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The anodization of aluminum (Al) in three alpha-hydroxy acids (AHAs): glycolic (GC), malic (MC), and citric (CC), was analyzed. Highly ordered pores in GC were obtained for the first time. However, the hexagonal cells were characterized by a non-uniform size distribution. Although common features of current density behavior are visible, the anodization in AHAs demonstrates some peculiarities. The electric conductivity (σ) of 0.5 M GC, MC, and CC electrolytes was in the following order: σ(CC) &gt, σ(MC) &gt, σ(GC), in accordance with the acid strength pKa(CC) &lt, pKa(MC) &lt, pKa(GC). However, the anodization voltage, under which a self-organized pore formation in anodic alumina (AAO) was observed (Umax), decreased with increasing pKa: Umax(CC) &gt, Umax(MC) ≥ Umax(GC). This unusual behavior is most probably linked with the facility of acid ions to complex Al and the active participation of the Al complexes in the AAO formation. Depending on the AHA, its tendency and different modes to coordinate Al ions, the contribution of stable Al complexes to the AAO growth is different. It can be concluded that the structure of Al complexes, their molecular mass, and the ability to lose electrons play more important roles in the AAO formation than pKa values of AHAs.

The influence of arsenazo-I additive on electrochemical anodizing of pure aluminum foil in malonic acid was studied. Aluminum dissolution increased with increasing arsenazo-I concentration. The addition of arsenazo-I also led to an increase in the volume expansion factor up to 2.3 due to the incorporation of organic compounds and an increased number of hydroxyl groups in the porous aluminum oxide film. At a current density of 15 mA·cm−2 and an arsenazo-I concentration 3.5 g·L−1, the carbon content in the anodic alumina of 49 at. % was achieved. An increase in the current density and concentration of arsenazo-I caused the formation of an arsenic-containing compound with the formula Na1,5Al2(OH)4,5(AsO4)3·7H2O in the porous aluminum oxide film phase. These film modifications cause a higher number of defects and, thus, increase the ionic conductivity, leading to a reduced electric field in galvanostatic anodizing tests. A self-adjusting growth mechanism, which leads to a higher degree of self-ordering in the arsenazo-free electrolyte, is not operative under the same conditions when arsenazo-I is added. Instead, a dielectric breakdown mechanism was observed, which caused the disordered porous aluminum oxide film structure.

A multi-variant process of cold rolling to different reductions of thickness (0–50%) and subsequent heating under different temperature was applied to Al substrates. Next, a comprehensive analysis of the influence of various microstructural, structural, and mechanical parameters of the Al substrates on the nanopore growth in anodic aluminum oxide (AAO), was performed. As an effect, new evidences concerning an important role of Al substrate state on the AAO growth were provided. The obtained results unambiguously show that the number of dislocations is the only parameter responsible for the regular pore formation is the AAO. Since the dislocation density is greatly reduced solely after recrystallization process, a significant reduction of the percentage of defects in AAO occurs only when Al substrate is annealed at the recrystallization temperature (483 °C), independent of percent cold work. All microstructural (elongation, circularity, and size of grains) or structural (texture, lattice strain) parameters are irrelevant for the pore formation, until the number of dislocations in the Al substrate remains high. Therefore, before anodization Al substrate should be annealed, at least for 1 h, under the temperature ≥ recrystallization temperature, not lower. Stress-relief anneal at temperature

In this work, aluminum (Al) anodization in malic acid electrolytes of different concentrations (0.15 M, 0.25 M, and 0.5 M) was studied. The close-packed hexagonal pore structure was obtained for the first time in this organic acid in a 0.5 M solution, at 250 V and temperature of 5 &deg, C. Moreover, the process was investigated as a function of the number of cycles carried out in the same electrolyte. A repetition of anodization under seemingly the same external electrochemical parameters (applied voltage, temperature, etc.) induced serious changes in the electrolyte. The changes were reflected in the current density vs. time curves and were most evident in the higher concentrated electrolytes. This phenomenon was tentatively explained by a massive incorporation of malate anions into anodic alumina (AAO) framework. The impoverishment of the electrolyte of the malate anions changed internal electrochemical conditions making easier the attraction of the anions to the Al anode and thus the AAO formation. The electrolyte modification was advantageous in terms of pore organization: In a 0.25 M solution, already after the second anodization, the pore arrangement transformed from irregular towards regular, hexagonal close-packed structure. To the best of our knowledge, this is the first observation of this kind.

In this work, the influence of a wide range anodizing temperature (5&ndash, 30 &deg, C) on the growth and optical properties of PAA-based distributed Bragg reflector (DBR) was studied. It was demonstrated that above 10 &deg, C both structural and photonic properties of the DBRs strongly deteriorates: the photonic stop bands (PSBs) decay, broaden, and split, which is accompanied by the red shift of the PSBs. However, at 30 &deg, C, new bands in transmission spectra appear including one strong and symmetric peak in the mid-infrared (MIR) spectral region. The PSB in the MIR region is further improved by a small modification of the pulse sequence which smoothen and sharpen the interfaces between consecutive low and high refractive index layers. This is a first report on PAA-based DBR with a good quality PSB in MIR. Moreover, it was shown that in designing good quality DBRs a steady current recovery after subsequent application of high potential (UH) pulses is more important than large contrast between low and high potential pulses (UH-UL contrast). Smaller UH-UL contrast helps to better control the current evolution during pulse anodization. Furthermore, the lower PSB intensity owing to the smaller UH-UL contrast can be partially compensated by the higher anodizing temperature.

Highly ordered, conical-pore anodic alumina (AAO) membranes with interpore distance (D c ) between ca. 530 and 620 nm and thickness ranging between 2.4 and 7.8 μm, were produced. In the fabrication process aluminum surface was first pre-patterned by the anodization in etidronic acid solution. Then, the regular arrays of Al concaves were used as nucleation sites to grow AAO during the second anodization, which was carried out in highly concentrated citric acid solution (20 wt%) and at relatively high temperature (33–35 °C). The conical pore shape was engineered by a multistep process combining anodization in the citric acid electrolyte and the subsequent chemical pore broadening in phosphoric acid solution. The morphological analyses has revealed that the geometrical parameters of the Al concaves were successfully transferred to the AAO membranes. Furthermore, FTIR spectra analysis confirmed that the electrolyte species, such as phosphonate and citric ions, are being embedded into the AAO framework during the anodization. The graded-index structure formed in AAO can be used for a production of antireflective coatings operating in a broad spectral range.

Three types of niobia nanostructured films (so-called native, planarized, and column-like) were formed on glass substrates by porous alumina assisted anodizing in a 0.2 M aqueous solution of oxalic acid in a potentiostatic mode at a 53 V and then reanodizing in an electrolyte containing 0.5 M boric acid and 0.05 M sodium tetraborate in a potentiodynamic mode by raising the voltage to 230 V, and chemical post-processing. Anodic behaviors, morphology, and optical properties of the films have been investigated. The interference pattern of native film served as the basis for calculating the effective refractive index which varies within 1.75–1.54 in the wavelength range 190–1100 nm. Refractive index spectral characteristics made it possible to distinguish a number of absorbance bands of the native film. Based on the analysis of literature data, the identified oxide absorbance bands were assigned. The effective refractive index of native film was also calculated using the effective-medium models, and was in the range of 1.63–1.68. The reflectance spectra of all films show peaks in short- and long-wave regions. The presence of these peaks is due to the periodically varying refractive index in the layers of films in two dimensions. FDTD simulation was carried out and the morphology of a potential 2-D photonic crystal with 92% (wavelength 462 nm) reflectance, based on the third type of films, was proposed.

In this work, the influence of various electrochemical parameters on the production of porous anodic alumina (PAA)-based DBRs (distributed Bragg reflector) during high-temperature-pulse-anodization was studied. It was observed that lowering the temperature from 30 to 27 &deg, C brings about radical changes in the optical performance of the DBRs. The multilayered PAA fabricated at 27 &deg, C did not show optical characteristics typical for DBR. The DBR performance was further tuned at 30 &deg, C. The current recovery (iamax) after application of subsequent UH pulses started to stabilize upon decreasing high (UH) and low (UL) voltage pulses, which was reflected in a smaller difference between initial and final thickness of alternating dH and dL segments (formed under UH and UL, respectively) and a better DBR performance. Shortening UH pulse duration resulted in a progressive shift of photonic stopbands (PSBs) towards the blue part of the spectrum while keeping intensive and symmetric PSBs in the NIR-MIR range. Despite the obvious improvement of the DBR performance by modulation of electrochemical parameters, the problem with regarding full control over the homogeneous formation of dH+dL pairs remains. Solving this problem will certainly lead to the production of affordable and efficient PAA-based photonic crystals with tunable photonic properties in the NIR-MIR region.

The influence of various pre-anodization voltage (Vs) on pore self-ordering process in anodic alumina (AAO) produced by hard anodization (HA) was investigated. It was demonstrated that the Vs has substantial effect on the pore arrangement in the AAO. Moreover, the results revealed that the pre-anodization under the mild condition is not necessary to form a long-range ordered AAO. When the HA process starts at relatively high Vs, only a small change of voltage and current during the second anodization stage is sufficient to rearrange the pores into the close-packed hexagonal structure. Excellent pore ordering was achieved when the samples were pre-anodized at 120 V.

Aluminum was anodized in mixtures of aqueous sulfuric and chromic acid in different ratios, with overall concentration of 1.0 M. It was found that the logarithm of current density (and consequently, oxide growth rate) is a square function of the anodizing voltage. Moreover, the barrier layer thickness at the pores bottom was found to increase exponentially with the voltage, and increased as well with the fraction of chromic acid in the electrolyte. Additionally, interpore distance of anodic aluminum oxide, formed at the same voltage, was found to increase exponentially with the molar fraction of the chromic acid. Altogether, the high impact of the composition of the electrolyte on the morphological features of the nanoporous arrays is revealed.

The influence of ethylene glycol (EG) on the morphology of anodic alumina fabricated during hard anodization (HA) was analyzed. A transformation from continuous nano-porous film to separated alumina nano-tubes was induced upon addition of EG to a 0.3 M oxalic acid solution. Moreover, while anodization under constant voltage in a 1:1 v / v EG:H 2 O acidic solution resulted in distinct current density ( i a ) reduction in reference with the i a registered for the HA in an EG-free solution, the anodization in a 1:4 v / v EG:H 2 O mixture was often accompanied by a sudden i a increase and irregular current oscillations. In the latter case mostly the alumina nanotubes with wall thickness variation were produced. In general, the separation phenomena was explained by extensive incorporation of soluble C 2 O 4 2 − and COO − ions into the oxide framework leading to an enhanced oxide dissolution rate and preferred cleavages along the cell boundaries. The current oscillation and the production of nanotubes with an altered diameter were ascribed to a higher value of e el ∙ e ox / η ratio (where e el and e ox are the dielectric coefficient of electrolyte and the formed anodic oxide, respectively, and η is the dynamic viscosity of the solvent) as compared to the value relevant for more viscous solvent.

Nanoporous anodic aluminum oxide was formed in sulfuric acid with addition of indigo carmine. During anodizing, the organic dye was incorporated into the porous oxide walls. X-ray photoelectron spectroscopy revealed the presence of nitrogen and sulfur in the anodic aluminum oxide. Two types of incorporated sulfur were found: belonging to the sulfate anions SO42 − of the electrolyte and belonging to the C-SO3− side groups of the indigo carmine. Raman spectroscopy confirmed the incorporation and showed that the inorganic–organic hybrid material inherited optical properties from the indigo carmine. Typical modes from pyrrolidone rings, unique for indigo carmine in the investigated system (650 and 1585 cm− 1), were found to be the strongest for the greatest anodizing voltages used. Despite the indigo carmine incorporation, the morphology of the oxide is still nanoporous and its geometry is still tuned by the voltage applied during aluminum anodization. This work presents an inexpensive and facile approach to doping an inorganic oxide material with organic compounds.

The chemical dissolution&mdash, in 0.1 M solutions of phosphoric, malonic, citric, sulfosalicylic, and tartaric acids and 0.6 M solutions of sulfuric, oxalic, malonic, phosphoric, tartaric, and citric acids&mdash, of aluminum (Al) and its barrier anodic oxide, with thicknesses of 240 and 350 nm, produced during the anodization of Al deposited on a sitall substrate and Al foil, respectively, in a 1% citric acid aqueous solution, was investigated. Signs of chemical dissolution for 0.1 M phosphoric acid solution and 0.6 M concentrations of all the listed solutions were found. It was shown that the dissolution rate and the nature of its change depend on the acid nature, the state of the sample surface, and the classification of the electrolytes according to their degrees of aggressiveness with respect to aluminum.

Highly ordered porous anodic alumina (PAA) with interpore distances ( D c ) tunable in the range of 250–400 nm have been fabricated in a two-step anodization. In the first step a modified hard anodization (HA) was developed, which allowed for extension of the maximal anodizing voltage up to 220 V, giving rise to larger pores intervals than usually attained in HA (∼300 nm). The HA process was carried out in 0.3 M ethanol-free (group I) and ethanol-modulated (group II) oxalic acid electrolytes. It was observed that ethanol worsens the hexagonal arrangement of Al concavities and strongly affects the D c performance with respect to applied voltages. The Al nano-concave arrays prepared in the modified HA were subsequently used in the second anodization, performed in 0.1 M phosphoric acid solution. The geometrical features and regularity of Al concavities were directly transferred to porous anodic alumina (PAA). An excellent long-range ordering of pores was achieved when the guidance effect of regular hexagonal Al concaves prepared in ethanol-free electrolyte was reinforced by the self-ordering conditions prevailing in 0.1 M phosphoric acid solution at voltages close to 195 V.

Anodization of AA1050 foil in 0.3 M oxalic acid at 50 V was performed and nanoporous alumina was formed. Four various approaches of electrochemical barrier layer thinning with varied voltage steps were applied to optimize the experimental conditions. Scanning electron microscopy was employed to investigate the bottoms of the anodic alumina after electrochemical barrier layer thinning. Values of the barrier layer thickness derived from Bode plots extrapolations confirmed the FE-SEM observations. For the most suitable barrier layer the thinning procedure was applied to successfully form nickel nanowires via pulse electrodeposition.

Technical purity aluminum (AA1050) was subjected to a cold rolling process up to 80% of thickness reduction. So prepared samples were anodized in 0.3 M oxalic acid at the voltage ranging from 20 to 60 V. On one hand, no significant changes in current density, or thickness of the formed oxide were noticed. On the other hand, a cold deformation of aluminum before the self-organized two step anodization caused formation of additional pores, among the hexagonal ones (anodization at 30 V of aluminum rolled to the thickness reduction of 80%). Moreover, cold rolling of aluminum before anodization caused interpore distance decrease (30 V) or increase (50–60 V), with relevant pore density increase and decrease. Additionally, fast Fourier transform based quantitative arrangement analysis revealed that rolling of aluminum prior to the anodization spoils the pore ordering. The results show clearly that substrate also plays an important role in the self-organized anodization.

UV plasmonics is of particular interest because of large variety of applications, where the higher energy plasmon resonances would advance scientific achievements, including surface-enhanced Raman scattering (SERS) with UV excitation, ultrasensitive label-free detection of important biomolecules absorbing light in the UV, or the possibility for exerting control over photochemical reactions. Despite its potential, UV plasmonics is still in its infancy, mostly due to difficulties in fabrication of reproducible nanostructured materials operating in this high energy range. Here, we present a simple electrochemical method to fabricate regular arrays of aluminum concaves demonstrating plasmonic properties in UV/violet region. The method enables the preparation of concaves with well-controlled geometrical parameters such as interpore distance ( D c ), and therefore, well controllable plasmon resonances. Moreover, the patterning is suitable for large scale production. The UV/violet properties of Al concaves can be further fine-tuned by Ag and Cu metals. The refractive index sensitivity (RIS) increases after the metals deposition as compared to RIS of pure Al nanohole arrays. The highest RIS of 404 nm/RIU was obtained for Cu coated Al nanoconcaves with the D c = 460.8 nm, which is similar or better than the RIS values previously reported for other nanohole arrays, operating in visible/near IR range.

Anodic aluminum oxide was formed in 0,3 M oxalic acid at various content of glycerol (from 0 to 100 vol. %), at voltage range from 20 to 60 V (30 °C, 1 h long steps). Due to the influence of viscosity on the ionic mobility, current density, as well as on the thickness of the grown oxide, current density and thickness of the grown oxide were found to be linear functions of the inversed viscosity. Quantitative FE-SEM images analyses revealed surprising relation: interpore distance increases with the viscosity (logarithmic function). Additionally, pore diameter, porosity and pores density were also influenced by viscosity of the electrolyte. We have found that viscosity of the electrolyte, one of the most important factors in electrochemistry, has significant influence on the anodic alumina formation.

Anodic aluminum oxide has been formed in 0.3 M oxalic acid with various additions of glycerol (0–100% vol. of glycerol) at voltage range from 20 to 60 V at 30 °C. It was found that properties of the electrolytes, as well as the voltage have strong influence on the type of formed oxide (porous, or non-porous barrier type oxide) and its arrangement. Quantitative arrangement analysis of the nanoporous oxide has been performed with the use of fast Fourier transforms of the FE-SEM images. The obtained data were confronted with criterion number estimated from the operating conditions and electrolytes properties. It was found that the best arrangement of the nanopores is when the criterion number is close to the literature data for other, aqueous electrolytes.

C. The resulting nanoporous alumina surfaces were characterized by high resolution scanning electron microscopy, and the images were quantitatively analysed by means of an innovative approach based on fast Fourier transform. The influence of operating anodization voltage and electrolyte temperature on nanopores geometry (pore diameter, interpore distance, porosity, pores density) and arrangement has been studied in details and compared to literature data and theoretical calculations. It was found that independently from the temperature, the best arrangement of the nanopores is for anodic aluminum oxide formed at voltages ranging from 40 to 50 V. Moreover, it was found that pore diameter and interpore distance increase linearly with voltage, what is in line with the literature data. Keywords: nanopores, anodization, self-organization, fast fourier transform.INTRODUCTIONAnodic aluminum oxide (AAO) is a nanostructured material formed with electrochemical oxidation of alu-minum foil. It consists of parallel, hexagonally arranged nanopores

Poorly arranged nanopores focus attention of the researchers due to the many fundamental aspects. Thus, quantitative arrangement analysis of non-ideally arranged pores is demanded. In the paper, radial average fast Fourier transform based arrangement analysis of the nanopores has been reported. This arrangement analysis tool has been applied in the quantification of arrangement of anodic alumina formed in 0.3 M chromic acid at voltage ranging from 20 to 50 V and temperature ranging from 20 to 50 °C. Presented approach, in contrary to typically applied methods, is based on the radial average of the fast Fourier transform (FFT). Averaged regularity ratio, estimated with the use of the reported tool, gives more repeatable results with smaller spread. Thus, reliable dependencies between operating conditions and the averaged regularity ratio can be also obtained for poorly arranged pores. It was found that the greater the voltage and temperature, the better nanopores arrangement.

A simple method of fabrication of anodic aluminum oxide at low voltages (from 5 to 15 V) in 0.3 M oxalic acid is presented. To overcome the issues concerning the anode's burning, a high velocity stirring was applied. It allowed to achieve nanopores with ultra-small diameters (even 12 nm), interpore distance (even 31 nm) and tremendously high pore densities (up to 980 pores per 1 µm 2 ) with simultaneous high porosity (up to 38%). Moreover, the presented approach (no modifier added to the electrolyte) resulted in the AAO with relatively well arranged ultra-small nanopores.

Anodic aluminum oxide (AAO) is typically applied as a template for nanofabrication. It was found that during anodization the anions of electrolytes are being incorporated into the nanopore walls giving rise to an emission in the UV/VIS range. In this paper, incorporation of copper-EDTA chelate ions was studied. The anodization of all studied samples was carried out in 0.02 M CuSO 4 +0.02 M EDTA in 2 wt% sulfuric acid, which has provided suitable conditions for self-organized two-step process of AAO growth. Nanoporous alumina was formed and studied with Scanning Electron Microscopy. Chemical composition analysis of the anodic alumina revealed that the greater the oxide growth rate, the more copper is being incorporated. The photoluminescence studies confirmed the presence of copper in the AAO pore walls.

In this work the quality, arrangement, composition, and regularity of nanoporous AAO formed on the low-purity (AA1050) and high-purity aluminum during two-step anodization in a mixture of sulfuric acid solution (0.3 M), water and glycol (3:2, v/v), at various voltages (15, 20, 25, 30, 35 V) and at temperature of −1 °C, are investigated. The electrochemical conditions have allowed to obtain pores with the size ranging from 30 to 50 nm, which are much larger than those usually obtained by anodization in a pure sulfuric acid solution (

Formation of the nanoporous alumina on the surface of Ni3Al intermetallic alloy has been studied in details and compared with anodization of aluminum. Successful self-organized anodization of this alloy was performed in 0.3 M citric acid at voltages ranging from 2.0 to 12.0 V using a typical two-electrode cell. Current density records revealed different mechanism of the porous oxide growth when compared to the mechanism pertinent for the anodization of aluminum. Electrochemical impedance spectroscopy experiments confirmed the differences in anodic oxide growth. Surface and cross-sections of the Ni3Al intermetallic alloy with anodic oxide were observed with field-emission scanning electron microscope and characterized with appropriate software. Nanoporous oxide growth rate was estimated from cross-sectional FE-SEM images. The lowest growth rate of 0.14 μm/h was found for the anodization at 0 °C and 2.0 V. The highest one – 2.29 μm/h – was noticed for 10.0 V and 30 °C. Pore diameter was ranging from 18.9 nm (2.0 V, 0 °C) to 32.0 nm (12.0 V, 0 °C). Interpore distance of the nanoporous alumina was ranging from 56.6 nm (2.0 V, 0 °C) to 177.9 nm (12.0 V, 30 °C). Pore density (number of pore occupying given area) was decreasing with anodizing voltage increase from 394.5 pores/μm2 (2.0 V, 0 °C) to 94.9 pores/μm2 (12.0 V, 0 °C). All the geometrical features of the anodic alumina formed by two-step self-organized anodization of Ni3Al intermetallic alloy are depending on the operating conditions.

Anodization of Ni3Al alloy is of interest in the field of industrial manufacturing, thanks to the formation of protective oxide layer on the materials working in corrosive environments and high temperatures. However, homogeneous surface treatment is paramount for technological applications of this material. The anodization conditions have to be set outside the ranges of corrosion and burning, which is the electric field enhanced anodic dissolution of the metal. In order to check against occurrence of these events, proper quantitative means for assessing the surface quality have to be developed and established. We approached this task by advanced analysis of scanning electron microscope images of anodized Ni3Al plates. The anodization was carried out in 0.3 M citric acid at two temperatures of 0 and 30°C and at voltages in the range of 2 12 V. Different figures can be used to characterize the quality of the surface, in terms of uniformity. Here, the concept of regularity ratio spread is used for the first time on surfaces of technological interest. Additionally, the Minkowski parameters have been calculated and their meaning is discussed.

The anodization of aluminum in 0.3 M chromic acid is studied. The influence of operating conditions (like anodizing voltage and electrolyte's temperature) on the nanoporous anodic aluminum oxide geometry (including pore diameter, interpore distance, the oxide layer thickness and pores density) is thoroughly investigated. The results revealed typical correlations of the anodic alumina nanopore geometry with operating conditions, such as linear increase of pore diameter and interpore distance with anodizing voltage. The anodic aluminum oxide is characterized by a low pores arrangement, as determined by Fast Fourier transforms analyses of the FE-SEM images, which translates into a high concentration of oxygen vacancies. Moreover, an optimal experimental condition where chromate ions are being successfully incorporated into the anodic alumina walls, have been determined: the higher oxide growth rate the more chromate ions are being trapped. The trapped chromate ions and a high concentration of oxygen vacancies make the anodic aluminum oxide a promising luminescent material.

The anodized alumina oxide (AAO) pattern-transferred Pd nanotubes occluded by Mg nanoparticles were easily manufactured by the use of vacuum evaporation technique at room temperature. The nanostructures were subjected to the H2 absorption at ambient conditions. The material can absorb H2 very fast (the equilibration between the hydrogen gas and within the material at each target pressure occurs in a time shorter than 110 s) reaching approximately 1 wt.% of H2 at only 0.4 kPa of H2 pressure. At 100 kPa the system absorbs up to 2 wt.% of H2 at room temperature. After hydrogenation a drastic change in the structure and morphology of the nanostructures, as an effect of the metal lattice expansion in all three dimensions, was observed which was characterized by the appearance of a regular array of columns with a substantial increase in volume as compared to the nanostructures before hydrogenation. The application of the system in a potential hydrogen sensing device is envisaged.

The anti-reflective properties of amorphous silicon (a-Si) films deposited directly on Al nanoconcave arrays with various interpore distances ( D c ) were studied. The Al anodized at 195 V ( D c =456.7 nm) and covered by a 277-nm a-Si layer demonstrated the best light- trapping efficiency. It absorbed up to ~86% of incident light. Average reflectivity under AM1.5 solar spectrum is strongly correlated to height alteration at the a-Si front surface. Further optimization found that D c and film thickness of 450–500 nm and 250–300 nm, respectively, take full advantage of the excellent optical properties of this simple, low cost material suitable for large scale production.