Carbazole-Based Halogenated SAMs as High Performing Hole Transporting Materials for Organic Photovoltaics
Mantas Marcinskas a, Tadas Malinauskas a, Vytautas Getautis a
a Department of Organic Chemistry, Kaunas University of Technology, Kaunas LT-50254, Lithuania.
Poster, Mantas Marcinskas, 069
Publication date: 24th October 2023

The emerging photovoltaic technologies are being improved at rapid speed in terms of quality, efficiency, and long-term stability. It is natural, that more and more materials are required to be tested, to attain the best possible results. Most of these technologies are inseparable from the organic semiconductors, of which hole transporting materials (HTMs) play an important role towards the improvement of overall photovoltaic systems operation.  Very recently, new generation of HTMs, which are nowadays referred as SAM (self-assembled monolayer) materials, have been successfully employed into a number of different solar cell technologies, by being able to form a monolayer of semiconducting substance on a commonly used metal oxides (e.g. ITO). Hereby, currently well-known carbazole-based SAM materials, such as 2PACz, MeO-2PACz and Me-4PACz, were successfully employed in perovskite solar cells, CIGSe/perovskite and silicon-perovskite tandems as HTMs. At a time, these devices demonstrated very high top-tier power conversion efficiency (PCE) results of 21.1 %, 23.26 % and 29.15 % respectively.[1,2]

Successful appearance and performance of carbazole-based SAMs in different types of solar cells made them attractive to be tested in other photovoltaics. One of the best examples is organic photovoltaics (OPVs) which demonstrated stable and high-efficient devices by applying 2PACz as a hole-selective interlayer. Its application led to the highest reported SAM-based OPV at a time, demonstrating 18.03 % PCE. Not long after 2PACz was outperformed by its successor Br-2PACz, which shown to be more suitable for OPVs due to its excellent hole-extracting characteristics and more suitable energetics, resulting in improved efficiency of 18.4 %. Given these results halogenated SAMs are promising materials for further investigation in OPVs. [3,4]

In this work, halogenated carbazole-based SAM derivatives containing different halogen atoms at different positions and varying length of aliphatic chain were synthesized, characterized, and tested in OPVs by prof. T. Anthopoulos research group (KAUST). This investigation demonstrated the influence of halogen substitutions on the energetics of synthesized molecules and overall performance, including stability under continuous illumination, setting Cl-2PACz to be the leader among tested materials, which yielded in the 18.5 % PCE (18.9 % with n-dopant benzyl viologen). [5]

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