Fig. 1. Illustrative overview of the topics covered in this review on black silicon (b-Si) nanostructures for solar energy conversion and photonic applications.

Fig. 2. (a) Light interaction mechanisms within nanostructured surfaces. (b) Four basic spatial refractive index profiles of thickness d: (i) refractive index profiles between air and a silicon substrate without any ARC; (ii) silicon wafer featuring a uniformly porous layer; (iii), (iv) silicon wafers with graded porosity profiles. Reproduced with permission,⁴⁸ © 2002 AIP Publishing. (c) Schematic of a graded density or refractive index behavior in a silicon nanostructure (Si NS) array layer. Symbols: nair, nSi, and neff represent the refractive indices of air, silicon, and the Si NS layer, respectively. Reproduced with permission,⁴⁹ © 2020 Springer Nature. (d) Illustration of the air-to-mc-Si boundary: left schematic presentation showing a sharp refractive index (n) shift at the boundary in region 1; right schematic presentation illustrating the gradual refractive index transition that enhances light trapping and absorption by introducing the nT-mc-Si layer in region 2. Reproduced with permission,⁵⁰ © 2015 Elsevier. (e) The development of enhanced b-Si absorption across the visible-NIR region to the MIR region. Reproduced with permission,⁵¹ © 2022 John Wiley and Sons.

Fig. 3. Schematic of b-Si fabrication methods: (a) electrochemical etching for macroporous silicon production, (b) stain etching process, (c) metal-assisted chemical etching (MACE) process, (d) reactive ion etching (RIE) process, (e) laser treatment process. Panels (a), (c), (d), and (e) are reproduced with permission,¹⁹ © 2014 John Wiley and Sons. The definitions of abbreviated characters (e.g., J, Nd, t, T, p, P, Φ) are provided in Table 1.

Fig. 4. (a) Typical experimental configurations for electrochemical HF etching of p-Si (left) and n-Si (right). Reproduced with permission,²⁰ © 2014 Royal Society of Chemistry. (b) Energy band diagrams of n-Si/HF and p-Si/HF junctions at the equilibrium. (c) Typical J–V curves for Si/HF junctions: p-Si/HF (red line) and n-Si/HF in the dark (blue line) and under illumination (blue dashed line). (d) Simulated etching kinetics and corresponding etched morphologies. The blue line and color map represent JPS and Ctip variations due to HF molecule diffusion within the pore. Etched profiles and current densities are shown for cases where the hole supply exceeds (yellow line), falls short of (green line), or perfectly matches (red line) HF diffusion. (b)–(d) Reproduced with permission,⁶² © 2024 John Wiley and Sons. (e) SEM picture of porous silicon fabricated by electrochemical etching. Reproduced with permission,⁶³ © 2007 IOP Publishing.

Fig. 5. (a) Stain etching process. Reproduced with permission,⁶⁹ © 2023 Ishik University. (b) Reflectance spectra of stain-etched porous silicon on an mc-Si wafer. Reproduced with permission,⁷⁰ © 2017 IOP Publishing; reproduced with permission,²⁰ © 2014 Royal Society of Chemistry. (c)–(e) SEM images of Si micropillars obtained by the stain etching under different conditions, showing various shapes: (c) Si tubes incorporating a homogeneous porous silicon layer etched from the BHNO solution; (d) Si tubes covered with porous silicon after etching in an HVO solution; and (e) Si micro-cones covered with porous silicon. Scale bar: 2μm. (c)–(e) Reproduced with permission,⁷¹ © 2017 IOP Publishing.

Fig. 6. (a) Schematic of the metal-assisted chemical etching (MACE) process. (b) Energy band diagrams (top) of the Au/Si interface and the corresponding MACE morphology schematic (bottom) for n- and p-type silicon. (c) SEM images of the experimental MACE morphologies for various n- and p-Si electrodes. (a)–(c) Reproduced with permission,⁸⁵ © 2016 American Chemical Society. (d) Top-view SEM image of commercially boron-doped (1 to 3 Ω cm), (100) oriented c-Si wafers etched for 15 min at 50°C in a 5 mmol/L Cu(NO3)2, 4.6 mol/L HF, and 0.55 mol/L H2O2 mixed solution. The inset shows the cross-sectional SEM image. Reproduced with permission,⁸⁶ © 2017 Royal Society of Chemistry. (e) FDTD simulation results illustrating the electric field intensity profiles for silicon with inverted pyramid structures (left) and upright pyramid structures (right). Reproduced with permission,⁸⁰ © 2017 Elsevier.

Fig. 7. (a) Schematic illustrating the fabrication of b-Si via plasma-assisted reactive ion etching (RIE) with inductively coupled plasma (ICP). (b) Process diagram showing alternating etching and deposition steps to create high-aspect-ratio features on Si wafers. (a) and (b) Reproduced with permission,⁹¹ © 2017 Royal Society of Chemistry. (c) Schematic representation of RIE-induced damage on a silicon substrate. Reproduced with permission,⁹² © 1999 American Vacuum Society; reproduced with permission,²⁰ © 2014 Royal Society of Chemistry. (d) Tilted top-view and (e) cross-sectional SEM images of SiNTs with a length of 1600 nm. (f) Absorption spectra comparison of SiNTs (filled squares) and crystalline silicon (solid line) across the UV-vis-NIR range. (d)–(f) Reproduced with permission,⁹³ © 2007 Springer Nature.

Fig. 8. (a) Illustration of the setup for producing b-Si with femtosecond laser pulses. The inset on the left depicts the vacuum chamber designed for placing the silicon samples. Reproduced with permission,⁹⁹ © 2016 IEEE. (b) SEM images of sharp conical spikes generated on Si (100) by 500 fs laser pulses (100 fs duration), viewed at 45 deg to the surface normal (top) and parallel to the surface (bottom). Reproduced with permission,¹⁰⁰ © 1998 AIP Publishing. (c) SEM image of the microgroove structures; inset is a photograph of the b-Si sample. (d) 3D optical image of microgroove structure with nano-textured patterns. (e) The reflectance spectrum of the b-Si fabricated through femtosecond laser ablation in an air environment. (c)–(e) Reproduced with permission,¹⁰¹ © 2011 Elsevier.

Fig. 9. (a) Potential technological advancements in silicon PVs based on historical efficiency improvements and ongoing research, including Al-BSF cells and PERCs. Homojunction c-Si solar cells here include Al-BSF and PERCs. Passivated contacts consist mainly of tunnel oxide-passivated contacts and silicon heterojunction-based c-Si PV modules. Reproduced with permission,¹²¹ © 2020 AIP Publishing. (b) Schematic diagram of an Al-BSF cell. (c) Schematic diagram of a PERC. (b) and (c) Reproduced with permission,¹²² © 2017 Elsevier. (d) Structure and carrier transport mechanisms of passivated contact solar cells. Reproduced with permission,¹²³ © 2022 Elsevier. (e) Spectral response of tandem-configured perovskite top cells and silicon bottom cells (left). Schematic of the device structures of 2T and 4T perovskite/silicon tandem solar cells (right). Reproduced with permission,¹²⁴ © 2021 John Wiley and Sons.

Fig. 10. (a) Diagram illustrating the mechanisms of excess carrier recombination in silicon nanostructures, in which photogenerated carriers (blue dots) are lost via Auger and surface recombination channels. (b) Carrier lifetime (τeff) of both polished and nanostructured silicon, with symbols in different colors indicating various surface area enhancement ratios. Zones I–III correspond to lifetime ranges influenced by sheet resistance. (c) J–V characteristics of nanostructured b-Si, polished silicon, and pyramid-textured silicon coated with a SiNx AR layer, measured under AM 1.5G solar simulation. (a)–(c) Reproduced with permission,¹²⁷ © 2012 Springer Nature. (d) Illustration of a laser-doped selective emitter solar cell based on b-Si. Reproduced with permission,¹²⁸ © 2016 Elsevier. (e) Diagram of a nanotextured b-Si solar cell featuring an n+-emitter/p-base configuration (left) and a high-resolution TEM image showing a SiNW coated with an Al2O3 layer (right). Reproduced with permission,¹²⁹ © 2013 American Chemical Society. (f) Illustration of an NBSi solar cell with an n+-emitter/p-base configuration (left). High-resolution TEM images reveal silicon nanowires surrounded by an Al2O3/TiO2 bilayer passivation structure (right). Reproduced with permission,¹³⁰ © 2015 American Chemical Society. (g) Structure of the IBC cell, where a thin Al2O3 layer is deposited on the nanostructured front surface. (h) Normalized photocurrent versus incidence angle of b-Si (circles) and reference (squares) solar cells. The inset defines the light incidence angle θ. (g) and (h) Reproduced with permission,¹³¹ © 2015 Springer Nature. (i) Cross-sectional schematic of NPP TOPCon solar cells. (j) Normalized short-circuit current density [J_sc(θ)/J_sc(0 deg)] for NPP and conventional reference cells across incidence angles of 0 deg to 70 deg. (i) and (j) are reproduced with permission,¹³² © 2022 Elsevier. (k) Schematic of monolithic perovskite/black-silicon tandems based on tunnel oxide passivated contacts. Inset: photographs of c-Si with the planar surface (left), reconstructed b-Si (middle), and pyramidal texture (right). (l) Device architecture of the nanotextured perovskite/silicon tandem solar cell architecture. (m) EQE and total absorbance (1–R, gray shading) spectra for planar and nanotextured perovskite/silicon tandem solar cells, where R represents reflectance. (n), (o) Crystallization mechanisms for (n) randomly oriented perovskite on planar surfaces and (o) vertically aligned perovskite on nanotextured surfaces. (k)–(o) Reproduced with permission,³³ © 2022 Elsevier.

Fig. 11. Diagram depicting key surface and interface engineering approaches for b-Si photoelectrodes and their mechanisms for enhancing PEC performance.

Fig. 12. (a) Microwire array featuring tandem junctions with an embedded homojunction (n-p+-Si) covered by layers of ITO and n-WO3. Reproduced with permission,¹⁵² © 2014 Royal Society of Chemistry. (b) Energy band diagrams for p-Si (left) and n+p-Si (right) photocathodes interfacing with the H+/H2 redox pair in solution in the dark (top) and under illumination (bottom). Ecb: conduction band edge, Evb: valence band edge, EF: Fermi level. EFp and EFn: quasi-Fermi levels for the holes and electrons under illumination. Reproduced with permission,¹⁵³ © 2011 American Chemical Society. (c) Diagram depicting the charge generation and oxygen evolution in the b-Si/TiO2/Co(OH)2 system. (d) Spectra showing light reflection and scattering for planar Si, b-Si, and b-Si/TiO2 samples. Inset: images of the wafers (size ∼2cm×2cm). (e) J_ph–t plots for b-Si/Co(OH)2 and b-Si/TiO2/Co(OH)2 under a fixed bias of 0.6 V versus SCE. (c)–(e) Reproduced with permission,¹⁵⁴ © 2017 Springer Nature. (f) Cross-sectional SEM image showing the AgNPs/PEDOT/SiNW arrays. (g) TEM image of a SiNW coated with PEDOT and AgNPs. (h) HRTEM of the AgNPs/PEDOT/SiNW arrangement. (i) IPCE of various photoanodes measured at 0 V versus SCE. (f)–(i) Reproduced with permission,¹⁵⁵ © 2014 American Chemical Society. (j) J–V curves comparing SiNWs, SiNWs/Ni-B (1 s), SiNWs/Co-B (10 s), and SiNWs/Pt (12 min) photocathodes under AM 1.5G illumination. Reproduced with permission,¹⁵⁶ © 2016 American Chemical Society. (k) Illustration of b-Si integrated with a-MoSx for PEC hydrogen generation. (l) LSV curves for planar and b-Si photocathodes with a-MoSx layers, showing significant reduction in overpotential for b-Si with a-MoSx. (k) and (l) Reproduced with permission,¹⁵⁷ © 2022 American Chemical Society.

Fig. 13. (a) Illustration of the setup for PEC N2 reduction using AuNPs/b-Si/Cr. (b) Ammonia production over 24 h on various substrates: (i) p-type silicon, (ii) b-Si, (iii) AuNPs/b-Si, (iv) AuNPs/b-Si/Cr, (v) Au/Si/Cr under illumination with two suns, and (vi) AuNPs/b-Si/Cr in the dark. (c) Quantum efficiency of ammonia generation by the AuNPs/b-Si/Cr PEC system across different wavelengths. (a)–(c) Reproduced with permission,¹⁷⁷ © 2016 Springer Nature. (d) Diagram of a bias-free photochemical diode device comprising a p-type SiNW biophotocathode (blue) and an n-type SiNW photoanode (green) separated by a bipolar membrane under red light. (e) Energy diagram of the photochemical diode under red light irradiation. Photovoltages of 0.4 V (photocathode) and 0.45 V (photoanode) couple the CO2 RR and GOR without external bias. Acetyl-CoA: acetyl coenzyme A; M_ox: oxidized mediators; M_red: reduced mediators. (f) Faradaic efficiencies of the cathodic product (blue), total anodic products (pink), and individual anodic products during bias-free operation of the photochemical diode under red light (740 nm, 20mW/cm2). (d)–(f) Reproduced with permission,¹⁷⁸ © 2024 Springer Nature.

Fig. 14. Applications of solar photothermal energy based on b-Si.

Fig. 15. (a) Optical absorption of In2O3−x(OH)y nanoparticles compared with the efficiency of photon capture in In2O3−x(OH)y/SiNW composite materials under solar light. Inset: diagram showing the process of CO2 conversion to CO under simulated sunlight. Reproduced with permission,¹⁸³ © 2016 American Chemical Society. (b) Reaction rates for methane synthesis using Ru-based catalysts on SiNW, glass, and polished Si substrates at 150°C and 45 psi. (c) Variation of methane production rates with temperature under conditions of no light (black) and simulated solar illumination (yellow). Dashed lines indicate exponential trends for batch reactions. Inset: Arrhenius plot (ln(k) versus 1000/T) employed to determine the activation energy for Ru/SiNW catalysts under light and dark. (a)–(c) are reproduced with permission,¹⁸⁴ © 2014 John Wiley and Sons. (d) Assembly of b-Si, generator, and cooling fin with thermally conductive silicone, with aluminum fins submerged in a mixture of ice and water. (e) Schematic showing the AR principle of optical wave propagation through surfaces with micropores (top) and micro-nanopores (bottom). (f) Variation in the voltage output of the thermoelectric generator over time. (d)–(f) are reproduced with permission,²⁵ © 2019 Elsevier. (g) Diagram of the photothermal experimental setup. (h) Real-time temperature of the surfaces of pristine silicon and b-Si samples. (g) and (h) Reproduced with permission,¹⁸⁵ © 2021 American Chemical Society. (i)–(k) Diagram illustrating the structural design of hydrovoltaic generator (HG) and solar steam generation (SSG). (i) Overview of the SiNWs-based HG/SSG device, which integrates electricity generation and freshwater production via a hydrophobic condensation layer during evaporation. (j) Detailed layout of the functional components in the SiNWs-based HG and SSG system. Carbon nanofiber (CNF) was created by selectively depositing CNTs onto nonwoven fabric, providing water transport and electrical conductivity for the HG. (k) Cross-sectional SEM image of SiNWs, with the inset displaying a photograph of a 6-in. SiNW sample prepared at wafer scale. (i)–(k) Reproduced with permission,¹⁸⁶ © 2022 John Wiley and Sons.

Fig. 16. (a) Diagram showing the vertical layout of a photodiode structure utilizing b-Si. Inset: SEM image of the b-Si active region coated with Al2O3. Reproduced with permission,²⁰² © 2016 Springer Nature. (b) 3D device structure of a Schottky heterojunction photodiode featuring b-Si and nanocrystal indium tin oxide (nc-ITO). (c) High-resolution SEM image showing a cross-section of nc-ITO/Si nanostalagmites. (d) Photographs comparing b-Si devices covered with different thicknesses of ITO films to a planar silicon surface without treatment. (e) 2D diagram illustrating the device layout. (b)–(e) Reproduced with permission,²⁰³ © 2022 John Wiley and Sons. (f) Mechanism of electrochemically enhanced SERS detection, highlighting changes in surface charge on Au-coated nanopillar SERS substrates and their engagement with melamine molecules. (g) Depiction of the customized electrochemical-SERS platform and its integrated interface. (h) Image of the assembled detection chamber paired with an SEM view of Au-capped nanopillar structures designed for SERS detection. (f)–(h) Reproduced with permission,²⁰⁴ © 2020 American Chemical Society. (i) Cross-sectional SEM images of b-Si (left) and AuBSi (right). (j) Schematic of fingerprint metabolite detection using functionalized AuBSi substrates. (i) and (j) Reproduced with permission,⁴³ © 2020 American Chemical Society. (k) Schematic illustration of the laser desorption and ionization process for NGQD@MoS2/SiNWs. Reproduced with permission,²⁰⁵ © 2022 American Chemical Society.