Electrothermal Tristability Causes Sudden Burn‐In Phenomena in Organic LEDs

Abstract

Organic light-emitting diodes (OLEDs) have been established as a mature display pixel technology. While introducing the same technology in a largearea form factor to general lighting and signage applications, some key questions remain unanswered. Under high-brightness conditions, OLED panels were reported to exhibit nonlinear electrothermal behavior causing lateral brightness inhomogeneities and even regions of switched-back luminance. Also, the physical understanding of sudden device failure and burn-ins is still rudimentary. A safe and stable operation of lighting tiles, therefore, requires an in-depth understanding of these physical phenomena. Here, it is shown that the electrothermal treatment of thin-film devices allows grasping the underlying physics. Configurations of OLEDs with different lateral dimensions are studied as a role model and it is reported that devices exceeding a certain panel size develop three stable, self heating-induced operating branches. Switching between them causes the sudden formation of dark spots in devices without any preexisting inhomogeneities. A currentstabilized operation mode is commonly used in the lighting industry, as it ensures degradation-induced voltage adjustments. Here, it is demonstrated that a tristable operation always leads to destructive switching, independent of applying constant currents or voltages. With this new understanding of the effects at high operation brightness, it will be possible to adjust driving schemes accordingly, design more resilient system integrations, and develop additional failure mitigation strategies.

A bulk‐surface model for the electrothermal feedback in large‐area organic light‐emitting diodes

“…Simulations of the full three‐dimensional $$p(x)$$‐thermistor model as performed for the paper [2], especially for large‐area organic LEDs with several organic layers, are very time‐consuming. Solving the effective model in ()–() is numerically cheaper since the thin layers do not have to be resolved and the lower‐dimensional current‐flow equations lead to a sparser system.…”

“…To understand the emergence of burn‐ins in organic LEDs due to high temperatures [2], it is interesting to consider the instationary version of the thermistor system. The derivation of an effective bulk‐surface model starting from a three‐dimensional, instationary $$p(x)$$‐Laplace thermistor model as treated in [10] and the investigation of its analytical properties is still open.…”

Accurate Wavelength Tracking by Exciton Spin Mixing

“…For such scenarios, it is feasible to employ phosphors with steep absorption edges, e.g., 2,2′,7,7′-tetrakis(diphenylamino)-9,9′-spirobifluorene (Spiro-TAD), [38] 2,2′,7,7′-tetra(N,N-di-p-tolyl)amino-9,9-spirobifluorene (Spiro-TTB), [39] or 2,2′,7,7′-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9′-spirobifluorene (Spiro-OMe-TAD), [40] which are established, temperature-stable organic light-emitting diode (OLED) materials. [41][42][43] Also the optimized emitter concentration can change the required exciton dynamics, as scanned in ref. [24].…”

Fine-grained phosphors for red-emitting mini-LEDs with high efficiency and super-luminance

“…With advances in the light extraction efficiency of minimized InGaN LED chips, mini-LED displays are attracting great attention in very recent years as they possess great advantages of high luminance, excellent contrast ratio (1,000,000:1), and superior high dynamic range (HDR) [1][2][3]. These promising properties make mini-LED displays affordable version of OLEDs but without screen burn-in issues [4][5][6], and thus are very attractive for use in a variety of applications, ranging from smart phones, large TVs, to automotive and industrial applications. Mini-LEDs can be used as the backlight source for a conventional liquid orystal display (LCD), or as self-emissive pixel emitters [3,7].…”