GB2585900A

GB2585900A - Photovoltaic cell and method of manufacturing a photovoltaic cell

Info

Publication number: GB2585900A
Application number: GB1910464.5A
Authority: GB
Inventors: Ogorodnov Sergey
Original assignee: Individual
Current assignee: Individual
Priority date: 2019-07-22
Filing date: 2019-07-22
Publication date: 2021-01-27
Also published as: GB201910464D0

Abstract

A photovoltaic cell 100 comprises a photovoltaic layer 40 having an upper surface 42, and protrusions (figure 2, 44) extending from the upper surface 42, wherein the protrusions are arranged to reduce the reflection of incident light. This increases the transmission of light to the photovoltaic layer 40, increasing the efficiency of the cell 100. The protrusions are arranged to imitate the cuticle of a firefly. Preferably the protrusions have a height in the range of 80-160 nm, a width in the range of 120-180 nm, and the spacing between protrusions in the range of 70-130 nm. A method of manufacturing is also claimed.

Description

Photovoltaic Cell and Method of Manufacturing a Photovoltaic Cell

Technical field

The present invention generally relates to a photovoltaic cell, a method of manufacturing a photovoltaic cell and a method of generating electricity by a photovoltaic cell. 1.0

Background

A photovoltaic cell or solar cell is a device which converts light into electricity. Photovoltaic, cells have numerous applications, ranging from powering small is portable devices such as calculators to the large scale generation of electricity for the power grid. Photovoltaic cells have also been used to power vehicles such as satellites and electric cars.

A limitation of photovoltaic cells is that only a relatively small proportion of light incident on the cells is converted to electricity. It would be desirable to improve the efficiency of the energy conversion.

Summary

In one aspect a photovoltaic cell is provided comprising a photovoltaic layer having an upper surface for receiving incident light; and protrusions extending from the upper surface; wherein the protrusions are arranged to imitate a cuticle of a light emitting organ of a firefly, so as to reduce reflection of incident light. The inventors have observed that the light emitting organ of a firefly includes an outer cuticle layer (referred to herein as a "firefly cuticle" for brevity) which is effective for reducing the reflection of incident light. It has been further observed that the loss of incident light by reflection is one factor which limits the efficiency of existing photovoltaic cells. Applying the concepts used in nature to reduce reflection may allow for improvements in efficiency.

In accordance with a more specific aspect the protrusions may extend by heights in the range 80 to 160 nm. In an even more specific aspect, the protrusions may extend by heights in the range of 110 to 130 nm. The protrusions found on the firefly cuticle may have heights within these ranges. It has also been observed that a particularly large reduction in reflectance can be observed when the height of the protrusions is roughly 120 nm (i.e., in the range of 110 to 130 nm).

The protrusions may have widths in the range 120 to 180 nm. In a more specific application, the protrusions may have widths in the range 140 to 160 nm. Widths in these ranges are similar to the widths of protrusions found on the firefly cuticle.

Spacings between adjacent protrusions may be in the range 70 to 130 nm. A more specific possible range is 90 to 110 nm, It may be observed that spacings between adjacent protrusions in a firefly cuticle are typically within these ranges.

Protrusions may be considered to imitate, resemble, or mimic the surface structure of a cuticle of a light emitting organ of a firefly when the protrusions have heights in the range 80 to 160 nm and/or widths in the range 120 to 180 nm and/or wherein spacings between adjacent protrusions are in the range 70 to 130 run.

The protrusions may comprise cylindrical protrusions. Cylindrical protrusions are one example which has been found to be effective for reducing reflections. The cylindrical protrusions may be arranged in a honeycomb pattern. This may imitate the striated appearance of a firefly cuticle.

The prot ions may comprise elongate protrusions. Further, the protrusions may be arranged in the same direction as one another. This may imitate closely the striated appearance of a firefly cuticle The photovoltaic layer may have a reflective index in the range 1.4 to 1.6. Firefly cuticles comprise chitin, which is reported to have a refractive index of approximately 1.5. Using a photovoltaic layer with a refractive index similar to that of firefly chitin may improve the reduction in reflection.

The photovoltaic cell may be a thin film photovoltaic cell. Thin film photovoltaic cells may have layer thicknesses comparable to thicknesses of layers found in the light-emitting organ of a firefly.

The photovoltaic layer may comprise an organic photovoltaic material. Organic photovoltaic materials, e.g. photovoltaic polymers, may be advantageous for some applications and in particular where a flexible photovoltaic cell is desired.

is The photovoltaic layer may comprise silicon.

The photovoltaic cell may further comprise a reflective layer arranged below the photovoltaic layer. The reflective layer may improve the efficiency of photovoltaic cell, by allowing capture of more light by the photovoltaic layer rather than allowing that light to exit the photovoltaic cell.

The reflective layer may comprises a particulate arranged to imitate a reflector layer of a light emitting organ of a firefly. The light-emitting organ of a firefly includes a reflector layer which has been observed to be particularly effective for reflecting light at wavelengths within the visible spectrum. In an example, the particulate may comprise hollow particles of silica. The hollow particles of silica may have walls with a thickness in the range of 0.05 pm to 0.15 pm. The particulate may comprise particles having a mass median diameter in the range 0.8 pm to 1.2 pm.

The reflective layer may have a thickness in the range 10 to 200 pm, and more particularly the reflective layer may have a thickness of at least 45 pm. This may approximate the thickness of the reflector layer found in a light emitting organ of a firefly.

The photovoltaic cell may be incorporated into a wide variety of devices. One particular example is a vehicle, such as an electrically-powered car, motorbike, scooter, or personal electric vehicle.

A further aspect provides a method of manufacturing the photovoltaic cell as described above, the method comprising producing the photovoltaic layer of the photovoltaic cell such that the protrusions on the upper surface of the photovoltaic layer resemble a cuticle of a light emitting organ of a firefly.. In accordance with a more specific aspect the method comprises 3-D printing the photovoltaic layer.

Another aspect provides a method of generating electricity by a photovoltaic cell comprising: receiving incident light by protrusions extending from an upper surface of a photovoltaic layer of the photovoltaic cell and arranged to resemble a cuticle of a light emitting organ of a firefly, and reducing reflection of incident light by the protrusions.

Brief Description of the Drawings

The present invention will become more fully understood from the detailed description and the accompanying drawings, in, which: Figure 1 is a cross-sectional view of a photovoltaic cell; Figure 2 is a plan view of a first example of a photovoltaic layer; Figure 3 is a plan view of a second example of a photovoltaic layer: Figure 4 is a scanning electron microscopy image of the surface of the cuticle of a light emitting organ of a firefly; and Figures 5 and 6 are flowcharts according to certain examples.

It will be appreciated that Figs. 1 to 3 of the drawings are schematic and are not to scale. Relative proportions of one or more components in Figs. 1 to 3 may be exaggerated for ease of representation.

Detailed Description

Directional terms such as "top" and 'bottom" are used herein for convenience of description, and refer to the orientation of the photovoltaic cell shown in Figure 1. For the avoidance of any doubt, this terminology is not intended to limit the orientation of the photovoltaic cell in an external frame of reference.

The expression "average particle size" as used herein refers to the mass 10 median diameter, D50, unless otherwise stated. Particle size may be measured by optical granulometry, for example.

The word "about" when used in connection with an numeric value means that the value may deviate from the stated value by ± 5%, and in particular aspects 15 by t 1%.

An example of a photovoltaic cell in accordance with the present invention will now be explained with reference to Figures 1 to 3.

Photovoltaic cell 100 comprises a substrate layer 10, a reflective layer 20, a lower electrode 30, a photovoltaic layer 40, upper electrodes 50a, 50b, and a covering layer 60.

The substrate layer 10 is the bottom layer of the photovoltaic cell 100. The substrate layer provides physical support for the other components of the photovoltaic cell. The material and construction of the substrate layer are not particularly limited and may be selected depending on the desired application of the photovoltaic cell. Examples of suitable materials include glass and polymer materials.

Reflective layer 20 is arranged on substrate layer 10. Light which reaches the reflective layer 20 may be reflected into the photovoltaic layer 40. This may improve the energy conversion efficiency of the photovoltaic device, by allowing for increased absorption of light by tho photovoltaic layer.

In the example shown in Figure 1, reflective layer 20 imitates the structure of the reflective layer of a firefly's light generating organ. This structure may provide high reflectivity, or in other words may reflect a particularly large proportion of light incident on the reflective layer.

Particularly, high reflectivity at wavelengths in the range 460 to 800 nm may be obtained. This range encompasses the majority of the visible range, 380 to 740 nm, and part of the near-infrared range, 750 to 1400 nm. The absorption spectra of photovoltaic materials typically have peaks in the visible range and/or near-infrared range. By reflecting light back into the photovoltaic layer, particularly light having wavelengths which can be absorbed by the photovoltaic layer, the reflective layer may improve the overall energy conversion efficiency of the device.

Reflective layer 20 comprises a particulate material. The particles of the particulate material are hollow microparticles. In this example, the particulate material is formed of silica, 102, The particles may have a diameter of about 1 pm. For example, the particulate may have a mass median diameter, QSc, in the range 0.8 to 1.2 pm. At least 90 % of the particles by weight may have a diameter in the range 0.8 to 1.2 pm.

The walls of the particles may have a thickness in the range of 0.05 pm to 0.15 A method for obtaining the particles may comprise preparing a template such as polystyrene particles; treating the particles with a reagent for forming a silica layer, such as a silane e.g. tetraethoxysilane; and removing the template using a solvent, for example toluene. One example of a suitable method is described by Chen et al, "Bioinspired photonic structures by the reflector layer of firefly lantern for highly efficient chemiluminescence", Sci. Rep. 5, 12965, the content of which is hereby incorporated by reference in its entirety.

It has been reported that the reflectivity of such a reflective layer varies with the thickness of the layer. The reflective layer 20 may have a thickness of at least 10 pm, arid in certain aspects a thickness of at least 45 pm. Thicknesses in the range 40 to 60 pm approximate the thickness of the reflective layer of the light organ of a firefly.

to A lower electrode 30 is arranged between the reflective layer 20 and photovoltaic layer 40. Upper electrodes 50a, 50b are arranged on an upper surface 42 of the photovoltaic layer 40. The lower electrodes and upper electrodes together provide electrical contacts for the photovoltaic layer. The construction of the upper and lower electrodes is not particularly limited so long is as appropriate electrical contact is achieved. The electrodes could take any suitable form, and could be disposed in any position in the photovoltaic device.

When lower electrode 30 is in the form of a layer, the lower electrode is suitably formed of a material which is transparent to light at wavelengths at which the photovoltaic layer absorbs light. Typically, the material may be transparent to light in the visible spectrum, 380 to 740 nm and/or near infrared spectrum 750 to 1400 nm.

A material may be considered transparent when, for example, at least 70 % of incident light at wavelengths in the desired range can be transmitted through a pm thick piece of the material.

Examples of materials which may be used to construct transparent electrodes include transparent conductive oxides such as indium tin oxide, aluminium-doped zinc oxide, or indium-doped cadmium oxide; transparent conducting polymer compositions such as poly(3,4-ethylenedioxythiophene), a mixture of poly(3,4-ethylenedioxythiophene) and poly(styrene sulfonate), and a mixture of poly(4,4-dioctyl cyclopentadithiophene) with iodine or 2,3-dichloro-5,6-dicyano1,4-benzoquinone; and carbon nanotubes.

Non-transparent materials such as copper can be used to construct electrodes s capable of transmitting light. Configuring an electrode as a mesh or network of spaced-apart wires may allow light to be transmitted through gaps in the electrode. In this arrangement, the wires may be nanowires, having a diameter of 500 nm or less.

la In Figure 1, photovoltaic layer 40 is arranged on the lower electrode 30.

Photovoltaic layer 40 may be formed from any suitable photovoltaic material. The photovoltaic layer may comprise an inorganic photovoltaic material such as silicon. The silicon may be monociystalline, muiticrystalline or amorphous. Further examples of inorganic photovoltaic materials include cadmium telluride and copper indium gallium (di)selenide. The photovoltaic layer may comprise an organic photovoltaic material. Examples of organic photovoltaic materials include polyacetyiene and poly(phenylene vinylene).

Photovoltaic layer 40 has an upper surface 42. Protrusions extend from the upper surface 42. The protrusions are arranged to imitate -a cuticle of a-light emitting organ of a firefly, so as to reduce reflection of incident light. By 'imitate' is meant that the protrusions resemble or approximate the structure of the firefly cuticle, particularly the outer surface of the firefly cuticle. Figure 4 is an example of a scanning electron microscopy image showing surface of a firefly cuticle.

Reducing reflection of incident light may improve the conversion efficiency of the photovoltaic cell, by increasing the amount of light which reaches the photovoltaic layer. It has been observed that the light emitting organs of fireflies include an outer cuticle layer which is particularly effective for reducing reflection of light in the visible range. The cuticle layer of a firefly's light emitting organ has a textured surface with a regular pattern.

Figure 2 shows one example of an upper surface 42 of a photovoltaic layer having protrusions 44 extending therefrom. In this example, protrusions 44 are generally cylindrical. The protrusions 44 extend from the upper surface 42 by a height in the range 80 to 160 nm. Heights in the range 110 to 130 nm, more particularly about 120 nm are particularly effective at reducing reflections at wavelengths in the visible range.

Protrusions 44 of the example shown in Fig. 2 have widths W1 in the in the range 120 to 180 nm, and particularly 140 to 160 nm. This imitates the widths to of the surface protrusions of the cuticle of a firefly's light organ.

Protrusions 44 in Fig. 2 are arranged in a honeycomb or checkerboard pattern. In other words, rows of protrusions extend in a length direction. Protrusions of adjacent rows are offset from one another in the length direction.

Protrusions 44 are spaced from adjacent protrusions by spacings P1 in the range 70 to 130 nm, for example 90 to 110 nm. These ranges approximate the spacings between protrusions on the cuticle of a light-emitting organ of a firefly.

zo In an example, the material of the photovoltaic layer is selected to have a refractive index in the range 1.4 to 1.6. This approximates the refractive index of the chitin which forms the cuticle of a firefly's light emitting organ.

By configuring the surface of the photovoltaic layer to approximate that of the cuticle of a firefly's light emitting organ, the reflection of incident light may be reduced. Although lens for LEDs including protrusions have been reported by Kim et al ("Biologically inspired LED lens from cuticular nanostructures of firefly lantern", PNAS, vol. 109, no. 46, pp. 18674-18678), the contents of which are hereby incorporated by reference, it is believed that this concept has not been applied to photovoltaic cells.

A further example illustrating the surface of a photovoltaic layer 42a is shown in Fig. 3. In this example, protrusions 44a are elongate protrusions which extend in a length direction, The protrusions have width W2. The protrusions are spaced from one another by spacings P2, with regions of flat surface 46a present between adjacent protrusions.

The width and height of the protrusions, and the spacing between protrusions, may all be within the ranges described above with reference to Fig. 2.

Elongate protrusions resemble closely the protrusions found on the cuticles of light organs of fireflies.

Although the protrusions illustrated in Fig. 2 are substantially straight, substantially parallel, and substantially identical to one another, this is not essential. Natural cuticles of fireflies have elongate protrusions which are ordered in the sense of being aligned in approximately the same direction as one another. Natural protrusions have variable shapes. widths and spacings although these parameters generally remain within the ranges described hereinabove. The width of any given protrusion may vary along its length, Referring again to Figure 1, upper electrodes 50a and bob provide electrical contact to the upper surface 42 of the photovoltaic layer. The construction of the upper electrodes is not particularly. limited. As with the lower electrode 30, upper electrodes 50a, 50b may usefully be constructed from a transparent conductive material.

A covering layer 60 is provided over photovoltaic layer 40. The covering layer may protect the photovoltaic layer 40 from physical damage, and/or from the ingress of water, dust or the like. Covering layer 60 may provide physical support for the photovoltaic device. Covering layer 60 is generally constructed from a transparent material, such as glass or a polymer.

Covering layer 60 may include an anti-reflective coating.

I I

In a variant, covering layer 60 may be provided with protrusions arranged to imitate a cuticle of a light emitting organ of a firefly, so as to reduce reflection of incident light. The protrusions may be as described previously with reference to the photovoltaic layer. Either covering layer 60 or photovoltaic layer 40 or both may include the protrusions.

Fig. 5 is a flow chart showing an illustrative method of manufacturing the photovoltaic device as described herein.

At block 502, a substrate layer as defined above is provided. The substrate layer may be as discussed above with reference to the photovoltaic device. In the present example, the photovoltaic device is built up on the substrate layer but this can also be provide otherwise.

is At block 504, a reflective layer is formed on the substrate layer. In an example, a mixture comprising hollow silica particles and a solvent such as water is applied to the substrate layer, and the solvent is allowed to evaporate thereby forming a layer of particulates arranged to imitate a reflector layer of a light emitting organ of a firefly. Layer thickness may be varied by varying the concentration of hollow silica particles in the mixture.

At block 506, a lower electrode layer is formed on the reflective layer. The process for forming the lower electrode layer is not particularly limited, and may be selected based on the desired electrode material and structure. Examples of techniques for forming layers include chemical vapour deposition and spin-coating.

At block 508, a photovoltaic layer is formed on the lower electrode layer. The layer is formed so that it comprises protrusions extending from the upper surface and imitating a cuticle of a light emitting organ of a firefly. Such formation is found to be advantageous in reducing reflection of incident light, and thus improving the efficiency of the cell.

The forming may comprise 3D-printing the photovoltaic layer to obtain a photovoltaic layer having a surface with protrusions extending from the surface, wherein the protrusions are arranged to imitate a cuticle of a light-emitting organ of a firefly so as to reduce reflection of incident light. 3D printing is an additive process where material is laid down, by progressively adding material to form a product of desired shape, size and appearance. In this specification term '3D printing' is used to refer to additive manufacturing processes that can be used to create physical objects based on three dimensional digital data. The additive manufacturing processes can be arranged to re-form a raw material by the addition of energy and positioning in a controlled manner. The control is provided by an appropriate control unit based on the digital data. A variety of 3D printing processes are available. Also, a variety of stock materials (polymers, metals, ceramics, etc.) in differing forms (e.g. liquids, powders, granules, and filament) are available. 1s

The material can be in liquid form and 3D printable. The printing can be provided on any appropriate surface. For example, protru,sion can be printed on a vehicle such as a car, boat or aeroplane during production instead of normal painting process.

At block 510, upper electrodes are formed on the photovoltaic layer. The method for forming the upper electrodes may be selected as appropriate.

At block 512, a covering layer is provided to cover the upper electrodes and photovoltaic layer. In examples where the covering layer is a polymer, the covering layer may be formed in situ. In examples where the covering layer is glass, the covering layer is typically prepared in advance and then attached to other components of the photovoltaic cell in an appropriate manner.

Fig. 6 is a flow chart illustrating a method of generating electricity by a photovoltaic cell. The photovoltaic cell includes a photovoltaic layer having an upper surface for receiving incident light and protrusions extending from the upper surface. The protrusions are arranged to imitate a cuticle of a light emitting organ of a firefly, so as to reduce reflection of incident light.

At block 602, incident light falls on the photovoltaic cell. Protrusions resembling a cuticle of a light emitting organ of a firefly provided on the surface of the s photovoltaic layer serve to reduce reflection of light by the photovoltaic layer at block 604. At block 606 incident light is converted to electricity by the photovoltaic layer. The reduction in reflection may increase the proportion of incident light which is converted to electricity.

In accordance with an example a photovoltaic device is constructed having a substrate layer, a transparent lower electrode, a photovoltaic layer, upper electrodes, and a covering layer. The photovoltaic layer is provided with cylindrical protrusions in the honeycomb configuration shown in Fig. 2 thus imitating the cuticle layer of a firefly's light emitting organ. The protrusions have is heights of about 120 nm and widths of about 150 nm. Spacings between adjacent protrusions are about 100 nm. The photovoltaic device is observed to have better light conversion efficiency than a comparative device that is otherwise identical but for the omission of the protrusions.

A further photovoltaic device is constructed as in the above example, differing in that a reflective layer is provided between the transparent lower electrode and the substrate. The reflective layer comprises a layer of hollow silicon dioxide particles obtained by the method reported by Chen et al, and is about 50 pm thick. The reflective layer imitates the reflective layer of a firefly's light emitting organ, and the photovoltaic layer's surface imitates the cuticle of a firefly's light emitting organ.

The further photovoltaic device is observed to have better light conversion efficiency than the device of the first example and the comparative device.

Claims

Claims 1.
A photovoltaic cell comprising: a photovoltaic layer having an upper surface for receiving incident light; and protrusions extending from the upper surface; wherein the protrusions are arranged to imitate a cuticle of a light emitting organ of a firefly, so as to reduce refiection of incident light.
3.0 2. A photovoltaic cell according to claim 1, wherein that the protrusions extend by heights in the range 80 to 160 nm.

3. A photovoltaic cell according to claim 2, wherein the protrusions extend by heights in the range of 110 to 130 nm.
4. A photovoltaic cell according to any preceding claim, wherein that the protrusions have widths in the range 120 to 180 nm.
5. A photovoltaic cell according to claim 4, wherein the protrusions have 20 widths in the range 140 to 160 nm.
6. A photovoltaic cell according to any preceding claim, wherein spacing between protrusions are in the range 70 to 130 nrn.
7. A photovoltaic cell according to claim 6, wherein spacings between protrusions are in the range 90 to 110 nm.
8. A photovoltaic cell according to any preceding claim, wherein the protrusions comprise cylindrical protrusions.
9. A photovoltaic cell according to claim 8, wherein the protrusions are arranged in a honeycomb pattern.
10. A photovoltaic cell according to any of claims 1 to 7, wherein the protrusions comprise elongate protrusions.
11. A photovoltaic cell according to claim 10, wherein the protrusions are arranged in the same direction as one another.
12. A photovoltaic cell according to any preceding claim, wherein the photovoltaic layer has a reflective index in the range 1.4 to 1.6.
13. A photovoltaic cell according to any preceding claim, wherein the photovoltaic cell is a thin film photovoltaic cell.
14. A photovoltaic cell according to any preceding claim, wherein the photovoltaic layer comprises an organic semiconductor.
15, A photovoltaic cell according to any of claims 1 to 14, wherein the photovoltaic layer comprises silicon.
16. A photovoltaic cell according to any preceding claim, further comprising a reflective layer arranged below the photovoltaic layer.
17. A photovoltaic cell according to claim 16, wherein the reflective layer comprises a particulate arranged to imitate a reflector layer of a light emitting organ of a firefly.
18. A photovoltaic cell according to claim 17, wherein the particulate comprises hollow particles of silica.
19. A photovoltaic cell according to claim 18, wherein the hollow particles of silica have walls with a thickness in the range of 0.05 pm to 0.15 pm.
20. A photovoltaic cell according to any of claims 17 to 19, wherein the particulate comprises particles having a mass median diameter in the range 0.8 pm to 1.2 pm.
21. A photovoltaic cell according to any of claims 16 to 20, wherein the reflective layer has a thickness in the range 10 to 200 p11).
22. A photovoltaic cell according to claim 21, wherein the reflective layer as a thickness of at least 45 pm.
23. A photovoltaic cell according to any preceding claim, wherein the o protrusions are 3D printed.
24. A vehicle comprising a photovoltaic cell as defined in any preceding claim.
25. A method of manufacturing a photovoltaic cell for receiving incident light cas defined in any of claims 1 to 23, the method comprising producing the photovoltaic layer of the photovoltaic cell such that the protrusions on the upper surface of the photovoltaic layer resemble a cuticle of a light emitting organ of a firefly.
26. A method according to claim 25 comprising 3D printing the photovoltaic layer.
27. A method of generating electricity by a photovoltaic cell comprising: receiving incident light by protrusions extending from an upper surface of a photovoltaic layer of the photovoltaic cell and arranged to resemble a cuticle of a light emitting organ of a firefly, and reducing reflection of incident light by the protrusions.