Role of oxygen within end group substituents on film morphology and charge carrier transport in thiophene/phenylene small-molecule semiconductors

In this study, the end group polarity of (5,5′)-biphenyl-(2,2’)-bithiophenes (PTTPs) was systematically varied from alkyl (1) to alkoxy (2) with one oxygen atom to glycol (3) with two oxygen atoms while the overall length of the end groups is kept constant. Thin films of the three compounds were sublimated at different substrate temperatures and their morphology, crystallinity and charge carrier transport in field-effect transistors was investigated to draw structure-property relationships for the PTTP derivatives. For all three compounds, the effective charge carrier mobility is improved with higher substrate temperatures at which films with higher crystallinity and larger grains are formed. The effective mobility decreases with higher polarity of the end groups from alkyl to alkoxy and glycol. The reliability factor of the alkyl (1) and alkoxy (2) substituted PTTPs decreases with higher substrate temperature, but at the same time this value is enhanced for the glycol substituted molecules (3). The transistors of 3 prepared at higher substrate temperatures also show a reduced threshold voltage and smaller hysteresis in the transfer characteristics. These insights are important for the understanding of the impact of oxygen incorporation into side chain/end group substituents of organic semiconductors and their implementation in organic electrochemical transistors, thermoelectrics and photovoltaics.