Improved Black Silicon for Photovoltaic Applications

Abstract

The morphology and the electronic properties of monocrystalline Si (c‐Si) with a nano‐textured “black” surface, obtained by a metal‐catalyzed wet etching process, and the improvement by an additional chemical treatment are examined with regard to solar cell applications. Photoluminescence and optical reflectivity measurements show the presence of a nano‐porous Si (np‐Si) phase in the as‐prepared nano‐texture. It is found that an additional wet chemical treatment with the standard clean 1 of the common RCA cleaning process removes the np‐Si fraction and significantly alters the surface of the nano‐structure. Cross‐sectional scanning electron microscopy images reveal a pronounced reduction of the surface area, to values of only 3–6 times that of a planar surface. Electron spin resonance measurements were performed to investigate the type and quantity of defects induced by the nano‐texturing process. The optimized nano‐texture exhibits a Si dangling bond density comparable to planar c‐Si wafers. Electrically detected magnetic resonance spectra reveal an additional paramagnetic defect present in the nano‐textured Si, linked to a hydrogen‐ or oxygen‐related double donor. In addition, initial results on the passivation of surface defects via atomic layer deposition of Al2O3 are presented. Photoconductance decay measurements of passivated samples show a tenfold increase of the effective lifetime for nano‐textures which have received the additional etching treatment. The improved electronic quality of the nano‐textured surface makes it an interesting candidate for application as an anti‐reflection surface in solar cells.

Black Silicon Photovoltaics

“…In this review article, we focus on the four most prominent fabrication methods of b-Si: dry reactive ion etching in an inductive coupled plasma (ICP-RIE), [ 15,46,51 ] electroless MACE with Ag [ 3,35,57 ] and Au [ 1,58 ] catalysts, photoelectrochemical anodization that leads to the formation of macP-Si, [ 12 ] and the irradiation of the surface with femtosecond (fs) laser pulses (L-Si) in vacuum. [ 39,40 ] The various black etching methods ( Figure 1 ) all lead to homogeneous macroscopically b-Si surfaces with a rather low refl ectance over the whole absorbing range of silicon as will be shown in section 1.8.…”

“…[ 58 ] It was also shown that the lifetime could be increased by about one order of magnitude if the SC1 was carried out after MACE and before Al 2 O 3 deposition by plasma-assisted ALD. A possible explanation is a defect-rich layer or contamination with deep-level impurities originating from the MACE process that cannot be passivated with Al 2 O 3 .…”

“…The consumed electrons are restored in the metal via electron transfer from the valence band of silicon trough the metal/Si interface. [ 3,58 ] Alternatively, the catalysis can be triggered by creating nanoparticles in situ during black etching by the reduction of metal salts. Subsequently, the holes diffuse to the semiconductor/electrolyte interface close to the particle.…”

Increasing efficiency of hierarchical nanostructured heterojunction solar cells to 16.3% via controlling interface recombination

“…Depositing a-Si:H on the as-processed Si MPs+NPs substrate resulted in a shunted device. Therefore, during i-a-Si:H deposition, the high dangling bond density on the (100) plane wafer reducing the a-Si:H precursor diffusion coefficient, resulting high-defects Si thin films occurs more readily on high-defect-density (100) planes than on (111) planes [22,23] . To ensure high Voc values, one needs to fabricate the interface carefully, so that it allows for effective carrier collection; this is not the case with defective interfaces and those corresponding to high Sit values.…”

Origin of the visible emission of black silicon microstructures

“…9,10 In addition, a simple femtosecond-laser (fs-laser) processing technique can drastically change the optical properties of silicon. In particular, the fs-laser processed silicon (black silicon) has gained increased interest due to the peculiar optical properties, including the enhanced sub-bandgap absorption [11][12][13] and visible light emission. 14,15 In this work, we carry out cathodoluminescence spectroscopy and imaging in the scanning electron microscope (SEM-CL) and in the scanning transmission electron microscope (STEM-CL) to determine the black silicon light emission properties from the microscale to the nanoscale.…”