Use of two-dimensional nanorod arrays with slanted ITO film to enhance optical absorption for photovoltaic applications

Yung-Chi Yao; Meng-Tsan Tsai; Hsu-Cheng Hsu; Li-Wei She; Chun-Mao Cheng; Yi-Ching Chen; Chien-Jang Wu; Ya-Ju Lee

doi:10.1364/OE.20.003479

1. Introduction

Recently, the elimination of Fresnel reflection from surface interfaces by the application of antireflection coatings (ARC) has become a topic of significant research interest [1–4]. The employment of ARC for reducing Fresnel reflection has mainly been applied to minimize reflections in optical components [5], to enhance the light extraction of light-emitting diodes [6,7], and to improve the coupling of sunlight into solar cells [8,9]. In particular, the development of novel ARCs that enhance the power generation efficiency of solar cells has attracted much attention because the demand for solar energy has become more intense than ever. For the day-to-day operation of solar cells, the design of ARCs with low reflectivity over the broadband spectrum and omnidirectional incidence is necessary to efficiently harvest energy from sunlight [10]. In accordance with these design principles, several artificial nanostructures with various porous geometries, such as rods [11–13], holes [14,15], cones [16,17], pyramids [18,19], and tips [20–22], have been proposed and fabricated. Among them, most nanostructures are created with top-down or bottom-up processes, and they typically demonstrate excellent ARC functionality, as required for the daily applications of solar cells. Additionally, depending on the continuity of the refractive index of the nanostructures, ARCs are mainly classified into two types, i.e., homogeneous and inhomogeneous layers [10]. Homogeneous ARCs (also known as step-index coatings) can reduce reflection by destructive interference of incident light that is reflected at different interfaces. For these coatings, the choice of coating material and the thickness of the deposited layer must be carefully considered for the incident wavelengths [23]. The other type of ARC is the inhomogeneous coating (also known as the graded-index coating), which is generally advantageous over its homogeneous counterpart because of its superior characteristics with respect to broadband spectra and omnidirectional incidence [8,24]. However, the gradual change of refractive index in inhomogeneous ARCs must be precisely controlled in the nanostructure to achieve the specific required outline during the fabrication process [25]. Therefore, to provide evidence for enhanced absorption in solar cells, the development of novel ARCs must consider the difference in the fundamental properties of various nanostructures and further manipulates them as necessary.

In the current study, a novel ARC that combines a 2-dimensional (2D) Si-nanorod array with a slanted indium-tin-oxide (ITO) film was proposed and demonstrated to enhance the quantum efficiency of solar cells. Here, we recognize the intrinsic antireflection effect of the 2D Si-nanorod array that arises from the sub-wavelength scale of the nanorods, improving optical absorption for photovoltaic applications. Moreover, to further reduce the Fresnel reflection that occurs at the interface between the 2D Si-nanorod array and the air caused by the large difference in their refractive indices, a slanted ITO film was applied to the top of nanorods to serves as an optical transparent layer with an intermediate refractive index (between that of the nanorods and that of the air). The ITO film also provided an important functionality as the electrode layer of the solar cells. Herein, we report the design, implementation and demonstration of 2D Si-nanorod arrays with a slanted ITO film that shows strong absorption over a broadband spectrum (λ = 400−1000nm) and a variety of incidence angles (θ = 0−80°), as well as high short-circuit current density (J_SC = 32.81 mA/cm²) and power generation efficiency (η = 22.70%) obtained by the numerical calculation.

2. Experiment

Figure 1(a) shows a schematic of the fabrication process for the proposed ARC. First, the 500μm thick single-crystalline bare silicon (Si) was cleaned with acetone in an ultrasonic bath, followed by an isopropyl alcohol and deionized water rinse. The 2D Si-nanorods were then fabricated with self-assembled nickel (Ni) clusters as a hard mask for inductively coupled-plasma (ICP) dry etching. Here, the 10nm Ni film was deposited on the cleaned Si by radio-frequency (RF) magnetron sputtering [step (i)]. The sample was then subjected to rapid thermal annealing (RTA) at 900 °C under a nitrogen atmosphere to produce nano-sized Ni clusters [step (ii)]. An RTA duration of 1 min was used to ensure that all Ni cluster were isolated. Figure 1(b) presents a scanning electron micrograph (SEM) of the nano-sized Ni cluster. Because of the nature of self-assembly under the RTA process, the Ni clusters are randomly arranged. As shown in the insert of Fig. 1(b), the diameter of an individual Ni cluster ranges from 40 to 90 nm, and exhibits a Gaussian-like distribution. On average, the diameter of the Ni clusters is approximately 60 nm, which is primarily determined and well controlled by the deposited thickness of the Ni film and the RTA condition [26]. The sample was then etched down to 350nm by an ICP system, using a mixture reactive gases (Cl₂ /Ar = 20/20 sccm) with an ICP power source, a bias power set at 200/100 W, and a chamber pressure of 5 mTorr for 1 min of etching time [step (iii)]. The hard mask of Ni clusters was removed by sulfuric acid solutions (H₂SO₄: H₂O₂: H₂O = 5:2:1) to expose the 2D Si-nanorod array. Finally, the slanted ITO film was grown by oblique-angle deposition using RF magnetron sputtering [step (iv)]. The apparatus used in the oblique-angle deposition has a sample stage on which the substrate is loaded, with controllable polar-angle rotation. During the deposition, an argon flux of 8 sccm is supplied at a working pressure of 6 mTorr, and there was no movement of the substrate. The sample stage maintains a fixed polar angle so that the substrate has a controlled tilt angle of θ = 60° with respect to the ITO vapor-flow direction. For our oblique-angle deposition system, the tilt angle (β) of deposited ITO is less than the incident angle of vapor-flow (θ), and follows the empirical tangent rule $\tan β = 1 / 2 \tan θ$ [27,28]. Additionally, because of the shadowing effect produced by the 2D Si-nanorod array, the incident ITO vapor-flow is deposited preferentially on the top of the individual nanorods [29]. As a result, a continuous and porous morphology was observed on the top surface of the slanted ITO layer, as shown in the SEM image in Fig. 1(c). The surface morphology of the sample was also examined by atomic force microscopy [right-hand in Fig. 1(c)]. The measured roughness (root-mean-square, RMS) of the top surface of our ARC is approximately 30.7nm. Compared to that of a planar ITO layer with identical thickness (RMS = 1.4 nm), a much larger RMS value was observed on our ARC, primarily because of the porous morphology of the slanted ITO layer. Additionally, the resistivity of the planar and slanted ITO films are ρ = 4.76 × 10⁻⁴ Ω-cm and ρ = 2.33 × 10⁻³ Ω-cm, respectively. The slightly higher resistivity obtained in the slanted ITO film is mainly due to the intrinsic nano-porosity that would scatter electrons, and thus hinders their transportation inside the material [15]. Nevertheless, for the slanted ITO film, the electrical property is not much degraded as compared to the planar ITO film, reducing the impact on the collection efficiency of photo-generated carriers.

Fig. 1 (a) Schematic of the fabrication of 2D Si-nanorod array with slanted ITO film. (b) SEM image of nano-sized Ni clusters, in which the scale bare is 1μm. Insert: statistics of the diameter of distributed Ni clusters. (c) SEM (left hand) and AFM (right hand) images of the slanted ITO film grown on top of the 2D Si-nanorod array. The scale bar of the SEM image is 1 μm.

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3. Results and discussion

To quantitatively examine the optical characteristics of our ARC, we first measured the refractive index (n) and extinction coefficient ( $κ$ ) of our samples, as shown in Fig. 2 . The wavelength-dependent complex refractive indices of the samples, $\tilde{n} (λ) = n (λ) - i κ (λ)$ , are determined by ellipsometry with a normal-incidence white light source (180W halogen-lamp). In Fig. 2(a), the n value of the bare Si (black solid-line) gradually decreases with increasing wavelength, exhibiting normal dispersion behavior. In contrast, the n value of the 2D Si-nanorod array (blue solid-line) is relatively stable and has a much smaller value of n = 2.18 for most of wavelengths (λ = 400−1000nm). The improved stability and reduction of n are caused by the low filling-fraction (f.f.) of the array. Generally, the effective refractive index of the 2D Si-nanorod array is defined as the average refractive index between the air and the bare Si, weighted by their respective volumes. It can be expressed as follows [30]:

n_{e f f} (λ) = n_{S i} (λ) + (1 - \frac{π D^{2}}{4 a^{2}}) [n_{a i r} - n_{S i} (λ)]

where n_Si and n_air denote to the refractive indices of bare Si and the air, respectively. D is the average diameter of the 2D Si-nanorod array, a is the average pitch between rod-to-rod, and πD²/4a² is the f.f. Accordingly, the f.f. of the 2D Si-nanorod array derived by Eq. (1) is f.f.~40%, which is consistent with the previous observation from the SEM image of Ni clusters in Fig. 1(b). More importantly, compared to that of the bare Si (black dashed-line), the

κ

value of the 2D Si-nanorod array (blue dashed-line) is remarkably enhanced, especially for the region of visible light (λ = 400−700nm). It is well known that the absorption coefficient (

α

) of a material is defined as

α \equiv 4 π κ / λ

, and the larger

κ

would be expected to lead to a larger value of

α

. As a result, the enhanced absorption of the 2D Si-nanorod array enables high quantum efficiencies for photovoltaic applications. The randomly distributed and non-planar geometry of the 2D Si-nanorod array is mainly responsible for the enhanced absorption coefficient because of it effectively scatters and traps incident photons [31]. Similarly, the refractive indices of the normal and slanted ITO films as a function of wavelength are shown in Fig. 2(b). Again, the refractive indices of both samples decrease with wavelength and exhibit the normal dispersion profile. The refractive index of the normally deposited ITO is higher than that of the slanted ITO. On average, the n values of the normal and slanted ITO films are 1.98 and 1.63, respectively. For the slanted film, that value corresponds to a porosity of ~34.8%, as evaluated by the Bruggemann effective medium approximation [32]. The extinction coefficients of both samples, however, are nearly identical and approximately zero, which allows most of the incident solar energy to propagate in a near-lossless fashion inside the film.

Fig. 2 Measured complex refractive indices of (a) bare Si and 2D Si-nanorod array, and (b) normal and slanted deposited ITO films.

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Figure 3(a) shows the cross-sectional SEM image of the normally deposited (planar-sheet) ITO film. SEM images of 2D Si-nanorod arrays (b) without and (c) with the slanted ITO film deposited on top are shown in Fig. 3(b) and Fig. 3(c), respectively. The variation of the refractive index along the z-direction of all samples is also plotted in the figure. In Fig. 3(a), the normally deposited ITO film was chosen as the quarter-wavelength ARC because its refractive index (n = 1.98) is approximately equal to the geometric mean of those of the air and bare Si [33,34]. Additionally, ITO films are widely used as the electrode layers of solar cells because of their good electrical conductivity. The thickness of the ITO film was controlled to d = 400nm (odd multiples of λ/4n), leading to destructive interference with extremely low reflection at certain incident wavelengths. For other incident wavelengths, the reflectivity of the ITO quarter-wavelength ARC is considerably increased, hindering the absorption of solar energy. In comparison to the ITO quarter-wavelength ARC, 2D Si-nanorod array has been suggested as a promising candidate for solar energy harvesting because of its advantageous optical property [35]. According to Fig. 3(b), each individual nanorod is well defined, with diameters between 40 and 90 nm, and each has a constant thickness of d = 350 nm. It is well known that a nanorod diameter that is comparable to or smaller than the incident wavelengths, produces a strong scattering effect and enhances the absorption of solar energy [36]. However, because of the porous structure (f.f. = 40%) of the 2D Si-nanorod array, its effective refractive index (n = 2.18) is much smaller than that of the bare Si (n = 3.95), which provides a similar functionality to the ITO quarter-wavelength ARC. Therefore, although the nanorod array itself can effectively trap incident photons, a significant amount of solar energy is still reflected and wasted because of the large difference between the refractive index of the air and the 2D Si-nanorod arrays. We recognize the intrinsic antireflection effect of the 2D Si-nanorod array. To further reduce the Fresnel reflection that occurs at its interface with the air, it is necessary to insert an optically transparency ( $κ$ ~0) intermediate layer (1<n<2.18). Here, we used the slanted ITO film with controllable porosity as the intermediate layer. As shown in Fig. 3(c), the slanted ITO film (d = 350nm), which consists of nearly continuous nanorods with tilt angle of β = 40° grown by oblique-angle deposition, has a lower refractive index (n = 1.68) than dense ITO (n = 1.98) because of its nano-porous nature. Furthermore, because of the shadowing effect provided by the 2D Si-nanorod arrays, the incident vapor of ITO vapor flow is deposited preferentially on top of the nanorods, and it eventually coalesces altogether. This coalescence forms an optically transparent thin film with a flat surface morphology, as previously discussed with reference to Fig. 1(c). In fact, an optically transparent and electrically conductive slanted ITO film with a nearly continuous surface morphology is extremely important for the subsequent fabrication of the electrode pads of solar cells.

Fig. 3 Cross-sectional SEM images of (a) normally deposited (planar-sheet) ITO film, 2D Si-nanorod arrays (b) without, and (c) with the slanted ITO film. The scale bar of 500nm in the top column applies to all images. The variation of (average) refractive index along the z-direction of each image is also presented in the figure.

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The following discussion concerns the optical characteristics of the samples of interest. Figure 4(a) plots the measured reflectivity versus the incident wavelength. The photographs of all samples (with the identical sizes of 2cm × 2cm) are also shown as inserts in the figure. Accordingly, the color images of all samples are uniform with reasonable fluctuations, suggesting that our fabrication processes for all samples are stable and reliable. The profile of the measured reflectivity of the bare Si (black solid line) decreases monotonically from R = 48.5% to R = 31.8% as the incident wavelength increases from λ = 400 nm to λ = 1000 nm, because of the slight decrease of the refractive index of Si with wavelength. On average, the reflectivity of the bare Si is R = 35.4%. The measured reflectivity is reduced for unmodified Si with the ITO quarter-wavelength ARC. The corresponding profile of the measured reflectivity (red solid line) oscillates decreasingly with respect to that of the bare Si substrate because of the influence of destructive interference of incident light. This film exhibits an average reflectivity of R = 18.7%, with an extremely low reflectivity of R<0.5% at incident wavelengths of λ = 470nm, λ = 640nm, and λ = 990 nm. Furthermore, the average reflectivity of the 2D Si-nanorod array decreases to R = 13.2% with non-significant oscillation fringes (green solid line). This decrease is primarily caused by the randomly distributed Si-nanorods, which effectively scatter and traps incident photons, thus decreasing their reflection and interference in the materials. Most importantly, by inserting the slanted ITO film as an intermediate layer, the Fresnel reflection at the interface between the air and the 2D Si-nanorod array can be further reduced, and the measured reflectivity and corresponding profile (blue solid line) become stable and independent of the incident wavelengths, especially over the visible light region. As a result, a low reflectivity with average value of R = 9.2% is achievable over a broad spectrum. It should be noted that, although the measured reflectivity of the samples is still higher than those ever reported in other studies [4,8], the interaction of the ARC design with the nanostructures accounts for the elimination of the Fresnel reflection, which is of primary concern in the current study.

Fig. 4 (a) Measured reflectivity as a function of normal-incident wavelength for bare Si without (black solid-line) and with (red solid-line) ITO quarter-wavelength ARC, and for the 2D Si-nanorod arrays without (green solid-line) and with (blue solid-line) the slanted ITO film. Insert: photographs of the fabricated samples with dimensions of 2cm × 2cm. (b) Calculated reflectivity as a function of normal-incident wavelength by the Airy formula for all samples. Insert: schematic of solar light emitted into the m-layer stack (c) Calculated absorption, A(θ, λ) = 1- R(θ, λ), obtained by the incidence of TM polarized light for all samples.

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To study the dependence of the reflectivity of the samples on the incident angle of solar illumination, the measured reflectivity at normal incidence was numerically fitted, and the result is shown in Fig. 4(b). The angular-dependent reflectivity of the incident wave, $R (θ, λ)$ , in the stack with m layers [inset of Fig. 4(b)] is expressed by the Airy formula as follow [37]:

R (θ, λ) = {| r_{12 \dots m} |}^{2} = {| \frac{r_{12} + r_{23 \dots m} e^{- i 2 φ_{2}}}{1 + r_{12} \cdot r_{23 \dots m} e^{- i 2 φ_{2}}} |}^{2}, φ_{2} = \frac{2 π}{λ_{0}} {\tilde{n}}_{2} d_{2}, {\tilde{n}}_{2} = n_{2} - i κ_{2}

r_{23 \dots m} = \frac{r_{23} + r_{34 \dots m} e^{- i 2 φ_{3}}}{1 + r_{23} \cdot r_{34 \dots m} e^{- i 2 φ_{3}}}, φ_{3} = \frac{2 π}{λ_{0}} {\tilde{n}}_{3} d_{3}, {\tilde{n}}_{3} = n_{3} - i κ_{3}

r_{34 \dots m} = \frac{r_{34} + r_{45 \dots m} e^{- i 2 φ_{4}}}{1 + r_{34} \cdot r_{45 \dots m} e^{- i 2 φ_{4}}}, φ_{4} = \frac{2 π}{λ_{0}} {\tilde{n}}_{4} d_{4}, {\tilde{n}}_{4} = n_{4} - i κ_{4}

r_{(m - 2) (m - 1) m} = \frac{r_{(m - 2) (m - 1)} + r_{(m - 1) m} e^{- i 2 φ_{m - 1}}}{1 + r_{(m - 2) (m - 1)} \cdot r_{(m - 1) m} e^{- i 2 φ_{m - 1}}}, φ_{m - 1} = \frac{2 π}{λ_{0}} {\tilde{n}}_{m - 1} d_{m - 1}, {\tilde{n}}_{m - 1} = n_{m - 1} - i κ_{m - 1}

r_{(m - 1) m} = \frac{{\tilde{n}}_{m - 1} \cos θ_{m - 1} - {\tilde{n}}_{m} \cos θ_{m}}{{\tilde{n}}_{m - 1} \cos θ_{m - 1} + {\tilde{n}}_{m} \cos θ_{m}} f o r T E, {\tilde{n}}_{m} = n_{m} - i κ_{m}

r_{(m - 1) m} = \frac{{\tilde{n}}_{m - 1} \cos θ_{m} - {\tilde{n}}_{m} \cos θ_{m - 1}}{{\tilde{n}}_{m - 1} \cos θ_{m} + {\tilde{n}}_{m} \cos θ_{m - 1}} f o r T M, {\tilde{n}}_{m} = n_{m} - i κ_{m}

where

φ_{N} = \frac{2 π}{λ_{0}} {\tilde{n}}_{N} d_{N}

is the phase difference induced by the Nth layer (

1 \leq N \leq m

), and

{\tilde{n}}_{N}

and

d_{N}

are the corresponding complex refractive index and the thickness of the Nth layer, respectively. The variable

θ_{N}

denotes the refractive angle of the solar light emitted into the Nth layer. Accordingly, the calculated results shown in Fig. 4(b) are in agreement with the measured reflectivity in Fig. 4(a), suggesting that the consideration of the 2D Si-nanorod array and the slanted ITO film as homogeneous materials with effective refractive indices is indeed feasible, when the propagation of incident waves between the layers is treated numerically.

To gauge the absorption ability of the samples, the calculated absorption values obtained by $A (θ, λ) = 1 - R (θ, λ)$ were plotted in Fig. 4(c), in which $R (θ, λ)$ was determined from the Airy formula for the TM polarized light, and the transmission of the samples is negligible because the thickness of the Si substrate is larger than 500 μm. Across the incident wavelengths studied here, the peak absorption of bare Si (upper left) is relatively low at normal incidence and increases at steeper incidence angles, until the Brewster angle (θ_B = 75.8°) is reached. This absorption profile limits the practical applications for photovoltaics. The average absorption of bare Si (θ = 0−80°; λ = 400−700 nm) is A = 75.85%. With the assistance of the ITO quarter-wavelength ARC (upper right), the normal-incidence absorption is significantly increased at certain wavelengths at which the destructive interference of incident light occurs, which causes $A (θ, λ)$ to exhibit a band-like profile. However, the enhancement of the overall absorption remains insignificant (A = 85.00%). As expected, the incorporation of 2D Si-nanorod arrays (lower left) can virtually eliminate the angular sensitivity of $A (θ, λ)$ , and increases the normal-incidence absorption, except for incident light in the visible region. On average, the absorption of the 2D Si-nanorod arrays is A = 88.09%. With the addition of the slanted ITO film (lower right), the average absorption of the 2D Si-nanorod arrays is increased to A = 92.70%, and $A (θ, λ)$ becomes nearly angle-independent over the broadband spectrum. Similarly, the absorption values of all samples for the TE polarized light were also calculated (not be shown here). The average absorption values (θ = 0−80°; λ = 400−700 nm) are A = 47.83% and A = 71.48% for the bare Si without and with the ITO quarter-wavelength ARC, respectively. The average absorption values of the 2D Si-nanorod arrays without and with slanted ITO films are A = 77.64% and A = 83.67%, respectively. The average absorption values of all samples obtained by the incidence of TM polarized light are obviously larger than those obtained by the incidence of TE polarized light, entirely due to the existence of the Brewster angle at which TM polarized light is perfectly transmitted through the surface of sample, and then absorbed by the silicon substrate underneath. Most importantly, for the incidence of both TE and TM polarized light, the 2D Si-nanorod arrays with the slanted ITO films exhibit high optical absorption over a broad range of wavelengths and incident angles.

To calculate the power generation efficiency (η) of the samples, we assume that each absorbed photon with energy larger than the band-gap energy of Si generates an electron-hole pair that reaches the electrical contacts. Therefore, the current density J versus the voltage V is expressed by the sum of the photon-generated current minus the intrinsic current generated by radiative recombination as follow [38]:

J (V) = \frac{q}{h c} \int_{0}^{\infty} λ \frac{d I}{d λ} A (λ) d λ - \frac{q (n^{2} + 1) E_{g}^{2} k T}{4 π ℏ^{3} c^{2}} e^{(\frac{e V - E_{g}}{k T})}

where

d I / d λ

represents the light intensity incident on the solar cell per unit wavelength (given by the ASTM AM 1.5 solar spectrum [39]),

A (λ)

is the absorption calculated by the Airy formula (as mentioned above), E_g is the band-gap energy of Si,

k T

is the thermal energy at the operating temperature T in Kelvin unit, and n is average refractive index of Si [40]. The resulting calculated J-V curves of all samples are shown in Fig. 5(a) . As depicted, the calculated short-circuit current densities are J_SC = 23.39mA/cm² and J_SC = 29.19 mA/cm² for the bare Si without and with the ITO quarter-wavelength ARC, respectively. The short- circuit current values of the 2D Si-nanorod arrays without and with slanted ITO films are J_SC = 31.20mA/cm² and J_SC = 32.81 mA/cm², respectively. Theoretically, the open-circuit voltage (V_OC) and the fill factor (FF) of all samples are identical and remain approximately V_OC = 0.8V and FF = 0.85, respectively. Compared to the bare Si, the enhancement of the power generation efficiency observed for the other samples is attributable to the enhanced short-circuit current density; i.e., it is attributable to the enhanced absorption according to Eq. (8). As a result, a power generation efficiency of η = 22.70% is achievable for the 2D Si-nanorod arrays with the slanted ITO film, corresponding to an improvement of approximately 42% with respect to that of the bare Si sample.

Fig. 5 (a) Calculated J-V curves using Eq. (8) for all samples. (b) A plot of A_avg calculations corresponding to each absorption A(θ,λ) shown in Fig. 4(c), showing the incident solar light and spectrally weighted absorption of each throughout the day.

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Finally, to distinguish the incident solar light and the spectrally weighted absorption of all samples throughout the operating day of a non-tracking solar cell, the overall fraction of the above band-gap photons that our samples would absorb, A_avg, was calculated based on a time-resolved reference spectrum of direct solar insolation in conjunction with the calculated angle- and wavelength-dependent absorption values, $A (θ, λ)$ . The fraction of above band-gap incident photons that would be absorbed from the reference spectrum, A_avg, is given by the following expression [41]:

A_{a v g} = \frac{\iint Γ (t, λ) A (θ (t), λ) \cos (θ (t)) d λ d t}{\iint Γ (t, λ) \cos (θ (t)) d λ d t}

where

θ (t)

describes the incidence angle of direct sunlight throughout the day and progresses at 15°/hr.

Γ (t, λ)

is the function that specifies the photon flux of direct normal radiation corresponding to reference spectrum at each hour (t) and wavelength (λ) throughout the day [41,42]. Figure 5(b) shows the A_avg calculations that correspond to the contour plot of absorption,

A (θ, λ)

, shown in Fig. 4(c). Importantly, compared to that of the bare Si sample, the 2D Si-nanorod arrays with the slanted ITO films have A_avg = 86.21%, which corresponds to a remarkable enhancement of ~50% and implies the fundamental optical concentration characteristic of the 2D Si-nanorod arrays.

4. Conclusion

In conclusion, we have successfully demonstrated that 2D Si-nanorod arrays with slanted ITO films have strong and angle-insensitive optical absorption over a broad range of incident wavelengths and that they are advantageous for photovoltaic applications. With the experimentally measured properties, such as the complex refractive index, optical reflectivity and physical thickness of the 2D Si-nanorod arrays and slanted ITO film, as the fitted parameters for the Airy formula, their combination and effect on the output performances of the silicon solar cells was numerically investigated. Additionally, the results of this study concerning the optical properties of 2D nanorod arrays and slanted ITO films are not limited to Si-based solar cell, and the revealed geometry suggests techniques to produce high efficiencies in other kinds of nanoscale photovoltaic devices.

Acknowledgments

The authors gratefully acknowledge the financial support from the National Science Council of Republic of China (ROC) in Taiwan under contract Nos. NSC–100–2112–M–003–006–MY3 and NSC–100–2112–M–006–002–MY3, and from the National Taiwan Normal University under award NTNU100-D-01.

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Use of two-dimensional nanorod arrays with slanted ITO film to enhance optical absorption for photovoltaic applications

Abstract

1. Introduction

2. Experiment

3. Results and discussion

4. Conclusion

Acknowledgments

References and links

Cited By

Figures (5)

Equations (9)

Optics Express