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Thickness reduction of crystalline silicon (c-Si) solar cell is one of the most efficient ways to reduce the cost of manufacturing process as well as Energy pack-back time (EPBT) and thus ecological footprint. However, thin solar cell goes with a lack of infra-red absorption of the solar spectra. To circumvent this effect, advanced light-trapping schemes involving nanostructured surfaces or black Silicon (b-Si) and/or pyramid textured surfaces are good candidates for light-trapping enhancement in thin bifacial n-PERT solar cell (80-180 µm). In order to identify an optical optimum for thickness reduction, several combinations of different morphologies (Flat, Pyramid, b-Si and Saw damage removal - SDR) were modelled in front and/or rear side. Unfortunately, sizes of nanostructures are in the same range as wavelength of absorbed light, therefore geometrical optics is no more available which makes nanostructure not easy to model with conventional electro-optical simulation software. In this study, we integrate absorptance and reflectance measurements of b-Si into c-Si solar cell modelling process and thus demonstrate a +1%abs efficiency with one side nanotexturing and without antireflection coating layer compared to standard bifacial n-PERT solar cells. Moreover, mixing rear pyramid and front nanotexturing does not enhance significantly J sc to be considered from a process integration perspective in simulated thickness range (80-180 µm). Nevertheless, such a combination reaches higher light trapping and thus a better resilience to thickness reduction. [ABSTRACT FROM AUTHOR]

In this work we report on the long-term stability of black silicon surfaces passivated with atomic layer deposited (ALD) 20 nm thick Al2O3 films on p- and n-type FZ c-Si substrates. The results are directly compared with random pyramid textured counterparts. The effective surface recombination velocity S eff has been measured within a time frame of one year after activation of surface passivation. The results demonstrate that after an initial slight degradation during the first month S eff values stabilize around 45 and 25 cm/s on p- and n-type black silicon samples, respectively. These values are enough to guarantee stable high efficiency in interdigitated back-contacted (IBC) c-Si(n) solar cells (> 24.5%) using black silicon nanostructures on the front side. Similar, although weaker, losses are also observed in surface passivation on textured samples covered by Al2O3 with equal thickness, indicating that the origin of the instability might be independent of surface morphology. [ABSTRACT FROM AUTHOR]

We study how different oxidants in atomic layer deposition of aluminium oxide (ALD Al 2 O 3 ) affect the surface passivation of black silicon. Here we show that processes using ozone cause higher fixed charge but surprisingly lead to lower lifetimes in black silicon samples as compared to water-based samples. In planar samples however, the best surface passivation is reached with O 3 -based processes. In case of water as oxidant, the planar wafers suffer from severe blistering and poorer surface passivation, while this seems to be the best process for black silicon. To find a reason for the lifetime differences we also study different Al 2 O 3 stacks where both H 2 O and O 3 are used as oxidants. In conclusion, surface texture seems to affect the optimal oxidant in the ALD process. [ABSTRACT FROM AUTHOR]

In this paper, we study the influence of modifying the geometry of nanotexture on its electrical properties. Nanotexture is formed by an industrially feasible dry-chemical etching process performed entirely in atmospheric pressure conditions. A surface modification process is developed that allows low surface recombination velocities ( S eff,min ≤ 10 cm/s) on nanotextured surfaces. By simultaneously improving the surface passivation and the emitter diffusion processes, we achieve an equivalent passivation level ( V OC,impl ≥ 670 mV) for nanotextured surfaces to that of reference textured surfaces after applying either PECVD or ALD based deposition techniques. [ABSTRACT FROM AUTHOR]

We present a 3-step metal-assisted chemical etching (MAE) texturing technique to fabricate nanoporous Si (MAE nSi) using mono- and multi-crystalline silicon (mc-si) substrate. We show that only a very short etch back is necessary to obtain low reflectance. The angular distribution of the reflected light suggests that most of the losses due to the surface reflectance can be recovered after encapsulation and this has been validated in this work. We demonstrate that by using the texturing method shown in this work, very low reflectance can be achieved upon encapsulation. The substantial reduction in reflectance, for some of the textures, upon encapsulation may be partly attributed to the angular distribution of light reflected from the textured surface. Texturing is shown to result in a modest increase in the surface area. However, the increase in the surface recombination rate is smaller than the increase in the surface area, and typically significantly less than a factor of 3, when samples are passivated with atomic layer deposited Al 2 O 3 . This result suggests neither the local curvature nor the predominant crystallographic orientation causes additional recombination at the MAE nSi surface. The combination of low surface reflectance and low surface recombination of the MAE nSi makes it a very interesting candidate for solar cell applications. [ABSTRACT FROM AUTHOR]

Abstract: Black silicon is an interesting surface texture for solar cells because of its extremely low reflectance on a wide wavelength range and acceptance angle. In this paper we present how black silicon (b-Si) texturization can be applied on the boron doped front surface of an n-type solar cell resulting in an efficiency of 18.7%. We show that the highly boron doped emitter can be formed on black silicon without losing its good optical properties and that atomic layer deposited aluminum oxide provides good surface passivation on these boron doped b-Si emitters. [Copyright &y& Elsevier]

Abstract: Optically black silicon nanostructures show excellent anti-reflection and light trapping properties minimizing reflection losses to less than 1.6% between 300 – 1100nm. Our light-trapping scheme enables an absorption enhancement factor of ∼10 at the band edge of silicon (1150nm) as compared to a simulated perfect ARC, where the Yablonovitch limit corresponds to a factor of 15. Just recently it was shown that similar wet-chemically black etched silicon surfaces can be exploited to fabricate high efficiency solar cells [1]. Towards the integration of our structures into a solar cell device, the passivation performance of atomic layer deposited thin Al2O3 films is investigated on a variety of black etched structures. The coatings lead to measured surface recombination velocities of less than 13cm/s on bifacially black structured as well as 12cm/s on polished 1 – 5Ωcm p-type Si CZ wafers. Thinner layers promise to be even more effective. This technology will enable high efficiencies on various solar cell concepts. [Copyright &y& Elsevier]

Abstract: In this work we present a novel tandem solar cell concept that is based on enhanced below band gap infrared absorption. The solar cell structure is based on silicon and infrared activated Black Silicon. Infrared active Black Silicon is produced by exposing silicon to fs-laser pulses. It features an enhanced IR absorption, when processed under a sulfur-containing atmosphere. Then sulfur is incorporated into the silicon lattice during laser processing providing energy states in the band gap. This silicon based tandem cell thus absorbs light with wavelengths beyond 1.1μm. This can potentially increase the overall efficiency. In this paper we present the first experimental realization of this concept. We use a standard aluminium-back-surface-field (Al-BSF) silicon solar cell and implement a Black Silicon solar cell on its rear side for enhanced IR absorption. Current and voltage measurements show the feasibility of our concept. [Copyright &y& Elsevier]

Abstract: Samples of crystalline silicon for use as solar cell material are structured and hyperdoped with sulfur by irradiation with femtosecond laser pulses under a sulfur hexafluoride atmosphere. The sulfur creates energy levels in the silicon band gap, allowing light absorption in the infrared wavelength regime, which offers the potential of a significant efficiency increase. This Black Silicon is a potential candidate for impurity or intermediate band photovoltaics. In this paper we determine the laser processed sulfur energy levels by deep-level transient spectroscopy (DLTS). We present how the number of laser pulses per sample spot influence the sulfur energy levels and hence the DLTS spectra. Further we show that changing the laser pulse by splitting it with a Michelson interferometer setup results in altered absorption which is most likely due to altered sulfur energy levels. This contribution focuses on the possibility of controlling the sulfur in Black Silicon through manipulating the laser pulse shape. As a first step samples of microstructured silicon are fabricated with doubled laser pulses at two different laser pulse distances and the absorption spectra by integrating sphere measurements are compared. [Copyright &y& Elsevier]

Abstract: We present a femtosecond laser pulse process that induces a texture-like surface structure on silicon wafers and optionally incorporates sulfur into the silicon lattice for emitter formation depending on the processing atmosphere. Such laser processed Black Silicon provides an easily adjustable surface roughness for good light trapping in silicon solar cells. The structure is independent of the silicon crystal orientation and is easily applied on one wafer side only. A sulfur emitter can be formed within the laser structuring process, and allows electric current extraction from a solar cell structure manufactured from this material. Then the advantage is that no further emitter formation step like diffusion is necessary compared to other Black Silicon solar cell approaches, where the Black silicon is created wet chemically. By incorporating sulfur in the silicon crystal lattice, we can show that this Black Silicon absorbs in the infrared wavelength regime. This characteristic can potentially be used to better exploit the energy in the sun spectrum. We manufacture a laser processed Black Silicon solar cell prototype without any emitter diffusion step and achieve the highest efficiency of 4.5% reported for this cell type. [Copyright &y& Elsevier]

Abstract: Upon inductive coupled plasma reactive ion etching (ICP-RIE) of Si surfaces needle-like nanostructures with aspect ratios up to 10 emerge showing excellent anti-reflection and light-trapping properties with absorption over 97% throughout the UV and VIS spectral range. In addition, the absorption at the band edge of silicon is enormously enhanced due to scattering. In this work we report on the feasibility to deposit conformal Al2O3 dielectric layers on these very rough silicon surfaces which enable adequate surface passivation. Lifetimes over 170μs have been measured on deep structured b-Si substrates. The optical properties of black silicon (b-Si) and the possible influence of applied alumina passivation layers as well as their passivation performance are discussed. [Copyright &y& Elsevier]

Abstract: The black multi-crystalline silicon (mc-Si) has been successfully produced by plasma immersion ion implantation. The microstructure and the reflectance of the black mc-Si have been investigated by atomic force microscope and spectrophotometer, respectively. Results show that the black mc-Si exhibits a hillock structure with a low reflectance. Besides, with decreasing the diffusion temperature, the external quantum efficiency of the black mc-Si solar cell increases below ∼550nm wavelength due to reduced surface recombination. The optimal conversion effieciency of the black mc-Si solar cell is 15.50% at the diffusion temperature of 825°C. Furthermore, it is interesting to find that there are something different between black mc-Si and acid etched mc-Si on the impact of diffusion. [Copyright &y& Elsevier]