Dominance of Exciton Lifetime in the Stability of Phosphorescent Dyes

Abstract

Many efforts have been made to understand OLED degradation behavior. [3][4][5][6][7][8][9][10] While extrinsic degradation mechanisms have been identified and minimized, [11] routine identification of primary intrinsic degradation mechanisms has not been developed. Indeed, diagnostic spectr oscopy at the microscopic level is challenging given the exceedingly rare processes involved. Red phosphorescent OLEDs, for example, exhibit a decay time to 95% of initial luminance (LT95) of ≈20 000 h at an operating brightness of 1000 nits. [12] Given that the active phosphorescent dye is present at 3% loading in a 30 nm thick emissive layer, this yields more than 60 billion excitons per dye to LT90. One approach to the experimental challenge is to build models that consider an array of possible phenomena that are then fit to degradation data for a specific combination of materials in a specific device structure. [3,4,[13][14][15][16][17][18] Due to the multitude of fitting parameters, however, such models do not allow for an unequivocal, and more importantly, generalized quantification of physical parameters governing degradation.Given the challenges that confront the rational design of OLED materials, it is important to determine general characteristics of OLED failure processes by isolating individual key parameters in direct experimental probes. We focus on long-lived triplet excitons, which are common to all OLEDs, and are hypothesized as an important energy source for degradation processes. The total energy stored in triplet excitons is dependent on the triplet exciton density which, in turn, is determined by the triplet exciton generation rate, G, and the triplet exciton lifetime, τ. Using these two key parameters, we construct a simple model of OLED degradation based on three primary classes of known failure mechanisms.As shown in Table 1, the first class (i) of degradation mechanisms is unimolecular pathways. Examples include spontaneous degradation of a given molecule in its excited state, or an impurity-assisted process such as photo-oxidation. [19] Unimolecular processes are distinguished from the other classes because they scale linearly with the number of triplet excited states in the OLED. The second class (ii) of degradation mechanism is triplet-charge interactions. [14,20] Here, triplet excitons collide with a charged molecule, forming a high energy state which initiates permanent damage to one of the molecules. Assuming that the charge density is determined by nongeminate recombination, Organic light-emitting devices (OLEDs) are widely used for mobile displays, but the relatively short lifetime of blue OLEDs remains a challenge in many applications. Typically, instability is viewed as a material-specific chemical degradation problem. It is known to be alleviated by reducing the operating current or otherwise decreasing the exciton density. It is shown here that this view is incomplete. For archetypical phosphorescent materials, it is observed that the dependence of photostability on the triplet exciton life...

Rational Tuning of Bis-Tridentate Ir(III) Phosphors to Deep-Blue with High Efficiency and Sub-microsecond Lifetime

“…A recent paper by Baldo and co-workers reported that the instability of OLEDs could be effectively alleviated by reducing the exciton density in the emitting layer, which was verified by the cubic dependence of device stability on triplet exciton lifetime. 28 With the possession of these new blue emitters reported above, one can thus predict a significant improvement in stabilities, a result of the fast radiative process and rapid depletion of energy stored in triplet excitons.…”

“…It has been reported that shorter the radiative lifetime, better the device efficiency due to the cubic dependence of device stability on triplet exciton lifetime. 28 In this work, we report an endeavor on how to attain blue emission with a short lifetime for a class of bis-tridentate Ir(III) phosphors using electron-deficient coordinative carbenes. 29−31 It is expected that replacement of parent imidazolylidene with imidazo [4,5-b]pyridin-2-ylidene appendages should lower the ππ* energy gap of the pincer carbene chelate in a way analogous to the cyclometalating carbene of [Ir(pmb) 3 ], [Ir(pmp) 3 ] and analogues.…”

Role of Singlet and Triplet Excitons on the Electrical Stability of Polymer Light‐Emitting Diodes

“…Furthermore, TTA also lowers the triplet lifetime, resulting in a lower steady-state triplet concentration in an OLED driven at constant current, which will enhance the stability as well. [14] Also in our SY:BPEA blend TTA could in principle enhance the efficiency of the PLED; a high triplet concentration on BPEA might enhance TTA upconversion to the singlet S1 state of BPEA, which then could be transferred to the S1 state of SY. If this recombination mechanism would be important a significant enhancement of the PLED efficiency upon addition of BPEA would be expected.…”

“…[1][2][3] A large disadvantage hindering commercialization of PLEDs is their low efficiency: external quantum efficiencies triplet excitons on lifetime has been extensively reported. [13][14][15][16] The device lifetime of ph-OLEDs has been increased by the incorporation of a managing molecule that dissipates the energy of highly excited states, resulting from triplet-triplet or triplet-polaron interactions, before they can lead to bond dissociation. [17] Furthermore, in TADF based OLEDs, triplet excitons have been pointed at as a cause of device degradation as well.…”

Negative Charge Management to Make Fragile Bonds Less Fragile toward Electrons for Robust Organic Optoelectronic Materials

“…In past decades, considerable effort was made to suppress these unwanted (photo)physical processes. 10, [17][18][19][20][21][22][23] However, completely avoiding them at the microscopic level was scarcely possible since even a small amount of deterioration product can result in a significant luminance loss. 10,24 Therefore, finding a way to restrain the induced (photo) chemical deterioration is still a critical need.…”