Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV18)
DOI: https://doi.org/10.29363/nanoge.hopv.2018.092
Publication date: 21st February 2018
Conjugated polymers based on the new chromophore indolo-naphthyridine-6,13-dione thiophene (INDT) are explored in OFET and organic photovoltaic applications. These polymers exhibit very narrow band gaps of ~1.2 eV and extremely high n-type mobility exceeding 3 cm2 Vs-1. The high n-type charge carrier mobility is correlated to remarkably high crystallinity along the polymer backbone with a correlation length in excess of 20 nm. OPV device efficiencies up to 4.1 % and charge photogeneration up to 1000 nm were demonstrated highlighting the potential of this novel chromophore in high-performance organic electronics. Although the efficiency of the devices are modest, these are some of the highest values reported for materials with such narrow band-gaps. External quantum efficiency measurements show that the majority of the photocurrent originates from the fullerene absorption; however there is also a clear and significant contribution from the polymer absorption.
These polymer blends were investigated further using ultra-fast transient absorption spectroscopy (TAS). It has been discovered that two methods of charge photogeneration are operational, depending upon whether the polymer or fullerene is excited. Fullerene excitation results in an unusually slow (nanosecond timescale) hole transfer process from the photoexcited fullerene to the polymer. Polymer excitation, in contrast, leads to standard electron transfer to the fullerene acceptor, despite the LUMO levels of the donor and acceptor being almost isoenergetic implying virtually zero driving force for charge separation. It is remarkable that such a high proportion of charge carriers are achievable through polymer excitation in ultra-low band gap polymers.
Different fullerenes were also trialed to assess the effect on charge photogeneration, using both TAS and pump-push photocurrent measurements. Interestingly, it was discovered that the INDT polymers may possibly generate an intramolecular CT state-like singlet exciton, which is only able to be efficiently separated in the presence of a fullerene with a deep enough LUMO.
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