In recent few years, the efficiency of organic-inorganic metal halide perovskite-based solar cells (PSCs) has been improved rapidly, because of the significant efforts in materials development and device fabrications. Hole transporting materials (HTMs) play an important role to the PSCs in charge extraction and interface modification. Currently, the most extensively studied and applied HTM for perovskite devices is 2,2’,7,7’-tetrakis(N,N’-di-p-methoxyphenylamine)-9,9’- spirobifluorene (Spiro-OMeTAD).[2,3] However, the spiro-OMeTAD possesses relatively low intrinsic conductivities, which need dopants like lithium bis(trifluoromethanesulfonyl)-imide (Li-TFSI), cobalt complexes, or 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquino-dimethane (F4-TCNQ) to enhance the material conductivity. Thus, for simplifying the device fabrication, reducing the cost, and solving some trade-off between photovoltaic properties and long-term stability and to circumvent the tight control of doping level and oxidation duration in the fabrication procedure, numerous dopant-free HTMs including small molecules and conjugated polymers have been vigorously explored. Side-chain polymers play important roles in the polymer and materials science with abundance structures, flexibility, and moisture protection ability. However, only a few side-chain polymers like the poly(9-vinylcarbazole) (PVK) derivatives were studied in the PSC fields with moderate efficiency of around 16%[4,5].
Herein, we designed new side-chain polymer structures by integrating a methoxytriphenylamineconjugated-thiophene moiety as the HTM repeating unit on a polystyrene side-chain. The polymer (Poly-HTM) was synthesized by the radical polymerization of its relevant monomer (Mono-HTM). The results showed that with similar device preparation and measurement conditions, the hole mobility of polymer was about 3 times of that of its small molecular monomer. PSCs based on Poly-HTM as the dopant-free HTM afford an impressive highest power conversion efficiency of 17.2%, which is much higher than those obtained from Mono-HTM (9.7%). Moreover, the devices based on the Poly-HTM presented significantly higher stability than the device based on its monomer. Hence, this study demonstrates that the side-chain polymer strategy is an effective approach to achieve high efficiency and highly stable PSCs using dopant-free HTMs.
1. Calió, L.; Kazim, S.; Grätzel, M.; Ahmad, S., Angew. Chem. Int. Ed. 2016, 55, 14522-14545.
2. Ding, I. K.; Tétreault, N.; Brillet, J.; Hardin, B. E.; Smith, E. H.; Rosenthal, S. J.; Sauvage, F.; Grätzel, M.; McGehee, M. D., Adv. Funct. Mater. 2009, 19, 2431-2436.
3. Jeon, N. J.; Lee, H. G.; Kim, Y. C.; Seo, J.; Noh, J. H.; Lee, J.; Seok, S. I., J. Am. Chem. Soc. 2014, 136, 7837-40.
4. Xu, Y.; Bu, T.; Li, M.; Qin, T.; Yin, C.; Wang, N.; Li, R.; Zhong, J.; Li, H.; Peng, Y.; Wang, J.; Xie, L.; Huang, W., ChemSusChem, 2017, 10, 2578–2584.
5.Yang, L., Yan, Y.; Cai, F.; Li, L.; Wang, T., Sol. Energy Mater. Sol. Cells, 2017, 163, 210–217.