Black Silicon Photovoltaics

Abstract

LT) can be achieved for weakly absorbed photons with energies close to the absorption edge of silicon. [ 15 ] These properties of b-Si are particularly useful for photovoltaic applications.The limiting effi ciency of a solar cell is given by the detailed balance of absorption and radiative recombination [ 16 ] and by nonradiative processes like Auger-and impurity recombination. [17][18][19] b-Si can help to approach those limits in two ways. On the one hand b-Si improves the coupling of light into the solar cell and the absorption of near band edge photons. This in turn increases the short circuit current and on a logarithmic scale also the open circuit voltage. On the other hand, due to excellent light-trapping properties b-Si might also allow reducing the solar cell thickness substantially below 100 µm while sustaining a high light absorption. This reduces nonradiative bulk recombination losses that scale linearly with the solar cell thickness [ 17,18 ] and hence, increases the open-circuit voltage. Of course, reducing the solar cell thickness also increases the cost effi ciency. Decreasing the amount of required silicon feedstock is a major industry concern as can be seen by the growing interest in kerf-free crystalline silicon solar cell technologies. [20][21][22] Unfortunately, besides bulk effects, surface recombination imposes a very critical limit to the solar This article presents an overview of the fabrication methods of black silicon, their resulting morphologies, and a quantitative comparison of their optoelectronic properties. To perform this quantitative comparison, different groups working on black silicon solar cells have cooperated for this study. The optical absorption and the minority carrier lifetime are used as benchmark parameters. The differences in the fabrication processes plasma etching, chemical etching, or laser processing are discussed and compared with numerical models. Guidelines to optimize the relevant physical parameters, such as the correlation length, optimal height of the nanostructures, and the surface defect densities for optoelectronic applications are given.

Anti-reflective surfaces: Cascading nano/microstructuring

“…6,7 In addition, these surfaces are required for other uses, including displays, optical lenses, and defense applications, which would benefit from suppressed reflectance. [8][9][10][11][12][13] One often overlooked but important benefit that these hierarchical nano/microstructures bring to anti-reflective surface applications is their applicability over very wide-ranging spectral windows, because they have a lower angular dependence of reflectivity (i.e., a higher omnidirectionality) than is typical for such multilayered dielectric films.…”

Enhancing light trapping of macroporous silicon by alkaline etching: application for the fabrication of black Si nanospike arrays

“…During the second stage of AE (after 1 min), the surface reflectance increased, due to the lowering of the aspect ratio of the nanostructures, as shown in Figure 2 We now pay special attention to the Si NSpike arrays that presented the best light trapping because they are very promising for energy conversion, SERS or biomimetic antibacterial devices. [10][11][12] More samples were prepared using the protocol described above (with an AE time of 1 min) and were further characterized, as shown in Figure 4. In the low magnification view (Figure 4a), it can be seen A c c e p t e d m a n u s c r i p t that the NSpikes covered the whole surface of the Si wafer with a grass-like fashion (see also Figure S9), based on analyses of SEM top views, the surface density was estimated to be 7 x10 4 mm -2 .…”

“…1,4,6,7 In this context, top down micro-and nanostructuration of mono-or polycrystalline Si surfaces is highly appealing for boosting photon absorption and enlarging surface areas while conserving the properties of bulk Si. The fabrication of structured and highly absorbing Si, referred as Black Si (BSi), is, therefore, a very active research topic in materials with applications for renewable energy and photonics [1][2][3][4][8][9][10] and also, as recently demonstrated, for designing efficient surface-enhanced Raman scattering (SERS) 11 surfaces or biomimetic antibacterial materials 12 . Several techniques can be used to enhance light absorption.…”

Wideband ‘black silicon’ for mid-infrared applications

“…Several methods are used to obtain black silicon including chemical etching, (deep) reactive ion etching, and laser treatment [24]. So far black silicon structures are presented in the visible and the near-infrared region mostly for photovoltaic and surface enhanced sensing applications [25][26][27].…”