Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV18)
DOI: https://doi.org/10.29363/nanoge.hopv.2018.064
Publication date: 21st February 2018
Electronic processes at the hetero-interfaces between electron donating and electron accepting molecules determine the photocurrent, photovoltage and ultimately, the power conversion efficiency of organic solar cells. While high incident-photon-to-extracted-charge conversion yields of over 85%, and absorbed photon-to-extracted-charge conversion yields of 90-100% have been achieved, the difference between the optical gap of main absorber and open-circuit voltage (Voc) is much larger than for inorganic and perovskite based solar cells. The main improvements of the Voc of organic solar cells have so far been made by tailoring donor-acceptor interfacial energetics, taking advantage of well-known principles of molecular design. Nevertheless, for most material systems we consistently find a large, almost constant difference (~0.6 eV) between eVoc and the energy of the intermolecular charge transfer (CT) state, ECT. Added to this, electron transfer losses are usually larger than 0.1 eV, resulting in overall voltage losses, often much larger than 0.7 eV. In this contribution, we will discuss the influence of molecular properties, such as the electronic coupling between electron donor and acceptor, the molecular reorganization energy as well as non-radiative triplet states on the free carrier recombination and the Voc. Furthermore, we show that, by reducing the physical interfacial area available for free charge carrier recombination, and by changes in the standard organic photovoltaic device architecture, it is possible to reduce the overall voltage losses to values below 0.6 eV. However, we further find that a fundamental lower limit of these voltage losses is the result of coupling of the CT state to high frequency molecular vibrations of the ground state. This has important implications for the efficiency upper limits of organic photovoltaics.
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International Conference on Hybrid and Organic Photovoltaics
International Conference on Hybrid and Organic Photovoltaics (HOPV) is celebrated yearly in May. The main topics are the development, function and modeling of materials and devices for hybrid and organic solar cells. The field is now dominated by perovskite solar cells but also other hybrid technologies, as organic solar cells, quantum dot solar cells, and dye-sensitized solar cells and their integration into devices for photoelectrochemical solar fuel production.
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