Difluorobenzothiadiazole based two-dimensional conjugated polymers with triphenylamine substituted moieties as pendants for bulk heterojunction solar cells

Headnote

Abstract. Three donor/acceptor (D/A)-type two-dimensional polythiophenes (PTs; PBTFA13, PBTFA12, PBTFA11) featuring difluorobenzothiadiazole (DFBT) derivatives as the conjugated (acceptor) units in the polymer backbone and tertbutyl-substituted triphenylamine (tTPA)-containing moieties as (donor) pendants have been synthesized and characterized. These PTs exhibited good thermal stabilities, broad absorption spectra, and narrow optical band gaps. The cutoff wavelength of the UV-Vis absorption band was red-shifted upon increasing the content of the DFBT units in the PTs. Bulk heterojunction solar cells having an active layer comprising blends of the PTs and fullerene derivatives [6,6] phenyl-C_61/71-butyric acid methyl ester (PC₆₁BM/PC₇₁BM) were fabricated; their photovoltaic performance was strongly dependent on the content of the DFBT derivative in the PT. Incorporating a suitable content of the DFBT derivative in the polymer backbone enhanced the solar absorption ability and conjugation length of the PTs. The photovoltaic properties of the PBTFA13-based solar cells were superior to those of the PBTFA11- and PBTFA12-based solar cells.

Keywords: polymer synthesis, difluorobenzothiadiazole, polythiophene derivative, and bulk heterojunction solar cells

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1. Introduction

Conjugated polymers with electron donor/acceptor (D/A) units in the polymer backbone have attracted much attention for their applications in optoelectronic devices, including polymer solar cells (PSCs), organic thin film transistors (OTFTs), and polymer light emitting displays (PLEDs) [1-7]. Major advances in the solar energy conversion efficiencies of PSCs have been accomplished by replacing the double-layer cell with a bulk-heterojunction (BHJ) based on a conjugated polymer/fullerene derivative blend as the photoactive layer [8-10]. The bipolar characteristics of D/A-type conjugated polymers, containing both electron- and hole-transporting moieties, can result in lower-energy band gaps and higher charge mobilities for the photoactive layers, thereby enhancing the PV efficiencies of PSCs [11-15]. Polythiophene (PT) derivatives bearing electron-donating and -withdrawing pendant groups, so-called two-dimensional (2-D) low band gap conjugated polymers, have been proposed by several groups for PSC applications [16-25]. The introduction of conjugated moieties as pendant units that absorb broadly in the UV and visible regions can, therefore, lead to the greater harvesting of solar light. Moreover, the conjugated pendant moieties can enhance the charge mobilities of the polymers.

The incorporation of electron-deficient moieties within the polymer backbone and the appending of electron-donating units would, ideally, promote charge balance and efficient conjugation...