The optoelectronic and photophysical properties of four regioregular poly[3-(carboxyalkyl)thiophene-2,5-diyl] (P3CAT) with different carboxyalkyl chain lengths (propyl to hexyl) are reported. Each P3CAT is combined with [6,6]-phenyl-C61-butyric acid methyl ester to form the photoactive bulk heterojunction layer for organic photovoltaic devices. The extent of hydrogen bonding and polymer crystallinity in the films has been determined through infrared spectroscopy and X-ray diffraction. The mechanical properties of films are analyzed with nanoindentation, and the measurements suggest that P3CATs are suitable for use in flexible devices. Power conversion efficiencies of up to 2.6% and 1.6% are obtained for devices fabricated in air, and supported on glass and flexible poly(ethylene terephthalate) substrates, respectively. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. ; peer reviewed: yes ; NRC Pub: yes