Acetate Anion Assisted Crystal Orientation ... - ACS Publications

Article Cite This: ACS Appl. Energy Mater. 2018, 1, 2730−2739

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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*,‡

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‡

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 S Supporting Information *

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 that does not use the antisolvent dripping process, where methylammonium acetate (MAAc) is incorporated into the precursor solution. It is disclosed that the MAAc additive causes the modulation of the initial orientation of PbI2 from (001) to (101) via formation of the intermediate adduct of PbI2·MAAc, and the sequential deconstruction-reconstruction procedures result in dense perovskite films composed of large grains along (112)/(200) planes, differing from the needle-like crystallites along (110)/(002) planes constructed 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%. Furthermore, anisotropic properties have been observed with respect to 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 has demonstrated considerable performance in the high-efficiency solar cell field1−3 because of its excellent optoelectronic features, namely, high absorption coefficient guaranteeing adequate light harvest;4,5 low exciton binding energy assuring active exciton dissociation;6,7 and long carrier diffusion length rendering efficient carrier transportation and collection.8−10 Tremendous efforts have been conducted to develop high-performance perovskite solar cells (PSCs) since the first demonstration in 2009,11 and significant advancement has been achieved in which power conversion efficiency (PCE) surpassing 22% has been gained recently.1 This makes PSCs comparable with the traditional solar cells such as multicrystalline silicon,12 copper indium gallium diselenide (CIGS),13 and CdTe,14 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, for example, solution process,20,21 vapor deposition,22,23 and the combination of thereof,24 have been explored. Among them, solution approaches dividing up one-step spin coating 20,21 and two-step sequential deposition5,22 are © 2018 American Chemical Society

commonly adopted as they provide the convenient and lowcost 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 pinholes or even bare substrates. Various efforts are proposed to control the grain growth, and the antisolvent extraction protocol is approved to be the most effective route to gain the high-quality film.25,26 However, the critical antisolvent dripping process during spin coating brings in the complex.17,18,27−29 The antisolvent dripping manner, such as solvent volume and dripping time, among other factors, has a considerable impact on resultant film quality.30 Especially, the antisolvent approach is inapplicable for the large-scale module fabrication. In two-step methods, metal halide and organic precursors are deposited sequentially on the substrate, and the organic precursor will infiltrate and react with the metal halide film to form a perovskite. However, nonstoichiometric perovskite film can be easily formed due to the insufficient infiltration and reaction.5 Meanwhile, all the parameters, such as thickness, annealing temperature, annealing Received: March 12, 2018 Accepted: May 17, 2018 Published: May 17, 2018 2730

DOI: 10.1021/acsaem.8b00400 ACS Appl. Energy Mater. 2018, 1, 2730−2739

Article

ACS Applied Energy Materials

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%.

time, rinse time, and so on, 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, and large-area scalability. In this contribution, an alternative approach bypassing the antisolvent dripping process is proposed via incorporating the methylammonium acetate (MAAc) in the precursor solution. Resultant films show good optoelectronic properties, and corresponding solar cells with the best efficiency of 18.91% is demonstrated. Interestingly, the 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 sizes 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 evolving 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 transport.31−34 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. 2731


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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. 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, and 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/

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 modification.34,35 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 into a vacuum chamber with solid desiccant (silica gel). The chamber pressure is maintained at 0.1 MPa for 24 h. The resultant colorless and viscous liquid was stored in a nitrogen-filled glovebox for further use. The perovskite precursor 2732


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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. to −0.2 V first (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., 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, and unsealed devices were stored in a nitrogen-filled glovebox in the dark.

square, AGC) were rinsed in detergent, acetone, ethanol, and DI water sequentially. Before spin-coating, the substrates were treated with UVOzone for 15 min. PEDOT:PSS (P VP Al4083, Heraeus Clevios) was filtered first with syringe-driven filter (0.45 μm, hydrophilic, Millex) and spin-cast on the substrate at 4000 rpm for 30 s. The resultant film with a thickness of ∼50 nm was then annealed at 130 °C for 5 min. The perovskite film with a thickness of ∼500 nm was fabricated by spin-casting 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 °C for 5 min. After the samples were cooled to room temperature, the PC61BM solution was spin-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 Å/s. 2.3. Characterizations. Film crystal structure was examined on a Rigaku ATX-XRD diffractometer with out-of-plane grazing incident diffraction mode. Cu Kα radiation (λ = 0.15406 nm) was employed as the radiation source, and 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

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 2733


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experience, the corresponding solar cell performance should be poor considering the serious shunt, which has also been confirmed and will be discussed subsequently. 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, whereas the needle-like crystals remain unchanged. This leads to an improved uniformity and coverage compared with the film fabricated without MAAc. By further increasing the MAAc ratio higher than 7%, needlelike crystals will be completely eliminated in the as-produced films, and compact layers fully composed of the homogeneous nucleus are obtained. This indicates that MAAc additive in perovskite precursor can effectively retard the disordered crystal growth that facilitates the formation of homogeneous nucleation sites, therefore, playing 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, a smooth perovskite film with a 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 does the MAAc additive retard the crystal growth in the as-produced film? Why do crystals with a prominent size discrepancy develop 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 2a,b. Films were deposited on ITO/ PEDOT:PSS substrate, and several weak peaks irrelevant to film fabrication conditions and presumably related to the ITO

Figure 5. PL decay curves for perovskite films with different content of MAAc.

Table 1. Fitting Parameters of PL Decaya MAAc content (%)

τ1

τ2

A1/(A1+A2)

A2/(A1+A2)

0 7 9 11 13 15 17

6.06 7.68 8.23 11.32 8.97 12.59 14.31

23.70 25.69 24.03 29.93 27.63 35.57 41.17

0.93 0.95 0.87 0.84 0.89 0.85 0.81

0.07 0.05 0.13 0.16 0.11 0.15 0.19

a

Fitting was conducted following a bi-exponential equation for perovskite films prepared on the glass substrate.

constructed without the assistance of MAAc additive, and the morphology remains 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 coating.36−38 Based on

Figure 6. (a) J−V curves and (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. 2734


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

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. 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, respectively.31,34 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. When we compared the XRD spectra before and after annealing, an accordant correspondence of the peak intensity between PbI2 (101) and MAPbI3 (112)/(200) can be observed

substrate have been excluded in the forthcoming discussion. In the spectra of the 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) phase.42,43 MAPbI3 peak intensity decreases, while the PbI2 (101) peak intensity increases monotonically with MAAc ratio. MAAc contains lone-pair electrons on the bare 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, which, accordingly, retards the perovskite crystallization. In addition, when there is 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 be released prominently from the system. Consequently, PbI2 will recrystallize again accompanied by the MAAc escape. This affords to the appearance of PbI2 phase in 2735


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The decay curve can be well fitted with biexponential 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 reduction of traps and improvement of film quality, which is vital for the high photovoltaic performance of resultant perovskite film.49 3.4. Photovoltaic Performance. As a consequence, highperformance 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 crosssectional 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 electrontransporting 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 6a, 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 6b. EQE curves show a very broad plateau between ∼400 and ∼700 nm, and the calculated short-circuit current (Jsc) shows a good agreement with the measured one. Notably, a PCE of 18.91% has been achieved when the MAAc ratio reaches to 11%, and corresponding J−V curves exhibit a negligible hysteresis between reverse and forward scan direction, as shown in Figure 6c. Because of the porous structure of perovskite film fabricated without MAAc, the corresponding PSC shows a serious shunt feature and degraded performance. PCE increases first and then tends to descend to the MAAc ratio. All the photovoltaic parameters are plotted in Figure 6d as a function of MAAc concentration. The upward tendency of PCE, accompanied by the increased 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 occurred with the MAAc ratio higher than 11%, while the tightly aligned large size MAPbI3 (112)/(200) crystals became 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 decline, which is probably responsible for the decline of device performance. The increased Rs will impede the efficient carrier transportation and, 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 6d. The 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, whereas it increases to 2 when the recombination is dominated by both carriers in the space charge region. The ideality factor decreases from 1.48 to

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 framework.45 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. With respect to to the Ostwald ripening, more PbI2 (101) phase formed in the as-produced film, larger domains of reconstructed framework are achieved, in order to reduce the surface energy.46−48 To intuitively explain the transformation of the initial position of PbI2, Figure 2c,d illustrates 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 transformed from standing to reclining. Because the difference of the Pb and I atoms density in (101) and (001) planes, the MAPbI3 crystal nuclei sites will be altered. 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 2e,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 minimal MAAc in the precursor solution. However, the PbI2 orientation changes from (001) to (101) under the assistance of MAAc, and 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 measurement, perovskite films were fabricated on the glass substrate. As depicted in Figure 4a, the 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 pinhole 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 provides evidence for an alternative strategy to advanced engineering of perovskite films for photovoltaic applications. In addition, the defect evolution as a function of the MAAc content can be evaluated according to the Urbach energy as shown in Figure 4b, which can be deduced from the slope of the exponential part of the Urbach tail,39 as illustrated in Figure S4 (Supporting Information). Urbach energy decreases significantly and remains at a low level when the MAAc ratio is higher than 11%. Therefore, localized states in the 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. 2736


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1.19 when the 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 with the further increase of MAAc ratio; thus, the radiative recombination of free carriers in perovskite film is probably becoming prominent when the MAAc ratio is higher than 11%. Considering that 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 demonstrated in the literature, the perovskite film shows anisotropic properties along different crystalline phases.31,34 Considering the principle of solar cells, there is carrier transport from the perovskite layer to electron and hole collection layers, respectively. Thus, the anisotropic nature of the 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 decreased carrier transport from perovskite to corresponding collection layer. The consistent phenomenon is also reported by another publication.50

†

(L.L., Z.T.) These authors contributed equally to this study. Y.D. and Z.T. developed the basic concept. Y.D. and L.L. designed and fabricated all of the PSCs; C.X. helped optimize the fabrication processes; S.Z. and S.A. measured UV-Vis-NIR spectrum; N.W. and L.F. helped SEM measurement; L.L., Z.T., and Y.D. analyzed the data and wrote the paper; S.Z., C.W., Q.H., and G.H. provided technical support regarding PSCs; Y.D., X.Z., and Y.Z. directed the project; All authors discussed the results and revised the paper. Notes

The authors declare no competing financial interest.

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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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4. CONCLUSIONS In summary, MAAc incorporation in precursor solution has been demonstrated to be an effective protocol without the antisolvent dripping process to facilely fabricate high-quality MAPbI3 films. The high efficiency of 18.91% is also achieved on an 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 by 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 with regard to the different perovskite crystal phase. MAPbI3 (112)/(200) phase possesses better light absorption nature; however, the carrier transport along the direction declined, which accounts for the degradation of solar cell performance.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.8b00400. SEM images, photo picture, crystal structure, photovoltaic parameters of the perovskite solar cells, absorption spectra, light-intensity-dependent Voc (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for Y.D.: [email protected]. *E-mail for X.Z.: [email protected]. ORCID

Yi Ding: 0000-0003-0163-9925 Xiaodan Zhang: 0000-0002-0522-5052 2737


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