Optical Design and Optimization of Highly Efficient Sunlight-like Three-Stacked Warm White Organic Light Emitting Diodes

Abstract

In this paper, we report an optical structure design method with the predicted performances of highly efficient three-stacked white organic light emitting diodes (WOLEDs) for solid state lighting applications. The efficiency and color properties of stacked WOLEDs are strongly affected by optical interference inside the thick cavity length; therefore appropriate emissive layer (EML) position is determined by thorough theoretical optical simulations to prevent such optical effect. The theoretically evaluated, three-stacked hybrid WOLED with entirely separated phosphorescent red and green as well as fluorescent blue EML has displayed a CRI and power efficiency of 91 and 33.5 lm/W, respectively. Based on our assumptions, design method, and optical simulation results, the fabricated three-stacked WOLEDs showed a CRI of 93, external quantum efficiency (EQE) of 49.4%, and power efficiencies of 33.4 lm/W. These experimentally measured characteristics are fully correlated with the performances of optically simulated devices. It is important to note that the driving voltage (10.0 V) of optically designed WOLEDs is almost identical to the summation of unit devices (9.9 V) because of good interconnecting units and the same charge balance in the tandem WOLEDs. In addition, the experimentally measured power efficiency of the tandem device is similar to an average value of the unit devices, and most importantly the EQE is nearly equal to the summation of the unit devices with an almost matched white spectrum.

Tandem white organic light-emitting diodes stacked with two symmetrical emitting units simultaneously achieving superior efficiency/CRI/color stability

“…The reduced blue and yellow intensity contributes to an increasing CRI value, reaching 80 and 82 for device W2 and W3 in the measurable ranges [8,17,18]. More importantly, these tandem WOLEDs inherit well the advantages of color stability of Y/B/Y-and R/Y/R-single unit WOLEDs [20,29,33]. Under a wide voltage range of 7-13 V (corresponding hundreds to tens of thousands of luminance), all devices W1-W3 exhibit extremely high color stability with CIE (Δx, Δy) coordinates change of < ± 0.007, ±0.011.…”

“…To obtain high CRI, three or more individual EL units need to be incorporated in the same tandem WOLED. Recently, Kwon et al fabricated three-color tandem WOLEDs containing blue, green, and red three-individual EL units by using two CGUs, and as expected the device demonstrated a maximum CRI of 93 [20]. However, this white device has a number of functional layers beyond 15, indicating very complicated device structure and long-time preparation process.…”

“…The structure for these devices is relatively simple and the total thickness of organic layers in these devices is only about 100−150 nm, inducing a time-efficient fabrication process [1-3, 5-12, 19-20]. However, these WOLEDs generally show short device lifetime (L T ), which can be ascribed to; (i) The thin thickness of organic layers makes them sensitive to the surface roughness and particles, and the materials in EMLs are more easily oxidized [20]; (ii) The L T of OLEDs usually follows a power law dependence with respect to the applied current density (J) (L T ∝ J -β , 1.5 < β < 3) [21,22], to reach a practical luminance of 3000−5000 cd/m 2 , these white devices need higher current density, which also drastically causes a reduced device lifetime.…”

“…For example, in single EML WOLEDs, different color emitters are doped into a host with very precise doping ratios, where a slight change in doping concentration will lead to an obvious change in EL spectra, and the excitons in emitter sites of low energy level are more easily saturated by energy transfer from high energy level emitters with increasing operating voltage, making EL spectra variation under different operating voltage [25,26]. In multiple-EMLs WOLEDs, carrier recombination zone shift in different EMLs also leads to a huge EL spectra change with the increase of operating voltage [20,27,28]. Indeed, the introduction of carrier or exciton blocking layer can effectively improve the color stability of multiple-EMLs WOLEDs, but such device structures generally cause a decrease in device efficiency [23,27].…”

“…Tandem OLEDs, with two or more individual EL units vertically stacked and electrically connected using charge generation unit (CGU), can effectively suppress the abovedefined issues in the single and multiple-EMLs WOLEDs [20,[29][30][31][32][33]. On the one hand, tandem OLEDs possess a thicker thickness of organic layers and exhibit a lower current density reaching the same luminance relative to the single emitting unit devices, which contribute to a substantially improved device lifetime [29,30].…”