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Optical losses
Optical losses
In the bottom emission QLEDs, the light escapes through the glass substrate. There are many lossy modes in the light extraction such as waveguide and substrate modes (due to total internal reflection from refractive index difference), absorption, reabsorption, and surface plasmon polariton modes. Due to these losses only a small amount of light actually exits the device.
One of the largest lossy mode is the substrate mode. One way to avoid the substrate mode is to make top emission QLEDs, also preferred by industry. In top emission the light does not enter the substrate and leaves through the top semi-transparent electrode.
Transparent conducting electrode (TCE)
Transparent conducting electrode (TCE)
While moving to top emission reduces the substrate modes, it introduces another challenge: the requirement of a high TCE. It is a challenge especially in the SWIR region. As the top electrode sits on the QD functional layers, it should be a soft deposition at room temperature. By doing a low-power sputtering, a high-transmission ITO was developed to use as a top electrode.
Microcavity effect: Sandwiching functional layers between a highly reflective bottom electrode and a semi-transparent top electrode, creates a nanoscale Fabry-Pérot resonator. This optical confinement, called ”the microcavity effect”, fundamentally enhances device performance. It selectively amplifies resonant wavelengths to achieve exceptional colour purity, concentrates emitted light in the forward direction to significantly boost apparent brightness, and accelerates the spontaneous emission rate of the quantum dots (the Purcell effect) to improve overall efficiency.
In order to validate the microcavity effect, we studied two different architectures: one with just transparent TCE and the other with a semitransparent TCE (ITO/Ag/ITO-IAI) to have light feedback into the cavity. I performed Transfer Method Matrix simulations with the help of AI to study the device structures. The structure with transparent TCE alone has a weak cavity compared to the device structure with optical feedback. The device structure with IAI TCE was chosen in the low Q-factor regime so that there is a good tolerance factor during the fabrication.
By using the IAI based TCE and taking advantage of the microcavity effect, I achieved a very bright PbS based top emission QLEDs in the SWIR region. These are the first demonstrated top emission in the SWIR region with radiance above 100 W·sr⁻¹·m⁻².
Current density vs Radiance
High radiance for TQLEDs with optical feedback compared to transparent electrode at a given current density means enhanced light outcoupling.
Normalised EL
As the cavity is tuned for particular wavelength, the other wavelengths get suppressed, thereby narrowing the emission wavelength.
Emission profile
The microcavity effect gives directional emission incontrast to Lambertian emission in the bottom emission.
Demonstration
Demonstration
The bright TQLEDs were used for first demonstrations of SWIR imaging with QLEDs at practical distances with light path of 0.5 m. We can see-through a silicon substrate in the SWIR imaging where the whole scene was illuminated only by TQLED.
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