1,720,997 research outputs found
Sub-10nm patterning by focused He-ion beam milling for fabrication of downscaled graphene nano devices
No full textDopant profiling based on scanning electron and helium ion microscopy
Full text linkIn this paper, we evaluate and compare doping contrast generated inside the scanning electron microscope (SEM) and scanning helium ion microscope (SHIM). Specialised energy-filtering techniques are often required to produce strong doping contrast to map donor distributions using the secondary electron (SE) signal in the SEM. However, strong doping contrast can be obtained from n-type regions in the SHIM, even without energy-filtering. This SHIM technique is more sensitive than the SEM to donor density changes above its sensitivity threshold, i.e. 1016 or 1017 donors cm-3 respectively on specimens with or without a p-n junction; its sensitivity limit is well above 2×1017 acceptors cm-3 on specimens with or without a p-n junction. Good correlation is found between the widths and slopes of experimentally measured doping contrast profiles of thin p-layers and the calculated widths and slopes of the potential energy distributions across these layers, at a depth of 1–3 nm and 5–10 nm below the surface in the SHIM and the SEM respectively. This is consistent with the mean escape depth of SEs in Si being about 1.8 nm and 7 nm in the SHIM and SEM respectively, and we conclude that short escape depth, low energy SE signals are most suitable for donor profiling
Stretchable and flexible crystalline silicon photovoltaic modules embodying an auxetic rotating-square structure for adjustable transmittance
No full textThis work describes the segmentation of commercial crystalline silicon solar cells into smaller sections and their subsequent restructuring into interconnected arrays, based on an auxetic rotating-square architecture, to produce a lightweight, flexible and stretchable solar module. As expected, the sectioning of the solar cells reduces their power conversion efficiency due to increased carrier recombination at the sawn edges. However, average cell section efficiencies are shown to be less than 1.8% lower than the original cells. Output voltage and current can be tailored according to the combination of series or parallel connections between solar cell sections in the design. Due to the negative Poisson's ratio of the auxetic structure, bidirectional expansion with uniaxial stretching is achieved, opening gaps in the module, which allows the light transmittance to be adjusted. Mechanical tests reveal that the structures are robust to repeated cycles of expansion and relaxation, aided by the joint rotation mechanism of expansion that avoids excessive strain on the joint material. The modules are fully expanded when each cell section is rotated by 45°. In this expanded state, modules made of 31.75 mm × 31.75 mm solar cell sections have a strain of 67% and transmittance of 41.9%. Modules incorporating the smaller 20 mm × 20 mm cell sections have a maximum strain of 60%, with a corresponding transmittance of 49.5%. A geometric model is used to show that by varying the design parameters, the transmittance maximum, minimum and range can be tuned, opening up various potential applications that include BIPV (e.g., partially shaded windows), AgriPV (e.g., greenhouse roofs), portable PV devices and wearables
Optical modelling of black silicon for solar cells using effective index techniques
No full textTexturing the surface with both micro and nano scale features to form black silicon is a promising approach in improving solar cell efficiency. In optical modeling of such a surface, it is difficult to balance the accuracy and computational resource. In this work, we develop on a semianalytical model, effective index technique (EIT), which utilizes a finite-difference time domain (FDTD) method to represent the nanoscale texturing as an effective medium, and then apply this to microscale structures, which can then be modeled using the transfer matrix method and ray-tracing. We fabricate and model both periodic and random nanoscale textures, and analyze the accuracy of several effective index models against measured reflectivity. The limitations in the model are identified and coherency of the films is studied. The semianalytical method is shown to perform better than the other effective medium approaches for modeling black silicon and is applicable to modeling multiscale textures, whereas full numerical methods such as FDTD are not. However, although the EIT approach predicts the trends in antireflective performance of a texture, it remains inaccurate when compared with the experiment. Also, as with all effective medium approaches, the EIT does not account for light trapping through scattering
Helium ion microscopy and energy selective scanning electron microscopy – two advanced microscopy techniques with complementary applications
No full textBoth scanning electron microscopes (SEM) and helium ion microscopes (HeIM) are based on the same principle of a charged particle beam scanning across the surface and generating secondary electrons (SEs) to form images. However, there is a pronounced difference in the energy spectra of the emitted secondary electrons emitted as result of electron or helium ion impact. We have previously presented evidence that this also translates to differences in the information depth through the analysis of dopant contrast in doped silicon structures in both SEM and HeIM. Here, it is now shown how secondary electron emission spectra (SES) and their relation to depth of origin of SE can be experimentally exploited through the use of energy filtering (EF) in low voltage SEM (LV-SEM) to access bulk information from surfaces covered by damage or contamination layers. From the current understanding of the SES in HeIM it is not expected that EF will be as effective in HeIM but an alternative that can be used for some materials to access bulk information is presented
Development of epitaxial interdigitated back contact (IBC) solar cell as a test platform for novel antireflection and light-trapping schemes
No full textLight capture
No full textThe efficient capture of light is an essential factor for consideration in all solar cell designs. This chapter explores antireflective and light trapping schemes designed to reduce optical losses in solar cells with the aim of improving device efficiency. After a survey of the different mechanisms available for antireflection and light trapping, the various schemes employing these mechanisms are described. This begins with the traditional methods of thin film antireflective coatings and large (micron) scale texturing before moving onto more recent developments in the use of subwavelength texturing, taking inspiration from natural ‘moth-eye’ antireflective surfaces. Finally, the rapidly emerging field of plasmonics for photovoltaics is explored in which metal nanoparticles scatter incoming light through the generation of localized surface plasmons. In each section, the simulation techniques used for design optimization are introduced and methods for experimental realization and implementation in a range of photovoltaic devices are described. The associated increases in cost and complexity conferred to the solar cell fabrication process are also considered because these are the main hindrances to wide scale adoption of new strategies of light captur
Proximity effect quantification and dose optimisation for high resolution helium ion beam lithography
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