In this paper we present a study of thermally interdiffused poly (3-octylthiophene) (P3OT) - C60 photovoltaic devices of varying polymer and fullerene layer thicknesses. It is found that overall device performance and external quantum efficiencies (EQEs) in the wavelength region of peak absorption are enhanced for thinner polymer as well as thinner fullerene layers. We present a simple, first level model for EQE curves that accounts for the effects of absorption throughout the film. The model considers 3 distinct regions; pure donor, interdiffused donor and acceptor, and pure acceptor. It is found that this model reproduces quite effectively the experimentally observed shapes and relative magnitudes for varying donor thickness when compared with a similar experimental study. It also reproduces the shapes of the EQE curves but is not as effective in reproducing the relative magnitudes for varying fullerene thickness devices.
The interface between polymer and fullerene in organic photovoltaic devices is improved by thermally induced interdiffusion. Starting from a bilayer of 2-methoxy-5-(2’-ethylhexyloxy)-1,4-phenylenevinylene copolymer (MEH-PPV) and the Buckminsterfullerene (C60) devices are heated in the vicinity of the glass transition temperature creating a gradient bulk-heterojunction. Interdiffused devices show photoluminescence quenching with concomitant improvements in photocurrents. Variation of the polymer layer thickness shows an increase in photocurrents with decreasing layer thickness within the examined thickness regime as transport of the separated charges out of the device is improved. The interdiffusion was observed in situ by monitoring the photocurrents during the heating step. Cross-sectional transmission electron microscopy reveals C60 clusters of up to 30 nm in diameter in the interdiffused devices. The clustering of the fullerene molecules puts a significant constraint on the interdiffusion process.