Hole Carrier Transport in Ternary Blend Polymer Solar Cells
Kohshiroh MIDORI a, Tomohiro FUKUHARA a, Yasunari TAMAI a, Hideo OHKITA a
a Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Japan
Asia-Pacific International Conference on Perovskite, Organic Photovoltaics and Optoelectronics
Proceedings of International Conference on Perovskite and Organic Photovoltaics and Optoelectronics (IPEROP19)
Kyōto-shi, Japan, 2019 January 27th - 29th
Organizers: Hideo Ohkita, Atsushi Wakamiya and Mohammad Nazeeruddin
Poster, Kohshiroh MIDORI, 140
Publication date: 23rd October 2018

In most polymer solar cells, a conjugated polymer and a fullerene derivative have been used as an electron donor and an electron acceptor, respectively.  However, it is difficult to harvest the solar light over the wide wavelength range from visible to near-IR region because fullerene derivatives have negligible absorption there.  In recent years, ternary organic solar cells have attracted great attention to improve the short-circuit current density (JSC) effectively by providing complementary absorption bands.  Interestingly, some ternary blend solar cells have shown an improvement in fill factor (FF) as well as JSC.  In our previous report, we found that a ternary device consisted of a wide-bandgap polymer PDCBT, a low-bandgap polymer PTB7-Th, and a fullerene derivative PCBM exhibited a higher FF than that of their binary analogues.  Herein, we study charge transport properties in the ternary blend to discuss the improvement in FF.  Hole mobility was estimated by space-charge limited current (SCLC) to be 2.8 × 10−4 cm2 V−1 s−1 for PTB7-Th, 1.6 × 10−4 cm2 V−1 s−1 for PDCBT neat films, and 9.0 × 10−4 cm2 V−1 s−1 for ternary blend films.  In other words, hole transport property is improved in ternary blend films compared to that in neat films.  For the assignment of hole polaron, we measured transient absorption spectra for PTB7-Th/PCBM, PDCBT/PCBM, and PTB7-Th/PDCBT/PCBM blends, and found that hole polarons were located in PTB7-Th domains even though PDCBT was selectively excited.  This finding suggests that hole polarons are transferred from PDCBT to PTB7-Th domains because of higher HOMO level in PTB7-Th or that hole polarons are generated after exciton transfer from PDCBT to PTB7-Th because of large spectral overlap between PDCBT emission and PTB7-Th absorption.  We therefore conclude that the improvement in FF is due to improved hole transport property in PTB7-Th domains in ternary blends.

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