Synthesis of a Phenothiazine-Quinoxaline Polymer for Photovoltaic Applications

Abstract

Polymer solar cells (PSCs) have attracted considerable attention because of their unique advantages of low cost, light weight, and potential use in flexible devices. Based on the concept of a bulk heterojunction (BHJ) structure, PSCs made by blending poly(3-hexylthiophene) (P3HT) as a p-type material and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) as an n-type material have been most intensively investigated and have shown power conversion efficiencies (PCEs) up to 5-6%. PCE of photovoltaic devices is proportional to the shortcircuit current density (Jsc), the open-circuit voltage (Voc), and the fill factor (FF). The Jsc of a solar cell is strongly affected by the absorption of the active layer, and thus an ideal p-type polymer should have a broad and strong absorption spectrum, which requires the polymer to have a low band-gap. The Voc is tightly related to the energy difference between the highest occupied molecular orbital (HOMO) of the p-type polymer and the lowest unoccupied molecular orbital (LUMO) of PCBM. Thus, the p-type polymer should have a low band-gap with appropriate energy levels of HOMO and LUMO. A facile method to synthesize low band-gap polymers is to combine electron-rich (electron-donor, D) and electrondeficient (electron-acceptor, A) monomers, forming alternating D-A type polymers. Quinoxaline was reported to be a good acceptor that can be combined with appropriate donors. For example, a thiophene-bis(3-octyloxyphenyl)quinoxaline polymer (TQ1) exhibited PCEs up to 6.0%. Alternating copolymers consisting of bis(3-alkoxyphenyl)quinoxaline and either dialkoxybenzene (LBPP) or fluorine (APFO) with a thiophene spacer showed PCEs up to 2.9% Another alternating copolymers of quinoxaline and either thieno[3,2-b]thiophene or carbazole with a thiophene spacer showed PCEs of 2.27% and 1.8%, respectively. A copolymer of 2,3-diphenylquinoxaline-based combined with a ladder type oligo-p-phenylene with a thiophene spacer showed a PCE of 3.04%. Some other types of quinoxaline-based copolymers with much lower PCEs were also reported. Poly(10-hexyl-10H-hexylphenothiazine-3,7-diyl) is a very strong electron donor and has a high ionization potential and has thus been used as a hole injection material in polymer light-emitting diodes. D-A type polymers consisting of phenothiazine and quinoxaline are also expected to be p-type materials with low band-gaps, but such polymers have not yet been reported in the literature. Thus, we attempted to synthesize a copolymer (PPTQX) consisting of alternating phenothiazine (PT) and dithienylquinoxaline (QX) segments for photovoltaic applications. As shown in Scheme 1, diboronic ester (compound 1) was reacted with dibromide (compound 2) in the presence of a palladium catalyst via Suzuki coupling to give the corresponding polymer, PPTQX. The polymer was purified using a Soxhlet extraction. The H NMR of the isolated polymer is shown in Figure 1, where the two different sets of aromatic protons (a, b) and four aliphatic protons (d) in the QX segment are easily identified along with two aliphatic protons (c) in the PT segment, indicating that PPTQX has been successfully synthesized. The numberand weight-average molecular weights of PPTQX were 7,000 and 12,000, respectively. The relatively low molecular weight of the polymer may be a result of the low reactivity of the QX monomer since its reacting sites are sterically hindered due to the two large alkoxyphenyl substituents. Molecular weights of LBPP and APFO, which were synthesized from QX monomers similar to ours, were also relatively low (Mn 12,000-15,000). 11