Concentration Gradient P3OT/PCBM Photovoltaic Devices Fabricated by Thermal Interdiffusion of Separately Spin-Cast Organic Layers

摘要

Polymeric photovoltaic devices offer the potential for inexpensive, large area, lightweight, flexible efficient renewable power sources. The highest efficiencies achieved so far are ～5% under AM1.5 simulated solar spectrum.1 Because the exciton diffusion distance is limited to <10 nm,2 it is essential to control the composition of the electron donor and acceptor components on the nanometer length scale. In standard blend, bulk heterojunction devices, one relies on phase separation of the two components to accomplish this in an averaged manner throughout the film. Blend devices provide good charge separation but charge transport is potentially limited due to discontinuous pathways. In contrast, bilayer devices provide optimized charge transport while the charge transfer is severely compromised. We have been developing an approach wherein a concentration gradient of the two components is achieved to maximize the concentration of the majority carrier component in the vicinity of each respective electrode. Thishas been accomplished through sublimation of a C_(60) layer on top a spin-cast polymer layer followed by thermally-induced interdiffusion of the two films. Here, we report on a series of experiments performed to study organic photovoltaic devices consisting of concentration gradients of poly (3-octylthiophene) (P3OT) and [6,6]-phenyl-C_(61) butyric acid methyl ester (PCBM). The devices were fabricated by thermally- induced interdiffusion of consecutively spin-cast layers of P3OT and PCBM from solvents of chloroform and pyridine, respectively.