Highly Efficient Dopant-Free Cyano-Substituted Spiro-Type Hole-Transporting Materials for Perovskite Solar Cells
Nobuko Onozawa-Komatsuzaki a, Daisuke Tsuchiya b, Shinichi Inoue b, Atsushi Kogo a, Toshiya Ueno b, Takurou Murakami N. a
a National Institute of Advanced Industrial Science and Technology (AIST)
b Nippon Fine Chemical Co., Ltd.
Poster, Nobuko Onozawa-Komatsuzaki, 066
Publication date: 21st November 2022

The hole transporting material (HTM) is an important component of perovskite solar cells (PSCs) because it plays a crucial role in achieving high performance. The most frequently used HTM is 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamino)-9,9'-spirobifluorene (Spiro-OMeTAD). Spiro-OMeTAD requires dopants such as Li(TFSI) to realize high power-conversion efficiencies (PCEs). However, these dopants cause severe instability issues in PSCs. The Spiro-OMeTAD layer also tends to undergo severe morphological deformation at high temperature that produces large voids, which further reduce the cell performance. These drawbacks must be overcome for the commercialization of PSCs. Therefore, it is strongly desirable to develop high-performance dopant-free HTMs to replace Spiro-OMeTAD. Here, we synthesize two cyano-substituted spirobifluorene-based HTMs, SF27 and SF48, for this purpose. These compounds are derived from Spiro-N [1], which is a Spiro-OMeTAD analogue obtained by replacement of the p-methoxy substituents with strong electro-donating p-N,N-dimethylamino groups. The influence of cyano substituents on the optoelectronic properties, PCEs, and charge-transport behavior in PSCs are investigated. These compounds have a high occupied molecular orbital (HOMO) energy level that is well matched to the valence band of perovskite layers. Mesoporous PSCs were fabricated using these new HTMs without dopants under relatively low HTM concentration conditions and the cell performance was compared to devices with Spiro-OMeTAD and Spiro-N. The PSC with non-doped SF48 exhibited a high PCE of 18.7%, which is comparable to the reference PSCs with doped Spiro-OMeTAD (18.6%). In addition, the thermal stability of SF48 at 85 °C in ambient air was superior to Spiro-OMeTAD, both with and without dopants.[2] Therefore, the SF48 spirobifluorene-based compound is determined to be quite effective as a high-performance dopant-free HTM for PSCs.

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