Organic photovoltaic devices (OPVs) have been an important focus of solar power research due to their low-cost fabrication and minimal environmental impact. Recent advancements in synthetic chemistry have enabled the tuning of organic semiconductor functionality at the molecular level, thereby enhancing device specificity and increasing OPV efficiency to levels near those required for commercial success. Specifically, oligothiophenes have been widely studied and demonstrate viability as charge transport materials in OPVs. In previous work, a series of highly redox-active fused tetrathiafulvalene-oligothiopene compounds were synthesized and exhibited complex electrochemical behavior ideal for organic electronic materials. The present study reports the synthesis of a novel family of bridged oligothiophenes with terthiophene, quinquithiophene, and septithiophene backbones (1, 2, 3) for application in OPV devices. The structures were studied with UV-Vis spectroscopy, cyclic voltammetry, and tested for OPV performance. Compound 3 exhibited the most red-shifted absorbance (461 nm) and the lowest electrochemical HOMO-LUMO gap (2.13 eV), and OPVs fabricated with this analog demonstrated successful photocurrent generation. Meanwhile, devices fabricated with 1 achieved PCEs of nearly 10 times those made with 3, even with less-desirable absorbance properties and redox behavior. Variations in the optical and electrochemical HOMO-LUMO energy levels among the three structures can be attributed to differences in the length of their conjugated backbones. While the properties of 3 were promising, its poor solubility was an obstacle for device processing and fabrication. This likely resulted in unfavorable film morphology, which impeded charge transport and limited photocurrent generation. Future efforts are needed to optimize the morphology and processing of films in order to best exploit the characteristics of bridged oligothiophenes and other organic semiconductors for energy applications.

Examining bridged oligothiophenes as small-molecule semiconductors in organic photovoltaic devices

249th National Meeting (2015)

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