Photo-Induced Charge Transfer and Spectroscopy Across a Series of Pbttt Model Compounds

Organic-based excitonic photovoltaics are a promising low-cost renewable energy source. New solution-processable polymer and small molecule electron donor materials often utilize solubilizing alkyl tails. Herein we study the effects of alkyl tails on charge transfer between a fullerene and a model compound of the highly-conducting PBTTT polymer with varied alkyl tail lengths, ranging from one to thirteen carbons.

First, we photophysically characterize the model compounds. Solution-phase spectroscopy and electrochemistry demonstrate similar energetics across the compounds series.  Intermolecular aggregation in films leads to two distinct film absorption and photoluminescence features, which interestingly does not appear to follow tail length or odd-even trends. Second, we utilize the electrodeless time-resolved microwave conductivity technique to study photo-induced charge transfer and carrier dynamics. Specifically we create a donor-acceptor heterojunction with the soluble fullerene PCBM. Last, we create an electron acceptor “dilute regime” to study the tail-length effects on charge transfer. Our transient microwave absorption experiments show that the charge generation yield is intimately tied to tail length across the model compound series. Alkyl tails, which are often incorporated to enable solubility, can in fact act as a charge transfer barrier in bulk heterojunctions.