In this work we demonstrate for the first time an efficient donor-acceptor effect in polythiophene-based photovoltaic materials prepared by post-polymerization electrochemical anodic halogenation of the polymers. Electrochemical, photoelectrochemical and Mott-Schottky measurements showed that the halogenated polythiophenes acted as efficient acceptor moieties and were able to facilitate the separation of primary photoexcitons generated in non-modified parent polymers. As a result, such halogenated photoelectrodes produced photocurrents in solution reaching 1.8 mA cm(-2) at 88 mW cm(-2) 405-nm monochromatic illumination and external quantum efficiency exceeding 8% at 480 nm. Working solid-state photovoltaic device prototypes were also fabricated that also showed a dramatic enhancement in photoefficiency upon halogenation. At the same time, it was also found that the materials prepared by electrochemical halogenation were susceptible to photoinduced cathodic dehalogenation, which gave rise to a relatively rapid decline in the photocurrent. Nevertheless, our results show that halogenated polythiophenes can act as efficient electron acceptors in donor-acceptor organic solar cells. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.005212jes] All rights reserved.