We study the effect of polymer thickness, hole mobility, and morphology on the device properties of polymer-based photovoltaics consisting of MEH-PPV as the optically active layer, ${\mathrm{TiO}}_2$ as the exciton dissociation surface, and ITO and Au electrodes. We demonstrate that the conversion efficiency in these polymer-based photovoltaics is primarily limited by the short exciton diffusion length combined with a low carrier mobility. For MEH-PPV devices with optimal device geometry, we achieve quantum efficiencies of 6% at the maximum absorption of the polymer, open circuit voltages of 1.1 V, current densities of 0.4 ${\mathrm{m}\mathrm{A}/\mathrm{c}\mathrm{m}}^2$ and rectification ratios greater than ${10}^5$ under 100 ${\mathrm{m}\mathrm{W}/\mathrm{c}\mathrm{m}}^2$ white light illumination. In addition, we achieve fill factors up to 42% at high light intensities and as high as 69% at low light intensities. We conclude by presenting a model that describes charge transport in ${\mathrm{s}\mathrm{o}\mathrm{l}\mathrm{i}\mathrm{d}-\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{p}\mathrm{o}\mathrm{l}\mathrm{y}\mathrm{m}\mathrm{e}\mathrm{r}/\mathrm{T}\mathrm{i}\mathrm{O}}_2$-based photovoltaics and suggest methods for improving energy conversion efficiencies in polymer-based photovoltaics.

• Received 5 June 2000

DOI:https://doi.org/10.1103/PhysRevB.64.125205