Bulk photoconductive gain in poly(phenylene vinylene) based diodes

We observe large, bulk photoconductive gain (> 100) in organic polymer diodes. Photoconductive gain was measured in diode structures employing the soluble polymer poly[2-methoxy,5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) as the active layer. The MEH-PPV layer was either undoped or doped by incorporation of a soluble C-60 derivative or PbSe quantum dots. The gain characteristics of the doped and undoped diodes are similar. We present the spectral response, transient response, and bias dependence of the gain. The photoconductive gain is due to the circulation of hole carriers through the diode in response to electrons trapped in the polymer layer. The bulk photoconductive gain reported here is distinct from the previous observations of gain in organic diodes that has been attributed to charge trapping near electrodes which increases the charge injection from that contact. The observed gain is consistent with estimates using previously established charge transport parameters of MEH-PPV. (c) 2007 American Institute of Physics.