Non-equilibrium quantum domain reconfiguration dynamics in a two-dimensional electronic crystal and a quantum annealer.

Relaxation dynamics of complex many-body quantum systems trapped into metastable states is a very active field of research from both the theoretical and experimental point of view with implications in a wide array of topics from macroscopic quantum tunnelling and nucleosynthesis to non-equilibrium superconductivity and energy-efficient memory devices. In this work, we investigate quantum domain reconfiguration dynamics in the electronic superlattice of a quantum material using time-resolved scanning tunneling microscopy and unveil a crossover from temperature to noisy quantum fluctuation dominated dynamics. The process is modeled using a programmable superconducting quantum annealer in which qubit interconnections correspond directly to the microscopic interactions between electrons in the quantum material. Crucially, the dynamics of both the experiment and quantum simulation is driven by spectrally similar pink noise. We find that the simulations reproduce the emergent time evolution and temperature dependence of the experimentally observed electronic domain dynamics.

Manipulation of fractionalized charge in the metastable topologically entangled state of a doped Wigner crystal.

Metastability of many-body quantum states is rare and still poorly understood. An exceptional example is the low-temperature metallic state of the layered dichalcogenide 1T-TaS2 in which electronic order is frozen after external excitation. Here we visualize the microscopic dynamics of injected charges in the metastable state using a multiple-tip scanning tunnelling microscope. We observe non-thermal formation of a metastable network of dislocations interconnected by domain walls, that leads to macroscopic robustness of the state to external thermal perturbations, such as small applied currents. With higher currents, we observe annihilation of dislocations following topological rules, accompanied with a change of macroscopic electrical resistance. Modelling carrier injection into a Wigner crystal reveals the origin of formation of fractionalized, topologically entangled networks, which defines the spatial fabric through which single particle excitations propagate. The possibility of manipulating topological entanglement of such networks suggests the way forward in the search for elusive metastable states in quantum many body systems.

Analysis and optimization of light outcoupling in OLEDs with external hierarchical textures.

Hierarchical textures (combining 2D periodic large and small micro textures) as an external outcoupling solution for OLEDs have been researched, both experimentally and by optical simulations. For the case of a red bottom emitting OLED, different hierarchical textures were fabricated using laser-based methods and a replication step and applied to the OLED substrate, resulting in an increased light outcoupling. Laboratory-size OLED devices with applied textured foils show a smaller increase in efficiency compared to the final large area devices. The results show that the full exploitation of textured foils in laboratory-size samples is mainly limited by the lateral size of the thin film stack area and by limited light collection area of the measuring equipment. Modeling and simulations are used to further evaluate the full prospective of hierarchical textures in large area OLED devices. Optimization of hierarchical textures is done by simultaneously changing the aspect ratios of the small and large textures and a potential of 57% improvement in EQE compared to devices without applied textures is predicted by simulations. Optimized hierarchical textures show similar outcoupling efficiencies compared to optimized single textures, while on the other hand hierarchical textures require less pronounced features, lower aspect ratios, compared to single textures to achieve the same efficiencies. Hierarchical textures also help in eliminating flat parts that limit outcoupling efficiency. Finally, the limiting factors that prevent higher outcoupling are addressed. We show that the dominant factor is non-ideal reflection from the organic thin film stack due to parasitic absorption. In addition, possible ways to further increase the outcoupling from a thick substrate are indicated.

Coupled modelling approach for optimization of bifacial silicon heterojunction solar cells with multi-scale interface textures.

For advanced optical analysis and optimization of solar cell structures with multi-scale interface textures, we applied a coupled modelling approach (CMA), where we couple the rigorous coupled wave analysis method with ray tracing and transfer matrix method. Coupling of the methods enables accurate optical analysis of solar cells made of thin coherent and thick incoherent layers and includes combinations of nano- and micro-scale textures at various positions in the structure. The approach is experimentally validated on standalone single- and both-side textured crystalline silicon wafers, as well as on complete silicon heterojunction (Si HJ) solar cell structures. Using CMA, fully encapsulated bifacial Si HJ solar cells are optically simulated first by applying single- and both-side illumination, and the effects of introducing nano inverted pyramids and random micro-pyramids at front and/or rear interfaces are analyzed. Secondly, an external light management foil with a three-sided pyramidal micro-texture is applied in simulations to the front and/or rear encapsulation glass, and the related improvements are quantified. For the optimal combination of internal textures in the analyzed structure (random micro-pyramids at the front and nano inverted pyramids at the back) and the use of the light management foil on both sides of the device, a 5.6% gain in the short-circuit current is predicted, compared to the reference case with no light management foil and with random micro-pyramids applied to the front and rear internal interfaces.

Performance analysis of rigorous coupled-wave analysis and its integration in a coupled modeling approach for optical simulation of complete heterojunction silicon solar cells.

A variety of light management structures have been introduced in solar cells to improve light harvesting and further boost their conversion efficiency. Reliable and accurate simulation tools are required to design and optimize the individual structures and complete devices. In the first part of this paper, we analyze the performance of rigorous coupled-wave analysis (RCWA) for accurate three-dimensional optical simulation of solar cells, in particular heterojunction silicon (HJ Si) solar cells. The structure of HJ Si solar cells consists of thin and thick layers, and additionally, micro- and nano-textures are also introduced to further exploit the potential of light trapping. The RCWA was tested on the front substructure of the solar cell, including the texture, thin passivation and contact layers. Inverted pyramidal textures of different sizes were included in the simulations. The simulations rapidly converge as long as the textures are small (in the (sub)micrometer range), while for larger microscale textures (feature sizes of a few micrometers), this is not the case. Small textures were optimized to decrease the reflectance, and consequently, increase the absorption in the active layers of the solar cell. Decreasing the flat parts of the texture was shown to improve performance. For simulations of structures with microtextures, and for simulations of complete HJ Si cells, we propose a coupled modeling approach (CMA), where the RCWA is coupled with raytracing and the transfer matrix method. By means of CMA and nanotexture optimization, we show the possible benefits of nanotextures at the front interface of HJ Si solar cells, demonstrating a 13.4% improvement in the short-circuit current density with respect to the flat cell and 1.4% with respect to the cell with double-sided random micropyramids. We additionally demonstrate the ability to simulate a combination of nano- and microtextures at a single interface, although the considered structure did not show an improvement over the pyramidal textures.

Detailed optical modelling and light-management of thin-film organic solar cells with consideration of small-area effects.

We present detailed numerical and experimental investigation of thin-film organic solar cells with a micro-textured light management foil applied on top of the front glass substrate. We first demonstrate that measurements of small-area laboratory solar cells are susceptible to a significant amount of optical losses that could lead to false interpretation of the measurement results. Using the combined optical model CROWM calibrated with realistic optical properties of organic films and other layers, we identify the origins of these losses and quantify the extent of their influence. Further on, we identify the most important light management mechanisms of the micro-textured foil, among which the prevention of light escaping at the front side of the cell is revealed as the dominant one. Detailed three-dimensional simulations show that the light-management foil applied on top of a large-area organic solar cell can reduce the total reflection losses by nearly 60% and improve the short-circuit current density by almost 20%. Finally, by assuming realistic open-circuit voltage and especially the realistic fill factor that deteriorates as the absorber layer thickness is increased, we determine the optimal absorber layer thickness that would result in the highest power conversion efficiency of the investigated organic solar cells.

Optical model for simulation and optimization of luminescent down-shifting layers filled with phosphor particles for photovoltaics.

We developed an optical model for simulation and optimization of luminescent down-shifting (LDS) layers for photovoltaics. These layers consist of micron-sized phosphor particles embedded in a polymer binder. The model is based on ray tracing and employs an effective approach to scattering and photoluminescence modelling. Experimental verification of the model shows that the model accurately takes all the structural parameters and material properties of the LDS layers into account, including the layer thickness, phosphor particle volume concentration, and phosphor particle size distribution. Finally, using the verified model, complete organic solar cells on glass substrate covered with the LDS layers are simulated. Simulations reveal that an optimized LDS layer can result in more than 6% larger short-circuit current of the solar cell.