Influence of Order and Disorder on the Photogeneration of Charge Carriers

• Applicants: Dr. Wichard J.D. Beenken; Privatdozent Dr. Harald Hoppe; Dr. Frédéric Laquai
• Subject Area: Experimental Condensed Matter Physics ; Organic Molecular Chemistry - Synthesis and Characterisation ; Theoretical Condensed Matter Physics
• Term: from 2014 to 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 255377421

The project (Influence of Order and Disorder on the Photogeneration of Charge Carriers) investigates 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 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 - respectively 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 suchcopolymers 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 calculations 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 electronicstructure of the transitions by means of transition and charge difference densities.Furthermore, the material system shall be experimentally investigated in respect of the photogeneration of charge carriers depending on the extent of order within the polymer. It should be mentioned 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 interface 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 generation and recombination) as well as space-charge limited current (SCLC) diodes for the investigation of charge transport (charge carrier mobility).

• DFG Programme: Research Grants
• International Connection: Austria
• Participating Institution: Fonds zur Förderung der wissenschaftlichen Forschung (FWF)
• Participating Person: Privatdozent Dr. Daniel Ayuk Mbi Egbe