Effects of annealing temperature on decaphenylcyclopentasilane-inserted CH3NH3PbI3 perovskite solar cells

Journal Pre-proofs Research paper Effects of annealing temperature on decaphenylcyclopentasilane-inserted CH3NH3PbI3 perovskite solar cells Masaya Taguchi, Atsushi Suzuki, Takeo Oku, Naoki Ueoka, Satoshi Minami, Masanobu Okita PII: DOI: Reference:

S0009-2614(19)30803-6 https://doi.org/10.1016/j.cplett.2019.136822 CPLETT 136822

To appear in:

Chemical Physics Letters

Received Date: Revised Date: Accepted Date:

16 August 2019 27 September 2019 3 October 2019

Please cite this article as: M. Taguchi, A. Suzuki, T. Oku, N. Ueoka, S. Minami, M. Okita, Effects of annealing temperature on decaphenylcyclopentasilane-inserted CH3NH3PbI3 perovskite solar cells, Chemical Physics Letters (2019), doi: https://doi.org/10.1016/j.cplett.2019.136822

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Effects of annealing temperature on decaphenylcyclopentasilane-inserted CH3NH3PbI3 perovskite solar cells Masaya Taguchi1, Atsushi Suzuki1, Takeo Oku1*, Naoki Ueoka1, Satoshi Minami2, and Masanobu Okita2

1

Department of Materials Science, The University of Shiga Prefecture, Hassaka 2500, Hikone, Shiga 522-8533, Japan

2

*

Frontier Materials Laboratories, Osaka Gas Chemicals Co., Ltd., Osaka 554-0051, Japan

Corresponding author E-mail address: [email protected] (T. Oku)

Abstract

Perovskite solar cells, in which decaphenylcyclopentasilane (DPPS) layers were formed on the surface of the CH3NH3PbI3 perovskite layer, were fabricated and characterized. The photovoltaic properties were improved by controlling the annealing temperature of the perovskite layer. For perovskite layers annealed at high temperatures of ~200 °C, the perovskite crystals were densely formed and the surface coverage of the perovskite layer was improved. The DPPS-laminated devices suppressed the formation of PbI2 crystals and the stability was improved by the DPPS layer.

Keywords: Perovskite solar cells, Polysilane, Stability

1

1. Introduction Recently, lead halide perovskite compounds have attracted immense research interest owing to their high carrier mobility, solution processability and excellent photovoltaic properties [1-7]. Therefore, perovskite solar cells are candidates as next-generation solar cells [8-10]. However, the stability of perovskite solar cells requires improving before they can be put into practical use [11-13]. Incorporating polymeric materials has been studied to improve the stability of perovskite solar cells [14-19], and the polymeric materials showed improvement of stability and promotion of crystal growth when introduced into the perovskite devices [18]. For example, by depositing a poly(methyl methacrylate) (PMMA) layer on the perovskite layer [20-23], the PMMA formed a compact layer via a cross-linked network and protected the device from oxygen and moisture [24]. Poly(propylene carbonate) was also similarly used and the resulting solar cell showed stability in various environments. In this case, large crystals were formed by crosslinking the perovskite particles and polymer which suppressed defects [25]. The general decrease in stability of perovskite solar cells is influenced by oxygen and moisture in the air, and can also be influenced by the hole transport layer (HTL). 2,2’,7,7’-tetrakis-(N,N-di(pmethoxyphenyl)amine)-9,9’-spirobifluorene (spiro-OMeTAD) is often used as a HTL for perovskite cells, but spiro-OMeTAD suffers from high cost and limited stability. Alternative hole transport materials with various improvements to stability have also been reported [26-30], and other hole transport materials, including polysilane derivatives, have been applied in organic solar cells [31-33]. Unlike ordinary polymer materials, polysilane derivatives have two advantages. The first is that the polysilane derivatives are a p-type semiconductor, and this facilitates hole transfer and rectification at the pn junction. The second is that the polysilane derivatives have high stability and are expected to act as a protective layer on the photoactive layer. Therefore, the polysilanes have also been applied as a HTL [34,35] and in a photoactive layer [36] for perovskite solar cells, and the photovoltaic performance was improved for a device incorporating decaphenylcyclopentasilane (DPPS) [36]. The polysilane derivatives were mixed with a perovskite precursor solution in that work [36], and the surface coverage of the perovskite layer increased and the conversion efficiency improved to ~10% in the device added with DPPS. However, it was desirable to improve the conversion efficiency further for practical use, and the device stability should be evaluated. It was also reported in another previous research [37] that the DPPS layer was deposited between the perovskite layer and the hole transport layer, and that the photovoltaic properties were improved by optimizing the concentration of DPPS solution. In that work, there were two problems; one is that

2

the conversion efficiency is still ~10%, and the other is that optimization of the annealing conditions was not sufficient. The device prepared at 160 °C provided the highest conversion efficiency of ~10% after 48 days, which suggested that the further improvement of the conversion efficiency is expected by annealing the devices at higher temperatures. The purpose of the present work was to investigate the photovoltaic properties and stabilities of CH3NH3PbI3 (MAPbI3) perovskite solar cells containing DPPS, which were prepared by annealing at high temperatures in ambient air. In previous research, the photovoltaic properties were improved by optimizing the concentration of DPPS and laminating it between the photoactive layer and the HTL. Furthermore, we tried to shorten the preparation time and improve the photovoltaic properties by raising the annealing temperature of the perovskite layer in the present work. The effects of DPPS insertion and annealing temperature on the photovoltaic properties and microstructures were investigated using light-induced current density voltage (J-V) characteristics, external quantum efficiency (EQE) measurements, X-ray diffraction (XRD), optical microscopy (OM), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDX).

2. Experimental procedures A schematic illustration of the fabrication process of the photovoltaic cells is shown in the previous report [37], and the detailed fabrication process was described in previous works [38-40]. Each layer was formed by a spin-coating method. F-doped tin oxide (FTO) substrates were cleaned using an ultrasonic bath with acetone and methanol and dried under nitrogen gas. Thereafter, the FTO substrates were treated with an ultraviolet ozone cleaner (Asumi Giken ASM401N) for 15 min. Precursor solutions of 0.15 and 0.30 M TiO2 were prepared from titanium diisopropoxide bis (acetyl acetonate) (Sigma Aldrich, 0.055 and 0.11 mL) with 1-butanol (Nacalai Tesque, 1 mL). The 0.15 M TiO2 precursor solution was spin-coated on the FTO substrate at 3000 rpm for 30 s (MIKASA Opticoat MSA100) and annealed at 125 °C for 5 min (AS ONE ND-1). The 0.30 M TiO2 precursor solution was spin-coated on the TiOx layer at 3000 rpm for 30 s, and the coated substrate was annealed at 125 °C for 5 min. This process of 0.30 M solution was performed twice, and the FTO substrate was sintered at 550 °C for 30 min (AS ONE SMF-2) to form a compact TiO2 layer. For the mesoporous TiO2 layer, the TiO2 paste was prepared with TiO2 powder (Aerosil, P-25, 100 mg) with poly(ethylene glycol) (Nacalai Tesque, PEG #20000, 10 mg) in ultrapure water (0.5 mL) [41]. This solution was mixed with acetylacetone (Fujifilm Wako Pure Chemical Corporation, 10 µL) and triton X-100 (Sigma Aldrich, 5 µL) for 30 min, and left for 24 h to suppress bubbles in the solution. After that, TiO2 paste was coated

3

on the substrate by spin-coating at 5000 rpm for 30 s. The cells were annealed at 125 °C for 5 min and sintered at 550 °C for 30 min to form the mesoporous TiO2 layer [42]. For preparation of the perovskite compounds, solutions of CH3NH3I (Tokyo Chemical Industry, 190.7 mg), PbCl2 (Sigma Aldrich, 111.2 mg) with the desired molar ratio in N,N-dimethylformamide (Sigma Aldrich, 0.5 mL), was mixed at 60 °C for 24 h. The perovskite precursor solutions were normally spin-coated during the first coating. During the second and third spin-coatings, an air blow method was applied [43]. Then, a standard device was annealed at 140 °C for 10 min to form the perovskite compound. When the starting material is CH3NH3I + PbI2, the suitable annealing temperature was 100 °C, which was confirmed in the previous work [38]. Whereas, the suitable temperature for the starting materials of 3CH3NH3I + PbCl2 was ~140 °C [44], and CH3NH3PbI3 with 2CH3NH3Cl (gas) were formed during the reaction. DPPS (Osaka Gas Chemicals, OGSOL SI-30-10, 10 mg) solutions were prepared using chlorobenzene (Fujifilm Wako Pure Chemical Corporation, 0.5 mL). The DPPS solution was dropped onto the perovskite layer during the last 15 s of spin-coating the perovskite precursor solutions. The devices with DPPS layers were annealed at temperatures in the range of 170 to 220 °C for 3 to 10 min. Then, a HTL was deposited by spin-coating at 4000 rpm for 30 s. For the HTL, a solution of spiroOMeTAD (Sigma Aldrich, 36.1 mg) in chlorobenzene (Fujifilm Wako Pure Chemical Corporation, 0.5 mL) and a solution of lithium bis(trifluoromethylsulfonyl)imide (Tokyo Chemical Industry, Li-TFSI, 260 mg) in acetonitrile (Sigma Aldrich, 0.5 mL) were stirred for 24 h. The spiro-OMeTAD solution with 4-tertbutylpyridine (Sigma Aldrich, 14.4 µL) was mixed with the Li-TFSI solution (8.8 µL) for 30 min at 70 °C. All procedures were carried out in air. Finally, Au electrodes were evaporated as top electrodes [43]. Layered structures of the present photovoltaic cells are denoted as FTO/TiO2/MAPbI3/(DPPS)/Spiro-OMeTAD/Au, as shown in a schematic illustration of Fig. 1. J-V characteristics of the photovoltaic cells were measured (Keysight B2901A) under illumination at 100 mW cm-2 by using an air mass (AM) 1.5 solar simulator (San-ei Electric XES-301S). The solar cells were illuminated through the side of the FTO substrates and the measurement area was 0.090 cm2. The EQEs of the solar cells were also measured for the same devices (Enli Technology QE-R). The microstructures of the perovskite layers were investigated using an X-ray diffractometer (Bruker D2 PHASER), while the surface morphologies of the perovskite layers were examined using an optical microscope (Nikon Eclipse E600) and a scanning electron microscope equipped with an EDX detector (JEOL JSM-6010PLUS-LA).

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3. Results and discussion J-V characteristics of FTO/TiO2/MAPbI3/(DPPS)/Spiro-OMeTAD/Au photovoltaic devices prepared at various temperatures are shown in Fig. 1(a). The measured photovoltaic parameters of the MAPbI3/(DPPS) devices are summarized in Table 1, where JSC is the short circuit current density, VOC is the open circuit voltage, FF is the fill factor, RS is the series resistance, Rsh is the shunt resistance, η is the conversion efficiency, and ηave is the average efficiency of four devices. The standard device without DPPS showed a η of 5.16% after annealing at 140 °C [37], while the device with DPPS annealed at 140 °C showed a η of 7.51%. On the other hand, the device with DPPS annealed at 220 °C showed a conversion efficiency of 11.99%. All photovoltaic parameters were improved for the devices annealed at high temperatures, and the FF values were especially increased compared with that of the standard device. Figure 1(b) shows EQE spectra of the as-prepared MAPbI3/(DPPS) photovoltaic devices with a structure model of DPPS, and the devices provided photoconversion efficiencies at wavelengths between 300 and 810 nm. J-V characteristics of the devices stored in the dark at 22 °C and ∼30% humidity for 4 weeks are shown in Fig. 1(c) and summarized in Table 1. The device with DPPS annealed at 190 °C showed the highest conversion efficiency of 13.36% after 4 weeks. The EQE spectra of MAPbI3/(DPPS) photovoltaic devices after 4 weeks are shown in Fig. 1(d). The devices annealed at high temperatures showed high and stable EQE values for wavelengths between 370 and 770 nm. Optical microscopy images of MAPbI3/(DPPS) devices are shown in Fig. 2(a). For the devices annealed at high temperatures with inserted DPPS, the sizes of the perovskite crystals were reduced and the perovskite crystals were densely packed. Therefore, the grain boundary area decreased and the surface coverage of the perovskite increased. Such microstructures resulted in charge traps and charge recombination in the grain boundaries being reduced and suppressed. Thus, charge loss was decreased as indicated by the decrease in RS and increase in JSC. SEM images and EDX mapping images (I, Pb and Cl) of the MAPbI3/(DPPS) devices are shown in Fig. 2(b). Compared with the standard device, the MAPbI3/(DPPS) devices annealed at high temperatures showed round perovskite crystals with smaller particle sizes. Uniform crystals were formed in the devices annealed at high temperatures, and the conversion efficiencies were improved because of the increase in surface coverage. Pb and I atoms were distributed between the large perovskite crystals. It was thought that larger perovskite crystals precipitated on the perovskite crystals penetrating in the mesoporous TiO2 layer. Therefore, the TiO2 layers were not exposed to the spiroOMeTAD. Measured compositions of the present perovskite solar cells are listed in Table 2. Although

5

desorption of iodine from the ordinary perovskite crystal have been observed [45], the compositions of iodine are almost constant even after annealing at 220 °C, which indicates excellent stabilization effect of the DPPS layer. XRD patterns of the as-prepared and after 4 weeks MAPbI3/(DPPS) devices are shown in Fig. 3(a) and (b), respectively. Enlarged XRD patterns are shown in Fig. 3(c) and (d), respectively, which show 100 peaks of perovskite and 001 peaks of PbI2. The measured lattice constants, crystallite sizes and peak intensity ratios of perovskite 100 to PbI2 001 of the devices are summarized in Table 3. The lattice constants and crystallite sizes did not change much over the 4 weeks. The as-prepared devices provided a small PbI2 peak, which could promote hole transfer and improve the photovoltaic properties. EDX mapping results suggested that a small amount of PbI2 crystals could be accumulated between the perovskite crystals, which may have prevented charge recombination [46,47]. Even after 4 weeks, the peak intensity of PbI2 did not change, which indicated that the DPPS acted as a protective layer of the perovskite layer and suppressed deterioration of the perovskite to PbI2. Stability measurements of the photovoltaic parameters up to 8 weeks after preparation are shown in Fig. 4. While all parameters decreased for the standard device, the parameters for the MAPbI3/(DPPS) devices annealed at high temperatures were fairly stable. This was because DPPS reduced the influence of spiro-OMeTAD, oxygen and moisture in air and thus suppressed decomposition of the perovskite compound. A schematic model showing the microstructures, carrier dynamics, and stability of the proposed devices is shown in Fig. 5(a). Because there was no DPPS layer for the standard device, the perovskite compound decomposed to PbI2 after 4 weeks by the desorption of CH3NH3 and influence of oxygen and H2O in the air. The number of holes and electrons generated in the perovskite layer was therefore reduced and the photovoltaic properties were degraded. However, the stability of the MAPbI3/(DPPS) devices improved because DPPS functioned as a protective layer and suppressed decomposition of the perovskite compound. From the XRD results in Fig. 3, the peak intensity of PbI2 for the 190 °C annealed device was weaker than those of the other devices, and the peak intensity of 100 perovskite increased after 4 weeks. The number of electrons and holes generated in the perovskite layer therefore increased and the photovoltaic properties were slightly improved after 4 weeks. In the present work, chlorobenzene was used as the solvent for the DPPS solution. The chlorobenzene dripping is often used to improve not only crystallinity of perovskite films but also photovoltaic performance of the perovskite solar cells [48,49]. Although the annealing temperature of 100 °C is enough for the chlorobenzene-dripped devices, the higher annealing temperature of ~200 °C is necessary for the present DPPS-dripped device. This indicates that the improvement mechanism of the photovoltaic properties would be different from that of the chlorobenzene dripping. The DPPS 6

layer could work as the effective protective layer improving the stability. In addition, the DPPS with the hole transport property would suppress the charge recombination and improve the photovoltaic properties. To explain the charge transport, an energy level diagram of the FTO/TiO2/MAPbI3/(DPPS)/SpiroOMeTAD/Au photovoltaic devices is shown in Fig. 5(b). In Fig. 5(b), previously reported values are used as the energy levels [50,51]. By irradiating light from the FTO substrate side, electrons and holes were generated in the perovskite layer. Electrons generated in the photoactive layer were transported through TiO2 to the FTO, and holes were transported through DPPS and spiro-OMeTAD to the Au. By incorporating DPPS between the perovskite and HTL, effective hole transport from the valence band of MAPbI3 to the Au electrode was induced. This smooth transport was also due to the molecular structure of DPPS [35].

4. Conclusions The effects of inserting a DPPS layer between the perovskite layer and HTL on the photovoltaic properties were investigated. The J-V characteristics indicated improvements to the devices upon introducing a DPPS layer on the perovskite layer after annealing at high temperatures. The device annealed at 220 °C showed the highest conversion efficiency of 11.99% among the as-prepared devices. After 4 weeks, the photovoltaic properties were improved further, and the device annealed at 190 °C showed the highest conversion efficiency of 13.36%. The surface morphology of the devices was evaluated using OM, SEM, and EDX, and the sizes of perovskite crystals with DPPS annealed at high temperatures were reduced. The grain boundary area decreased, and the surface coverage increased, which reduced the leakage current and improved the JSC values. Crystal structures of the perovskite were analyzed using XRD, and PbI2 formation was suppressed for the DPPS-inserted devices annealed at high temperatures even after 4 weeks. This suppression of PbI2 formation was due to the protective effect of the DPPS layer, and the stability of the performance was maintained for up to 8 weeks. These findings indicate that DPPS insertion and annealing at high temperatures are effective for improving the performance and stability of the perovskite solar cells.

Declaration of Competing Interest

The authors declare no conflict of interest.

Acknowledgements We thank Aidan G. Young, PhD, from Edanz Group (www.edanzediting.com/ac) for editing a draft of this manuscript. 7

References [1] V. L. Pool, B. Dou, D. G. V. Campen, T. R. Klein-Stockert, F. S. Barnes, S. E. Shaheen, M. I. Ahmad, M. F.A.M. V. Hest, M. F. Toney, Nat. Commun. 8 (2017) 14075. [2] G. A. Sewvandi, K. Kodera, H. Ma, S. Nakanishi, Q. Feng, Sci. Rep. 6 (2016) 30680. [3] M. D. Sampson, J. S. Park, R. D. Schaller, M. K. Y. Chan, A. B. F. Martinson, J. Mater. Chem. A 5 (2017) 3578. [4] J. F. Wang, L. Zhu, B. G. Zhao, Y. L. Zhao, J. Song, X. Q. Gu, Y. H. Qiang, Sci. Rep. 7 (2017) 14478. [5] W. Xu, L. Zheng, X. Zhang, Y. Cao, T. Meng, D. Wu, L. Liu, W. Hu, X. Gong, Adv. Energy Mater. 10 (2018) 1703178. [6] F. Wang, S. Bai, W. Tress, A. Hagfeldt, F. Gao, NPJ. Flex. Electron 2 (2018) 22. [7] D. Gedamu, I. M. Asuo, D. Benetti, M. Basti, I. Ka, S. G. Cloutier, F. Rosei, R. Nechache, Sci. Rep. 8 (2018) 12885. [8] D. Cortecchia, H. A. Dewi, J. Yin, A. Bruno, S. Chen, T. Baikie, P. P. Boix, M. Gratzel, S. Mhaisalkar, C. Soci, N. Mathews, Inorg. Chem. 55 (2016) 1044. [9] Y. Y. Pan, Y. H. Su, C. H. Hsu, L. W. Huang, K. P. Dou, C. C. Kaun, J. Adv. Nanomater. 1 (2016) 33. [10] C. H. Ng, T. S. Ripolles, K. Hamada, S. H. Teo, H. N. Lim, J. Bisquert, S. Hayase, Sci. Rep. 8 (2018) 2482. [11] N. Li, Z. Zhu, C. C. Chueh, H. Liu, B. Peng, A. Petrone, X. Li, L. Wang, A. K. Y. Jen, Adv. Energy Mater. 7 (2017) 1601307. [12] X. Zhang, J. Yin, Z. Nie, Q. Zhang, N. Sui, B. Chen, Y. Zhang, K. Qu, J. Zhao, H. Zhou, RSC Adv. 7 (2017) 37419. [13] H. Dong, Z. Wu, J. Xi, X. Xu, L. Zuo, T. Lei, X. Zhao, L. Zhang, X. Hou, A. K. Y. Jen, Adv. Funct. Mater. 10 (2017) 1704836. [14] S. S. Mali, C. S. Shim, C. K. Hong, NPG Asia Mater. 7 (2015) e208. [15] H. W. Chen, T. Y. Huang, T. H. Chang, Y. Sanehira, C. W. Kung, C. W. Chu, M. Ikegami, T. Miyasaka, K. C. Ho, Sci. Rep. 6 (2016) 34319. [16] S. Zhang, Y. Lu, B. Lin, Y. Zhu, K. Zhang, N. Y. Yuan, J. N. Ding, B. Fang, Sol. Energy Mater. Sol. Cells 170 (2017) 178. [17] L. Zuo, H. Guo, D. W. deQuilettes, S. Jariwala, N. D. Marco, S. Dong, R. De Block, D. S. Ginger, B. Dunn, M. Wang, Sci. Adv. 3 (2017) e1700106.

8

[18] H. Zhang, J. Shi, L. Zhu, Y. Luo, D. Li, H. Wu, Q. Meng, Nano Energy 43 (2018) 383. [19] Z. Chen, Q. Dong, Y. Liu, C. Bao, Y. Fang, Y. Lin, S. Tang, Q. Wang, X. Xiao, Y. Bai, Y. Deng, J. Huang, Nat. Commun. 8 (2017) 1890. [20] D. Bi, C. Yi, J. Luo, J. D. Decoppet, F. Zhang, S. M. Zakeeruddin, X. Li, A. Hagfeldt, M. Gratzel, Nat. Energy 1 (2016) 16142. [21] F. Wang, M. Endo, S. Mouri, Y. Miyauchi, Y. Ohno, A. Wakamiya, Y. Murata, K. Matsuda, Nanoscale 8 (2016) 11882. [22] S. D. Stranks, G. E. Eperon, G. Grancini, C. Menelaou, M. J. P. Alcocer, T. Leijtens, L. M. Herz, A. Petrozza, H. J. Snaith, Science 342 (2013) 341. [23] M. Taguchi, A. Suzuki, H. Tanaka, T. Oku, AIP Conf. Proc. 1929 (2018) 020012. [24] F. Wang, A. Shimazaki, F. Yang, K. Kanahashi, K. Matsuki, Y. Miyauchi, T. Takenobu, A. Wakamiya, Y. Murata, K. Matsuda, J. Phys. Chem. C 121 (2017) 1562. [25] T. H. Han, J. W. Lee, C. Choi, S. Tan, C. Lee, Y. Zhao, Z. Dai, N. D. Marco, S. J. Lee, S. H. Bae, Y. Yuan, H. M. Lee, Y. Huang, Y. Yang, Nat. Commun. 10 (2019) 520. [26] M. M. Tavakoli, R. Tavakoli, D. Prochowicz, P. Yadav, M. Saliba, Mol. Syst. Des. Eng. 3 (2018) 717. [27] A. Dubey, N. Adhikari, S. Mabrouk, F. Wu, K. Chen, S. Yang, Q. Qiao, J. Mater. Chem. A 6 (2018) 2406. [28] D. Wang, K. H. Chan, N. K. Elumalai, M. A. Mahmud, M. B. Upama, A. Uddin, S. Pillai, Phys. Chem. Chem. Phys. 19 (2017) 25016. [29] S. Mabrouk, M. Zhang, Z. Wang, M. Liang, B. Bahrami, Y. Wu, J. Wu, Q. Qiao, S. Yang, J. Mater. Chem. A 6 (2018) 7950. [30] E. A. Gaml, A. Dubey, K. M. Reza, M. N. Hasan, N. Adhikari, H. Elbohy, B. Bahrami, H. Zeyada, S. Yang, Q. Qiao, Sol. Energy Mater. Sol. Cells 168 (2017) 8. [31] T. Oku, J. Nakagawa, M. Iwase, A. Kawashima, K. Yoshida, A. Suzuki, T. Akiyama, K. Tokumitsu, M. Yamada, M. Nakamura, Jpn. J. Appl. Phys. 52 (2013) 04CR07. [32] J. Nakagawa, T. Oku, A. Suzuki, T. Akiyama, M. Yamada, S. Fukunishi, K. Kohno, Green Sustain. Chem. 7 (2017) 20. [33] T. Oku, N. Hibi, A. Suzuki, T. Akiyama, M. Yamada, S. Fukunishi, K. Kohno, Jpn. Soc. Appl. Phys. Conf. Proc. 3 (2015) 011404. [34] Y. Shirahata, Y. Yamamoto, A. Suzuki, T. Oku, S. Fukunishi, K. Kohno, Phys. Status Solidi A 214 (2017) 1600591. [35] Y. Shirahata, T. Oku, S. Fukunishi, K. Kohno, Mater. Sci. Appl. 8 (2017) 209. [36] T. Oku, J. Nomura, A. Suzuki, H. Tanaka, S. Fukunishi, S. Minami, S. Tsukada, Int. J. Photoenergy 9

2018 (2018) 8654963. [37] M. Taguchi, A. Suzuki, T. Oku, S. Fukunishi, S. Minami, M. Okita, Coatings 8 (2018) 461. [38] T. Oku, M. Zushi, Y. Imanishi, A. Suzuki, K. Suzuki, Appl. Phys. Express 7 (2014) 121601. [39] T. Oku, K. Suzuki, A. Suzuki, J. Ceram. Soc. Jpn. 124 (2016) 234. [40] T. Oku, Y. Ohishi, A. Suzuki, Y. Miyazawa, J. Ceram. Soc. Jpn. 125 (2017) 303. [41] Y. Li, W. Sun, W. Yan, S. Ye, H. Rao, H. Peng, Z. Zhao, Z. Bian, Z. Liu, H. Zhou, Adv. Energy Mater. 6 (2016) 1601353. [42] N. Ueoka, T. Oku, Y. Ohishi, H. Tanaka, A. Suzuki, Chem. Lett. 47 (2018) 528. [43] T. Oku, Y. Ohishi, N. Ueoka, RSC Advances 8 (2018) 10389. [44] T. Oku, Y. Ohishi, J. Ceram. Soc. Jpn. 126 (2018) 56. [45] N. Ueoka, T. Oku, ACS Appl. Mater. Interfaces 10 (2018) 44443. [46] Q. Chen, H. Zhou, T. B. Song, S. Luo, Z. Hong, H. S. Duan, L. Dou, Y. Liu, Y. Yang, Nano Lett. 14 (2014) 4158. [47] C. R. Carmona, P. Gratia, I. Zimmermann, G. Grancini, P. Gao, M. Graetzel, M. K. Nazeeruddin, Energy Environ. Sci. 8 (2015) 3550. [48] N. J. Jeon, J. H. Noh, Y. C. Kim, W. S. Yang, S. Ryu, S. Seok, Nat. Mater. 13 (2014) 897. [49] M. Xiao, F. Huang, W. Huang, Y. Dkhissi, Y. Zhu, J. Etheridge, A. G. Weale, U. Bach, Y. B. Cheng, L. Spiccia, Angew. Chem. Int. Ed. 53 (2014) 9898. [50] J. H. Noh, S. H. Im, J. H. Heo, T. N. Mandal, S. I. Seok, Nano Lett. 13 (2013) 1764. [51] Y. Haga, Y. Harada, Jpn. J. Appl. Phys. 40 (2001) 855.

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Table 1. Measured photovoltaic parameters of MAPbI3/(DPPS) perovskite solar cells. The above and below show parameters of as-prepared cells and cells after 4 weeks, respectively.

As-prepared Annealing

JSC

VOC

FF

Rs

Rsh

η

ηave

(Ω cm )

(Ω cm )

(%)

(%)

0.349

28.0

105

5.16

2.07

0.903

0.660

8.13

916

7.51

6.06

19.38

0.889

0.640

6.64

1505

11.03

10.16

180

20.32

0.887

0.564

7.23

250

10.17

9.02

190

19.74

0.874

0.670

5.77

2758

11.57

11.29

210

20.32

0.908

0.637

6.56

969

11.75

11.33

220

21.23

0.894

0.632

5.83

661

11.99

11.18

FF

Rs

Rsh

η

ηave

(Ω cm2)

(Ω cm2)

(%)

(%)

67

2.32

2.05

-2

(°C)

(mA cm )

(V)

140*

17.77

0.833

140

12.61

170

2

2

After 4 weeks Annealing

JSC

VOC

(°C)

(mA cm-2)

(V)

140*

13.14

0.703

0.251

190

22.06

0.881

0.687

5.11

2419

13.36

10.30

210

20.78

0.899

0.655

5.92

1571

12.23

11.61

220

20.04

0.888

0.621

6.15

499

11.05

10.62

29.5

* MAPbI3 perovskite cells without DPPS.

Table 2. Measured compositions of the present perovskite solar cells. Annealing (°C)

Pb (at.%)

I (at.%)

Cl (at.%)

C : N (at.%)

140*

25.2

68.7

6.1

61.8 : 38.2

170

23.2

69.8

7.0

51.6 : 48.4

180

23.7

71.4

4.9

44.8 : 55.2

190

24.2

70.3

5.5

48.2 : 51.8

210

26.0

68.2

5.8

45.1 : 54.9

220

25.0

68.9

6.1

48.1 : 51.9

* MAPbI3 perovskite cells without DPPS.

11

Table 3. Microstructural parameters of MAPbI3 crystals. The above and below show parameters of asprepared cells and cells after 4 weeks, respectively.

As-prepared

After 4 weeks

Annealing (°C)

Lattice constant (Å)

Crystallite size (Å)

I100/PbI2

190

6.289

457

16.7

210

6.287

462

4.90

220

6.286

477

7.15

Annealing (°C)

Lattice constant (Å)

Crystallite size (Å)

I100/PbI2

190

6.276

474

16.9

210

6.274

495

4.88

220

6.272

522

7.49

12

(b) 90

(a) 24

As-prepared 220 °C

21

210 °C

70

18

170 °C 190 °C

15

60

180 °C 140 °C

12 9

180 °C 220 °C

50 40

Si Si

30

6

Si Si

Si

20

3

10

100 mW cm-2 AM 1.5

0

0 0

(c)

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Voltage (V)

24

1

(d) 90

After 4 weeks

21

300

400

190 °C

190 ° C

70

18

210 °C

15

220 °C

800

After 4 weeks

210 °C

60

12 140 °C

9

500 600 700 Wavelength (nm)

80

EQE (%)

Current density (mA cm-2)

As-prepared

190 °C

80

EQE (%)

Current density (mA cm-2)

210 °C

220 °C

50

40 30

6

20

3

10

100 mW cm-2 AM 1.5

0

0 0

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Voltage (V)

1

300

400

500 600 700 Wavelength (nm)

800

Fig. 1. (a), (c) J-V characteristics and (b), (d) EQE spectra of as-prepared MAPbI3/(DPPS) cells and cells after 4 weeks, respectively.

13

(a)

(b) 170 °C

170 °C

20 μm

IK

20 μm

Pb K

20 μm

Cl K

20 μm

180 °C 40 μm 180 °C

20 μm

190 °C

IK

20 μm

Pb K

20 μm

Cl K

20 μm

190 °C

20 μm

40 μm

40 μm

IK

20 μm

Pb K

20 μm

Cl K

20 μm

210 °C

220 °C

210 °C

20 μm

IK

20 μm

Pb K

20 μm

Cl K

20 μm

220 °C

40 μm

40 μm

20 μm

IK

20 μm

Pb K

20 μm

Cl K

20 μm

Fig. 2. (a) Optical microscopy images and (b) SEM images with EDX mappings of as-prepared MAPbI3/(DPPS) solar cells.

14

100

(a)

As-prepared

After 4 weeks

100

220 °C

Intensity (a. u.)

110

Intensity (a. u.)

PbI2

220 °C

200 FTO 210 FTO 111

210 °C

PbI2

110

210 °C

190 °C

15

(b) 5000

20 25 2θ (degree)

4500

Intensity (Counts)

3500

30

35

100

4500

190 °C 210 °C 220 °C

2500 2000 1500 PbI2

20 25 2θ (degree)

After 4 weeks

3500

30

35

100

190 °C 210 °C 220 °C

3000

2500 2000 1500

1000

500 0 12.0

15

4000

3000

1000

10

5000 As-prepared

4000

190 °C

Intensity (Counts)

10

200 FTO FTO 210 111

PbI2

500

12.5

13.0 13.5 2θ (degree)

14.0

14.5

0 12.0

12.5

13.0 13.5 2θ (degree)

14.0

14.5

Fig. 3. XRD patterns of (a) as-prepared MAPbI3/(DPPS) cells and (b) cells after 4 weeks.

15

(a) 14

(b) 24

12

21 18 JSC (mA cm-2)

η (%)

10 8

6 4

12

9 6

140 °C 190 °C 210 °C 220 °C

2

15

140 °C 190 °C 210 °C 220 °C

3

0

0

0

1

2

3 4 5 Time (week)

6

7

8

(c) 0.95

0

1

2

3 4 5 Time (week)

6

7

8

3 4 5 Time (week)

6

7

8

(d) 0.8

0.90 0.7 0.85 0.6

0.75

FF

VOC (V)

0.80

0.70 0.65

0.5 140 °C 190 °C 210 °C 220 °C

0.4 140 °C 190 °C 210 °C 220 °C

0.60 0.55

0.3

0.50

0.2

0

1

2

3 4 5 Time (week)

6

7

8

0

1

2

Fig. 4. Stability measurements of (a) η, (b) JSC, (c) VOC, and (d) FF of MAPbI3/(DPPS) perovskite solar cells.

16

(a) Annealing

As-prepared

(b)

After 4 weeks Perovskite

Spiro-OMeTAD

PbI2

140 °C

-1.7

-2.0 TiO2

TiO2 DPPS

190 °C TiO2

SpiroOMeTAD

-3.0

Energy level (eV)

PbI2

TiO2

210 °C

e⁻ e⁻

-4.0

-3.9

-4.2 -4.4

FTO

TiO2

Perovskite TiO2

-5.0 TiO2

DPPS

-5.2 -5.5 h⁺

-5.4 h⁺

-4.8 Au

h⁺

-6.0

hole

220 °C electron TiO2

TiO2

Fig. 5. (a) Schematic illustration of microstructures, carrier dynamics and stability for perovskite solar cells under different annealing temperatures. (b) Energy level diagram of the cell.

17

Effects of annealing temperature on decaphenylcyclopentasilane-inserted CH3NH3PbI3 perovskite solar cells

Masaya Taguchi, Atsushi Suzuki, Takeo Oku, Naoki Ueoka, Satoshi Minami, and Masanobu Okita Graphical Abstract Annealing

As-prepared

After 4 weeks Perovskite

Spiro-OMeTAD

PbI2

140 °C

-1.7

-2.0 TiO2

TiO2 DPPS

190 °C TiO2

SpiroOMeTAD

-3.0 Energy level (eV)

PbI2

TiO2

210 °C

e⁻ e⁻

-4.0

-3.9

-4.2 -4.4

FTO

TiO2

Perovskite TiO2

-5.0 TiO2

-5.2 -5.5

h⁺ -6.0

hole

220 °C electron TiO2

TiO2

18

DPPS

-5.4

h⁺ h⁺

-4.8

Au

Highlights



Decaphenylcyclopentasilane-inserted perovskite solar cells were characterized.



Conversion efficiencies of DPPS-inserted cells were improved by annealing at 220°C.



Surface coverage of the perovskite layer was improved by the DPPS insertion.



The DPPS layer suppressed the PbI2 formation and improved the device stability.



After 4 weeks, the 190°C-heated device provided the highest conversion efficiency.

19