Doped hydrogenated silicon oxide layers (SiO_X:H) have recently been successfully integrated as front window layers, back reflector layers, intermediate reflector layers, passivation layers, and junction layers in thin-film silicon solar cells. Depending on the deposition conditions of the SiO_X:H layers, some devices suffer from a degradation in performance in time. In this paper, we demonstrate the responsible mechanism involved. It is demonstrated that oxidation of the p-Type doped (p-)SiO_X:H with a high crystallinity and, therefore, poor passivation of crystalline grains is responsible for this degradation. The oxidation of p-SiO_X:H is caused by the in-diffusion of water vapor from the ambient air. Stable p-SiO_X:H can be obtained if the material is processed at higher pressure. In addition, the degradation can be prevented if the cell is well encapsulated, like using dense n-Type (n-)SiO_X:H in the back reflector of the cell.

@en

Vertical space bears great potential of solar energy especially for congested urban areas, where photovoltaic (PV) windows in high-rise buildings can contribute to both power generation and daylight harvest. Previous studies on sun-tracking PV windows strayed into the trade-off between tracking performance and mutual shading, failing to achieve the maximum energy generation. Here we first build integrated models which couple the performance of sun-tracking PV windows to the rotation angles. Secondly, one-degree-of-freedom (DOF) and two-DOF sun tracking are mathematically proven to be not able to gain either maximum power generation or non-glare daylighting under reasonable assumptions. Then we derive the optimum rotation angles of the variable-pivot-three-degree-of-freedom (VP-3-DOF) sun-tracking elements and demonstrate that the optimum VP-3-DOF sun tracking can achieve the aforementioned goals. When the restriction of the proposed model is relaxed, the same performance can be achieved by the optimum one-DOF sun tracking with extended PV slats and particular design of cell layout, requiring less complicated mechanical structures. Simulation results of nine global cities show that the annual energy generation and average module efficiency are improved respectively by 27.40% and 19.17% via the optimum VP-3-DOF sun tracking over the conventional perpendicular sun tracking. The proposed optimum sun-tracking methods also reveal better protection against sun glare. The optimum VP-3-DOF sun tracking is also demonstrated to be applicable to horizontal PV windows, as those applied in the sun roof of a glass greenhouse.@en

We fabricated and studied quadruple-junction wide-gap a-Si:H/narrow-gap a-Si:H/a-SiGex:H/nc-Si:H thin-film silicon solar cells. It is among the first attempts in thin-film photovoltaics to make a two-terminal solar cell with four different absorber materials. Several tunnel recombination junctions were tested, and the n-SiOx:H/p-SiOx:H structure was proven to be a generic solution for the three pairs of neighboring subcells. The proposed combination of absorbers led to a more reasonable spectral utilization than the counterpart containing two nc-Si:H subcells. Besides, the use of high-mobility transparent conductive oxide and modulated surface texture significantly enhances the total light absorption in the absorber layers. This work paved the way toward high-efficiency quadruple-junction cells, and a practical estimation of the achievable efficiency was given.

@en

Photoelectrochemical (PEC) devices for solar water splitting require not only high solar to hydrogen conversion efficiency but also high chemical stability in strong acidic or alkaline electrolytes for long-term operation. Titanium dioxide (TiO₂) has been considered as a highly promising protection layer to achieve high chemical stability for solar water splitting devices, especially for silicon based monolithic photovoltaic electrochemical (PV-EC) systems, while there is a trade-off relationship between activity and stability in these devices: the high charge transport barrier at the PV (silicon based thin film solar cells)/TiO₂ interface and the high ohmic loss in TiO₂ films hinder the device performance, especially when a thick TiO₂ protection layer (preferred to enhance the chemical stability in the electrolyte) is used. Herein, we show that a hydrogen doped TiO₂ protection layer can break this traditional trend to increase the activity without deteriorating the stability, when thick protection layers are employed to ensure stability. We demonstrated significant performance enhancement in hydrogenated amorphous silicon/silicon germanium (a-Si:H/a-SiGe:H) photocathodes through this approach. On one hand, the H-doping can shift up the Fermi level and reduce the electron transport barrier at the interface of the PV/TiO₂ protection layer. On the other hand, the higher carrier density via H-doping leads to the enhancement of electron transport in TiO₂ films and a shorter depletion layer barrier. Thus, the H-doping results in a higher photocurrent output at 0 V vs. reversible hydrogen electrode (RHE), indicating the high potential of the H-doped TiO₂ protection layer for achieving stable and efficient monolithic solar water splitting devices.

@en

A hybrid tandem solar cell consisting of a thin-film, nanocrystalline silicon top junction and a siliconheterojunction bottom junction is proposed as a supporting solar cell for photoelectrochemical applications.Tunneling recombination junction engineering is shown to be an important consideration indesigning this type of solar cell. The best hybrid cell produced has a spectral utilization of 30.6 mA cm2a JSC of 14 mA cm2, a VOC of 1.1 V, a fill factor of 0.67 and thus an efficiency of 10.3%. A high solar-tohydrogenefficiency of 7.9% can be predicted when using the hybrid cell in conjunction with current a-SiCphotocathode technology.@en

The effect of decoupled front/back textures and the application of photonic and plasmonic nanostructures on the performance of thin silicon solar cells was studied. New light trapping concepts based on diffraction on periodic photonic nanostructures and scattering using plasmonic structures have potential to outperform the currently used randomly textured structures. The study demonstrates that supporting layers of solar cells, such as transparent conductive oxides, doped layers and back reflectors, are responsible for significant parasitic absorption losses that prevent achieving 4n2 enhancement of light absorption in solar cells with silicon absorbers.@en