Correlation of work-function tunability and conformation of dipole moment of alcohol-soluble polyelectrolytes as a hole-transporting layer in polymer solar cells
Doo Kyung Moon a, Eui Jin Ko a, Yong Woon Han a, Doo Hun Kim a, Min Hee Choi a
a Nano & information materials lab., Konkuk University, 1 Hwayang-dong, Gwangjin-gu, Seoul, 143, Korea, Republic of
Asia-Pacific International Conference on Perovskite, Organic Photovoltaics and Optoelectronics
Proceedings of International Conference Asia-Pacific Hybrid and Organic Photovoltaics (AP-HOPV17)
Yokohama-shi, Japan, 2017 February 2nd - 4th
Organizers: Tsutomu Miyasaka and Iván Mora-Seró
Poster, Min Hee Choi, 011
Publication date: 7th November 2016

Three alcohol-soluble conjugated polyelectrolytes (CPEs) based on thiophene derivatives were designed and synthesized. The CPEs were successfully incorporated into polymer solar cells (PSCs) as pH-neutral hole-transporting layers (HTLs) via solution process at room temperature. The dipole moment and work function (WF) of CPE-coated indium tin oxide (ITO) were controlled by the dipole moment of the polymer, as determined by ultraviolet photoelectron spectroscopy. By coating ITO with CPEs having different π-linkers with stronger dipole moments, the secondary cut-off of the CPE-covered ITO electrodes in the range 16.70–16.60 eV successively shifted to lower binding energies due to an increase in the electrostatic repulsion in the molecules. The dipole moment of the polymer was found to be determined by the stereochemical properties of the molecular structure. The strength and direction of the dipole moment of the monomers and CPEs were calculated using density functional theory. The results confirmed that the strength and direction of the dipole moment changed according to the number of heteroatoms and configuration in the polymer backbone. P1 with a single thiophene unit had a deeper WF than that of polymers with atactic thiophene monomers. Bulk heterojunction (BHJ) solar cells fabricated with P1 as the HTL showed the strongest generated ion-induced dipoles(μID), and the short-circuit current (JSC) and hole mobility of the devices increased. The device fabricated with P1 exhibited the best power conversion efficiency (PCE) up to 7.3%, with a JSC of 15.6 mA·cm-2, open circuit voltage (VOC) of 0.697 V, and fill factor (FF) of 67.4%. The initial PCE of this device was maintained, accompanied by a 34% improvement in the stability compared with that of the PEDOT:PSS-based device. These results indicate the importance of judicious selection of a π-conjugated monomer with optimal electrical and structural properties to obtain effective CPEs for PSCs. Moreover, drawbacks of the instability of PEDOT:PSS may potentially be overcome by P1 based on its pH-neutrality and maximized dipole moment.

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