Angular-Shaped 4,10-Dialkylanthradiselenophene and Its Donor–Acceptor Conjugated Polymers: Synthesis, Physical, Transistor, and Photovoltaic Properties

An angular-shaped and isomerically pure 4,10-di­(2-octyl)­dodecyl­anthradiselenophene (aADS) was successfully developed. The expedient synthesis to form the framework of aADS with two lateral side chains regioselectively at its 4,10-positions is via a base-induced propargyl–allenyl isomerization/6π-electrocyclization/aromatization protocol. This pentacyclic distannylated aADS unit was then copolymerized with dithienyl­diketopyrrolopyrrole (DPP) and dithienyl-5,6-difluoro-2,1,3-benzothiadiazole (DTFBT) acceptors with different alkyl side chains to afford four donor–acceptor copolymers: PaADSDPP, PaADSDTFBT-C₄, PaADSDTFBT-C₈, and PaADSDTFBT-C₈C₁₂. UV–vis spectroscopy and cyclic voltammetry revealed that PaADSDPP has the narrowest energy band gap, and PaADSDTFBT-C₈C₁₂ has larger band gap than PaADSDTFBT-C₄ and PaADSDTFBT-C₈. Two layer ONIOM (our own <i>n</i>-layered integrated molecular orbital and molecular mechanics) calculations were implemented to investigate the disparity in optical, electrochemical, and device properties between these polymers. Both experimental and theoretical data suggest that the aliphatic side chains play a significant role in determining the physical, transistor, and photovoltaic properties of the polymers. PaADSDTFBT-C₄ and PaADSDTFBT-C₈ exhibited organic-field-effect-transistor hole mobilities of 2.7 × 10^–2 and 1.0 × 10^–2 cm² V^–1 s^–1, greatly outperforming that of PaADSDTFBT-C₈C₁₂ with a mobility of 5.4 × 10^–6 cm² V^–1 s^–1. Polymer solar cells were fabricated on the basis of ITO/PEDOT:PSS/polymer:PC₇₁BM/Ca/Al configuration. The efficiency decreased as the increase of bulkiness of the aliphatic side chains installed on DTFBT units (4.4% for PaADSDTFBT-C₄, 3.5% for PaADSDTFBT-C₈, 0.3% for PaADSDTFBT-C₈C₁₂). Atomic force microscopy images reveal that the degree of aggregation for the polymer:fullerene blends is influenced significantly by the bulkiness of aliphatic side chain installed on DTFBT. Noticeable aggregation was found for the PaADSDTFBT-C₈C₁₂:PC₇₁BM blend. These results are in good agreement with the computational results elucidating that the intermolecular interactions between the polymers and PC₇₁BM are sterically hindered by the bulky 2-octyldodecyl groups. This work not only presents a promising selenophene-based aADS building block but also provides insights into the side-chain engineering for donor–acceptor conjugated copolymers

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

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-<i>alt</i>-benzothiadiazole) (PDTGCBT), poly­(dithienogermolocarbazole-<i>alt</i>-dithienylbenzothiadiazole) (PDTGCDTBT), poly­(dithienogermolocarbazole-<i>alt</i>-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 lower-lying 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 <i>V</i>_oc of 0.84 V, <i>J</i>_sc of 9.87 mA/cm², 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 <i>V</i>_oc of 0.84 V, a <i>J</i>_sc of 11.19 mA/cm², and an FF of 47.7%