Solution-processable polymer based photovoltaic devices with concentration graded bilayers made via composition control of a poly(3-hexylthiophene)/[6,6]- phenyl C₆₁-butyric acidmethyl ester

The bulk-heterojunction (BHJ) active layer, prepared by a spin-coating process, has been extensively investigated for its large donor-acceptor interfacial contact area, which can result in increased photo-current density. It is known that a concentration graded structure would enhance not only the charge generation but also charge transport, but, it is very difficult to exhibit such structure in BHJ system. The reason is that the morphology of the BHJ originates from self organization during the thermal annealing step. In this paper, polymer based photovoltaic (PV) devices with concentration graded bilayers have been successfully fabricated for the first time by solution process with various composition controlled poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C₆₁-butyric acidmethyl ester (PCBM) mixtures. The concentration profiles have been confirmed by Auger electron spectroscopy. The optimized bilayer devices with uniform morphology show enhanced photocurrent density and power conversion efficiency when compared to those exhibited by BHJ based PV devices.