The Resonant-Cavity-Enhanced Organic Photo-detectors and Photovoltaics (RCE-OPP) project, aims to enhance the performance of current state-of-the-art narrowband near-infrared organic photodetectors (NIR-OPDs) by using new active materials combined with novel one dimensional optical cavities to extend to long detection wavelengths (> 1200 nm). Additionally, strong light-matter coupling within a one dimensional optical cavity device architecture has been exploited to reduce voltage losses and improve the performance of the state-of-the-art organic solar cells (OSCs).
In order to efficiently extend the detection window toward lower energy infrared light, of organic semiconductor devices, two fundamental material design criteria need to be addressed. First, the intermolecular energy gap or charge-transfer state energy, approximated by the difference between the highest occupied molecular orbital (HOMO) of the donor and lowest unoccupied molecular orbital (LUMO) of the acceptor molecules, must be decreased to make lower energy sub-bandgap transitions available. Second, the CT transitions must occur sufficiently frequent to achieve a threshold absorption coefficient for which cavity enhancement enables a reason-ably high external quantum efficiency (EQE) at the resonance wavelength. In the RCE-OPP project, high-performance detecting wavelength-tunable narrowband cavity NIR-OPDs were fabricated and the detection region was pushed deeper into the NIR, up to ~1350 nm. The broad wavelength tuning range achieved using a single polymer:fullerene blend renders this system an ideal candidate for printable miniature NIR spectrophotometers.
Currently, OSCs using nonfullerene acceptors have been steadily improving to above 18%, but still suffer from relatively large non-radiative recombination voltage losses as compared to inorganic or perovskite solar cells, typically around 250 mV. In the RCE-OPP project, a narrower electroluminescence (EL) line-width was demonstrated to have lower non-radiative losses. Based on this finding, an extremely low non-radiative voltage loss of 155 mV using an aggregation-less donor:acceptor combination (PM6:Y16F) was achieved, and subsequently near-infrared light-emitting devices based on this material were fabricated and their performance are amongst the best reported fluorescent OLEDs emitting around 900nm. This work indicates that new acceptors should be designed with narrow emission linewidth which is essential to further enhance the performance of OSCs. In the long-term, this research achievement has relevance for OSC, enabling for them to enter earlier into solar energy market helping the world to greatly reduce the carbon emissions and alleviate global warming.
