ReMorphOPV · Recombination in Organic Photovoltaics: Impact of Morphology and Long-Range Non-Equilibrium Transport
Horizon 2020 — Marie Skłodowska-Curie Actions
- Duration
- 2019-01-15 → 2021-02-28
- EU contribution
- €173,857
- Participants
- 2
- Scheme
- MSCA-IF-EF-ST
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Results in brief
Recombination in Organic Photovoltaics: Impact of Morphology and Long-Range Non-Equilibrium Transport
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.
Data: CORDIS, © European Union
Project objective
The global transition towards clean energy requires new ways to generate electricity. One promising approach are organic bulk heterojunction (BHJ) solar cells. These devices are based on a phase-separated network of two organic materials and hold the potential to make solar power cheap and sustainable. However, there is still a lack of fundamental understanding in key areas. One important open question concerns the charge recombination. Although identified as main loss mechanism in BHJ solar cells, its underlying principles remain mysterious. ReMorphOPV comes to address these limitations by developing a new recombination model. The basic hypothesis is that a successful theoretical description must properly consider two key features of a BHJ blend: the complex nanoscale morphology and the dispersive type of charge transport. To account for both aspects, ReMorphOPV will make use of extensive kinetic Monte Carlo simulations with high spatial and temporal resolution. The proposed numerical approach includes most realistic assumptions on the nanostructure (domain size, phase purity, molecular miscibility etc.) and previously overlooked phenomena of charge transport, namely the non-equilibrium and long-range motion of carriers. The predictions of the simulations will be validated by experiments on different prototype material systems. A feedback loop between experiment and numerical model will be initialised to refine the theoretical description and define new parameterisations of the recombination rate that enable easy dissemination to other researchers. With such a model at hand, it will be possible to find design rules for organic solar cells with minimised recombination losses even at large thickness. These results are of great relevance for the photovoltaics community and will help to reinforce Europe's world-leading position in renewable energies.
Original text from CORDIS.
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Data: CORDIS, © European Union