Organic sensitizers for application in photonic and photovoltaic devices

Field: Chemistry (Physical Sciences)

With the increasing demand for reliable and efficient devices with minimal environmental impact, novel organic materials gain extreme interest in the research community and industry. In this work we present synthetic strategies towards new organic compounds as promising materials for dye sensitized solar cells (DSCs) and low-cost integrated optics along with the investigation of these materials in devices. Despite the recent hype in the research community around DSCs, increasing efficiency of DSCs is still a challenge. In principle, one promising way to obtain DSCs with significantly enhanced efficiency lies in connecting an n-type photoelectrode (i.e. n-Dye/TiO2) with a p-type one (i.e. p-Dye/NiO) leading to a tandem cell composed by two serially connected photoactive electrodes, each contributing with its own photovoltage to the total photovoltage delivered by the cell. Applying such concept could theoretically lead to organic photovoltaic devices with up to 40% overall conversion yield. One of the main limitations in p-type systems, commonly based on NiO, arises from fast charge recombination between the photoinjected hole in NiO, and the reduced dye. Therefore it is crucially important to develop p-type chromophores which could produce a long-lived charge separated state and minimize back recombination. We were thus triggered to explore new organic structures for potentially efficient chromophores for p-type devices, by considering that the intramolecular charge transfer, at the basis of efficient charge separation in donor-acceptor dyes, is strongly dependent on the electron-withdrawing ability of the acceptor. Herein we present charge separators based on organic push-pull systems of tryphenylamine donors and branched electron acceptors (SK2-3-4) based either on Dalton (SK2) or benzothidaziole acceptor groups (SK3-4) which were synthesized and characterized by steady state spectroscopic, electrochemical and computational means. All the dyes exhibit strong charge transfer bands in the visible regions with ground and excited state energetics which are favourable to the sensitization of NiO electrodes. The computational investigation revealed a clear directionality of the lowest excited state exhibiting a marked charge transfer character, shifting the electron density to the acceptor branches, an electronic situation which is favourable to the hole injection in p-type semiconductors. When tested in p-type DSCs the SK series was found capable to sensitize NiO electrodes. The charge recombination kinetics, probed by considering the charge transfer resistance at the NiO/electrolyte interface at a comparable chemical capacitance, showed that the dyes behaved similarly and that the higher Voc observed with the SK4 dye is ostensibly due to a positive shift of the valence band edge, consistent with the shift in the anodic current threshold observed in dark conditions. The second part of this work is dedicated to synthesis and characterisation of metallo-organic materials for optoelectronic devices. Optical amplification plays crucial role in the transmission and manipulation of optical signals in modern telecomunications. Nowadays amplifiers, which rely on erbium ions in a glass matrix, suffer from difficulties in fabrication and the need of high pump power densities to produce gain. Here we show a newly synthesised series of organic fully halogenated optical amplifier materials. We will compare the ability of materials with different halogen atoms in complexes with transition metals to provide population of triplets which together with the lack of CH or OH oscillators in the molecule, can be potentially used as an efficient chromophore to sensitise the erbium ions in a long-lifetime erbium complex. Finally by doping Er(FTPIP)3 with newly designed Zn and Co complexes, we aim to find differences in the lifetime emission from erbium at the important telecommunication wavelength of 1.5 μm.