Semiconducting dye-functionalised poled polymers for photovoltaic applications

In order to build efficient single-layer polymer photovoltaic devices, the realization of an equivalent distributed p-n junction is proposed. Orientation in the initially centrosymmetric material is obtained by ordering polar molecules contained in a polymer matrix with a DC field. The molecular rectification effect induced in an oriented polymer film improves the efficiency of polymeric semiconducting devices like solar cells. The first experiments were conducted with a poly(methylmethacrylate) (PMMA) matrix containing azo-dye compounds (such as the Disperse Red 1, DR1) as the polar molecules, grafted onto the chains of the polymer backbones. Although this material allows for high orientation efficiencies, it is not adapted for photovoltaic applications, since DR1-MMA has very poor semiconducting properties due to the dielectric nature of PMMA. Organic semiconductors are uncommon and it is difficult to find an appropriate polymer system. We present preliminary results obtained for two intrinsically semiconducting and dye-functionalised polymer systems which may be found applicable in the fabrication of organic photovoltaic solar cells using the concept of polar molecular ordering. We use polymer systems with a covalent bond between the dye molecule and conjugated backbone to obtain a high dye content and to avoid phase separation problems. To achieve the largest stable molecular polar orientation possible and to optimise the electric field stored in an oriented structure, we studied the orientation parameters of samples, i.e., the orientation field, temperature, poling duration, and cooling conditions.