Acetate Anion Assisted Crystal Orientation Reconstruction in Organic

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Acetate Anion Assisted Crystal Orientation Reconstruction in Organic-Inorganic Lead Halide Perovskite Lin Liu, Zeguo Tang, Chenguang Xin, Shijie Zhu, Shichong An, Ning Wang, Lin Fan, Changchun Wei, Qian Huang, Guofu Hou, Ying Zhao, Yi Ding, and Xiaodan Zhang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00400 • Publication Date (Web): 17 May 2018 Downloaded from http://pubs.acs.org on May 17, 2018

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ACS Applied Energy Materials

Acetate Anion Assisted Crystal Orientation Reconstruction in Organic-Inorganic Lead Halide Perovskite Lin Liu†, ‡, Zeguo Tang†, §, Chenguang Xin‡, Shijie Zhu‡, Shichong An‡, Ning Wang‡, Lin Fan‡, Changchun Wei‡, Qian Huang‡, Guofu Hou‡, Ying Zhao‡, Yi Ding*, ‡ and Xiaodan Zhang*,‡ ‡

Institute of Photoelectronic Thin Film Devices and Technology, Key Laboratory of Photoelectronic Thin Film Devices and Technology of Tianjin, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Key Laboratory of Optical Information Science and Technology of Ministry of Education, Nankai University, Tianjin 300071, P. R. China § Research Center for Advanced Science and Technology (RCAST), The University of Tokyo, 4-6-1, Komaba, Meguro-Ku, Tokyo 153-8904, Japan.

ABSTRACT The crystalline reconstruction along (112)/(200) planes rather than (110)/(002) planes of methylammonium lead iodide (MAPbI3) is realized via an alternative one-step fabrication protocol evading anti-solvent dripping process, where methylammonium acetate (MAAc) is incorporated into the precursor solution. It is disclosed that the MAAc additive causes the modulation of initial orientation of PbI2 from (001) to (101) via

formation

of

the

intermediate

adduct

of

PbI2·MAAc,

sequential

deconstruction-reconstruction procedures results in dense perovskite films composed of large grains along (112)/(200) planes, differing from the needle-like crystallites along (110)/(002) planes constructing without MAAc assistance. The resultant films present superior optoelectronic properties, and corresponding solar cells with efficiency as high as 18.91% have been achieved at the MAAc content of 11%. 1

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Furthermore, anisotropic properties have been observed respecting different crystal orientation, which also plays a crucial role in device performance. KEYWORDS: crystal orientation, acetate anion, MAPbI3, solar cells, anisotropic, Lewis base 1. INTRODUCTION Organometal halide perovskite achieved considerable triumph in the high-efficiency solar cells field1–3 thanks to the excellent optoelectronic features, namely, high absorption coefficient guaranteeing adequate light harvest4,5; low exciton binding energy assuring active exciton dissociation6,7; long carrier diffusion length rendering efficient carrier transportation and collection8–10. Tremendous efforts have been conducted to develop high performance perovskite solar cells (PSCs) since the first demonstration in 200911, and significant advancement has been achieved that the power conversion efficiency (PCE) surpassed 22% have been gained recently1. This makes PSCs comparable with the traditional solar cells such as multi-crystalline silicon12, copper indium gallium diselenide (CIGS)13, and CdTe14, demonstrating the potential of PSC as the leading technique for next-generation solar cells. High-quality perovskite film with large grain size15,16 and low trap density17–19 is essential for high-efficiency solar cell and the efficiency breakthrough is attributed to the advancement of fabrication technique more or less. A variety of fabrication approaches, e.g. solution process20,21, vapor deposition22,23, and the combination of thereof24, have been explored. Among them, solution approaches dividing up one-step 2


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spin coating[20–21] and two-step sequential deposition[5,22] are commonly adopted as they provide the convenient and low-cost fabrication route for high-quality perovskite film. In theory, the one-step spin-coating is the most facile approach to gain the perovskite thin-film. However, the fast evaporation of solvents for dissolving the chemicals results in the pinholes or even bare substrates. Various efforts are proposed to control the grain growth and the anti-solvent extraction protocol is approved to be the most effective route to gain the high-quality film25,26. But the critical anti-solvent dripping process during spin coating brings in the complex [17,18,27,29]. The anti-solvent dripping manner, such as solvent volume, dripping time etc., has a considerable impact on resultant film quality30. Especially, the anti-solvent approach is inapplicable for the large-scale module fabrication. In two-step methods, metal halide and organic precursors are deposited sequentially on the substrate, organic precursor will infiltrate and react with metal halide film to form perovskite. However, nonstoichiometric perovskite film can be easily formed due to the insufficient infiltration and reaction5. Meanwhile, all the parameters, such as thickness, annealing temperature, annealing time, rinse time etc. are required to be controlled precisely to gain high efficiency. Thus, it is necessary to explore the facile process to fulfill the requirements of high efficiency, high reproducibility, large-area scalability. In this contribution, an alternative approach bypassing the anti-solvent dripping process is proposed via incorporating the methylammonium acetate (MAAc) in the precursor solution. Resultant films show good optoelectronic properties, and 3



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corresponding solar cells with the best efficiency of 18.91% is demonstrated. Interestingly, preferred orientation transition from (110)/(002) to (112)/(200) is observed as the increasing content of MAAc in the precursor solution. Grains growing along (112)/(200) planes are getting larger with increasing MAAc content and even larger than 5 µm are achieved when the MAAc volume ratio is higher than 13%. The incorporation of Ac anion results in the crystal reconstruction along (112)/(200) planes rather than (110)/(002) planes and thus avoiding the pinhole or bare substrate. The film presents excellent light absorption, carrier lifetime and low trap density. However, the efficiency of the corresponding solar cell is not consistently evolution with the grain size. Considering the (112)/(200) planes are parallel to the substrate, the conversion efficiency reduction can probably be attributed to the anisotropic carrier transport31–34. Furthermore, an acetate anion assisted crystal reconstruction model is proposed. All the results demonstrate that this protocol is facile and practical for high-quality perovskite film fabrication. 2. EXPERIMENTAL SECTION 2.1 Materials. All the chemicals and solvents were purchased from commercial sources and used as received unless stated otherwise. MAAc were synthesized through the method reported by the literature with a little modification34,35. Briefly, 50 mL of ethanol, 5 mL of acetate (99.5%, Aladdin Reagents) and 12 mL of methylamine (30-33 wt. % in ethanol, Aladdin Reagents) were added sequentially in a 250 mL round-bottom flask. The solution was stirred at room temperature for 2 h and then put 4


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into a vacuum chamber with solid desiccant (silica gel). The chamber pressure keeps at 0.1 MPa for 24 h. Resultant colorless and viscous liquid was stored in a nitrogen-filled glovebox for the further use. Perovskite precursor solution was prepared by dissolving 1.3 M of PbI2 and MAI in DMF and DMSO blend solvent (DMF:DMSO=9:1 (v/v)), in which MAAc was also introduced with different volume ratio. The solution was stirred for 1 h at room temperature before utilization. PC61BM (>99.9%, Sigma-Aldrich) was dissolved in chlorobenzene with a concentration of 15 mg/ml, the solution was stirred for 1 h at room temperature before spin coating. Bathocuproine (BCP) (96%, Sigama-Aldrich) was dissolved in isopropanol with a concentration of 1 mg/ml, and the solution was stirred for 6 h for homogeneous dispersion. 2.2 Solar cell fabrication. Planar p-i-n type solar cells with a structure of glass/ITO/PEDOT:PSS/Perovskite/PCBM/BCP/Au were fabricated as follows. ITO covered glass substrates (10 Ohm/square, AGC) were rinsed in detergent, acetone, ethanol and DI water sequentially. Before spin-coating, the substrates were treated with UV-Ozone for 15 min. PEDOT:PSS (P VP Al4083, Heraeus Clevios) was filtered firstly with springe-driven filter (0.45 µm, hydrophilic, Millex) and spin cast on the substrate at 4000 rpm for 30 s. The resultant film with thickness of ~50 nm was then annealed at 130 ℃ for 5 min. Perovskite film with a thickness of ~500 nm were fabricated by spin cast the precursor solution on PEDOT:PSS surface at 1000 rpm for 10 s and then 3500 rpm for 70 s. Samples were annealed again at 100 ℃ for 5 min. After cooling down the samples to room temperature, the PC61BM solution was spin 5



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cast on it at 1500 rpm for 60 s, and then BCP solution was spin cast above at 3000 rpm for 60 s. Finally, a 100 nm-thick Au electrode was evaporated on the top with an evaporation speed of ~1 angstrom/s. 2.3 Characterizations. Film crystal structure was examined on a Rigaku ATX-XRD diffractometer with out-of-plane grazing incident diffraction mode, Cu K-alpha radiation (λ = 0.15406 nm) was employed as the radiation source, the incident angle was fixed to 1.2 degree and the out-of-plane exit angle was scanned from 10 degree to 50 degree; Sample morphology was characterized by using a scanning electron microscope (JSM-6700F, JEOL); Film transmittance, reflectance, and absorbance were measured with a UV-Vis-NIR spectrophotometer (Cary 5000, VARIAN); Time resolved photoluminescence (TR-PL) measurement was performed at room temperature with laser wavelength and power of 532 nm and 0.2 mW, respectively (Hamamatsu C12132); Device photocurrent density–voltage (J–V) curves were measured under 1 sun illumination with a Keithley2400 Digital Source Meter. Unless otherwise specified, bias scan from 1.2 V to -0.2 V firstly (reverse scan) and return back (forward scan) with a step of 70 mV and scan speed of 30 mVs-1. Reverse curve is mainly adopted to evaluate the device performance. The light source was calibrated by Asahi Spectra Co.,Ltd. Incident light power was checked before every measurement by using a standard silicon photodiode. Light intensity dependent J–V curves were also measured with the same equipment, while the light intensity was adjusted carefully by using a series of ND filters. The external quantum efficiency (EQE) spectrum was measured on an EQE system (CEP-2000MLQ, Bunkoukeiki Co., 6


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Ltd.) in the DC mode without any voltage bias. The excitation light intensity was calibrated using a Si photodiode. A black mask was used to confirm the photoactive area of 0.1 cm2 during J-V and EQE measurements. Device stability has also been evaluated by measuring the J-V curves under 1 sun illumination once per day, unsealed devices were stored in a nitrogen-filled glovebox in the dark. 3. RESULTS AND DISCUSSION 3.1 Morphology Surface morphology of MAPbI3 films fabricated on ITO/PEDOT:PSS substrate with different MAAc volume ratio is systematically evaluated by scanning electron microscope (SEM). Figure 1 shows top-view SEM images of representative films before and after annealing, and SEM images for all the films can be found in Figure S1. Also, the photos of all films are assembled in Figure S2. Clearly, dendritic crystals randomly cover the substrate when the crystal is constructed without the assistance of MAAc additive and the morphology keeps identical regardless of annealing treatment. This is consistent with other reported literature and the reason is attributed to the too rapid crystallization owing to the fast solvent evaporation during spin coating36–38. Based on the experience, corresponding solar cell performance should be poor considering serious shunt, which has also been confirmed in the later section. By introducing 7% of MAAc, the film surface will be dominated by small crystal nucleus, leaving a little portion of needle-like crystals exposed in the certain areas. The nucleus starts to grow modestly with annealing, while, the needle-like 7



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crystals remain unchanged. This leads to an improved uniformity and coverage compared to the film fabricated without MAAc. By further increasing MAAc ratio higher than 7%, needle-like crystals will completely eliminate in the as-produced films, and compact layers fully composed of the homogeneous nucleus is obtained. This indicates that MAAc additive in perovskite precursor can effectively retard the disordered crystal growth facilitate the formation of homogeneous nucleation sites, hence, plays a crucial role in controlling the film morphology. In addition, the size of tightly arranged crystals, growing horizontally along the substrate surface, becomes larger with MAAc ratio. Grain size as large as ~1 µm can be achieved when the MAAc ratio reaches 11%. Moreover, smooth perovskite film with grain size even larger than 5 µm can be realized when the MAAc ratio is equal to and higher than 15%. However, perplexing questions associated with MAAc introduction have been raised. Why and how MAAc additive retards the crystal growth in the as-produced film; meanwhile, why crystals with prominent size discrepancy have been developed after annealing, although the corresponding films before annealing exhibited the similar morphology, especially when the MAAc ratio is higher than 9%. 3.2 Crystal structure With these doubts, XRD spectra of films before and after annealing are systematically characterized as shown in Figure 2 (a) and (b). Films were deposited on ITO/PEDOT:PSS substrate, several weak peaks irrelevant to film fabrication 8


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conditions, and presumably related to the ITO substrate have been excluded in the coming discussion. In the spectra of as-produced film, besides the predominant MAPbI3 (110)/(002) and (220) peaks centered around 14.1° and 28.4°, respectively; a weak peak assigned to PbI2 (001) at ~12.6° can be observed when the MAAc ratio is 7% and 9%39–41. Additionally, a new peak around 25.1° can be detected when the MAAc ratio is higher than 9%, which belongs to the PbI2 (101) phase42,43. MAPbI3 peak intensity decreases, while the PbI2 (101) peak intensity increases monotonically with MAAc ratio. MAAc contains lone-pair electrons on the bear oxygen in acetate anion, thus it can probably work as a Lewis base, while PbI2 is known to be a Lewis acid. Hence, similar to the mechanism proposed by Nam-gyu Park et al.44, an intermediate compound of PbI2•MAAc will be formed through strong interactions in the precursor. This impedes the direct reaction between PbI2 and MAI, accordingly, retards the perovskite crystallization. In addition, more MAAc incorporated, more intermediate compound will be formed. Therefore, the XRD peak intensity of MAPbI3 phase decreases monotonically with MAAc. However, the intermediate compound is metastable as the MAAc is thermally unstable and will release prominently from the system. Consequently, PbI2 will recrystallize again accompanied with the MAAc escape. This affords to the appearance of PbI2 phase in as-produced film. Nevertheless, PbI2 tends to preferentially recrystallized in (101) orientation when the MAAc ratio is higher than 9%, which might be because that the PbI2 formation energy in (101) orientation is declined adequately in the circumstance with high ratio of MAAc. 9



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After annealing, PbI2 phase vanishes in the resultant films, probably because of the further reaction accompanied by remnant MAI in the film. The peak intensity of MAPbI3 (110)/(002) and (220) decreases and fades away; meanwhile, new peaks around 20.0° and 40.6° become prominent and exclusive when the MAAc ratio increases higher than 13%. MAPbI3 (112) and/or (200) diffraction would be responsible for the peak at 20.0°. However, it is hard to distinguish from XRD, as the interplanar spacing of {112} and {200} has a very similar value of 4.446 and 4.425 Å, respectively31,34. Further study is necessary regarding the specific identification. In this work, we tend to annotate it as MAPbI3 (112)/(200), and the peak at 40.6° is recognized subsequently as the MAPbI3 (224)/(400) phase. Consequently, it is plausible to declare that the orientation of MAPbI3 crystals can be modulated from (110)/(002) to (112)/(200) by utilizing and precisely controlling of MAAc ratio in the precursor solution. Comparing the XRD spectra before and after annealing, an accordant correspondence of the peak intensity between PbI2 (101) and MAPbI3 (112)/(200) can be observed in terms of the MAAc ratio. This implies that the existence of PbI2 (101) phase in the as-produced film might be responsible for the transformation of perovskite orientation. It has been proposed that the perovskite crystal orientation is predetermined by the initial orientation of the PbI6 octahedral framework45. Hence, the PbI2 (101) remnant will presumably induce the reconstruction of the PbI6 octahedral framework through annealing, which eventually leads to the perovskite recrystallization in (112)/(200) direction after the intercalation of MAI molecules. 10


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Respecting to the Ostwald ripening, more PbI2 (101) phase formed in the as-produced film, larger domains of reconstructed framework achieved, in order to reduce the surface energy46–48. To intuitively explain the transformation of the initial position of PbI2, Figure 2 (c) and (d) illustrate the PbI2 crystal parallel to the (001) and (101) planes as well as view along the and direction, respectively. The drawings are produced by the software of VESTA. It looks like the PbI2 crystal transform from standing to reclining. Because the difference of the Pb and I atoms density in (101) and (001) planes, the MAPbI3 crystals nuclei sites will alter. Consequently, MAPbI3 crystals grow along the (112)/(200) plane rather than (110)/(002) plane. Analogously, the MAPbI3 crystals parallel to the (110) and (112) planes as well as view along the and directions are plotted in Figure 2 (e) and (f) respectively for a clear view. The corresponding (002) and (200) planes are drawn up in Figure S3. The crystals growth mechanism is summarized in the Figure 3. The initial orientation of PbI2 is parallel to the (001) plane and the resulting MAPbI3 films show (110)/(002) orientation while there is lacking MAAc in the precursor solution. However, the PbI2 orientation alters from (001) to (101) under the assistance of MAAc, as a consequence, the MAPbI3 films present (112)/(200) orientation. 3.3 Absorption spectra and defect evaluation Optical properties of resultant perovskite films have been evaluated by UV-vis absorption and TR-PL decay characterizations. For the UV-vis absorption 11



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measurement, perovskite films were fabricated on the glass substrate. As depicted in Figure 4 (a), resultant films exhibit a broad absorption over the range from 300 nm to ~800 nm, which in turn offers the remarkable light-harvesting capacity of the devices. Perovskite film prepared without MAAc additive shows weaker light absorption, which can be ascribed to the pin-hole in the film. Furthermore, the light absorbance increases and saturates when the MAAc ratio increases higher than 11%, which correlates to the film composed of prominent (112)/(200) phase, indicating the dense grain stacked along (112)/(200) phase as well as the excellent light absorption nature. This expects an alternative strategy to advanced engineering of perovskite films for photovoltaic applications. In addition, the defects evolution as a function of the MAAc content can be evaluated according to the Urbach energy as shown in Figure 4 (b), which can be deduced from the slope of the exponential part of the Urbach tail39, as illustrated in Figure S4 (Supporting Information). Urbach energy decreases significantly and keeps at a low level when the MAAc ratio is higher than 11%. Therefore, localized states in band tail, commonly generated by possible structural defects or trap centers located around grain boundaries, have been suppressed effectively with MAAc, implying an enhanced film uniformity and crystallization, which has also been confirmed from SEM and XRD results. TR-PL decay as shown Figure 5 has been measured to further investigate the effect of MAAc on the carrier lifetime. The decay curve can be well fitted with bi-exponential components and the fitting parameters are listed in Table 1. Both the fast decay and the slow decay lifetimes increase with MAAc ratio, suggesting a 12


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reduction of traps and improvement of film quality, which is vital for the high photovoltaic performance of resultant perovskite film49. 3.4 Photovoltaic performance As a consequence, high performance solar cells are expected by employing highly crystallized films qualified with enhanced light absorption and suppressed trap centers. Perovskite solar cells with an inverted device architecture have been fabricated, and the typical cross-sectional SEM image of the device is shown in Figure S5. Perovskite film with a thickness of ~500 nm, fabricated with the identical conditions, was sandwiched by a ~50 nm-thick PEDOT:PSS hole transporting layer and a PC61BM electron transporting layer. Device performance was evaluated by measuring photocurrent density versus voltage (J–V) and external quantum efficiency (EQE) spectra. Typical J-V curves measured in the reverse scan direction are shown in Figure 6 (a), device parameters derived from the corresponding J-V curves are summarized in Table S1. EQE spectra of perovskite solar cells and the integrated photocurrents calculated from the integral of the EQE spectra are shown in Figure 6 (b). EQE curves show a very broad plateau between ~400 and ~700 nm and calculated Jsc shows a good agreement with the measured one. Noteworthy Power conversion efficiency (PCE) of 18.91% has been achieved when the MAAc ratio reaches to 11%, corresponding J-V curves exhibit a negligible hysteresis between reverse and forward scan direction, as shown in Figure 6 (c). Due to the porous structure of perovskite film fabricated without MAAc, 13



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corresponding PSC shows serious shunt feature and degraded performance. PCE increases firstly and then tends to descend to MAAc ratio. All the photovoltaic parameters are plotted in Figure 6 (d) as a function of MAAc concentration. The upward tendency of PCE, accompanied with the promoted short-circuit current (Jsc), open-circuit voltage (Voc), fill factor (FF) and shunt (Rsh) resistivity, is mainly attributed to the improved uniformity and optoelectronic properties of absorber layers through moderated MAAc incorporation in the precursors. In addition, Device stability has also been evaluated, and a ~5% degradation in PCE after a 12-day duration has been observed, as shown in Figure 7, although progressive study needs to be conducted to further improve the device stability in the coming works. However, an unanticipated decline in device performance is occurred with the MAAc ratio higher than 11%, while the tightly aligned large size MAPbI3 (112)/(200) crystals become dominant in the perovskite film, moreover, enhanced light harvest and suppressed trap concentration has been demonstrated. A pronounced increase of Rs can be observed with the PCE declining, which is probably responsible for the decline of device performance. The raised Rs will impede the efficient carrier transportation, in turn, result in a considerable Voc drop from 1020 mV to 980 mV. Furthermore, it will substantially accelerate the recombination of free carriers as reflected by the ideality factor of the device, as shown in Figure 6 (d). Ideality factor of the device can be obtained by fitting the dependence of Voc on light intensity as shown in Figure S6. This value would be equal to the unit when the recombination is dominated by minority carriers injected into the neutral regions of the junction, while 14


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it increases to 2 when the recombination is dominated by both carriers in the space charge region. Ideality factor decreases from 1.48 to 1.19 when MAAc ratio increases up to 11%, indicating the suppressed recombination. This is reasonable because, as discussed above, the film quality and carrier lifetime have been improved progressively with MAAc ratio. However, this value increases reversely with the further increase of MAAc ratio, thus the radiative recombination of free carriers in perovskite film is probably getting prominent when the MAAc ratio is higher than 11%. Considering the identical device structure has been employed in this work, therefore, the increase of Rs is probably originated from the enhanced resistivity of perovskite film along the carrier transportation direction. As be approved in many kinds of literature that the perovskite film shows anisotropic properties along different crystalline phase31,34. Considering the principle of solar cells, the carriers transport from perovskite layer to electron and hole collection layers, respectively. Thus, the anisotropic of perovskite layer has a crucial impact on the carrier collection efficiency. It is demonstrated that the carrier could smoothly transport in the (112)/(200) planes 34, but in the solar cell, the carrier is required to transport along the direction. Thus, the efficiency decline can be attributed to the descending carriers transport from perovskite to corresponding collection layer. The consistent phenomenon is also reported by other publication50. 4. CONCLUSIONS 15



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In summary, MAAc incorporation in precursor solution has been demonstrated to be an effective protocol without the anti-solvent dripping process to facilely fabricate high-quality MAPbI3 films. The high efficiency of 18.91% is also achieved on inverted perovskite solar cells. It has been demonstrated that the MAAc additive can retard the disordered crystallization by forming PbI2•MAAc intermediate compounds in the precursor; and PbI2 will recrystallize again in the as-produced film accompanied with the MAAc release from the intermediate compounds; however, PbI2 will preferentially crystallize in (101) phase when the MAAc ratio is higher than 9%; which will induce the reconstruction of PbI6 octahedral framework of perovskite during annealing; and eventually leads to the transformation of perovskite orientation from (110)/(002) to (112)/(200). Anisotropic electrical properties have been observed regarding the different perovskite crystal phase. MAPbI3 (112)/(200) phase possesses better light absorption nature, however, the carrier transport along the direction is declined, which accounts for the degradation of solar cell performance.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: SEM images, photo picture, crystal structure, photovoltaic parameters of the perovskite solar cells, absorption spectra, light intensity dependent Voc. AUTHOR INFORMATION Corresponding author: * Email: [email protected] (Y. Ding); [email protected] (X. Zhang) 16


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† These authors contributed equally to this study. Present Address ‡Institute of Photoelectronic Thin Film Devices and Technology, Key Laboratory of Photoelectronic Thin Film Devices and Technology of Tianjin, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Key Laboratory of Optical Information Science and Technology of Ministry of Education, Nankai University, Tianjin 300071, P. R. China §Research Center for Advanced Science and Technology (RCAST), The University of Tokyo, 4-6-1, Komaba, Meguro-Ku, Tokyo 153-8904, Japan. Author Contributions Yi Ding, Zeguo Tang developed the basic concept. Yi Ding and Lin Liu designed and fabricated all of the PSCs; Chenguang Xin helped optimize the fabrication processes; Shijie Zhu and Shichong An measured UV-Vis-NIR spectrum; Ning Wang and Lin Fan helped SEM measurement; Lin Liu, Zeguo Tang and Yi Ding analyzed the data and wrote the paper; Shijie Zhu, Changchun Wei, Qian Huang and Guofu Hu provided technical support regarding PSCs; Yi Ding, Xiaodan Zhang and Ying Zhao directed the project; All authors discussed the results and revised the paper. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (61504069 and 61474065), International cooperation projects of the Ministry of Science and Technology (2014DEF60170), Tianjin Research Key Program of Application Foundation and Advanced Technology (15JCZDJC31300), and the 111 Project (B16027). The Authors would like to thank Prof. Bo Zhou in Northwest University, China, for fruitful discussions on the crystal structure of perovskite film. Also, the authors express their appreciation to the team of VESTA software.

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Figure 1. Surface SEM images for perovskite films with different MAAc contents before and after annealing, (a), (b) 0%; (c), (d) 7%; (e), (f) 11%; (g), (h) 15%.

18


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Figure 2. XRD patterns for perovskite films with various content of MAAc without (a) and with (b) annealing; (c) PbI2 crystal structure as shown (001) plane parallel to substrate and viewed along the direction; (d) PbI2 crystal structure as shown (101) plane parallel to substrate and viewed along the direction; (e) MAPbI3 crystal structure as shown (110) plane parallel to substrate and viewed along the direction; (f) MAPbI3 crystal structure as shown (112) plane parallel to substrate and viewed along the direction. 19



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Figure 3. Schematic of MAPbI3 crystal reconstruction (a) without and (b) with acetate anion assistance.

Figure 4. (a) Absorbance spectra for MAPbI3 films prepared by precursors with various contents of MAAc; (b) Urbach energy evolution as a function of MAAc content, the inset table shows the exact value.

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Figure 5. PL decay curves for perovskite films with different content of MAAc. Table 1. The fitting parameters of PL decay. Fitting was conducted following a bi-exponential equation for perovskite films prepared on the glass substrate. MAAc content (%)

τ1

τ2

A1/(A1+A2)

A2/(A1+A2)

0

6.06

23.70

0.93

0.07

7

7.68

25.69

0.95

0.05

9

8.23

24.03

0.87

0.13

11

11.32

29.93

0.84

0.16

13

8.97

27.63

0.89

0.11

15

12.59

35.57

0.85

0.15

17

14.31

41.17

0.81

0.19

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Figure 6. (a) J-V curves (b) EQE spectra for perovskite solar cells prepared by precursors with different MAAc concentrations. (c) J-V curves for best solar cells prepared at the MAAc content of 11%. (d) All the photovoltaic parameters evolution as a function of MAAc content.

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Figure 7. Stability test of perovskite solar cells prepared by precursors containing different MAAc.

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Acetate Anion Assisted Crystal Orientation Reconstruction in Organic

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