A figure of merit for efficiency roll-off in TADF-based organic LEDs

Abstract

Organic light-emitting diodes (OLEDs) are a revolutionary light-emitting display technology that has been successfully commercialized in mobile phones and televisions^1,2. The injected charges form both singlet and triplet excitons, and for high efficiency it is important to enable triplets as well as singlets to emit light. At present, materials that harvest triplets by thermally activated delayed fluorescence (TADF) are a very active field of research as an alternative to phosphorescent emitters that usually use heavy metal atoms^3,4. Although excellent progress has been made, in most TADF OLEDs there is a severe decrease of efficiency as the drive current is increased, known as efficiency roll-off. So far, much of the literature suggests that efficiency roll-off should be reduced by minimizing the energy difference between singlet and triplet excited states (ΔE_ST) to maximize the rate of conversion of triplets to singlets by means of reverse intersystem crossing (k_RISC)^5-20. We analyse the efficiency roll-off in a wide range of TADF OLEDs and find that neither of these parameters fully accounts for the reported efficiency roll-off. By considering the dynamic equilibrium between singlets and triplets in TADF materials, we propose a figure of merit for materials design to reduce efficiency roll-off and discuss its correlation with reported data of TADF OLEDs. Our new figure of merit will guide the design and development of TADF materials that can reduce efficiency roll-off. It will help improve the efficiency of TADF OLEDs at realistic display operating conditions and expand the use of TADF materials to applications that require high brightness, such as lighting, augmented reality and lasing.

Fig. 1. Examples of efficiency roll-off.

a, Efficiency roll-off of prototypical fluorescent (Alq₃), phosphorescent (Ir(ppy)₂acac) and TADF (4CzIPN) OLEDs^,,. b, Schematic graph showing definition of J₉₀. c, Graph of the relation between J₉₀ and EQE_1,000 for TADF^,,,–, fluorescent (Fluo.)^,– and phosphorescent (Phos.)^–,– devices emitting in the red (R), green (G) and blue (B) regions of the spectrum (for the references, see Methods).

Fig. 2. Simplified Jablonski diagram of a TADF emitter.

The excited singlet and triplet states S₁ and T₁, respectively, are shown in equilibrium due to the occurrence of both intersystem crossing (k_ISC) and reverse intersystem crossing (k_RISC) enabled by the small energy gap (ΔE_ST) between S₁ and T₁. A TADF OLED emits light by radiative decay from ${\text{S}}_1\left(k_{\text{r}}^{\text{S}}\right)$, whereas non-radiative decay from both ${\text{S}}_1\left(k_{\text{nr}}^{\text{S}}\right)$ and ${\text{T}}_1\left(k_{\text{nr}}^{\text{T}}\right)$, as well as the negligible radiative decay from ${\text{T}}_1\left(k_{\text{r}}^{\text{T}}\right)$ are other deactivation pathways of excited species.

Fig. 3. Data analysis.

J₉₀ of the reported TADF OLEDs with respect to k_RISC (Spearman correlation ρ = 0.638). Red crosses present TADF molecules containing heavy atoms that benefit from enhanced SOC to increase k_RISC. Data inside the dashed boxes are for comparison.

Fig. 4. FOM.

Correlations between J₉₀ and $k_{\text{r}}^{\text{S}}{K}_{\text{eq}}$ with a Spearman correlation coefficient of ρ = 0.700. Red crosses identify TADF molecules containing heavy atoms that enhance SOC, which leads to an increase in k_RISC. In grey circles, the correlation of J₉₀ and k_RISC from Fig. 3 is shown for comparison.

Extended Data Fig. 1. Correlation with delayed fluorescence lifetime τ_DF.

(a) Dependence of J₉₀ on τ_DF with a Spearman correlation, ρ, of  −0.685. (b) Dependence of τ_DF on k_RISC (ρ = −0.709). (c) Dependence of τ_DF on $k_{\text{r}}^{\text{S}}{K}_{\text{eq}}$ (ρ = −0.801). Red crosses identify TADF molecules containing heavy atoms that enhance SOC, which leads to an increase in k_RISC.

Extended Data Fig. 2. Simplified Figure of Merit.

(a) Correlation between J₉₀ and the simplified FOM of $k_{\mathrm{r}}^{\mathrm{S}}$k_RISC/k_ISC with a Spearman correlation of ρ = 0.680, showing a better correlation than k_RISC but a less precise predictor than the FOM of $k_{\text{r}}^{\text{S}}{K}_{\text{eq}}$. Red crosses identify TADF molecules containing heavy atoms that enhance SOC, which leads to an increase in k_RISC. In grey circles, the correlation of J₉₀ and k_RISC from Fig. 3 is displayed for comparison. (b) Deviation between the FOM of $k_{\text{r}}^{\text{S}}{K}_{\text{eq}}$ and its simplification of $k_{\mathrm{r}}^{\mathrm{S}}{k}_{\mathrm{RISC}}/k_{\mathrm{ISC}}$ for k_RISC = 10⁷ s^–1 showing a deviation between the FOMs for systems with competitive $k_{\text{r}}^{\text{S}}$ and k_ISC.

Extended Data Fig. 3. Device influence on Roll-off.

Comparison of efficiency roll-off of two literature 2CzPN OLEDs showing different J₉₀ because of different efficiency rise at low current densities^,.

Extended Data Fig. 4. Impact of STA and TTA on roll-off.

The impact of STA and TTA on the correlation between J₉₀ and $k_{\text{r}}^{\text{S}}{K}_{\text{eq}}$ calculated for a simplified three-level system with $k_{\text{r}}^{\text{S}}$ between 10⁵ s^–1 and 10¹⁰ s^–1, ${k_{\text{ISC}}/k}_{\text{r}}^{\text{S}}$ between 10^–1 and 10³ and $k_{\text{r}}^{\text{S}}{K}_{\text{eq}}$ between 10² s^–1 and 10⁸ s^–1 (a) for three different TTA rate constants and (b) for a particular STA rate, a particular TTA rate and a particular combination of STA and TTA rate.