Singlet fission is usually the only reaction channel for excited states in rubrene-based organic light-emitting diodes (OLEDs) at ambient temperature. Intriguingly, we discover that triplet fusion (TF) and intersystem crossing (ISC) within rubrene-based devices begin at moderate and high current densities (j), respectively. Both processes enhance with decreasing temperature. This behavior is discovered by analyzing the magneto-electroluminescence curves of the devices. The j-dependent magneto-conductance, measured at ambient temperature indicates that spin mixing within polaron pairs that are generated by triplet-charge annihilation (TQA) causes the occurrence of ISC, while the high concentrations of triplets are responsible for generating TF. Additionally, the reduction in exciton formation and the elevated TQA with decreasing temperature may contribute to the enhanced ISC at low temperatures. This work provides considerable insight into the different mechanisms that occur when a high density of excited states exist in rubrene and reasonable reasons for the absence of EL efficiency roll-off in rubrene-based OLEDs.

Triplet fusion (TF) and singlet fission (SF) are two important spin-coupled exciton interactions that occur in rubrene-based organic light-emitting diodes (OLEDs). TF produces additional singlets, which increases fluorescence efficiency, while SF consumes singlets and lowers the fluorescence efficiency. In an effort to adjust the SF and TF processes in rubrene-based OLEDs, we changed the average molecular spacing (d) of rubrene by doping it at varying concentrations in the high triplet energy material 1,3-bis­(9-carbazolyl)­benzene (mCP). Using magneto-electroluminescence (MEL), we observed that TF increased, while SF decreased at ambient temperature as d was increased from 1.8 to 5.0 nm. This was further confirmed using MEL at different temperatures and current intensities. We found that the efficiency of rubrene-based OLEDs was improved by altering the value of d, with the highest efficiency being observed at d = 3.8 nm because of complete conversion of SF to TF (SF → TF). The SF → TF was explained using a model that describes Dexter- and Förster-energy transfer in SF and TF processes with functions that have a different dependence on d. This difference causes the rate constant of SF to decrease more rapidly than that of TF. The TF will be primary when d goes between the Dexter and Förster radii, leading to complete SF → TF at ambient temperature. This work presents a promising approach to improve the efficiency of rubrene-based OLEDs.

Non-emissive triplet excited states in devices that undergo thermally activated delayed fluorescence (TADF) can be up-converted to singlet excited states via reverse intersystem crossing (RISC), which leads to an enhanced electroluminescence efficiency. Exciton-based fluorescence devices always exhibit a positive magneto-electroluminescence (MEL) because intersystem crossing (ISC) can be suppressed effectively by an external magnetic field. Conversely, TADF devices should exhibit a negative MEL because RISC is suppressed by the external magnetic field. Intriguingly, we observed a positive MEL in TADF devices. Moreover, the sign of the MEL was either positive or negative, and depended on experimental conditions, including doping concentration, current density and temperature. The MEL observed from our TADF devices demonstrated that ISC in the host material and RISC in the guest material coexisted. These competing processes were affected by the experimental conditions, which led to the sign change of the MEL. This work gives important insight into the energy transfer processes and the evolution of excited states in TADF devices.

Lowering the driving voltage of organic light-emitting diodes (OLEDs) is an important approach to reduce their energy consumption. We have fabricated a series of bifunctional devices (OLEDs and photovoltaics) using rubrene and fullerene (C60) as the active layer, in which the electroluminescence threshold voltage(~1.1 V) was half the value of the bandgap of rubrene. Magneto-electroluminescence (MEL) response of planner heterojunction diodes exhibited a small increase in response to a low magnetic field strength (<20 mT); however, a very large decay was observed at a high magnetic field strength (>20 mT). When a hole-transport layer with a low mobility was included in these devices, the MEL response reversed in shape, and simultaneously, the EL threshold voltage became larger than the bandgap voltage. When bulk heterojunction device was examined, the amplitude of MEL curves presented an anomalous voltage-dependence. Following an analysis of the MEL responses of these devices, we proposed that the EL of half-bandgap-voltage device originated from bimolecular triplet-triplet annihilation in the rubrene film, rather than from singlet excitons that formed via an interface auger recombination. This work provides critical insight into the mechanisms of OLED emission and will help advance the applications of bifunctional devices.

Although it is known that triplet excitons can be quenched via triplet-charge interaction (TQI), it is still unclear how this process occurs in rubrene-based devices. We found that magneto-conductance (MC) can be used to probe the detailed mechanism of TQI in rubrene-based organic light-emitting diodes and observed the coexistence of negative and positive MC responses in the high-field region when holes and electrons were the dominant charged species at different interfaces adjoining the rubrene layer, respectively. Further analysis suggests that the negative MC response was originated from the dissociation of triplet excitons by holes, while the positive MC response was due to electron scattering by triplet excitons. The MC responses of the devices were examined under different injection currents and temperatures to confirm our hypothesis. This work gives significant insight into mechanisms of TQI in organic semiconductors, which will allow for the design of new and improved devices.