COMMUNICATION Perovskite Solar Cells
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Tin-Based Perovskite with Improved Coverage and Crystallinity through Tin-Fluoride-Assisted Heterogeneous Nucleation Min Xiao, Shuai Gu, Pengchen Zhu, Mingyao Tang, Weidong Zhu, Renxing Lin, Chuanlu Chen, Weichao Xu, Tao Yu, and Jia Zhu* similar method to formamidinium tin triiodide (FASnI3), demonstrating that the addition of SnF2 could suppress the oxidation of Sn2+ to Sn4+.[22] Soon, Seok et al. applied SnF2–pyrazine complex as additive and achieved a PSC with high PCE of 4.8%.[28] Recently, Yan et al. applied SnF2 to FASnI3 with a simple inverted device architecture and obtained a quite high efficiency (6.22%) of tin-based PSC as of now.[27] It is generally believed that the addition of SnF2 can increase the formation energy of tin vacancy, the main point defect in tin-based perovskite, therefore leading to decreased concentration of this kind of defect.[19] While there is a general observation that addition of SnF2 can improve the morphology, so far the mechanism behind it is still not clear. Here in this study, we clearly demonstrate that when applying SnF2 additive to methylamine tin halide (MASnIBr2) perovskite, SnF2 can play a crucial role by serving as heterogeneous nucleation sites, not only facilitating the formation of more tin-based perovskite nucleuses, but also enabling more homogeneous crystal growth with full coverage. Both crystals and thin films of tin-based perovskite with and without SnF2 are employed to verify this mechanism. Because of the nucleation assisted film process enabled by the addition of SnF2, MASnIBr2 film with 30 mol% SnF2 addition can achieve full coverage and uniform morphology. Meanwhile, the grain size is more than twice as the one without SnF2 doping. An MASnIBr2-based PSC with a high and stable PCE of 3.70% is demonstrated. To examine the effect of SnF2 addition, tin-based perovskite films are first used based on a traditional one-step spin-coating process.[2,3] Figure 1 shows two different nucleation-growth processes of tin-based perovskite with and without SnF2. Limited solubility of SnF2 plays a critical role: similar to lead chloride (PbCl2),[33] SnF2 is the least soluble compound among SnX2 series (X = F, Cl, Br or I) in common solvents because the bonding energy of tin halides increases with the decrease in the ionic radius of halide, see Table S1 in the Supporting Information.[34] During spin-coating, SnF2 will first precipitate when the solvent evaporates. Then these precipitated homogenous SnF2 particles will function as heterogeneous nucleation sites to facilitate the growth of MASnIBr2 crystals and enable a more uniform perovskite film.
Tin fluoride (SnF2) is widely used as an effective additive for lead-free tinbased perovskite solar cells. However, the function of SnF2 and the mechanism in improving the film morphology are still not clear. In this work, it is clearly demonstrated that SnF2 can play a crucial role in the crystal nucleation process. Due to the limited solubility, SnF2 creates more nucleuses for the crystal growth and therefore enables more uniform thin film with high coverage. It is confirmed that this mechanism can be applied to the growth of both thin film and single crystal. As a result of tin-fluoride-assisted heterogeneous nucleation, an MASnIBr2-based perovskite solar cell with a high and stable power conversion efficiency of 3.70% is demonstrated.
With the power conversion efficiency (PCE) of organic–inorganic halide perovskite solar cells (PSCs) exceeding 22%,[1–18] the toxicity related to lead remains as a major concern for large scale applications of the PSCs. Since the year of 2014, several groups have reported tin-based PSCs.[19–30] However, so far the performance of tin-based PSCs is far below the lead analog mainly because of two reasons. First, it is challenging for tinbased perovskite to form a homogeneous and full-covered film due to the uncontrollable crystallization during the solution process.[22,27,28] Second, tin-based perovskite film shows significant instability in its Sn2+ oxidation status when exposed to air. This inevitable process will lead to p-type carrier doping, which will limit the carrier diffusion length.[20,31,32] Tin fluoride (SnF2) has been introduced to address these two problems.[19,22–24,27] For example, Mathews et al. added 20 mol% SnF2 additive in cesium tin triiodide (CsSnI3), resulting in a PSC with PCE of 2.02%.[19] The same group applied the M. Xiao, S. Gu, P. Zhu, M. Tang, R. Lin, C. Chen, W. Xu, Prof. J. Zhu National Laboratory of Solid State Microstructures College of Engineering and Applied Sciences, and Collaborative Innovation Center of Advanced Microstructures Nanjing University Nanjing 210093, China E-mail:
[email protected] W. Zhu, Prof. T. Yu National Laboratory of Solid State Microstructures and Eco-Materials and Renewable Energy Research Center (ERERC) at Department of Physics Nanjing University Nanjing 210093, China
DOI: 10.1002/adom.201700615
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When further increasing SnF2 concentration (40 mol%), the compact film is separated by some needlelike structures, which have been reported by Mathews.[22] Therefore, it is clear that SnF2 plays a critical role in the formation of uniform MASnIBr2 film. We also carefully examine the effect of SnF2 on crystallinity of MASnIBr2 perovskite films. Figure 3a shows X-ray diffraction (XRD) patterns for films of pristine MASnIBr2 and those with 10, 20, 30, and 40 mol% SnF2, demonstrating that (110) and (220) planes are preferably developed. Besides, it is clearly that the crystallinity of MASnIBr2 perovskite films increases with the increase of the amount of SnF2 for the narrower peak width as is shown in Figure 3b. By Scherrer’s equation, we estimate those crystallites to be around 25.18, 34.91, 41.93, 53.15, and 58.09 nm for the film with 0, 10, 20, 30, and 40 mol% SnF2. Furthermore, there is not any new peak appearing (such as SnF2) or any Figure 1. Illustration of two different nucleation-growth processes of tin-based perovskite with and without SnF2. peak position shifting when adding SnF2, which means SnF2 does not exist as crystal phase and F− does not substitute I− or Br− for their ionic radii The top-view and cross-section images of MASnIBr2 with and without SnF2 are shown in Figure 2. The pristine MASnIBr2 film with remarkable difference, as is previously reported.[20] To furwithout SnF2 shows less than half of coverage, with large area of ther explore the tendency of the crystallinity of MASnIBr2 peromesoporous TiO2 layers below exposed clearly (Figure 2a), which vskite films change with the concentration of SnF2, we made hinders the formation of capping layer (Figure 2e). In contrast, a XRD test of film with 50% SnF2. The result is presented in if 20 mol% SnF2 additive was added to the precursor solution, Figure S2 in the Supporting Information. As a result, the crystallinity of MASnIBr2 perovskite films will improve as the conthe film becomes more uniform but the surface is still unevenly covered, which might lead to a recombination of electrons and centration of SnF2 increases, and finally reach a platform due holes on the interface of perovskite and electron transport layer to the limited solvent and too much excess SnF2 as the impu(Figure 2b). The most uniform and compact film was obtained rity phase. The X-ray photoelectron spectroscopy (XPS) data for when increasing SnF2 concentration to 30 mol% (Figure 2c). This the pristine MASnIBr2 film and film with 30 mol% SnF2 are film shows uniform and full coverage over large area (Figure S1, depicted in Figure 3c, the peak of fluoride in the sample with Supporting Information) with a capping layer on mesoporous SnF2 revealing that SnF2 is probably present as an amorphous titanium dioxide (TiO2) of ≈200 nm thickness (Figure 2f). phase. Figure 3d represents the absorption spectra of these
Figure 2. a–d) Top-view SEM images of MASnIBr2 films with different concentrations of SnF2: a) 0, b) 20, c) 30, and d) 40 mol%. The scale bar is 3 µm. e,f) Cross-section SEM images of MASnIBr2 films with 0 and 30 mol% SnF2. The scale bar is 500 nm.
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Figure 3. a) XRD patterns of MASnIBr2 films with 0–40 mol% SnF2. b) The dependence of full width at half maximum (FWHM) for the main peak (110) of MASnIBr2 perovskites with 0–40 mol% SnF2. c) XPS spectra of MASnIBr2 films with 0 and 30 mol% SnF2. d) Absorption spectra of MASnIBr2 films with 0–40 mol% SnF2.
films. It is clearly that the absorption of films is enhanced with the addition of SnF2. At the same time, sharper absorption edge of the cut-off lines with addition of SnF2 indicates that SnF2 additive facilitates to reduce disorders and defects thus further improves film crystallization. To further confirm the role of SnF2 on the nucleation and growth of tin-based perovskite crystal, we carefully compared methylammonium tin tribromide (MASnBr3) crystal.[35,36] Methylamine bromide (MABr), tin oxide (SnO) with and without SnF2 were interacted in a mixed solution of HBr/H3PO2 at 80 °C in the ambient air. The H3PO2 solution was used to prevent the oxidation of Sn2+ during the crystal growth. The photos taken every hour which can clearly reflect the change of the crystal growth process are shown in Figure 4. After 3 h, the colors of these two solutions both deepened from pale yellow to light golden, indicating that the solutions were mostly saturated. After 4 h, the MASnBr3 crystals were precipitated from the solution involved SnF2 with a mass of nucleation site while only bits of MASnBr3 crystals precipitated from the other solution after 5 h. The XRD pattern of pure MASnBr3 single crystal was shown in Figure S3 in the Supporting Information, which indicates that it crystallized in the pseudocubic structure at
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Figure 4. Time-varying crystallization products of MASnBr3 without (left) and with 30 mol% SnF2 (right).
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room temperature. Therefore, it is confirmed that SnF2 can serve as an excess nucleation center to accelerate heterogeneous nucleation and crystal growth, leading to a large and uniform crystalline grain. To examine the effect of SnF2 on device performance, we applied a cell structure of fluorine-doped tin oxide (FTO)/compact TiO2/mesoporous TiO2/MASnIBr2/spiro-OMeTAD/Au and assessed their performance by adding 10, 20, 30, and 40 mol% SnF2 to the precursor perovskite solutions. It is essential to anneal perovskite films at a suitable temperature to acquire higher the efficiency of PSCs, and we handpicked 100 °C as the annealing temperature after comparison (Figure S4, Supporting Information). Figure 5a; Table 1 show the J–V characteristics of the champion PSCs with different SnF2 concentrations fabricated with an active area of 0.09 cm2 under AM 1.5G solar illumination, which are measured in nitrogen-filled glove box without any encapsulation. The pristine MASnIBr2 PSC produces efficiency below 0.08% due to its extremely low photocurrent and photovoltage. Instead, with increasing SnF2 concentration, the open-circuit voltage (Voc), short-circuit current (Jsc), fill facor (FF), and PCE all improve largely and reach the summit at 30 mol% SnF2. The surface morphology of films with 30 mol% SnF2 is more continuous, smooth, and uniform than the others, which is helpful to increase shunt resistance and suppress charge recombination. Even though Jsc of PSCs with 10–30 mol% SnF2 is much higher than the pure one, they distinct slightly among each other. This is because they have similar absorption property (see Figure 3d). Furthermore, we have performed the Hall measurement to examine the carrier density of MASnIBr2 films with different SnF2 molar concentrations. Figure S5 in the Supporting Information shows the dependence of carrier density on SnF2 molar concentration in MASnIBr2 films. The result indicates a strong p-type behavior of tin-based perovskite due to the high levels of defects. A relatively high carrier density of 6.34 × 1018 cm−3 was obtained from the pure MASnIBr2 film without SnF2, with one order of magnitude decrease with the addition of SnF2. Similar as the case of FASnI3,[27] with the addition of SnF2, the carrier density does not change dramatically with different SnF2 concentrations. However, when adding more SnF2, the Jsc of PSC drops sharply, yielding a loss of PCE. This is probably caused by the uneven film shown in Figure 2d, which impedes the transport of carriers and reduces the recombination resistance as a kind of defect. In addition, we used 20 solar cells from five different patches for each doping concentration to calculate the average values. The variation line chart of each parameter with different SnF2 concentration is shown in Figure S6 in the Supporting Information. Over all, the best cell performs an efficiency of 3.46% (3.70%), with a Voc of 0.43 V (0.45 V), a Jsc of 13.73 mA cm−2 (13.78 mA cm−2), and a FF of 59.58% (57.30%), under forward (reverse) scan. Its J–V characteristics and external quantum efficiency (EQE) spectra are shown in Figure 5b,c. The integrated Jsc calculated from EQE spectra (12.91 mA cm−2) is in good agreement with the measured Jsc. In order to verify the measured performance, the steady-state current output at the maximum power point was measured. Figure 5d shows that the steady-state current at a constant bias of 0.34 V for 2 min is 10.65 mA cm−2. We also fabricated 30 devices in several batches to verify the reproducibility of our
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PSCs and the PCE distribution histogram of these devices is shown in Figure 5e. In order to study the function of SnF2 additive in carrier transport process, an electrochemical impedance spectroscopy (EIS) test was employed under dark condition. The spectra of PSCs with and without SnF2 doping with different bias voltage from 0 V to Voc are showed in Figure S7 in the Supporting Information, which can be fitted to the same equivalent circuit as is shown in inset of Figure S7 in the Supporting Information.[37] Figure 5f represents the recombination resistance (Rrec) fitted by Z-View software. As is shown, the Rrec decreases obviously as the bias voltage increases. Besides, the PSC with 30 mol% SnF2 exhibits one order of magnitude higher Rrec than the undoped one, yielding a more excellent Voc. The steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) measurements for both films were also performed to further testify the advantage of SnF2 in carrier transportation, see Figure S8 in the Supporting Information. Steady-state PL intensity of MASnIBr2 with 30% SnF2 exhibits a nearly five-fold improvement compared to that in the pristine one without any shift in peak position, which indicates that radiative recombination is enhanced without changing bandgap. For the TRPL spectra, a biexponential fitting y = A1 exp(−t/τ1) + A2 exp(−t/τ2) can be fitted to both curves. The fast decay component has been ascribed to a trap-mediated nonradiative recombination (τ1) while the slow decay component was considered as radiative recombination (τ2).[38–40] Apparently, a much faster decay of 223 ps in the pristine film, dominated by the fast component, suggests severe recombination of photocarriers caused by the high density of the trap-state. After adding 30 mol% SnF2, the amplitude of τ1 is decreased from nearly 100% to 67%, and the film exhibits a longer lifetime due to the longer τ2. This is probably because that SnF2 could reduce defects of tin-based perovskite and then decrease the concentration of recombination centers. In addition, the MASnIBr2 PSCs with 30 mol% SnF2 exhibit a good stability. After stored in nitrogen-filled glove box for 60 days, the device maintained over 80% of its initial efficiency, see Figure 5g. In summary, we found that SnF2 can serve as heterogeneous nucleation sites to facilitate crystal growth of tin-based perovskite crystal. The addition of SnF2 with appropriate concentration can enable tin-based perovskite film with full coverage and improved crystallinity and reduced carrier recombination. As a result, a tin-based PSC with stable efficiency of 3.70% is demonstrated.
Experimental Section Synthesis of MASnBr3 Crystals: 0.674 g SnO (Sigma-Aldrich, 99%) and 0.795 g MABr (Sigma-Aldrich, 99%) with and without 0.235g SnF2 were dissolved in a mixture of 12 mL HBr (J&K, 48 wt%) and 6 mL H3PO2 (J&K, 50 wt%), followed by stirring at 80 °C under an ambient atmosphere. After forming a bright yellow solution, the solutions were partially transferred to a 10 mL beaker sealed by filter paper with a certain amount of holes and heated at 85 °C. After several hours, many tiny reddish-brown MASnBr3 single crystals were precipitated from two solutions. Device Fabrication: A fluorine-doped tin oxide-coated (FTO) glass substrate was etched with Zinc metal powder and 12 wt% HCl diluted in deionized water. The substrates were then cleaned by ultrasonication with deionized water, acetone, and ethanol, and finally treated under UV ozone (UVO) cleaner for 30 min. A 50 nm thick TiO2 compact layer was deposited
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Figure 5. a) J–V curves of PSCs with different SnF2 concentrations under 100 MW cm−2 AM 1.5G illumination. b) J–V curves of the champion device with 30 mol% SnF2 in forward (from Jsc to Voc) and reverse (from Voc to Jsc) scans. c) EQE spectra and integrated Jsc. d) Steady-state Jsc at a constant bias of 0.34V. e) PCE histogram of 30 devices with 30 mol% SnF2. f) Recombination resistance at different bias for the devices with and without SnF2. g) Stability of a PSC with 30 mol% SnF2 stored and measured in nitrogen-filled glove box. on the substrates by spray pyrolysis process with titanium diisopropoxide bis (acetylacetonate) solution (Sigma-Aldrich, 75% in 2-propanol) diluted with isopropanol (J&K, 99.7%) in volume ratio of 1:9. The mesoporous TiO2 layer was formed by spin coating DYESOL18NR-T paste (DYESOL) at 4000 r.p.m. for 30 s, which was diluted with ethanol in a weight ratio of 1:3,
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and then sintered at 540 °C for 30 min. After cooled to room temperature, the substrate was treated under UVO cleaner for 30 min. MASnIBr2 with 0–40 mol% SnF2 (Sigma-Aldrich, 99%) was dissolved in dimethyl sulfoxide (DMSO) (J&K, 99.8%) at a molar ratio of 1.25 M by stoichiometric ratio of MAI and SnBr2 (Sigma-Aldrich, 99%) while
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Table 1. J–V parameters of PSCs with different SnF2 concentrations (20 solar cells for each sample). Voc [V]
Jsc [mA cm−2]
FF [%]
PCE [%]
Max.
0.19
1.37
40.27
0.08
Sample
MASnIBr2
MASnIBr2 + 10 mol% SnF2
MASnIBr2 + 20 mol% SnF2
MASnIBr2 + 30 mol% SnF2
MASnIBr2 + 40 mol% SnF2
Min.
0.16
1.00
27.32
0.06
Mean
0.17
1.17
35.14
0.07
Max.
0.28
12.32
43.61
1.41
Min.
0.25
7.85
38.85
1.14
Mean
0.27
11.26
41.25
1.24
Max.
0.43
13.92
56.65
2.78
Min.
0.41
11.46
42.62
2.06
Mean
0.42
12.36
49.26
2.53
Max.
0.45
13.77
59.58
3.70
Min.
0.42
12.30
55.30
3.15
Mean
0.44
13.14
58.00
3.36
Max.
0.44
10.64
49.13
1.98
Min.
0.39
8.86
37.28
1.62
Mean
0.43
9.67
41.97
1.75
stirring at 70 °C for 2 h. 60 µL of this solution was used per substrate to spin coat on TiO2 substrates at 2000 r.p.m. for 40 s, followed by dried at 100 °C for 15 min to remove the solvent. The hole transport material (HTM) film was then deposited by spin coating at 4000 r.p.m. for 30 s. The spin-coating formulation was prepared by dissolving 72.3 mg spiro-OMeTAD, 30 µL 2, 6-lutidine, 17.5 µL of a stock solution of 520 mg mL−1 lithium bis (trifluoromethylsulphonyl) imide in acetonitrile in 1 mL chlorobenzene. Finally, 50 nm of gold was thermally evaporated on top of the device to form the back contact. All the fabrication are performed in glovebox filled with Nitrogen. Film and Device Characterization: MASnIBr2 films were imaged by a focused ion beam microscope (Dual-beam FIB 235, FEI Strata). The crystal structure of the films with 0, 10, 20, 30, and 40 mol% SnF2 was characterized on a Rigaku Ultima X-ray IV diffractometer using a Cu Kα radiation at 3° min−1. The absorbance spectra of perovskite films on mesoporous TiO2 with sarin sealing were measured by a Shimadzu-UV3600 (UV–vis–NIR) spectrophotometer attached with an integrating sphere (ISR-3100) in the UV–VIS–NIR range (200 nm ∼ 2.5 µm). The XPS spectra were obtained on Thermo Fisher Scientific K-Alpha.The J–V characteristics of the devices were measured by a solar simulator (Enli Technology Co., Ltd, SS-F5-3A) equipped with a Keithley 2400 digital source meter. The NREL-calibrated solar (Enli Technology Co., Ltd, SRC2020) was used to adjust the light intensity into one sun illumination (100 MW cm−2). All the J–V curves are performed with a scan rate of 100 mV s−1 in glovebox filled with nitrogen. The EQE was measured by a system with a monochromator (Newport74125) and a power meter (Newport2936-C). The Hall measurement was performed with Lake Shore Hall test system 8040A. EIS test was meatured on a CHI660E workstation (CH Instruments). A 20 mV voltage perturbation was applied at different dc voltages ranging from 0 to Voc with frequencies between 105 and 1 Hz under dark condition. The results were fitted using the software Z-view. The steady-state photoluminescence (PL) and TRPL measurements were tested with an 530 nm output of a 5.6 MHz, picosecond supercontinuum fiber laser (EXR-15, from NKT Photonics).
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
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Acknowledgements The authors acknowledge the micro-fabrication center of National Laboratory of Solid State Microstructures (NLSSM) for technique support. This work was jointly supported by the National Key Research and Development Program of China (Grant No. 2017YFA0205700) and the State Key Program for Basic Research of China (Grant No. 2015CB659300), National Natural Science Foundation of China (Grant Nos. 11621091 and 11574143), Natural Science Foundation of Jiangsu Province (Grant Nos. BK20150056), the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and the Fundamental Research Funds for the Central Universities (Grant Nos. 021314380068, 021314380089, and 021314380091).
Conflict of Interest The authors declare no conflict of interest.
Keywords crystal nucleation, lead free, perovskite solar cells, tin-fluoride Received: June 28, 2017 Revised: September 26, 2017 Published online:
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