Polymer solar cells (PSCs) have attracted considerable attention because of their unique advantages of low cost, light weight, and potential use in flexible devices.1,2 Based on the concept of a bulk heterojunction (BHJ) structure, 3PSCs made by blending poly(3-hexylthiophene) (P3HT) as a p-type material and [6,6]-phenyl-C61-butyric acid methyl ester (PC 61 BM) as an n-type material have been most intensively investigated and have shown power conversion efficiencies (PCEs) up to 5-6%. 4,5PCE of photovoltaic devices is proportional to the shortcircuit current density (J sc ), the open-circuit voltage (V oc ), and the fill factor (FF). The J sc 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 V oc 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. 6A 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.7-9 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%. 10Alternating copolymers consisting of bis(3-alkoxyphenyl)-quinoxaline and either dialkoxybenzene (LBPP) or fluorine (APFO) with a thiophene spacer showed PCEs up to 2.9% 11Another 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.12,13 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%.14 Some other types of quinoxaline-based copolymers with much lower PCEs were also reported. 15,16Poly(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. 17D-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 1 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) ...

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Recently, research has rapidly progressed on the development of bulk heterojunction polymer solar cells that use low band-gap polymers (p-type semiconducting polymers or electron donors) blended with (6,6)-phenyl-butyric acid methyl ester (PCBM, a n-type organic molecule or an electron acceptor) in the active layers, and their power conversion efficiency (PCE) has been increased over 8%.1 One approach to further improve the performance of polymer solar cells is through the use of new conjugated polymers with ideal properties such as low band-gap, high charge mobility, and appropriate energy levels of the highest/lowest molecular orbital (HOMO/LUMO).Low band-gap polymers have been developed which allow polymer solar cells to harvest more sunlight. Low band-gap polymers for use in solar cells have been synthesized from many different types of electron-rich and electron-poor monomers.2 One strategy for obtaining low bandgap polymers is to synthesize alternating copolymers by combining appropriate electron-rich monomers with electron-poor monomers (so-called "push-pull" architecture).The charge mobility of active layers in polymer solar cells has been increased using various methods such as thermal annealing 2b and the use of additives. The energy difference between the HOMO level of the p-type polymer and the LUMO level of PCBM is closely related to the open-circuit voltage (V oc ) of a solar cell, which increases as the HOMO/ LUMO difference increases. Thus, p-type conjugated polymers with low (or deep) HOMO levels are preferred. The energy levels of the HOMO/LUMO of p-type polymers can be tuned by combining appropriate electron-rich monomers with electron-poor monomers.Dialkoxynaphthalene (DN) is planar and can form intermolecular π-π stacks, leading to high charge mobility.3 Alternating polymers based on DN that contain benzodithiadiazole, thiazolothiazole, bithiophene, and (Z)-2,3-di(thiophen-2-yl)acrylonitrile units have been used to fabricate solar cells. The PCEs of solar cells fabricated using these p-type polymers were in the range of 0.06-2.90%. Diketopyrrolo [3,4-c]pyrrole (DPP) is a strong electron acceptor that can lower the HOMO level. Some DPP-containing conjugated polymers exhibited high hole mobilities, up to 2 × 10 −3 cm 2 / V·s, and PCEs up to 6.5%.4 Thieno [3,4-c]pyrrole-4,6-dione (TPD) is also a strong electron acceptor, and some TPDcontaining conjugated polymers were reported to have deep HOMO levels, leading to a high V oc and the highest reported PCE of 7.30%. 5A DPP-naphthalene polymer (PDPP-TNT) with a thiophene spacer between the DPP and naphthalene units has been reported to have a PCE of 4.7%.6 A DPP-containing polymer analog (PDPP-TDNT) is also interesting since alkoxy substituents can tune the HOMO/LUMO levels by increasing the electron-donating properties of the naphthalene moiety.