Ultrahigh Efficiency Fluorescent Single and Bi‐Layer Organic Light Emitting Diodes: The Key Role of Triplet Fusion

Abstract

A new family of anthracene core, highly fluorescent emitters is synthesized which include diphenylamine hole transport end groups. Using a very simple one or two layer organic light emitting diode (OLED) structure, devices without outcoupling achieve an external quantum efficiency of 6% and photonic efficiencies of 20 cd/A. The theoretical maximum efficiency of such devices should not exceed 3.55%. Detailed photophysical characterization shows that for these anthracene based emitters 2T1≤Tn and so in this special case, triplet fusion can achieve a singlet production yield of 0.5. Indeed, delayed electroluminescence measurements show that triplet fusion contributes 59% of all singlets produced in these devices. This demonstrates that when triplet fusion becomes very efficient, fluorescent OLEDs even with very simple structures can approach an internal singlet production yield close to the theoretical absolute maximum of 62.5% and rival phosphorescent‐based OLEDs with the added advantage of much improved stability.

Efficient Triplet Exciton Fusion in Molecularly Doped Polymer Light‐Emitting Diodes

“…Maximum external quantum efficiencies (EQE) of about 6% or above have been obtained for DPA, perylene, and rubrene ( Figure 3c and Table S1, Supporting Information). These values exceed the EQE limit (5%) of conventional electrofluorescence (assuming an optical outcoupling factor of 0.2), [6][7][8] suggesting a significant contribution from triplets toward the total EL. We found that a high emitter doping concentration (20%) results in optimum OLED performance ( Figure S2, Supporting Information), consistent with the view that a small intermolecular separation is required for efficient bimolecular TTA process.…”

“…[5] Another strategy to utilize triplet excitons in fluorescent OLEDs is through the generation of singlets by triplet-triplet annihilation (TTA) or "triplet fusion." [6][7][8] At the same time, photon upconversion through TTA has been widely considered for next-generation photovoltaic applications. [9][10][11][12][13][14] Effective upconverters have been demonstrated using combinations of a triplet sensitizer such as platinum octaethylporphyrin (PtOEP) [3] and a triplet acceptor/annihilator such as 9,10-diphenylanthracene (DPA), [15] perylene, [16] or rubrene.…”

“…f outcoupling is assumed to be 0.2. [6][7][8] A more practical quantity we investigate is the TTA-UC quantum yield (or triplet-to-photon quantum yield), Φ TTA-UC , which calculates the number of TTA-generated photons emitted per triplet exciton entering the upconverter. It is given by the following equation: …”

“…[9][10][11][12]18] A summary of the efficiencies of solution systems [17,18,[31][32][33] is shown in Table S2 (Supporting Information). Comparisons of our devices with other solid-state TTA-UC systems [13,14,19,31,34] and other TTA-enhanced OLEDs [7,8,[35][36][37] are presented in Tables S3 and S4 (Supporting Information), respectively. The remarkably high TTA-UC efficiencies in our best devices suggest that only singlets are formed during the TTA process.…”

Effect of various red phosphorescent dopants in single emissive white phosphorescent organic light-emitting devices

“…Considerable attention has been paid to white organic light-emitting devices (OLEDs) in flat panel displays (FPDs) and solid-state lighting in the next generation due to several potential advantages such as diffusive emission of surfaces, large-range area manufacturability, ecofriendliness, and effective fabrication process for the cost [1][2][3][4][5][6][7][8][9] . Such interesting advantages in white OLEDs have activated the field and, though they are entering the marketplace, outstanding challenges in achieving high efficiency and color rendering index (CRI) still remain.…”