Advanced Functional Materials and Devices (AFMD) Group

Abstract:

The well-known organic semiconductor C60 is attracting renewed attention due to its centimetre-long electron diffusion length and high performance of solar cells containing 95% fullerene. Yet, its photophysical properties remain poorly understood. Here, we elucidate the dynamics of Frenkel and intermolecular (inter- C60) charge transfer (CT) excitons in neat and diluted C60 films from high quality femtosecond transient absorption (TA) measurements, performed at low fluences and free from oxygen or pump-induced photo-dimerization. We find from preferential excitation of either species that the CT excitons give rise to a strong electro-absorption signal but are extremely short-lived. The Frenkel exciton relaxation and triplet yield depend strongly on the C60 aggregation. Finally, TA measurements on full devices with applied electric field allow us to optically monitor the dissociation of CT excitons into free charges for the first time and to demonstrate the influence of cluster size on the spectral signature of the C60 anion.

Abstract:

© 2018 Walter de Gruyter GmbH, Berlin/Boston 2018. The effciency of organic solar cells is not only determined by their absorber system, but also strongly dependent on the performance of numerous interlayers and charge transport layers. In order to establish new custom-made materials, the study of structure-properties relationships is of great importance. This publication examines a series of naphthalenetetracarboxylic diimide molecules (NTCDI) with varying side-chain length intended for the use as n-dopable electron transport materials in organic solar cells. While all compounds basically share very similar absorption spectra and energy level positions in the desired range, the introduction of alkyl chains has a large impact on thin film growth and charge transport properties: both crystallization and the increase of conductivity by molecular doping are suppressed. This has a direct influence on the series resistance of corresponding solar cells comprising an NTCDI derivative as electron transport material (ETM) as it lowers the power conversion efficiency to 1%. In contrast, using the side-chain free compound it is possible to achive an efficiency of 6.5%, which is higher than the efficiency of a comparable device comprising n-doped C60as standard ETM.