Anti-solvent free fabrication of FA-Based perovskite at low temperature towards to high performance flexible perovskite solar cells

Journal Pre-proof Anti-solvent Free Fabrication of FA-Based Perovskite at Low Temperature towards to High Performance Flexible Perovskite Solar Cells Wenbin Deng, Faming Li, Jianyang Li, Ming Wang, Yuchao Hu, Mingzhen Liu PII:

S2211-2855(20)30062-8

DOI:

https://doi.org/10.1016/j.nanoen.2020.104505

Reference:

NANOEN 104505

To appear in:

Nano Energy

Received Date: 11 October 2019 Revised Date:

10 January 2020

Accepted Date: 14 January 2020

Please cite this article as: W. Deng, F. Li, J. Li, M. Wang, Y. Hu, M. Liu, Anti-solvent Free Fabrication of FA-Based Perovskite at Low Temperature towards to High Performance Flexible Perovskite Solar Cells, Nano Energy, https://doi.org/10.1016/j.nanoen.2020.104505. 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. © 2020 Published by Elsevier Ltd.

Graphical Abstract

FA-based flexible perovskite solar cell fabricated at low temperature without using anti-solvent exhibits an efficiency of 18.5% and excellent mechanical stability.

Anti-solvent Free Fabrication of FA-Based Perovskite at Low Temperature towards to High Performance Flexible Perovskite Solar Cells Wenbin Denga,b, , Faming Lia,b, , Jianyang Lia,b, Ming Wanga,b, Yuchao Hua,b, and Mingzhen Liua,b,∗ #

#

a

School of Materials and Energy, University of Electronic Science and Technology of China, Chengdu 611731, P. R. China. b Center for Applied Chemistry, University of Electronic Science and Technology of China, Chengdu 611731, P. R. China ∗

Corresponding authors: [email protected]

#

These authors contributed equally to this work.

TOC Graphic

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Abstract Flexible solar cells have attracted increasingly attentions due to its possible extensive applications in wearable, portable, flyable and deployable devices. The fabrication of flexible perovskite solar cells (PSCs) strictly requires the fabrication temperature below 150°C due to the limitation of flexible substrate. For retaining the process at low temperature, applying anti-solvents has becoming to the most popular method. However, the involvement of anti-solvents requires very precise control and brings toxicity in the fabrication procedures, which limits its large-scale industrial use. Here, we propose a strategy that simply replacing the usual anti-solvents with CH3NH3Br (CH3NH3+, MA) into dynamic spinning of perovskite precursor significantly reduces the formation energy of α-phase FA-perovskites to 40°C. An optimized annealing temperature at 100°C provides high-quality morphology and crystallization of FA-based perovskite films with superior optoelectric properties, yielding a power conversion efficiency (PCE) of flexible PSCs to 18.5% and retaining over 80% of its initial PCE after 1200 bending cycles. This is the best performance so far of preparing anti-solvent free flexible PSCs at low temperature (≤100°C). Our results offer new insights into producing high-quality FA-based perovskite materials at low temperature, which paves the way for further development of flexible PSCs to industrial commercialization.

Keywords： flexible perovskite solar cells; low temperature; anti-solvent free; mechanical stability; phase transformation

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1. Introduction Hybrid organic-inorganic perovskite solar cells (PSCs) have drawn enormous attentions because its rapid growth in power conversion efficiency (PCE), which has exceeded to 25.2% in a just few years.1, 2 The excellent photovoltaic performance is ascribed to the outstanding properties of perovskite materials, such as high charge carrier mobility,3 high absorption coefficients,4 long diffusion lengths,5 and low exciton binding energy.6 Compared to the conventional photovoltaic technologies (e.g., silicon), perovskite materials possess a noteworthy advantage of fabricating at low-temperature, which is beneficial to the production of flexible devices. In recent years, flexible PSCs have also shown remarkable development in PCE, where single-junction has achieved over 19%7 and perovskite tandem has exceeded to 21%.8 The critical challenge of reaching high performance of flexible PSCs is limited to the tolerable temperature (restricting to 150°C)9-11 of the most commonly-used flexible polymer substrate such as PEN/ITO and PET/ITO. A number of various approaches have been proposed to address this challenge to reduce the fabrication temperature of perovskites, such as compositional engineering,10,

12-22

additive coordination23 and gas-assisted

passivation.7, 24 In particular, compositional engineering that involves selection of components for perovskites, has been widely utilized for the perovskite deposition to accomplish low energy crystallization with high-quality film. Among all categories of compositions, formamidinium (NH=CHNH3+, FA)-based flexible PSCs has yielded impressive progress during the past several years due to the more suitable bandgap for photovoltaic devices. However, introduction of highly toxic anti-solvents is always applied during the processing of fabrication FA-based perovskites for achieving efficient flexible PSCs. The purpose of using anti-solvent is to oversaturate the perovskite components while spinning leading to a rapid formation of compact and uniform films with the help of low temperature annealing to further evaporate the residual solvent.25 For example, a PCE of 17.9% for flexible PSCs base on FAPbI3-xBrx was obtained through employing toluene as the anti-solvent with fabrication temperature below 150°C.17 Use of diethyl ether is also very popular to assist the formation of MA1−xFAxPbI3 perovskite films for obtaining the efficiency as high as to 18.36% for flexible PSCs.26 In addition, chlorobenzene always possesses as an useful choice in both conventional and flexible PSCs.9, 15, 27-31 Most recently, a champion flexible device with KRbCsMAFA-quintuple cations based perovskites has exceeded the PCE to 19% as result of the passivation of chlorobenzene.22 However, the application of anti-solvent is extremely sensitive to the processing conditions such as dropping time and amount of solvent, which requires very precise control in fabrication procedures. In addition, most of anti-solvents are always toxic and harmful with strong volatility, which could cause serious pollution to environment. Thus, it still remains as a challenge that the involvement of toxic anti-solvents in the production of highly efficient flexible PSCs hampers its further industrial application. 2

Here, we propose a facile strategy that dropping small amount of MABr directly after spinning the perovskite precursor (i.e., PbI2 and FAI in a mixed solution of DMF/DMSO) without using any anti-solvents during the process. We notice that the application of MABr significantly reduces the essential δ-α phase transformation energy of FA-based perovskites, leading to a reduction in annealing temperature. These FA-based perovskites can be crystalized to α phase at the annealing temperature from 40°C, possessing a best photoelectric properties at 100°C. Such reduction in fabrication temperature of FA-perovskites guarantees the prerequisites of fabricating flexible devices. Thus, we achieve a power conversion efficiency (PCE) up to 18.5%, retaining its performance after 1200 bending cycles in ambient condition. Our findings offer new insights into this important fabrication process, which can lead to better implementation and further optimization in producing high-quality FA-based flexible PSCs with improved efficiency and bending property. 2. Experimental section 2.1 Perovskite precursor preparation All the chemicals were used as received and were used without further purification unless otherwise stated, including PbI2 (99%, Sigma-Aldrich), MABr (99%, DYESOL), FAI (>99%, Greatcell), N, N-dimethylformamide (DMF, 99.8%, Sigma-Aldrich), Dimethyl sulfoxide (DMSO, 99.8%, Sigma-Aldrich), isopropanol (IPA, 99.9%), Tin oxide colloidal solution (SnO2, 15wt% in H2O, Alfa Aesar), 2,2’,7,7’-tetrakis (N, N-di-pmethoxyphenylamine) 9,9’-spirobifluorene (Spiro-OMeTAD, 99.8%, Borun New Material Technology), chlorobenzene (Sigma-Aldrich), bis(trifluoromethane) sulfonamide lithium salt (Li-TFSI, Sigma-Aldrich), acetonitrile (J&K), 4-tert-Butylpyridine (tBP, 98%, Aldrich). PEN/ITO substrate (Peccell, Japan). PbI2 (1.3M) and FAI (0.85M) were first dissolved in a mixed solvent of DMF: DMSO = 4:1 (v/v) to form perovskite precursor and MABr (8mg) was dissolved in 1ml IPA to form MABr solution. These solutions were heated at 70°C and stirring overnight just before use. The solution of hole transport material was achieved by dissolving 80mg of Spiro-OMeTAD in 1ml chlorobenzene containing 17.5µL lithium Li-TFSI in acetonitrile (520mg/mL) and 28.8µL 4-tert-butylpyridine (tBP). 2.2 Device fabrication. Planar

heterojunction

flexible

PSCs

were

fabricated

with

a

configuration

of

PEN/ITO/SnO2/perovskite/Spiro-OMeTAD/Au. PEN/ITO was used as the substrate. The flexible PEN/ITO films were pasted onto the glass substrates for the following experiments. A 3.75wt% suspension of SnO2 nanoparticles was directly spin-coated onto flexible PEN/ITO substrates, spun at 3000rpm for 30s. Afterwards, spin-coated SnO2 2

films were annealed at 90°C for 60 minutes before the deposition of perovskite films. To form FA1-xMAxPbIyBr3-y perovskite films, the perovskite precursor was spin-coated on the substrate at 3000rpm. The MABr solution of 100µL was then added to the precursor film during the spinning process in a glovebox filled with nitrogen. Subsequently, these films were annealed at 40°C, 70°C, 100°C, 120°C and 150°C for 40 minutes. The Spiro-OMeTAD precursor solution of 40µL was spin-coated onto the cooled perovskite films at 4000rpm for 30s. Finally, 100nm of Au electrode was deposited on the substrates by thermal evaporation. All the preparative work to deposit perovskite and Spiro-OMeTAD was done inside a N2-filled glovebox. For the fabrication of Space-charge-limited current method devices, NiOx films were prepared at room temperature according

to

a

previous

report.32

The

purpose

of

using

hole-only

devices

(PEN/ITO/NiOx/perovskite/Spiro-OMeTAD/Au) is to prevent perovskite layer from changing its properties due to secondary heating. 2.3 Characterizations X-ray diffraction (XRD) patterns of FA1-xMAxPbIyBr3-y films on glass substrates films were detected by a Bruker D8 Advance diffractometer with Cu Kα radiation (λ = 1.5418Å) and LYNXEYE_XE detector. A FEI Inspect F50 electron microscope field-emission electron microscope (SEM) was used to acquire surface morphology images of samples. The electron energy is 10keV. Steady-state photoluminescence(PL) and Time-resolved photoluminescence measurement were conducted by using FluoTime 300 (PicoQuant). The space-charge-limited current (SCLC) measurements were carried out by Keithley 2400 digital source-meter under dark condition. The J-V curves of the PSCs devices were measured using a Keithley 2400 series digital source-meter unit under simulated AM 1.5G irradiation (100mW cm-2, xenon-lamp, Newport). The effective area of one cell is 0.09cm2. Electrical impedance spectroscopy (EIS) was obtained by Zahner Ennium electronchemical work-station (ZAHNER ENNIUM Pro). Infrared thermal imaging was photographed by Flir C2. 3

Results and discussion

The flexible perovskite solar cells here is based a planar heterojunction structure, where the perovskite layer is sandwiched between SnO2 (electron transporting materials, ETM) and hole transport material (HTM) Spiro-MeOTAD with PEN/ITO (Peccell, Japan) as the flexible substrate. We notice that it’s essential to keep the fabrication temperature below 150°C, as the PEN/ITO is getting bended when the annealing temperature goes to 150°C (Figure 1 (a)). Thus, we fabricate all the charge transport layers and light absorbing layer at low 2

temperature (<150°C). Figure 1 (b) illustrates the process of fabricating the FA-based perovskite films with/without dropping MABr solution directly after spinning the perovskite precursor (PbI2 and FAI in a mixed solution of DMF/DMSO). We observe that the films with MABr treated requires much lower annealing temperature for crystallization compared to the usual formation temperature of FA-based perovskite films.33-35 The films with MABr treated present black colour once the annealing temperature excesses 70°C, while the films without MABr treated remain transparent even the temperature up to 150°C (Figure 1 (c)). It suggests that incorporation of MABr promotes the nucleation and crystallization of FA-based perovskites from δ phase to α phase (Figure 1 (b)), which significantly reduces the requisite annealing temperature for fully crystalizing the films. In order to distinguish the role of IPA which is used as the solvent for the MABr in our work, we fabricated FA1-xMAxPbIyBr3-y perovskite films (Figure S1-S3) with different compositions, as well as flexible devices (Figure S4) while only using IPA as the anti-solvent. We found that using IPA as the anti-solvent cannot reduce the annealing temperature of FA-based perovskites while achieving high-quality film morphology and crystallization (Figure S2&S3). Thus, we confirm that in our work IPA only works as solvent for organic halide (i.e., MABr) rather than anti-solvent for passivation effects. To investigate the effects of MABr on the crystallization of FA-based perovskite films, scanning electron microscopy (SEM) along with X-ray diffraction (XRD) were conducted systematically. SEMs illustrate that the introduction of MABr significantly improves the film morphology and crystallization of the FA-based perovskites for different annealing temperatures at 40°C, 70°C, 100°C, 120°C and 150°C, respectively (Figure 2 (a)). The films with MABr-treated show a smooth film with highly dense and composed crystallites, owning grain sizes at several hundred nanometers. We notice that the perovskite films present uniform films with full coverage when the annealing temperature from 40°C, and the grain size is getting larger with better definition in the grain boundaries as the temperature increases. While the temperature reaches to 100°C, bright and small crystals are observed, which have been suggested as PbI2, beneficial for achieving highly efficient PSCs.36 XRD further confirms the change in crystallization as a function of annealing temperature (Figure 2 (b)). Typical α-phase of FAPbI3 is assigned to peaks at 14.1°, 28.2° and 31.9°, corresponding to (110), (220) and (310) planes respectively,37, 38 where δ-phase usually takes place at 11.8°.38 Thus, we observe that the film with MABr heated at 40°C exhibits mixed phases (i.e., mixture of α-phase and δ-phase), where additional small XRD peaks at low angles (6.55°, 7.21° and 9.17°) are attributed to the MAI–PbI2–DMSO intermediate phase.25 Once the temperature exceeds to 70°C, the α-phase dominates the crystal formation. This confirms that the involvement of MABr significantly reduces the δ-α phase transformation temperature of FA-based perovskites. As the temperature further increases, the α-phase of FA-based perovskite becomes more crystallized with sharper and higher intensity 2

in XRD. Meanwhile, the peak at 12.8° referring to PbI2 crystals also appears which agrees the information seen from SEMs when the temperature exceeds to 100°C (Figure 2 (c)). Thus, we propose this crystallization process schematically in Figure 2 (d). we suspect that the nucleation of MA-based perovskite is partially formed while dropping MABr due to its lower formation energy39 that would act as seeds to promote the growth of cubic phase FA-based perovskite with less thermal-annealing energy. In this way, a high quality FA-based perovskites can be fabricated with lower annealing temperature through this MABr treating method. We also assess the effects of annealing temperature on the defect-states of the FA1-xMAxPbIyBr3-y films through Space-charge-limited current method. Figure 3 (a) and Figure S5 present the dark current–voltage curves of the devices with a configuration of PEN/ITO/NiOx/perovskite/Spiro-OMeTAD/Au. The devices show a linear Ohmic-type response at low bias voltages. Once the applied voltage exceeds the kink point, the current displays significant enhancement due to the filling of trap states by injected carriers. Thus, the kink voltage is also known as trap-filled limit voltage (VTFL) and the trap density can be calculated through the equation:

(1) Where e, ε, ε0 and L represent the elementary charge, the vacuum permittivity, the relative dielectric constant of the perovskite, the electron charge and the thickness of the perovskite films, respectively.40 Here ε of 46.9 is adopt from single-crystal FAMA perovskite.41 Thus, we found that the perovskite films annealed at 100°C possesses the lowest trap density (Figure 3 (b) ), which could be beneficial to the device performance. The steady state photoluminescence (PL) spectra (Figure 3 (c)) further confirm the suppression in defect-states of the films with 100°C annealing, where PL peaks present blue-shift from 40°C to 100°C and followed by red-shift when exceeding 100°C. The slight blue-shift of the emission peak is very likely due to the spontaneous radiative recombination between the suppressed band-edge trap states.42 Meanwhile, the perovskite films annealed at 100°C also present the longest lifetime (τ1=415ns) in time-resolved photoluminescence (TRPL) (Figure 3 (d)) as compared to other films (i.e., τ1=181ns, 259ns, 274ns and 194ns for perovskite films annealed at 40°C, 70°C, 120°C and 150°C, respectively (Table S1)), indicating slower recombination process with less defects. To further evaluate the effects of different temperature on devices, we conduct the electrical impedance spectroscopy (EIS) with a configuration of PEN/ITO/SnO2/perovskite/Spiro-OMeTAD/Au on the flexible substrate, measured at a bias of 0.85V under dark conditions (Figure 4 (a)). The Nyquist plots of the devices with perovskite films annealed at different temperatures (i.e., 40°C, 70°C, 100°C, 120°C, 150°C) show that the device with perovskite annealed at 100°C has the smallest semicircles in the high-frequency region, suggesting a lowest charge transform resistance of the device.43, 44 To investigate the recombination mechanism in flexible PSCs based on 2

perovskite films annealed at different temperature, we also measured the dependence of short current density (JSC) and open voltage (VOC) on light intensity ranging from 5 to 100mW/cm2 (in Figure 4 (b) and Figure S6). The JSC varied with light intensity has a linear relation at double logarithmic scale, where the fitting slope values are 0.984, 0.986, 0.994, 0.992 and 0.989 for flexible PSCs based on the perovskite films annealed at 40°C, 70°C, 100°C, 120°C, 150°C, respectively (Table S2). The perovskite film with annealing temperature at 100°C presents a slope closer to 1, indicating that this device experiences less space-charge-limited photocurrent than others since an ideal photovoltaic device with no space charge limit owns a slope of α = 1.26 Benefitting from the reduced fabrication temperature of FA-based perovskite films, the device performance on flexible substrate (i.e., PEN/ITO) was also evaluated. We assess the devices statistically with the perovskite films annealed at different temperatures (with sample size at 40 cells for each annealing temperature) and notice that even the devices with 40°C annealing temperature exhibit PCE. Further to optimize the annealing temperature, the devices with films annealed at 100°C show the best performance (Figure 4 (c)), which is consistent with the film characterization above. We achieve the champion device based on the perovskite annealed at 100°C with a PCE of 18.5%, yielding VOC, JSC and fill factor of 1.124V, 22.2mA/cm2 and 0.739, respectively (Figure 4 (d)). Such champion device delivers a steady-state power output of 17.7% at the maximum power output point of 0.915V (Figure 4 (e)). We further test the bending fatigue properties of the flexible PSCs in Figure 4 (f). At a bending radius of 7mm, the VOC and JSC almost keep a constant with increasing bending cycles, while the FF decreases dramatically after 1200 bending cycles, leading to a reduction in PCE. The increased sheet resistance of the flexible PEN/ITO substrate with increased bending cycles is attributed predominantly to the decrease in performance.26 4

Conclusion In summary, we proposed a new method that dropping MABr directly after perovskite precursor during the

dynamic spinning processing leads to a remarkable reduction in the formation energy of α-phase FA-perovskites to 40°C. By carefully controlling the fabrication procedures of perovskite films, we achieved an optimal annealing temperature at 100°C essential for obtaining high-quality FA-based perovskite films with highly-orientated crystallization, uniform morphology and low defect-states. Based on the optimized perovskite films, we obtained the PCE of the flexible devices up to 18.5% with excellent bending properties of retaining over 80% initial PCE after 1200 bending cycles. Our findings open a new route for the fabrication of high-quality FA-based perovskite films at low temperature, which are ideal for environmentally friendly application of flexible PSCs.

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Conflicts of interest There are no conflicts of interest to declare.

Acknowledgments This work was supported by the National Key R&D Program of China (2017YFA0207400), the National Natural Science Foundation of China (61604032), the Special Program for Sichuan Youth Science and Technology Innovation Research Team (2019JDTD0006) and the Fundamental Research Funds for the Central Universities of China (ZYGX2016J206).

Appendix A. Supporting Information Supplementary data associated with this article can be found in the online version at doi: xxxxxxxxx

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Figures

Figure 1. (a) Infrared thermal imaging of PEN/ITO substrate annealed at different temperatures of 40°C, 70°C, 100°C, 120°C and 150°C. (b) Schematic illustration of perovskite formation and the structure of FAPbI3 and FA1-xMAxPbIyBr3-y (with MABr treated). (c) photo-images of FAPbI3 and FA1-xMAxPbIyBr3-y perovskite films on bare glass annealed at different temperatures of 40°C, 70°C, 100°C, 120°C and 150°C.

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Figure 2. (a) SEM images of FA1-xMAxPbIyBr3-y films annealed at different temperatures of 40°C, 70°C, 100°C, 120°C and 150°C and FAPbI3 film annealed at 100°C. (b)XRD pattern of the FA1-xMAxPbIyBr3-y films annealed at different temperatures of 40°C, 70°C, 100°C, 120°C, 150°C and FAPbI3 film annealed at 100°C. The peak marked “∗” corresponds to MAI-PbI2-DMSO intermediate phase; the peak marked “♣” corresponds to δ-phase of FAPbI3; the peak marked “•” corresponds to PbI2 phase; the peak marked “♦” corresponds to α-phase of FA1-xMAxPbIyBr3-y. (c) Enlarged XRD pattern from 11° to 15° to highlight the effect on different annealing temperatures of 40°C, 70°C, 100°C, 120°C and 150°C. (d) The schematic diagram of FA1-xMAxPbIyBr3-y film crystallization process.

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Figure 3. (a) The Logarithmic plotted dark I-V curves of hole-only devices (PEN/ITO/NiOx/perovskite/Spiro-OMeTAD/Au) with FA1-xMAxPbIyBr3-y films annealed at 100°C. (b) Trap density of hole-only devices as a function of perovskite films annealed at different temperatures of 40°C, 70°C, 100°C, 120°C and 150°C. (c) PL spectra of FA1-xMAxPbIyBr3-y films annealed at different temperatures of 40°C, 70°C, 100°C, 120°C and 150°C. (d) Time-resolved PL decay of FA1-xMAxPbIyBr3-y films annealed at different temperatures of 40°C, 70°C, 100°C, 120°C and 150°C.

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Figure 4. (a) Nyquist plots of flexible PSCs with perovskite films annealed at different temperatures of 40°C, 70°C, 100°C, 120°C and 150°C. (b) Jsc versus light intensity of flexible PSCs with perovskite films annealed at different temperatures of 40°C, 70°C, 100°C, 120°C and 150°C. (c) PCE of devices with perovskite films annealed at different temperatures of 40°C, 70°C, 100°C, 120°C and 150°C, with sample size at 40 cells for each annealing temperature. (d) J-V curves of the champion device (perovskite films annealed at 100°C) measured under AM1.5G 100 mW/cm2. (e) Steady state PCE and current density of the champion device measured at maximum-power point of 0.915V. (f) Normalized performance parameters of a flexible PSCs (perovskite films annealed at 100°C) as a function of bending cycles (bending radius = 7 mm)

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Wenbin Deng received his bachelor's degree in University of Electronic Science and Technology of China (UESTC). He is currently a master student at the School of Material and Energy, UESTC. His research interests focus on flexible perovskite solar cells.

Dr. Faming Li is currently an associate professor at School of Materials and Energy, University of Electronic Science and Technology of China (UESTC). He received his Ph.D degree in Department of Physics from Nanjing University, China. He joined the UESTC in 2016, and his research interests include materials, fabrication and engineering of perovskite solar cells.

Jianyang Li received his bachelor's degree in University of Electronic Science and Technology of China (UESTC). He received his master’s degree from UESTC in 2019. His research interests focus on the long-term stability of perovskite solar cells.

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Ming Wang received his bachelor's degree in Materials Science and Engineering from Southwest Jiaotong University. He is currently a master student in School of Material and Energy, University of Electronic Science and Technology of China. His research interests focus on perovskite semiconductors and light converting devices, lead-free perovskites and vapor deposition technology.

Dr. Yuchao Hu received his Ph.D in Power Engineering and Engineering Thermophysics from Xi’an Jiaotong University. He worked as a visiting scholar in Chemistry Department of Northwestern University from 2014~2015. He is currently a postdoctoral fellow in School of Material and Energy，University of Electronic Science and Technology of China. His research interests focus on application of semiconductor materials in transferring of solar energy，including photocatalytic water splitting and photovoltaic solar cells.

Prof. Mingzhen Liu got her undergraduate degree at University of Bristol and pursued her MPhil at University of Cambridge. Later she got Ph.D. degree at University of Oxford. She became a professor at the University of Electronic Science and Technology of China (UESTC) since 2015. Prof. Liu was selected as a member of the National Youth Thousand Talents Program in 2016. In 2018, Prof. Liu joined the School of Materials and Energy in UESTC and became the vice dean. Her current research interests are to develop low-cost, high-efficiency, and long-lifetime perovskite solar cells. 22

Highlights Flexible PSCs exhibit a champion photovoltaic power conversion efficiency of 18.5%. This is the best performance so far of preparing anti-solvent free flexible PSCs at low temperature (≤100°C). Obtain

high-quality

perovskite

films

with

highly-orientated

crystallization, uniform morphology and low defect-states. Significantly reduce the formation energy of α-phase FA-perovskites to 40°C without using any anti-solvents

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: