Publication date: 3rd December 2021
Perovskite solar cells (PSCs) efficiency was improved in the last decade from 3.8% to 25.5%. Although the achieved performance is impressive, the long-term operational stability of PSCs is still a distant goal.
Recently, it has been shown that using hole-transport materials (HTM) in the standard n-i-p device configuration can improve dramatically PSCs stability. For this purpose, the HTMs should provide decent charge transport characteristics without additional doping agents. Furthermore, HTMs must be chemically inert towards perovskite materials and possess low gas permeability to prevent leakage of the perovskite degradation products from the absorber layer. In this regard, conjugated polymers can be considered as promising HTMs, since they are intrinsic semiconductor materials with easily tunable physicochemical and optoelectronic properties. Many reports presented the design of HTMs based on conjugated polymers which provide high efficiency and improved stability of PSCs. However, the selection of the materials still looks purely empiric. In order to perform a rational design of polymer-based HTMs for improving the efficiency and stability of perovskite solar cells, some basic guidelines have to be defined.
In this work, we systematically explored a broad range of conjugated polymers as HTMs to reveal the impact of their molecular structure and properties on the PSCs performance and operational stability. The selected set of the polymers comprises materials with various backbone structures, different solubilizing side chains, and HOMO energy levels varied from -5.8 to -5.1 eV. We showed that PSCs with high efficiency of up to 18.9% can be fabricated using the best polymeric HTMs without additional doping. A systematic study of the device aging behavior within 1000 h under continuous light soaking was performed and we identified the series of polymers comprised of octyloxy-substituted benzoxadiazole and/or carbazole units enabling long-term operational stability of PSCs.
The obtained results allowed us to establish important correlations between molecular structure of polymers, and photovoltaic performance as well operational stability of devices. These relationships may guide further rational design of polymer-based HTMs for efficient and stable perovskite photovoltaics.
The investigation of P26 was funded by Russian President Science Foundation МК-1103.2020.3. General support was also provided by the Ministry of Science and Higher Education of the Russian Federation within the project No. AAAA-A19-119101590029-0. We gratefully thank Dr. Troshin P.A. for providing the equipment of FMEM laboratory and helpful discussion.
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