Organic-inorganic or fully inorganic metal halide perovskite solar cells (PSCs) have widely received considerable interest in recent years as one of the most promising candidates for solar to energy conversion devices.[1][2] Their power conversion efficiency (PCE) has lately approached 24 %.[3] However, to date, most of the efficient PSCs still rely on highly expensive hole-transporting materials (HTMs), such as spiro-OMeTAD and PTAA, which require hygroscopic dopants (e.g. lithium and cobalt salts) to realize high hole mobility. Nevertheless, those dopants negatively affect PSCs since they can easily decompose the perovskite crystalline structure, resulting in a dramatic decrease of PCE over time. To address this issue, we have designed and synthesized novel dopant-free HTMs based on dithioketopyrrolopyrrole (DPP) polymers via a low-cost synthetic approach. The excellent hole mobilities and favorable energy levels of DPP-based polymers, typically usually used in the context of organic field-effect transistors, make them potential candidates as dopant-free HTMs for PSCs.[4] By employing a one-step thionation reaction, the C=O bonds are successfully replaced by C=S bonds, and the resulting DTPP polymers show not only enhanced hole mobility (from 0.82 to 1.20 cm2 V-1 S-1), but also increased HOMO energy level (from -5.45 to -5.23 eV vs. vacuum level), which is well compatible with the perovskite valence band for efficient hole injection.[5] Furthermore, synthetic costs analysis reveals that our HTMs are 3 times cheaper than spiro-OMeTAD and 23 times cheaper than PTAA. Time-resolved absorption and photoluminescence characterizations demonstrate that swift hole injection and retarded charge recombination dynamics occur at the interface of perovskite and these new HTMs. In addition, these materials show absorption onset at around 1000 nm, which could effectively enhance the overall light harvesting for higher photocurrent generation. When PSCs are fabricated in ambient conditions (HR~50%), a high open circuit voltage (~1.1 V) is achieved, and the overall PCE reaches nearly 10 %, while that of the controlled device with spiro-OMeTAD is 13 %. By monitoring the decrease in PCE as a function of storage time in ambient conditions, the cells employing the new HTMs exhibit quite stable performance with less than 5 % decreased PCE in portion after one week. On the other hand, the devices based on spiro-OMeTAD HTM show fast degradation, with nearly five-fold PCE decrease after only 2 days of storage in air. We attribute the enhanced stability of the cells with new HTMs to the lack of dopants and to the surface passivation by sulfur binding to perovskite, which effectively protects its crystalline structure and improves the charge transfer as well. We believe that, after fine-tuning the performance of PSCs based on the proposed new dopant-free HTMs, these materials could advance the state-of-the-art on low-cost HTMs for efficient and stable PSCs towards their commercialization.

References

[1] D. Yang, R. Yang, K. Wang, C. Wu, X. Zhu, J. Feng, X. Ren, G. Fang, S. Priya, S. Liu, Nat. Commun. 2018, 9, 3239.

[2] M. Liu, M. Endo, A. Shimazaki, A. Wakamiya, Y. Tachibana, ACS Appl. Energy Mater. 2018, 1, 3722.

[3] “NREL Best Research-Cell Efficiencies”, 2018

https://www.nrel.gov/pv/assets/pdfs/pv-efficiency-chart.20181221.pdf

[4] J. Yao, C. Yu, Z. Liu, H. Luo, Y. Yang, G. Zhang, D. Zhang, J. Am. Chem. Soc. 2016, 138, 173.

[5] H. Zhang, K. Yang, K. Zhang, Z. Zhang, Q. Sun, W. Yang, Polym. Chem. 2018, 9, 1807.