Abstract
Two alternating copolymers of dithienosilole (DTS) were designed and synthesized with small optical band gaps, flanked by thienyl units as electron-donor moieties and benzothiadiazole dicarboxylic imide (BTDI) as electron-acceptor moieties. The BTDI moieties were anchored to two different solubilizing side chains, namely 3,7-dimethyloctyl and n-octyl chains. An analysis of the effect of the electrochemical, optical, thermal, and structural characteristics of the resulting polymers along with their solubility and molecular weight is the subject of this paper. The Stille polymerization was used to synthesize PDTSDTBTDI-DMO and PDTSDTBTDI-8. The average molecular weight of PDTSDTBTDI-DMO and PDTSDTBTDI-8 is 14,600 and 5700 g mol−1, respectively. Both polymers have shown equivalent optical band gaps around 1.4 eV. The highest occupied molecular orbital (HOMO) levels of the polymers were comparable, around −5.2 eV. The lowest unoccupied molecular orbital (LUMO) values were −3.56 and −3.45 eV for PDTSDTBTDI-DMO and PDTSDTBTDI-8, respectively. At decomposition temperatures above 350 °C, both copolymers showed strong thermal stability. The studies of powder X-ray diffraction (XRD) have shown that they are amorphous in a solid-state.
Highlights
We synthesized two alternating copolymers comprising benzothiadiazole dicarboxylic imide (BTDI) with varying substituents flanked by two thienyl repeat units as electron-accepting moieties and DTS as electron donor moieties
The results indicated that the PDTSDTBTDI-8 had lower solubility since the DTS and BTDI units have n-octyl chains
The highest occupied molecular orbital (HOMO) values for both PDTSDTBTDI-DMO and PDTSDTBTDI-8 polymers are equivalent the former polymer’s lowest unoccupied molecular orbital (LUMO) energy level is lower than that of the latter. These findings show that adding different side chains onto BTDI units had an insignificant effect on the polymers’ degree of HOMO
Summary
Polymer solar cells (PSCs) using conjugated organic semiconductors have received enormous interest owing to their inherent benefits of low cost, light weight, manufacturing flexibility and large-scale devices through solution processing. The power conversion efficiencies (PCEs) of this type of system has been documented at more than 17%, which is a critical step towards industrial applications of OPVs. The fast production of new photoactive products, which entails the careful consideration of device optimization and interface constituents and the features of donors and acceptors (e.g., absorption, energy levels and band gaps), suggest a promising future for industrial applications of PSC systems. A PCE of 5.59% was achieved from blending PDTSDTTTz-3 with PC71 BM, while the PCE for PDTSDTTTz-4–PC71 BM improved to yield 5.88% with a considerable FF of 71.6% This was due to the more planar structure of PDTSDTTTz-4, which had a higher hole mobility than its PDTSDTTTz-3 analogue [28]. The results of the present study emphasize that copolymerization can be a novel approach to designing and synthesizing conjugated copolymers with lowor small-energy band gaps
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