A study of photoconductivity in a planar photodiode based on the uniaxially aligned, semicrystalline polymer, poly(2,5-bis(3-alkylthiophene-2-yl)thieno[3,2-b]thiophene) (PBTTT) is presented. The charge carrier photogeneration yield in bulk conjugated polymers is usually low but it could possibly be enhanced by applying a sufficiently large electric field, particularly along the direction of the polymer backbone. PBTTT is, in principle, an ideal system to study any anisotropy in carrier photogeneration because directional alignment of fully extended polymer chains can be achieved over macroscopic length scales. However, we have found that electron trapping in this polymer results in high photoconductive gain obscuring measurements of intrinsic photogeneration. We study the mechanism of photoconductive gain using steady-state and transient photocurrent measurement. Constant background illumination and the deposition of a surface layer of an electron acceptor material have been found to be effective in reducing the gain effects.