Correlated Disorder Substrate‐Integrated Nanodisk Scatterers for Light Extraction in Organic Light Emitting Diodes

A major loss mechanism in organic light emitting diodes (OLEDs) is the coupling of the emitter molecule light field to waveguide modes in the OLED thin film stack. In this work, a disordered 2D array of TiO$_{2}$ nanodisk scatterers is integrated into the OLED substrate to enable efficient light extraction from these waveguide modes. Fabrication of the nanodisks is based on a bottom-up, colloidal lithography technique and subsequent pattern transfer into high refractive index TiO$_{2}$ via reactive ion etching. The substrates are completed by spin-coating a polymer planarization layer before applying the OLED thin film stack. This ensures reproducible optoelectronic properties of the OLED through leaving the electrically active layers planar. Simultaneously, the nanodisks in close vicinity to the thin film stack ensure efficient out-of-plane scattering of waveguide modes. In a monochromatic OLED (center wavelength λ$_{0}$ = 520 nm), a 44.2%$_{rel}$ increase in external quantum efficiency is achieved in comparison to a device without scattering structure. An in-depth numerical analysis reveals that this significant enhancement is only partly due to the out-coupling of waveguide modes. Additional enhancement is suspected to result from out-coupling of substrate modes through scattering by the nanodisks. Further improvements to the scattering structure are numerically evaluated

Tailored Light Scattering through Hyperuniform Disorder in Self-Organized Arrays of High-Index Nanodisks

Arrays of nanoparticles exploited in light scattering applications commonly only feature either a periodic or a rather random arrangement of its constituents. For the periodic case, light scattering is mostly governed by the strong spatial correlations of the arrangement, expressed by the structure factor. For the random case, structural correlations cancel each other out and light scattering is mostly governed by the scattering properties of the individual scatterer, expressed by the form factor. In contrast to these extreme cases, it is shown here that hyperuniform disorder in self-organized large-area arrays of high refractive index nanodisks enables both structure and form factor to impact the resulting scattering pattern, offering novel means to tailor light scattering. The scattering response from the authors’ nearly hyperuniform interfaces can be exploited in a large variety of applications and constitutes a novel class of advanced optical materials

Antireflective Huygens’ Metasurface with Correlated Disorder Made from High-Index Disks Implemented into Silicon Heterojunction Solar Cells

A large variety of different strategies has been proposed as alternatives to random textures to improve light coupling into solar cells. While the understanding of dedicated nanophotonic systems deepens continuously, only a few of the proposed designs are industrially accepted due to a lack of scalability. In this Article, a tailored disordered arrangement of high-index dielectric submicron-sized titanium dioxide (TiO$_{2}$) disks is experimentally exploited as an antireflective Huygens’ metasurface for standard heterojunction silicon solar cells. The disordered array is fabricated using a scalable bottom-up technique based on colloidal self-assembly that is applicable virtually irrespective of material or surface morphology of the device. We observe a broadband reduction of reflectance resulting in a relative improvement of a short-circuit current by 5.1% compared to a reference cell with an optimized flat antireflective indium tin oxide (ITO) layer. A theoretical model based on Born’s first approximation is proposed that links the current increase in the arrangement of disks expressed in terms of the structure factor S(q) of the disk array. Additionally, we discuss the optical performance of the metasurface within the framework of helicity preservation, which can be achieved at specific wavelengths for an isolated disk for illumination along the symmetry axis by tuning its dimensions. By comparison to a simulated periodic metasurface, we show that this framework is applicable in the case of the structure factor approaching zero and the disks’ arrangement becoming stealthy hyperuniform

Increasing the efficiency of solar modules by femtosecond laser written blazed phase gratings in the volume of soda lime glass

We inscribed phase gratings into the cover glass of a solar module using femtosecond laser pulses to guide light around the front side metallization. Photocurrent and efficiency of the module increased by 1 % (relative)

Passivation of Optically Black Silicon by Atomic Layer Deposited Al2O3

AbstractOptically black silicon nanostructures show excellent anti-reflection and light trapping properties minimizing reflection losses to less than 1.6% between 300 – 1100nm. Our light-trapping scheme enables an absorption enhancement factor of ∼10 at the band edge of silicon (1150nm) as compared to a simulated perfect ARC, where the Yablonovitch limit corresponds to a factor of 15. Just recently it was shown that similar wet-chemically black etched silicon surfaces can be exploited to fabricate high efficiency solar cells [1]. Towards the integration of our structures into a solar cell device, the passivation performance of atomic layer deposited thin Al2O3 films is investigated on a variety of black etched structures. The coatings lead to measured surface recombination velocities of less than 13cm/s on bifacially black structured as well as 12cm/s on polished 1 – 5Ωcm p-type Si CZ wafers. Thinner layers promise to be even more effective. This technology will enable high efficiencies on various solar cell concepts

Thermodynamics of a nanowire solar cell: Towards the ultimate limit

A lossless solar cell operating at the Shockley-Queisser (S-Q) limit generates an open-circuit voltage (VOC) equal to the radiative limit. At VOC, the highly directional beam of photons from the sun is absorbed and subsequently externally re-emitted into a 4p solid angle, providing a large photon entropy loss. Moreover, due to many total internal reflections and low internal radiative efficiency, a lot of light is lost in nonradiative recombination events. In our research, we perform a nanophotonic optimization of a semiconductor nanowire geometry with a top microlens in order to decrease the photon entropy loss and to increase the photon escape probability for the nanowire, therefore increasing the output voltage. The optimization leads us to a maximum VOC of 1178 mV which is 141 mV higher than the radiative limit and 172 mV lower than the ultimate limit. The photon entropy loss is also studied fundamentally from the thermodynamics point of view to better understand where the entropy is generated during the absorption-emission processes