• Energy Research

A 2-D photonic crystal was integrated experimentally into a thin-film crystalline-silicon solar cell of 1-��m thickness, after numerical optimization maximizing light absorption in the active material. The photonic crystal boosted the short-circuit current of the cell, but it also damaged its open-circuit voltage and fill factor, which led to an overall decrease in performances. Comparisons between modeled and actual optical behaviors of the cell, and between ideal and actual morphologies, show the global robustness of the nanostructure to experimental deviations, but its particular sensitivity to the conformality of the top coatings and the spread in pattern dimensions, which should not be neglected in the optical model. As for the electrical behavior, the measured internal quantum efficiency shows the strong parasitic absorptions from the transparent conductive oxide and from the back-reflector, as well as the negative impact of the nanopattern on surface passivation. Our exemplifying case, thus, illustrates and experimentally confirms two recommendations for future integration of surface nanostructures for light trapping purposes: 1) the necessity to optimize absorption not for the total stack but for the single active material, and 2) the necessity to avoid damage to the active material by pattern etching. Authors' postprint version - Editor's pdf published online on Nov. 5

Dry plasma etching, commonly used by the Photonics community as the etching technique for the fabrication of photonic nanostructures, could be a source of device performance limitations when used in the frame of silicon photovoltaics. So far, the lack of silicon solar cells with state‐of‐the‐art efficiencies utilizing nanophotonic concepts shows how challenging their integration is, owing to the trade‐off between optical and electrical properties. In this study we show that dry plasma etching results in the degradation of the silicon material quality due to (i) a high density of dangling bonds and (ii) the presence of sub‐surface defects, resulting in high surface recombination velocities and low minority carrier lifetimes. On the contrary, wet chemical anisotropic etching used as an alternative, leads to the formation of inverted nanopyramids that result in low surface recombination velocity and low density of dangling bonds. The proposed inverted nanopyramids could enable high efficiency photonic assisted solar cells by offering the potential to achieve higher short‐circuit current without degrading the open circuit voltage. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)

ALD is used to deposit Pt nanoparticles capped by ultra-thin SiO2 layers of various thicknesses to enhance the performance and stability of Si photocathodes used for solar water splitting. Enhanced stability is achieved compared to the reference case.

In order to relax the mechanical constraints of processing thin crystalline Si wafers into highly efficient solar cells, we propose a process sequence, where a significant part of the process is done on module level. The device structure is an interdigitated-back-contact cell with an amorphous silicon back surface field. The record cell reaches an independently confirmed efficiency of 18.4%. Although the device deserves further optimization, the result shows the compatibility of processing on glass with efficiencies exceeding 18%, which opens the door to a high-efficiency solar cell process where the potentially thin wafer is attached to a foreign carrier during the full processing sequence.

In this paper, we present the integration of an absorbing photonic crystal within a thin film photovoltaic solar cell. Optical simulations performed on a complete solar cell revealed that patterning the epitaxial crystalline silicon active layer as a 1D and 2D photonic crystal enabled to increase its integrated absorption by 37%abs and 68%absbetween 300 nm and 1100 nm, compared to a similar but unpatterned stack. In order to fabricate such promising cells, a specific fabrication processes based on holographic lithography, inductively coupled plasma etching and reactive ion etching has been developed and implemented to obtain ultrathin patterned solar cells.

Abstract Monolithic solar water splitting devices implemented in an integrated design approach, i.e. submerged in the electrolyte, pose a significant limitation when it comes to up-scaling. The ion transport distances around the monolith are long and consequently, the ionic Ohmic losses become high. This fact turns out to be a bottleneck for reaching high device efficiency and maintaining optimum performance upon up-scaling. In this paper, we propose a new device design for integrated monolithic solar water splitting based on porous multi-junction silicon solar cells. Simulation results highlight that porous monoliths can benefit from lower ionic Ohmic losses compared to dense monoliths for various pore geometries and monolith thicknesses. In particular, we show how micrometer scale pore dimensions could greatly reduce Ohmic losses, thereby minimizing overpotentials. A square array of holes with a diameter of 20 µm and a period of 100 µm was fabricated on single-junction and multi-junction amorphous and microcrystalline silicon solar cells. A small impact on the open circuit voltage (Voc) and short circuit current density (Jsc) was obtained, with porous triple junction solar cells reaching Voc values up to 1.98 V. A novel device design is proposed based on porous triple-junction silicon-based solar cells.

Bottom-up large-scale nanopatterns are fabricated conformally onto highly non-planar substrates using colloidal lithography.

In this paper, we present the integration of an absorbing photonic crystal within a monocrystalline silicon thin film photovoltaic stack fabricated without epitaxy. Finite difference time domain optical simulations are performed in order to design one- and two-dimensional photonic crystals to assist crystalline silicon solar cells. The simulations show that the 1D and 2D patterned solar cell stacks would have an increased integrated absorption in the crystalline silicon layer would increase of respectively 38% and 50%, when compared to a similar but unpatterned stack, in the whole wavelength range between 300 nm and 1100 nm. In order to fabricate such patterned stacks, we developed an effective set of processes based on laser holographic lithography, reactive ion etching and inductively coupled plasma etching. Optical measurements performed on the patterned stacks highlight the significant absorption increase achieved in the whole wavelength range of interest, as expected by simulation. Moreover, we show that with this design, the angle of incidence has almost no influence on the absorption for angles as high as around 60°.