Efficient planar perovskite solar cells based on high-quality perovskite films with smooth surface and large crystal grains fabricated in ambient air conditions

Solar Energy 155 (2017) 942–950

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Efficient planar perovskite solar cells based on high-quality perovskite films with smooth surface and large crystal grains fabricated in ambient air conditions Lei Zhang, Xuezhen Zhang, Yang Yu, Xiaoxia Xu, Jie Tang, Xin He, Jihuai Wu, Zhang Lan ⇑ Engineering Research Center of Environment-Friendly Functional Materials, Ministry of Education, Institute of Materials Physical Chemistry, Huaqiao University, College of Materials Science & Engineering, Huaqiao University, Xiamen 361021, China

a r t i c l e

i n f o

Article history: Received 21 May 2017 Received in revised form 8 July 2017 Accepted 11 July 2017

Keywords: Planar heterojunction perovskite solar cell Anti-humidity fabrication protocol Multi-cycle short-time dipping reaction Ambient air condition

a b s t r a c t Due to the hygroscopic feature of methylammonium component, it is still a big challenge to prepare efficient perovskite solar cells in ambient air conditions with high relative humidity about 60–80%. In the humid air conditions, it can only obtain the network-like PbI2 films and porous perovskite films with poor surface morphology when utilizing the typical two-step method and usually used precursor solution. Because of the key role of the morphology of PbI2 films in determining the final quality of perovskite films, the first challege is to fabricate high-quality PbI2 films. Later, another challege is how to eliminate the influence of moist air when converting PbI2 films into perovskite films. For overcoming the challenges, a special additive of n-butyl amine is used to prepare full coverage and continuous PbI2 films, and then a low-hydrophilic n-butanol solvent and a novel multi-cycle short-time dipping reaction are used to successfully convert these PbI2 films into high-quality perovskite films with smooth surface and large crystal grains similar as that of the ones prepared with one-step anti-solvent method in glove box. The best performance device based on the high-quality perovskite film achieves an average power conversion efficiency (average of the forward and reverse scans) of 17.56% with slight hysterisis and good reproducibility. Ó 2017 Elsevier Ltd. All rights reserved.

1. Introduction After a short period of substantial work, perovskite solar cells (PSCs) have rapidly become a front-runner in the photovoltaic community in pace with the rocketing power conversion efficiency (PCE) from initial 3.8% in 2009 to exceed 20% (Kojima et al., 2009; Tan et al., 2017). The ABX3 structure of organic-inorganic perovskite materials, where A = [Cesium Cs+; methylammonium (MA) CH3NH+3; formamidinium (FA) CH3(NH2)+2]; B = (Pb2+; Sn2+); and X = (Cl
; Br
; I
) (Lee et al., 2012; Hao et al., 2014; Fu et al., 2017; Huang et al., 2014, 2016), have outstanding optoelectronic properties including high light extinction coefficient (Yang et al., 2015a,b,c), low exciton-binding energy (Miyata et al., 2015), long carrier lifetime and high mobility (Dong et al., 2015a,b; Wehrenfennig et al., 2014a,b), long and balanced electron-hole diffusion lengths (Wehrenfennig et al., 2014a,b), and so on. In addition, PSCs can be prepared either with vacuum-assisted ‘‘noble process” or with low cost solution-assisted ‘‘humble process” (Cai ⇑ Corresponding author. E-mail address: [email protected] (Z. Lan). http://dx.doi.org/10.1016/j.solener.2017.07.039 0038-092X/Ó 2017 Elsevier Ltd. All rights reserved.

et al., 2017). These advantages make PSCs ideal candidates in researching new kinds of high-efficient and low-cost photovoltaics. Unfortunately, the organic-inorganic perovskite materials are very sensitive to moisture due to the hygroscopic nature of the organic components (Yang et al., 2015a,b,c). The moist air can cause poor surface morphology and even decomposition of perovskite films. To avoid moisture influence most groups prepare PSCs in a dry atmosphere. On the other hand, some groups reported that high-efficient PSCs could be fabricated at certain environment of humidity (You et al., 2014; Zhou et al., 2014; Ko et al., 2015). Eperon and coworkers highlighted the importance of controlled moisture exposure for fabricating high-performance PSCs (Eperon et al., 2015). For future mass production, it is the best choice to develop new protocols suitable for ambient air conditions without special controlled humidity. Many groups have been trying to fabricate PSCs on this condition (Luo et al., 2015; Guo et al., 2016; Tai et al., 2016). Accordingly, the PCE of PSCs manufactured under ambient air without special controlled humidity is seldom or even not over 16%. So it is still a big challenge for preparing efficient PSCs in normal ambient air conditions (Troughton et al., 2017; Yang et al., 2015a,b,c).

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Here we report the successful fabrication of efficient planar heterojunction PSCs with PCE over 17% in ambient air conditions with relative humidity (RH) between 60 and 80%. The following three measures are used to overcome the challenges and finally enable us to prepare high-quality PbI2 films and pervoskite films with smooth surface, large crystal grains and good photovoltaic performance. Firstly, we add small amount of n-butyl amine (BTA) to enhance the quality of PbI2 films. Secondly, we elaborately choose n-butanol other than isopropanol to minimize the effect of moisture when converting PbI2 films into perovskite films becuse of the less hydrophilic property of n-butanol as that of isopropanol. Finally, we use a novel multi-cycle short-time dipping reaction (STDR) to substitue the long-time dipping reaction (LTDR) in the traditional two-step dipping fabrication protocol for fully converting PbI2 films into high-quality perovskite films with smooth surface and large crystal grains.

20 mL n-butanol) for 30 s and then dried by solvent cleaning with 0.5 mL n-butanol and spinning at 4000 rpm for 10 s. The processes were done for several cycles to completely convert lead iodide into perovskite. Finally, the perovskite layers were heated at 100 °C for 30 min. All of the above mentioned processes were done in indoor lab ambient air conditions with high RH about 60–80%. After the preparation of perovskite layers, the hole transporting layers were fabricated on the perovskite layers by spin-coating 20 lL of the mixture of 72.3 mg spiro-OMeTAD, 1 mL chlorobenzene, 28.8 lL 4-tert-butyl pyridine and 17.5 lL lithium bis (trifluoromethane sulfonyl) imide (LI-TFSI) solution (520 mg LI-TSFI dissolved in 1 mL acetonitrile) at 4000 rpm for 30 s. Finally, 80 nm thick of Au electrodes were deposited by thermal evaporation under vacuum through a shadow mask.

2. Material and methods

The scanning electron microscopy (SEM) measurements were conducted using a SU8000 SEM. The X-ray diffraction (XRD) patterns were recorded with a Bruker D8 Advance X-ray diffractometer using Cu Ka radiation (k = 0.15418 nm). The absorption spectra were measured by a Lamda 950 UV–Vis–NIR spectrophotometer. Intensity-modulated photovoltage spectroscopy (IMVS) and intensity-modulated photocurrent spectroscopy (IMPS) measurements were carried out on a CIMPS-4 system (Zahner, Zenium) with a frequency response analyzer under a modulated green light emitting diodes (530 nm) driven by a source supply (Zahner, PP211). The current density -voltage (J-V) characteristic curves of PSCs were recorded with a computer-controlled Keithley 2400 source meter under simulated AM 1.5 G solar illumination at 100 mW cm
2 with #94043A solar simulator (PVIV-94043A, Newport, USA) in air. The incident-photo- to-current conversion efficiency (IPCE) curves were measured using the Newport IPCE system (Newport, USA). The PSCs with the active area of 0.12 cm2 (0.3  0.4 cm2) and without any encapsulation were prepared for measurements.

2.1. Materials Unless specifically noted, all reagents used in this work were purchased from Sigma-Aldrich Corp. Methylammonium iodide (MAI) and methylammonium chloride (MACl) salts were supplied by Xi’an Polymer Light Technology Corp, China. The 2,20 ,7,70 -Tetra k i s ( N,N-di-p-methoxyphenylamine)-9,90 -spirobifluorene(spiroOMeTAD) was supplied by Luminescence Technology Corp, Taiwan, China. TiO2 nanoparticles with size about 3.6 ± 0.4 nm were synthesized according to the references Que et al. (2014) and Tu et al. (2015). Fluorine doped tin oxide conductive glasses (FTO glasses, sheet resistance 15 X h
1, purchased from Nippon Glass Corp. JP) were used as substrates for preparing PSCs. 2.2. Device fabrication The TiO2 electron transporting layers (ETLs) were prepared on the cleared and patterned fluorine doped tin oxide conductive (FTO) glasses by spin-coating toluene solution of 3.6 ± 0.4 nm TiO2 nanoparticles (20 mg mL
1) at a rate of 4000 rpm for 30 s. Then, the TiO2 ETLs were heated at 100 °C for 30 min and 500 °C for 30 min. The multi-cycle STDR was used to fabricate perovskite layers on the TiO2 ETLs according to the Scheme 1. Firstly, 60 lL of the mixture of 1.3 g PbI2, 0.9 mL dimethyl sulfoxide (DMSO), 0.45 mL n-butyl amine and 2.1 mL N,N-dimethylformamide (DMF) was spin-coated on the TiO2 ETLs at 3500 rpm for 30 s and then heated at 70 °C for 10 min (Li et al., 2016). Secondly, after cooled down to room temperature, the lead iodide layers were dipped into a n-butanol solution of mixed MAI and MACl with MAI/MACl mole ratio of 4/1 (0.432 g MAI, 0.0462 g MACl and

2.3. Film and device characterization

3. Results and discussion It has been widely demonstrated that the morphology of perovskite is essential for high-performance PSCs (Zheng et al., 2017). In the typical two-step dipping fabrication protocol, controlling the PbI2 morphology is a key strategy for controlling the final perovskite morphology (Wu et al., 2014; Liu et al., 2017). Unfortunately, it is failing to prepare high quality PbI2 thin films on FTO substrates with TiO2 ETLs under indoor humidity with RH higher than 60%, even via the addition of DMSO in PbI2 solution, as illustrated by the SEM image shown in Fig. 1a. We find that the addition of a small amount of BTA into PbI2 solution can solve this

Scheme 1. The multi-cycle STDR protocol for preparing CH3NH3PbI3
xClx films.

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Fig. 1. SEM images of PbI2 films prepared without (a) and with (b) BTA additive; and CH3NH3PbI3
xClx films (c and d) converted from (a) and (b), respectively. Scale bars, 1 lm. J-V curves of PSCs assembled with the CH3NH3PbI3
xClx films from poor-quality PbI2 film without BTA additive and prepared with traditional LTDR; from high-quality PbI2 films with BTA addive and prepared with multi-cycle STDR (with BTAa) and traditional LTDR (with BTAb) protocols, respectively (e).

(CH3NH3PbI3
xClx) other than the pure CH3NH3PbI3 because it is not so sensitive to the moisture and has a longer photogenerated carrier lifetime compared with the latter one (Stranks et al., 2013). Subsequently, we find that the commonly used isopropanol is not suitable for the two-step dipping fabrication protocol under indoor humidity condition. Isopropanol can easily absorb moisture from the air because of its hydrophilic property, which will result in degradation of perovskite in situ. Hence, a less hydrophilic solvent of n-butanol is chosen to minimize the effect of moisture when turning PbI2 into CH3NH3PbI3-xClx films. The photographs presented in Fig. 2 show that the freshly prepared CH3NH3PbI3
xClx films using the two solvents are almost identical. However, clear differences are appeared after either rapid solvent cleaning combining with spin drying or natural drying. That is, the colour of CH3NH3PbI3
xClx films prepared with isopropanol solution changes from black to grey after drying; reversely, that of the one prepared with n-butanol solution is not different with the freshly prepared sample after rapid solvent cleaning combining with spin drying, which means that the methodology can work well under high RH. Fig. 2c and f reveal that by going with the natural drying process the appearance of MAI and MACl salts on the surface of the CH3NH3PbI3
xClx films results in obvious colour fading. So the simpler successive multi-cycle solution coating is not suitable for the case (Dong et al., 2015a,b). When using the typical two-step dipping fabrication protocol to prepare perovskite, the PbI2 film should be immersed into a solution containing organic halide salts for at least 10 min to complete the reaction (Chang et al., 2015). The intercalation reaction between PbI2 and organic halide salts is very quickly, which tends to form small size crystal grains and produces a perovskite film consisting many discrete crystal grains (Mokhtar et al., 2017). In order to improve the quality of the CH3NH3PbI3
xClx film, we adjust the traditional methodology with one time of LTDR to the new protocol with multi-cycle STDR when turning PbI2 films to perovskite layers.

problem. A full coverage and continuous PbI2 film is indeed obtained with the help of BTA (see Fig. 1b). Owing to the well infiltrative property and strong Lewis base nature of BTA, it is feasible to homogeneously spread out the PbI2 solution on the substrates and greatly slow down the crystallization rate to form high quality PbI2 films. As displayed in Fig. 1c and d, the high-quality perovskite film composed of densely packed big crystal grains without any pinholes is prepared with the good quality PbI2 film from the PbI2 solution containing BTA. Nevertheless, the poor quality perovskite film with porous structure is turned from the network-like PbI2 film. The different qualities of the perovskite films cause completely different photovoltaic performance of PSCs (see Fig. 1e and Table 1). When utilizing the same LTDR methodology (the traditional dipping method), the PSC assembled with the perovskite film from the poor-quality PbI2 film without BTA additive shows poorer photovoltaic performance (with a PCE as low as 6.37%) compared with that of the one assembled with the perovskite film from the high-quality PbI2 film with BTA additive (with a PCE of 13.40%). And the best-performance PSC with a PCE as high as 17.37% is achieved by assembling the high-quality perovskite film prepared with the multi-cycle STDR protocol in the device. In the worst-performance PSC, the existed many holes in the perovskite film can produce very low shunt resistance (Rsh) (352.36 X cm2 VS 2623.98 X cm2 as listed in Table 1) and probably result in serous recombination for the direct contact between spiro-OMeTAD and TiO2 ETL (Eperon et al., 2014), which cause extremely low fill factor and open-circuit voltage of the PSC. Due to the hygroscopic nature of the organic components in perovskite materials (Ciro et al., 2017), it is another big challenge to convert PbI2 films into high quality perovskite films under ambient air conditions with high RH, especially in our lab where the local RH is usually higher than 60%. Therefore, the protocol should be well-designed to minimize the moisture effect. Above all, we select the perovskite with iodide and chloride hybrid ions

Table 1 The J-V key parameters of PSCs presented in Fig. 1e. PSC a

With BTA With BTAb W/O BTA a b

VOC/V

JSC/mA cm
2

FF

PCE/%

Rs/X cm2

Rsh/X cm2

1.07 1.02 0.90

22.55 20.21 18.16

0.72 0.65 0.39

17.37 13.40 6.37

4.03 6.30 13.39

2623.98 750.44 352.36

The perovskite film in the PSC is prepared with multi-cycle STDR protocol. The perovskite film in the PSC is prepared with traditional LTDR protocol.

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Fig. 2. Photographs of fleshly prepared CH3NH3PbI3
xClx film (a), after solvent cleaning and spin drying (b), and after natural drying (c) with isopropanol; that of the counterparts (d, e, f) with n-butanol. Scale bars, 1 lm.

The SEM top-view images of the CH3NH3PbI3
xClx films prepared with multi-cycle STDR are shown in Fig. 3a–d. It is apparent that the PbI2 film can convert into high-quality CH3NH3PbI3
xClx film with large size crystal grains and clean surface after fivecycle STDR. Similar as the results of multi-cycle solution coating process reported by Huang’group (Dong et al., 2015a,b), the multi-cycle STDR protocol also can induce the abnormal grain growth behavior. In the latter case, not only the MACl additive in the precursor solution but also the deliberately interrupting each

time of intercalation reaction between PbI2 and organic halides can promote secondary grain growth by slowing the crystal formation process to allow the nucleus to adjust the orientation to minimize the total Gibbs free energy and finally form large size crystal grains. So it is quite different with that of the LTDR protocol, which induces quick formation of many discrete CH3NH3PbI3
xClx particles because of the randomness in the nucleation process by the uncontrollable and high-speed crystallization process (Sharenko and Toney, 2016).

Fig. 3. SEM images of CH3NH3PbI3
xClx films fabricated with three cycles (a), five cycles (b), seven cycles (c), and ten cycles (d) of STDR processes. Scale bars, 1 lm.

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For the new methodology, finding the suitable cycles of STDR is important because the insufficient reaction results in small size crystal grains and overdose of PbI2 residues (see Figs. 3a and 4a); nonetheless, the excessive cycles of STDR cause partial dissolution of CH3NH3PbI3
xClx and then re-growing as sheet-like crystal grains dispersing on the surface (see Fig. 3c and d). The SEM cross-sectional micrographs shown in Fig. 4a and b suggest that the STDR-deposited CH3NH3PbI3
xClx film has larger size crystal grains and some single grains can run throughout the film thickness, whereas the LTDR-deposited one is stacked with many different sizes of crystal grains and some pinholes are appeared. The XRD spectra of the PbI2 film and the as-prepared CH3NH3PbI3
xClx films with multi-cycle STDR are shown in Fig. 5a. The relevant characteristic peaks are also marked in the figure. One can observe that after five cycles of STDR the diffraction strength of the PbI2 characteristic peak declines to a very low value. The extending cycles of STDR, such as seven or ten cycles, can still slightly decrease the diffraction strength of the PbI2 characteristic peak but cannot completely eliminate the peak. It is not a bad thing because a small amount of residual PbI2 can bring about a passivation effect for high-performance PSCs (Bi et al., 2016). Meanwhile, the measure of increasing cycles of STDR can increase crystallinity of the CH3NH3PbI3
xClx films because the main diffraction peaks in the XRD patterns become stronger gradually. The absorption spectra are also performed and presented in Fig. 5b. Although the sample prepared with three-cycle STDR still contains overdose of PbI2 residues as revealed in the XRD pattern in Fig. 5a, it has already shown the CH3NH3PbI3
xClx characteristic absorption shoulder at around 780 nm and its absorbance is much higher than that of the initial PbI2 film. The other three samples prepared with five, seven and ten cycles of STDR, respectively, maintain similar absorbance in the long wavelength from 500 nm to 800 nm but show increased absorbance in the wavelength shorter than 500 nm, which can be attributed to the increased crystallinity of CH3NH3PbI3
xClx films as previously analyzed. The photovoltaic performance of PSCs based on the CH3NH3PbI3
xClx films prepared with multi-cycle STDR is illustrated in Fig. 5c and Table 2. Due to the overdose of PbI2 residues in the CH3NH3PbI3
xClx film prepared with three cycles of STDR, the photovoltaic performance of the device is very poor, just giving an open-circuit voltage (Voc) of 0.98 V, a short-circuit current density (Jsc) of 15.83 mA cm
2, a fill factor (FF) of 0.65, and a PCE of 10.08%. Except for it, the other three devices based on the CH3NH3PbI3
xClx films prepared with five, seven, and ten cycles of STDR show enhanced performance. In particular, the best performance is achieved when using five-cycle STDR to prepare the CH3NH3PbI3
xClx film, yielding a Voc of 1.07 V, a Jsc of 22.55 mA cm
2, a FF of 0.72, and a PCE of 17.37%. The data in Table 2 reveal that the difference Rsh of the latter three devices is obvious. It has been confirmed that Rsh is associated with the loss of photocurrent by carrier recombination within the device, especially at the interfaces of each layer (Wu et al., 2015). A higher Rsh indicates a lower power loss in the device, resulting in a higher FF and Voc. As

aforementioned, with the excessive cycles of STDR in preparing the CH3NH3PbI3
xClx films, particularly by ten-cycle STDR, many sheet-like crystal grains are perpendicularly standing on the surface, resulting in a defective surface and a low Rsh. The CH3NH3PbI3
xClx film prepared with five-cycle STDR shows a higher light absorbance in the wavelength range from 400 nm to 550 nm compared with that of the film prepared with LTDR as shown in Fig. 6a. The corresponding IPCE spectra in Fig. 6b indicate that the device prepared with five-cycle STDR shows a broader plateau of over 80% IPCE across the wavelength range from 400 nm to 750 nm than the one prepared with LTDR. Also, the integrated Jscs agree well with the values from J-V curves. Fig. 6c compares the J-V curves (under both forward and reverse scanning directions) of the two best performance devices fabricated with the novel multicycle STDR and the traditional LTDR, respectively. The key parameters are summarized in Table 3. Apparently, the multi-cycle STDR is more advantage than LTDR methodology because almost all of the key parameters of the device prepared with five-cycle STDR are superior to the latter one. In detail, the device prepared with five-cycle STDR achieves an average PCE (average of the forward and reverse sweeps) of 17.56% with a Voc of 1.08 V, a Jsc of 22.58 mA cm
2, and a FF of 0.72; yet the one prepared with LTDR just gives a much low average PCE of 14.25% with a Voc of 1.04 V, a Jsc of 20.28 mA cm
2, and a FF of 0.68. Importantly, the device prepared with five-cycle STDR exhibits a minor hysteretic effect because the calculated hysteresis index (HI) is very low (0.015) (Wang et al., 2015). The charge carrier transfer dynamics in the devices are investigated with IMPS and IMVS (Pockett et al., 2016). From the characteristic frequency minimums of IMPS (fd) and IMVS (fr), the relevant charge carrier transfer parameters including transporttime (sd), recombination life-time (sr), and collection efficiency (gcc) can be directly calculated by using the expressions of sd = 1/(2pfd), sr = 1/(2pfr), and gcc = (1
sd/sr)  100% (Roose et al., 2016). The plots of sd, sr, and gcc depending on incident photon flux in Fig. 6d–f show that the device prepared with five-cycle STDR has shorter sd, longer sr, and higher gcc than the device prepared with LTDR on the same photon flux. This suggests that the device prepared with five-cycle STDR has better charge carrier dynamics than the latter one because the shorter sd means the faster charge transport rate; the longer sr represents the slower charge recombination; and the combination of the two factors finally result in higher gcc. The better charge carrier dynamics in the device prepared with five-cycle STDR are conducive to achieve high-efficiency and low hysteresis performance. Reversely, the inferior charge carrier dynamics in the device prepared with LTDR contribute to serious recombination, poor photovoltaic performance, and high hysteresis. To certify the capability of efficient and stable power output of the devices, the steady-state Jsc and PCE of the PSCs prepared with five-cycle STDR and LTDR protocols, respectively, are recorded as a function of time when the devices are held at an external bias close to the maximum power point in the related J-V curves (Jeon et al.,

Fig. 4. Cross-sectional SEM views of CH3NH3PbI3
xClx films constructed by the five-cycle STDR methodology (a) and the typical LTDR protocol (b).

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Fig. 5. XRD patterns (a) and absorption spectra (b) of PbI2 film prepared with BTA additive and CH3NH3PbI3
xClx films prepared with different cycles of STDR; J-V curves (forward scanning) of PSCs prepared with the corresponding CH3NH3PbI3
xClx films (c).

Table 2 The J-V key parameters of PSCs presented in Fig. 5c. PSC

VOC /V

JSC/mA cm
2

FF

PCE/%

Rs/X cm2

Rsh/X cm2

3 cycles 5 cycles 7 cycles 10 cycles

0.98 1.07 1.06 1.04

15.83 22.55 22.19 22.21

0.65 0.72 0.71 0.66

10.08 17.37 16.70 15.24

6.73 4.03 4.09 5.38

689.17 2623.98 1668.93 1170.52

Fig. 6. Absorption spectra of CH3NH3PbI3
xClx films prepared with LTDR and five-cycle STDR (a); IPCE (b), J-V (forward and reverse scanning) (c) curves, incident photon flux dependence of sd (d), sr (e), and gcc (f) of PSCs prepared with LTDR and five-cycle STDR.

Table 3 The J-V key parameters of PSCs presented in Fig. 6c. PSC STDR

LTDR

Forward Reverse Average Forward Reverse Average

VOC /V

JSC/mA cm
2

FF

PCE/%

Rs/X cm2

Rsh/X cm2

HIa

1.07 1.09 1.08 1.02 1.06 1.04

22.55 22.61 22.58 20.21 20.34 20.28

0.72 0.72 0.72 0.65 0.70 0.68

17.37 17.74 17.56 13.40 15.09 14.25

4.03 3.95 – 6.30 4.22 –

2623.98 13182.00 – 750.44 1500.88 –

0.015

0.097

Where JRS(0.8 VOC) and JFS(0.8 VOC) represent the photocurrent density at 80% of the VOC for the reverse scanning (from open curcuit to short circuit) and forward scanning (from short circuit to open circuit), respectively (Wehrenfennig et al., 2014a,b). a OC Þ
J FS ð0:8V OC Þ The hysteresis index (HI) values are calculated according to the following equation: HI ¼ JRS ð0:8V . J ð0:8V OC Þ RS

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Fig. 7. Steady-state output of Jsc and PCE (a and b) and normalized PCE of PSCs (c) stored in high humidity (RH about 60–80%) without any encapsulation. The devices are prepared with five-cycle STDR and LTDR, respectively.

Fig. 8. PCE histograms along with the Gaussian fitting curves for independently prepared 20 cells with five-cycle STDR and traditional LTDR protocols, respectively, by forward and reverse scanning.

2015). As shown in Fig. 7a and b, the Jsc and PCE of PSC prepared with five-cycle STDR can stabilize at 21.25 mA cm
2 and 17.13% for 200 s, respectively, indicating well stable performance. These values are obviously higher than that of the counterpart prepared with LTDR, which shows a stable Jsc and a stable PCE of 18.67 mA cm
2 and 13.23%, respectively. Furthermore, the long-time stability of the PSCs prepared with five-cycle STDR and LTDR protocols, respectively, is also studied through measuring the changed values of PCE of the PSCs stored in high humidity (RH about 60– 80%) without any encapsulation. As shown in Fig. 7c, the device prepared with five-cycle of STDR shows good stability at the initial 36 h with slightly decreased PCE. Even extending the storing time to 72 h, the decreasing amplitude of PCE is still small. The longtime stability of the device prepared with five-cycle STDR is clearly superior to that of the counterpart prepared with LTDR because the PCE of the latter one starts to decrease at the beginning of 24 h, and then quickly decrease after 48 h. It has been demonstrated that the perovsktie films with compact and large crystal grains are more tolerant to moisture attack than that of the films composed with small and discrete crystal grains (Mahmud et al., 2017), so the high quality of CH3NH3PbI3-xClx film prepared with five-cycle STDR contributes to the high stability of the corresponding device.

Moreover, the PCE histograms along with the Gaussian fitting curves for independently prepared 20 cells with the two protocols, respectively, are illustrated in Fig. 8. The average PCEs of the devices prepared with five-cycle STDR are 16.11 ± 0.71% and 16.86 ± 0.57% versus the values of the counterparts as 11.29 ± 0.96% and 13.96 ± 0.79% for forward and reverse scanning directions, respectively. So the multi-cycle STDR can also enhance the reproducibility of device performance.

4. Conclusions In conclusion, we have demonstrated that high-quality CH3NH3PbI3-xClx films with large crystal size grains can be fabricated under ambient air conditions with high RH about 60–80% by the novel and robust anti-humidity fabrication protocol. The utilization of BTA additive contributes to forming good quality PbI2 films; and the application of low-hydrophilic n-butanol and multi-cycle STDR ensure the successfully converting of these PbI2 films into high-quality perovskite films. The best performance device achieves an average PCE (average of the forward and reverse sweeps) of 17.56% with slight hysterisis and good reproducibility,

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