Universal Electroluminescence at Voltages below the Energy Gap in Organic Light‐Emitting Diodes

Abstract

annihilation (TTA) to form an emitting singlet [1,[6][7][8] and band-to-band recombination. [9,10] EL has even been observed in devices without such heterojunction, for which it was proposed that electrons could be injected directly into a low-energy triplet state, followed by emissive singlet exciton formation via triplet fusion. [7] However, it should be noted that EL at subgap voltages has been widely observed in diodes based on inorganic semiconductors, in which processes such as TTA likely do not play a role. [11][12][13][14] Here, we demonstrate that EL at subgap voltages is a universal feature of OLEDs. Even down to zero applied voltage, EL is theoretically present, which is the result of the recombination of diffused and thermally generated charge carriers. Consequently, for an ideal OLED without traps and Ohmic contacts, the electrical quantum efficiency (EQE, Γ EL ) for EL equals unity down to zero applied voltage. Experimentally, EL at voltages below the energy gap is clearly observed in OLEDs based on emitters with negligible energy difference between the singlet and triplet excited stated, ruling out energy up-conversion mediated by low-energy triplet states. Furthermore, also in OLEDs with a strongly reduced free carrier density due to trapping the EL at subgap voltages occurs, ruling out Auger processes that require high carrier densities.

Bright short-wavelength infrared organic light-emitting devices

“…Due to thermodynamic constraints, EL emission at voltages below emitted photon energy is hardly ever accessible but can occur without violating the energy conservation law through the recombination of diffusion-driven and thermally generated charge carriers 46 . Under any voltage above 0 V, diffusion current due to the gradient of the carrier concentrations starts to flow, following an exponential dependence on the applied voltage.…”

Single‐Layer Blue Organic Light‐Emitting Diodes With Near‐Unity Internal Quantum Efficiency

“…The turn‐on voltage at 1 cd m −2 equals 2.53 V, which is lower than the optical gap of 2.88 eV (Figure S5, Supporting Information), which can be traced back to the recombination of diffused and thermally generated charge carriers below the built‐in voltage, the concentration of which is maximized due to the ohmic contacts. [ 24 ] For this reason, single‐layer OLEDs with ohmic contacts are capable of operating at very low voltages. A control device with a non‐ohmic PEDOT:PSS electrode reaches an EQE of 14% (Figure S6, Supporting Information), after electrical conditioning.…”

Acceptor Interlocked Molecular Design for Solution‐Processed Stable Deep‐Blue TADF and Hyper Fluorescence Organic LED Enabling High‐Efficiency

“…As shown in Figure 6e,f, the luminance-voltage and current density-voltage curves, the current density of KCTBC based device was higher than that of KCCz, which may be attributed to the difference between the highest occupied molecular orbital (HOMO) of the host and the lowest unoccupied molecular orbital (LUMO) of KCTBC generated charge trapping, decreasing the current density of KCCz. [56,57] The addition of tert. butyl group in KCTBC reduced the HOMO and LUMO gap between the host and KCTBC, which supports the carrier injection and transport in the emitting layer of KCTBC based device.…”