Since the first introduction of sensitized solar cells with liquid configuration in 2009 ^[1], organolead halide perovskite materials have garnered great attention around the world. Using different compositions, these perovskite materials ^{[2, 3, 4, 5]} have been successfully applied as light absorbers in photovoltaic devices with various support materials ^{[6, 7, 8, 9, 10, 11]} and structures ^{[1, 2, 12, 13, 14, 15, 16, 17]}. Among them, mesoscopic structures based on metal oxides (TiO₂ ^{[3, 4, 6, 7, 8, 9, 12]}, Al₂O_{3 ­}^{[2, 18]}, etc.) are probably the most studied category, and exhibit very impressive photovoltaic performance. The unusual properties of these perovskite materials, such as large diffusion length ^{[19, 20, 21]} and low recombination ^{[12, 22]} enable the fabrication of planar-structured thin film photovoltaic devices with excellent performance, and promise for applications in flexible ^{[23, 24]} and tandem solar cells ^{[25, 26]}. However, in comparison with sensitized mesoscopic structured cells, fabricating high-quality perovskite films without supporting mesoscopic metal oxides is a greater challenge ^{[27, 28]}.

The vacuum thermal evaporation adopted by Snaith’s group ^[17] is a good approach which yields perovskite films with uniform thickness and high device efficiency. However, use of this method greatly increases manufacturing costs and thus is unfavorable for large-scale solar cell fabrication. One step spin-coating is a simple way to prepare perovskite materials, but does not produce films with homogeneous grain size and uniformity. Two-step deposition processes including sequential solution deposition ^{[29, 30]} and vapor-assisted two-step reaction ^{[31, 32]} have been exploited to make perovskite films with high uniformity, but these multi-step deposition procedures extend the overall processing time. Cheng’s group ^[33] invented a one-step, fast crystallization method by drop-casting chlorobenzene (CB) during the spin-coating of perovskite N,N-dimethylformamide (DMF) solution to quickly induce crystallization, yielding very flat, highly uniform CH₃NH₃PbI₃ (MAPbI₃) thin films. Seok’s group ^[34] developed a similar spin-coating and drop-casting method, mainly focused on mesoscopic systems. A mixture of γ-butyrolactone (GBL) and dimethyl sulfoxide (DMSO) was used as the solvent for the perovskite, followed by toluene drop-casting. The introduction of DMSO caused a uniform CH₃NH₃I-PbI₂-DMSO intermediate phase film ^{[34, 35]} to form, which enabled the subsequent formation of a highly uniform and dense MAPbI₃ film after annealing. GBL was reported to work solely as a high-evaporation component in this mixed solvent GBL-DMSO. Despite these findings, it is still necessary to carry out a systematic study of solvent engineering for perovskite materials.

Herein, we have investigated the influence of the type and proportion of the mixed solvents on the morphology of MAPbI₃ thin film in detail using a modified spin-coating method. We found that adding 20%~40% of DMF in the solvent mixture (DMF-DMSO) led to uniform MAPbI₃ films with large grain size and increased film thickness. As-prepared MAPbI₃ thin films were further assembled into planar-structured perovskite solar cells, which exhibited power conversion efficiency (PCE) as high as 16.5%. Because the entire solar cell fabrication process was carried out at temperatures lower than 100 °C, the present fabrication method could be easily extended to flexible photovoltaic devices on plastic substrates.

PbI₂, lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI), 4-tert-butyl pyridine (tBP), CB, and acetonitrile were purchased from Sigma-Aldrich. Spiro-MeOTAD was provided by Shenzhen Feiming Technology Co., China. CH₃NH₃I (MAI) was synthesized according to a literature method ^[31]. GBL was purchased from Aladdin Industrial Inc., Shanghai, China. DMF, DMSO and all other reagents were purchased from Sino Chem. Co., China.

A TiO₂ dense film, working as both a hole blocking layer (bl-TiO₂) and electron extraction layer, was synthesized by TiCl₄ chemical bath deposition according to the literature ^[36]. FTO glass substrates were ultrasonically cleaned with water, ethanol, acetone and 2-propanol, and then treated in an O₂-plasma cleaner for 30 min. The treated FTO substrates were immersed into 200 mmol/L TiCl₄ aqueous solution and kept at 70 °C for 1 h, followed by washing with DI water and ethanol, and were finally dried at 100 °C for 1 h before further use.

The spin-coating solution was prepared by dissolving 0.530 g PbI₂ and 0.183 g MAI in 1 mL of DMF-DMSO or GBL-DMSO solvent mixtures of varying ratios (volume fractions) under stirring at 60 °C for 12 h. The resulting solutions were spin-coated onto the prepared bl-TiO₂ layer at 1000 rpm for 10 s and 6000 rpm for 55 s at room temperature (~20 °C). During the second spin-coating step, after 25 s of spin-coating at 6000 rpm, 0.75 mL chlorobenzene was quickly dropped onto the center of the spinning substrate. After annealing for 15 min on a 100 °C hotplate, the as-prepared films turned from colorless to dark brown, indicative of the formation of MAPbI₃ perovskite. A spiro-MeOTAD solution was prepared by dissolving 72.3 mg of spiro-MeOTAD in 1 mL of chlorobenzene, into which 27.8 μL of tBP and 17.5 μL of Li-TFSI solution (520 mg Li-TFSI in 1 mL acetonitrile) were added. The spiro-MeOTAD solution was spin-coated on the perovskite film at 5000 rpm for 30 s. Finally, a gold electrode was thermally evaporated onto the spiro-MeOTAD-coated film to a thickness of ~60 nm.

The morphological characterization of the perovskite film was carried out by scanning electron microscopy (SEM; FEI Quanta200F scanning electron microscope). The crystal phase was identified by X-ray diffraction (XRD; X’Pert Pro) using Cu-K_α radiation of λ = 0.154 nm. The UV-Vis absorption of the film was measured on a Varian Cary 5000 UV-Vis spectrophotometer. The photocurrent density-voltage (J-V) characteristics of the solar cells were obtained using a Keithley 2400 source meter under illumination with simulated sunlight (AM 1.5, 100 mW/cm²) provided by a solar simulator (Newport 69907) with an AM 1.5 filter. A metal aperture of 0.09 cm² was used during the measurement to define the active area of the device and avoid light scattering through the sides. The incident photon-to-current efficiency (IPCE) of the device was measured on a QTest Station 2000ADI system (Crowntech Inc. USA) in AC mode with a tungsten-halogen lamp (150 W) as the light source. The monochromatic light intensity used in the IPCE efficiency measurements was calibrated with a reference silicon photodiode.

In this work, DMSO functioned both as a solvent and as a coordination reagent in the form of a PbI₂-CH₃NH₃I-DMSO complex, while DMF and GBL only functioned as solvents with relatively higher evaporation rates than that of DMSO (vapor pressure data are provided in Table 1). CB was used as a drop-casting solution to wash out surplus components remaining in solution to leave a uniform and flat intermediate-phase film ^[34]. Generally, to prepare the spin-coating solution, 0.530 g PbI₂ and 0.183 g MAI were dissolved in 1 mL of the mixed solvent, and the resulting solutions were spin-coated onto the FTO/bl-TiO₂ (TiO₂ blocking layer) substrates according to a modified method ^[34], as described in detail in the experimental section.

Table 1 Physical properties of organic solvents used in the spin-coating process. Solvent Molecular weight (g/mol) Density (g/mL, 25 °C) Boiling point (°C) Vapor pressure (mmHg, 20 °C) Viscosity (mPa·s, 25 °C) DMF (C3H7NO) 73.09 0.9445 153 2.7 0.8 DMSO (C2H6OS) 78.13 1.0955 189 0.42 2.0 GBL (C4H6O2) 86.09 1.124 204 1.5 1.7 CB (C6H5Cl) 112.56 1.106 131 11.8 0.799 (20 °C)

Table 1 Physical properties of organic solvents used in the spin-coating process.

The SEM images in Fig. 1 show the changes in morphology of the as-prepared perovskite films. In the case of pure DMF, a number of large bundles of perovskite with length of over 5 μm were woven together in a network on the surface of the substrate. When pure GBL was used, clusters of round grains with diameter of 0.5-2 μm were observed. In both cases, the coverage of the substrates was very low and a large area of the substrate was left exposed, which is very unfavorable for solar cell application. When pure DMSO was used as the solvent, the surface of the perovskite film was very smooth and uniform with nearly complete substrate coverage. This was mainly ascribed to the formation of PbI₂-MAI-DMSO intermediate-phase film, which retarded the rapid reaction between PbI₂ and MAI during the evaporation of DMF or GBL ^[34]. Because of its relatively low evaporation rate and high viscosity (see Table 1), pure DMSO was not very appropriate for forming a thin film by spin-coating. Therefore, DMF and GBL were added to DMSO to study the influence of mixed solvents on the morphology of the obtained perovskite. Previous studies have demonstrated that the morphology of the perovskite exerts a significant influence on the device performance of planar-junction perovskite solar cells ^[27]. When 20%~40% DMF was added, the film surface exhibited a reasonably uniform morphology predominated by micrometer-scale polygonal grains. Statistical analysis showed that the average grain size was 1.7 ± 0.2 μm for the sample fabricated with 20% DMF, and 1.3 ± 0.3 μm for 40% DMF in the processing solution, both values much larger than those previously reported ^{[33, 34]}. When the DMF fraction was further increased to 60%, the grain size became much smaller and the grain boundaries became blurred. When the DMF fraction was further extended to 80%, no clear grain boundaries could be found on the surface of the perovskite film. The use of 20%~40% of GBL in the DMSO (Fig. 1(i) and 1(j)) led to significantly smaller grains and much more blurred grain boundaries in comparison with the corresponding samples with DMF additive. When the GBL fraction was increased to 60%~80%, more and larger pinholes were formed in the perovskite films, making the surface of the resulting films much rougher, which was expected to deteriorate the cell performance significantly.

Fig. 1. SEM images of MAPbI3 films spin-coated using different solvents. (a) Pure DMF, (b) Pure GBL, (c) and (d) Pure DMSO, (e)-(h) DMF-DMSO mixtures containing DMF volume fractions of 20%, 40%, 60%, and 80% respectively, (i)-(l) GBL-DMSO mixtures containing GBL volume fractions of 20%, 40%, 60% and 80% respectively.

Cross-sectional SEM images of the planar solar cells fabricated from the continuous perovskite films prepared with pure DMSO and mixed solvents are shown in Fig. 2. The thickness of the perovskite layer fabricated with pure DMSO was ~300 nm. For samples fabricated with DMF-DMSO solvent mixtures, the thickness of the perovskite film increased with the ratio of DMF to a certain degree (~330 nm at 40% DMF, and ~350 nm at 80% DMF) Interestingly, a reverse trend occurred for the films made with GBL-DMSO solvent mixtures (~270 nm film thickness at 40% GBL and ~250 nm at 80% GBL). Because all three solvents had low vapor pressure at room temperature (see Table 1), the evaporation rate had limited effect on the spin-coated film thickness. Considering the very high rotation speed employed (6000 r/min), the difference in density of each solvent (0.9445, 1.0955 and 1.124 g/mL for DMF, DMSO, and GBL respectively, Table 1) might be the main reason for the observed variation in film thickness. It was also found from the cross-sectional SEM images that most of the pinholes had diameters no more than 100 nm, much smaller than the perovskite layer thickness. The sample fabricated with 80% GBL in the solvent mixture exhibited more pinholes (Fig. 1(i)) and was thinner (Fig. 2(e)). Some of the pinholes even formed channels across the perovskite layer that may provide route for hole transport materials (HTMs) to penetrate through, thereby obviously increasing charge recombination and deteriorating cell performance.

Fig. 2. Cross-sectional SEM images of planar solar cells based on perovskite films fabricated with different spin-coating solvents. (a) 80% and (b) 40% DMF in DMF-DMSO, (c) pure DMSO, (d) 40% and (e) 80% GBL in GBL-DMSO.

To reveal the crystal structure of the as-made perovskite films, the X-ray diffraction (XRD) patterns of samples prepared with pure DMF, DMSO, and GBL and DMF-DMSO and GBL-DMSO mixtures were measured (Figs. 3(a) and 3(b)). The three intense diffractions at 14.08°, 28.40°, and 31.86° could be respectively assigned to the (110), (220), and (310) reflections of the tetragonal MAPbI₃ phase ^[29]. The samples made with DMSO, both pure and mixed, exhibited MAPbI₃ phase without impurities. However, MAI diffraction peaks (9.77°, 19.66°) could be observed in the patterns of the pure DMF-processed perovskite film and a PbI₂ diffraction peak (12.74°) was observed for pure GBL-processed film. These small residual phases may have been caused by the huge difference in the solubilities of the PbI₂ and MAI, hence causing them to separate from the processing solution during the evaporation of the pure DMF or GBL solvents ^{[33, 34, 37]}.

UV-Vis absorption spectra of the MAPbI₃ films prepared with pure and mixed solvents are shown in Fig. 3(c). All the samples displayed absorption onsets at ~800 nm and a sharp rise at ~780 nm, characteristic of the band-gap absorption of MAPbI₃. As a result of the poor film coverage of the substrate for the samples made with pure DMF or GBL solvent, their absorption was relatively lower than those of the films fabricated with DMSO and the mixed solvents, which was more prominent in the short wavelength region (350-550 nm). Increasing the ratio of DMF enhanced the absorption intensity, while increasing the ratio of GBL reduced the absorption intensity, which was consistent with the observed trends in the variation of film thickness.

Fig. 3. a) XRD patterns of MAPbI3 films spin-coated with pure solvents (DMF, DMSO, and GBL) and mixed solvents (DMF-DMSO and GBL-DMSO with 40% DMF or GBL). (b) Enlarged image of (a) in the range of 2θ: 8°-22°. (c) UV-Vis absorption spectra of MAPbI3 films spin-coated with different solvents: pure DMF, (80-20) % DMF in DMF-DMSO, pure DMSO (dashed curve), (20-80) % GBL in GBL-DMSO, and pure GBL.

To investigate the application of the perovskite films in a photovoltaic device, a series of planar solar cells were fabricated and designated as device 1 to 11 as presented in Table 2. The main configuration of the devices was FTO/bl-TiO₂ layer/MAPbI₃/spiro-MeOTAD/Au (see Fig. 2(c)). The complete fabrication of the solar cell was carried out at temperatures no greater than 100 °C. This fabrication process could thus be easily extended to plastic substrates for flexible photovoltaic devices. The main photovoltaic parameters for the fabricated perovskite film-formed cells are presented in Table 2 and Fig. 4. For device 1 (made with pure DMF) and 11 (pure GBL), the values of all photovoltaic parameters (J_SC, V_OC, FF, PCE, and IPCE) were much lower than those of the devices fabricated from films made with DMSO (pure and mixed solvents), which may have arisen from cell shunting and poor light absorption caused by the low coverage of the perovskite films on the substrates. Fig. 4(a) reveals that J_SC was enhanced with increasing DMF content during spin-coating (device 6 to 2), and decreased with increasing GBL content (device 6 to 10), consistent with the variations in light absorption observed in Fig. 4(a). Additionally, for all devices made with DMF-DMSO (device 2 to 10), the average V_OC was found to be around 1.00 V except for a small rise to 1.03 V when using 20 or 40% DMF in the processing solvent. Considering the very large, micrometer-scale grains and reduced grain boundaries in these cases (Fig. 1(e) and (f)), the improved V_OC obtained may have been caused by reduced charge recombination and energy loss. Fig. 4(c) shows that the average FF of the devices 6 to 2 decreased progressively from 79.3% to 70.5% with increasing DMF content (0% to 80%), which probably arose from the increased recombination and series resistance in the thicker perovskite films (Fig. 2(a)-(c)). As a result, the average PCEs of the devices fabricated using DMF (device 2 to 6) had a peak value of 15.9% for device 4 (see Table 2 and Fig. 4(d)). In contrast, for the thinner devices made with DMSO-GBL (device 7 to 10), the FF was a little higher when the GBL volume fraction was ≤60%. However, the FF was greatly reduced l to ~70% when 80% GBL was used in the solvent mixture (device 10). This may have been caused by the increased number of pin-holes in the film (Figs. 1(l) and 2(e)) and enhanced charge recombination in the cells. The average PCEs of the devices (6 to 10) decreased with increasing GBL content. Fig. 4(e) shows typical J-V curves of the devices made with pure solvents and some of those made with mixed solvents (Table 2). The IPCE spectra (Fig. 4(f)) of these devices all displayed an onset at about 800 nm and a sharp rise around 780 nm, reaching a peak value of over 85% at about 480 nm, confirming the high charge production and collection efficiencies of the devices.

Fig. 4. Dependence of the main photovoltaic parameters on the volume fraction of DMF or GBL in the mixed spin-coating solvent with DMSO. (a) Short-circuit current density (JSC); (b) Open circuit voltage (VOC); (c) Fill factor (FF); and (d) power conversion efficiency (PCE); (e) J-V curves; (f) Incident-photon-to-current conversion efficiency (IPCE) spectra for the devices made with (1) pure DMF, (2) 80% DMF, (4) 40% DMF, (6) pure DMSO, (8) 40% GBL, (10) 80% % GBL, and (11) pure GBL, as designated in Table 2.

Table 2 Average photovoltaic parameters with standard deviations obtained with different spin-coating solvents. At least 8 identical devices were measured in each case. No. Volume fraction (%) JSC (mA/cm2) VOC (V) FF (%) PCE (%) PCEmax (%) DMF DMSO GBL 1 100 0 — 13.0 ± 0.5 0.86 ± 0.01 59.5 ± 6.2 6.7 ± 0.8 7.9 2 80 20 — 19.8 ± 0.3 1.00 ± 0.02 70.5 ± 7.7 14.0 ± 1.6 16.2 3 60 40 — 19.5 ± 0.3 1.01 ± 0.02 75.6 ± 2.8 14.9 ± 0.9 16.1 4 40 60 — 19.8 ± 0.2 1.03 ± 0.01 77.8 ± 1.8 15.9 ± 0.4 16.5 5 20 80 — 19.0 ± 0.1 1.03 ± 0.01 78.8 ± 0.9 15.5 ± 0.3 15.9 6 0 100 0 18.3 ± 0.2 1.01 ± 0.02 79.3 ± 1.2 14.7 ± 0.3 15.1 7 — 80 20 17.9 ± 0.2 1.00 ± 0.01 77.7 ± 0.9 13.8 ± 0.2 14.1 8 — 60 40 17.0 ± 0.2 1.00 ± 0.01 79.9 ± 1.1 13.6 ± 0.2 13.8 9 — 40 60 16.3 ± 0.3 1.00 ± 0.01 79.9 ± 1.7 13.0 ± 0.3 13.4 10 — 20 80 16.7 ± 0.1 0.99 ± 0.02 70.4 ± 7.0 11.6 ± 1.3 13.1 11 — 0 100 13.2 ± 0.5 0.87 ± 0.02 71.9 ± 8.8 8.3 ± 1.1 9.0

Table 2 Average photovoltaic parameters with standard deviations obtained with different spin-coating solvents. At least 8 identical devices were measured in each case.

The best-performing devices (40% DMF in DMF-DMSO, device 4) exhibited a maximum PCE (PCE_max) of 16.5%, with a J_SC of 19.8 mA/cm², V_OC of 1.05 V, and FF of 79.3% obtained from the J-V curve (Fig. 5(a)), under reverse scan at ~0.1 V/s and standard AM 1.5 conditions. We further characterized the hysteresis in the J-V curves by sweeping the applied voltage in forward scan, obtaining 19.8 mA/cm² (J_SC), 0.96 V (V_OC), 62.3% (FF), and 11.9% (PCE) for this cell. It has been demonstrated that reverse scanning of a perovskite solar cell will overestimate the PCE, while forward scanning results in an underestimate ^[34]. The most accurate evaluation of device efficiency is to measure its power output under working conditions (i.e., stabilized conditions). Therefore, the photocurrent of the cell was also recorded at a fixed forward bias of 0.88 V (Fig. 5(b)). The photocurrent stabilized within a short time to about 16.3 mA/cm², yielding a power conversion efficiency of 14.4% after over 100 s of irradiation, which was higher than reported data for similar device structures ^{[33, 36]}.

Fig. 5. Photovoltaic performance of the best cell (40% DMF in DMSO, device 4). (a) J-V curves obtained from reverse scan and forward scan under simulated AM 1.5 G solar irradiation. (b) Steady-state photocurrent density and PCE measured as a function of time for the same cell held at a maximum power point (0.88 V). The cell was placed in the dark prior to the start of the measurement.

We have described the solvent engineering of mixed solvents (DMF-DMSO and GBL-DMSO) for spin-coating of uniform perovskite films. Both the grain size and film thickness varied significantly with the volume fraction of the mixed solvents. At 20%~40% DMF in DMSO, micrometer-scale perovskite grains and reduced grain boundaries were observed in the obtained films, and the final devices yielded average PCEs of over 15%, a highest PCE of 16.5% during reverse scan, and a stabilized PCE of 14.4% at 0.88 V under standard AM 1.5 conditions. Because the perovskite film solar cells exhibited high device performance and were fully fabricated at temperatures no greater than 100 °C, the present perovskite solar cells show promise for flexible devices and tandem solar cells.