DOI: https://doi.org/10.29363/nanoge.aohm.2019.022
Publication date: 8th January 2019
Many organic photovoltaic devices suffer from an irreversible deterioration in performance when illuminated by 1 sun irradiation in inert atmospheres. This is typically a biphasic degradation with a rapid initial drop in the first few 10’s of hours, followed by a slower phase of degradation that continues indefinitely during irradiation. Recently, with the development of new non-fullerene acceptors (NFAs), burn-in free devices have been demonstrated and T80 lifetimes of almost 10 years have been reached.[1,2] Yet, it is not fully understood why these systems show superior stability. In this study we have systematically tested a range of benchmark NFAs in combination with several high-performance donor polymers and found that the device photostability is highly dependent on the chosen acceptor. Photoluminescence, electroluminescence, UV-vis and Raman spectroscopy were used alongside transient photovoltage measurements to investigate the degradation processes. Of the investigated NFAs, Eh-IDTBR and O-IDTBR showed the most promise with excellent photostability when used in combination with PCE10, PCE11 and PCE12. This was attributed to exceptional photochemical and morphological stability. Other devices based on ITIC and M-ITIC were much less stable and lost up to 40% of their initial performance in less than 5 days of light soaking in a nitrogen atmosphere. Chemical degradation of the acceptor and morphological evolution were both demonstrated to be contributing factors to this loss of performance. Whilst it may be possible to resolve morphological instability by modifying device fabrication procedures, photochemical stability is a more fundamental property of a material. We show that by utilising Raman spectroscopy, it may be possible to identify the degradation location on NFA molecules. This information could allow for improved molecular design and the development of more photochemically stable NFAs.
The authors would like to thank all sources of funding that made this work possible including funding from the European Social Fund via the Welsh Government, EPSRC projects EP/L015099/1 & EP/M025020/1, Innovate UK project 133701, the Welsh Assembly Government of the Ser Cymru Solar Program, the Welsh Assembly Government Ser Cymru II Fellowship Scheme, the National Research Network in Advanced Engineering and Materials and Eight19.
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