A New Ladder-Type Germanium-Bridged Dithienocarbazole Arene and Its Donor–Acceptor Conjugated Copolymers: Synthesis, Molecular Properties, and Photovoltaic Applications

Abstract

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%.

Medium‐Bandgap Conjugated Polymer Donors for Organic Photovoltaics

“…They designed and synthesized a new germanium‐bridged dithienogermolocarbazole (DTGC) unit with low polarity of carbon‐germanium bonds with good chemical and thermal ability. When the DTGC unit was copolymerized with dithienylbenzothiadiazole (DTBT) and dithienyldifluorobenzothiadiazole (DTFBT), two new polymers P143 and P144 were synthesized, when PC71BM was used as acceptor, P143 and P144 showed PCE of 1.63% and 4.11%, respectively . P144 exhibited low‐lying HOMO and LUMO energy level than P143, which was due to the two electron‐withdrawing fluorine atoms.…”

Germanium‐Bridged 2‐Phenylbenzoheteroles as Luminophores Exhibiting Highly Efficient Solid‐State Fluorescence

“…The common structural feature that these molecules share is bilateral symmetry (Ar 1 =Ar 2 ). In sharp contrast, bilaterally unsymmetrical Ge‐bridged biaryls (Ar 1 ≠Ar 2 ) remain unexplored except for benzo[ b ]germafluorene, dithienogermolocarbazole, germa‐indacenodithiophene, and (benzofurano)(benzothieno)germole . As a lack of symmetry can significantly affect the properties of molecules and their aggregates, such as crystallinity, morphology, solubility, dipole moment, and electronic structure, the design of novel types of bilaterally unsymmetrical Ge‐bridged biaryls, and the disclosure of their structures and properties for the further advancement of Ge‐based functional materials is very intriguing.…”

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

“…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][9][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][12][13][14][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][17][18][19][20][21][22][23][24][25].…”