Efficiency-limiting processes in photovoltaic blends of DPP-type copolymers and PC71BM

Julian Ochsmann^a, Frédéric Laquai^a, Kwang-Sup Lee^b, Pramod Kandoth Madathil^b, Deepak Chandran^b^,^c, Husna Anwar^d

^aMax Planck Institute for Polymer Research, Mainz, Ackermannweg, 10, Mainz, Germany

^bHannam University, 133, Ojeong-dong, Daejeon

^cDublin City University, Glasnevin, Dublin 9

^dMount Holyoke College, 50 College Street, South Hadley, MA 01075

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Roma, Italy, 2015 May 11th - 13th

Organizer: Filippo De Angelis

Poster, Julian Ochsmann, 118
Publication date: 5th February 2015

A promising approach to improve the performance of bulk‑heterojunction (BHJ) organic solar cells (OSC) is to use low‑bandgap polymers as electron donor materials, as they enhance the photon absorption of the photoactive layer in the near-infrared spectral range and thereby create larger photocurrents. In addition, low‑bandgap polymers are attractive for tandem solar cells, since their absorption is complementary to that of common mid‑bandgap polymers such as P3HT or PCDTBT, which allows for photocurrent matching of front and back cells.^[1] In this respect diketopyrrolopyrrole (DPP)-based donor-acceptor copolymers have recently attracted large interest.^[2] Here we investigate the photovoltaic performance and photophysics of two DPP‑based copolymers, namely PTDPP‑TT and PFDPP‑TT, blended with PC₇₁BM and applied in single junction BHJ solar cells. Optimized blends yield power conversion efficiencies of 2.3 % and 2.4 % for the PTDPP-TT:PC₇₁BM and PFDPP-TT:PC₇₁BM cells, respectively.^[3] The charge generation and carrier loss processes in devices were investigated by broadband transient absorption pump-probe spectroscopy (TA). Interestingly, we found that a major loss channel is the comparably short polymer singlet exciton lifetime of less than 20 ps, which prevents quantitative exciton quenching at the polymer:PCBM interface. Furthermore, at high excitation fluences, that is high carrier densities and open circuit conditions, a significant population of the polymer triplet state is formed on the sub-nanosecond timescale. We discuss whether the formation of polymer triplet states via non-geminate recombination is an issue under solar illumination conditions in systems in which the triplet state is energetically accessible.

[1] J. You, L. Dou, K. Yoshimura, T. Kato, K. Ohya, T. Moriarty, K. Emery, C.-C. Chen, J. Gao, G. Li, Y. Yang, Nature communications 2013, 4, 1446. [2] Y. N. Li, P. Sonar, L. Murphy, W. Hong, Energy & Environmental Science 2013, 6, 1684. [3] J.R. Ochsmann, D. Chandran, H. Anwar, P.K. Madathil, K.-S. Lee, and F. Laquai, Manuscript under submission

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