Investigation of electrostatic self-assembly as a means to fabricate and interfacially modify polymer-based photovoltaic devices

Abstract

This work focuses on studying a water-based processing method for fabricating and modifying polymer-based photovoltaic devices based on donor–acceptor type complexes. Electrostatic self-assembly is a simple technique that involves immersion of a substrate into dilute aqueous solutions of positively and negatively charged polymers. Extremely thin layers of these polymers are adsorbed onto the surface and their structure can be tailored by manipulating deposition conditions such as the concentration, pH, and salt content. Poly(p-phenylene vinylene) (PPV) containing bilayers were examined as the donor block and water soluble, functionalized C60 molecules were investigated for the acceptor block. By varying the number of bilayers deposited in each individual block (i.e., the block thickness), we have been able to demonstrate a peak in device performance. By controlling the thickness of both the donor and acceptor blocks, we have determined the optimal device architecture for this system. Additionally, we have demonstrated that the interface between the donor and acceptor layers can be modified by inserting thin interfacial layers in between the blocks. The insertion of only two interfacial bilayers apparently combines the donor and acceptor functionalities such that the efficiency can be improved by a factor of 3. From this, it is apparent that one strength of electrostatic self-assembly lies in the modification of surfaces and interfaces, which is a key capability for further development of polymeric photovoltaics.