Online Seminar Announcement: Wednesday 3 February 2021

Dr. Hellen Jin
The University of Queensland

Graphene has shown tremendous potential as a transparent conductive electrode (TCE) for flexible organic solar cells (OSCs). However, the trade-off between electrical conductance and transparency as well as surface roughness of the graphene TCE with increasing layer number limits power conversion efficiency (PCE) enhancement and its use for large-area OSCs. Here, we use a 300 nm-thick poly[(2,5-bis(2-hexyldecyloxy)phenylene)-alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c]-[1,2,5]thiadiazole)]:[6,6]-phenyl-C71-butyric acid methyl ester as the photoactive layer and a benzimidazole (BI)-doped graphene as the transparent anode to demonstrate efficient OSCs with good flexibility. It is found that 3 layer (L) graphene had the best balance between sheet resistance, optical transmittance and surface roughness for optimized cell design. A 0.2 cm2 cell with a 3L BI-doped graphene anode achieves a PCE of 6.85%, which is one of the highest PCE values reported so far for flexible graphene anode-based OSCs. The flexible cells are robust, showing only a small performance degradation during up to 250 flexing cycles. Moreover, the combination of the thick photoactive layer with the optimized 3L BI-doped graphene TCE enabled production of 1.6 cm2 flexible OSCs with a PCE of 1.8%. Our work illustrates the importance of graphene TCE development for flexible OSCs as well as other wearable optoelectronic devices.

Dr. Patrick Conaghan
University of Sydney / University of Cambridge

Emitting materials in organic light-emitting diodes are predominantly phosphorescent iridium complexes or all-organic thermally activated delayed fluorescence materials. We have demonstrated that the carbene-metal-amide molecular structure of coinage-metal complexes can also be used to produce organic light-emitting diodes with efficient electroluminescence from both singlet and triplet excited states.
We have shown that the emission colour can be changed through variation of the electron-donating strength of the donor moiety and the polarity of the host environment and have used both of these effects to produce devices with emission across the visible range. By engineering excited state energies we have fabricated green-emitting devices with a maximum electroluminescence quantum efficiency of 26.9 % and blue-emitting devices (Commission Internationale de l’Éclairage co‐ordinates [0.17, 0.17]) with external quantum efficiency of 20.9 %.
Transient photoluminescence measurements at varying temperatures show that the emission process is thermally activated with short excited-state lifetimes (<1 μs) and along with device data show that the energy of local triplet excited states imposes an energy limit on efficient emission. Carbene-metal-amides do not show a strong concentration-dependent luminescence quenching in the solid state, which has allowed us to fabricate host-free devices with external quantum efficiency of up to 23 % which, to our knowledge, is the highest reported for host-free organic light-emitting diodes.