1. Introduction
Long
before humans began constructing artificial nanostructures to control
the propagation of light, insects like moths were using such structures
to improve their vision systems. By covering their eyes' facets with
nanostructured nipple arrays, moths create an artificial layer, which
acts as an almost perfect broadband anti-reflection coating. The
protuberances on the eye's facets act as an optically matched layer
between the facets and the surrounding air. This layer greatly reduces
reflectivity and maximizes the incoupling of light in the eye. Photonic
structures such as these can be used as an inspiration to improve the
in- or outcoupling of light in optical sensors, solar cells and light
emitting diodes (LEDs) [1–3].
Bernhard and Miller were the first to seriously occupy themselves with corneal nipple structures in insect compound eyes [4]. Arrays of corneal nipples, radial or parallel ridges and even corneal hairs have, since then, been observed in a variety of insects [5–7] and a few crustaceans [8, 9]. Corneal nipples are particularly common in nocturnal moths, where they were thought to have a variety of roles to play [5, 10, 11]. In order to utilize and mimic the nanostructured corneal nipple arrays for usage in technological applications, it is imperative to understand the propagation of light in such a system and then to further optimize it to achieve the best performance. The influence of the nipple array on the vision of moths has been investigated by several biologists (e.g. [12, 13]), and effects of nipple height, periodicity and spacing of the nipples on the wave propagation have been investigated by scientists and engineers trying to replicate the structures [14–16]. The influence of the shape of the corneal nipple arrays on the optics of insect eyes has so far been studied only by very few authors (e.g. [11]). Stavenga and coworkers analysed the dimensions and shapes of the nipple arrays by atomic force microscopy (AFM) and fitted the surface profile to determine the effective refractive index of the nipple array. The effective refractive index was used as input parameter to simulate the wave propagation by a one-dimensional transfer matrix multiplication method [11]. However, such a model is only valid for wavelengths distinctly larger than the dimensions of the nipples. To the best of our knowledge no full electromagnetic study of nanotextured nipple arrays has been carried out to date.
In section 2, we introduce the insects investigated in this study. Moreover, a brief description on the optical model used to solve the Maxwell's equations in three dimensions is also presented in this section. The results of the optical modelling are described in section 3. In the first part of the results section, we look into the optical properties of corneal nipple arrays for different geometries of the nipple arrays. In the second part, the nanotextured moth-eye nipple arrays were used as the template to construct the amorphous silicon solar cell in substrate configuration. The surface texture reduces the reflectivity of the solar cell and enhances the absorption in the silicon diode layer.
Bernhard and Miller were the first to seriously occupy themselves with corneal nipple structures in insect compound eyes [4]. Arrays of corneal nipples, radial or parallel ridges and even corneal hairs have, since then, been observed in a variety of insects [5–7] and a few crustaceans [8, 9]. Corneal nipples are particularly common in nocturnal moths, where they were thought to have a variety of roles to play [5, 10, 11]. In order to utilize and mimic the nanostructured corneal nipple arrays for usage in technological applications, it is imperative to understand the propagation of light in such a system and then to further optimize it to achieve the best performance. The influence of the nipple array on the vision of moths has been investigated by several biologists (e.g. [12, 13]), and effects of nipple height, periodicity and spacing of the nipples on the wave propagation have been investigated by scientists and engineers trying to replicate the structures [14–16]. The influence of the shape of the corneal nipple arrays on the optics of insect eyes has so far been studied only by very few authors (e.g. [11]). Stavenga and coworkers analysed the dimensions and shapes of the nipple arrays by atomic force microscopy (AFM) and fitted the surface profile to determine the effective refractive index of the nipple array. The effective refractive index was used as input parameter to simulate the wave propagation by a one-dimensional transfer matrix multiplication method [11]. However, such a model is only valid for wavelengths distinctly larger than the dimensions of the nipples. To the best of our knowledge no full electromagnetic study of nanotextured nipple arrays has been carried out to date.
In section 2, we introduce the insects investigated in this study. Moreover, a brief description on the optical model used to solve the Maxwell's equations in three dimensions is also presented in this section. The results of the optical modelling are described in section 3. In the first part of the results section, we look into the optical properties of corneal nipple arrays for different geometries of the nipple arrays. In the second part, the nanotextured moth-eye nipple arrays were used as the template to construct the amorphous silicon solar cell in substrate configuration. The surface texture reduces the reflectivity of the solar cell and enhances the absorption in the silicon diode layer.
2. Samples and methods
2.1. Insect sample
Corneal nipple arrays of the chestnut leafminer Cameraria ohridella (Deschka and Dimic, 1986) were examined by scanning electron microscopy (SEM). The chestnut leafminer (shown in figure 1(a)), known as a widespread pest species in Europe (e.g. [17]
and papers cited within), is a moth of small body size, i.e. 4–5 mm.
For the SEM-investigation, dried samples of the chestnut leafminer were
coated with a layer of approximately 15 nm gold in a sputter coater
(Quorum Q150T S, Quorum Technologies Ltd, East Grinstead, UK) and
observed under a JEOL JSM-5900 SEM, operated at 20 kV.
An overview of the head morphology is shown in figure 1(b).
One compound eye, consisting of about 400 hexagonal facets, is located
on either side of the scale-covered head. The facets are arranged
regularly in lines (figure 1(c)).
Higher magnifications of the corneal surface reveal the presence of
highly organized corneal protuberances, the so-called corneal nipples
(figure 1(d)).
In order to describe the extent and shape of these corneal nipples in
more detail, uncoated samples were investigated by AFM. The AFM
(Nanosurf Mobile S) was operated in the tapping mode and cantilevers for
dynamic scans were used (ACLA-type).
Figure 1. (a) Image of the insect sample used in this study: the chestnut leafminer Cameraria ohridella. (b) SEM image giving an overview of the head morphology. (c) and (d) Higher magnifications of the corneal surface, revealing the corneal nipples.
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2.2. Optical modelling
An atomic force microscope image of the nipple arrays on the moth eye facet is shown in figure 2.
The arrays are closely packed and arranged in a hexagonal grid. A line
scan of surface profile from the nipple array in figure 2(a) is shown in figure 2(b).
The one-dimensional profile exhibits an almost periodic arrangement of
protuberances, with an average diameter of 200 nm. Each nipple has an
almost parabolic outline with a height of 70–80 nm. The height profile, h(x, y), of a nipple can be described by
where h0 is the maximal height of a nipple and rn is the radius of a nipple. The equation is valid for x2 + y2 ≤ r2n.
In order to investigate the influence of the nipples on the optical
properties we numerically modelled the facets using different shapes. By
simulating the optics of the different shapes, we investigated the
reflectivity of parabolas, cones and pillars. Simulations were performed
by solving Maxwell's equations in three dimensions using a finite
difference time domain (FDTD) algorithm. Perfectly matched layer (PML)
boundary conditions were set in the direction of the propagation of
light. Periodic boundary conditions were assumed in the other two
boundary directions. In other words, we assumed a periodic nipple array
in the x- and y-directions. A sketch of the hexagonal
simulation grid and isometric projections of the different nipple array
shapes are shown in figure 3.
Figure 2. (a) AFM scan of corneal nipple arrays on the facets of moth eye. (b) Line scan of the corneal nipple array. Periodic array with period of 200 nm and height around 70 nm is observed.
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Figure 3. Hexagonal
simulation grid for the simulations is shown. Isometric projections of
the three shapes used for the study, pillars, cones and parabolas, are
also depicted.
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The refractive index of the nipples was set to nn = 1.52, which is similar to that of most insect corneae [18–20]. The diameter of the nipples was kept fixed at dn = 2rn = 200 nm, while the height of the nipples, h0, was varying from 0 to 250 nm. The nipple arrays were positioned in a hexagonal grid where ri and ro represent the inner and outer radius of a hexagonal cell, respectively. In the case of maximal packing, the nipple radius rn
is equal to the inner radius of the hexagonal grid. In this case 90% of
surface is covered by nipples. For the pillar-shaped arrays, the radius
of the pillar was taken such that the proportion of the pillars covers
50% of the area of the unit cell.
Figure 2. (a) AFM scan of corneal nipple arrays on the facets of moth eye. (b) Line scan of the corneal nipple array. Periodic array with period of 200 nm and height around 70 nm is observed.
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3. Results
3.1. Reflectivity of corneal nipple arrays
The
anti-reflection properties of the nipple arrays were investigated by
varying the shape of the protuberances on the moth eye facet. The
reflectivity from such surfaces as a function of the incident wavelength
is shown in figure 4. Reflectivity plots for the pillar, conical and paraboloids are shown in figures 4(a)–(c). The reflectivity plot for nipple arrays modelled as circular pillars is shown in figure 4(a).
The dashed curve is shown as a reference for reflectivity of a flat
substrate. The reflectivity was calculated assuming perpendicular
incidence of light, R = [(nn − 1)/(nn + 1)]2.
The reflectivity from circular pillar arrays exhibits strong
modulations with changing heights of the pillar. 50% of the hexagonal
unit cell base area is not covered with the pillars; as a result the
interference of reflections from top and base of the pillars causes the
maxima and minima of the curves. For a specific height of the pillar, we
observe that the reflectivity is reduced to almost zero, albeit only
for a very small band of the incident wavelengths. Therefore, circular
pillar arrays will not be ideal for applications where a low
reflectivity for a broader band is needed (e.g. optical sensors, solar
cells). The reflectivity from cone-shaped arrays is shown in figure 4(b).
The modulation of reflectivity as seen for arrays with pillars is not
seen for cone-shaped arrays. Moreover, the reflectivity steadily
decreases as the height of the cone array is increased. For array
heights higher than 200 nm, reflectivity is almost zero for wavelengths
up to 500 nm. By changing the shape to parabolic arrays, the
reflectivity can be minimized for a much broader spectral range. The
reflectivity curves in figures 4(c)
for parabolic shapes of arrays are almost zero for array heights larger
than 200 nm. By adjusting the height of the parabola the reflectivity
of the broadband anti-reflection layer can be tuned.
Figure 4. Reflection as a function of the incident wavelength from moth eye facets covered with corneal nipple arrays with three different shapes—(a) pillar, (b) cone and (c) parabola. The dashed line represents the reference for a moth eye facet without corneal nipples.
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The
height of the nipples on the surface of the moth eye was approximately
70 nm. In such a case the reflectivity is mainly reduced for short
wavelength down to 300 nm. All investigated moths so far show a spectral
sensitivity of around 300–600 nm with peaks in the UV (345–360 nm), in
the blue (440–460 nm) and the green (515–530 nm) [21, 22]. A limited number of moths possess an additional fourth peak, positioned in the red at 560–600 nm [21]. Assuming Cameraria ohridella
possesses the same spectral sensitivity as other investigated moths,
the reduced reflectivity by the corneal nipple arrays would have its
greatest effect on wavelengths of the UV-band, although reflectivity is
also reduced in the blue and green and thus should increase the total
amount of light that can be absorbed by the retinae of the moth's eyes.
For nipples with periods smaller than λ/(2nn) the propagation of light can be explained by using effective medium theory [23, 24]. The periodic array acts as a refractive index gradient and allows the incident light to couple into the propagating medium with reduced reflection losses. In figure 5, the effective refractive index of an air/nipple interface is shown as the incident light passes through the nipple array. The effective refractive index, neff, can be calculated based on
where An(h) is the area of the nipple as a function of the height of the nipple. Abase is the base area of a nipple. nn and nair
are the refractive index of the nipple and the surrounding media, which
is in this case air. In the case of a parabolic-shaped nipple the
effective refractive index is given by
where h is the height of the nipple, h0 is the maximal height of the nipple, rn is the radius of the nipple and ri is the inner radius of the hexagonal grid. In figure 5
it is assumed that the radius of the nipple is equal to the inner
radius of the hexagonal grid. The straight line for the parabola-shaped
nipples represents a PML resulting in the lowest possible reflection.
Therefore, the reflection properties, as seen in figure 4,
are consistent with the refractive index gradient arising from the
shape of the nipple arrays. Due to the abrupt change of the refractive
index observed for the circular pillar arrays, the reflectivity from
such a surface is higher compared to a cone or parabola-shaped nipple
array.
Figure 5. Change of effective refractive index as the incident light travels from air (n = 1) into the moth eye (n = 1.52). Normalized height of 1 represents the peak of the corneal nipple arrays, and 0 represents the base surface of the eye facet.
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Reflection
properties from moth eye facets covered with a nipple array were so far
discussed for the normal incidence of light. The reflectivity for a
nipple array as a function of the incident angle is shown in figure 6.
Figure 6. Reflection as a function of the incident angle from moth eye facets with parabola-shaped corneal nipple arrays. A circularly polarized monochromatic incident light of wavelength 400 nm was assumed.
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The
heights of the paraboloid nipple arrays were 50, 100 and 200 nm with
their period fixed at 200 nm. The reflectivity was calculated for a
monochromatic wavelength of 400 nm. The polarization of the incident
light was assumed to be circular. Similar to what is observed for normal
incidence, the reflectivity is reduced as the height of the corneal
nipples is increased. The reflectivity does not change much up to an
angle of incidence of 25°.
Hence, nipple arrays on the facet of moth eyes act as an anti-reflection coating. The shape of the nipples allows for almost optimal anti-reflection properties over a broad spectral range. The parabolic nipple arrays are superior to numerous technical solutions, where simply arrays of pillar-shaped (or tapered pillar) structures are used to minimize optical reflection [24]. Furthermore, the reflection is reduced for all angles of incidence.
Figure 4. Reflection as a function of the incident wavelength from moth eye facets covered with corneal nipple arrays with three different shapes—(a) pillar, (b) cone and (c) parabola. The dashed line represents the reference for a moth eye facet without corneal nipples.
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For nipples with periods smaller than λ/(2nn) the propagation of light can be explained by using effective medium theory [23, 24]. The periodic array acts as a refractive index gradient and allows the incident light to couple into the propagating medium with reduced reflection losses. In figure 5, the effective refractive index of an air/nipple interface is shown as the incident light passes through the nipple array. The effective refractive index, neff, can be calculated based on
Figure 5. Change of effective refractive index as the incident light travels from air (n = 1) into the moth eye (n = 1.52). Normalized height of 1 represents the peak of the corneal nipple arrays, and 0 represents the base surface of the eye facet.
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Figure 6. Reflection as a function of the incident angle from moth eye facets with parabola-shaped corneal nipple arrays. A circularly polarized monochromatic incident light of wavelength 400 nm was assumed.
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Hence, nipple arrays on the facet of moth eyes act as an anti-reflection coating. The shape of the nipples allows for almost optimal anti-reflection properties over a broad spectral range. The parabolic nipple arrays are superior to numerous technical solutions, where simply arrays of pillar-shaped (or tapered pillar) structures are used to minimize optical reflection [24]. Furthermore, the reflection is reduced for all angles of incidence.
3.2. Solar cells with nipple arrays
Reducing
the cost and increasing the conversion efficiency is a major objective
of research and development on solar cells. In this study we will focus
on the improved incoupling of light in a silicon thin film solar cell by
utilizing anti-reflection coatings. As a consequence the absorption of
the solar cell is increased, which results in an increased short circuit
current and conversion efficiency. Most anti-reflection coatings (ARC)
are based on quarter wavelength thick layers. Ideally the refractive
index of an optimal quarter wavelength anti-reflection coating is given
by 
, where n1 and n2
are the refractive indices of the two materials involved. Furthermore,
the extinction coefficient of the anti-reflection coating should be as
low as possible, so that almost no light is absorbed in the
anti-reflection coating. The operation principle of the quarter
wavelength layer is based on the destructive interference of waves being
reflected at the interface between medium 1/ARC and ARC/medium 2.
However, the condition for destructive interference is only fulfilled
for a narrow band of wavelengths. For other wavelengths the reflection
distinctly increases. The relationship between the thickness of the
anti-reflection layer and the optimum wavelength is given by 4dARCnARC = λ, where dARC
is the thickness of the anti-reflection coating and λ is the incident
wavelength. A significant increase of the reflection is observed for
wavelengths different from the optimal wavelength.
The investigation discussed in the previous subsection has shown that parabola-shaped nipple arrays exhibit excellent anti-reflection properties over a broad spectral range. In this subsection we discuss the influence of a moth-eye surface texture on the wave propagation and the absorption of light in an amorphous silicon thin film solar cell. The optical properties of hydrogenated amorphous silicon (a-Si:H) and aluminium-doped zinc oxide (ZnO:Al) layers, which form the diode and front electrode layers of thin film silicon solar cells, are shown in figure 7. In order to illustrate a comparison between amorphous silicon and classical crystalline silicon (c-Si), the optical properties of crystalline silicon are also highlighted in figure 7. The optical data for crystalline silicon have been reproduced from [25]. The refractive indices (or the real part of the complex refractive indices) of a-Si:H, c-Si and ZnO:Al as a function of the incident wavelength are shown in figure 7(a). For wavelengths larger than 400 nm, the refractive indices of a-Si:H and c-Si are comparable. The refractive index of ZnO:Al remains mostly unchanged for varying wavelengths of the incident optical spectrum. In figure 7(b) the absorption coefficients of a-Si:H, c-Si and ZnO:Al as a function of the incident wavelength are shown. The absorption coefficients of a-Si:H and c-Si exhibit large differences. For incident wavelengths of 400–700 nm a-Si:H has a significantly higher absorption coefficient than c-Si. Thus, compared to the thickness of classical wafer-based mono- or multi-crystalline silicon solar cells (around 200–300 µm), the thickness of amorphous silicon solar cells can be much thinner (around 300–400 nm only). The absorption coefficient of ZnO:Al is very low which allows for high transmission into the silicon cell.
Figure 7. (a) Refractive indices (real part of the complex refractive index) and (b) absorption coefficient of hydrogenated amorphous silicon (a-Si:H), crystalline silicon (c-Si) and aluminium-doped zinc oxide (ZnO:Al) as a function of the incident wavelength of light.
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The
layer sequence of the solar cell used in this study is consistent with
amorphous silicon solar cells prepared in industry and academia [26–28].
We modified the classical layer sequence of the solar cell by
introducing a back contact with a nipple texture. The n-i-p amorphous
silicon solar cell in substrate configuration (light enters the solar
cell through the front transparent conductive oxide layer) is
conformally prepared on top of the textured back contact. A schematic
cross section of such a textured amorphous silicon solar cell is shown
in figure 8(a).
The surface profile of the individual nipples on the surface was
assumed to be parabolic. The back contact of the solar cell consists of
an 80 nm thick zinc-oxide layer along with a perfectly reflecting metal
contact. Following the back contact, the solar cell consists of a 300 nm
thick (n-i-p) hydrogenated amorphous silicon solar cell. The n-layer
and p-layer both were assumed to be 10 nm thick. Finally, in our
simulation study, the solar cell structure is completed with a 500 nm
thick aluminium-doped zinc-oxide layer as the front contact electrode.
Experimentally, the zinc oxide layer is prepared by sputtering and the
silicon layers are prepared by plasma-enhanced chemical vapour
deposition (PECVD) [26].
The quantum efficiencies of the solar cells with varying period and
height of the parabolic nipple arrays are shown in figures 8(b).
Figure 8. (a)
Schematic cross section of 300 nm thick amorphous silicon solar cell
deposited on a textured metal back contact with moth-eye
(parabola-shaped) nipple arrays. (b) Quantum efficiency plots for parabola-shaped textured solar cells with period 200 nm and heights of 70 nm and 200 nm. (c)
Reflectivity plots of the corresponding textured solar cells. The
quantum efficiency and reflectivity for a solar cell deposited on a
smooth substrate are also shown as reference in (b) and (c).
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The
quantum efficiency is defined as the ratio of the power absorbed by the
absorber (i-layer) of the solar cell to the total power incident on the
unit cell. The quantum efficiency was calculated using the equation
where c is the speed of light in free space; ε0 is the permittivity of free space; α is the energy absorption coefficient (α = 4πk/λ), with n and k being the real and imaginary parts of the complex refractive index; λ is the wavelength; E is the electric field and Popt is the optical input power. Based on the quantum efficiency, the short circuit current can be calculated as
where q is the elementary charge, h is Planck's constant and S(λ)
is the weighted sun spectrum (AM 1.5 spectral irradiance). A more
detailed description of these calculations for the quantum efficiency
and short circuit current density is given in [29].
The quantum efficiency of a 300 nm thick amorphous silicon solar cell
deposited on a smooth substrate is also shown in figure 8(b).
The quantum efficiency on a smooth substrate is used as a reference.
The modulation of the quantum efficiency of the solar cell on the smooth
substrate is caused by the optical interference of light in the layers
of the amorphous silicon solar cell. Along with it, the quantum
efficiencies of solar cells with parabolic nipple arrays with period of
200 nm and heights of 70 and 200 nm are shown. These values for the
heights were chosen since the moth eyes exhibit nipple heights in the
range of 70 nm (which have good anti-reflection properties for shorter
wavelengths <400 nm), whereas our investigation from the previous
section has shown that broadband anti-reflection property (up to 800 nm
wavelength) was achieved for nipple heights higher than 200 nm. With the
introduction of the texture, an increase in the quantum efficiency up
to a wavelength of 600 nm is observed. For shorter wavelengths (<600
nm), the enhancement is achieved due to better incoupling of the
incident light into the solar cell. Similar to what was observed in
figure 4(c),
the higher height of 200 nm of the nipple array resulted in better
incoupling of the incident light compared with the nipple array of
height of 70 nm. A slight increase of the quantum efficiency in the
longer wavelength region is also observed. The reflections from the
simulated solar cells on a smooth substrate and with parabolic nipple
arrays of two different heights are also shown in figure 8(c).
Consistent with the quantum efficiency plots, the reflectivities from
the solar cells exhibit opposite behaviour. The solar cell with
parabolic nipple arrays of height of 200 nm shows the least reflectivity
up to the wavelength of 630 nm. The solar cell on a smooth substrate,
on the other hand, reflects the most since it does not have any optical
design for better incoupling of the incident light. The reflectivity for
all three solar cells increases significantly as the incident
wavelength gets close to the optical bandgap of a-Si:H (around 720 nm).
Based on the quantum efficiency, the achievable short circuit current
density was calculated assuming a standard (AM1.5) sun spectrum. The
calculated short circuit current values are given in table 1.
In terms of fabrication of such textured arrays, several research groups have already demonstrated that moth-eye anti-reflection coatings grown on silicon wafers or transparent conductive zinc oxide can greatly reduce the reflection losses from the surface [31–34]. Such moth-eye anti-reflection coatings can be of major importance for solar cells where the overall optical wave propagation is dominated by incoupling of light and not by diffraction and scattering of the incident light. This is the case for amorphous silicon solar cells (as discussed herein) and organic solar cells. In both cases the thickness of the solar cell is small or very small in comparison to the wavelength of the incident light. Thus, the proposed concept of integrating the moth-eye structures into the solar cells provides a promising route to enhance the performance of thin and ultra-thin solar cells.
, where n1 and n2
are the refractive indices of the two materials involved. Furthermore,
the extinction coefficient of the anti-reflection coating should be as
low as possible, so that almost no light is absorbed in the
anti-reflection coating. The operation principle of the quarter
wavelength layer is based on the destructive interference of waves being
reflected at the interface between medium 1/ARC and ARC/medium 2.
However, the condition for destructive interference is only fulfilled
for a narrow band of wavelengths. For other wavelengths the reflection
distinctly increases. The relationship between the thickness of the
anti-reflection layer and the optimum wavelength is given by 4dARCnARC = λ, where dARC
is the thickness of the anti-reflection coating and λ is the incident
wavelength. A significant increase of the reflection is observed for
wavelengths different from the optimal wavelength.The investigation discussed in the previous subsection has shown that parabola-shaped nipple arrays exhibit excellent anti-reflection properties over a broad spectral range. In this subsection we discuss the influence of a moth-eye surface texture on the wave propagation and the absorption of light in an amorphous silicon thin film solar cell. The optical properties of hydrogenated amorphous silicon (a-Si:H) and aluminium-doped zinc oxide (ZnO:Al) layers, which form the diode and front electrode layers of thin film silicon solar cells, are shown in figure 7. In order to illustrate a comparison between amorphous silicon and classical crystalline silicon (c-Si), the optical properties of crystalline silicon are also highlighted in figure 7. The optical data for crystalline silicon have been reproduced from [25]. The refractive indices (or the real part of the complex refractive indices) of a-Si:H, c-Si and ZnO:Al as a function of the incident wavelength are shown in figure 7(a). For wavelengths larger than 400 nm, the refractive indices of a-Si:H and c-Si are comparable. The refractive index of ZnO:Al remains mostly unchanged for varying wavelengths of the incident optical spectrum. In figure 7(b) the absorption coefficients of a-Si:H, c-Si and ZnO:Al as a function of the incident wavelength are shown. The absorption coefficients of a-Si:H and c-Si exhibit large differences. For incident wavelengths of 400–700 nm a-Si:H has a significantly higher absorption coefficient than c-Si. Thus, compared to the thickness of classical wafer-based mono- or multi-crystalline silicon solar cells (around 200–300 µm), the thickness of amorphous silicon solar cells can be much thinner (around 300–400 nm only). The absorption coefficient of ZnO:Al is very low which allows for high transmission into the silicon cell.
Figure 7. (a) Refractive indices (real part of the complex refractive index) and (b) absorption coefficient of hydrogenated amorphous silicon (a-Si:H), crystalline silicon (c-Si) and aluminium-doped zinc oxide (ZnO:Al) as a function of the incident wavelength of light.
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Table 1. Calculated
short circuit values of a-Si:H thin film solar cells deposited on a
smooth substrate and with parabolic nipple array texture with heights of
70 and 200 nm. The corresponding quantum efficiencies of the solar
cells are shown in figure 8(b).
Compared with the short circuit current density of 12 mA cm−2
for a solar cell on a smooth substrate, a gain of 27% and 47% is
achieved for solar cells with parabolic nipple arrays of 200 nm
periodicity and heights of 70 and 200 nm, respectively. The gain in the
short circuit current is caused by improved incoupling of the shorter
wavelengths.| Short circuit current (mA cm−2) | |
| Smooth substrate | 12.00 |
| Parabola texture (P-200 nm, H-70 nm) | 15.20 |
| Parabola texture (P-200 nm, H-200 nm) | 17.65 |
In terms of fabrication of such textured arrays, several research groups have already demonstrated that moth-eye anti-reflection coatings grown on silicon wafers or transparent conductive zinc oxide can greatly reduce the reflection losses from the surface [31–34]. Such moth-eye anti-reflection coatings can be of major importance for solar cells where the overall optical wave propagation is dominated by incoupling of light and not by diffraction and scattering of the incident light. This is the case for amorphous silicon solar cells (as discussed herein) and organic solar cells. In both cases the thickness of the solar cell is small or very small in comparison to the wavelength of the incident light. Thus, the proposed concept of integrating the moth-eye structures into the solar cells provides a promising route to enhance the performance of thin and ultra-thin solar cells.
4. Summary
Moths
can efficiently couple light into their eyes with the aid of the
corneal nipple arrays on their eye facets. Corneal nipples with a
parabolic shape create an artificial coating on the facet, which shows
broadband anti-reflection properties. These are achieved due to the
linear change of the refractive index arising from the geometry of the
protuberances' shapes. In the next step parabolic nipple arrays were
applied for texturing amorphous silicon solar cells. The nipple arrays
cause a reduction in the reflectivity and an increase of the quantum
efficiency for the shorter wavelengths. Compared with the solar cell on a
smooth substrate, quantum efficiencies and the short circuit current of
the textured solar cells were significantly enhanced.
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- source: iopscience.iop.org
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