Photophysics of substituted polythiophenes

We have measured the photoluminescence quantum yield and photoluminescence lifetime for a series of polythiophenes, in solution, and the quantum yield for the same polymers in thin films. Depending on the different substituents, quantum yields ranged between 0.38 and 0.01 for polymers dissolved in chloroform; for spin-coated films, quantum yields as high as 0.24 were measured. We observe that increasing the bulkiness of the substituent increases the quantum yield in solution. For spin-coated films an increased ordering can either increase or decrease the quantum yield, depending on the separation of the conjugated backbones. Polythiophenes in which the electronic band gap is increased by steric hindrance show very low quantum yield, both for film and solution. This low quantum yield originates from a fast nonradiative decay.