Conductive polymer brushes: New electro- and photo- active molecular architectures
Final Report Abstract
IPF: To graft conductive polymers Kinds of "grafting-trough" methods that is a capture of polymerized in bulk solution chains by the properly functionalized surfaces were treated. To this end the following polymerization techniques were checked: electro-polymerization of symmetrical tris-thiophene condensation between Schiff-bases and aldehydes, Ni-catalyzed Kumada polycondensation, Pd-catalyzed Suzuki-polycondensation. Surfaces were modified by respective functional groups reactive toward growing polymer chains. Besides the electrochemical grafting (see below), all others approaches fail to produce CP brushes with reasonable grafting densities. Even if an appropriate anchoring group, able to couple with the monomer, is immobilized on the surface, the process leads to the mixture of grafted chains and a large amount of unattached polymers. Polymerization products formed faster in solutions, precipitate onto the surface and hinder the growth from the surface. Electrochemical grafting of (2,2':5',2"-3,3"-dioctyl-terthiophene, DOTT) onto ITO-glass modified with self-assembled monolayer (SAM) with aligned oxidative potentials of the monomer and the SAM, was successful. Photovoltaic properties of these grafted polythiophene films were tested by Prof. K. Leo group. However, most promising results we obtained in the development of selective surface-initiated polycondensation based on Kumada coupling reaction. In particular, we developed a new method to polymerize 2-bromo-5-chloromagnesio-3-alkylthiophene selectively (without polymerization in bulk solution above the substrate) from photo-cross-linked poly(-4-bromostyrene) films or brushes of poly(-4-iodostyrene) modified with Ni(PPh3)4. The process leads to regioregular head-to-tail P3HT covalently grafted to polystyrene-based anchoring layers. Unfortunately, the present performance of the reaction does not allow to realize the polymerization from ultra-thin initiating layers, such as SAMs with an appropriate chemistry. The grafting process is quite demanding to any impurities that deactivate the initiator at the surface. This is especially critical in the case of ultra-thin initiating layers. This circumstance, namely that the polymerization could be performed only from quite thick and insulating anchoring layer, is of course, undesired for photovoltaic devices. That is, most probably, why the obtained samples showed quite poor PV performance. We, however, optimistic to believe that surface initiating polycondensation process could be further optimized to become suitable for polymerizations from short anchors that directly connected to electrically conductive surfaces. This issue will be a subject of future projects. In addition a facile method of the preparation of mono- and hexa-functional bipyridyl-Niinitiators that mediate catalyst-transfer polycondensation into 6-arms P3HT stars have been developed. An important advantage of the Ni(bipy)-based catalytic system is an of easiness the preparation of a number of Ar-Ni(bipy)Br initiators from readily available aryl halides and possibility to define in a such way the place from which the polycondenstaion to be started. We believe that the developed method would be suitable for the synthesis of diblock copolymers consisting of only conjugated blocks, as well as for the preparation other architectures of P3HT-containing conductive polymers. Since, the requested budget for IPF group was reduce factor of two, synthesis of "hairy P3HT nanoparticles" was not developed, as planned. As a result, one paper was published and two manuscripts are in preparation. IAPP: Research in the recent years confirmed the assumption that controlling the nanomorphology of the donor-acceptor blend is a key issue in organic photovoltaics (OPV). Polymer brushes are an interesting approach for controlling the morphology at the nanoscale. However, synthesising conductive polymer brushes turned out not to be straight forward and for much of the project no semiconducting polymers grafted to a conducting substrate were available. During that time the basic properties of the polymers that were planned to be grafted were tested in solar cells. The active polymers were all based on poly-thiophenes, the currently used standard material class in polymer-based OPV. The materials and devices were characterised with photoluminescence and corresponding quenching experiments, current voltage measurement and the determination of the external quantum efficiency. A promising candidate was PDOTT, a variation of the commonly used P3HT in which every second hexyl side-chain was missing. The material parameters looked promising, but the missing hexyl side-chains had a profound effect on the photovoltaic properties and the results were disappointing. As literature shows, other groups are currently working with the same material with comparable results, which was published at the time of our experiments. Towards the end of the project CPs that were grafted to conducting surfaces were available. However, the desired effects, i.e. the beneficial self-ordering in the donor-acceptor structures, were not observed in the results. The devices showed poor photovoltaic response with the best values at 0.25% (not corrected for spectral mismatch). The grafted polymers contribute to the photocurrent, i.e. they are photovoltaic active. However, from current-voltage characteristics is it clear that charge transport is severely inhibited and the devices are subject to rapid degradation. Problems in the charge transport most likely originate from the fact that the CP were attached as side chains to a non-conducting brush in case of P4VP-PS-g-P3HT or having the undesired morphology in grafted PDOTT:PCBM blends as already observed for the nongrafted CP. Furthermore it appears that the exciton separation itself is not as efficient as in the non-grafted equivalent polymers The overall goal of the project was very ambitious, because for the currently used standard materials in polymer-based OPV it took years of optimising the production parameters until efficiencies of 5% were reached. Large progress was only obtained when the influence of the morphology was discovered and process parameters and strategies were developed to tune the phase separation at the nanoscale. Taking this into account the feasibility of semiconducting polymer brushes for organic photovoltaic was shown. It is now possible to synthesise CPBs grafted to conducting substrates in a sufficient amount such that a further investigation of the fundamental physical properties and an optimisation of the devices is possible in future.
Publications
- Conductive Polymer Brushes of Regioregular Head-to-Tail Poly(3-alkylthiophenes) via Catalyst-Transfer Surface-Initiated Polycondensation. J. Am. Chem. Soc. 2007,129, 6626-6632
Senkovskyy, V.; Khanduyeva, N.; Komber, H.; Oertel, U.; Stamm, M.; Kuckling, D.; Kiriy, A.
