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Improving the Performance of Inverted Formamidinium Tin Iodide Perovskite Solar Cells by Reducing the Energy-Level Mismatch Xiao Liu, Yanbo Wang, Fengxian Xie, Xudong Yang, and Liyuan Han ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b00383 • Publication Date (Web): 11 Apr 2018 Downloaded from http://pubs.acs.org on April 12, 2018

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ACS Energy Letters

Improving the Performance of Inverted Formamidinium Tin Iodide Perovskite Solar Cells by Reducing the Energy-Level Mismatch Xiao Liu,1,2 Yanbo Wang,2 Fengxian Xie,1 Xudong Yang,2 and Liyuan Han1,2* Dr. X. Liu, Y. Wang, Dr. X. Yang, Dr. L. Han 1 Research Network and Facility Services Division National Institute for Materials Science Tsukuba, Ibaraki 305–0047, Japan Dr. X. Liu, Dr. F. Xie, Dr. L. Han 2 State Key Laboratory of Metal Matrix Composites Shanghai Jiao Tong University, 800 Dong Chuan Road Shanghai 200240, China *Corresponding Author: [email protected] Abstract A new hole-transport layer, poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate (PEDOT:PSS) with intercalated polyethylene glycol (PEG), was introduced into Pb-free inverted Formamidinium Tin Triiodide (FASnI3) perovskite solar cells to reduce the energy-level mismatch between FASnI3 and PEDOT:PSS. Intercalation of PEG into the PEDOT:PSS enhanced the power conversion efficiency from 2.01% to 5.12% in the forward scan and resulted in high reproducibility. The device based on PEG-PEDOT:PSS showed a reduced hysteresis and a stabilized efficiency of 4.91%. Finally, the devices with the new hole-transport layer retained more than 90% and 95% of their initial power conversion efficiencies upon storage in, respectively, an ambient environment for 4 h and a N2 environment for 30 days. These findings suggest that this new material will facilitate the development of environment-friendly Pb-free perovskite solar cells for real-world applications.

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ACS Energy Letters

Considerable research effort has been devoted to organic–inorganic Pb halide perovskite solar cells (PSCs), led to the power conversion efficiency (PCE) was rapidly boosted from 3.8% to above 22%.1-9 However, the toxicity of Pb severely limits the commercial utility of Pb-based PSCs, so the development of Pb-free PSCs is an important goal. Many potential replacements for Pb have been proposed, including Ge2+, Sn2+, Sb3+, and Bi3+.10-19 In particular, Sn-based perovskites have attracted attention because both Sn and Pb are group 14 elements and have similar structural properties and electronic characteristics.11,12 At beginning, Sn-based PSCs used a conventional structure based on mesoporous TiO2 as the electron-transport layer (ETL) and 2,2’,7,7’-tetrakis(N,N-di-p-methoxyphenylamino)-9,9’spirobi-fluorene (Spiro-MeOTAD) as the hole-transport layer (HTL). Snaith’s group reported a MASnI3 PSC with a maximum PCE of approximately 6% but with low reproducibility.11 By using pure DMSO as a solvent and hydrazine vapor to improve the film quality and retard the oxidation of Sn2+ to Sn4+, Kanatzidis’s group achieved PCEs ranging from 3% to 6%.20-22 However, the hydroxyl and hygroscopic property of TiO2, and the hygroscopic property of lithium salts in Spiro-MeOTAD layer might accelerate the oxidization rate of Sn2+ to Sn4+, a process that rapidly decreases device performance.23,24 Recently, researchers have directed their attention to inverted device architectures that use

poly

(3,4-ethylenedioxythiophene):poly

(styrene

sulfonate)

(PEDOT:PSS)

and

[6,6]-phenyl C61 butyric acid methyl ester (PCBM) as HTL and ETL, respectively. A few devices with this architecture are reported to exhibit PCEs over 6%,25-27 but these results were not so easy to repeatable. More commonly, the performance of such devices is lower than expected, and large hysteresis is observed, which hinders accurate measurement of the PCE.18



In contrast, the phenomenon of large hysteresis was not observed with a methylammonium lead iodide (MAPbI3)-based PSC with the same device architecture.28 The perovskite materials MAPbI3 and FASnI3 have different band gaps: 1.55 eV for the former and approximately 1.4 eV for the latter.29,30 The narrower bandgap of the Sn-based PSC results in an increase in the energy of the valence band maximum (VBM) or a decrease in the energy of the conduction band minimum (CBM) of the perovskite layer, even though the energy levels of the two adjacent layers are the same in the two types of cells. Therefore, the large hysteresis observed for Sn-based PSCs may be due to the misaligned energy levels. In fact, the VBM of FASnI3 is very shallow and only around -4.7 eV,29,30 and the work function (WF) of PEDOT:PSS (~ -5.1 eV) is too deep to accommodate the relatively large energy-level difference between the VBM of FASnI3 and WF of PEDOT:PSS. Additives such as lithium salts have been introduced into the PEDOT:PSS layer to move the WF to a shallower level.31 However, the adverse effects of ion movement and moisture sensitivity resulting from the presence of additives often shortens the lifetime of electronic devices in which they are used. In this study, we developed a HTL with a tunable WF by using a PEDOT:PSS with intercalated polyethylene glycol (PEG) in an inverted FASnI3 PSC, this strategy effectively reduced the energy level mismatch between FASnI3 and PEDOT:PSS without compromising the stability of device. By using PEG-PEDOT:PSS instead of neat PEDOT:PSS, we were able to increase the PCE from 2.01% to 5.12% in the forward scan with high reproducibility. More importantly, The device based on PEG-PEDOT:PSS showed a reduced hysteresis and a stabilized efficiency of 4.91%. Finally, a stability test showed that the devices based on PEG-PEDOT:PSS maintained more than 90% of their initial PCEs when


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ACS Energy Letters

stored under ambient conditions for 4 h and more than 95% when stored in a N2 environment for 30 days. Our results show that device performance can be improved by minimizing the interfacial energy bandgap mismatch between FASnI3 and PEDOT:PSS. The fabrication of a FASnI3 PSC with reliable physical characteristics and device performance requires the advance preparation of a dense, uniform FASnI3 film. To accomplish this, we used a common FASnI3 precursor formulation based on a SnF2 additive in a solvent mixture solution with different volume ratios of DMF: DMSO.32 SnF2 (12% mol) was added to the precursor solution as an anti-oxidant to penetrate the Sn2+ vacancies. Although we used DMSO as the predominant solvent, we added DMF in various proportions to control the crystallization rate of the FASnI3 film. Figure 1 shows scanning electron microscopy (SEM) images of three different FASnI3 films. A film derived from pure DMSO showed very high surface coverage and uniformity on the PEDOT:PSS layer. However, the grain size of the FASnI3 crystals was relatively small, averaging 500–600 nm (Figure 1a). In a film derived from 10% DMF in DMSO, the amount of the SnI2·3DMSO intermediate phase was lower than that in the film derived from pure DMSO, a result that was attributed to the faster crystallization of FASnI3 from the DMF-containing solution than from pure DMSO. Therefore, the FASnI3 film derived from 10% DMF in DMSO had larger grains (~1.5 µm, Figure 1b). In general, the larger grain size is known to not only reduce the number of defects at the grain boundaries but also hinder ion migration at the boundaries.33-36 However, when we increased the proportion of DMF to 25%, due to the too fast crystallization rate of FASnI3 from DMF solvent,20 the film did not completely cover the PEDOT:PSS layer: a few pinholes were found. By the way, some small white dots (Figure 1a, b, c) would come from the SnF2



additive,12 X-ray diffraction (XRD) analysis of the FASnI3 film derived from different volume ratios of DMF in DMSO confirmed that the film was highly crystalline with an orthorhombic lattice structure. Compared with XRD peak intensity of FTO glass, the relative low XRD peak intensity of FASnI3 films (Figure 1d) might be due to the thickness of our FASnI3 films is thin, only around 280 nm (Figure 3a). There were no obvious SnF2 peaks in FASnI3 XRD patterns (Figure 1d). This result indicates there was no obvious phase segregation of FASnI3 and SnF2. For this study, we used FASnI3 PSCs with the following inverted device structure: fluorine-doped tin oxide (FTO)/(PEG-)PEDOT:PSS/FASnI3/PCBM/bathocuproine (BCP)/Ag. To understand the energy diagram for this device structure, we estimated the VBM of FASnI3 and the WF of PEDOT:PSS by means of photoelectron spectroscopy in the air (PESA; Figures S1 and S2, respectively). The bandgap of the FASnI3 film was estimated from its absorption spectrum and its Kubelka–Munk-transformed diffuse reflectance spectrum (Figure S1), and the VBM of FASnI3 was determined to be -4.74 eV and the WF of PEDOT:PSS to be -5.10 eV (Figures S1 and S2, respectively); therefore, the energy-level mismatch between the FASnI3 and neat PEDOT:PSS was approximately 0.36 eV. As mentioned above, we then added PEG to the PEDOT:PSS solution at a volume ratio of 0.05%, 0.1%, or 0.2% to adjust the WF of the PEDOT:PSS to a shallower level. The WFs of the PEG-PEDOT:PSS films were -4.98, -4.88, and -4.79 eV at PEG volume ratios of 0.05%, 0.1%, and 0.2%, respectively (Figure S2). That is, the WF gradually decreased with PEG volume ratio. We also found that increasing the PEG volume ratio to greater than 0.2% had little effect on the WF; even when we increased the volume ratio to 1%, the WF was


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ACS Energy Letters

-4.76 eV (data not shown), which is only 0.03 eV lower than the WF at 0.2%. The proposed mechanism of PEG-induced change in the WF of PEDOT:PSS in our study can be explained as follows. According to the previous report,37 PEDOT:PSS film with a PSS-rich surface layer would show a deeper WF due to the surface dipole. When PEG is added to the PEDOT:PSS solution, PEG forms hydrogen bonds with the SO3H+ groups of the PSS, reducing the Coulombic interaction between PEDOT and PSS (Figure S3).38 Eventually, segregation of excess PSS results in PEDOT-rich domains on the film surface. Then the dipole will disappear and lead to a decrease in the WF of PEDOT:PSS. In our case, after we introduce 0.2% volume ratio of PEG in the PEDOT:PSS solution, the PSS-rich surface layer may almost change to PEG-rich surface layer, so the WF of PEDOT:PSS does not decrease apparently when we further increase the amount of PEG in PEDOT:PSS. We also found when the PEG volume ratio exceeded 0.2%, the morphology of the resulting PEG-PEDOT:PSS film deteriorated, with some strips appearing on the film. Therefore, for subsequent experiments, we used the 0.2% PEG-PEDOT:PSS, which had a smooth surface and a WF (-4.79 eV) that was well-matched to the VBM of FASnI3. With the above information, the energy band diagram could be drawn in Figure 2b. Having characterized the FASnI3 film and determined the WF of 0.2% PEG-PEDOT:PSS, we proceeded to the main focus of our work, which was to investigate the effect of using PEG-PEDOT:PSS in a FASnI3-based PSC. We fabricated a device consisting of an FTO transparent electrode, a 0.2% PEG-PEDOT:PSS HTL (~40 nm thick), a FASnI3 perovskite layer (~280 nm), a PCBM/BCP layer (~80 nm), and a Ag back electrode (~100 nm) (Figure 3a); and we measured the current density-voltage (J–V) curve of this device, as well



as that of a device with a neat PEDOT:PSS HTL (Figure 3b). Owing to the large grain size and uniformity of the FASnI3 film, the neat PEDOT:PSS device showed fair PCEs of 2.01% and 1.56%, with Voc values of 0.29 and 0.23 V, Jsc values of 14.38 and 13.76 mA/cm2, and

FF values of 47.9% and 49.2%, in the forward and reverse scan directions, respectively (Table 1). When we used the 0.2% PEG-PEDOT:PSS as the HTL, we observed a clear improvement in the performance of the resulting FASnI3 PSC. In forward scans, the PCEs of the PEG-PEDOT:PSS-based devices gradually increased with increasing PEG volume ratio to 3.00%, 4.05%, and 5.12%, respectively (Figures 2b, S4, and S5). The best performance was obtained with the 0.2% PEG-PEDOT:PSS-based device, which had PCEs of 5.12% and 5.03%, Voc values of 0.37 and 0.36 V, Jsc values of 22.06 and 21.90 mA/cm2, and FF values of 62.7% and 63.8% in the forward and reverse scan directions, respectively. So the hysteresis of J-V curve for FASnI3 device based on 0.2% PEG-PEDOT:PSS is smaller than the FASnI3 device based on pristine PEDOT:PSS (Figure 2b). Encouragingly, we found that the Jsc, Voc, and FF values were all higher in the PEG-PEDOT:PSS-based device than in the PEDOT:PSS-based device, a result that we attributed to the reduction in the energy-level mismatch between FASnI3 and PEDOT:PSS, which led in turn to reduced charge carrier accumulation and recombination, so that the carriers could be successfully extracted from the FASnI3. The PCE values for all four devices are also summarized in Figure S6. Considering that several other studies of similar FASnI3 formulations showed average efficiencies of 1%–4%,18,20,22 our devices, with a maximum efficiency of 5.12% under ambient conditions without encapsulation, have the potential to be useful for future studies and real-world applications.


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ACS Energy Letters

The incident photo-to-current conversion efficiency (IPCE) spectra and integrated Jsc values of devices based on neat PEDOT:PSS and on 0.2% PEG-PEDOT:PSS are shown in Figure 3c. Over the entire wavelength region from 300 to 1000 nm, the IPCE spectrum for the PEG-PEDOT:PSS device was higher than that for the neat PEDOT:PSS device, and the integrated Jsc values for the two devices were calculated to be 22.17 and 14.41 mA/cm2, respectively; these results are consistent with the J–V curves of the two devices. Furthermore, we evaluated reproducibility by measuring the performance characteristics of 18 FASnI3 devices based on 0.2% PEG-PEDOT:PSS (Figure 3d): the average PCE, Voc, Jsc, and FF values were 4.64 ± 0.35%, 0.35 ± 0.02 V, 20.78 ± 0.52 mA/cm2, and 60.8 ± 2.8%, respectively, under forward scan conditions. These results indicate that intercalation of 0.2% PEG into the PEDOT:PSS HTL led to enhanced performance and high reproducibility. The instability of Sn-based PSCs is a well-known problem awaiting a solution, so we compared the temporal dependences of the power outputs of PSCs based on neat PEDOT:PSS and PEG-PEDOT:PSS. When we measured the steady-state current and stabilized power output of the PEDOT:PSS-based device at an applied voltage of 0.17 V, we found that Jsc started to decrease immediately and that after 180 s under continuous 100 mW/cm2 AM 1.5G illumination, the PCE had dropped by 18.9% from the initial value of 1.64% (Figure 4a). However, when we measured steady-state current and stabilized power output of a device with a 0.2% PEG-PEDOT:PSS HTL at an applied voltage of 0.26 V, we observed a greatly improved stabilized PCE of ~4.91%, with a steady-state current density of 18.90 mA/cm2 (Figure 4b). We also compared the performance of an unencapsulated cell based on 0.2% PEG-PEDOT:PSS stored in a dark place under ambient conditions (~40%



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humidity, temperature ~25 °C) with the performance of a similar cell stored in a glove box under N2 (O2 < 0.1 ppm, H2O < 0.01 ppm, temperature ~22 oC) (Figure 4c,d). We found that the cell stored under ambient conditions maintain more than 90% of its initial PCE after 4 h, although the PCE eventually dropped to about 65% of the initial value after 8 h; nevertheless, the cell was more stable than previous reported Sn halide cell, the efficiency of which drops substantially

after

only

a

few

minutes

under

ambient

conditions.11

When

a

PEG-PEDOT:PSS-based cell was stored in a N2-filled glove box, it maintained more than 95% of its initial PCE value for up to 30 days, indicating that O2 and H2O contributed strongly to performance degradation. In summary, a PEDOT:PSS with intercalated PEG HTL was used to reduce the energy-level mismatch between FASnI3 and the HTL in PSCs with an inverted structure. By employing 0.2% PEG-PEDOT:PSS, we could tune the WF of PEDOT:PSS to 4.79 eV; and using this new HTL, we achieved a maximum PCE of 5.12% and high reproducibility. The device showed a reduced hysteresis and a stabilized efficiency of 4.91%. Furthermore, the use of the PEG-PEDOT:PSS HTL prolonged device stability during storage under ambient conditions for up to 4 h and in a N2 environment for 30 days. Taken together, our results provide valuable information that may facilitate the development of highly efficient and stable Pb-free PSCs. Supporting Information Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1002/((please add manuscript number)).


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ACS Energy Letters

Experimental section and additional characterization, including UV–vis absorption spectrum, photoelectron spectroscopy, molecular structures, current density-voltage curve. Author Information and Notes Corresponding author *Email: [email protected] ORCID Xiao Liu: 0000-0003-0245-8982 Xudong Yang: 0000-0002-3877-7830 Liyuan Han: 0000-0001-9766-9015 Notes The authors declare no competing financial interest. Acknowledgements This work was partially supported by the New Energy and Industrial Technology Development Organization (NEDO), National Natural Science Foundation of China (11574199 and 11674219). The authors thank Dr. C. C. Chen for his suggestions on this work, and thank Mr. T. Shimizu and Mr. T. Ishikawa of the National Institute for Materials Science, Japan, for assisting us with experiments. ((will be filled in by the editorial staff)) ((will be filled in by the editorial staff)) ((will be filled in by the editorial staff))



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ACS Energy Letters

(22) Song, T. B., Yokoyama, T., Stoumpos, C. C., Logsdon, J., Cao, D. H., Wasielewski, M. R., Aramaki, S., Kanatzids, M. G. Importance of Reducing Vapor Atmosphere in the Fabrication of Tin-Based Perovskite Solar Cells. J. Am. Chem. Soc. 2017, 139, 836-842. (23) Adli, H. K., Harada, T., Septina, W., Hozan, S., Ito, S., Ikeda, S. Effects of Porosity and Amount of Surface Hydroxyl Groups of a Porous TiO2 Layer on the Performance of a CH3NH3PbI3 Perovskite Photovoltaic Cell. J. Phys. Chem. C 2015, 119, 22304-22309. (24) Liu, J., Wu, Y., Qin, C., Yang, X., Yasuda, T., Islam, A., Zhang, K., Peng, W., Chen, W., Han, L. A Dopant-Free Hole-Transporting Material for Efficient and Stable Perovskite Solar Cells. Energy. Environ. Sci. 2014, 7, 2963-2967. (25) Liao, W., Zhao, D., Yu, Y., Crice, C. R., Wang, C., Cimaroli, A. J., Schulz, P., Meng, W., Zhu, K., Xiong, R. G., Yan, Y. Lead-Free Inverted Planar Formamidinium Tin Triiodide Perovskite Solar Cells Achieving Power Conversion Efficiencies up to 6.22%.

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ACS Energy Letters

(37) Nardes, A. M., Kemerink, M., De Kok, M. M., Vinken, E., Maturova. K., Janssen, R. A. J. Conductivity, Work Function, and Environmental Stability of PEDOT:PSS Thin Films Treated with Sorbitol. Org. Electron. 2008, 9, 727-734. (38) Mengistie, D. A., Wang, P. C., Chu, C. W. Effect of Molecular Weight of Additives on the Conductivity of PEDOT:PSS and Efficiency for ITO-Free Organic Solar Cells. J.

Mater. Chem. A 2013, 1, 9907-9915.



Figure 1. Scanning electron microscopy images of FASnI3 films on PEDOT:PSS prepared from precursor solutions with DMF:DMSO volume ratios of (a) 0:100, (b) 10:90, and (c) 25:75 (scale bars = 5 μm) and (d) X-ray diffraction pattern of the FASnI3 film shown in panel a, b and c.

Figure 2. (a) Configuration of the FASnI3 PSCs used in this study and (b) energy diagram for devices with this configuration.


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ACS Energy Letters

Figure 3. (a) Cross-section scanning electron microscopy image of a typical device; (b) J–V curves and (c) IPCE spectrum and integrated Jsc values for FASnI3 PSCs based on PEDOT:PSS and PEG-PEDOT:PSS; and (d) PCE histogram of 18 FASnI3 devices based on 0.2% PEG-PEDOT:PSS.

Figure 4. Steady-state current density and stabilized PCE tests for FASnI3 PSCs based on (a) PEDOT:PSS and (b) 0.2% PEG-PEDOT:PSS under continuous 100 mW/cm2 AM 1.5G 19 ACS Paragon Plus Environment


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illumination. Device stability tests of FASnI3 PSCs based on 0.2% PEG-PEDOT:PSS during storage (c) under ambient conditions and (d) in a N2-filled glovebox. Table1. Device parameters for best-performing PEDOT:PSS- and PEG-PEDOT:PSS-based FASnI3 PSCs. Scan Voc (V) direction Forward 0.29 PEDOT:PSS Reverse 0.23 Forward 0.31 0.05% Reverse 0.27 PEG-PEDOT:PSS Forward 0.35 0.1% PEG-PEDOT:PSS Reverse 0.33 Forward 0.37 0.2% PEG-PEDOT:PSS Reverse 0.36 HTL

Jsc (mA/cm2) 14.38 13.76 16.21 15.92 18.77 18.92 22.06 21.90


FF (%)

PCE (%)

47.9 49.2 59.7 59.8 61.6 60.8 62.7 63.8

2.01 1.56 3.00 2.57 4.05 3.80 5.12 5.03

Improving the Performance of Inverted ... - ACS Publications

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