Improvement on the performance of perovskite solar cells by doctor-blade coating under ambient condition with hole-transporting material optimization

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Improvement on the performance of perovskite solar cells by doctor-blade coating under ambient condition with hole-transporting material optimization Deng Wang a,1, Jiming Zheng a,c,1, Xingzhu Wang a,b,∗, Jishu Gao a, Weiguang Kong a, Chun Cheng a,∗, Baomin Xu a,∗

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Department of Materials Science and Engineering, Southern University of Science and Technology, Shenzhen 518055, Guangdong, China Academy for Advanced Interdisciplinary Research, Southern University of Science and Technology, Shenzhen 518055, Guangdong, China c School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, Heilongjiang, China b

a r t i c l e

i n f o

Article history: Received 9 January 2019 Revised 6 March 2019 Accepted 18 March 2019 Available online xxx Keywords: Hole-transporting material NiOx Perovskite solar cells Thermally assisted blade-coating Ambient condition Fabrication

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a b s t r a c t Numerous fabrication methods have been developed for high-efficiency perovskite solar cells (PSCs). However, these are limited to spin-coating processes in a glove box and are yet to be commercialized. Therefore, there is a need to develop a controllable and scalable deposition technique that can be carried out under ambient conditions. Even though the doctor-blade coating technique has been widely used to prepare PSCs, it is yet to be applied to high-efficiency PSCs under ambient conditions (RH∼45%, RT∼25°C). In this study, we conducted blade-coating fabrication of modified high-efficiency PSCs under such conditions. We controlled the substrate temperature to ensure phase transition of perovskite and added dimethyl sulfoxide (DMSO) to the perovskite precursor solution to delay crystallization, which can contribute to the formation of uniform perovskite films by doctor-blade coating. The as-prepared perovskite films had large crystal domains measuring up to 100 μm. Solar cells prepared from these films exhibited a current density that was enhanced from 17.22 to 19.98 mA/cm2 and an efficiency that increased from 10.98% to 13.83%. However, the open-circuit voltage was only 0.908 V, probably due to issues with the hole-transporting layer. Subsequently, we replaced poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) with NiOx as the hole-transporting material and then prepared higher-quality perovskite films by blade-coating under ambient conditions. The as-prepared perovskite films were preferably orientated and had large crystal domains measuring up to 200 μm; The open-circuit voltage of the resulting PSCs was enhanced from 0.908 to 1.123 V, while the efficiency increased from 13.83% to 15.34%. © 2019 Published by Elsevier B.V. and Science Press on behalf of Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences

1. Introduction Organic-inorganic hybrid perovskite solar cells (PSCs) have attracted widespread attention since first being developed [1–3] because of their economic advantages and flexibility [4,5]. Thus far, the efficiency of PSCs has been enhanced to more than 22% [6]; however, devices with such high efficiencies are usually prepared by spin-coating in a glovebox. For commercial applications in the future, it will be imperative to fabricate PSCs under ambient

∗

Corresponding author at: Department of Materials Science and Engineering, Southern University of Science and Technology, Shenzhen 518055, Guangdong, China. E-mail addresses: [email protected] (X. Wang), [email protected] (C. Cheng), [email protected] (B. Xu). 1 These authors contributed equally to this work.

condition [7]; hence, it is vital to find alternatives to spin-coating that can meet the requirements of large-scale production [8]. Compared to spin-coating, doctor-blade coating is a simple and costeffective technique that can utilize the rheological properties and drying dynamics of the precursor solution [9,10]. In particular, doctor-blade coating has been widely applied because it is suitable for production under ambient condition [11,12], and can be easily used in conjunction with other continuous deposition techniques such as roll-to-roll fabrication [13,14]. N,N-Dimethylformamide (DMF) is the most common solvent employed to dissolve a perovskite precursor [9–11]. However, owing to its relatively low boiling point, the evaporation of perovskite precursor usually causes the doctor blade to start moving. This leads to the formation of non-uniform perovskite films. The removal of dimethyl sulfoxide (DMSO) [15,16] from the precursor is more difficult than that of DMF on account of the stronger

https://doi.org/10.1016/j.jechem.2019.03.023 2095-4956/© 2019 Published by Elsevier B.V. and Science Press on behalf of Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences

Please cite this article as: D. Wang, J. Zheng and X. Wang et al., Improvement on the performance of perovskite solar cells by doctorblade coating under ambient condition with hole-transporting material optimization, Journal of Energy Chemistry, https://doi.org/10. 1016/j.jechem.2019.03.023

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coordination of DMSO with lead (Pb) [17] and its higher boiling point. Hence, in order to obtain highly crystalline perovskite films, we used DMSO as an additive in the DMF-based precursor solution to delay crystallization. Thus, high-quality perovskite films could be produced by doctor-blade coating under ambient condition (RH∼45%, RT∼25°C), and the efficiency of the PSCs increased from 10.98% to 13.80%. The majority of inverted planar heterojunction PSCs fabricated by doctor-blade coating are based on PEDOT:PSS as the holetransporting layer (HTL). However, PEDOT:PSS is not good for the long-term stability of the device under ambient condition owing to its high acidity and hygroscopicity [18]. Therefore, different hole transport materials have been employed to replace PEDOT:PSS in inverted PSCs. These include organic materials such as poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine (PTAA) and (di4-tolylamino)phenyl) cyclohexane (TAPC) and biaxially-extended octithiophene-based conjugated polymers (PII2T8T) [19–22] and inorganic materials such as NiOx . The wide bandgap, good transmittance, good chemical stability, and convenient energy level alignment of NiOx with the perovskite facilitate hole collection and electron blocking [23–26]. Moreover, the good wettability of NiOx films with the perovskite precursor can reduce the required amount of raw materials, and enable large-scale production of PSCs [27]. In this study, we replaced PEDOT:PSS with NiOx as a hole transport layer to prepare the perovskite film by doctor-blade coating under ambient condition (RH∼45%, RT∼25°C), in order to fully meet the requirements of industrial-level production. This resulted in the formation of highly oriented perovskite layers, and the size of the grain domains reaching 200 nm. The efficiency of the PSCs was also enhanced from 13.83% to 15.34%.

2. Experimental 2.1. Materials FTO coated glass with sheet resistance of 15 /sq was purchased from Ying Kou You Xuan Trade Co. Ltd. Poly(3,4 ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) aqueous solution (Clevious PVP AI 4083) was purchased from Shanghai Mater Win New Materials Co., Ltd. 2,9-dimethyl-4,7-diphenyl1,10-phenanthroline (BCP), phenyl-C61-butyric acid methyl ester (PC61BM) and Methylammonium iodide (MAI) were purchased from Xi’an Polymer Light Technology Corp. All of the other materials were purchased from Sigma-Aldrich, including 2-propanol (IPA, 99.5%), sulfoxide (DMSO, 99.5%), N,N-dimethylformamide (DMF, 99.5%), chlorobenzene (CB, 99.5%), poly(4-styrenesulfonic acid) (PSSH), PbI2 (99.999%), ethanolamine (EA, 99%) and nickel (II) acetate tetrahydrate (99.99%). These commercially available materials were used directly without further purification.

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2.2. Preparation of NiOx films

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The NiOx films were prepared according to a previously reported procedure [28]. Briefly, 0.1 mmol Ni(Ac)2 ·4H2 O was dissolved in 1.0 mL of isopropanol with ethanolamine (NH2 CH2 CH2 OH). The mole ratio of Ni2+ /EA in solution was maintained at 1:1. The solution was kept in a sealed glass vial and stirred for 4 h in air at 75°C until deep green solution was obtained. The NiOx precursor solution filtered with 0.45 μm polytetrafluoroethylene (PTFE) filters before device fabrication. NiOx film was formed by spin-coating at 1500 rpm for 30 s on a FTO substrate, and annealed at 280°C for 1 h in air.

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2.3. Preparation of PEDOT:PSS films

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The modified PEDOT:PSS solution was prepared by adding PSSH [29] into PEDOT:PSS solution in volume ratio of 1:4. The modified PEDOT:PSS solution was stirred for 10 min at room temperature then filtered with 0.45 μm polytetrafluoroethylene (PTFE) filters before device fabrication. PEDOT:PSS film was formed by spincoating a PEDOT:PSS mixed solution at 50 0 0 rpm for 35 s on a FTO substrate, and annealed at 150°C for 20 min in air.

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2.4. Device fabrication

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The as-prepared perovskite precursor solution was prepared by PbI2 and MAI with mole ratio of 1:1 with concentration of 1 M in a mixture of DMF and DMSO (The volume fraction of DMSO is respectively 0, 15%, 30% and 45%) then was filtered using 0.45 μm PTFE filter. The MAPbI3 (MA=methyl ammonium) perovskite thin films were fabricated by blade-coating onto HTLcoated (PEDOT:PSS and NiOx ) substrates under ambient condition with RH∼45%. In brief, the blade was operated at 25 mm/s with the blade gap of 100um, and the substrate temperature was maintained at 135°C. The resulting films on FTO glass were sequentially annealed at 90°C for 15 min. Then, PC61BM solution in chlorobenzene (20 mg/mL, filtered with a 0.45 μm PTFE filter, 40 μL) was spin-coated onto perovskite film at 20 0 0 rpm for 30 s in the glove box filled with nitrogen. After coating, the device was set aside for half an h before next step. Then, BCP solution (0.5 mg/mL, 80 μL) in isopropanol was spin-coated onto PC61BM films at 40 0 0 rpm for 30 s. Finally, 120 nm thick silver electrode was deposited by thermal evaporation 1.0 × 10−4 Pa through a shadow mask.

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2.5. Characterization

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The morphology of perovskite films was characterized by scanning electron microscopy (SEM, TESCAN MIRA3), at a 10 kV accelerating voltage. The optical properties of samples were measured on a UV/vis/NIR spectrophotometer equipped with an integrating sphere (PerkinElmer Lambda 950). Photocurrent density-voltage (JV) curves were measured under AM 1.5 G one sun illumination (100 mW/cm2 ) with a solar simulator (Enlitech SS-F7-3A) equipped with a 300 W xenon lamp and a Keithley 2400 source meter. The light intensity was adjusted by an NREL-calibrated Si solar cell. The active area of studied devices is 1.5 × 1.5 mm2 , it defined as 0.1 mm2 by a shadow mask with spherical aperture for the J-V measurements. The XRD spectra of the prepared films were measured using a Bruker eco D8 with 40 kV and 25 mA. Film thickness is measured by KLA Tencor d-120. Contact angle is measured by AST VCAoptima.

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3. Results and discussion

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3.1. Optimization of solvents

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In order to explore the effects of solvents in the perovskite precursor on the crystallization quality and surface convergence of the perovskite films, we adopted a typical inverted planar heterojunction architecture for PSCs– FTO/PEDOT:PSS/MAPbI3 /PC61BM/BCP/Ag. PEDOT:PSS and PC61M served as the HTL and electron-transporting layer (ETL), respectively. Although recent studies have focused on mixed cation and mixed halide formulations of perovskite [30–32], MAPbI3 —as the simplest perovskite formulation—was used during deposition via doctor-blade coating. The quality of perovskite films can be ensured by precise control of some critical parameters, including the concentration of the precursor solution, height of the gap, temperature of the substrate, and speed of the moving blade. Furthermore, the choice of solvent is very important, as highly

Please cite this article as: D. Wang, J. Zheng and X. Wang et al., Improvement on the performance of perovskite solar cells by doctorblade coating under ambient condition with hole-transporting material optimization, Journal of Energy Chemistry, https://doi.org/10. 1016/j.jechem.2019.03.023

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Fig. 1. Schematic diagrams of perovskite films formed by doctor blade coating with (a) DMF-based and (b) DMF/DMSO-based precursor solutions.

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polar solvents are required to completely dissolve the lead halide. DMF, DMSO, and their mixtures have been widely employed in such situations. It is generally accepted that the coordination chemistry of solvents will determine the crystallization kinetics [33,34]. Compared with DMF, DMSO has a higher boiling point and stronger coordination with Pb, which makes its removal from the perovskite film difficult. In this study, except for the most commonly used DMF, DMSO was selected as the secondary solvent in the perovskite precursor solution for doctor-blade coating. In order to achieve complete phase transition and high performance of the MAPbI3 perovskite film, the temperature of the substrate should exceed 120°C and the optimum temperature retain at 130°C [35,36]. Fig. S1 plots the PCE changes of DMF-based devices as a function of substrate temperature and related results can be seen in Table S1. In this case, when the blade passes the substrate, evaporation of the DMF-based perovskite precursor solution commences immediately, accompanied by crystallization (Fig. 1a). As

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for the DMF/DMSO-based perovskite precursor solution, the crystallization of the perovskite is delayed, and a dry black film starts to form until the blade has passed the entire substrate (Fig. 1b). Therefore, DMSO can be regarded as an indispensable solvent for doctor-blade coating, as it assists the crystallization of perovskite. The results obtained by solvent optimization are as follows. Fig. 2(a) shows the current density-voltage (J-V) characteristics of the PSCs under a simulated solar irradiance of AM1.5 g (100 mW/cm2 ); the devices were prepared by blade-coating using different ratios of DMSO to DMF in the perovskite precursor solution. The dependence of the photovoltaic parameters on the DMSO content is shown in Fig. 2(c), and the relevant performance parameters are presented in Table 1. A reference device, prepared without adding DMSO to the perovskite solution, exhibits a power conversion efficiency (PCE) of 10.98%, with an open-circuit voltage (Voc ) of 0.872 V, short-circuit current density (Jsc ) of 17.22 mA/cm2 , and fill factor (FF) of 73.09%. When the DMSO content in the precursor solution was 15 vol% (DMF:DMSO=8.5:1.5), the resulting device showed a significantly enhanced PCE of 13.83%, with a Voc of 0.908 V, Jsc of 19.98 mA/cm2 , and FF of 76.26%. Compared to the device prepared without DMSO, the average values of all photovoltaic parameters, especially Jsc and FF, were improved for the devices prepared from the perovskite solution with 15% DMSO. However, further increase in the DMSO content did not improve the device performance. Therefore, a DMSO content of 15% was selected to prepare the cells for subsequent studies. The spectra of the external quantum efficiency (EQE) of the best devices (Fig. 2b) confirm that the integrated current densities of the devices prepared from perovskite solutions with DMSO are much higher

Fig. 2. Performance of perovskite solar cells (PSCs). (a) Current density-voltage (J-V) characteristics of PSCs with different DMSO content in precursor solutions. (b) Plot of external quantum efficiency (EQE) and integrated current density as functions of wavelength for PSCs with different DMF and DMSO content. (c) Dependence of various photovoltaic parameters on DMSO content.

Please cite this article as: D. Wang, J. Zheng and X. Wang et al., Improvement on the performance of perovskite solar cells by doctorblade coating under ambient condition with hole-transporting material optimization, Journal of Energy Chemistry, https://doi.org/10. 1016/j.jechem.2019.03.023

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Fig. 3. (a) UV-vis absorption spectra of perovskite films prepared by blade-coating of different precursor solutions; (b) X-ray diffraction (XRD) spectra of perovskite films prepared by blade-coating of different precursor solutions.

Fig. 4. SEM images of perovskite films prepared by doctor blade-coating of different precursor solutions: (a) without DMSO; (b) with 15% DMSO; (c) with 30% DMSO; (d) with 45% DMSO. Table 1. Performance parameters of PSCs with different DMSO contents. DMSO (vol%)

Jsc (mA/cm2 )

0 15 30 45

16.60±0.48 19.97±0.77 19.25±0.78 18.93±0.67

a b

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a

(17.221) (19.98) (19.25) (19.34)

b

Voc (V)

FF (%)

PCE (%)

0.872±0.008 (0.872) 0.903±0.020 (0.908) 0.897±0.019 (0.902) 0.894±0.020 (0.871)

69.26±2.81 (73.09) 71.55±2.91(76.26) 70.31±2.61 (75.04) 67.54±3.47 (73.38)

10.03±0.55 (10.98) 12.90±0.82 (13.83) 12.13±0.59 (13.03) 11.42±0.65 (12.36)

Averaged values with a standard deviation are calculated from more than 15 devices from different batches; The values corresponding to the best devices are in the parenthesis.

than that of the device without adding DMSO. Moreover, the EQE values of the devices with DMSO are higher than that of the device without DMSO throughout the visible region. These results imply that electron transport was improved by the addition of DMSO, which can contribute to the balance of the charge-carrier transport within the device. The UV-vis absorption spectrum in Fig. 3(a) suggests that all perovskite films prepared by blade-coating with different

controlled ratios of DMSO to DMF in the perovskite solutions exhibit strong absorption for wavelengths in the visible range (400 to 700 nm). Notably, the absorption of all perovskite films decreases distinctly in the range of 70 0–80 0 nm, which matches well with the EQE results shown in Fig. 2(b). We attribute the change in the perovskite structure to the additive DMSO. Although the perovskite films with DMSO have the same phase, the big difference in the crystal domains and film morphology

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Fig. 5. (a) Device architecture transformation from PEDOT:PSS to NiOx . (b) XRDspectra of NiOx film. (c) Top-view SEM image of NiOx film. (d) Cross-section SEM image of a typical NiOx -based perovskite solar cell. (e) Energy level diagram of the various in the perovskite device.

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may lead to variation in their light absorption properties. The X-ray diffraction (XRD) patterns in Fig. 3(b) confirm the role of the additive DMSO in the growth of perovskite crystals. In the case of pure DMF, the XRD spectra present the strongest diffraction peak at 14.14°, corresponding to diffraction from (110) [37]. However, in the presence of DMSO, the diffraction peak at 20° shows the highest intensity, in contrast, other peaks are almost negligible. The reason may be the change in the boiling point and polarity of the solvent; further, the different volatility dynamics combined with the stress of the blade resulted in a change in the orientation of the crystal. To clarify the effect of adding DMSO to the perovskite precursor solution, we evaluated the coverage and morphology of the perovskite films, which are mainly governed by nucleation and crystal-growth dynamics [38]. Fig. 4 shows the SEM images of the perovskite films fabricated by doctor-blade coating. All perovskite films based on PEDOT:PSS exhibited compact and dense crystal domains with a coffee-ring like structure [39], indicating that doctor-blade coating contributes to the nucleation and crystal dynamics under ambient condition. It is clear that for the perovskite

film without DMSO (Fig. 4a), the size of the crystal domains is not uniform. This is because the collective effect of solvent volatilization power and blade coating stress led to uneven grain growth. On the other hand, perovskite films with 15% and 30% DMSO in the precursor solution (Fig. 4b, c) exhibit good surface flatness and crystal domains of almost the same size, approximately up to 100 μm. The former has fewer grain boundary defects. However, on increasing the DMSO content to 45%, the crystallization of the perovskite is inhibited because of the delay of crystallization, and Fig. 4(d) indicates that the film has a smaller crystal domain size. Thus, larger crystal domains, better film uniformity, and fewer domain defects will enable efficient charge transport and excellent light absorption.

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Considering the limitation of PEDOT:PSS as a hole transport material, we used NiOx to improve the device performance. Fig. 5(a) shows the device architectures of the NiOx -based and PEDOT:PSSbased PSCs. Integrating the previous work, the perovskite precursor

Please cite this article as: D. Wang, J. Zheng and X. Wang et al., Improvement on the performance of perovskite solar cells by doctorblade coating under ambient condition with hole-transporting material optimization, Journal of Energy Chemistry, https://doi.org/10. 1016/j.jechem.2019.03.023

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Fig. 6. (a) J-V curves of the best devices based on PEDOT:PSS and NiOx HTLs. (b) EQE spectra and integrated current densities of best devices based on PEDOT:PSS and NiOx HTLs. (c) The conversion efficiency distribution of 25 devices based on PEDOT:PSS and NiOx HTLs. (d) UV-vis absorption spectra, (e) XRD spectra and (f) steady-state photoluminescence (PL) of perovskite films based on PEDOT:PSS and NiOx HTLs.

Table 2. Photovoltaic parameters for the two groups of devices. Hole contact

PEDOT:PSS NiOx

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Scan direction

Forward Reverse Forward Reverse

Voc

0.903 0.908 1.123 1.113

FF (%)

73.155 76.261 68.041 68.777

2

Jsc (mA/cm ) J-V

EQE

20.222 19.983 19.873 20.031

19.063 19.752

PCE (%)

13.36 13.83 15.19 15.34

solution with 85% DMF and 15% DMSO was selected to form the perovskite film by doctor-blade coating. The diffraction peaks of NiOx in Fig. 5(b) indicate a cubic structure, with three characteristic peaks at 37.2°, 43.1°, and 62.6°, respectively corresponding to the (111), (200), and (220) planes. The top-view SEM image of NiOx in Fig. 5(c) indicates a very smooth and uniform surface morphology, which can facilitate the formation of the perovskite film. The cross-section SEM image in Fig. 5(d) shows a dense, compact, and uniform perovskite film with thickness ∼500 nm on the NiOx film. The energy level arrangements of the functional layers are shown in Fig. 5(e), in which the energy levels of NiOx , MAPbI3 , PC61 BM, BCP, and Ag are from literatures [28,40]. Fig. 6(a) shows the J − V curves of the best devices based on PEDOT:PSS and NiOx HTLs; the photovoltaic parameters of the devices are listed in Table 2. The device based on a PEDOT:PSS HTL exhibited a Voc of 0.908 V, Jsc of 19.98 mA/cm2 , and FF of 76.261%, resulting in a PCE of 13.83%. For the device using the NiOx HTL, a remarkable PCE of 15.34% was obtained in the reverse scan with a Jsc of 20.031 mA/cm2 , Voc of 1.113 V, and FF of 68.78%. Both devices exhibited negligible hysteresis, which is common for inverted PSCs. Compared to the PEDOT:PSS-based device, the NiOx -based device exhibited a higher Voc , which can be attributed to the alignment of the valence band (VB) of NiOx with that of perovskite (MAPbI3 ) [41]. The EQE spectra of the best devices are shown in Fig. 6(b), together with the integrated current densities. Within the range of 300–450 nm, the EQE of the NiOx -based device increased continuously, whereas a plateau occurred for the EQE of the PEDOT:PSSbased device. The values of the integrated Jsc determined from the

EQE spectra (listed in Table 2) are extremely close to those obtained from the J-V curves, thus validating the PCE values to some extent. The PCE distribution of PSCs based on PEDOT:PSS and NiOx is presented in Fig. 6(c). The NiOx -based devices display better photovoltaic performance and excellent reproducibility. A similar variation can be observed from the UV-vis spectra in Fig. 6(d). The XRD spectra in Fig. 6(e) present similar characteristic peaks of perovskite on PEDOT:PSS and NiOx , indicating that a change in HTLs can hardly change the phase of the perovskite. The perovskite films prepared by doctor-blade coating based on different HTLs both show high light absorption in the range of 40 0–80 0 nm, which is consistent with the EQE results. In addition, photoluminescence (PL) spectra of the perovskite films deposited on different layers were measured; the obtained results are shown in Fig. 6(f). A decrease in the highest intensity for 750–800 nm indicates more effective charge extraction by NiOx HTL from the perovskite layer [42]. The formation process of the perovskite precursor is the same; apart from the energy level, the main difference is the wettability of the NiOx film and PEDOT:PSS film with the perovskite precursor. Fig. S2 shows that the infiltration of the perovskite precursor solution into the NiOx layer is better than that onto the PEDOT:PSS layer; the infiltration angle is only 32.45° for the former but 56.8° for the latter. This better infiltration enhanced the blade-coating of the precursor solution onto NiOx , thus contributing to the nucleation and crystal dynamics for the formation of a high-quality perovskite film under ambient conditions. Moreover, as shown in Fig. 7(a), the same 30 μL of the precursor solution are enough to cover the surface of NiOx , which serves to improve the utilization of raw materials in industrial production. From Fig. 7(b), we note that the largest crystal domain (>200 μm) in the perovskite film that was blade-coated onto NiOx , was almost twice as large as that in the perovskite film blade-coated onto PEDOT:PSS. Furthermore, the former was more uniform, exhibited better crystallographic orientation, and showed fewer defects in crystal domains than the latter. Thus, a larger crystal domain, better crystallographic orientation, and higher uniformity result in more efficient charge transport and efficient light absorption [43].

Please cite this article as: D. Wang, J. Zheng and X. Wang et al., Improvement on the performance of perovskite solar cells by doctorblade coating under ambient condition with hole-transporting material optimization, Journal of Energy Chemistry, https://doi.org/10. 1016/j.jechem.2019.03.023

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Fig. 7. (a) Diagram of solution drops before doctor blade-coating. (b) SEM image of perovskite films prepared on NiOx layer.

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4. Conclusions

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High-quality perovskite films were prepared by doctor-blade coating under ambient condition, with DMSO added to the perovskite precursor solution to delay crystallization. The as-prepared perovskite film had fewer defects and better uniformity than those without DMSO, which enabled efficient charge transport and excellent light absorption. The current density of the resulting perovskite solar cell was enhanced from 17.22 to 19.98 mA/cm2 , while the efficiency increased from 10.98% to 13.83%. Furthermore, when NiOx was used instead of PEDOT:PSS as the HTL, good wettability and higher work function were obtained, which is appropriate for preparing high-efficiency PSCs via blade-coating under ambient conditions. The as-prepared perovskite film was preferably orientated and had large grain domains sized up to 200 μm, a smoother surface, and even less obvious boundaries between the grain domains. Thus, the open-circuit voltage of a PSC prepared from this film was enhanced from 0.908 to 1.123 V, and the efficiency was enhanced from 13.83% to 15.34%.

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Acknowledgments

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This work was supported by the National Key Research and Development Project funding from the Ministry of Science and Technology of China (Grants Nos. 2016YFA0202400 and 2016YFA0202404), the Peacock Team Project funding from Shenzhen Science and Technology Innovation Committee (Grant No. KQTD2015033110182370), and the Fundamental Research (Discipline Arrangement) Project funding from Shenzhen Science and Technology Innovation Committee (Grant No. JCYJ20170412154554048), and the National Natural Science Foundation of China (Grant No. 51473139).

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Supplementary material

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Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jechem.2019.03.023.

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Please cite this article as: D. Wang, J. Zheng and X. Wang et al., Improvement on the performance of perovskite solar cells by doctorblade coating under ambient condition with hole-transporting material optimization, Journal of Energy Chemistry, https://doi.org/10. 1016/j.jechem.2019.03.023

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