Low band gap polymers for solar cells: The influence of chemical structure on electronic structure, interfacial properties and self-organization of thin films

Abstract

Intrinsically semiconductive polymers are a promising class of materials for the application in various electronic devices. Their electrical/electronic and morphological properties in the thin films can be fine-tuned by the methods of organic synthesis. Hence, in this work, the low band gap thiophene-based copolymers with related chemical structure are selected into two pairs (according to their electron accepting subunit): PCPDTBT & PCPDTTBTT and PCPDTBBT & PCPDTTBBTT, and studied as promising donor materials for photovoltaic devices. The main focus of the study was to provide an insight into the relationship between the chemical structure of studied polymers with their electronic structure, energy level alignment (interaction) at different interfaces, and self-organization properties in the thin films. The thin films of polymers are firstly prepared on the plethora of different substrates (model electrodes) and their electronic structure and interaction/energetics at interfaces were probed by means of UV-vis-NiR , X-ray and ultraviolet photoemission spectroscopy. It was found that the presence of different electron accepting moiety in the monomer unit of the polymer (e.g. PCPDTBT vs. PCPDTBBT) has a greater influence on the electronic structure of polymer than the presence of additional (hexyl)thiophene rings (e.g. PCPDTBT vs. PCPDTTBTT). Also, it was found that polymers within pair show similar behaviour (energetics) at interfaces, but rather different between pairs. This observed differences further effect and determine the interaction with organic accepting materials in studied donor/acceptor interfaces. Moreover, for the case of the thin film of PCPDTBT on ITO, where the hole injection barrier of 1.0 eV was found, the p-doping of the PCPDTBT thin film by F4-TCNQ molecule was performed, and collected results showed an improved (much lower) hole injection barrier value, thus favourable energy level alignment for the application in solar cells was obtained. Lastly, a series of near edge X-ray absorption fine structure spectroscopy (NEXAFS) measurements were done on the pristine, annealed and blend (with PCBM) thin films of PCPDTTBTT and PCPDTTBBTT polymers, and compared with the same for PCPDTBT and PCPDTBBT polymers. As a main conclusion, it was inferred that the presence of the side chains may have a strong effect on the self-organization ability of studied polymers in thin films.