Methylammonium iodide bismuthate ((CH3NH3)3Bi2I9) (MIB) perovskite has attracted research attention because of its low toxicity and stability in air. However, MIB perovskite also has its inherent defects, e.g., MIB films have too many pinholes and large indirect band gap. Our group has also done some efforts to successfully improve the performance of MIB-based PSC. One of the recent works focused on the use of diethyl ether (DEE) as anti-solvent to improve film compactness and reduce pinhole defects. The characterization indicated that the films treated by DEE (DEE/MIB) were compact with fewer pinholes. The devices fabricated with DEE/MIB based on mesoporous TiO2 (mp-TiO2) architecture exhibited good performance in terms of Voc (885 mV), Jsc (0.807 mA cm−2), and FF (49.71%), with a power conversion efficiency of 0.343%, which were much higher than that of the contrast test and results of our previous work. The present devices also exhibited superior long-term stability in ambient air (approximately 30% humidity) of more than 200 days.
During the fabrication of perovskite film, it tends to agglomerate into crystal lumps because of the high surface tension. Gaps are then formed between perovskite lumps, resulting in a greater loss of incident light . Therefore, a superior crystal film is the key to achieve high-quality perovskite solar cells (PSCs) . An inferior perovskite film has a great impact on charge separation, transport, and photoelectric conversion performance . A uniform film can not only absorb incident light, but also avoid direct contact between the hole transport layer and the electron transport layer as well as the serious problem of charge recombination. Therefore, exploring and optimizing the process of fabricating perovskite films are important to control the surface coverage of crystal film and crystal size, thus producing superior consecutive film without pinholes.
To obtain high-quality films, researchers have explored different methods such as one-step spin-coating (1-S method), two-step spin-coating (2-S method), and chemical vapor deposition [4,5,6]. Although vapor deposition can produce density films, the preparation process needs a certain vacuum and high temperature, which has the disadvantage of time-consuming and energy-consuming. The 2-S methods will make the reaction between the reactants insufficient. The 1-S method has the advantage of low cost as well as simple and easy operation, but it has poor repeatability, and cannot control the thickness and morphology of the perovskite layer. Consequently, the grain size of perovskite is uneven, pinholes are formed in perovskite layer, and the substrate is not fully covered , which result in poor photoelectric properties of the devices. Therefore, fabrication of compact films by the 1-S method is important, which can be achieved by choosing mixing solvent [8, 9], adding additive [10, 11], and anti-solvent treatment [12, 13]. Among these methods, anti-solvent treatment is widely used because it can accelerate the crystallization of perovskite crystal, resulting in extremely dense and compact film. The most common anti-solvents are methylbenzene, dichloromethane, diethyl ether (DEE), chlorobenzene (CB), ethyl acetate (EA), n-hexane, and trichloromethane [14,15,16,17,18]. Given that the solubilities of PbI2 and CH3NH3I (MAI) vary in N,N-dimethylformamide (DMF), during the perovskite formation, DMF volatilizes slowly and PbI2 (whose solubility is low) will separate out from DMF firstly, and then, the CH3NH3I attaches to the separated PbI2, which results in a perovskite crystal morphology similar to that of PbI2 crystal [3, 19]. The solution can be extracted instantly via dropping anti-solvent, when using spin-coating method to fabricate perovskite films, making PbI2 and MAI supersaturated and precipitate at the same time, which can eliminate PbI2 and MAI segregation.
Li developed a new method using CB to treat PbI2 film during 2-S method and successfully acquired compact and equable perovskite film . Considering that the solubilities of solute and solvent are different, DMF can be removed quickly; PbI2 could crystallize quickly, and reduce the volume of the PbI2 film at the same time. When the PbI2 film is dipped in a MAI solution, the mesoscopic architecture PbI2 film provides enough space for the reaction of PbI2 and MAI, so that the reaction does not occur on the PbI2 film surface.
Solvent engineering and compositional engineering [21,22,23] indicate that Bismuth-based perovskite crystal structure is not simply a regular perovskite structure formed by replacing lead in lead-based perovskite with element Bi. Because Bi3+ is trivalent and Pb2+ is bivalent, Bi substitutes for Pb to form a perovskite-like structure consisting of two coplanar octahedral (Bi2I9)3− group surrounded by cations, which results in a hexagonal lamellar structure of Bismuth-based perovskite crystal. During spin-coating, the crystal growth preferentially grows along the C axis, which causes the morphology of Bi-based perovskite thin films being a porous structure formed by the accumulation of hexagonal crystals. Therefore, the nucleation and growth of bismuth-based perovskite crystals must be controlled if we want to get a better Bismuth-based perovskite film.
Sawanta fabricated (CH3NH3)3Bi2I9 PSCs based on mp-TiO2 architecture and achieved a PCE of 0.069% . After treating with CB, they obtained dark-color films, large grain size, and high PCE of 0.356%. The performance of the devices was better than the devices fabricated by traditional 1-S method, with Voc, Jsc, and FF of 0.653 V, 1.1 mA cm−2, and 49.6%, respectively. Optical absorption measurement exhibits a strong absorption band about 550 nm, and the band gap is approximately 2.1 eV. The reason may be that adding CB reduces the nucleation rate of MIB and controls the crystal growth process, thereby decreasing the grain boundary number and improving the quality of MIB films. As a result, the performance of PSCs based on MIB films is further improved. Therefore, treatment of MIB films with anti-solvent is beneficial and needs further investigation.
Because mixed solvents (DMF, DMSO) are advantageous to the preparation of Perovskite thin films , DMSO is advantageous to the anti-solvent effect , and the use of DEE as anti-solvent to treat the MIB film and to investigate the performance of devices has not been explored. Hence, in this work, CB and DEE was used as anti-solvent to treat MIB films and obtain high-quality films, which were then applied to mp-TiO2 architecture organic–inorganic hybrid PSCs as optical absorption layer. The performances of the devices based on the films treated by different anti-solvents and fabricated by traditional 1-S method were further compared. X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM), UV–Vis spectrophotometry (UV–Vis), and current density–voltage (J–V) curves were used to characterize the films and explore the influence of film compactness on the performance of the devices.
MIB film fabrication
The (CH3NH3)3Bi2I9 precursor solution (1 mol L−1) was obtained by mixing methylammonium iodide (CH3NH3I, 99%, Shanghai Mater Win New Materials Co., Ltd.) and BiI3(99.999%, Sigma-Aldrich Co., LLC) at a ratio of 1.5:1 in 1 mL of DMF (99.8%, Shanghai Mater Win New Materials Co., Ltd)/dimethyl sulfoxide (DMSO, 99.8%, Shanghai Mater Win New Materials Co., Ltd.) solvent mixture (4:1 by volume) and filtered through a syringe filter of 0.45 µm after stirring in dark (sealed). The precursor solution was deposited on top of the mesoporous TiO2 (mp-TiO2) film through spin-coating method at 2000 rpm (revolutions per minute) for 6 s followed by 4000 rpm for 30 s. Then, 100 µL of CB (99.9%, Sigma-Aldrich Co., LLC) or DEE (99.9%, Sigma-Aldrich Co., LLC) was pipetted onto the substrate in the process of high-speed spin-coating, followed by annealing on a hot plate at 100 °C for 30 min (Fig. 1).
Fabrication of solar cell
Conductive fluorine-doped tin oxide (FTO)-coated glasses (sheet resistance 14 Ω/square), as the devices substrates, were cleaned by ultra-sonication for 20 min in deionized water and detergent mixture, deionized water, ethanol, acetone, and isopropanol, respectively. Then, the substrates were treated by ultraviolet ozone cleaner (UVO) for 25 min after drying. Direct-frequency magnetron sputtering (DFMS) was used to deposit the compact TiO2 (cp-TiO2) layer onto the substrates at 100 W for 30 min. The cp-TiO2 films were annealed at 500 °C for 15 min. After cooling the substrates to room temperature, the mp-TiO2 was deposited by spin-coating at 4000 rpm for 30 s on the top of cp-TiO2 layer using a commercial TiO2 paste (18-NRT, Mater Win) diluted in ethanol (1:4, weight ratio). After drying at 100℃, the films were heated to 500 °C for 60 min to form mp-TiO2. Then, the MIB films were fabricated by the method mentioned above. Subsequently, 30 µL of HTM was pipetted for onto the top of MIB film and spin-coated at 3000 rpm for 30 s, and then oxidized in the air for approximately 8 h. Finally, the gold counter electrodes were deposited to obtain the final devices by thermal evaporation.
Elements analysis was carried out by XPS (Thermo Fisher) on a Scientific Escalab 250Xi instrument with monochromatized Al K-Alpha radiation. The XRD patterns of MIB films fabricated on mp-TiO2 FTO glass were collected using a Bruker-AXS D8 Advance from 10° to 60° (2θ) at room temperature. AFM (Nanoscope-IIIa) and FE-SEM (Jeol JEM 6510LV) were employed to characterize the surface roughness and surface morphology. The UV–Vis absorption measurements were obtained by a UV-3600 spectrophotometer (Shimadzu). The spectral test range is 300–900 nm, the light source slit width is 5 nm, and the test interval is 1 nm. The J–V characteristics were analyzed under simulated AM 1.5 G (100 mW cm−2 irradiance) using a solar simulator (NEWPORT, 91192-1000) and a source meter (Keithley 2400, USA). Photoluminescence (PL) spectra and time-resolved PL (TRPL) decay measurement were measured by Fluo Time 300 (Pico Quant Ltd., Germany) with an excitation of 520 nm at room temperature (20.0 MHZ, 25.0 ps), and instrument test parameters: room temperature; 485 nm picosecond pulsed laser (LDH-P-C-485 nm); 530 nm filter; emission wavelength 536 nm; total output average power 2 mW. Test results were analyzed using FluoFit analysis software.
Results and discussion
Figure 2a shows the solution of MIB/DMF (vivid red). Figure 2b shows the annealed films fabricated by traditional 1-S method treated with CB and DEE, respectively. As shown in Fig. 2, it is obvious that the color of the film fabricated by traditional 1-S method is lighter than the other two films; DEE/MIB is the most conspicuous, which would lead to a better compactness and stronger intensity of optical absorption, and thus, the device based on the film would have a higher PCE.
The elemental composition and valence of the near-surface atoms of the MIB films fabricated on bare FTO glass were analyzed by XPS. Figure 3f shows the peaks of I, Bi, C, N, and O. The peaks of I 3d3/2 (630.44 eV) and I 3d5/2 (618.98 eV) are attributed to the spin orbital of I−. The peaks Bi 4f5/2 (163.98 eV) and Bi 4 f7/2(158.65 eV) can be attributed to the characteristic signals of Bi3+, which indicated that Bi was mainly in the 3+ oxidation state. Moreover, the peak of Bi 4f7/2 can be attributed to both Bi–O and Bi–I bonds . Two characteristic peaks of C1s in Fig. 3c with the binding energy of 285.78 and 284.68 eV corresponded, respectively, to C–N and C–C bonds in CH3NH3+ [25, 26]. The N1s signal peak at 402.08 eV (Fig. 3d) was from CH3NH3+. These results indicated that the MIB was successfully synthesized.
The XRD patterns of MIB fabricated on mp-TiO2 layer are shown in Fig. 4 to further confirm MIB formation. The diffraction peaks of 12.5°, 16.2°, 37.6°, and 41.56°, according with previous work [26,27,28], indicated that the MIB fabricated by 1-S method belong to the hexagonal system with the P63/mmc space group , which confirmed the formation of the MIB perovskite. In addition, a peak of anatase TiO2 around 42.5° was observed, which indicated that the film of MIB did not cover the substrate completely. In the XRD patterns of MIB films without treatment by anti-solvent, a weak diffraction peak of BiI3 can be observed, which were not detected in the samples treated by anti-solvent, which suggested an almost completely transformation of BiI3 into MIB. In the XRD patterns of CB/MIB and DEE/MIB, the peak of anatase TiO2 was very weak, which indicated that the films were more compact.
AFM measurement further compares the surface topography and roughness of the MIB films fabricated on mp-TiO2 by different methods. Figure 5a shows the surface topography of the MIB films fabricated by traditional 1-S method, in which numerous holes are observed in the film. The large roughness of 188 nm is shown in Fig. 4a-1. Figure 4b and b-1 shows a better surface topography of CB/MIB film, which is more compact with the roughness reduced to 33.1 nm. Figure 4c and c-1 exhibits the highest compactness and smooth surface of DEE/MIB film with the smallest roughness of 27.5 nm and largest grain. These results indicated that the films treated by anti-solvent are more compact and smoother.
To validate the above analysis, the surface morphology of different MIB films was characterized by SEM. The images in Fig. 6 show that the compact and crystal morphology of the film untreated with anti-solvent (Fig. 6a) is inferior so that the mp-TiO2 layer can be observed. The grain size is the smallest, which in good agreement with the XRD result of the strong TiO2 peaks, which will cause many defects in the films (‘traps’), increase the exciton recombination rate, even cause shunting paths in the device and limiting the ultimate efficiency. Figure 6b and c shows SEM images of CB/MIB and DEE/MIB, respectively. The films are more compact, and the size of the crystal increases distinctly as compared with Fig. 6a. The compactness and grain size of the DEE/MIB are best in this study; this may be ascribed to different solubility of BiI3 and MAI. When no anti-solvent is used, DMF slowly volatilizes, and low-solubility BiI3 precipitates first, and then high-solubility MAI precipitates to cause nucleation and growth unevenness of the perovskite crystal. As anti-solvent, DEE has a lower boiling point and stronger extraction ability than CB. DEE can faster extract DMF and volatilize, so that iodide and MAI can be supersaturated at the same time, which is more favorable for MIB nucleation. In the presence of DMSO, a BiI3-DMSO-MAI mesophase is formed. After heating, the DMSO is removed and the MIB crystals are gradually grown to form a uniform and dense MIB film. Hence, the perovskite layer has the largest coverage, which may increase the absorption intensity and PCE. Figure 6a-1 to c-1 shows the cross-sectional images of different films, which show that the films treated by anti-solvent are more compact, with thickness of about 300 nm.
The absorption spectra of the MIB films on the FTO substrate are shown in Fig. 7a. The absorption range of all films was between 300–600 nm, and the absorption edge was about in 700 nm. Compared with other films, the absorption intensity of the DEE/MIB was the strongest because of their high crystallinity and high coverage, which can be supported by the SEM images. Moreover, the absorption intensity of crystals enhanced with the increase in grain size . The stronger absorption intensity produces more excitons, which is beneficial to the device performance. Figure 7b shows the Tauc curves of DEE/MBI. We calculated the band gap of DEE/MBI (2.16 eV) by plotting the diagram of (αhυ)2 against hυ obtained from the UV–Vis absorption spectra.
Figure 8a, c, and d shows the different diagrams of MIB PSCs, including cp-TiO2 layer, mp-TiO2 layer, MIB layer, HTM layer, and Au counter electrode. The valence band (− 5.9 eV) of MIB is lower than the work functions of Spiro-OMe TAD, and thus, the holes can be transported easily through perovskite layer and collected by the gold electrode. The conduction band (− 3.8 eV) of MIB matched well with that of TiO2 (− 4.2 eV). The wider pattern of energy conductor of TiO2 facilitates the electron transfer from MIB layer to TiO2 layer .
All fabrication procedures of devices were carried out in ambient atmosphere, with humidity of > 50%. The photoelectric performance of all devices was evaluated under simulated AM 1.5 G (100 mV cm−2). The J–V characteristics of MIB PSCs based on the films treated with different anti-solvent are shown in Fig. 9, and the relevant parameters of the photoelectric performance are shown in Table 1. The highest PCE of the devices based on the MIB films fabricated by transitional 1-S method is 0.082%, with the Voc, Jsc, and FF of 715 mV, 0.300 mA cm−2, and 38.23%, respectively. The PCE of the devices based on the MIB films treated with anti-solvent enhanced obviously. The devices based on the CB/MIB film exhibited a PCE of 0.185%, and the Voc (844 mV), Jsc (0.502 mA cm−2), and FF (43.67%) were also enhanced. As expected, the devices based on DEE/MIB film exhibited the highest PCE of 0.343%. These devices also had the best photoelectric performance in terms of Voc (885 mV), Jsc (0.807 mA cm−2), and FF (49.71%). The best performance of the devices fabricated by DEE/MIB film can be attributed to the smoother, lower defects, and more compacter MIB layer with larger grain and lower charge trap density.
Eight devices based on DEE/MIB film were selected in a batch of devices fabricated in same experiment condition and processes to test the repeatability. The corresponding photoelectric parameters are shown in Fig. 10 and Table 2. The PCE of 85.7% of the devices was over 0.3%, and the Voc, Jsc, and FF of these devices were almost higher than 800 mV, 0.8 mA cm−2, and 43%, respectively, which indicated that the devices based on DEE/MIB film had good repeatability.
PCE decay was measured to test the stability of the devices based on DEE/MIB film. As shown in Fig. 11 and Table 3, the PCE of these devices remained high at 0.3% in ambient air (approximately 30% humidity) for 200 days, although the devices showed a decaying trend. The small PCE decrease of 7.58% indicated that the PSCs based on DEE/MIB film exhibited excellent long-term stability.
The J–V curves demonstrated that the devices based on films treated by anti-solvent have a high FF, which indicated that the films treated by anti-solvent had fewer defects. However, the PCE of all devices was still low. Besides the high exciton binding energy, the internal defects of MIB films are also an important factor influencing the FF and PCE values [32, 33]. To further investigate the defect state of the films, PL curves were obtained on the quartz glass substrates, as shown in Fig. 12. The strongest photoluminescence intensity of DEE/MIB film revealed that DEE/MIB had less nonradiative recombination, which indicated fewer trap density states of DEE/MIB. Therefore, the devices based on DEE/MIB have the highest FF. However, all carrier lifetime of the MIB film fabricated in this work is much smaller than that of Pb-based perovskite films. Hence, the performance of all devices based on the MIB films should be optimized further.
Table 3 summarizes the photoelectric properties of all bismuth-based perovskite solar cells (MIB PSCs) prepared by using anti-solvent methods in previous studies and in our work. Sawanta and co-workers improved the performance of MIB PSCs by treatment with chlorobenzene . Our work mainly used chlorobenzene and diethyl ether as anti-solvents to treat MIB films. Then, we compared different MIB films in terms of morphology, internal defects, and photoelectric performance. When we used diethyl ether (DEE) as anti-solvent instead of CB, the devices exhibited superior performance in terms of Voc (885 mV), Jsc (0.807 mA cm−2), FF (49.71%), and PCE (0.343%). The PCE of the devices in our work is comparable with that of devices reported by Sawanta, but the devices treated by DEE in our work have higher Voc and FF. In our work, all performances of the MIB PSCs treated by DEE are higher than the devices treated by CB. Compared to our previous works [34, 35], the performance of MIB PSCs treated by anti-solvents is much higher than those of the devices fabricated by traditional method. The photoelectric properties of the devices treated by DEE are also the best among all devices.
This work introduced DEE as anti-solvent in 1-S method for the first time to improve MIB films for fabricated PSCs. MIB, CB/MIB, and DEE/MIB films were prepare and tested. Results showed that DEE/MIB was the most compact, with the largest grains and lowest roughness. The device based on DEE/MIB exhibited the best photoelectric property, which should attribute to the strong extraction function DEE, which can extract the solvent of perovskite immediately. Given that the boiling point of DEE is lower than that of CB, DEE can volatilize at room temperature, which can remove DMF to facilitate the separation of BiI3 and MAI. The PSCs based on DEE/MIB achieved highest PCE and long-term stability. This study also showed that the MIB films can be improved through optimization of process parameters and selection of appropriate anti-solvent.
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We thank Pico Quant (ETSC Technologies Co., China) for the TRPL decay measurement. This work was supported by the NSFC (51572072 and 21402045). This work was also financially supported by Wuhan Science and Technology Bureau of Hubei Province (2013010602010209), Educational Commission of Hubei Province (D20181005), and Department of Science & Technology of Hubei Province of China (2015CFA118 and 2014CFB167).
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Liu, Z., Chen, M., Wan, L. et al. Anti-solvent spin-coating for improving morphology of lead-free (CH3NH3)3Bi2I9 perovskite films. SN Appl. Sci. 1, 706 (2019). https://doi.org/10.1007/s42452-019-0727-6
- (CH3NH3)3Bi2I9 (MIB)
- Anti-solvent spin-coating
- Lower toxicity
- Longer stability