The long road to universal electrification:A critical look at present pathways and challenges

Nearly 840 million people still lack access to electricity, while over a billion more have an unreliable electricity connection. In this article, the three different electrification pathways-grid extension, centralized microgrids, and standalone solar-based solutions, such as pico-solar and solar home systems (SHS)-are critically examined while understanding their relative merits and demerits. Grid extension can provide broad scale access at low levelized costs but requires a certain electricity demand threshold and population density to justify investments. To a lesser extent, centralized (off-grid) microgrids also require a minimum demand threshold and knowledge of the electricity demand. Solar-based solutions are the main focus in terms of off-grid electrification in this article, given the equatorial/tropical latitudes of the un(der-)electrified regions. In recent times, decentralized solar-based off-grid solutions, such as pico-solar and SHS, have shown the highest adoption rates and promising impetus with respect to basic lighting and electricity for powering small appliances. However, the burning question is-from lighting a million to empowering a billion-can solar home systems get us there?The two main roadblocks for SHS are discussed, and the requirements from the ideal electrification pathway are introduced. A bottom-up, interconnected SHS-based electrification pathway is proposed as the missing link among the present electrification pathways.Management SupportDC systems, Energy conversion & StorageElectrical Sustainable Energ

Solar harvesting based on perfect absorbing all-dielectric nanoresonators on a mirror

The high-index all-dielectric nanoantenna system is a platform recently used for multiple applications, from metalenses to light management. These systems usually exhibit low absorption/scattering ratios and are not efficient photon harvesters. Nevertheless, by exploiting far-field interference, all-dielectric nanostructures can be engineered to achieve near-perfect absorption in specific wavelength ranges. Here, we propose - based on electrodynamics simulations - that a metasurface composed of an array of hydrogenated amorphous silicon nanoparticles on a mirror can achieve nearly complete light absorption close to the bandgap. We apply this concept to a realistic device, predicting a boost of optical performance of thin-film solar cells made of such nanostructures. In the proposed device, high-index dielectric nanoparticles act not only as nanoatennas able to concentrate light but also as the solar cell active medium, contacted at its top and bottom by transparent electrodes. By optimization of the exact geometrical parameters, we predict a system that could achieve initial conversion efficiency values well beyond 9% - using only the equivalent of a 75-nm thick active material. The device absorption enhancement is 50% compared to an unstructured device in the 400 nm - 550 nm range and more than 300% in the 650 nm - 700 nm spectral region. We demonstrate that such large values are related to the metasurface properties and to the perfect absorption mechanism. (C) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreemen

Designing a hybrid thin-film/wafer silicon triple photovoltaic junction for solar water splitting

Solar fuels are a promising way to store solar energy seasonally. This paper proposes an earth-abundant heterostructure to split water using a photovoltaic-electrochemical device (PV-EC). The heterostructure is based on a hybrid architecture of a thin-film (TF) silicon tandem on top of a c-Si wafer (W) heterojunction solar cell (a-Si:H (TF)/nc-Si:H (TF)/c-Si(W)) The multijunction approach allows to reach enough photovoltage for water splitting, while maximizing the spectrum utilization. However, this unique approach also poses challenges, including the design of effective tunneling recombination junctions (TRJ) and the light management of the cell. Regarding the TRJs, the solar cell performance is improved by increasing the n-layer doping of the middle cell. The light management can be improved by using hydrogenated indium oxide (IOH) as transparent conductive oxide (TCO). Finally, other light management techniques such as substrate texturing or absorber bandgap engineering were applied to enhance the current density. A correlation was observed between improvements in light management by conventional surface texturing and a reduced nc-Si:H absorber material quality. The final cell developed in this work is a flat structure, using a top absorber layer consisting of a high bandgap a-Si:H. This triple junction cell achieved a PV efficiency of 10.57%, with a fill factor of 0.60, an open-circuit voltage of 2.03 V and a short-circuit current density of 8.65 mA/cm2. When this cell was connected to an IrOx/Pt electrolyser, a stable solar-to-hydrogen (STH) efficiency of 8.3% was achieved and maintained for 10 hours.</p

Efficient Water-Splitting Device Based on a Bismuth Vanadate Photoanode and Thin-Film Silicon Solar Cells

A hybrid photovoltaic/photoelectrochemical (PV/PEC) water-splitting device with a benchmark solar-to-hydrogen conversion efficiency of 5.2 % under simulated air mass (AM) 1.5 illumination is reported. This cell consists of a gradient-doped tungsten–bismuth vanadate (W:BiVO_4) photoanode and a thin-film silicon solar cell. The improvement with respect to an earlier cell that also used gradient-doped W:BiVO4 has been achieved by simultaneously introducing a textured substrate to enhance light trapping in the BiVO4 photoanode and further optimization of the W gradient doping profile in the photoanode. Various PV cells have been studied in combination with this BiVO_4 photoanode, such as an amorphous silicon (a-Si:H) single junction, an a-Si:H/a-Si:H double junction, and an a-Si:H/nanocrystalline silicon (nc-Si:H) micromorph junction. The highest conversion efficiency, which is also the record efficiency for metal oxide based water-splitting devices, is reached for a tandem system consisting of the optimized W:BiVO_4 photoanode and the micromorph (a-Si:H/nc-Si:H) cell. This record efficiency is attributed to the increased performance of the BiVO_4 photoanode, which is the limiting factor in this hybrid PEC/PV device, as well as better spectral matching between BiVO_4 and the nc-Si:H cell

Geometrical Impact on Guided Mode Excitation in Solar Cells

We investigate the influence of patterned metallic back reflector on absorption in thinfilm silicon photovoltaics. The impact of symmetry, periodicity, and angle of incidence of light is discussed by simulation and experiment

Gradient dopant profiling and spectral utilization of monolithic thin-film silicon photoelectrochemical tandem devices for solar water splitting

A cost-effective and earth-abundant photocathode based on hydrogenated amorphous silicon carbide (a-SiC:H) is demonstrated to split water into hydrogen and oxygen using solar energy. A monolithic a-SiC:H photoelectrochemical (PEC) cathode integrated with a hydrogenated amorphous silicon (a-SiC:H)/nano-crystalline silicon (nc-Si:H) double photovoltaic (PV) junction achieved a current density of −5.1 mA cm^(−2) at 0 V versus the reversible hydrogen electrode. The a-SiC:H photocathode used no hydrogen-evolution catalyst and the high current density was obtained using gradient boron doping. The growth of high quality nc-Si:H PV junctions in combination with optimized spectral utilization was achieved using glass substrates with integrated micro-textured photonic structures. The performance of the PEC/PV cathode was analyzed by simulations using Advanced Semiconductor Analysis (ASA) software