Synthesis and Electro-Optical Properties of Fluorene-Based Copolymer for Organic Photovoltaic Device Application

Abstract

The following soluble π-conjugated polymer was synthesized by palladium-catalyzed Suzuki polymerization: poly[(9,9-dioctylfluorene)-alt-(3-hexylthiophen-5-yl)-(4′-hexylthiazol-5′-yl)] poly(FO-HThHTha) derived from poly(9,9-dioctylfluorene) (PFO) and poly[(3-hexylthiophen-5-yl)-2,2′-(4′-hexylthiazol-5′-yl)] poly(HThHTa). The polymer, poly(FO-HThHTa) was soluble in common organic solvents and easily spin coated onto indium-tin oxide (ITO) coated glass substrates. Bulk heterojunction photovoltaic cells with an ITO/PEDOT/poly(FO-HThHTa):PC₇₀BM/TiO_x/Al configuration were fabricated and the photovoltaic cell using poly(FO-HThHTa):PC₇₀BM (1:4) showed the best photovoltaic performance compared with those using poly(FO-HThHTa):PC₇₀BM (1:2 and 1:6). The optimal bulk heterojunction photovoltaic cell exhibits a power conversion efficiency (PCE) of 0.39% (V_oc = 0.44 V, J_sc = 2.57, FF = 0.34) with poly(FO-HThHTa):PC₇₀BM (1:4) measured by using an AM 1.5 G irradiation (100 mW/cm²) on an Oriel Xenon solar simulator (Oriel 300 W).

Keywords:

• Bulk heterojunction
• low-band gap polymer
• organic photovoltaics

Acknowledgment

This work was supported by the New & Renewable Energy program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant (No. 20103020010050) funded by the Ministry of Knowledge Economy, Republic of Korea.

Notes

^a  M_w and PDI of the polymers were determined by GPC using polystyrene standard.

^b Determined by DSC at a heating rate of 10°C/min under N₂ atmosphere.

^c TGA was measured at temperature of 5% weight loss for the polymer.

^d Measured in the thin film onto the quartz substrate.

^e Band gap estimated from the onset wavelength of the optical absorption.

^a V_oc: open-circuit voltage.

^b J_sc: short-circuit current density.

^c FF: fill factor.

^d PCE: power conversion efficiency.