Munib Wober

In the last years, the use of multispectral images in different applications has been increasing due to access to industrial multispectral cameras that can analyze the composition of different materials. In another scenario, the use of high-dynamic-range (HDR) imaging is not only related to different exposure times for analyzing the light but also to produce pretty pictures thanks to the access to portable cameras with HDR capabilities, like in the smartphones. In this work, we present a first approach to produce HDR images using nine spectral images acquired using a novel prototype of a multispectral camera using nanowires sensors. The methods analyzed were multiple linear regression, support vector machines, and the k-nearest neighbors algorithm. The approach is tested using a Macbeth chart of 24 colors and real, natural objects.

In the last years, the use of multispectral cameras have been increasing due to their ability to analyze different characteristics of materials. Thus, multiples methods to develop cameras with multispectral filters arrays have been studied. These cameras must always have the option to combine the multispectral information to create a regular color image. In this work, we present a method to estimate the regular colors from a novel prototype of a multispectral camera that been developed using nanowires sensors. Real information from nine frequencies are analyzed using classification and regression methods.

Flexible crystalline silicon radial junction photovoltaics with tapered microwires are presented, which simultaneously show high flexibility and efficiency.

Here, a novel microgrid top electrode for highly efficient radial‐junction Si microwire solar cells is demonstrated. The microgrid electrode minimizes optical and electrical losses, thus ensuring proper function of the shallow (sheet resistance of ≈100 Ω sq⁻¹) junction emitter. This leads to effective collection of the photocarriers from the shallow junction emitter through the top electrode without severe Auger/surface recombination, improving the overall power conversion efficiency of the Si microwire solar cell. With an optimized microgrid structure, 1 cm² microwire solar cells show a conversion efficiency of up to 16.5%, with an open‐circuit voltage of 565.2 mV and a short‐circuit current density of 35.9 mA·cm⁻²; this conversion efficiency is 72% higher than that of solar cells with an edge electrode (9.6%). Further, an ≈1 μm thick Ni electrode that is formed by electroplating considerably reduces the metal and contact resistances, which reproducibly yields a fill factor of over 80% (max 81.2%). Thus, the use of a novel microgrid to construct an ideal metal/emitter interface presents a unique opportunity to develop highly efficient microwire solar cells.

A systematic study was conducted into the use of metal-assisted chemical etching (MacEtch) to fabricate vertical Si microwire arrays, with several models being studied for the efficient redox reaction of reactants with silicon through a metal catalyst by varying such parameters as the thickness and morphology of the metal film. By optimizing the MacEtch conditions, high-quality vertical Si microwires were successfully fabricated with lengths of up to 23.2 μm, which, when applied in a solar cell, achieved a conversion efficiency of up to 13.0%. These solar cells also exhibited an open-circuit voltage of 547.7 mV, a short-circuit current density of 33.2 mA/cm² and a fill factor of 71.3% by virtue of the enhanced light absorption and effective carrier collection provided by the Si microwires. The use of MacEtch to fabricate high-quality Si microwires therefore presents a unique opportunity to develop cost-effective and highly efficient solar cells.

The current trend toward image sensors with ever-increasing pixel counts is prompting continual reductions in pixel area, leading to significant cross-talk and efficiency challenges. The realization of image sensor pixels containing waveguides presents a means for addressing these issues. The fabrication of such pixels is however not straightforward. Conventional waveguides employed in integrated optics are horizontal, but waveguides needed for the proposed sensor must be vertical and integrated with photodetectors. Here, the authors describe a fabrication process for vertical silicon nitride waveguides integrated with silicon photodetectors. The authors describe the etching, deposition, and planarization techniques that enable the formation of silicon nitride waveguides embedded in silicon dioxide. They also describe a fabrication process for silicon photodetectors, including a means for ensuring that their photosensitive areas have sizes consistent with those of photodetectors employed in conventional image sensors. In addition, the authors perform optical and electrical characterization of the fabricated devices. The results demonstrate the ability of the fabricated waveguides to guide light onto the photodetectors with high efficiency.

The organic dye filters of conventional color image sensors achieve the red/green/blue response needed for color imaging, but have disadvantages related to durability, low absorption coefficient, and fabrication complexity. Here, we report a new paradigm for color imaging based on all-silicon nanowire devices and no filters. We fabricate pixels consisting of vertical silicon nanowires with integrated photodetectors, demonstrate that their spectral sensitivities are governed by nanowire radius, and perform color imaging. Our approach is conceptually different from filter-based methods, as absorbed light is converted to photocurrent, ultimately presenting the opportunity for very high photon efficiency.

Silicon microwire arrays have attracted considerable attention recently due to the opportunity they present as highly efficient and cost-effective solar cells. In this study, we report on efficient Si microwire array solar cells with areas of 1 cm² and Air Mass 1.5 Global conversion efficiencies of up to 10.6%. These solar cells show an open-circuit voltage of 0.56 V, a short-circuit current density of 25.2 mA/cm², and a fill factor of 75.2%, with a silicon absorption region that is only 25 μm thick. In particular, the maximum overall efficiency of the champion device is improved from 8.71% to 10.6% by conformally coating the wires with a 200 nm thick SiO₂ layer. Optical measurements reveal that the layer reduces reflection significantly over the entire visible range.

We demonstrate that vertical silicon nanowires function as photodetectors whose spectral sensitivities are controlled by choice of nanowire radius. The measured external quantum efficiencies of fabricated devices are compared to electromagnetic simulations.