Fabrication of efficient formamidinium perovskite solar cells under ambient air via intermediate-modulated crystallization

Fabrication of efficient formamidinium perovskite solar cells under ambient air via intermediate-modulated crystallization

Solar Energy 187 (2019) 147–155 Contents lists available at ScienceDirect Solar Energy journal homepage: www.elsevier.com/locate/solener Fabricatio...

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Solar Energy 187 (2019) 147–155

Contents lists available at ScienceDirect

Solar Energy journal homepage: www.elsevier.com/locate/solener

Fabrication of efficient formamidinium perovskite solar cells under ambient air via intermediate-modulated crystallization

T

Gaoxiang Wanga,b, Lipeng Wanga,b, Jianhang Qiua, , Zheng Yanc, Kaiping Taia, Wei Yud, , ⁎ Xin Jianga, ⁎



a

Shenyang National Laboratory for Materials Science (SYNL), Institute of Metal Research (IMR), Chinese Academy of Sciences (CAS), Shenyang 110016, China School of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, China c College of Energy and Environment, Shenyang Aerospace University, Shenyang 110136, China d Hebei Key Laboratory of Optic-electronic Information and Materials, National-Local Joint Engineering Laboratory of New Energy Photoelectric Devices, College of Physics Science and Technology, Hebei University, Baoding 071002, China b

ARTICLE INFO

ABSTRACT

Keywords: Perovskite solar cell Formamidinium Ambient-air fabrication Intermediate Crystallization

Developing simple methods to synthesize perovskite layers under ambient air can facilitate the industrial production of perovskite solar cells (PSCs). While limited progress has been made for the ambient-air fabrication of formamidinium lead triiodide perovskite (α-FAPbI3) layers due to the coexisting non-perovskite polymorph (δFAPbI3). Here, a facile N-methyl pyrrolidone (NMP) additive-based method is proposed to produce FA-perovskite (FAPbI3) layers in ambient air and smooth, highly-crystallized and δ-phase free perovskite films are produced under humidity of ∼40%. The resultant PSC delivers a power conversion efficiency (PCE) of 17.29%, ∼20% enhancement from that of PSC produced from traditional dimethyl sulfoxide (DSMO) additive. It is found that a distinct FAI·PbI2·NMP intermediate phase forms during the film growth, providing an alternative crystallization process different to that of traditional δ-FAPbI3 intermediate route. The fine nucleation of FAI·PbI2·NMP intermediate contributes a smooth and dense morphology to the final film. The rapid detachment of NMP molecule from the intermediate enables a direct formation of α-FAPbI3 film, averting the incomplete δto α- phase conversion under ambient air. Due to the high film quality, a PCE of 13.55% is still remained after 30-day storage for unencapsulated device, demonstrating the reliability of our method for FA-perovskite ambient-air fabrication.

1. Introduction Organic-inorganic hybrid perovskite processes many excellent photoelectric properties such as high absorption coefficient, long carrier diffusion length and fast carrier mobility (Lee et al., 2012; Liu et al., 2013; Stranks et al., 2013; Zhou et al., 2014). Its structure can be summarized in ABX3, where A is monovalent cation e.g., methylammonium (MA), formamidinium (FA) or Cs+, B is divalent metal cation Pb2+ or Sn2+ and X is halide ion. As a pioneer material in perovskite family, MAPbI3 debuted in the field of photovoltaics in 2009, reaching a power conversion efficiency (PCE) of 3.8% for the perovskite solar cell (PSC) (Kojima et al., 2009). Since then, various perovskites composed of different cations and anions have been investigated, in hope of fully exploiting the material potential (Eperon et al., 2014; Jena et al., 2018; Jeon et al., 2015; Li et al., 2015; Liang et al., 2018; McMeekin et al., 2016; Mozaffari et al., 2018; Qiu et al., 2013, 2016,



2017; F. Wang et al., 2015). Due to the desirable combination of superb thermal stability and favorable light-harvesting bandgap (Eperon et al., 2014; Weller et al., 2015), FA-based perovskites are adopted by many researchers and become the preferred material for efficient PSC fabrication (Correa-Baena et al., 2017). Nowadays, the PCE of FA-based PSC has been rocking over 23% (Jiang et al., 2019; Saliba et al., 2016; Tan et al., 2017; Yang et al., 2015). Considering the high efficiency already achieved, the commercialization of PSC has become the next challenge in this field (Ono et al., 2017). Fabricating perovskite layers under ambient air is an appealing strategy for low-cost and large-scale production of PSCs. Due to the adverse influence of moisture on the crystallization process, the airproduced perovskite layers normally suffer from problems, such as poor morphology, low crystallinity and impurity inclusion (Ono et al., 2017). To address above issues, strategies of utilizing the additive-induced intermediate to control the perovskite crystallization are widely

Corresponding authors. E-mail addresses: [email protected] (J. Qiu), [email protected] (W. Yu), [email protected] (X. Jiang).

https://doi.org/10.1016/j.solener.2019.05.033 Received 21 January 2019; Received in revised form 6 May 2019; Accepted 15 May 2019 Available online 22 May 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.

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investigated (He et al., 2018; Huang et al., 2019; Rong et al., 2017; Sun et al., 2017; Zhang et al., 2018). Wang et al. applies the 2-pyridylthiourea additive to ambient-air fabrication of MAPbI3 layers, wherein strong coordination between 2-pyridylthiourea and PbI2 slows down the perovskite formation, leading to uniform, continuous and compact films with larger grains (Sun et al., 2017). By using the synergistic effect of moisture and intermediate on crystallization process, the growth of perovskite is retarded during thermal annealing but accelerated under moisture condition. A preferentially oriented MAPbI3 film is thus obtained (Rong et al., 2017). During fabricating of PSCs by a two-step thermal engineering method under ambient air, it is found that there is a NH4PbI3 intermediate formed by deficient hot treatment, whereby the formation of impurity is suppressed under high temperature annealing process (Moyez and Roy, 2018). As for the state-of-theart FA-based perovskite layers, facile ambient-air fabrication methods are limited due to the strong hygroscopic nature of FAI and the coexistence of lower-temperature none-perovskite polymorph (δ-FAPbI3). Recently, in fabricating (Cs)0.15(FA)0.85PbI3-based PSC, a pin-hole free film is obtained through suppressing moisture ingress during the spincoating process, where the designed PbI2-(CsI)0.3-FAI intermediate reduces the required concentration of FAI solution and the preheating treatment enables a rapid solvent evaporation. This work shows the feasibility of obtaining high-quality FA-perovskite films through controlling the crystallization (Xu et al., 2018). To the best of our knowledge, there is no report on protocols of utilizing the additive-induced intermediate for ambient-air fabrication of FA-perovskite layers. Here, we propose a facile method to fabricate high-quality FA-perovskite layers under ambient air with N-methyl pyrrolidone (NMP) additive, wherein the nucleation and growth of perovskite film is modulated via an additive-induced FAI·PbI2·NMP intermediate. It is found that smooth, highly-crystalized and δ-phase free layers can be produced under ambient air, and a best PCE of 17.29% is achieved for the corresponding PSCs. Combining morphology observation, phase characterization and thermogravimetric analysis, the underlying mechanism of the intermediate-modulated crystallization process has been discussed in detail, which is significant for the production of highperformance PSC for large-scale industrial application.

precursor solution was spin-coated on the substrate at 5000 rpm for 25 s, during which 1 mL diethyl ether was dropped at the spinning film in final 5 s. The obtained wet intermediate film was immediately annealed at 140 °C for 5 min on hot plate. The as-prepared perovskite films were spincoated with a 2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenyl-amine)-9,9′spirobifluorene (spiro-MeOTAD) solution at 4000 rpm for 20 s, which was obtained by dissolving 73.2 mg spiro-MeOTAD in 1 mL chlorobenzene with addition of 29 μL 4-tert-butylpyridine (tBP), 17.5 μL lithium bis(trifluoromethane)sulfonimide (Li-TFSI) stock solution (520 mg Li-TFSI in 1 mL acetonitrile) and 29 μL tris(2-(1H-pyrazol-1-yl)-4-tert-butylpyridine) cobalt(III) tri[bis(trifluoromethane)sulfonimide] (FK209, Aldrich) (300 mg FK209 in mL acetonitrile) stock solution. Except that the spiro-MeOTAD solution was prepared in the glovebox, all the other manipulations were carried out in ambient air with relative humidity of ∼40%. Finally, a 100 nm Au layer was thermally evaporated on the spiro-MeOTAD layer as counter electrode. 2.3. Properties characterization Morphologies of the synthesized perovskite films were characterized by an optical microscope (OM, Olympus BX51) and a field emission scanning electron microscope (SEM, Hitachi SU-70) operated at 10 kV. X-ray diffraction (XRD) patterns were obtained using a diffractometer (Rigaku RINT2000 X-Ray) with Cu Kα radiation at a scan rate of 4° min−1 under operation condition of 50 kV and 80 mA. The crystallite size was estimated based on Scherrer analysis. The calculation was conducted on the Jade 6 software and the instrument broadening was corrected using the standard silicon sample. Ultraviolet–visible diffuse absorption spectra were recorded on a spectrophotometer (Hitachi U3900). The samples for OM, SEM, XRD and absorbance characterization were prepared on TiO2-coated FTO substrates. Fourier transform infrared spectroscopy (FTIR) measurements were performed on a Bruker FTIR spectrometer (Vertex 70) equipped with MIRacle ATR accessory. PbI2·X (or FAI·PbI2·X) complexes, where X is DMF, DMSO or NMP, were prepared by dissolving 461 mg PbI2 (or 461 mg PbI2 and 172 mg FAI) in 625 µL DMF, DMSO or NMP. The mixture of 2 µL asprepared solution and 0.001 g KBr was compressed to form a Φ13 tablet for FTIR measurement. Thermogravimetric analysis (TGA) was performed on a TA Q1000 instrument at 10 °C min−1 under ambient air. Samples for TGA were prepared by peeling off wet intermediate films from FTO substrate. Steady-state photoluminescence (PL) spectra were obtained by an optical spectrometer system (Horiba HR Evolution) with a 100 mW laser (λ = 532 nm) as excitation source. The time-resolved PL measurements were carried out in an Edinburgh FLS920 spectrofluorometer with excitation wavelength at 470 nm. Samples for PL measurements were prepared on the slide glass. The electrochemical impedance spectroscopy (EIS) analysis was performed on an electrochemical workstation (Autolab 302N) in the frequency range between 0.1 MHz and 0.1 Hz using a 10 mV perturbation signal. The current density–voltage (J–V) curves of electron-only devices (FTO/c-TiO2/FAperovskite/PCBM/Ag) and photovoltaic devices (FTO/c-TiO2/mesoTiO2/FA-perovskite/Spiro-OMeTAD/Au) were measured using a Keithley 4200 semiconductor parameter analyzer. The electron-only devices were monitored from 0.02 V to 10 V by a log-scan mode under dark condition. The photovoltaic devices were measured under AM 1.5 G one sun illumination (100 mW/cm2) with a solar simulator (Newport 94021A) calibrated by a Si-reference cell certified by NREL. The J–V curves of photovoltaic devices were recorded by reverse (from opencircuit to short-circuit) mode with a linear scan rate of 10 mV s−1. The external quantum efficiency (EQE) values were measured by an Oriel IQE200 system under direct current (DC) mode, where a 300 W xenon lamp was used as the light source for generating monochromatic beam. Steady-state efficiency measurements were carried out when devices were held at the maximum power point voltage determined from the JV scans. Photovoltaic devices were measured under ambient air by masking the active area with a shadow mask (0.09 cm2 in area) and

2. Materials and methods 2.1. Materials Chemicals were purchased from Aladdin without further purification. PbI2 was prepared as yellow precipitate by mixing aqueous solution of 0.05 mol Pb(NO3)2 and 0.1 mol KI, followed by drying in a vacuum oven at 50–60 °C for 24 h. 1.6 M FAPbI3 precursor solutions with different additives were prepared by dissolving equimolar FAI (Dalian Youxuan Trading Co.) and PbI2 in N,N-Dimethylformamide (DMF) solvent (none-additive), or DMF and additive mix solutions (4:1 vol ratio). The DMSO and NMP additives were investigated in this work. 1.0, 1.2 and 1.4 M solutions with NMP additive were prepared in similar manners. 2.2. Device fabrication Fluorine doped tin oxide (FTO) glasses were etched with Zn powder and 37% HCl to obtain the required pattern. The etched FTO substrates were cleaned by sequential sonication in detergent, deionized water, acetone, isopropanol and ethanol for 15 min respectively, followed by UVozone treatment for 15 min. A TiO2 blocking layer was deposited on the cleaned FTO by spin-coating a 0.15 M titanium diisopropoxide bis(acetylacetonate) (75 wt% in isopropanol) in 1-butanol at 2000 rpm for 40 s, followed by heating at 125 °C for 5 min. 0.128 g mL−1 TiO2 paste (Dyesol, 30NR-T) solution was spin-coated on the TiO2 blocking layer at 4000 rpm for 10 s, and dried at 120 °C for 10 min on hot plate. After that, the resultant substrates were annealed at 550 °C for 30 min. The FAPbI3 148

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Fig. 1. (a) SEM images and (b) XRD patterns of FA-perovskite films fabricated from different additives. The scale bars are 1 μm.

placed in the dark prior to the start of the measurement without any voltage applied. J-V curves of the large area (1 cm2) for the best performance devices were also recorded for the reliability study.

interface modification according recent research (Jiang et al., 2016). The above analyses manifest that a smooth, highly-crystalized and δphase free FA-perovskite layer is obtained through NMP-additive method under ambient-air fabrication. The photogenerated carrier behavior within different FA-perovskite films was characterized by the PL measurements. In the steady-state PL spectra (Fig. 2a), a significantly enhanced peak intensity indicates that the nonradiative recombination is efficiently suppressed through NMPadditive method, which may originate from the reduced defect density and trap centers due to the improved film quality. Fitting results of the time-resolved PL spectra (Fig. 2b) with biexponetial decay are consistent with the steady-state PL analysis (Table S1) (Zeng et al., 2017). The bulk lifetime is determined to be 297.8, 142.5 and 552.2 ns for layers prepared by none-additive-, DMSO additive- and NMP-additiveproduced films methods, respectively. We assessed the trap density (ntrap) of corresponding FA-perovskite films using space-charge-limited current (SCLC) measurement (Li et al., 2017; Niu et al., 2018; Wang et al., 2018). Fig. 2c and Fig. S1 shows the current density–voltage (J–V) characteristics of electron-only devices (FTO/c-TiO2/FA-perovskite/PCBM/Ag) based on films from different additives. Three distinctive stages can be identified on J–V plots in the double logarithmic coordinate system, indicative of a linear ohmic response at low bias (slope n = 1, grey region), a trap-filling limited (TFL) regime at intermediate bias (slope n > 3, red regime) and a SCLC regime at high bias (slope n = 2, blue region) (Niu et al., 2018). The knee-point voltage between the ohmic regime and the TFL regime is known as the TFL voltage (VTFL), which is determined by the ntrap as the following equation (Bube, 1962): VTFL = en trap d 2/2 0 r , where e is the elementary charge, d is the thickness of the film, ε0 is the vacuum permittivity and the εr is the relative dielectric constant. The calculated ntrap is determined to be 6.3 × 1015, 1.2 × 1016, 3.8 × 1015 cm−3 for none-, DMSO- and NMP-additive methods respectively, demonstrating an obvious decrease in trap density of film fabricated from NMP-additive route. The improved photogenerated carrier behavior of FA-perovskite films facilitates a better device performance as confirmed by the electronic measurements. Charge transport processes of the photovoltaic

3. Results and discussion 3.1. Characterizations on FA-perovskite films and PSCs Fig. 1 displays the morphologies and phase compositions of airfabricated FA-perovskite films produced from none-additive, traditional DMSO additive and NMP additive. The SEM images are compared in Fig. 1a. Whereas rough surfaces with impurity inclusion are observed in films from none-additive and DMSO-additive methods, smooth and dense film composed of micrometer-scale grains can be produced with NMP-additive method. The XRD spectra of resultant films are compared in Fig. 1b. The peaks at 14.0°, 19.8°, 24.4°, 28.2°, 31.5°, 34.6°, 40.2° and 42.7° in all spectra are indexed to (1 0 0), (1 1 0), (1 1 1), (2 0 0), (2 1 0), (2 1 1), (2 2 0) and (2 2 1) of cubic α-FAPbI3 with Pm3¯m symmetry, respectively (Liu et al., 2017; Weller et al., 2015). Despite of obtaining desirable α-FAPbI3 from all investigated methods, the reduced full width at half maximum (FWHM) of (1 0 0) peak, from 0.333 and 0.402 of none-additive and DMSO additive to 0.216 of NMP additive demonstrates the enhanced α-FAPbI3 crystallization from NMP-additive method. Using Scherrer analysis, the crystallite sizes of FA-perovskite films made from none-additive, DMSO and NMP routes are estimated to be 36.6 ± 0.7, 28.9 ± 0.4, and 52.1 ± 3.6 nm, respectively (Zhang et al., 2015). Normally, the crystallite size calculated by Scherrer equation is an estimation of crystallite dimension perpendicular to the substrate, which is much smaller than the grain size derived from the plane view in SEM analysis due to the thickness limit. While these two results confirm the same trend that the NMP method enhances the crystallinity apparently. As the common byproduct in ambient-air fabrication, the δ-FAPbI3 impurity indicated by peak of 11.9° is resident in films produced from none-additive and DMSO-additive methods (Liu et al., 2017; Z. Wang et al., 2015; Xu et al., 2018). Only films obtained through NMP additive can eliminate the δ inclusion. To note, small trace of residual PbI2 (12.7°) in perovskite films is seen as favorable to 149

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Fig. 2. (a) Steady-state and (b) time-resolved PL spectra of FA-perovskite films produced from different additives. (c) Current density–voltage (J–V) characteristics of electron-only device (FTO/c-TiO2/FA-perovskite/PCBM/Ag) fabricated from NMP route. (d) Nyquist curves of the photovoltaic devices (FTO/c-TiO2/meso-TiO2/FAperovskite/Spiro-OMeTAD/Au) prepared with different additives under 0.8 V applied bias in darkness. The inset picture is the equivalent circuit.

of DMSO-additive device to 23.30 mA/cm2 of NMP additive device. The integrated Jsc calculated from the EQE spectra (JIntegrated) shows tolerable variation with Jsc deduced from J–V curves, validating the facticity of photocurrent density (Fig. 3a-b, Table 1). The statistics of photovoltaic parameters of each batch display small standard deviation, confirming the reproducibility of our method (Fig. S2 and Table S3). We also carried out the steady-state efficiency measurement for the as-prepared cells (Figs. 3c and S3). The NMP device delivers a stabilized PCE of 16.69% under a constant bias voltage of 0.82 V, indicative of a stable and efficient device operation under ambient-air condition (Fig. 3c). Moreover, J–V curves for best-performing large-area (1 cm2) devices fabricated based on different additive methods were also recorded. A moderate PCE of 12.16% is obtained for NMP additive-produced device in contrast with low PCEs (< 10%) of those devices fabricated from none-additive and DMSO additive (Fig. 3d and Table S4), suggesting the potential of our NMP-additive route in large-scale fabrication.

devices with FTO/c-TiO2/meso-TiO2/FA-perovskite/Spiro-OMeTAD/ Au architecture were investigated by electrochemical impedance spectroscopy (EIS). Fig. 2d shows the Nyquist curves of the photovoltaic devices prepared with different additives under 0.8 V applied bias in darkness. There are two distinct arcs at high- and low-frequency region in Fig. 2d, which can be assigned to the interface transport and recombination process respectively. Assisted by the equivalent circuit insert in Fig. 2d, we characterized the two processes with the contact resistance (Rco) and recombination resistance (Rrec) correspondingly and listed the calculation results in Table S2. The Rco displays negligible variation among devices from different additives, indicating the comparable interfacial charge transport ability for the as-prepared films. While the value of Rrec deduced from NMP device increased 32.9% and 46.6% from none-additive and DMSO-additive devices. As the Rrec is inversely proportional to the recombination rate, it can be demonstrated that nonradiative recombination process in NMP devices is efficiently suppressed due to lower trap density. We compared the photovoltaic performance of PSCs based on FAperovskite films produced from different additive-based methods with a FTO/c-TiO2/meso-TiO2/FA-perovskite/Spiro-OMeTAD/Au device architecture. A 10-cell batch was fabricated for each additive route and the best-performing cells of respective batch are displayed in Fig. 3 and Table 1. It can be recognized that the best-performing device is achieved by NMP-additive method with a PCE of 17.29%, contrast with the 15.53% of none-additive and 14.01% of DMSO-additive. We first attribute the over 10% PCE enhancement to the suppressed nonradiative recombination due to reduced trap density in NMP films, which is demonstrated in PL, SCLC and EIS analyses. As a result, a larger shunt resistance of 11.59 kΩ cm2 and a significantly enhanced Voc of 1.00 V are obtained in the NMP device. The suppressed shunt current also gives rise to a dramatic enhancement in Jsc, which increases from 22.68 mA/cm2 of none-additive device and 21.32 mA/cm2

3.2. Crystallization analysis The improved film quality assisted by NMP additive can be fundamentally attributed to the distinct interaction between NMP molecules and perovskite precursors, as it affects the types of intermediate formed during the film fabrication (Ahn et al., 2015; Lee et al., 2016; Zhang et al., 2018). The FTIR spectra of bare DMF (common solvent), DMSO and NMP as well as possibly-formed PbI2·X and FAI·PbI2·X complexes, where X is DMF, DMSO or NMP, are demonstrated in Fig. 4 and Fig. S4. In Fig. 4a and b, the stretching vibration peaks of C]O for bare DMF at 1677 cm−1 and bare S]O for DMSO at 1064 cm−1 shift to 1658 cm−1 and 1018 cm−1 upon addition of PbI2, respectively. The decreased stretching vibration frequency can be attributed to the weakened bond strengths of C]O and S]O as a consequence of the formation of 150

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Fig. 3. (a) J–V characteristics and (b) EQE spectra of FA-PSCs fabricated from different additives. (c) Steady-state efficiency measurement for the FA-PSC fabricated from NMP additive. The applied bias voltage for measurement was 0.82 V. (d) J–V curves of the large-area (1 cm2) FA-PSCs produced from different additives.

PbI2·DMF and PbI2·DMSO complexes (Lee et al., 2018; Lee et al., 2016). However, the vibration frequency is scarcely changed upon further addition of FAI, indicating that the interactions between FAI and DMF/ DMSO are negligible. In contrast, the stretching vibration of C]O for bare NMP at 1690 cm−1 exhibits successive shifts to 1674 cm−1 upon reaction with PbI2 alone and to 1654 cm−1 with further addition of FAI, suggesting that both PbI2·NMP and FAI·PbI2·NMP complexes can be formed with NMP additive. The types of intermediates induced from different additive-precursor interactions are identified in Fig. 5. For the intermediate film deposited without additive (Fig. 5a), the XRD pattern coincides with that of pure δ-FAPbI3 with hexagonal P63mc symmetry (Fig. S5). In Fig. 5b, the intermediate film produced from DMSO additive yields a dramatically intensified peak at 11.9°, indicative of developing highlytextured δ-FAPbI3 along (1 0 0) direction (Liu et al., 2017). As for NMPinduced intermediate (Fig. 5c), the main diffraction peak shifts ∼0.7° to lower degree (Fig. S6), suggesting an expansion in the crystal cell. A rational reason for this shift can be the formation of FAI·PbI2·NMP intermediate, within which the strong interaction between NMP and FAPbI3 precursors weakens the original bonding and results in an enlarged cell. Refine analysis of low-degree regions (2θ < 10°) provides the information of the possibly-formed PbI2·X or FAI·PbI2·X complexes (Ahn et al., 2015; Jeon et al., 2014; Liu et al., 2017; Wu et al., 2017). In Fig. 4d, intermediate without additive shows no peaks in the low-degree range, coinciding with the negligible interaction within FAI and

DMF and the poor stability of PbI2·DMF complex (Ahn et al., 2015). The faint low-degree peaks at 9.4° and 9.9° in the intermediate produced from DMSO additive can be assigned to the trace amounts of PbI2·DMSO complex (Fig. 5e) (Cao et al., 2016). In Fig. 5f, except for the weak diffraction of PbI2·NMP phase at 8.1° (Jo et al., 2016), unknown peaks at 8.7° and 9.1° can be only features of the FAI·PbI2·NMP intermediate. In addition, no characteristic peaks of FAI are observed in all XRD spectra (Fig. S7). Thus, the phase composition is determined to be the δFAPbI3 phase, δ-FAPbI3 phase with slight PbI2·DMSO complex and FAI·PbI2·NMP complex with slight PbI2·NMP complex for intermediate films produced from none-additive, DMSO additive and NMP additive, respectively. Photos of these intermediate films reflect the corresponding microstructure features as well (inserts in Fig. 5). Yellow and opaque intermediates from none-additive and DMSO additive resemble the pure δ-FAPbI3 film (insert in Fig. S5), among which the DMSO sample delivers a rougher surface as a consequence of preferred orientation growth. While, a relatively transparent film obtained by NMP additive implies a different intermediate composition. The morphologies of the intermediate films obtained under different fabrication conditions are shown in Fig. 6. The top panels compare the films obtained merely after spin-coating different precursor solutions, which eliminates the interference of antisolvent treatment to disclose the intrinsic nucleation features of the different intermediates. The intermediate film fabricated without additive is composed of scattered δFAPbI3 grains with an average length of ∼ 14 μm (Fig. 6a). Upon

Table 1 Photovoltaic parameters of best-performing FA-PSCs fabricated from different additives. Additive

Jsc (mA/cm2)

Voc (V)

FF (%)

PCE (%)

Rs (Ω cm2)

Rsh (kΩ cm2)

JIntegrated (mA/cm2)

None DMSO NMP

22.68 21.32 23.30

0.97 0.96 1.00

70.62 68.42 74.23

15.53 14.01 17.29

7.03 7.36 6.37

6.00 0.86 11.59

20.23 19.31 21.59

151

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Fig. 4. FTIR spectra for fingerprint regions for (a) C]O, (b) S]O and (c) C]O stretching vibrations of bare DMF, DMSO, NMP and their corresponding complexes.

addition of DMSO, the δ-FAPbI3 grains elongate to ∼28 μm (Fig. 6b). In contrast, FAI·PbI2·NMP intermediate tends to form refined grains with improved surface coverage (Fig. 6c). These intrinsic nucleation features are retained in the antisolvent dripping-assisted nucleation process (bottom panels in Fig. 6). Compared with intermediate from none-additive (Fig. 6d), elongated grains grow in the intermediate film from DMSO additive (Fig. 6e), coinciding with the development of (1 0 0) texture confirmed by XRD patterns (Fig. 5b). In Fig. 6f, uniform film with reduced grain size is obtained due to the fine nucleation of FAI·PbI2·NMP intermediate. Consequently, final film transited from FAI·PbI2·NMP intermediate reveals a more smooth morphology in contrast with those without NMP additive (Fig. 1a). The transition of FAI·PbI2·NMP intermediate under annealing was investigated. For the traditional δ-FAPbI3 intermediate route, it has been proved that the δ- to α-phase conversation is inhibited by the synergistic effect from substrate confine and moisture atmosphere

(Chen et al., 2016; Koh et al., 2013; Z. Wang et al., 2015). Thus, final films fabricated with none-additive and DMSO additive deliver relatively low crystallinity with unconverted δ-FAPbI3 phase (Fig. 1b). As for FAI·PbI2·NMP intermediate, its transition process is investigated in details in Fig. 7. Fig. 7a plots the TGA spectra of FAI·PbI2·NMP intermediate. The obvious weight loss of 13.6% before 144 °C can be attributed to the detachment of NMP molecules, corresponding to its weight ratio of 13.5% in FAI·PbI2·NMP. Additional gentle weight loss up to 200 °C is assigned to the slow evaporation of FAI component under ambient-air heating (Lee et al., 2016). In the middle of the TGA spectrum, a steep slope region indicative of rapid escape of NMP is observed above 100 °C and achieves its maximum rate around the reported δ- to α-phase transition temperature of ∼140 °C (Eperon et al., 2014; F. Wang et al., 2015; Z. Wang et al., 2015). This feature is unique for the FAI·PbI2·NMP intermediate when compared with the negligible weight loss of none-additive sample and very gentle weight loss of DMSO-

Fig. 5. Full range and low-degree region of XRD spectra for intermediate films produced from (a, d) none-additive, (b, e) DMSO additive and (c, f) NMP additive. The inserts are photos of corresponding intermediate films. All scale bars are 1 cm. 152

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Fig. 6. Morphologies of intermediate films produced from intrinsic nucleation (top panels) and antisolvent-assisted nucleation (bottom panels) with (a, d) noneadditive, (b, e) DMSO additive and (c, f) NMP additive. The top panels are optical microscope images of films obtained after merely spin-coating precursor solutions. The scale bars are 25 μm. The bottom panels are SEM images of films assisted by antisolvent dripping treatments. The scale bars are 1 μm.

additive sample at the same temperature range (Fig. S8). We correlate this process with the phase composition obtained at different annealing temperature in Fig. 7b, where the relative α-FAPbI3 fraction was estimated by the identical peak intensity in XRD spectra (Fig. S9) (Ma et al., 2018). A dramatically increased α-FAPbI3 fraction is observed when elevating the annealing temperature from 100 to 140 °C. Apparently, the detachment of NMP molecules triggers the transition of FAI·PbI2·NMP intermediate to α-FAPbI3. Annealing at low temperature results in a low fraction of α-FAPbI3 due to the thermodynamic disadvantage. When annealing around the phase transition point (∼140 °C), the intermediate directly converts to α-FAPbI3, averting the undesirable δ- to α-phase transition route. The evolution of films at different annealing temperatures was also monitored by the absorption spectra and SEM images (Fig. 7c and d). With elevating the annealing temperature from 100 to 140 °C, an enhanced absorbance in visible region is observed (Fig. 7c). The grains in the as-prepared films endure a gradual Ostwald ripening process with increasing temperature, growing from nanometer-scale at intermediate stage (Fig. 6f) to micrometer-scale grains at 140 °C (Fig. 7d). While higher annealing temperature leads to a reduced absorbance (Fig. 7c), which can be ascribed to the enlarged void in films due to the loss of solute as evidently shown

by SEM images (Fig. 7d). The optical band gaps of as-prepared films were calculated via the absorption edges in Fig. 7c (Fig. S10 and Table S5). A band gap of 1.48 eV is obtained at 140–160 °C, comparing with the band gap of 1.49 eV obtained under lower or higher annealing temperature. Thus improved light harvesting at near-infrared band can be achieved with appropriate heating treatment. It can be concluded that annealing at 140 °C makes the direct formation of α-FAPbI3 at its thermodynamically favorable temperature, averting the traditional incomplete δ- to α-phase transition process and avoiding significant solute loose, finally fulfilling the high-quality FA-perovskite fabrication under ambient air. It is interesting that the none-interacting DMSO additive induces an adverse impact on the ambient-air fabrication of FA-perovskite. We speculate that DMSO additive of high boiling point and low vapor pressure slows down the evaporation of liquid and amplifies the impact of moisture under ambient-air manipulation. Investigations on NMPadded precursor solutions with a series of diluted concentrations (1.6, 1.4, 1.2, 1.0 M) confirm this assumption. Poorer films with increasing fractions of δ-FAPbI3 and PbI2 are obtained with lower concentration precursor solution with NMP additive (Fig. S11).

Fig. 7. (a) TGA of the FAI·PbI2·NMP intermediate powders. (b) Plot of α-FAPbI3 phase fraction in FA-perovskite film produced from NMP additive dependence on annealing temperature. The phase fraction was qualitatively estimated by the identical peak intensity in XRD spectra. (c) Absorption spectra and (d) SEM images of FA-perovskite films produced from NMP additive under different annealing temperatures. The scale bars are 2.5 μm.

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Fig. 8. XRD spectra of FA-perovskite films fabricated with (a) none-additive, (b) DMSO additive and (c) NMP additive under 30-day storage. The inserts are water contact angles on the respective films. (d) Long-term stability of the unencapsulated FA-PSCs from different additives.

3.3. Stability measurement

intermediate can form as a consequence of strong interaction between NMP additive and FAPbI3 precursor, by which the nucleation and growth of the perovskite films are modulated obviously. The fine nucleation of this FAI·PbI2·NMP intermediate relieves the surface roughness of usual air-processed layers. The rapid detachment of NMP molecules from intermediate under annealing results in a direct formation of α-FAPbI3, averting the indirect and incomplete δ- to α-phase conversion in the traditional route. Due to the improved film stability, a PCE of 13.55% is still retained after 30-day storage for unencapsulated PSC. The simple method developed in this work promises a prospect of facile fabrication for FA-perovskite in atmospheric circumstance. Moreover, the detailed crystallization process revealed in this work deepens the understanding of useful intermediate-modulated crystallization strategy for PSC ambient-air fabrication.

The stability of films fabricated from different additives and the resultant PSCs are compared in Fig. 8. As shown in Fig. 8a–c, the degeneration of FA-perovskite films is mainly consisted of decomposition to PbI2 phase (12.7°) and degradation to δ-FAPbI3 phase (11.9°). These two processes are progressively proceeding in the films from none-additive during 30-day storage (relative humidity < 20%) (Fig. 8a). In DMSO samples, a large-scale conversion to δ-FAPbI3 occurs after a week and a nearly completely degraded to δ-FAPbI3 occurs in 30 days (Fig. 8b). For films from NMP additive, only decomposition to PbI2 is obvious and the conversion to δ-FAPbI3 is negligible (Fig. 8c). The different degeneration behaviors can be ascribed to the different quality of initial films. Dense film produced from NMP additive is more resistant to moisture with large water contact angles (Fig. 8a–c inserts) and has less conversion sites due to the elimination of δ-FAPbI3, resulting in an improved stability (Zeng et al., 2017). The PCE decays of unencapsulated devices produced from different additives are shown in Fig. 8d. Consistent with the FA-perovskite films degradation analysis, slow decay of FA-perovskite allows a 13.55% of PCE retaining for the NMP device after 30-day storage, higher than 10.4% of the none-additive and completely deteriorated DMSO ones.

Acknowledgements The authors gratefully thank the support of the National Nature Science Foundation of China (No. 51402308). This work was also supported by the Young Talent Program of Shenyang National Laboratory for Materials Science (No. 2017FP29). Appendix A. Supplementary material

4. Conclusion

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.solener.2019.05.033.

In this work, a facile method is developed to fabricate high-quality FA-perovskite layers in ambient air with NMP additive. Smooth, highlycrystalized and δ phase-free films are obtained under humidity of ∼40%. The reduction of nonradiative recombination within layers produced from NMP route benefits the photovoltaic performance of the resultant PSCs. A best efficiency of 17.29% is achieved for the airprocessed FA-PSCs, ∼10% and ∼20% increasing from PSCs produced from none-additive and traditional DMSO additive, respectively. The microstructure analysis demonstrates that a distinctive FAI·PbI2·NMP

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