Nanostructuring for the purpose of reflectance reduction has been widely investigated for Silicon based solar applications.
Bare Silicon surfaces reflect between 50 and 60 % of the incident light and are thus unsuitable for absorbing significant
amounts of sunlight. A typical approach to addressing this is to use an anti-reflective coating on top of the Silicon which
reduces reflectance via destructive interference. Since this interference is mainly dependent on the thickness of film this
type of anti-reflection layer can only be optimized for a certain wavelength and thus is inherently limited. To reduce the
reflectance over a broad range of wavelengths a structuring based approach is necessary. A common approach to
implementing this is by wet etching the top surface of a crystalline solar cell to create pyramid structures based on the
crystalline dependence of the etching process. Since this approach exploits the crystalline structure it is most suited for
crystalline Si. Dry etching based nanostructuring can offer a high level of control over the resulting structure with the
crystalline dependence being less concern. One approach is to etch cylindrical holes arranged in a periodic fashion into the
top surface of the device to create a photonic crystal lattice. Here we present a systematic analysis of a photonic crystal
slabs in Silicon and how the geometry affect the reflectance of the device. Lumerical’s FDTD solution is used to vary the
pitch, diameter and depth of the cylindrical holes making up the Photonic Crystal structure. The analysis reveals that air
fill fraction and hole depth are the most significant determinants of the overall reflectance.
Perovskite nanocrystals of the form FAPbBr3 display significant promise in the field of optoelectronics. In particular, these nanocrystals could bridge the `green gap' of LED technology, and also serve to down-convert ultraviolet light for harvesting using silicon-based photovoltaic cells. To remain competitive with traditional devices, optimising the energy transfer between the nanocrystal and the device is crucial, however very little investigation has been performed into this subject.
Here, we characterise the energy transfer dynamics of FAPbBr3 nanocrystals on a silicon substrate using time resolved photoluminescence. We also use deposited `spacer layers' to vary the displacement of the nanocrystals from the silicon in order to observe the effect on the energy-transfer dynamics. We find that the overall photo luminescent lifetime increases when reducing the distance between between the nanocrystals and the silicon layer, which runs counter to the expected behaviour. This suggests that the presence of an optically-active substrate suppresses photo luminescent lifetime and, further, suggests that nanocrystal-to-nanocrystal transfer is highly efficient.
In this work, we probe the photodegradative behaviour of CsPbBr3 perovskite nanocrystals under illumination intensities in excess of 1 W=cm2. In doing so, we uncover optical behaviours unique to this extreme form of degradation namely a pronounced period of increasing photoluminescent intensity at the outset of degradation along with a red-shifted emission lobe.
We also compare the photochemical lifetimes of CsPbBr3 to the relating organic-inorganic hybrid of FAPbBr3 and show that FAPbBr3 can withstand such high intensities for approximately ten times longer than CsPbBr3. This marks out FAPbBr3 as a potential successor to CsPbBr3 in optoelectronic applications.
LED surface structuring has been widely used to increase light extraction[1]. Due to the high refractive index of the thick GaN epitaxy layers, most emitted light becomes trapped and reabsorbed by the epitaxial layers. While random structuring can effectively scatter trapped light out of the LED, it gives little control over the resulting beam-shapef[2]. Photonic crystals however provide a means to simultaneously improve light extraction efficiency and control beam directionality. Furthermore, P-side up LEDs normally utilize a transparent top contact layer in order to allow top light emission whilst maintaining good electrical properties. In this paper we investigate a novel photonic crystal LED configuration with a nontransparent metal top contact layer, and cylindrical holes etched through the top contact layer and deep into the underlying epitaxy. In this novel configuration light emission is only possible from the etched holes giving rise to extreme beam steering effects. We utilize broadband spectroscopic reflectometry to experimentally investigate beam shape and optical properties from fabricated devices. We observe a range of achievable beam patterns with extreme deviations from the normal Lambertian. We investigate the effect of square and triangular photonic crystal lattices on beam directionality.
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