMSO)x complex with MAI and/or FAI/MAI. Figure 1b and 1c display the influence of the DMSO concentration on the structure of as-formed perovskite in the FAI/MAI system before and after annealing process, respectively. Clearly, the presence of DMSO allows tuning the crystallinity and phase of perovskite. In particular, in the case without any DMSO, there are large amount of PbI2 prior to annealing process, whereas the coexistence of δ-FAPbI3 phase, PbI2 and α-FAxMA1-xPbI3 was found after thermal treatment. The residual of PbI2 is likely attributed to the lack of FAI/MAI at the bottom of thin film and the residual of α-FAxMA1xPbI3is

owning to the excess of FAI/MAI on the surface that reacted with the PbI2(DMSO)x

complex. The DMSO20 (the concentration of DMSO is 20µL of 240µL in the mixed solution of DMF and DMSO) exhibited best crystallinity and pure α-MAxFA1-xPbI3 phase, which was similar to the pure MAI system.16 With the concentration of DMSO increasing (DMSO240), the


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δ-FAPbI3 phase appeared in the meantime, the α-FAxMA1-xPbI3 phase almost disappeared, which was certified to be negative for the performance of solar cell36, 39 It turns out the presence of DMSO can effectively control the synthesizing rate of perovskite and reaction activity between PbI2(DMSO)x complex and MAI/FAI by means of tuning the molecular spacing of PbI2. As an evidence, with the concentration of DMSO increasing, the higher reaction activity will shrinkage the formation of PbI2 in the final perovskite thin film as shown in Figure 1b and 1c. To further certify the effect of DMSO on the reaction priority between MAI and FAI with PbI2(DMSO)x complexes as well as the particular schematics in final FAxMA1-xPbI3 thin films, we performed the XPS characterizations of as-obtained FAxMA1-xPbI3 thin films prepared with different concentration of DMSO treatment. Considering that there is no report concerning the binding energy of FAI and MAI powder, we carried out the XPS of pure FAI and MAI powder, respectively (Figure S3). Figure 2 shows the N1s peak of the film in different cases, which were fitted into two main peaks that were associated with FAI (400.3 eV) and MAI (402.1 eV), respectively. Clearly, with increasing DMSO concentration, the MAI ratio in the final FAxMA1xPbI3

thin film tent to decrease. In the case of DMSO20, the composition of the perovskite film

was FA0.86MA0.14PbI3. In the case of DMSO240, there was almost no MAI inside the FAxMA1xPbI3

thin film.

In order to further elucidate the dynamic process of the perovskite formation, we built an insitu photoluminescence (PL) detection system (Figure 3a), by which the PL intensity as the function of reaction time between PbI2(DMSO)x complex with FAI/MAI can be acquired. Here, the increase in the emission intensity before the quenching was an optical artefact that resulted from opening the sample compartment to add the MAI/FAI precursor solution. The PL quenching time represent the dynamic process of perovskite formation with respect to the self-


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absorption of the luminescence by the perovskite formed.5 Figure 3b displays the changes in PL intensity at 762 nm monitored during the transformation, which was associated with the reaction of MAI and PbI2(DMSO)x complex. Note that the PL spectra remained stable during the reaction process of pure FAI solution and PbI2(DMSO)x complex, which yielded “yellow” δ-FAPbI3 without detectable PL signal, thus the possibility of FAI-induced variation of PL intensity was excluded. In the case of pure MAI system, the introduction of DMSO could capably catalyze the formation of perovskite, which was associated with the fact that the DMSO can facilitate the intramolecular exchange between the PbI2(DMSO)x complex and MAI with respect to the suitable molecular spacing of PbI2. On the other hand, in the cases of PbI2 with and without DMSO, the presence of FAI in the mixture system of FAI/MAI could retard the formation of perovskite by means of restricting the reaction between PbI2 and MAI, which turns out the introduction of FAI dominate the reaction of PbI2/PbI2(DMSO)x with mixed cation species. Consequently, in the present mixed cations system, the role of MAI could be considered to simply supply a vacancy deficiency once the reaction of FAI with PbI2/ PbI2(DMSO)x could not be entirely carried out. Here, the reaction between PbI2(DMSO)x complexes and FAI/MAI is assumed to be with selectivity with respect to the size of reaction channel in the stretched lattice structure. The stretching extent of perovskite crystal lattice is usually considered to be directly proportional to the concentration of DMSO solvent. Consequently, the appropriate DMSO content enables the FAI to react homogenously and stoichiometrically with PbI2(DMSO)x complexes, by which the δ-FAPbI3 phase could not be formed anymore. Again, the presence of DMSO coupling with FAI can effectively slow down the reaction of MAI with PbI2(DMSO)x complexes. 3.2 Reducing residual solvent via DE treatment


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The superfluous solvent induced degradation of perovskite film would definitely deteriorate the solar cell.40-41 In this scenario, the removal of unnecessary DMSO appears to be essential toward the formation of pure α-FAxMA1-xPbI3 phase, which was high crystallinity and propitious to efficient perovskite solar cell. Taking into account the fact that the flashing annealing process that usually gives rise to severe pinholes and/or cracks, here, we introduced the DE as an effective catalytic solvent in the present system to remove the superfluous DMSO. Figure 4 displays the top-down morphology of the FAxMA1-xPbI3 thin film before/ after annealing process with/without DE treatment. Despite the film morphology evolution of PbI2(DMSO)x complexes as the function of DMSO concentration was similar to that in the previous report (Figure S4),16 the morphology of MAPbI3 and FAxMA1-xPbI3 thin films derived from the same PbI2(DMSO)x complex was quite different (Figure S5). Compared the perovskite films without annealing process, the films with the DE treatment showed more compacted grains and clear grain boundaries (Figure 4a and b). Meanwhile, the DE treatment accelerates the formation of perovskite phase, which was further certified by the UV-vis absorbance spectra (Figure S6). Then in the annealed films, some pinholes were produced for the FAxMA1-xPbI3 thin film without DE treatment (Figure 4c), which was ascribed to the evaporation of the residual solvents DMSO in FAxMA1-xPbI3 thin film with the flashing annealing process. The corresponding perovskite thin films prepared with DE treatment have been proven to process almost full coverage and highly crystalized quality (Figure 4d). In addition, the (110) and (220) diffraction face was greatly improved with partially preferred orientation, shown in Figure 1c. Likewise, the root mean square (RMS) derived from the AFM images was demonstrated to be only 28 nm, which is much lower than that of the thin film prepared without DE treatment (44 nm) (Figure 4e and 4f). Such phenomenon implies that the DE treatment played a crucial role in


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the process of the crystallization by means of cleaning off the DMSO solvent as concluded previously from XRD pattern in Figure 1c. It is worth noting that, the composition of the DE treated films was not changed so much, which was proved by XPS results in Figure 3. When the DE treatment introduced in the process, the fundamental question in such system, i.e., how does the intermediate phase change to perovskite phase, is raised. With this doubt, we characterized the DMSO-interrelated system in the intermediate phase by Fourier transform infrared spectroscopy (FTIR). Figure 5a demonstrates the FTIR spectra of DMSO liquid, PbI2DMSOx thin film, FAI-DMSO thin film and perovskite thin film before annealing, respectively. In the case of DMSO liquid, stretching vibration of S=O (ν(S=O)), appeared at around 1045 cm−1, which certified the presence of DMSO.33 The S=O stretching vibration was observed to shift to 1022 cm−1 while the PbI2 complexed with DMSO. The stretching vibration at 1022 cm−1 is in well agreement with the stretching frequency observed for the 1:1 adduct of PbI2-DMSO.42 In the case of FAI-DMSO-PbI2 thin film, the S=O stretching vibration was further shift to 1018 cm−1. The frequency of vibration is generally considered to be proportional to square root of force constant in terms of the harmonic motion for diatomic model.43 Here, the gradually decreased S=O stretching vibration was associated with a weaker bond strength between sulfur and oxygen as a consequence of the formation of FAI-DMSO-PbI2 intermediate phase. Note that the S=O stretching vibration displays a further shift toward lower wavenumber (1016 cm−1) while FAI combined with DMSO. Figure 5b exhibits the FTIR spectra of FAxMA1-xPbI3 prepared with/without DE treatment, which was normalized according to the intensity of δ(N-H) at 1711cm-1 to confirm the relative amounts of DMSO in films (Figure S7). In all cases, stretching vibration at 1045 cm−1 was ascribed to be the S=O, which was associated with DMSO as we mentioned above. Besides, in the case of DMSO20 before annealing, a stretching


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frequency at 1018 cm-1 was attributed to the intermediate phase, i.e., FAI(MAI)-DMSO-PbI2. After DE treatment, the intermediate phase was completely removed. This suggested that the DE treatment is capable of “washing off” the residual DMSO and promoting the transformation from intermediate phase to perovskite phase. Meanwhile, such intermediate phase can also be removed by annealing treatment. In this regard, the DE treatment assumed to play an important role in removing the superfluous DMSO as the annealing treatment did, which enable the postthermal treatment to be significantly simplified. Note that either DE treatment or annealing process could not entirely remove the residual DMSO and FAI(MAI)-DMSO species, the combination of both approached appears to be essential for removing the residuum. Combining all of the conclusions above, we can now elucidate the potential mechanism concerning what is the intermediate phase and how it is favorable to finalize the intramolecular exchange process in such sequential two-step solution-process. As shown in Figure 6, the complex of PbI2(DMSO)x was obtained in the first stage. It is worthy of noting that the DMSO is more likely to combine with PbI2 in comparison to DMF.15,44 Afterward, the as-formed PbI2(DMSO)x films combine with FAI/MAI by means of hydrogen bond, and the intermediate can be certified to be FAI/MAI-DMSO-PbI2. In the third stage, coupling with the annealing process, the DE treatment are favorable to remove the superfluous DMSO and thereby facilitate the reaction of FAI/MAI with PbI2. In this sense, the DMSO and DE here we used can be considered as a kind of catalyst to accelerate the so-called intramolecular exchange process. As shown in Figure S8, the thermal stability of perovskite thin film prepared with DE treatment outperformed the other thin films. Both the crystallization quality and absorbance performance of the films remains stable with the DE treatment. Consequently, the denser and higher crystallinity films via DE treatment were favorable to the better thermal stability of the films.


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3.3 Photovoltaic performances To obtain decent photovoltaic performances, we have systematically studied the effect of the concentration of DMSO involved in PbI2 precursor solution on the photovoltaic properties. We varied the involved amount of DMSO in the range of 0-240 µL. As shown in Figure S9, the lower concentration of DMSO offered a poor photovoltaic performance, which was associated with the severe pinholes of perovskite thin film as shown in Figure S5. The photovoltaic parameters including JSC, VOC, and FF were gradually enhanced with increasing the DMSO concentration. The best properties of the cell can be obtained while the amount of DMSO increased up to 20 µL, the corresponding perovskite thin film has a complete coverage and superior crystallinity as displayed in Figure 4d and Figure 1c, respectively. It assumes that the coverage and crystallinity of thin film are critical factors to dominate the photovoltaic performances as reported elsewhere.45, 46 Afterward, its performance started to quench with the increased DMSO concentration, despite the grain size increased in comparison to the case of lower DMSO concentration. Combining all of the optimized parameters above, we fabricated a series of solar cells toward high efficiency. Figure 7a gives the J-V curves with an optimized FA0.88MA0.12PbI3 solar cell, which exhibit a high JSC of 23.50 mA/cm2, a VOC of 1.08 V and a FF of 0.75, resulting in a PCE of 19.1 %. Note that an average PCE of around 18 % can be obtained by the catalytic treatment with DMSO and DE solvent (inset histograms of Figure 7a and Table 1). It is worthy of noting that the as-prepared cells still have slight hysteresis effects with the ratio of 12.6% (Figure 7a). To rule out the effects of hysteresis on the photovoltaic performances, the photocurrent density and stabilized power output of the cells were further measured, which remained stable within 100 s, and a highly stable PCE over 17.6 % can be obtained (Figure 7b). Figure 7c displays the


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external quantum efficiency (EQE) spectrum together with EQE date-based integrated JSC. Clearly, the measured JSC is consistent with the integrated JSC of 23.0 mA/cm2 estimated from external quantum efficiency in Figure 7c. Afterward, we have compared the thermal stability of the cells prepared with and without DE treatment in Figure 7d. The as-prepared solar cells were stored at 85ºC in dark with a humidity of 20-30%. The specific PCE as the function of time was obtained in order to clarify the PCE evolution of solar cells with thermal treatment. It turns out that the thermal stability was enhanced when the FA+ introduced in the films. Moreover, the cell prepared with DE treatment exhibited superior thermal stability within 30 days, whereas the PCE of the cell prepared without DE treatment quenched by a factor of >50%. The removal of superfluous organic solvent by DE treatment benefits to stabilized the solar cell. The outstanding thermal stability of the cell with DE treatment suggests that the present study gives the possibility of an efficient and thermally stable perovskite solar cell prepared by the solvent engineering.

4. CONCLUSIONS In summary, we explored the molecular configuration of intermediate phase as well as the potential dynamic mechanisms in the crystal growth process of FAxMA1-xPbI3 thin film prepared by a subsequent two-step solution-process. (i)

In view of molecular structure, the DMSO molecular was likely to be a “bridge” that linked the FAI/MAI and PbI2, which gave rise to the formation of FAI/MAI-DMSO-PbI2.

(ii)

The DMSO linker was proven to be easily removed by the DE treatment, which speeded up the reaction of FAI/MAI with PbI2(DMSO)x complexes and generation of perovskite phase.


16

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(iii)

The removal of superfluous DMSO was demonstrated to be capable of highly increasing the thermal stability of perovskite solar cells.

(iv)

The highest PCE over 19.1% with slight hysteresis effect was eventually acquired with a FA0.88MA0.12PbI3 solar cell, which exhibited a constant power output within 100 s upon light soaking and stable PCE output within 30 days with thermal treatment.

This study provided significant development toward identification the critical role of intermediate phases in the growth kinetics of FAxMA1-xPbI3 films and reproducible way of controlling the solution-processing via solvent catalytic treatment for highly efficient and thermally stable perovskite solar cells.

FIGURES

(a)

(b)

& &

30s & &

(C)

DMSO240

x

20s

DMSO240

DMSO20-DE

DMSO20-DE # FTO + PbI2

5s # FTO + PbI2

x δ-FAPbI3

o α-FAxMA1-xPbI3

#o

o

0s 10

15

20

25

x δ-FAPbI3 ∀ MAPbI3

DMSO20

x δ-FAPbI3

+

# 30

2Theta/degree

35

o o #

#+ 40

5

10

15

o ∀

DMSO0

o

+

DMSO20

o α-FAxMA1-xPbI3

& FA3PbI5

3s

5

# FTO + PbI2

o α-FAxMA1-xPbI3

Intensity/a.u.

x

10s

Intensity/a.u.

Intensity/a.u.

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


20

25

o #

30

35

40

2Theta/degree

5

10

15

o DMSO0

o #

x+

#+

20

25

# 30

35

# 40

2Theta/degree

Figure 1. XRD patterns of (a) perovskite thin film prepared via different reaction time/soaking time (= time between dropping the M20F25 solution and starting the spinning) between PbI2(DMSO)x complex with MAI/FAI; (b) perovskite thin film prepared by different concentration of DMSO with and without DE treatment before annealing and (c) after annealing.


17

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

Intensity/a.u.


FA 0.99 MA0.01PbI 3

DMSO240

FA 0.88MA0.12PbI 3

DMSO20-DE

FA0.86MA0.14PbI3

DMSO20

FA0.58MA0.42PbI3

DMSO0

MA

406

404

Page 18 of 29

FA 402

400

398

396

Binding Energy/eV

Figure 2. XPS spectra of N1s in FAxMA1-xPbI3 thin film prepared by different concentration of DMSO and with/without DE treatment.

Figure 3. (a) Schematic images of the in-situ PL detection system; (b) The PL intensity as the function of reaction time of PbI2(DMSO)x complex with MAI/FAI.


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Figure 4. Top-down SEM of FAxMA1-xPbI3 thin film (a), (c) before and (b), (d) after annealing, without and with DE treatment. AFM images of FAxMA1-xPbI3 thin film (e) without and (f) with DE treatment.

DMSO PbI2-DMSO

DMSO

FAI-DMSO PbI2-DMSO FAI-DMSO PbI2-DMSO-FAI PbI2←DMSO→FAI

1150

1100

1050

Wavenumber/cm

1000 -1

950

Transmittance/%

(b)

(a) Transmittance/%

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


DMSO→FAI(MAI)

DMSO DMSO20-DE DMSO20 DMSO20-DE before annealing DMSO20 before annealing

PbI2←DMSO→FAI(MAI)

1150

1100

1050

Wavenumber/cm

1000

950

-1

Figure 5. (a) FTIR spectra of DMSO liquid, PbI2-DMSO thin film, FAI-DMSO thin film and perovskite thin film before annealing, respectively (b) Expanded the fingerprint region for the S=O vibrations of FAxMA1-xPbI3 film with/without DE treatment before and after annealing.


19


Figure 6. Schematic view of the crystal growth process of perovskite thin film by the catalytic treatment with DMSO and DE. (b) 30

20 15 10 5

Current Density/mA cm-2

Reverse Forward Dark Current

25 20 15 10 5 0

16

17

18

19

20

PCE/%

0 -5 0.0

0.2

0.4

0.6

0.8

1.0

1.2

20

25

SPO Current Density

15

10

10 5 5 0 0

20

40

Voltage/V

60

15

40

10

20

5

0

400

500

600

700

800

0

(d) Norm. PCE

20

80

60

80

0 100

Time/s

25

100

Integrated JSC/mA cm-2

(c)

15

20

SPO/%

30

Number of Cells

Current Density/mA cm-2

(a)

EQE/%

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

Page 20 of 29

1.0

FAxMA1-xPbI3/DE

0.8

MAPbI3

FAxMA1-xPbI3

0.6 0.4 0.2 0.0 0

5

Wavelength/nm

10

15

20

25

30

Time/day

Figure 7. (a) J-V curves of the FA0.88MA0.12PbI3 solar cells in dark and upon illumination, respectively, inset is the histograms of PCE with 91 cells; (b) Photocurrent density and stabilized power output (SPO) as a function of time for the same cell held close to 0.75 V forward bias;(c) External quantum efficiency (EQE) spectrum of perovskite solar cells with EQE date-based integrated JSC; (d) PCE evolution of the as-fabricated solar cell as a function of time.


20

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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 photovoltaic performances of perovskite solar cells with/without DE treatments. Samples

JSC/mA·cm-2 a)

VOC/V b)

FF c)

PCE/% d)

DE average f)

23.4±0.4

1.06±0.02

0.72±0.02

17.7±0.8

Forward

23.5

1.03

0.69

16.7

Reverse

23.6

1.08

0.75

19.1

23.2±0.3

1.04±0.08

0.65±0.05

15.6±1.3

Forward

23.2

0.99

0.66

15.1

Reverse

23.2

0.73

18.0

SPO/% e)

17.6

DE champion Control average g)

14.1

Control champion a)

JSC, short-circuit current density;

stabilized power output;

f,g)

b)

VOC, open circuit voltage;

1.06 c)

d)

FF, fill factor; PCE, power conversion efficiency; e)SPO,

DE/Control average, statistics of PCE from the reverse scan.

ASSOCIATED CONTENT Supporting Information. The UV-vis absorbance of FAxMA1-xPbI3 thin films with different ratio of MAI/FAI as well as the photovoltaic performance of corresponding solar cells. The UV-vis absorbance of the intermediate phase and formed perovskite thin film prepared with different concentration of DMSO and DE treatment. Top-down SEM images of FAxMA1-xPbI3 thin films prepared with and without DE treatment. The characterizations of thermal stability of as-fabricated solar cells, and the the photovoltaic performances of perovskite solar cells that was affected by the concentration of DMSO. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author


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†E-mail: (L. W.) [email protected] Phone: +86 10 62788802 ‡E-mail: (J. F.) [email protected] Present Addresses †Key Lab of Organic Optoelectronics & Molecular Engineering of Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084, China ‡ Institute of New Energy Technology, College of Information and Technology, Jinan University, Guangzhou, 510632, China Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The research was funded by the National Natural Science Foundation of China (No. 51273104 , 91433205, and 51672111), National Key Basic Research Program of China “973 program” early projects (No.2014CB260405), Natural Science Foundation of Hebei Province (No.F2015201189), “100 Talents Program of Hebei Province” (E2014100008). REFERENCES (1) Etgar, L.; Gao, P.; Xue, Z.; Peng, Q.; Chandiran, A. K.; Liu, B.; Nazeeruddin, M. K.; Grätzel, M. Mesoscopic CH3NH3PbI3/TiO2 Heterojunction Solar Cells. J. Am. Chem. Soc. 2012, 134, 17396-17399.


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Table of Contents Graphic


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High Performance of Perovskite Solar Cells via Catalytic Treatment in

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