Organic photovoltaic devices: Concentration gradient of donor and acceptor materials in the molecular scale

In organic photovoltaic devices, a concentration gradient of electron donor and acceptor materials has been introduced in layers of molecular dimension via layer-by-layer electrostatic self-assembly. The photovoltaic devices based on graded-heterojunctions have been compared to single-, multiple- and bulk-heterojunction devices. Concentration gradient of the donor and the acceptor provided an extended exciton dissociation region and, at the same time, pathways for carrier transport without any energy barriers. The film deposition technique enabled us to vary the width over which concentrations of donor and acceptor materials have been graded. Importance of dielectric properties on device performance has also been stressed.