Microcavity Resonance Design for a Near-Infrared Phototherapy OLED
Cell proliferation effect of deep-penetrating microcavity tandem NIR OLEDs with therapeutic trend analysis
Authors: Yongjin Park, Hye-Ryung Choi, Yongmin Jeon, Hyuncheol Kim, Jung Won Shin, Chang-Hun Huh, Kyoung-Chan Park, and Kyung-Cheol Choi
Journal: Scientific Reports 12, 10935 (2022) · Comparison target: Figure 1c and Figure 3
Skin scatters light above 700 nm more weakly, so the same optical power penetrates deeper — which is why a phototherapy OLED pushes its emission peak into the near infrared. The resonance wavelength is set by transport-layer thicknesses of a few tens of nanometres: ten-odd extra nanometres of hole-transport layer move the resonance from 730 nm to 770 nm.
This case builds three devices — non-cavity single, non-cavity tandem, and microcavity device A — checks the non-cavity emission baseline, reproduces the cavity-mode thickness map of paper Figure 3 with one two-dimensional sweep, and finally verifies the peak of device A.
Each panel holds two parallel bright ridges, the first- and second-order cavity modes. As the target wavelength rises from 710 nm to 770 nm, both ridges move toward thicker transport layers.
Background
The front-direction spectrum of a microcavity OLED is the product of the emitter's intrinsic spectrum and the cavity gain, which comes jointly from the Fabry–Perot resonance between the two metal mirrors and the two-beam interference between the emission point and the cathode.
The paper uses three device types. The ITO-anode non-cavity device has no front mirror, so its front-direction lineshape is set mainly by the emitter-to-Al distance; it validates the emission-spectrum input. Replacing ITO with 30 nm Ag makes both faces mirrors, the resonance condition becomes a function of the hole- and electron-transport-layer thicknesses, and the front spectrum collapses onto the cavity mode. Device A is the single-unit microcavity device, measured at 727 nm.
Structure
The three stacks run from the glass substrate side. The 730 nm optical inputs are the interpolated values actually used.
| Layer | Non-cavity single | Non-cavity tandem | Microcavity device A | Optical input at 730 nm |
|---|---|---|---|---|
| Glass substrate | 1 mm | 1 mm | 1 mm | , , treated as incoherent |
| ITO anode | 150 nm | 150 nm | — | Matching entry in the built-in database |
| Ag semi-transparent anode | — | — | 30 nm | , |
| MoO₃ hole-injection layer | 10 nm | 10 nm | 10 nm | , |
| NPB hole-transport layer | 40 nm | 40 nm | 90 nm | , |
| Bebq₂:Ir(piq)₃ emissive layer | 30 nm | 30 nm | 30 nm | Alq₃ proxy, , |
| Alq₃ or Alq₃:LiH electron-transport layer | 20 nm | 20 nm | 40 nm | , |
| MoO₃ charge-generation layer | — | 10 nm | — | , |
| NPB hole-transport layer, second unit | — | 40 nm | — | , |
| Bebq₂:Ir(piq)₃ second emissive layer | — | 30 nm | — | Alq₃ proxy, , |
| Alq₃:LiH electron-transport layer, second unit | — | 20 nm | — | , |
| Liq electron-injection layer | 1 nm | 1 nm | 1 nm | , |
| Al cathode | 100 nm | 100 nm | 100 nm | , |
All thicknesses come from the paper's Methods, with four supplied here: the glass substrate is taken as 1 mm and treated as incoherent; ITO is taken as 150 nm; Bebq₂ has no published optical data and is proxied by Alq₃, another metal-quinoline chelate with in the red and near infrared; and Alq₃ doped with 2 wt% LiH is treated as optically identical to Alq₃.
The non-cavity single device, with 1 mm glass flagged incoherent in the first row:
The tandem device adds four layers between the first unit's electron-transport layer and Liq, with an emitter in each of the two emissive layers:
Microcavity device A replaces ITO with 30 nm Ag and thickens NPB to 90 nm and Alq₃ to 40 nm:
Optical and Emitter Settings
All three models share the same emitter and detector settings and differ only in structure.
| Setting | Value |
|---|---|
| Emission point | Centre of the emissive layer, Delta distribution |
| Emission spectrum | Digitized Ir(piq)₃ thin-film photoluminescence from paper Figure S1, peak 631.5 nm |
| Spectrum Unit | Power |
| Dipole orientation | Isotropic (not published; only horizontal dipoles contribute at 0°, so the value does not affect the normalized spectrum) |
| Quantum efficiency, conversion efficiency, multiplier | All 1 |
| Relative strength of the two tandem emission units | Equal weight (not published) |
| Detectors | Emission Intensity + Propagation R |
| Wavelength sampling | Sweep, 400–800 nm, step 2 nm |
| Emission angle | Single, 0° |
The comparison quantities are the Normalized Spectrum view and the Peak Wavelength and Envelope FWHM derived from it.
The Figure 3 thickness map is generated with Sweep on the device A stack: NPB thickness 20–310 nm in 5 nm steps and Alq₃ thickness stepped across the values being compared, 472 thickness combinations across two jobs.
Run for individual device spectra and Sweep for the two-dimensional thickness map. It does not use Optimize.Simulation Results and Comparison with Figure 1c and Figure 3
Non-cavity emission baseline
Paper Figure 1c gives the measured spectra of the non-cavity single and tandem devices together with the authors' own simulation:
All three curves on one set of axes:
The raw results in the app. Non-cavity single device:
Non-cavity tandem device, with a clearly raised 670–720 nm shoulder:
| Quantity | Paper measurement | Paper simulation | Dreapex TMM | vs. paper measurement |
|---|---|---|---|---|
| Single-unit peak | 630.1 nm | 623.4 nm | 630.7 nm | +0.5 nm |
| Single-unit envelope FWHM | 88.0 nm | 73.0 nm | 81.4 nm | −6.6 nm |
| Tandem peak | 626.4 nm | 624.7 nm | 632.4 nm | +6.0 nm |
| Tandem envelope FWHM | 102.6 nm | 97.0 nm | 98.9 nm | −3.7 nm |
Point-by-point RMS lineshape differences over 580–778 nm: the single device is 7.8 % from the paper measurement and 3.6 % from the paper simulation; the tandem device is 4.1 % and 5.3 %. For reference, the paper's own simulation sits 9.6 % and 3.1 % from its measurements.
Cavity-mode thickness map
90 nm NPB, 40 nm Alq₃, and a measured peak of 727 nm, and in the 730 nm panel the first-order ridge on the Alq₃ = 40 nm row sits at 89.4 nm. Reading it as MoO₃ plus NPB introduces a 10 nm systematic offset.The published figure:
Taking the 80 nm electron-transport-layer row as a profile and overlaying the sweep:
| Cavity order | Points compared | Mean difference | RMS difference | Maximum difference |
|---|---|---|---|---|
| First | 16 | +0.16 nm | 1.56 nm | 3.20 nm |
| Second | 20 | +0.47 nm | 1.62 nm | 3.70 nm |
| Total | 36 | +0.33 nm | 1.59 nm | 3.70 nm |
Across 36 extrema the largest difference is 3.70 nm, below the 10 nm grid of the paper's own data cursor. For an 80 nm electron-transport layer the first-to-second ridge spacings are 202.1, 209.5, 215.2, and 222.5 nm at 710, 730, 750, and 770 nm; the half-wave optical-path condition gives 203.6, 209.3, 215.0, and 220.8 nm, a maximum difference of 1.7 nm.
Peak of microcavity device A
With 90 nm NPB and 40 nm Alq₃ entered, one Run gives:
| Quantity | Paper | Dreapex TMM | Difference |
|---|---|---|---|
| Electroluminescence peak | 727 nm | 726.84 nm | 0.16 nm |
| Envelope FWHM | 52 nm | 69.1 nm | +17.1 nm |
The peak lands 0.16 nm from the published value. The FWHM is recorded only: the paper's 52 nm is a measurement at 30 mA/cm².
Deviation Notes
Alq₃ proxies for Bebq₂, which has no published optical data. The peak's sensitivity to that index is about +87 nm per unit of refractive index; since the emissive layer is only 30 nm thick, the proxy shifts the cavity ridges by about 0.9 nm, below the measured 1.59 nm RMS difference.
The non-cavity tandem peak sits 6.0 nm high. The paper's measurement shows the tandem device blue-shifted by 3.8 nm relative to the single unit, while the paper's own optical model gives a 1.4 nm red shift and this run gives 1.7 nm — two independent transfer-matrix calculations agreeing to 0.3 nm and both opposite in sign to that measured number. The paper publishes neither the relative strength of the two emission units nor the actual recombination positions, and both change the peak of the combined spectrum.
The ITO thickness is an assumption supplied here; across 80–220 nm the single-unit peak moves by only 3.9 nm, and neither the microcavity device nor the Figure 3 comparison contains ITO.