Molecular Spacing Modulated Conversion of Singlet Fission to Triplet Fusion in Rubrene-Based Organic Light-Emitting Diodes at Ambient Temperature

Abstract

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.

Exploiting Singlet Fission in Organic Light‐Emitting Diodes

“…We chose TBRb because, even though its emission spectrum and absorption coefficients (Figure S4, Supporting Information) are similar to those of rubrene, its four tertiary‐butyl groups reduce intermolecular interactions between neighboring host molecules. Since the formation of intermediate charge‐transfer states between neighboring molecules is critical to the singlet‐fission process, the reduced intermolecular interactions for TBRb compared with rubrene should decrease the singlet‐fission efficiency. In 2 mol% ErQ 3 :TBRb co‐deposited films, NIR emission weaker than that from the rubrene‐based films was observed (Figure a).…”

Identify triplet-charge interaction in rubrene-based diodes using magneto-conductance: Coexistence of dissociation and scattering channels

“…For example, the energy of a singlet exciton within rubrene is resonant with that of two triplet excitons ( [1][2][3][4][5], which facilitates bimolecular reactions, including singlet fission (STT) and triplet fusion (TTA), ( + + [4,[6][7][8]. STT can lead to highly efficient organic solar cells (OSCs) [9], while TTA can increase the theoretical quantum efficiency of fluorescent organic light-emitting diodes (OLEDs) up to 62.5% [10,11].…”

The evolution of microscopic process mediated by current and temperature in Rubrene-based organic light emitting diodes

“…这也与Bai等人 [11] 特征曲线 [11] , 低场部分出现了ISC过程, 且这2种过程 都随电流的增大而增强. 为了进一步定量分析, EL 在低场的增加(low field effect, LFE)和EL在高场的减 小(high field effect, HFE)我们定义 [13] : HFETF=MEL(20 的三重态极化子对(PP 3 ) [19] . 其中, 除了部分极化子对 kS和kT的相对大小 [20] .…”