Over the past few years, tremendous research effort has been made on all-solution processed bulk-heterojunction polymer solar cells (PSCs) in order to realize low-cost, lightweight, largearea and flexible photovoltaic devices. 1 To achieve high efficiency of PSCs, the most critical challenge at the molecular level is to develop the p-type conjugated polymers that possess (1) sufficient solubility to guarantee solution processability and miscibility with an n-type material, (2) low band gap (LBG) for strong and broad absorption spectrum to capture more solar photons and (3) high hole mobility for efficient charge transport. The general approach to produce a LBG polymer is to incorporate electron-rich donor and electrondeficient acceptor segments along the conjugated polymer backbone. Based on these polymers, researchers have made a breakthrough in fabricating PSC devices with PCEs over 5%. 2 Planarization of polyaromatic system facilitates π-electron delocalization and elongates effective conjugation length, providing another effective way to reduce the band gap. 3 Moreover, coplanar geometries and rigid structures can suppress the rotational disorder around interannular single bonds and lower the reorganization energy, which in turn enhances the intrinsic charge mobility. 4 Tricyclic 2, 7-carbazole 5 unit is an ideal electron-rich building block to construct donorÀacceptor polymers because its derivatives exhibit deeplying HOMO energy levels and good hole-transporting properties which are crucial prerequisites to achieve high open-circuit voltages (V oc ) and short circuit currents (J sc ), respectively. Poly-(2,7-carbazole-alt-dithienylbenzothiadiazole) (PCDTBT) has been shown to act as a superior p-type photoactive material for the application in PSCs (Scheme 1). 5 Inspired by the skeletons of the PCDTBT polymers, for the first time, we have successfully utilized a facile Friedel-Craft cyclization to develop a novel carbazole-based coplanar π-conjugated system, carbazole-dicyclopentathiophene (CDCT), where the 3-positon of two outer thiophenes are covalently connected with the 3,6-position of central carbazole cores by a sp 3 -hybridized carbon bridge (Scheme 1). 6 By copolymerizing this heptacyclic structure with electron-deficient benzothiadizole unit, an alternating poly(carbazole-dicyclopentathiophene-alt-benzothiadiazole) (PCDCTBT) was synthesized. 6 The two cyclopentadiene rings embedded in the CDCT structure allows for introducing four 4-(2-ethylhexoxy)phenyl groups to guarantee solubility and
We have developed a new germanium-bridged heptacyclic arene, dithienogermolocarbazole (DTGC), in which two outer thiophene subunits are covalently fastened to the central 2,7-carbazole core by two dibutylgermanium bridges. The germole moieties embedded in the DTGC structure were successfully constructed by one-pot nucleophilic cyclization in a high yield of 88%. Because of the relatively lower polarity of carbon−germanium bonds, the DTGC unit is chemically stable under basic conditions, rendering its more versatile functionalization. Comparison of germanium-bridged DTGC with the carbon-bridged DTCC (dithienocyclopentacarbazole) and silicon-bridged DTSC (dithienosilolocarbazole) analogues reveals that the HOMO energy level of DTGC lies between those of DTCC and DTSC and so does the LUMO energy level of DTGC. Density functional theory (DFT) calculations suggest that DTSC and DTGC have more bent structures than DTCC, which plays an important role in determining their frontier orbital energies. The structural disparity could be amplified in their corresponding polymers. The DTGC unit was copolymerized with four different comonomers, including benzothiadiazole (BT), dithienylbenzothiadiazole (DTBT), difluorobenzothiadiazole (FBT), and dithienyldifluorobenzothiadiazole (DTFBT) to yield a series of new alternating donor−acceptor copolymers, poly(dithienogermolo-carbazole-alt-benzothiadiazole) (PDTGCBT), poly-(dithienogermolocarbazole-alt-dithienylbenzothiadiazole) (PDTGCDTBT), poly(dithienogermolocarbazole-alt-difluorobenzothiadiazole) (PDTGCFBT), and poly(dithienogermolocarbazole-alt-dithienyldifluorobenzothiadiazole) (PDTGCDTFBT). Because of the two additional thiophene rings in the repeating units on the backbone to facilitate π-electron delocalization, PDTGCFDTBT showed a lower optical band gap than PDTGCFBT. Furthermore, PDTGCDTFBT also showed the lowerlying LUMO and HOMO energy levels than PDTGCDTBT as a result of the electron-withdrawing fluorine atoms. Consequently, the bulk heterojunction solar cell incorporating PDTGCDTFBT delivered the highest performance with V oc of 0.84 V, J sc of 9.87 mA/cm 2 , FF of 48.8%, and PCE of 4.05%. By adding 3 vol % 1-chloronaphthalene to tailor the morphology, the solar cell using PDTGCDTFBT with higher molecular weight exhibited the improved efficiency of 4.50% with a V oc of 0.84 V, a J sc of 11.19 mA/cm 2 , and an FF of 47.7%.
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