Strategies to improve the semitransparency of organic solar cells investigated on different donor/acceptor combinations
Bernadette Ortner a b, Georg Haberfehlner b, Thomas Rath a, Gregor Trimmel a
a Institute for Chemistry and Technology of Materials, NAWI Graz, TU Graz, 8010 Graz
b Institute for Electron Microscopy and Nanoanalysis, Graz University of Technology, Graz 8010
Poster, Bernadette Ortner, 197
Publication date: 6th February 2024

The possibility to reach high semitransparency in certain wavelength ranges and defined coloration are very beneficial features of organic solar cells, which are useful for many different applications like building integrated PV, wearable electronics or vehicle integrated PV. This work concentrates on various strategies to obtain semitransparent organic cells with optimal balance between efficiency and semitransparency. In search for suitable materials, the focus fell on efficient conjugated polymers such as D18, PTQ10 or PTB7-Th and near-infrared absorbing NFAs such as M3, L8-BO and IEICO-4F. While IEICO-4F has the lowest band gap of the investigated NFAs, the overall obtainable efficiencies are typically significantly lower due to increased energetic disorder. Among the tested materials, the D18:L8-BO system showed the highest power conversion efficiency (16.3%) with a VOC of 0.90 V, a JSC of 24.8 mA/cm2 and a fill factor of 0.73. Compared to that, PTB7-Th:IEICO-4F based solar cells led to PCEs of 6.91%.

In this regard, we investigated possible benefits of using the most efficient systems instead of IEICO-4F acceptors and to reduce the absorption in the visible range as much as possible. This was investigated with experiments such as reducing the amount of the conjugated polymer in the bulk heterojunction absorber layer in order to have the main absorption in the NIR region, reducing the active layer thickness, or layer by layer processing combined with conjugated polymer films with reduced thickness. The impact of these strategies on the performance and semitransparency between 400 and 750 nm of the three most efficient material combinations was investigated in detail and compared to the properties of IEICO-4F based solar cells. While the reduction of the active layer thickness decreases the efficiency significantly when the absorber layer thickness goes below 80 nm, reducing the donor content seems to be a promising method to increase the semitransparency in the visible wavelength range, while still achieving good PCEs. As the donor:acceptor phase separation and the formation of continuous donor phase pathways play a crucial role when using low donor contents, this aspect was thoroughly investigated using analytical transmission electron microscopy to visualize the absorber layer morphology based on D18:L8-BO as model system. The results of this TEM-study on a range of different donor:acceptor ratios were correlated to the photovoltaic properties of these bulk heterojunction absorber layers. Interestingly, the hole mobilities as well as the efficiencies remain comparably high also with low donor contents below 20w% allowing an increased semitransparency in the visible wavelength range.

This work was funded by the Climate and Energy Fund as part of the PEROPTAM research project, which is conducted in a collaboration of JKU & TUG.

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