Enhanced optoelectronic quality of perovskite films

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Apr 26, 2017 - photoemission spectroscopy shows that no metallic Pb0 exists in the perovskite film and the I/Pb ... exciton quenching for perovskite light-emitting diodes [28]. However, MAPbBr3 film ... (b) Architecture of mesoscopic perovskite solar cells. ..... [1] Kim H S et al 2012 Lead iodide perovskite sensitized all-solid-.
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Enhanced optoelectronic quality of perovskite films with excess CH3NH3I for high-efficiency solar cells in ambient air

This content has been downloaded from IOPscience. Please scroll down to see the full text. 2017 Nanotechnology 28 205401 (http://iopscience.iop.org/0957-4484/28/20/205401) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 115.236.14.176 This content was downloaded on 27/04/2017 at 01:38 Please note that terms and conditions apply.

Nanotechnology Nanotechnology 28 (2017) 205401 (10pp)

https://doi.org/10.1088/1361-6528/aa6956

Enhanced optoelectronic quality of perovskite films with excess CH3NH3I for high-efficiency solar cells in ambient air Yunhai Zhang1, Huiru Lv1, Can Cui1,4, Lingbo Xu1, Peng Wang1, Hao Wang1, Xuegong Yu2,4, Jiangsheng Xie2, Jiabin Huang2, Zeguo Tang3,4 and Deren Yang2 1

Center for Optoelectronics Materials and Devices, Department of Physics, Zhejiang Sci-Tech University, Hangzhou 310018, People’s Republic of China 2 State Key Lab of Silicon Materials and School of Materials Science & Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China 3 Ritsumeikan Global Innovation Research Organization, Ritsumeikan University, Nojihigashi, Kusatsu, Shiga 525-8577, Japan E-mail: [email protected], [email protected] and [email protected] Received 15 February 2017, revised 19 March 2017 Accepted for publication 27 March 2017 Published 26 April 2017 Abstract

Solution-processed polycrystalline perovskite films contribute critically to the high photovoltaic performance of perovskite-based solar cells (PSCs). The inevitable electronic trap states at grain boundaries and intrinsic defects such as metallic lead (Pb0) and halide vacancies in perovskite films cause serious carrier recombination loss. Furthermore, the film can easily decompose into PbI2 in a moist atmosphere. Here, we introduce a simple strategy, through a small increase in methylammonium iodide (CH3NH3I, MAI), molar proportion (5%), for perovskite fabrication in ambient air with ∼50% relative humidity. Analysis of the morphology and crystallography demonstrates that excess MAI significantly promotes grain growth without decomposition. X-ray photoemission spectroscopy shows that no metallic Pb0 exists in the perovskite film and the I/Pb ratio is improved. A time-resolved photoluminescence measurement indicates efficient suppression of non-radiative recombination in the perovskite layer. As a result, the device yields improved power conversion efficiency from 14.06% to 18.26% with reduced hysteresis and higher stability under AM1.5G illumination (100 mW cm−2). This work strongly provides a feasible and low-cost way to develop highly efficient PSCs in ambient air. Supplementary material for this article is available online Keywords: perovskite solar cells, CH3NH3PbI3, excess methylammonium iodide, metallic lead, ambient air, stability (Some figures may appear in colour only in the online journal) (>1 μs) [3, 4] and high carrier mobility (∼10 cm2 V−1 s−1) [5]. The band-gap of perovskite is reported to be tailored in the range from the ultraviolet to infrared region through varying the components [6], and the bonding energy of the Wannier excitons is below 25 meV [7]; therefore, the excitons can easily dissociate into free charges for the cost of a tiny thermal dynamic drive [8]. With these intriguing properties, the theoretical limited efficiency of perovskite solar cells

Introduction The organic–inorganic hybrid perovskite has emerged as one of the most promising materials for photovoltaic applications due to its remarkable optoelectronic properties, such as large optical absorption coefficient [1, 2], long carrier lifetime 4

Authors to whom any correspondence should be addressed.

0957-4484/17/205401+10$33.00

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(PSCs) is predicted to be 31% and a maximum certified power conversion efficiency (PCE) of 22.1% has recently been achieved [9, 10], indicating the great potential of the PSC device in the future photovoltaic industry. To date, most PSCs are based on methylammonium lead iodide (MAPbI3, where MA is CH3NH3+) with a solution filming process [11–13]. Careful control of perovskite filming and the crystallization process is crucial for high efficiency devices. Due to the sensitive chemical properties of perovskite [14], pinholes are easily generated in MAPbI3 films under moist conditions during the decomposition of MAPbI3 via a MAPbI3·H2O intermediate, retaining PbI2 in the film [15, 16]. This irreversible morphological feature and imbalance of the I/Pb molar ratio are fatal for charge transport. Thus, most fabrication processes are strictly controlled under dry or indoor conditions to avoid decomposition of the MAPbI3 films [17–19]. On the other hand, solution-processed perovskite film is polycrystalline and the inevitable grain boundaries cause carrier recombination loss [8, 20], which might be the main reason for the current–voltage hysteresis behavior in PSCs [21, 22]. Previous works utilized additions such as hydriodic acid (HI) [19], hydrochloric acid (HCl) [23], sulfoxide (DMSO) [24] and 1, 8-diiodooctane (DIO) [25] to inhibit perovskite crystallization in order to form large grains. Other reports have focused on suppressing nonradiative recombination at the grain boundaries with a fullerene/perovskite heterojunction film [21] or passivating trap states by adding thiophene and pyridine [26, 27]. Besides forming compact and large grain-sized polycrystalline perovskite films, it is very important to control the inner traps in bulk perovskite. The intrinsic metallic Pb0 and halide vacancies in the perovskite film, which dominate the fast photoluminescence (PL) quenching, can emerge even if the precursor species of PbI2 and MAI are mixed with a stoichiometric ratio of 1:1 [22, 28, 29], especially in ambient air [30]. This would lower the quasi-Fermi level of perovskite films, resulting in a reduced open-circuit voltage (Voc) of the photovoltaic device. Several reports have tried to solve this problem by adding hypophosphorous acid (HPA) [31] in the precursor solution or using a polymer-templated nucleation method with poly(methyl methacrylate) (PMMA) [32]. Recently, Himchan Cho et al have applied a small increase of methylammonium bromide (MABr) molar proportion into the perovskite precursor to suppress metallic Pb0 and inhibit the exciton quenching for perovskite light-emitting diodes [28]. However, MAPbBr3 film exhibits low absorption intensity and decreased grain size, which are not favorable for PSCs. On the other hand, these strategies are limited in to a glove box or dry air, which requires precise conditions and thus increased manufacturing costs. To date, there is still a lack of fast, facile methods to suppress the inner traps and obtain high-quality perovskite film in an ambient atmosphere. In this work, we report a simple and effective strategy to fabricate large grain-sized MAPbI3 film in ambient air (with ∼50% relative humidity, RH) by adding excess MAI into the precursor solution.

Morphology analysis demonstrates that excess MAI profits the grain growth without decomposition or pinholes. Additionally, it effectively improves the stoichiometry ratio of I/ Pb and eliminates the metallic Pb0 in the film. PSCs fabricated with an optimization of excess MAI (5% molar ratio) and annealing time (20 min) exhibit much higher and more reproducible photovoltaic performance, with smaller hysteresis and more enhanced stability than the control devices.

Results and discussion Figure 1(a) schematically illustrates the synthesis process of perovskite films in ambient air using a modified one-step method [33], in which a slight excess of MAI is added to the precursor solution. Figure 1(b) represents the schematic architecture of mesoscopic PSCs that are employed to investigate the effect of excess MAI on the device performance. The corresponding cross-sectional SEM image is shown in figure 1(c), where layers of glass/FTO/compact TiO2 (∼40 nm)/mesoscopic TiO2 (∼150 nm)/MAPbI3 (∼450 nm)/spiro-oMeTAD (∼200 nm)/Au electrode (∼100 nm) are clearly observed. Figures 2(a) and (b) show top-view SEM images of the as-prepared perovskite films. The control sample prepared with MAI: PbI2=1:1 consists of irregular perovskite crystals and a few pinholes (figure 2(a)). It has been reported that the formation of these pinholes is attributed to the decomposition of perovskite films via a MAPbI3·H2O intermediate in a moist atmosphere [34, 35]. As shown in figure 2(b), the perovskite film prepared with MAI: PbI2=1.05:1 appears more homogenous and possesses larger grain size than the control sample. Furthermore, the corresponding surface roughness decreases from 8.5 nm to 5.5 nm (figure S1 is available at stacks.iop.org/NANO/28/205401/mmedia). The corresponding crystal structures of MAPbI3 perovskite films were analyzed by x-ray diffraction (XRD) patterns in figure 2(c). The peaks located at 14.1°, 28.4° and 31.8° are assigned to the (110), (220) and (310) crystal planes of the tetragonal crystal phase of the perovskite [36, 37], respectively. The peak at 12.6° is related to the PbI2 impurity phase, which appears in the control sample but disappears in the perovskite film with MAI: PbI2=1.05:1. The full width at half-maximum (FWHM) of the (110) peak of the perovskite decreases from 0.215° to 0.185° while the intensity increases as the MAI: PbI2 ratio increases from 1:1 to 1.05:1, suggesting that slightly excess MAI can induce higher crystallization of the perovskite film. The morphology of the perovskite film prepared with excess MAI depends on the subsequent annealing time in ambient air, and a moisture-assisted grain growth process [38] is found over an extended annealing time from 10 to 30 min (figure S2). However, on increasing the annealing time up to 30 min, the PbI2 impurity phase is observed as a result of the decomposition of MAPbI3 (figure S3). Figure 2(d) shows the UV–visible absorption spectra of the perovskite films. In comparison with the control sample, the perovskite films prepared with excess MAI demonstrate an improved absorption

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Figure 1. (a) Fabrication processes of perovskite films without and with excess MAI. (b) Architecture of mesoscopic perovskite solar cells.

(c) Corresponding cross-sectional SEM image of the device.

Figure 2. Top-view SEM images of the perovskite films: (a) MAI: PbI2=1:1, (b) MAI: PbI2=1.05:1. (c) XRD patterns of the perovskite films. (d) UV–visible absorption spectra of the perovskite films on the mesoscopic TiO2 substrate.

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Figure 3. Photovoltaic parameters of perovskite solar cells based on films with varied MAI: PbI2 ratio and annealing time: (a) PCE, (b) Voc, (C) Jsc, (d) FF. The left solid boxes show the values obtained from the forward scan direction and the right dashed boxes present the results measured from the reverse scan direction.

intensity, which would allow the absorption of more photons for the generation of charge carriers [37]. The influence of excess MAI and annealing time on the photovoltaic performance of PSCs was investigated. The statistics of PCE, Voc, short-circuit current density (Jsc) and fill factor (FF) are shown in figure 3, and the detailed parameters are summarized in table S1. As displayed in figure 3(a), the optimized control devices with MAI: PbI2=1:1 demonstrate an average PCE of 13.03% in the forward voltage scan direction, while they exhibit anomalous hysteresis behavior with a relatively low average PCE of 9.92% in the reverse voltage scan direction. For the devices based on the MAPbI3 perovskite layer with MAI: PbI2=1.05:1, the average PCE is higher than that of the control device, regardless of the evolution of annealing time. The PCE in the forward scan direction reaches the highest value of 17.84% for devices based on the perovskite film with an annealing time of 20 min. The hysteresis behavior of the devices is also obviously supressed, with an average PCE of 17.39% in the reverse voltage scan direction (table S1). Further extending the annealing time to 30 min leads to a decline of device performance with an average PCE of 16.65% accompanied by an increasing hysteresis behavior due to I− ion migration in the bulk perovskite film [39]. Combined with the analysis of SEM and XRD in figures S2 and S3, this efficiency decrease is likely related to the decomposition of the perovskite films. In contrast, for the device with 20 min annealing time, the additional MAI was

able to reduce the device hysteresis owing to the suppression of I− ion migration. Voc, Jsc and FF are shown in figures 3(b)– (d), respectively, and they exhibit a similar tendency to the PCE in figure 3(a). In addition, the effects of larger amounts of excess MAI (10% and 20% molar ratio) on the growth of perovskite films and device performance were investigated and the results are displayed in figure S4. Although large grain-sized perovskite films are obtained with higher MAI content (figures S4(a) and (b)), the corresponding PSCs demonstrate relatively lower photovoltaic performance, and a longer annealing time is required for the perovskite film to reach maximum efficiency (figure S4(c)). In order to further explore the mechanism of excess MAI (5%) on the improvement of photovoltaic performance and the suppression of hysteresis behavior, time-resolved photoluminescence (TRPL) spectra were measured and are presented in figure 4. The TRPL decay curves of the MAPbI3 perovskite films on glass at 775 nm emission peak were fitted to determine the characteristic time constants with a biexponential decay function: I (t ) = A1 ⋅ exp ( - t t1) + A2 ⋅ exp ( - t t2) + y0

(1 )

in which τ1(τ2) represents the time constant of the fast (slow) decay process [40]. The fast decay (τ1) component is attributed to non-radiative recombination by trap states in perovskite films and the slow decay (τ2) component is associated with recombination through radiative channels from the bulk perovskite. For the control perovskite film (figure 4(a)), τ1 is 4

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Figure 4. Time-resolved PL spectra for the MAPbI3 perovskite films on glass: (a) control perovskite films; (b), (c), (d) perovskite films (MAI:

PbI2=1.05:1) with different annealing time of 10, 20 and 30 min, respectively. Filled circles represent measured data and solid lines represent the corresponding fitting results.

2.9 ns with a fraction weight of 59.5% and τ2 is 25.4 ns with a fraction weight of 40.5%, suggesting that the non-radiative recombination caused by the large density of electronic trap states dominates the reduction of photo-generated charges. For perovskite films with MAI: PbI2=1.05:1, the value of τ2 is much higher than that of the control film, and it rises to a maximum value of 101.8 ns with an improved fraction weight of 89.25% when the annealing time is 20 min. Although τ1 (7.2 ns) of perovskite films with MAI: PbI2=1.05:1 is larger than that of the control perovskite film, its fraction weight dropped significantly from 59.5% to 10.75%. This indicates the effective suppression of non-radiative recombination in perovskite films by adding excess MAI [21, 28]. The corresponding steady-state PL spectra in figure S5 also verified the improved quality of MAPbI3, where the PL intensity of the MAPbI3 perovskite film with excess MAI (5%) and 20 min annealing is twice as high as that of the control sample [41–43]. Previous attempts to use excess ammonium species to obtain large grain-size perovskite films or to form a MAI layer at the grain boundaries to reduce recombination have been reported [12, 44–46], while the effect on inner traps such as Pb0 and I/Pb ratio has not been extensively discussed. To clarify the origin of the enhanced carrier lifetime in this work, x-ray photoemission spectroscopy (XPS) analysis was carried out to map the chemical changes in the MAPbI3 layers. In

figure S6, the peaks of I (∼619 and 613 eV), Pb (∼138 and 143 eV), C (∼285 eV) and N (∼413 eV) in the survey spectra agree with the values in previous reports [28, 30]. In figure 5, the systematic deconvolution of Pb 4f (figures 5(a) and (c)) and I 3d (figures 5(b) and (d)) spectra into summations of Lorentzian–Gaussian curves reveals the nature of the chemical bonds in MAPbI3. The strong peaks at 138.1 and 142.9 eV for the control perovskite film (figure 5(a)) correspond to the Pb 4f7/2 and Pb 4f5/2 levels, respectively. Each of these peaks is associated with a smaller peak (marked with a star) that is shifted 1.70 eV to the lower binding energy; these smaller peaks are ascribed to metallic Pb0 [28]. It is obvious that these peaks disappear in figure 5(c), indicating that the generation of metallic Pb0 has been suppressed when the ratio of MAI: PbI2 increases from 1:1 to 1.05:1. In general, metallic Pb0 can act as a recombination center and thus cause PL quenching and decrease the PSC’s performance [22, 28–30]. The I/Pb ratio in the MAPbI3 perovskite films is calculated from the XPS data and it increases from 2.51 to 2.91 as the ratio of MAI:PbI2 increases from 1:1 to 1.05:1. The I/Pb ratio (2.91) for the perovskite film with MAI: PbI2=1.05:1 is very close to that of the stoichiometric MAPbI3 (I/Pb=3) [30, 47]. Therefore, the XPS analysis reveals that the defect states of metallic Pb0 and the imbalance of the I/Pb ratio, which are also indispensable factors for the improvement of carrier lifetime and device performance, have been suppressed by 5

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Figure 5. High-resolution XPS spectra of Pb 4f and I 3d for the perovskite films. (a) Pb 4f and (b) I 3d for perovskite films with MAI:

PbI2=1:1; (c) Pb 4f and (d) I 3d for perovskite films with MAI: PbI2=1.05:1. Peaks marked with stars represent metallic Pb0.

adding slightly excess MAI. It has been reported that excess MAI would occupy the grain boundaries of perovskite films when the deposition process was carried out under dry conditions [12]. In our work, the perovskite films were prepared under humid conditions with 50% RH. According to the XRD patterns of perovskite films with different annealing times (figure S1), as well as previous reports about the chemical reaction between MAPbI3 and H2O [34, 35, 38], we infer that the excess MAI at the film surface might partially react with H2O and decompose into MA and HI during annealing, protecting the bulk perovskite film from decomposition. Meanwhile, some MAI remains at the grain boundaries of the bulk perovskite film to suppress non-radiative recombination [12]. Therefore, we conclude that slightly excess MAI and an appropriate annealing time can assist the grain growth of high-quality MAPbI3 perovskite films without decomposition and thus give rise to higher carrier lifetime. The current density–voltage (J–V ) curves of champion PSC devices measured under AM 1.5G conditions (100 mW cm−2) are shown in figure 6(a) and the corresponding parameters are listed in table 1. The reference PSC device based on the control perovskite film yields a PCE of 14.06% with Voc of 1.05 V, Jsc of 19.59 mA cm−2 and FF of 68.37% in the forward direction. The champion PSC device with MAI: PbI2=1.05:1 shows an improved PCE of 18.26% with Voc of 1.11 V, Jsc of 21.62 mA cm−2 and FF of 76.09% under the same voltage scan direction and sweeping rate. As discussed above, the control perovskite film demonstrates fast PL quenching due to non-radiative recombination by

detrimental trap states. These trap states lower the quasiFermi level of the perovskite films, resulting in a reduced Voc and FF. However, with excess MAI, this recombination has been substantially suppressed. Therefore, Voc and FF are obviously enhanced from 1.05 to 1.11 V and from 68.37% to 76.09%, respectively. The J–V curves of the PSC devices measured in the reverse voltage scan direction are also shown in figure 6(a). It is apparent that excess MAI suppresses the hysteresis behavior to a large extent [22]. The improvement of Jsc is further confirmed by external quantum efficiency (EQE) and integrated Jsc, as shown in figure 6(b). The PSC device with MAI: PbI2=1.05:1 exhibits enhanced EQE values compared to the reference PSC with MAI: PbI2=1:1 in a broad-spectrum range from 300 to 800 nm. The value of integrated Jsc calculated from the EQE spectrum is also improved from 18.8 to 20.7 mA cm−2, in agreement with the value obtained from the J–V measurement. In addition, to obtain a reliable and stable PCE as suggested by Snaith and coworkers [48], the steady-state current densities of the devices were measured at the maximum power point for 100 s. Figure 6(c) shows the Jsc and PCE of the PSC devices as a function of irradiation time. The PSC device with MAI: PbI2=1.05:1 measured under a bias voltage of 0.905 V exhibits a stabilized Jsc of 20.19 mA cm−2, and yields a stabilized PCE of 16.9%, which is slightly lower than that from the J–V result (figure 6(a)). In contrast, the control PSC device with MAI:PbI2=1:1 measured under a bias voltage of 0.836 V demonstrates a stabilized PCE of 11.9% and a relatively lower Jsc of 16.67 mA cm−2 in comparison with the 6

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Figure 6. (a) J–V curves of the champion PSCs based on perovskite films of varied MAI: PbI2 ratio measured at different voltage scan directions under AM1.5G illumination. (b) EQE spectra and the integrated Jsc of the champion PSCs. (c) Steady-stable test of Jsc and PCE for the PSCs. (d) Statistic histogram of the PCE from 50 pieces for each group of PSCs.

It has been confirmed that the mesoscopic structured PSC possesses better stability than the planar device [44, 46, 49]. However, the quality of the perovskite films would also affect the device stability. In this work, we tested unpackaged devices in ambient air (humidity ∼30%) for 100 h under AM1.5G light soaking. Figure 7 shows the normalized device parameters of Voc, Jsc, FF and PCE as a function of light soaking time in ambient air. It is obvious that PSCs suffer inevitable degradation under light soaking. However, the device with excess MAI maintains better device stability than the control sample. With excess MAI, the device can retain over 80% of its original efficiency, while the control device only retains 70% of its initial efficiency. It can be inferred that the control device with pinholes in the perovskite film could enable moisture and oxygen to enter the device layers and decrease the device stability. Also, the retained PbI2 may contribute to the instability of the device under light soaking [50]. In contrast, the adding of excess MAI dramatically reduces PbI2 and pinholes and thus suppresses degradation of the device.

Table 1. Characteristics of photovoltaic parameters of the champion

PSCs in different scan directions. Forward: from 1.4 to 0 V; reverse: from 0 to 1.4 V. MAI: PbI2

Direction

Voc (V)

JSC (mA cm−2)

FF (%)

PCE (%)

1:1

Forward Reverse Forward Reverse

1.05 0.95 1.11 1.09

19.59 18.89 21.62 22.05

68.37 52.15 76.09 70.79

14.06 9.35 18.26 17.01

1.05:1

values deduced from the J–V curve due to the severe hysteresis. Moreover, the PSC device with excess MAI induces a faster rising photocurrent to its maximum value during immediate light illumination than that of the control device (figure 6(c)). The similar values of the steady-state PCE to the efficiency deduced from the J–V curve, the fast photocurrent response and the prompt photocurrent saturation in the PSC device with MAI: PbI2=1.05:1 further confirm the suppression of the electronic trap states in the perovskite film with excess MAI. Figure 6(d) shows a statistical histogram of the PCE from 50 pieces of PSC devices. It demonstrates that half of the devices with 5% excess MAI deliver a PCE over 17.5%, compared with 12.9% of the control devices. Thus, the strategy of adding a slight amount of excess MAI into the precursor solution ensures excellent reproducibility of highperformance devices in ambient air.

Conclusions In summary, we have successfully developed a simple strategy to improve the optoelectronic quality of MAPbI3 perovskite films in ambient air (∼50% RH) by adding a slight 7

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Figure 7. Stability of normalized parameters of perovskite solar cells with and without excess MAI after 100 h light soaking in ambient air:

(a) VOC, (b) JSC,(c) FF and (d) PCE.

excess of MAI (5%) into the precursor solution. This strategy promotes the growth of high-quality perovskite films with large grain size. The excess MAI can effectively eliminate the formation of metallic Pb0, improve the ratio of I/Pb and reduce the non-radiative recombination of photo-generated charges in perovskite films. As a result, in comparison with the control PSC with MAI: PbI2=1:1, Jsc, Voc and FF of PSCs with MAI: PbI2=1.05:1 are improved from 19.59 to 21.62 mA cm−2, from 1.05 to 1.11 V and from 68.37% to 76.09%, respectively. Therefore, the ambient-processed PSC devices achieve a maximum PCE of 18.26% under a standard illumination condition (AM 1.5G, 100 mW cm−2). Moreover, the slight excess of MAI effectively suppresses the hysteresis behavior and improves the stability and reproducibility of the PSC devices. This work provides a feasible and low-cost method to develop highly efficient PSCs in ambient air.

99.8%, Sigma-Aldrich). For perovskite films prepared by the modified one-step method, excess MAI (5%–20%, molar ratio) was added to the same perovskite precursor solution. This solution was first spin-coated on TiO2 electron transport layers at 1000 rpm for 5 s and 5000 rpm for 20 s, and then 0.6 ml antisolvent of diethyl ether was drop-casted rapidly on the rotating substrate at the 20th second. After spinning, the control perovskite film was heated at 70 °C for 1 min and then 100 °C for 10 min. The perovskite films with excess MAI were heated at 70 °C for 1 min followed by 100 °C for 10∼30 min. Device fabrication

The mesoscopic PSCs were fabricated in ambient air and the device structure is illustrated in figure 1(b). Specifically, fluorine-doped tin oxide (FTO) glass (Nippon Sheet Glass Co., Japan) with a sheet resistance of 8 Ω−2 and an optical transmission greater than 80% in the visible range was etched with 2 mol l−1 HCl and zinc powder. Then the FTO glass was cleaned sequentially with HellmanexTM detergent, deionized water, acetone and ethanol for 10 min in an ultrasonic bath; subsequently it was dried by N2 flow and cleaned with UVozone for 30 min. A compact TiO2 layer was deposited on the cleaned FTO by spray pyrolysis at 550 °C with 0.25 M Ti(IV) bis(ethylacetoacetal)-diisopropoxide in ethanol solution. After that, a mesoscopic TiO2 layer was spin-coated on the compact TiO2 with commercialized TiO2 paste (0.1 g ml−1 in ethanol) and then dried at 125 °C to form a substrate, which was annealed at 550 °C for 1 h. The perovskite films were

Experimental details Perovskite layer preparation

Figure 1(a) schematically illustrates the one-step synthesis process of the perovskite film in ambient air (∼50% RH) with diethyl ether as anti-solvent. For the control perovskite film (MAI: PbI2=1:1), the precursor solution was prepared with 461 mg PbI2 (99%, Sigma-Aldrich), 159 mg CH3NH3I (99.5%, Xi’an Polymer Light Tech.) and 71 μl DMSO (99.5%, SigmaAldrich) mixing in 625 μl N, N-dimethylformamide (DMF, 8

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fabricated on the substrate as mentioned above. Afterwards, a hole transport layer (HTL) was spin-coated on the perovskite layer at 3000 rpm for 30 s. The HTL solution consisted of 72.25 mg 2,2’,7,7’-Tetrakis[N,N-di(5-methoxyphenyl)amino]9,9’-sporobifluorene (spiro-oMeTAD), 28.75 μl of 4-tert-butyl pyridine and 17.5 μl of lithium bis(trifluoromethanesulfonyl) imide (Li-TFSI) solution (520 mg LI-TSFI in 1 ml acetonitrile, Sigma-Aldrich, 99.8%) and 1 ml chlorobenzene solvent. Finally, a 100 nm Au layer was thermally evaporated as an electrode through a shadow mask. The cell area was 0.12 cm2. The whole process was carried out in atmosphere. Characterizations

The crystal structures of the perovskite films were characterized by XRD (Bruker D8 ADVANCE) with Cu Kα radiation (λ=1.5406 Å). XPS measurements were conducted using a photoelectron spectrometer (Kratos Inc., AXIS-Ultra DLD). The spectra were calibrated with the C 1s peak (285.00 eV). Surface and cross-sectional morphologies were investigated with a field emission scanning electron microscope (FE-SEM, HITACH S4800). UV–visible absorption spectra were measured using a spectrophotometer (UV4100, Hitachi). PL spectra and TRPL decay were measured using a fluorescence spectrophotometer (FLS920, Edinburgh Instruments) with a 535 nm laser. The illuminated current density–voltage (J–V ) curves were measured with a Keithley 2400 source meter under AM 1.5G illumination (100 mW cm−2) provided by a solar simulator (94022A, Newport®). The light intensity was calibrated with a standard Si solar cell (PVM937, Newport®). The measuring parameters are: fForward: from 1.4 to 0 V; reverse: from 0 to 1.4 V; decay time: 100 ms; sweeping rate: 10 mV/step. The EQE of solar cells was measured by an EQE measurement system (QEX10, PV Measurement Inc.).

Acknowledgement This work was supported by National Natural Science Foundation of China (No. 61604131), Natural Science Foundation of Zhejiang Province (No.LY17F040005), 521 Talents Project of Zhejiang Sci-Tech University, and Xinmiao Undergraduate Student Talents Program of Zhejiang Province (2016R406001 and 2016R406024). Author contributions Y Z, L X, P W, J X and J H performed the experiments. Y Z and C C wrote the draft. All authors discussed and revised the manuscript.

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