Pre-crystallisation applied in sequential deposition approaches to improve the photovoltaic performance of perovskite solar cells

Journal Pre-proof Pre-crystallisation applied in sequential deposition approaches to improve the photovoltaic performance of perovskite solar cells Yulong Zhang, Zhaoyi Jiang, Weijia Zhang, Lanqin Yan, Chaoqun Lu, Cong Ni PII:

S0925-8388(19)34862-5

DOI:

https://doi.org/10.1016/j.jallcom.2019.153616

Reference:

JALCOM 153616

To appear in:

Journal of Alloys and Compounds

Received Date: 6 October 2019 Revised Date:

19 December 2019

Accepted Date: 30 December 2019

Please cite this article as: Y. Zhang, Z. Jiang, W. Zhang, L. Yan, C. Lu, C. Ni, Pre-crystallisation applied in sequential deposition approaches to improve the photovoltaic performance of perovskite solar cells, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2019.153616. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

Credit author statement Yulong Zhang: Conceptualization, Methodology, Investigation, Formal analysis, Writing - Original Draft, Writing - Review & Editing. Zhaoyi Jiang: Investigation, Methodology, Formal analysis, Writing - Review & Editing. Weijia Zhang: Methodology, Resources, Formal analysis, Writing - Review & Editing, Supervision, Project administration, Funding acquisition. Lanqin Yan: Investigation, Resources. Chaoqun Lu: Investigation, Formal analysis. Cong Ni: Investigation, Formal analysis.

Pre-crystallisation applied in sequential deposition approaches to improve the photovoltaic performance of perovskite solar cells Yulong Zhang a,#, Zhaoyi Jiang a,#, Weijia Zhang a,*, Lanqin Yan b, Chaoqun Lu a, Cong Ni a a Center of Condensed Matter and Material Physics, School of Physics, Beihang University, Beijing 100191, People’s Republic of China b National Center for Nanoscience and Technology, Beijing 100190, China * Corresponding author. E-mail: [email protected]. Telephone number: 011+86+10+82338147 # These authors contributed equally. Abstract The fabrication process for perovskite films is critical for their microstructure and photo-electrical properties. In this study, a pre-preparation step for perovskite crystal particles with a conventional two step spin-coating method was introduced for improving the quality of the prepared perovskite films. By adding CH3NH3I into the PbI2 precursor solution and performing anti-solvent extraction during the preparation process, a small amount of perovskite crystal particles was produced in the PbI2 layers. We demonstrated that the pre-preparation of perovskite crystal particles enhanced the grain size and coverage of the perovskite films due to modification of the surface morphology and crystallisation of the PbI2 layers. Furthermore, after optimisation, the crystal size in the perovskite films reached almost 3 µm, which tremendously enhanced the generation and transportation of photo-generated carriers in photo-voltaic devices. Under illumination, the fill factor (FF) and short circuit current density (Jsc) of the corresponding devices increased, and the photoelectric conversion efficiency (PCE) was enhanced from 11.7% to 17.5%. Further, the stability of the devices in dark humid air was improved. Our results demonstrate that pre-preparing the perovskite crystal particles is useful for the fabrication of high-quality perovskite films. This approach can also be applied to devices based on perovskite films with other compositions. Keywords: perovskite, solar cells, Crystallization, sequential, deposition, Solvent engineering 1. Introduction Recently, hybrid organic-inorganic perovskite film-based photo-electrical devices have attracted considerable attention. Owing to their strong light absorption, low binding energy, long carrier lifetime, bipolar charge transfer, and easy preparation, perovskite films have been widely applied to thin film solar cells [1-10]. The photoelectric conversion efficiency of perovskite solar cells (PSCs) has been increased to over 22% since the 3.8% increase reported by Kojima in 2009 [2][11][12] . The generation and transmission of photogenerated carriers in the device are mainly related to the microstructure of the perovskite film, in particular, the coverage, crystallinity, and number of defects [13-16]. However, perovskite films contain a large number of grain boundaries, where the light absorption and ion mobility are lower than elsewhere in the crystal [17-19]. Therefore, a perovskite film with large crystal grains is favourable for enhancing the PCE of PSCs. Sequential deposition [20-22] and solvent engineering [23-27] are conventional methods for preparing perovskite films. Owning to its high coverage on substrates, easy doping, and low cost,

the two-step spin-coating method has been commonly used for preparing perovskite films with excellent microstructures and photo-electrical properties. Nevertheless, the fabrication methods and composition of perovskite films can be improved in various ways. Yang et al. proposed an intramolecular exchange process based on a two-step process and obtained a Lewis adduct PbI2·DMSO solid film in the first deposition step; then, they spin coated the organic ammonium halide MAX solution. The FAPbI3 film obtained after annealing reached a grain size of approximately 500 nm [28]. Chien-Hung et al. used H2O as an additive in a two-step MAI solution to prepare a perovskite film with crystal grain diameter of 1 µm [29]. Further, after annealing in DMF vapour, the grain size was expanded to 3 µm to obtain a single-layered perovskite film. Bo et al. used MAC1 and DMSO as additives, and the size of colloidal clusters in the standard perovskite precursor solution increased significantly under the action of coordination interaction [30] . They spin-coated the large colloidal cluster of ordered rows on the substrate to form a mesophase monolayer film, which grew to form large grains with an average diameter of 3 µm. By adjusting the ratio of DMF to DMSO in the mixed cationic perovskite precursor solution, Min et al. found that the Lewis base adduct in the mixed cationic perovskite system still existed in the form of PbI2·MAI·DMSO, although the composition of the pure cationic perovskite precursor of PbI2 was different [31]. A perovskite film with a grain size of approximately 500 nm was prepared using a precursor solution with a DMF and DMSO ratio of 3:5. The increase in grain size was accompanied by a reduction in the number of defects, which facilitated efficient carrier transport and suppression of recombination, providing a basis for the preparation of high-efficiency PSCs [32] . In this work, a small amount of CH3NH3I and PbI2 was added in the PbI2 precursor solution as an additive. After spin-coating and anti-solvent treatment, the generation of perovskite crystals in PbI2 layers was detected by X-ray diffusion and absorption spectroscopy. By inducing perovskite crystals in the PbI2 layers, high quality perovskite films with larger grain size (approximately 3 µm) were obtained, which, in turn, promoted the transport of photo-generated carriers in the films. Then, by optimisation, the PCE and stability in air of the corresponding photo-voltaic devices were enhanced, and their influence on the microstructure of the perovskite films and the photovoltaic properties of the corresponding photo-voltaic devices were evaluated. 2. Experimental Details 2.1 Materials synthesis N-dimethylformamide (DMF, 99.8%, Sigma-Aldrich); dimethyl sulfoxide (DMSO, 99.5%, Sigma-Aldrich); anhydrous isopropanol (IPA, 99.5%, Sigma-Aldrich); chlorobenzene (CB, 99.8%, Sigma-Aldrich); Acetonitrile (CAN, 99.9%, Sigma-Aldrich); PbI2 (99.999%, Xi’an polymer Light Technology Corp.); CH3NH3I (MAI, 99.5%, Xi’an polymer Light Technology Corp.); CH3NH3Br (MABr，99.5%, Xi’an polymer Light Technology Corp.）; Spiro-OMeTAD（99.98%，Xi’an polymer Light Technology Corp.); bistri-uoromethanesulfonimide lithium (Li-TFSI, 99.95%, Sigma-Aldrich); 4-tert-butylpyridine (tBP, >96.0%, Sigma Aldrich); urea (Sinopharm Chemical Reagent Co., Ltd); HCl (Sinopharm Chemical Reagent Co., Ltd); mercaptoacetic acid (Sinopharm Chemical Reagent Co., Ltd); SnCl2·2H2O (98%, Alfa Aesar); SnO2 (SnO2 15% in H2O colloidal dispersion, Alfa Aesar); gold electrode (99.99%, Zhongnuo Advanced Material Technology Co. Ltd).

2.2. Preparation of the precursor solutions The PbI2 precursor solution was prepared by a conventional two-step process. We added 461 mg of PbI2 and 0.2 m of DMSO to 1 mL of DMF solvent, dissolved the mixture by magnetic stirring, and prepared the solution with a molar concentration of 1 M PbI2. The pre-crystallised precursor solution was prepared by adding equimolar amounts of PbI2 and MAI to the PbI2 precursor solution and dissolving it at 60 °C by magnetic stirring. The pre-crystallised precursor solution was defined as PbI2+x(PbI2+MAI), with x values of 0, 0.1, 0.15, 0.2, and 0.25. The x is the molar ratio of (PbI2 + MAI) to PbI2. The second solution preparation step was to add 31.8 mg of MAI and 6 mg of MABr to 1 mL of IPA solvent and dissolve them by magnetic stirring to obtain a solution with a molar concentration of 0.20 M MAI and 0.05 M MABr. The HTM Spiro-OMeTAD solution was prepared by dissolving 72.3 mg of Spiro Spiro-OMeTAD in 1 mL of CB solvent, adding 17.5 mL of Li-TFSI solution (520 mg of Li-TFSI dissolved in 1 mL of ACN) and 28.8 µL of tBP, and dissolving with magnetic stirring. All solutions were filtered using a 0.44 µm pore PVDF syringe filter after 12 h. 2.3 Device fabrication The glass with ITO coating was ultrasonically washed with deionised water, acetone, and alcohol for 15 min, dried with N2, and placed in an ultraviolet (UV) irradiation machine for UV ozone treatment for 30 min. The electron transport layer was prepared by spin coating and water bath methods. The diluted SnO2 hydrocolloid was spin-coated on the cleaned ITO glass at 3000 rpm for 30 s; the dilution ratio was 1:5, and the concentration after dilution was 3%. The sample was then placed on a 180 °C heating table for 1 h. The water bath solution was prepared by dissolving 1 g of urea in 80 ml of deionised water, adding 20 µL of thioglycolic acid and 0.5 mL of HCl, and finally dissolving 0.01 M of SnCl2·2H2O in the solution. After stirring for 10 min, the sample was placed vertically, heated to 70 °C and held at this temperature for 3 h. After being dried with N2, it was placed on a heating table at 180 °C for 1 h. Once the sample cooled to room temperature, it was moved into the glove box. As shown in Fig. 1, the first step of the pre-crystallisation two-step sequential deposition of the perovskite film was to apply 100 µL of a PbI2 precursor solution to a SnO2 substrate and spin-coat at 3000 rpm for 30 s. Then, 200 µL of anti-solvent CB was added dropwise during the last few seconds of the spin coating process to accelerate the crystallisation process. The wet film was placed on a heating table at 80 °C for 10 min. Once the temperature is lowered, the second step of spin coating was performed. A certain amount of the second-step solution was spin-coated on the sample at 3000 rpm for 30 s. The sample was then annealed on a hot plate at 100 °C for 40 min. After cooling to room temperature, 100 µL of Spiro solution was spin-coated at 4000 rpm for 30 s on the surface of the sample. The sample was placed in a dry box and oxidised for 12 h in an environment with a humidity of <10%, an oxygen content of 21%, and a temperature of 20 °C. Finally, an Au electrode of approximately 50 nm was deposited by thermal evaporation at a pressure of 3×10-6 Pa. 2.4 Characterisation

The FEI NOVA NANOSEM 450 cold field scanning electron microscopy (SEM) system was used to observe the surface morphology of the films prepared on Schott glass. The X-ray diffraction (XRD) patterns of the films in the 10°−60° region were

collected by a Bruker D8 X ray diffractometer with Cu Ka radiation and 1.5418 Å wavelength. The absorption spectra of the films were measured with a Hitachi U-4100 UV-Vis spectrophotometer. The steady-state photoluminescence spectra (SSPL) of the films were recorder with an Edinburgh Instrument FLS920 with a 450 W xenon lamp as excitation light source. The J-V curves of the devices under AM1.5 illumination (standard 100 mW·cm-2) were measured with an Agilent B1500A Semiconductor parameter analysis instrument with a 450 W xenon lamp light source. The external quantum efficiencies (EQE) were measured with a SPIQE200-5031 AC/DC I/EQE system in AC mode (Newport Corporation). 3. Results and Discussion During the pre-crystallisation process, a PbI2 film is deposited on the surface of the substrate after the first step, as shown in Fig. 2(a). Because the first step solution contained a small amount of MAI, the colour of the PbI2 film was gradually deepened. The colour of the sample with x=0 is yellow, which is the usual colour of PbI2 films. The colour of the sample with x>0 gradually changed to brown because the film contained a small amount of perovskite. Figure 2 (b) shows the XRD patterns of the PbI2 film. The (001) characteristic peak of PbI2 at 12.7° the (002) characteristic peak at 25.5°, and the (003) characteristic peak at 38.6° are clearly observed in all five samples. The sample with x>0 exhibits the (110) characteristic peak of MAPbI3 at 14.2°, the (220) characteristic peak at 28.5°, and the (310) characteristic peak at 32°. This indicates that, in this sample, MAPbI3 crystals are produced in the PbI2 film, which also proves that the colour darkening in the photograph is caused by the MAPbI3 crystal. Figure 2(c) shows the XRD pattern between 11° and 15° containing the (001) peak of PbI2 and the (110) peak of MAPbI3. it was found that the intensity of the main peak at 12.7° PbI2 increased with increasing amount of (PbI2+MAI); this indicates that the incorporation of (PbI2+MAI) assisted the crystallisation of PbI2 crystals effectively. The main peak of the MAPbI3 was observed at 14.2°. As the amount of (PbI2+MAI) increased, the intensity of MAPbI3 crystal increased gradually. However, even in the sample with x = 0.25, the peak at 14.2° is much lower than that at 12.7°, which indicates that the content of MAPbI3 grown in the first step is very low, and the MAI is not completely reacted. Figure 2 (d) shows the UV-vis absorption spectrum of the PbI2 film. It can be seen in the figure that the characteristic light absorption of PbI2 from 400 to 520 nm increases with increasing x, indicating an enhanced light-scattering effect. At wavelengths longer than 520 nm, the light absorption also increases with x, which is caused by the MAPbI3 crystal formed by the reaction of PbI2 and MAI in the film. Figure 3(a)−(e) shows a top-view SEM image of the PbI2 samples with x=0−2.5, respectively. It can be seen from this figure that as x increases, the crystal structure changes, gradually transforming from granular to rod-shaped, and the gap between crystals gradually increases. This phenomenon can be understood as a PbI2 crystals growth-induction effect by the MAI or MAPbI3 crystals. To obtain the two-step prepared perovskite film, 100 µL of the second-step solution was spin-coated on the surface of the PbI2 film, which was then annealed. Figure 4(a) shows the XRD patterns of the PbI2 film. Significant peaks of (110), (220), (310), and (224) planes of MAPbI3 are observed for each sample in the figure at 14.2°, 28.5°, 32°, and 40.8°, respectively. However, the

(001) and (003) characteristic peaks of PbI2 were still observed in samples with x < 0.2 at 12.7° and 38.5°, respectively. This indicates that an equal amount of the second-step solution causes different degrees of PbI2 conversion to MAPbI3 in different samples. As x increases, more PbI2 converts to MAPbI3. Although PbI2 passivates defects in the perovskite film[33-35], we expect PbI2 to react fully. The complete conversion of PbI2 indicates that the first-step PbI2 film prepared by the pre-growth process reacts more easily with the second-step solution, which prevents the issue incomplete PbI2 conversion in the sequential deposition process. To induce the complete reaction of the different (PbI2+MAI) amounts in the PbI2 film, the amount of the second-step solution was increased for the samples with x=0, x=0.1, and x=0.2. After the appropriate amount of the second-step solution was spin-coated on the surface of the PbI2 film and the film was annealed, the XRD patterns of the perovskite film were obtained, as shown in Fig. 4(b). After improving the amount of solution in the second step, the PbI2 characteristic peaks of the five samples could not be observed, indicating that PbI2 completely converted into MAPbI3. In addition, MABr is contained in the second-step solution, but no Br-doped perovskite characteristic peak was observed in the perovskite film because MABr induces growth during the reaction, and the perovskite grown will not contain the Br component after it is completely converted. The characteristic peak intensity and full-width-at-half-maximum of the five samples at 14.2° are shown in Fig. 4(c). The crystallinity and grain size of the perovskite film increase monotonically with the amount of (PbI2+MAI) incorporated [36]. Similarly, in the top-view SEM image of the perovskite film in Fig. 3(f)−(j), it can be clearly seen that the crystal size of MAPbI3 increases with x. In the increase of the process, x=0.1 and x=0.15 samples were observed, and the crystal sizes in the same samples differed significantly. With x = 0.2, the crystal reached a size of approximately 3 µm and the perovskite film is continuous, no holes, fully covered on the substrate, and free of holes. However, with x = 0.25, although the crystal size was further increased, a large hole appeared in the crystal. This indicates that a lower amount of (PbI2+MAI) should be incorporated. The blue shift of the emission peak in the steady-state PL spectra of the perovskite film (growth on the glass) in Fig. 3(d) also indicate the increase in the grain size of the perovskite (the calcium-titanium with x=0−0.25). The emission peaks of the perovskite films are 769, 764, 761, 759, and 757 nm, respectively. At the same time, the strength of PL varies greatly. As x increases, the PL intensity first increases and then decreases, reaching the highest intensity at x=0.2, which indicates a low recombination at this time[37]. When x> 0.2, the PL intensity begins to decrease, which indicates that the carrier recombination rate is increased, which is caused by defects such as holes in the perovskite film, as shown in Figure 3 (j). Figure 4(e) shows the UV-vis absorption spectrum of each perovskite film sample. As the amount of (PbI2+MAI) incorporated increases, the light absorption of the films between 420−800 nm increases. With x = 0.25, the absorption is below 500 nm, indicating the presence of nano-sized pores in the film, which is consistent with Fig. 3(j). From the SEM images of the x=0 and x=0.2 perovskite films in Fig. 5(a) and (b), respectively, it can be seen that the x=0.2 film is larger than the x=0 film, and its crystal size is larger. The figures show that a perovskite film composed of one layer of perovskite crystals is obtained. The photovoltaic properties of the MAPbI3 film were further investigated. The device was assembled with ITO/SnO2/MAPbI3/Spiro/Au, as shown in Fig. 6(a). Figure 6(b) the J-V curve with a scan rate of 50 Mv/s of a solar cell prepared with a pre-growth process under a standard

AM 1.5 solar illumination intensity of 100 mW cm-2. The performance parameters are summarised in Table 1 and are shown in Fig. 7. It can be clearly seen that all performance parameters are affected by the amount of (PbI2+MAI) added. The solar cell with x=0.2 has a maximum PCE of 17.57% and the highest average PCE among the solar cells tested (16.53%); these values are much higher than that of the device without (PbI2+MAI). The average PCEs of the solar cells with x=0.1 and x=0.15 were 12.86% and 15.57%, respectively, which are still higher than those without (PbI2+MAI). The enhancement in the PCE results from the higher Jsc and Voc and increased FF. In turn, the improvement in Jsc resulted mainly from the increase in the crystal size of the perovskite, which led to a decrease in the number of grain boundaries. The maximum Jsc was 21.97 mA/cm2 with x=0.2. With x=0.2, the highest Voc was 1.079 V. This slight increase was due to the enhanced crystallinity of the perovskite crystal, resulting in a decrease in radiation recombination. The FF also increased due to the higher series resistance of the larger perovskite crystal (78.2 Ω for series resistance at x = 0 and 12.6 Ω for series resistance at x = 0.2) and the highest FF (76.6%) was also obtained with x = 0.2. Excessive (PbI2+MAI) content will cause a sharp drop in Jsc and the FF. The shorter error bars in Fig. 7 indicate that the pre-growth process produces a perovskite film with good repeatability, and the obtained PSC has a narrower efficiency distribution. Table 1 Summary of solar cell photovoltaic performance parameters prepared based on pre-growth processes with different x value (20 groups of each x value). x

Jsc (mA/cm2)

Voc (V)

FF (%)

PCE (%)

0

16.27±0.72

0.99±0.025

66.5±3.9

11.08±0.70

0.1

17.81±0.85

1.02±0.028

68.6±2.9

12.86±1.04

0.15

20.11±0.94

1.04±0.021

72.1±3.6

15.57±0.86

0.2

21.11±0.86

1.05±0.029

73.4±3.2

16.53±1.04

0.25

17.44±1.09

1.00±0.027

61.5±2.9

10.69±1.34

Figure 8(a) shows the external QE (EQE) spectrum. The efficiency of the x=0 sample is higher than 70% at 369−538 nm wavelength and lower than 60% at wavelengths longer than 596 nm. The photocurrent at 800 nm is consistent with the optical band gap of MAPbI3. The lower EQE under long wavelengths is caused by offset charge extraction and light trapping, resulting in a relatively low Jsc of the solar cell. The x=0.15 and x=0.2 samples showed a significant increase over the entire spectrum; in particular, the x=0.2 maintained over 80% within the long-wavelength region between 397−686 nm with a peak value of over 89%. The enhanced light absorption of the perovskite film is shown, which is consistent with the absorption spectrum of Figure 4(e), which makes the sample show high injection photon conversion efficiency in the long wave region. The enhanced EQE has also led to a significant increase in Jsc. The integrated current densities of the three are 16.19 (x=0), 19.87 (x=0.15), and 21.31 (x=0.2), which are consistent with the J-V curve test results. The PSCs experience J-V curve hysteresis depending on the quality of the perovskite layer and the interface charge accumulation. The efficiency of negative sweeping (scanning from high voltage to low voltage) is higher than that of positive sweeping (from low voltage to low voltage). Figure 8 (b) (c) and (d) show typical positive and negative sweep curves of a perovskite solar device prepared by a precursor solution with x = 0, 0.15, and 2, respectively. The prepared scanning efficiency of the devices with x = 0, 0.15, and 2 reached 75.3%, 84.7%, and 88.7% of the

anti-sweep efficiency, respectively; the hysteresis factors calculated according to the literature were 24.7, 15.3, and 11.3, respectively. The decrease in hysteresis factor was due to the decrease in grain boundaries as the grain size increases. The hysteresis factor of the PSC prepared by the pre-growth process is close to the typical hysteresis factor of high quality PSCs. This shows that the pre-growth process can improve the quality of the perovskite film. The stability of the PCSs is important for their application and fabrication. The main factors that led to the breakdown of the perovskite films were the illumination and the concentration of oxygen and moisture in air. Figure 9 (a) and (b) shows the results of the PSCs stability tests performed inside a glove box and in dark air, respectively. The humidity of the dark air is about 30RH and the oxygen content is about 21%. It is clear that the PSCs prepared by the pre-crystallisation process are very stable even in air. Furthermore, the decrease in the photoelectric conversion efficiency of the corresponding devices was lower than that of reference samples, which can be attributed to the lager grain size and denser structures in the films. The PCE of the devices inside the glove box and under illumination also showed a decline with time. Because phase segregation should not occur in the prepared MAPbI3 films, the decline of the devices mainly resulted in the breakdown induced by illumination. Similar to the results obtained in dark air, the denser structure and larger grain size resulted in the superior performance of photo-voltaic devices over the reference samples. Hence, the high-quality films prepared by the pre-preparation method benefitted from the stability of the corresponding devices in air.

4. Conclusion In summary, the conventional two-step spin-coating method for fabricating high-quality perovskite films was improved. By adding small amounts of CH3NH3PbI3 in the PbI2 precursor solution, perovskite crystals grew in the PbI2 layers, influencing the microstructures. With increasing amounts of CH3NH3PbI3 in the PbI2 precursor solution, the grain size improved (the average grain size was almost 3 µm ) and the surface morphology enhanced the absorption and charge-carrier transport in the films. However, excessive concentrations of CH3NH3PbI3 produced pin-holes in the films, which deteriorated the surface morphology. When using the optimal amount of CH3NH3PbI3, the Jsc and FF values of the corresponding devices significantly increased, and consequently, the PCE increased from 11.7% to 17.5%. Furthermore, the compact structure and enlarged grains size enhanced the stability of the photo-voltaic devices in air. The perovskite film fabrication process can be implemented by introducing the pre-preparation of the perovskite crystal particles, which improves the quality of the perovskite films. This process can also be applied to photo-electrical devices based on perovskite films with other compositions. Acknowledgements The authors gratefully acknowledge Beihang University for providing access to experimental equipment to be used for samples fabrication and measurement. The project was supported by the National Natural Science Foundation of China (grant no.51572008).

Reference [1] S. Yang, W. Fu, Z. Zhang, H. Chen, C.-Z. Li, Recent advances in perovskite solar cells:

efficiency, stability and lead-free perovskite, J. Mater. Chem. A. 5 (2017) 11462-11482 [2] L.K. Ono, Y. Qi, Research progress on organic–inorganic halide perovskite materials and solar cells, J. Phys. D Appl. Phys. 51 (2018) 093001 [3] C.R. Kalaiselvi, N. Muthukumarasamy, D. Velauthapillai, M. Kang, T.S. Senthil, Importance of halide perovskites for next generation solar cells – a review, Mater. Lett. 219 (2018) 198-200 [4] I. Mesquita, L. Andrade, A. Mendes, Perovskite solar cells: materials, configurations and stability, Renew. Sustain. Energy Rev. 82 (2018) 2471-2489 [5] A. Abate, J.P. Correa-Baena, M. Saliba, M.S. Su'ait, F. Bella, Perovskite solar cells: from the laboratory to the assembly line, Chem. Eur J. 24 (2018) 3083-3100 [6] H. Zhang, Y. Wang, H. Wang, M. Ma, S. Dong, Q. Xu, Influence of drying temperature on morphology of MAPbI3 thin films and the performance of solar cells, J. Alloys Compd. 773 (2019) 511-518 [7] M. Ouafi, L. Atourki, L. Laânab, E. Vega, B. Mari, M. Mollar, B. Jaber, Hot airflow deposition: Toward high quality MAPbI3 perovskite films, J. Alloys Compd. 790 (2019) 1101-1107 [8] X. Zeng, T. Zhou, C. Leng, Z. Zang, M. Wang, W. Hu, X. Tang, S. Lu, L. Fang, M. Zhou, Performance improvement of perovskite solar cells by employing a CdSe quantum dot/PCBM composite as an electron transport layer, J. Mater. Chem. A. 5 (2017) 17499-17505 [9] M. Wang, H. Wang, W. Li, X. Hu, K. Sun, Z. Zang, Defect passivation using ultrathin PTAA layers for efficient and stable perovskite solar cells with a high fill factor and eliminated hysteresis, J. Mater. Chem. A. 7 (2019) 26421-26428 [10] M. Wang, Z. Zang, B. Yang, X. Hu, K. Sun, L. Sun, Performance improvement of perovskite solar cells through enhanced hole extraction: The role of iodide concentration gradient, Sol. Energy Mater. Sol. Cells, 185 (2018) 117-123 [11] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, Organometal halide perovskites as visible-light sensitizers for photovoltaic cells, J. Am. Chem. Soc. 131 (2009) 6050-6051 [12] W.S. Yang, B.-W. Park, E.H. Jung, N.J. Jeon, Y.C. Kim, D.U. Lee, S.S. Shin, J. Seo, E.K. Kim, J.H. Noh, S.I. Seok, Iodide management in formamidinium-lead-halide–based perovskite layers for efficient solar cells, Science. 356 (2017) 1376 [13] A. Kojima, M. Ikegami, K. Teshima, T. Miyasaka, Highly luminescent lead bromide perovskite nanoparticles synthesized with porous alumina media, Chem. Lett. 41 (2012) 397-399 [14] M. Radulescu, L. Arsenie, O. Oprea, B.S. Vasile, Optical and photocatalytic properties of copper (II) doped zinc oxide, Rev. Chim. 67 (2016) 2596-2599 [15] U. Khan, Z.N. Yu, A.A. Khan, A. Zulfiqar, N. Ullah, High-performance CsPbI 2 Br perovskite solar cells with Zinc and manganese doping, Nanoscale Res. Lett. 14 (2019) 116 [16] A.A. Khan, Z.N. Yu, U. Khan, L. Dong, Solution processed trilayer structure for high-performance perovskite photodetector, Nanoscale Res. Lett. 13 (2018) 399 [17] P. Zhao, B.J. Kim, X. Ren, D.G. Lee, G.J. Bang, J.B. Jeon, W.B. Kim, H.S. Jung. Antisolvent with an ultrawide processing window for the one-step fabrication of efficient and large-area perovskite solar cells, Adv. Mater. 30 (2018) 1802763 [18] X. Zheng, B. Chen, J. Dai, Y. Fang, Y. Bai, Y. Lin, H. Wei, X. Zeng, J. Huang, Defect passivation in hybrid perovskite solar cells using quaternary ammonium halide anions and cations, Nat. Energy. 2 (2017) 17102-17112

[19] B. Shen, Y. Wang, Z. Hu, S. Tang, Y. Chen, J. Zhang, Y. Zhu, Growth of monolithically grained CH3NH3PbI3 film by a uniform intermediate phase for high performance planar perovskite solar cells, J. Alloys Compd. 776 (2019) 250-258 [20] W.S. Yang, B.W. Park, EH. Jung, N.J. Jeon; Y.C. Kim; D.U. Lee, S.S. Shin, J. Seo, E.K. Kim, J.H. Noh, S.I. Seok, Iodide management in formamidinium-lead-halide-based perovskite layers for efficient solar cells, Science. 356 (2017) 1376-1379 [21] Y. Zhou, M. Yang, J. Kwun, O.S. Game, Y. Zhao, S. Pang, N.P. Padture, K. Zhu, Intercalation crystallization of phase-pure α-HC(NH2)2PbI3 upon microstructurally engineered PbI2 thin films for planar perovskite solar cells, Nanoscale. 8 (2016) 6265-6270 [22] Z. Xiao, C. Bi, Y. Shao, Q. Dong, Q. Wang, Y. Yuan, C. Wang, Y. Gao, J. Huang, Efficient, high yield perovskite photovoltaic devices grown by interdiffusion of solution-processed precursor stacking layers, Energy & Environmental Science. 7 (2014) 2619-2623 [23] N. J. Jeon, H. Na, E. H. Jung, T.-Y. Yang, Y. G. Lee, G. Kim, H.-W. Shin, S. I. Seok, J. Lee, J. Seo, A Fluorene-Terminated Hole-Transporting Material for Highly Efficient and Stable Perovskite Solar Cells, Nature Energy. 3 (2018) 682-689 [24] J. Liu, M. Ozaki, S. Yakumaru; T. Handa, R. Nishikubo, Y. Kanemitsu, A. Saeki, Y. Murata, R. Murdey, A. Wakamiya, Lead-Free Solar Cells based on Tin Halide Perovskite Films with High Coverage and Improved Aggregation, Angewandte Chemie, International Edition. 57 (2018) 13221-13225 [25] M. Yang, Z. Li, M.O. Reese, O.G. Reid, D.H. Kim, S. Siol, T.R. Klein, Y. Yan, J.J. Berry, H.M. van; K. Zhu, Perovskite ink with wide processing window for scalable high-efficiency solar cells, Nature Energy. 2 (2017) 17038 [26] J. Zhang, G. Zhai, W. Gao, C. Zhang, Z. Shao, F. Mei, J. Zhang, Y. Yang, X. Liua, B. Xu, Accelerated formation and improved performance of CH3NH3PbI3-based perovskite solar cells via solvent coordination and anti-solvent extraction, J. Mater. Chem. A. 5 (2017) 4190 [27] K. Sun, C. Gao, C. Yang, H. Liu, Z. Hu, J. Zhang, Y. Zhu, Direct formed tri-iodide ions stabilizing colloidal precursor solution and promoting the reproducibility of perovskite solar cells by solution process, Electrochimica Acta. 311 (2019) 132-140 [28] W.S. Yang, J.H. Noh, N.J. Jeon, Y.C. Kim, S. Ryu, J. Seo, S.I. Seok, High-performance photovoltaic perovskite layers fabricated through intramolecular exchange, Science. 348 (2015) 1234– 1237 [29] C.-H. Chiang, M. K. Nazeeruddin, M. Grätzelc, C. Wu, The synergistic effect of H2O and DMF towards stable and 20% efficiency inverted perovskite solar cells, Energy & Environmental Science. 10 (2017) 808-817 [30] B. Li, M. Li, C. Fei, G. Cao, J. Tian, Colloidal engineering for monolayer CH3NH3PbI3 films toward high performance perovskite solar cells, J. Mater. Chem. A. 5 (2017) 24168 [31] M. Wang, F. Cao, K. Deng, L. Li, Adduct phases induced controlled crystallization for mixed-cation perovskite solar cells with efficiency over 21%, Nano Energy. 63 (2019) 103867 [32] Z. Chu, M. Yang, P. Schulz, D. Wu, X. Ma, E. Seifert, L. Sun, X. Li, K. Zhu, K. Lai, Impact of grain boundaries on efficiency and stability of organic-inorganic trihalide perovskites, Nat. Commun. 8 (2017) 2230 [33] Q. Jiang, Z. Chu, P. Wang, X. Yang, H. Liu, Y. Wang, Z. Yin, J. Wu, X. Zhang, J. You, Planar-structure perovskite solar cells with efficiency beyond 21%, Adv. Mater. 29 (2017) 1703852

[34] T. Zhang, N. Guo, G. Li, X. Qian, Y. Zhao, A controllable fabrication of grain boundary PbI2 nanoplates passivated lead halide perovskites for high performance solar cells, Nano energy. 26 (2016) 50-56 [35] L. Wang, C. McCleese, A. Kovalsky, Y. Zhao, C. Burda, Femtosecond time-resolved transient absorption spectroscopy of CH3NH3PbI3 perovskite films: evidence for passivation effect of PbI2, J. Am. Chem. Soc. 136 (2014) 12205-12208 [36] Y. Zhou, M. Yang, J. Kwun, O.S. Game, Y. Zhao, S.P. Pang, P. Nitin, K. Zhu,, Intercalation crystallization of phase-pure α-HC(NH2)2PbI3 upon microstructurally engineered PbI2 thin films for planar perovskite solar cells, Nanoscale. 8(2016) 6265-6270 [37] I.L. Braly, D.W. Dequilettes, L.M. Pazos-Outón, S. Burke, M.E.Ziffer, D.S. Ginger, H.W. Hillhouse, Hybrid perovskite films approaching the radiative limit with over 90% photoluminescence quantum efficiency, Nature Photonics. 12(2018) 355-361

Figure 1. Schematic of CH3NH3PbI3 film fabrication process by pre-crystallisation two-step sequential deposition

Figure 2. (a) Photograph of PbI2 films; (b) XRD patterns of PbI2 films at 10°−50°; (c) XRD patterns of PbI2 films at 11°−15°; (d) PbI2 film light absorption at 400−800 nm.

Figure 3. (a)−(e) Top-view SEM images of PbI2 films; (f)−(j) Top-view SEM images of perovskite films.

Figure 4. XRD patterns of perovskite film after spin coating of (a) 100µL and (b) varying volumes of the second-step solution; (c) XRD patterns of calcium after spin coating of the second-step solution of different volumes; the black and blue lines indicate the peak intensity and full-width-at-half-maximum of XRD patterns, respectively; (d) PL spectra and (e) light absorption of perovskite film (growth on the glass).

Figure 5. SEM image of a cross section of a perovskite film with (a) x = 0 and (b) x = 0.2.

Figure 6. (a) Schematic of PSC device structure; (b) PCE of PSC prepared with the pre-growth process

Figure 7. Photovoltaic performance parameters of PSCs prepared with the pre-growth process with different x value (20 groups of each x value).

Figure 8. (a) EQE of x = 0, 0.15, and 0.2 samples prepared according to different x-value pre-crystallisation processes; Forward and reverse scan JV curves of (b) x = 0, (c) x = 0.15, and (d) x = 0.2 samples.

Figure 9. Stability of PSCs (a) in the glove box and (b) in the atmosphere

1. 2. 3. 4.

Highlights Sequential deposition method has been improved for the preparation of perovskite films. The high quality perovskite films were prepared by the pre-crystallisation. The crystal size in the perovskite films reached almost 3 µm. The PCE of perovskite solar cells have been greatly improved.

Conflict of interest statement We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled “Pre-crystallisation applied in sequential deposition approaches to improve the photovoltaic performance of perovskite solar cells” No conflict of interest exits in the submission of this manuscript, and manuscript is approved by all authors for publication. I would like to declare on behalf of my co-authors that the work described was original research that has not been published previously, and not under consideration for publication elsewhere, in whole or in part. All the authors listed have approved the manuscript that is enclosed.