Photoconductivity anisotropy study in uniaxially aligned polymer based planar photodiodes

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.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Photoconductivity anisotropy study with field orientation along the polymer chain and perpendicular to it. ► Highly oriented PBTTT nanoribbon was employed in a lateral geometry photodiode. ► Electron trapping give rise to large photoconductive gain. ► Constant background illumination and deposition of fullerene reduce the photoconductive gain.