Black silicon contains high-aspect-ratio micro/nanostructures with greatly suppressed front-surface reflection, thus possessing superior property in photoelectric devices. In this report, by a two-step copper-assisted chemical etching method, we have fabricated pyramid n+p-black silicon with optimized morphology and anti-reflectance capability, through systematically tuning the concentration of both copper ions and reducing agents, as well as the etching time. The improved optical absorption and superior charge transfer kinetics validate n+p-black silicon as a highly active photocathode in photoelectrochemical cells. The onset potential of 0.21 V vs. RHE and the saturation photocurrent density of 32.56 mA/cm2 are achieved in the optimal n+p-black silicon. In addition, the nanoporous structure with lower reflectance is also achieved in planar p-silicon via the same etching method. Moreover, the photodetectors based on planar p-black silicon show significantly enhanced photoresponsivity over a broad spectral range. This study offers a low-cost and scalable strategy to improve the photoelectric-conversion efficiency in silicon-based devices.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Black silicon (Si) with surface micro/nanostructures has attracted much attention due to its low reflectance and strong light-trapping over a broad spectral range, demonstrating great potential in energy conversion applications. It has been widely reported that black silicon is a promising material candidate in high-efficiency photovoltaic solar cells, without employing any antireflection coating or vacuum processing steps [1–3]. Recently, the use of black silicon as the photoelectrode in photoelectrochemical system has been applied to many reactions such as water splitting and synthesis of ammonia [4–6]. Furthermore, as a newly developed micro/nanomaterial with near blackbody absorption properties in the visible to near infrared wavelengths, black Si promotes the idea of fabricating photodetectors in the silicon-based photonic integration circuits .
Generally, reactive ion etching, electrochemical etching, laser micro structuring, and metal-assisted catalyzed etching (MACE) are developed for the fabrication of black silicon . To date, MACE method has been adopted to practical applications in high efficiency photoelectric devices, owing to its inherent simplicity, low cost, and ease of scalable production [9–12]. Although Ag, Au, Pt, and Pd nanoparticles are the most commonly used metal catalysts [13–18], inexpensive metals such as Cu, Fe and Ni are also employed in MACE for economic fabrication of the black silicon [19–21]. Among them, Cu is particularly attractive since it has been already used widely in the commercial Si integrated circuits. Besides, Cu-based MACE shows slow etching velocity and no preferential etching direction [22,23], which is beneficial for the uniform formation of the nanopores and the reduction of the electron-hole recombination sites.
Recently, a one-step Cu-assisted catalyzed etching method was used to synthesize inverted pyramid nanopore-type black Si with Cu2+/H3PO3/HF  and Cu2+/H2O2/HF [24,25] systems, in which the anti-reflection capability of the Si surface was significantly improved. The optical reflectivity of the Cu-etched inverted pyramids is much lower than that of the upright pyramids obtained by conventional alkaline etching . However, in the one-step MACE method, the metal deposition and the electroless chemical etching occur simultaneously, which makes finding and controlling the optimal etching conditions quite challenging. Toor et al. prepared nanoporous black Si surfaces by using Cu2+/H2O2/HF system via two steps (i.e. the catalyzed etching of silicon takes place after the Cu deposition) . They found that it is crucial to use high HF to H2O2 ratio in the etch solution for effective Cu-etching, which is completely different from that used in usual Au-based MACE. To achieve low-cost and scalable production of black Si, it is necessary to conduct a systematic and comprehensive study on the impacts of main factors on the Si morphology during Cu-assisted catalyzed etching procedure, by taking the advantages of two-step procedure. In addition, while Cu-etched black Si is mainly applied to photovoltaic solar cells , the study of such black Si as a photocathode or photodetector material is still rare in the literature.
Herein, we report a two-step Cu-assisted chemical etching method using H2O2 as a reducing agent in the CuSO4/H2O2/HF system. Commercially available single crystalline pyramid n+p-Si and planar p-Si are used to fabricate black Si. Systematic experiments have been conducted to investigate the effects of the concentration of Cu ions, the composition of etchants, and the etching time, on the wafer morphology and corresponding surface reflectivity. Finally, the performance of the photoelectrochemical cells and photodetectors based on black-Si were evaluated.
2. Experimental section
2.1. Fabrication of black Si
Pyramid-textured monocrystalline n+p-type silicon (100) wafers and planar p-type Si (100) wafers (Canadian Solar Inc., resistivity of 1-3 Ω cm and total thickness of 180 μm) were used. In the commercial pyramid n+p-Si, 200∼300 nm thick n-type Si layer is fabricated on the front surface of p-type Si wafer by phosphorus doping, and the Al bottom electrodes were manufactured by a standard screen-printing process. All the Si photocathodes were finally laser-cut into 1.5×1.5 cm2. The detailed procedure of black silicon fabrication is as follows. First, the silicon wafers were cleaned by sonication using acetone, ethanol, and ultrapure water for 10 minutes, sequentially. Then, the cleaned silicon wafers were immersed in 4 vol % HF solution for 8 minutes to remove the native oxide on the surface. Copper deposition was performed in a CuSO4·5H2O solution with different concentrations by sonication for 1 min. After the deposition, the samples were etched in mixture aqueous solution, which contains 10 vol % HF and different concentrations of H2O2, for different time in ultrasonication. Then, the silicon wafer was immersed in concentrated nitric acid for 20 minutes to remove residual copper particles on the surface. Finally, the sample is rinsed with deionized water and blown dry with nitrogen.
2.2. Material characterization
The morphology of different samples was characterized by field emission scanning electron microscope (SEM, Hitachi Regulus 8100). The reflectance spectra of the samples were measured by a UV-vis-NIR scanning spectrophotometer (UV3600, Shimazu, Kyoto, Japan). A commercial contact angle meter (SL200A, KINO Industry Ltd. USA) was used to measure the hydrophilicity by the sessile drop technique.
2.3. Photoelectrochemical and photodetection performance measurement
Photoelectrochemical (PEC) measurement: Ohmic contact was achieved by applying the Cu wire on the Al bottom electrode in the back of the Si. The exposed backside and edges of electrodes are covered with a non-conductive, chemically resistant epoxy resin (PKM12C-1, Pattex). During the PEC measurement, a 300W xenon lamp (oriel, Newport company) with infrared cutoff filter was used as the light source and the light intensity was kept at 100 mW/cm2. The photoelectrochemical performance of the silicon photocathode was measured with a standard electrochemical analyzer (Shanghai Chenhua Instrument Co., Ltd. CHI-660E). The test voltage range of the linear sweep voltammetry curve is -1.2 ∼ +0.2 V, and the scan rate is 0.005 V/s. The frequency range of the electrochemical impedance spectroscopy test was from 0.01 Hz to 10 MHz, and the AC voltage was set to 10 mV. The test voltage area of the Mott Schottky curve was -1 V ∼ +1 V, and the frequency was set to 1000 Hz. A three-electrode system was used in the experiment. The silicon photocathode was used as the working electrode, the platinum wire was used as the counter electrode, and the Ag/AgCl electrode was used as the reference electrode. The electrolyte used in the measurement was the 1 M HClO4 aqueous solution. After testing, the electrode potential was converted: ERHE=EAg/AgCl+0.197 V+0.0591×PH.
Photodetection performance measurement: First, indium tin oxide (ITO) electrodes with dimension of 300×300 μm2 and spacing of 400 μm were deposited on the surface of p-Si by radio frequency magnetron sputtering. A supercontinuum laser (Wuhan Yangtze Soton Laser Co.,Ltd. OYSL, SC-Pro) was used as the laser source. All measurements of photodetection performance were carried out by the semiconductor characterization system (Tektronix Inc., Keithley 4200-SCS) on the probe station, with the laser incident perpendicular to the surface of p-Si. During the current-voltage (I-V) measurement, the applied voltage range was -1V∼ +1 V, with the sweep rate of 0.002 V/s.
3. Results and discussion
The underlying principles of constructing black Si are based on the electrochemical reaction between Si and Cu nanoparticles, in which the driving force is the electrochemical potential difference between these two materials. The reaction can be described as two half-cell reactions identical to the well-known MACE method for fabricating various Si nanostructures, as follows.
In our experiment, the CuSO4/H2O2/HF system is composed of a corrosion-type redox couple. Since Cu has a higher electronegativity than Si, the Cu ions withdraw electrons from the silicon substrate and are deposited in a form of metallic Cu nanoparticles. These Cu nanoparticles grow further by attracting the Cu2+ ions from the etchant solution. Simultaneously, Si atoms underneath the Cu nanoparticles are continuously oxidized, which are then etched by the HF so that nanopores are formed around Cu nanoparticles.
The effect of CuSO4 concentration on the surface morphology of the single crystalline n+p-Si is shown in Fig. 1, in which the H2O2 concentration and etching time are fixed at 1.2 vol % and 5 min, respectively. All samples exhibit the typical pyramid structure with the size in the range of 1–2 μm, while the (111) oriented pyramid structures are perpendicular to the wafer with a height of 2–3 μm. No nanopores are formed on the Si surface in the absence of CuSO4. By introducing a small amount of Cu ions, a few shallow pits start to emerge on the Si surface. Once the concentration of CuSO4 increases to 0.01 M, numerous nanopores are distributed uniformly on the whole area of the black Si wafers. Such a density-graded Si structure is effective in suppressing the optical reflection, as shown in Fig. 1(f). The broadband suppression in reflectance can be explained by existence of the light trapping effect. The incident light will be reflected multiple times between the nanotextured structures, which is accompanied by the increase of the number and probability of absorption of incident light by black silicon . As the concentration of CuSO4 is increased to 0.015 M, the originally induced nanopores connect with each other and form irregular shallow carters, which decreases the nanopore density and is detrimental for further decrease of the reflectivity.
Besides Cu2+, the concentration of H2O2 also plays an important role in the formation of the nanopores. Energetically, the electrochemical potential of H2O2 is much more positive than both the valence band of Si and the reduction potential of Cu2+/Cu nanoparticles. Thus, the presence of H2O2 may promote holes injection into the valence band of Si as well as into the Cu nanoparticles. Figure 2 shows SEM images of n+p-black Si synthesized with various H2O2 concentrations, in which the CuSO4 concentration and etching time are fixed at 0.01 M and 5 min, respectively. Without or at low concentration of H2O2, the formation and growth of Cu nanoparticles would be limited. Once the concentration of H2O2 increases to 1.2 vol %, the reduction of Cu2+ ions is accelerated, allowing more Cu nanoparticles to be formed on the Si surface, which results in the development of nanopores. At high H2O2 concentration (1.8 vol %), the reduction of Cu2+ will be faster, allowing more and larger Cu nanoparticles to be formed on the wafer surface. This situation will induce overlapping regions of oxidation and accordingly more isotropic Si etching, leading to wrinkle shaped pits rather than nanopores. Compared to the n+p-black Si fabricated with lower H2O2 concentration, these wrinkle shaped pits weaken the anti-reflection ability of Si (Fig. 2(f)).
Figure 3 shows SEM images of n+p-black Si synthesized with different etching time, in which the CuSO4 and H2O2 concentrations are fixed at 0.01 M and 1.2 vol %, respectively. As long as the etchant is sufficient, the Cu nanoparticles can continue catalyzing the Si etching, with a proper etching time of 5 min. Once the etching time reaches 10 min, the surface becomes hierarchically rough, and the pyramid structure of the sample will be destroyed. For further extended etching time, e.g. 15 min, the nanoporosity disappears and low aspect ratio microwells are observed on the surface. Such phenomenon indicates that the preferential etching on the pyramid structure quickly dissolves, and the lowering of the pyramid height and nanopore density are unfavorable for the anti-reflection of the Si surface (Fig. 3(f)).
After the characterization of the morphology and reflection of the different samples, we choose n+p-black Si with the lowest reflection (i.e. the concentrations of CuSO4 and H2O2, and the etching time are 0.01 M, 1.2 vol %, and 5 min, respectively) for the photoelectrochemical (PEC) measurement, as shown in Fig. 4(a). Current density-potential (J-V) curves of both n+p- and n+p-black Si photocathodes were obtained through linear sweep voltammetry. As shown in Fig. 4(b), both samples show very low current densities (in the range of 1 μA/ cm2) in the dark, while exhibiting cathodic photocurrent density under illumination, which is consistent with the p-type semiconducting behavior of Si photoelectrodes. The photocurrent onset potential (Vonset, defined as the potential required to reach -0.1 mA/cm2) of the n+p-Si photocathode is about 0.05 V vs. the reversible hydrogen electrode (RHE), whereas the n+p-black Si shows a larger Vonset of about 0.21 V vs. RHE. Moreover, the saturation photocurrent densities (Jsc) are 26 and 32.56 mA/cm2 for n+p-Si and n+p-black Si photocathodes, respectively. These results indicate the enhancements of both Vonset and Jsc in n+p-black Si. J-V curves under chopped light for both samples further verify that the black Si is a highly active photocathode (Fig. 4(c)).
The photoelectrochemical performance of the pyramid Si photocathode is related to the optical absorption, the carrier separation and transfer kinetics. It has been known that the carrier generation efficiency is closely related to the amount of light absorption. As shown in Figs. 1–3, the n+p-black Si has much higher light absorption than that of the n+p-Si, which improves the photoelectrochemical performance. The other factors are the separation and transport kinetics of the carriers. In order to check the band bending at the photocathode/electrolyte interface, Mott–Schottky measurements on the different samples were performed. The n+p-Si and n+p-black Si exhibit the flat band potential (Efb) value of 0.22 and 0.24 V vs. RHE (Fig. 4(d)) respectively, implying nearly the same band bending at the solid/electrolyte interface.
In order to gain deeper insight into the charge transfer process of the PEC cell, electrochemical impedance spectroscopy (EIS) measurement was carried out without illumination. Analysis of the Nyquist plots revealed a large difference in impedance between two samples (Fig. 4(e)). By fitting with the equivalent circuit model, the charge transfer resistance across the electrode-electrolyte interface, Rct, was found to be 1739 and 703 Ω for n+p-Si and n+p-black Si, respectively, which suggests superior charge transfer kinetics for n+p-black Si. The difference in resistance may arise from an unequal rate of surface degradation, and the reduced resistance in black Si can be attributed to the better hydrophobicity. It is reasonable that when the Si wafer is immersed in the electrolyte, the formation of silicon oxides (SiOx) would quickly happen on the surface of Si, leading to higher resistance at the electrode/electrolyte interface. As shown in Fig. 4(f), the measured water droplet contact angles are 82 ± 1.5° and 134 ± 2.5° for n+p-Si and n+p-black Si wafers, respectively. Such result indicates that the electrolyte cannot easily infiltrate through the nanopores of the black Si samples, and therefore the formation of SiOx will be effectively hindered, which gives rise to smaller interfacial charge-transfer resistance. Based on the results and analysis above, we attribute the enhancements of PEC performance to the improved optical absorption and superior charge transfer kinetics. We also notice that Cu can easily diffuse into Si and form deep level defects, which may degrade the photoelectric performance, even though our black silicon sample is treated with concentrated nitric acid. On the other hand, however, the buried metal nanoparticles are reported to improve the interfacial contact to the nanoporous silicon surface and result in high PEC performance . The detailed effects of acid immersion or other post-treatments deserve further investigation, by leveraging the key idea of two-step procedure.
The evolution of surface nanostructures by Cu-assisted etching in HF/H2O2/H2O can also be realized in planar p-Si. During the etching of planar p-Si, the optimal concentrations of CuSO4·5H2O and H2O2 remain the same as those for Si with pyramid structure, except that the etching time is increased to achieve the lowest optical reflection. Figures 5(a) and 5(b) show the SEM images of the planar p-Si and p-black Si. Numerous nanopores are observed at the surface after etching, confirming the formation of the black Si layer. Figure 5(c) shows the reflectance spectra of Si samples with different etching time. The mean reflectance of non-etched p-Si is about 20% from 300 to 800 nm. On the contrary, the 45 min-etched p-black Si has mean reflectance as low as less than 8%.
To show the validity of applying the planar p-black Si with enhanced optical absorption to high efficiency photoelectric devices, we have tested the photodetection properties. Figure 5(d) illustrates the device architecture of the photodetector, in which ITO is used as the contact electrodes. The equivalent circuit of planar Si-based photodetector can be deemed as a Si semiconductor channel connected by two back-to-back Schottky diodes. The I-V characteristics of planar p-Si and p-black Si photodetectors are shown in Figs. 6(a) and 6(b), respectively, both in dark and under illumination with different laser wavelengths. It is obvious that the photocurrent of the detector is greatly increased in p-black Si. Furthermore, the change of I-V curve from nonlinear to linear implies that the Schottky barrier at the metal-semiconductor interface is efficiently eliminated and nearly Ohmic contact is established, facilitating the photocarrier transport in the circuit. The underlying mechanism is proposed as follows. Compared to non-etched sample, p-black Si have a large surface area with rich surface trap states. And the nanopores and trap states serve as an additional degree of the energy band-edge modulation, leading to reconstructed barrier height [28,29], which in our case results in the substantial lowering of the Schottky barrier to form nearly Ohmic contact at the interface. Therefore, the significant improvement of the photodetection performance of the device based on p-black Si stems from the synergy effect of enhanced absorption (i.e. lower reflectance) and optimized band alignment. At the bias of 1 V, the photoresponsivity (R) as a function of wavelength of the laser illumination is plotted. Figures 6(c) and 6(d) are the R values for planar p-Si and p-black Si, respectively, where an enhancement of more than 2 orders of magnitude is obtained for p-black Si over a broad spectral range, from 450 to 850 nm.
In summary, we have reported the fabrication of black Si through a two-step Cu-assisted catalyzed etching process in the CuSO4/H2O2/HF system. The effects of etching conditions on morphology and corresponding surface reflectance are systematically investigated. By optimizing the concentration of Cu2+ and H2O2, and the etching time, we realize nanoporous structures and suppressed reflectance in both single crystalline pyramid n+p-Si and planar p-Si. The corresponding PEC cells and photodetectors based on the black Si exhibit greatly improved performances. This work delivers a facile and cost-effective approach for high-efficiency photoelectric devices based on black silicon.
National Natural Science Foundation of China (11774249, 12074278); Major Basic Research Project of the Natural Science Foundation of the Jiangsu Higher Education Institutions (18KJA140004, 20KJA140001); Priority Academic Program Development of Jiangsu Higher Education Institutions; Soochow University; Jiangsu Specially-Appointed Professors Program; China Postdoctoral Science Foundation (227238); Guangzhou Science and Technology Program (202002030142); Open Project of Provincial Key Laboratory of Soochow University (KJS1943).
The authors declare no conflicts of interest.
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
1. J. Oh, H.-C. Yuan, and H. M. Branz, “An 18.2%-efficient black-silicon solar cell achieved through control of carrier recombination in nanostructures,” Nat. Nanotechnol. 7(11), 743–748 (2012). [CrossRef]
2. S. Koynov, M. S. Brandt, and M. Stutzmann, “Black nonreflecting silicon surfaces for solar cells,” Appl. Phys. Lett. 88(20), 203107 (2006). [CrossRef]
3. C. Huo, J. Wang, H. Fu, X. Li, Y. Yang, H. Wang, A. Mateen, G. Farid, and K.-Q. Peng, “Metal-assisted chemical etching of silicon in oxidizing HF solutions: origin, mechanism, development, and black silicon solar cell application,” Adv. Funct. Mater. 30(52), 2005744 (2020). [CrossRef]
4. Y. Zhao, N. C. Anderson, K. Zhu, J. A. Aguiar, J. A. Seabold, J. Van De Lagemaat, H. M. Branz, N. R. Neale, and J. Oh, “Oxidatively stable nanoporous silicon photocathodes with enhanced onset voltage for photoelectrochemical proton reduction,” Nano Lett. 15(4), 2517–2525 (2015). [CrossRef]
5. B. Wang, L. Yao, G. Xu, X. Zhang, D. Wang, X. Shu, J. Lv, and Y.-C. Wu, “Highly efficient photoelectrochemical synthesis of ammonia using plasmon-enhanced black silicon under ambient conditions,” ACS Appl. Mater. Interfaces 12(18), 20376–20382 (2020). [CrossRef]
6. Y. Yu, Z. Zhang, X. Yin, A. Kvit, Q. Liao, Z. Kang, X. Yan, Y. Zhang, and X. Wang, “Enhanced photoelectrochemical efficiency and stability using a conformal TiO2 film on a black silicon photoanode,”,” Nat. Energy 2(6), 17045 (2017). [CrossRef]
7. Z. Jia, Q. Wu, X. Jin, S. Huang, J. Li, M. Yang, H. Huang, J. Yao, and J. Xu, “Highly responsive tellurium-hyperdoped black silicon photodiode with single-crystalline and uniform surface microstructure,” Opt. Express 28(4), 5239–5247 (2020). [CrossRef]
8. Y. Nishijima, R. Komatsu, S. Ota, G. Seniutinas, A. Balčytis, and S. Juodkazis, “Anti-reflective surfaces: Cascading nano/microstructuring,” APL Photonics 1(7), 076104 (2016). [CrossRef]
9. X. Liu, P. R. Coxon, M. Peters, B. Hoex, J. M. Cole, and D. J. Fray, “Black silicon: fabrication methods, properties and solar energy applications,” Energy Environ. Sci. 7(10), 3223–3263 (2014). [CrossRef]
10. K.-Q. Peng, X. Wang, L. Li, X.-L. Wu, and S.-T. Lee, “High-performance silicon nanohole solar cells,” J. Am. Chem. Soc. 132(20), 6872–6873 (2010). [CrossRef]
11. K. Gao, Y. Liu, Y. Fan, L. Shi, Y. Zhuang, Y. Cui, S. Yuan, Y. Wan, W. Shen, and Z. Huang, “High-efficiency silicon inverted pyramid-based passivated emitter and rear cells,” Nanoscale Res. Lett. 15(1), 174 (2020). [CrossRef]
12. Z. G. Huang, K. Gao, X. G. Wang, C. Xu, X. M. Song, L. X. Shi, Y. Zhang, B. Hoex, and W. Z. Shen, “Large-area MACE Si nano-inverted-pyramids for PERC solar cell application,” Sol. Energy 188, 300–304 (2019). [CrossRef]
13. N. A. M. Noor, S. K. Mohamad, S. S. Hamil, M. Devarajan, and M. Z. Pakhuruddin, “Effects of annealing temperature towards surface morphological and optical properties of black silicon fabricated by silver-assisted chemical etching,” Mater. Sci. Semicond. Process. 91, 167–173 (2019). [CrossRef]
14. H. Zhong, A. Guo, G. Guo, W. Li, and Y. Jiang, “The enhanced light absorptance and device application of nanostructured black silicon fabricated by metal-assisted chemical etching,” Nanoscale Res. Lett. 11(1), 322 (2016). [CrossRef]
15. K. Chen, T. P. Pasanen, V. Vähänissi, and H. Savin, “Effect of MACE parameters on electrical and optical properties of ALD passivated black silicon,” IEEE J. Photovoltaics 9(4), 974–979 (2019). [CrossRef]
16. H. M. Branz, V. E. Yost, S. Ward, K. M. Jones, B. To, and P. Stradins, “Nanostructured black silicon and the optical reflectance of graded-density surfaces,” Appl. Phys. Lett. 94(23), 231121 (2009). [CrossRef]
17. Y. Matsui and S. Adachi, “Optical properties of “black silicon” formed by catalytic etching of Au/Si (100) wafers,” J. Appl. Phys. 113(17), 173502 (2013). [CrossRef]
18. H. Asoh, F. Arai, K. Uchibori, and S. Ono, “Pt–Pd-embedded silicon microwell arrays,” Appl. Phys. Express 1(6), 067003 (2008). [CrossRef]
19. K.-H. Kuo, W.-H. Ku, and B. T.-H. Lee, “Photoluminescent or blackened silicon surfaces synthesized with copper-assisted chemical etching: for energy applications,” ECS J. Solid State Sci. Technol. 9(2), 024006 (2020). [CrossRef]
20. Y.-T. Lu and A. R. Barron, “Anti-reflection layers fabricated by a one-step copper-assisted chemical etching with inverted pyramidal structures intermediate between texturing and nanopore-type black silicon,” J. Mater. Chem. A 2(30), 12043–12053 (2014). [CrossRef]
21. O. V. Volovlikova, G. O. Silakov, S. A. Gavrilov, A. A. Dudin, G. O. Diudbin, and Y. I. Shilyaeva, “Investigation of the morphological evolution and etching kinetics of black silicon during Ni-assisted chemical etching,” J. Phys.: Conf. Ser. 987, 012039 (2018). [CrossRef]
22. J. P. Lee, S. Choi, and S. J. Park, “Extremely superhydrophobic surfaces with micro-and nanostructures fabricated by copper catalytic etching,” Langmuir 27(2), 809–814 (2011). [CrossRef]
23. F. Toor, J. H. Oh, and H. M. Branz, “Efficient nanostructured ‘black’ silicon solar cell by copper-catalyzed metal-assisted etching,” Prog. Photovolt.: Res. Appl. 23(10), 1375–1380 (2015). [CrossRef]
24. Y. Wang, L. Yang, Y. Liu, Z. Mei, W. Chen, J. Li, H. Liang, A. Kuznetsov, and X. Du, “Maskless inverted pyramid texturization of silicon,” Sci. Rep. 5(1), 10843 (2015). [CrossRef]
25. S. Zhao, G. Yuan, Q. Wang, W. Liu, R. Wang, and S. Yang, “Quasi-hydrophilic black silicon photocathodes with inverted pyramid arrays for enhanced hydrogen generation,” Nanoscale 12(1), 316–325 (2020). [CrossRef]
26. P. K. Singh, R. Kumar, M. Lal, S. N. Singh, and B. K. Das, “Effectiveness of anisotropic etching of silicon in aqueous alkaline solutions,” Sol. Energy Mater. Sol. Cells 70(1), 103–113 (2001). [CrossRef]
27. Z. Fan, D. Cui, Z. Zhang, Z. Zhao, H. Chen, Y. Fan, P. Li, Z. Zhang, C. Xue, and S. Yan, “Recent progress of black silicon: from fabrications to applications,” Nanomaterials 11(1), 41 (2020). [CrossRef]
28. X. Liu, L. Gu, Q. Zhang, J. Wu, Y. Long, and Z. Fan, “All-printable band-edge modulated ZnO nanowire photodetectors with ultra-high detectivity,” Nat. Commun. 5(1), 4007 (2014). [CrossRef]
29. X. Zhao, F. Wang, L. Shi, Y. Wang, H. Zhao, and D. Zhao, “Performance enhancement in ZnO nanowire based double Schottky-barrier photodetector by applying optimized Ag nanoparticles,” RSC Adv. 6(6), 4634–4639 (2016). [CrossRef]