Project R-2079

Charge transfer in semiconducting polymer / CVD diamond heterostructures (Research)

This project is related to the fundamental study of solar energy conversion using novel self-assembled monolayers of organic molecules and their interfaces to inorganic surfaces. Specifically, the project involves two materials that receive significant attention these days: electro-active organic molecules and CVD diamond. By the formation of novel organic/inorganic systems, more specifically polymer/diamond heterostructures, this project envisages the fundamental study of charge transfer mechanisms between these materials, generating knowledge that is indispensable for future novel photovoltaic solar cells, molecular switches and other electroactive optical components. The study of heterojunctions formed by polymers and diamond is still in its early stage as evidenced by the scarce literature on this topic, mainly carried out by a small number of groups. The earliest reports deal with thin polymer layers on diamond, usually cast from solution [Lee99, Yos99], i.e. spin-coating, or thermally evaporated [Chu03]. The role of diamond, single crystals or thin microcrystalline films, was limited: as reservoir for hole injection into the polymer (e.g. MDMO-PPV/diamond [Yos99], -naphthyl-phenyldiamine/ diamond [Chu03]), or just as UV-protective window (C60-doped poly(3-dodecylthiophene/diamond) [Lee99]. By large due to the work of the Hamers group, demonstrating the superior stability of diamond as a material platform that can be functionalised [Yan02], the number of investigations in the chemical modification of diamond surfaces and particles is steadily increasing [Har04, Chr06, Krü08, Ouy08]. A novel material that can bring important progress in the area of electrochemical photovoltaic conversion cells is p-type B-doped nanocrystalline diamond (B-NCD), prepared by chemical vapour deposition (CVD) routes. Recently, based on a collaboration between UHasselt-IMO and the National University of Singapore, it was demonstrated that photovoltaic cells using organic dyes attached to diamond achieved a factor 2 to 3 higher photocurrent than in the case of indium tin oxide-dye systems [Zho08]. Semi-conductive polymers can be synthesised with different functionalities depending on functional groups, conduction mechanism, chain length, etc. The electronic properties are then determined by the corresponding molecular electronic states, i.e. HOMO and LUMO. The position of these levels and their alignment with respect to the conduction band minimum (CBM), valence band maximum (VBM) and Fermi level (Ef) of diamond, will determine whether an efficient and direct charge transfer will take place between polymer molecule and diamond [Stu06]. The main advantage of using diamond to study these phenomena is that the position of CBM and VBM with respect to the vacuum level can be tuned by terminating the diamond surface with hydrogen or oxygen. Due to the presence of a negative electron affinity (NEA) for the H-terminated surface, the ionisation potential can be varied between 4.2 eV for H-termination to 7.2 eV for O-termination [Ris06]. The detailed study of the proposed heterostructures using photoelectric and surface characterisation methods will allow an optimal matching of the HOMO and LUMO with the CBM or VBM respectively, achieving optimal charge transfer performance.

01 January 2010 - 31 December 2013