Photovoltaic devices based on photo induced charge transfer in polythiophene: CN-PPV blends

Abstract

¨We have investigated the photovoltaic properties of polymer devices based on a blend of two polymers, (poly(3-(2’-methoxy-5’-octylphenyl) thiophene) (POMeOPT) and poly(2,5,2’,5’-tetrahexyloxy-7,8’-dicyanodi-p-phenylenevinylene) (CN-PPV), where photo induced charge transfer takes place at the interfaces formed by the interpenetrating network of the donor and the acceptor. The polymer blend films resulting from the spin coating were phase separated and the photo response of the devices depended on the morphology of the segregation. We have studied different blend ratios by photoluminescence quenching, scanning force microscopy and photovoltaic measurements. External quantum efficiencies of 4.5% at 500nm were achieved for the less segregated polymer blend. I Introduction The possibility in using the photovoltaic effect for energy conversion has been motivated the research on this field along the years. The focus was mostly in the use of inorganic materials. Recently, with the use of photo induced charge transfer across the internal or external donor-acceptor (D/A) heterojunctions, [1] the organic photovoltaic devices have been demonstrated to be highly efficient, attracting attention. [2-10] Organic materials in general and polymers in particular have been extensively studied due to the advantages these materials present when compared to inorganic semiconductors. Polymers are easily spin-coated from solution and do not require ultra-clean environment during any processing procedure. In addition, these materials can be chemically tuned so that their absorption and emission color covers the entire visible spectrum. [11] High quality polymers are being synthesized with high degree of reproducibility, which leads to a great development in stability, lifetimes and performance in all areas of application. The use of polymer blends as the active layer in optoelectronic devices is an alternative to change and improve the device characteristics and performance. LEDs in which the emission color varies as a function of the applied voltage were made by blending conjugated polymers that have different emission spectrum. [12] Efficient photodiodes have been fabricated from mixtures of electron-accepting materials and hole-accepting polymers. [2-10] The photo generated excitons are dissociated at the interfaces, which are dispersed in the bulk of the blend film. Charges are transported through the respective phase, driven by the internal electric field. The photovoltaic properties of CN-PPV/POMeOPT blends have been investigated in this work, where (poly(3(2’-methoxy-5’-octylphenyl) thiophene) (POMeOPT) [11,14] was the “donor” and poly(2,5,2’,5’-tetrahexyloxy7,8’-dicyano-di- p-phenylenevinylene) (CN-PPV), [5] was used as the electron-accepting material. The chemical structures of the polymers are presented in Fig. 1. The blend was investigated by comparing morphology, photoluminescence and action spectra of several compositions.