Langmuir and Langmuir-Blodgett films of low-bandgap polymers
Résumé
Low‐bandgap conjugated polymers have provided a considerable increase in organic photovoltaic efficiencies, however, an understanding of class‐specific nanostructures, necessary to further improve device qualities, remains scarce. Their self‐assembly and associated electronic behaviors in Langmuir−Blodgett (LB) films are used here to provide relationships specific to each polymer, clarifying their structure−property characteristics. The behavior of two low‐bandgap polymers based on cyclopentadithiophene (PCPDTBT) and dithienosilole (Si‐PCPDTBT) units in the Langmuir trough were investigated and it is shown that it is possible to fabricate nanostructured films of low‐bandgap polymers on solid substrates with the LB deposition technique. The polymers were mixed with amphiphilic molecules at well‐defined concentrations to improve the formation of the LB films. The polymers were also deposited by drop‐casting and LB techniques onto interdigitated electrodes to evaluate their electrical properties, and the LB films were characterized for their optical and morphological properties. It was found that both LB and drop‐cast films of PCPDTBT showed higher electrical conductivities than those of Si‐PCPDTBT. Importantly, LB films resulted in higher electrical conductivities – by an order of magnitude − compared to their equivalent mixtures with stearic acid in drop‐cast films, although drop‐cast films without stearic acid gave higher conductivities. This fine‐tuning of the molecular architectures of the films is thus demonstrated to directly affect the physical properties and may lead to an improvement in device efficiencies in future applications.