Photoelectrochemical cells have been fabricated from electrochemically deposited polyterthiophene films. The thickness of the photoactive layer was systematically varied by altering the charge density of the electrodeposited film. The morphology, optical absorption and photovoltaic performance of these devices have been characterised as a function of film thickness in order to gain a better understanding of the structure-function relationships in photoelectrochemical cells. Two distinct growth modes are observed. Initially a compact two-dimensional 'precursor' polymer film is formed up to a critical thickness of several hundred nanometres whereupon the growth of a 'bulk' three-dimensional film structure occurs. The photovoltaic efficiency of these cells appears to be governed by the 'precursor' film, which is characterised by a dense, uniform morphology of polymer of increasing conjugation length. On the other hand, the 'bulk' overlayer, which manifests as a nodular and highly cracked surface layer, appears to be primarily responsible for limiting device efficiency. (c) 2007 Elsevier B.V. All rights reserved.