Synthesis of a low-bandgap fluorinated donor-acceptor copolymer and its optoelectronic application.

We demonstrate the synthesis of a new copolymer which is composed of dialkyl thienylated benzodithiophene and perfluororalkyl-carbonyl thienothiophene (DTBDT-TTFO) and the characterization of its optoelectronic properties. The introduction of thienyl groups enabled the extended delocalization of π electrons in the DTBDT-TTFO backbone and efficient intermolecular charge transport as proved by the fairly high field effect mobility of 0.02 cm(2)/(V s). The introduction of perfluororalkyl-carbonyl side chains resulted in a significant red-shift of DTBDT-TTFO in the absorption spectra and a decrease in the HOMO and LUMO levels. The resulting energy levels of DTBDT-TTFO were not satisfactory for solar cell applications, especially in terms of charge separation at the polymer/PCBM interfaces. Rather, the DTBDT-TTFO showed better energy level matching with the colloidal nanocrystals (NCs) of CdSe. A photodetector based on the bulkheterojunction of DTBDT-TTFO and CdSe NCs with coplanar device geometry resulted in a high photoconductive gain (responsivity higher than 1A/W under a low operating voltage of 1 V), possibly arising from electron trapping at CdSe NCs such that the hole can travel along the detector and its surrounding circuit. More importantly, the photodetector revealed a time constant of a few hundreds of microseconds, which means that the response speed of the photodetector is fast enough for lag-free imaging applications.