Influence of Order and Disorder on the Photogeneration of Charge Carriers

The project (Influence of Order and Disorder on the Photogeneration of Charge Carriers) investi- gates the influence of the chemical side-chain structure of potentially semi-crystalline conjugated PPE-PPV copolymers on the resulting conformational order of the polymer within donor-acceptor bulk heterojunctions with fullerene derivatives (e.g. PCBM). The applicants already have more than 10 years of experience with some representative copolymers of this material system. The performance of some representatives of this material class is at least comparable with that of the most studied poly(3-hexylthiophene) (P3HT). While for the PPE-PPV interactions with fullerene derivatives may lead to improved polymer order, the order of P3HT becomes distorted in case of non-annealed blends with PCBM. The advantage of this material system is furthermore that the polymer conformation can be finely tuned between ordered, semi-crystalline versions of the AnE- PV copolymer and completely disordered, amorphous representatives solely by the choice - re- spectively substitution - of solubility inducing side-chains. In addition, there are no annealing processes required to improve the order in the polymer-rich phase of the blend with PCBM in order to obtain good optoelectronic properties, as required in case of P3HT. By mixing such co- polymers with different degrees of order and therefore solubility for PCBM, there is much space for variation of the optoelectronic properties of these materials. In the present project we will use a systematic synthetic approach for optimizing the system in this respect. Quantum chemical cal- culations will complement the synthetic efforts on the one hand by predictions concerning the secondary structure/conformation of the copolymers and on the other hand by determination of UV/VIS spectrum of the polymer and the polymer-fullerene complex, including an analysis of the electronic structure of the transitions by means of transition and charge difference densities. Fur- thermore, the material system shall be experimentally investigated in respect of the photogenera- tion of charge carriers depending on the extent of order within the polymer. It should be men- tioned that in general the polymer exits in two different phases of different conformation: 1) in polymer-rich crystallites and 2) in a rather amorphous polymer-fullerene blend phase at the inter- face to the fullerene-rich phase. Based on that, we aim to develop an improved understanding about the structure of the interface between polymer and fullerene. Finally it shall be revealed how the polymer conformation within a bulk heterojunction with fullerene derivatives influences on charge transport and recombination. We aim for a quantitative answer to this question based on ultrafast spectroscopy (charge generation and recombination), solar cell devices (charge gen- eration and recombination) as well as space-charge limited current (SCLC) diodes for the investi- gation of charge transport (charge carrier mobility).

The PhotogenOrder project is based on the collaborative research of three German groups, Harald Hoppe (Technische Universität Ilmenau/ Friedrich Schiller Universität Jena), Wichard J. D. Beenken (Technische Universität Ilmenau) as well as Frédéric Laquai (Johannes Gutenberg Universität Mainz), and an Austrian team, Daniel A. M. Egbe (Johannes Kepler Universität Linz). The goal of the project is to advance the understanding of order/disorder in the active layer and its impact on the performance of organic solar cells. The active layer is a key element in organic photovoltaic devices, commonly comprising a light absorbing organic semiconductor (e.g. conjugated polymer) and an electron-acceptor material (e.g. fullerene). The interaction within these materials as well as at their interfaces can affect the performance characteristics (e.g. charge generation and transport) drastically. Commonly, equipping conjugated backbones with linear side chains increases order and aggregation tendencies within donor polymers, while branched side chains foster disorder (amorphousness) and solubility. The right balance is essential. The objective for us at the JKU Linz (Austria) was to prepare light absorber materials, based on a well-established conjugated layout, and equipped with linear (octyloxy) and branched side chains (2-ethylhexyloxy) in various ratios and configurations. One of our main challenges was the synthesis of building blocks with specific mixed (linear/branched) side chain configurations. The first synthesis attempts yielded mixtures of regioisomers. However, through optimization and adaptation of a regiospecific synthesis protocol, both desired side chain regioisomers could be synthesized selectively and effectively from the same starting material. The specific building blocks as well as the mixed species were utilized to synthesize an assembly of polymers possessing identical conjugated backbone units but individual sequences of octyloxy and 2-ethylhexyloxy side chains in random to highly defined successions. Side chain configuration specific characteristics were revealed in NMR spectra as well as in photophysical investigations. UV-Vis absorption and photoluminescene measurement of the polymers were performed in solution as well as in thin solid film, and showed clear leads for structure-properties relationships. The differentiation of the spectral features (e.g. red shifts) due to the positioning of linear and branched side chain along the repeating unit is highly evident. The preparation of solar cells as well as further detailed optical and photovoltaic studies are to be conducted by our German partners.