Novel materials for bulk heterojunction thin film organic photovoltaic devices - research and application

Abstract

The majority of organic semiconductors developed are more easily oxidized (p-type) than reduced (n-type). However, n-type organic semiconductors are important for a range of optoelectronic applications including organic photovoltaic devices, light-emitting diodes and field effect transistors (FETs). In spite of this there has been significantly less development of n-type compounds. This project focused on developing novel n-type semiconductors for use in organic photovoltaic devices. To identify the organic acceptor candidates, a series of novel materials were synthesized and characterized. Amongst these materials, 2-[(7-{9,9-di-n-propyl-9H-fluoren-2-yl}benzo[c][1,2,5]thiadiazol-4-yl)methylene]malononitrile (K12) was identified as a lead candidate. K12 has the remarkable property in that it can be deposited from solution or by evaporation. K12 exhibits a tendency to order in the solid state at room temperature, and with mild heating the process can be accelerated. The extent of ordering in the films was followed by Polarized Optical Microscopy, Atomic Force Microscopy, and Wide Angle X-ray Diffraction. K12 was found to be a good electron acceptor candidate for bulk heterojunction organic solar cells and as an active channel in organic field effect transistors (OFETs). A 0.7% power conversion efficiency under AM1.5 light was achieved in a bulk heterojunction solar cell configuration with poly(3-n-hexylthiophene) as the donor. The thermal properties of K12 enable the film morphology to be controlled at easily accessible temperatures allowing the charge mobility to be tuned over two orders of magnitude. The electron mobility in the films was found to be independent of the initial processing conditions (solution or evaporation). The electron mobility measured in a FET configuration was of order 10-3 cm2/Vs for films prepared via either processing method whilst Photoinduced Charge Extraction in Linearly Increasing Voltage (PhotoCELIV) for annealed films gave a mobility of order 10-4 cm2/Vs. To further tune the properties of K12 several chemical modifications were made. To harvest more of the solar spectrum derivatives with electron donating groups such as thiophene (K12-Th) and diphenylamine (K12-Diph) were prepared to extend the conjugation length and form a Donor-Acceptor structure. Amongst the derivatives K12-Diph gave a greatly broadened absorption ranging between 200 nm and 800 nm. The dipole moment of K12 was found to play an important role in its morphology and hence K12b, which has a smaller dipole moment was prepared to investigate this effect further. It was found that the different dipole moments of K12 and K12b had dramatic effects on both the physical properties, reflected in their thermal properties and molecular packing. The morphological differences in films comprised of the each of the materials were observed to have a direct impact on the device performance in both OFET and OPV devices. The propensity of K12 to crystallize was found to give rise to better charge transport than K12b. Macromolecular structures K12-3 and K12-6 (with three and six K12 chromophores around a central benzene ring, respectively) were prepared. Although they had similar fundamental optoelectronic properties to K12 their physical properties were altered and although they could be solution processed did not form films that gave enhanced charge transport due to aggregation effects.