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Recombination in Organic Photovoltaics: Impact of Morphology and Long-Range Non-Equilibrium Transport

Periodic Reporting for period 1 - ReMorphOPV (Recombination in Organic Photovoltaics: Impact of Morphology and Long-Range Non-Equilibrium Transport)

Reporting period: 2019-01-15 to 2021-01-14

Photovoltaics, i.e. the direct conversion of sunlight into electricity, is a key technology for meeting the European Union's climate targets. One particularly promising and emerging approach are organic photovoltaics (OPVs). In contrast to conventional inorganic devices, OPVs are based on abundant materials such as polymers and small molecules that can be processed from solution. This makes OPVs not only attractive for cost-effective mass production, but also opens up new fields of application such as flexible and transparent solar cells. However, there is still considerable need for improvement in terms of efficiency and long-term stability, accompanied by a lack of fundamental understanding in key areas. In particular, it is still not well understood how the nanoscale morphology of the active layer, typically a bulk-heterojunction (BHJ) blend of an electron donor and an electron acceptor material, affects elementary processes such as charge transport and recombination.

ReMorphOPV aimed to contribute to the fundamental understanding by developing a realistic numerical device model. The model makes accurate assumptions about the complex BHJ morphology, while fully taking into account the hopping nature of charge transport and non-equilibrium effects. It has been calibrated and extensively tested using experimental data from a range of recent material systems. In particular, it was shown that the model can predict complete current-voltage characteristics of OPVs, and thereby predict all relevant performance parameters such as the power conversion efficiency. With this powerful tool in hand, general design rules for better performing OPVs were then established.

One main result of ReMorphOPV is that the presence of aggregates of high crystalline quality within the donor or acceptor phase are a key to reduce losses due to charge recombination. This demonstrates that a theoretical model of recombination in OPVs must make much more complex assumptions about morphology than the two-phase descriptions commonly used to date. In addition, elementary insights into the role of non-equilibrium effects on device performance were gained. It was shown that the open-circuit voltage of OPVs is in fact higher than would be expected according to prevailing equilibrium concepts. The reason for this is the slow relaxation of charge carriers in the disorder-broadened density of states. Harvesting these non-thermalized carriers opens up entirely new routes for device optimization of OPVs.
To achieve the objectives of ReMorphOPV, a kinetic Monte Carlo (KMC) model was used. The model includes all relevant morphological features such as phase separation, aggregation and phase purity/intermixing. To cover the hopping nature of transport, the Gaussian disorder model was implemented. In the first step of ReMorphOPV, the KMC model was extended to describe complete current-voltage characteristics and calibrated experimentally. It turned out that an accurate description of charge extraction/injection at the contacts as well as the presence of aggregates in the active layer are key requirements to accurately predict the device performance of real OPVs.

The calibrated model was then used to describe experimental data for a range of OPV material systems. It was shown that aggregates in the active layer are not only key to reduce losses due to charge recombination, but also to boost charge separation and transport on ultrafast time scales. It was shown that these statements hold for both fullerene and the more recent non-fullerene systems. Furthermore, the results of ReMorphOPV indicate that the open-circuit voltage of OPVs is substantially enhanced by non-equilibrium effects. In particular, it has been demonstrated that even under open-circuit conditions there is an exchange of charge carriers between the active layer and the contacts, so that the excess energy due to incomplete thermalization can actually be harvested.

The results of ReMorphOPV have been published in articles in peer-reviewed scientific journals and a book chapter (see publication list). Other channels used to disseminate the results include presentations at conferences and workshops, the project website and activities in social networks. Further publications are currently in preparation and will be added gradually.
ReMorphOPV has shown that a realistic description of the BHJ morphology, which goes beyond the common two-phase donor-acceptor description, is elementary to understand charge recombination and transport in OPVs. Furthermore, it was clearly demonstrated that the often overlooked non-equilibrium effects play an important role in macroscopic device performance. This also challenges existing equilibrium concepts and requires new interpretation schemes. These results are of key importance to the OPV research community and were/will be communicated through respective publications and presentations.
ReMorphOPV: Towards a general microscopic description of organic bulk-heterojunction solar cells