WO2010143096A2

WO2010143096A2 - Solar cells and their manufacture

Info

Publication number: WO2010143096A2
Application number: PCT/IB2010/052454
Authority: WO
Inventors: Peter Douglas Fairley; Frank Anton Van Abeelen; Steffen Reymann
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2009-06-10
Filing date: 2010-06-02
Publication date: 2010-12-16
Also published as: WO2010143096A3

Abstract

A method of fabricating a solar cell comprises forming a flat flexible film structure including an optical absorbing layer, and sandwiching the flexible film structure between upper and lower substrates, thereby deforming the flexible film to have a profile corresponding to non-flat upper and lower surfaces of the substrates. This method improves the efficiency and reduces the cost of existing thin film solar cells. A non-flat optical structure enables long wavelength photons to pass through the absorbing layer several times, thereby absorbing more of the longer wavelength photons. The provided structure also enables recycling of light reflected off material interfaces, thereby reducing losses due to these reflections. The use of a flexible solar cell substrate enables a simplified manufacturing process to be employed.

Description

Solar cells and their manufacture

FIELD OF THE INVENTION

This invention relates to solar cells, and more particularly to thin film solar cells.

BACKGROUND OF THE INVENTION

Thin film solar cells have existed for many years. Their advantages over bulk solar cells include simplified fabrication processes, a reduction in the amount of material in the active layer, and the possibility of their deposition onto flexible substrates. These factors lead to cost reductions and offer the possibility of producing low cost solar cells that can be deployed over large areas in an increasingly energy conscious world. However these advantages do not come without drawbacks. One of the key tradeoffs in thin film solar cells is between the efficiency of the solar cell and the thickness of the active layer. In the materials typically used for the active layer, the absorption coefficient varies with wavelength, becoming much weaker at longer wavelengths. This means that thick layers are necessary in order to absorb the longer wavelengths. The obvious solution is to make the thickness of the active layer equal to the absorption depth for the longest wavelength that offers above-bandgap excitation.

However, the solar cell efficiency is determined by not only the number of photons that are absorbed but also by the number of thus-generated electron-hole pairs that are collected in the external circuit. The photovoltaic semiconductor structures that are typically used to separate the electron-hole pairs include pn and p-i-n types, in which the plane of the junction is presented perpendicularly to the rays of incident light. In such structures, carriers are optically generated throughout the semiconductor, and in order to generate a current in the external circuit they must diffuse away from the point of generation so that the electrons reach the n region, and the holes reach the p-type region before they recombine with each other. The recombination is characterised by a carrier lifetime which is also expressed as a distance, the diffusion length, which is the distance following which 1/e of the original carriers will have recombined. To suit both requirements for solar cell efficiency the ideal solution is to i) make the active layer thickness equal to the absorption depth, thereby ensuring most photons are absorbed, and additionally ii) to ensure that the diffusion length exceeds the active layer thickness.

In the presence of a long diffusion length, such as that which exists in high purity silicon bulk solar cells, this situation is readily satisfied, and the solution is to use a thick absorbing region (typically around 100 μm in silicon) that absorbs both the short and long wavelength photons. The longest wavelength of interest is that which corresponds to above-bandgap excitation as it is this process that results in carrier generation. The long carrier lifetime ensures that the carriers reach their respective regions. In contrast, the processes that are typically used to deposit thin film solar cells typically produce a much shorter carrier lifetime, giving rise to a diffusion length that is shorter than the absorption depth. Since only the carriers that diffuse to the n and p type regions are useful, the trade-off favours using a material thickness that is equal to the diffusion length. Consequently the longer wavelength photons are poorly absorbed and therefore wasted.

Taking amorphous silicon as an example, the short carrier lifetime in thin film solar cells is in part due to the less-ordered (frequently amorphous) state of the atoms in the active layer which results in improperly terminated bonds. Pure amorphous silicon has strong absorption of the visible spectrum at all wavelengths but its bonds are not properly terminated, leading to a very short carrier lifetime and indeed the material's band structure is so poorly defined that it barely behaves as a semiconductor. As hydrogen is incorporated into the lattice the carrier lifetime increases, which improves the carrier collection efficiency, but the absorption decreases at the long wavelength end as the molecular structure changes. Beyond about 8 % hydrogen the carrier lifetime starts to plateau, so the collection efficiency of generated carriers will improve little beyond this point. Additionally, beyond 8 % hydrogen the optical absorption continues to decrease, so there is little point in exceeding this concentration. Approximately 8 % hydrogen concentration is therefore optimal from a carrier lifetime aspect, but at this concentration the absorption of the carriers at the long wavelength end is already very weak. Little is achieved by moving to a lower hydrogen concentration as although more carriers are generated, they cannot be collected. In the case of amorphous silicon (a: Si-H) the trade-off is typically met with a material thickness of about lμm but results in poor absorption of these longer wavelength photons.

Consequently, in order to avoid wastage of the longer wavelength photons and thereby improve the efficiency of thin film solar cells, there has been much innovation in the area of making optical structures that create multiple passes of the longer wavelength photons through the absorbing layer. Such inventions typically reflect the light through the same absorbing layer several times, effectively making this absorbing layer appear much thicker to the longer wavelength photons, using a combination of optical structures, mirrors and total internal reflection (TIR).

SUMMARY OF THE INVENTION

According to the invention there is provided a method of fabricating a solar cell, comprising (a) forming a flat flexible film structure including an optical absorbing layer and a reflective conductive electrode underneath it; (b) forming a lower substrate having a non-flat upper surface; (c) forming an upper substrate having a non-flat lower surface corresponding in profile with the upper surface of the lower substrate; and (d) sandwiching the flexible film structure between the upper and lower substrates, thereby deforming the flexible film to have a profile corresponding to the non-flat upper and lower surfaces.

This method improves the efficiency and reduces the cost of existing thin film solar cells. A non-flat optical structure enables long wavelength photons to pass through the absorbing layer several times, thereby absorbing more of the longer wavelength photons. The use of a flexible solar cell structure enables a simplified manufacturing process to be employed.

The film structure can comprise (a) a plastic substrate; (b) the reflective conducting lower electrode; (c) the optical absorbing layer; (d) a top electrode. This structure defines the solar cell with its associated electrodes.

The top electrode is preferably transparent, and the bottom electrode is preferably made from a reflective conductor. The top electrode in this case is defined as the one oriented to receive incoming light - it does not necessarily face upwards.

The top electrode can then comprise ITO and/or the bottom electrode can comprise aluminium. The optical absorbing layer can comprise amorphous silicon.

In one example, the optical absorbing layer is made using an EPLaR process (discussed further below). An example of this process uses a plastic substrate of polyimide and can result in a film structure having a thickness of less than 20 μm.

The non-flat surfaces can have a symmetric sawtooth profile. The invention also provides a solar cell comprising: (a) a lower substrate having a non-flat upper surface; (b) an upper substrate having a non-flat lower surface corresponding in profile with the upper reflective surface of the lower substrate; and (c) a flexible film structure including an optical absorbing layer sandwiched between the upper and lower substrates, wherein the flexible film structure comprises: (i) a plastic substrate; a reflective conducting lower electrode; (ii) the optical absorbing layer; and (iii) a top electrode.

This design provides efficient reflections within the structure, and enables the desired deformation of the film structure to be implemented by a clamping process. Preferably, the top electrode having a thickness t, wherein t is substantially equal to an odd integer multiple of the value λ/(4n), where n is the refractive index of the material of the top electrode and λ is an optical wavelength close to the peak of the sun's emission spectrum.

This design uses the top electrode of the solar cell structure as an antireflection coating.

The material of the top electrode can be ITO and for example λ equals 550 nm.

BRIEF DESCRIPTION OF THE DRAWINGS

An example of the invention will now be described in detail with reference to the accompanying drawings, in which:

Fig. 1 shows a known design for providing multiple paths through the optical absorbing layer of a solar cell;

Fig. 2 shows a first example of design of the invention;

Figs. 3a and 3b show the light paths through the design of Fig. 2; Fig. 4 shows a reference design for the purposes of comparison;

Fig. 5 shows a second example of design of the invention for comparison with Fig. 4; and

Fig. 6 shows how ITO can provide antireflection properties.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The invention provides a method of fabricating a solar cell to provide the solar cell structure with non-flat upper and lower surfaces, to provide light confinement within the solar cell structure.

Typical constructions that are used to optically confine the longer wavelength photons have the active absorbing layer on one surface and form the optical structures that perform the optical confinement on the other surface (the reflective surface, 14) as shown in Fig. 1. The thin films that comprise the active absorbing layer are typically fabricated on flat substrates, so the necessary optics are typically provided by shaping the reflective surface 14 of the transparent substrate 12, in this case to provide the mirrors on the lower layer in Fig. 1. With reference to Fig. 1, the construction and operation of an existing design is as follows.

An active thin film 10, comprising n type and p type regions, is fabricated on the upper surface of a transparent substrate 12. The lower surface of the transparent substrate 12 is patterned with grooves that are inclined at an angle α to the horizontal and coated with silvered material to act as mirrors. Light that is at normal incidence to the incidence surface 13 passes initially through the n and p type regions of the semiconductor, partially being absorbed and thereby generating electron-hole pairs.

Light that is not absorbed during the first pass through the active layer propagates through the transparent substrate toward the lower patterned surface where it is reflected back toward the absorbing layer after being bent through an angle 2α. Following its refraction in the active thin film (which occurs in the event of a difference in refractive index between the transparent substrate and the active thin film) the ray then meets the incident surface again. At this point, providing its incidence angle exceeds the critical angle, the ray is reflected back through the active layer and toward the reflective lower surface again, where the process repeats itself until either the ray has been extinguished, or the incident angle with the front surface no longer meets the condition required for its total internal reflection.

The invention provides a different approach for light confinement in thin film solar cells and makes use of flexible substrates to permit the absorbing layer to be shaped after processing. The invention also relates to the provision of an antireflection method without additional layers. The resulting simplification of solar cell design leads to reduced cost and potentially to higher efficiency solar cells.

The general principle of the arrangement of the invention is shown in Fig. 2 whose construction and operation is as follows. This example again uses an optical absorbing layer comprising amorphous silicon, and the required doping is not shown in Fig. 2 for simplicity.

The top electrode 20 is transparent and typically made from ITO, the bottom electrode 22 is made from a highly reflective ductile conductor such as Aluminium. The ductility permits its flexing into the desired shape without damage. In this example design, the active absorbing layer is made using the EPLaR process.

The EPLaR process is a laser release process for releasing a plastic flexible substrate (e.g. polyimide) from a rigid carrier. A plastic coating is applied to the rigid carrier substrate (e.g. glass) using a wet casting process, the plastic coating forming a plastic substrate. Thin film electronic elements or other layers can be formed over the plastic substrate. The rigid carrier substrate is released from the plastic substrate by a heating process, which expands the plastic substrate. Examples of this process are described in detail in WO2005/050754 and in WO2008/050300.

However, many other flexible thin film flexible solar cell fabrications are equally applicable.

The EPLaR process leaves a thin (as thin as 7 μm) polyimide layer on the back of the solar cell in order to support it, giving a total EPLaR stack thickness as thin as lOμm.

In Fig. 2 the solar cell itself comprises what is labelled as the "EPLaR stack", of top and bottom electrodes 20,22, polyimide release layer 24 and amorphous silicon optical absorbing layer 26

The solar cell is fabricated by depositing the respective layers on top of the polyimide substrate, in combination with doping processes. Typical solar cell constructions are well known to those in the semiconductor field and include p-n junction and p-i-n junction types. The EPLaR stack is sandwiched between a transparent top substrate 28 and a lower substrate 30, the latter of which need not necessarily be transparent.

In the simplest design shown in Fig. 2, the absorbing solar cell is shaped in the form of a symmetric sawtooth with adjacent edges inclined and declined at an angle α to the horizontal. Alternative structures are described below.

Two example paths for normally incident rays are shown in Figs. 3a and 3b. These show ray paths for a large proportion of the rays that are incident in the structure and are used to describe its general effect.

Fig. 3a shows the path for rays encountering the most encounters with the absorbing layer and Fig. 3b shows the path for rays encountering the least encounters with the absorbing layer. Optical simulations are used to confirm this by taking into account all possible rays. With reference to the ray diagrams in Figs. 3a and 3b, a normally incident ray first passes through the transparent top substrate and is partially absorbed by the EPLaR stack underneath. Following this, the ray is reflected back toward the surface by the aluminium layer at the bottom of the EPLaR stack, thereby undergoing two passes through the absorbing active layer, and is bent by an angle 2α.

Providing this angle exceeds the critical angle for the air-transparent top substrate interface, then this ray will undergo total internal reflection and subsequently be reflected back toward the solar cell region again where another two passes will be made through the absorbing active layer. Following this the ray will be bent to an angle 4α from the vertical, undergo another two passes through the absorbing active layer, be bent to an angle (180-6α) from the vertical and again, providing the incidence angle exceeds the critical angle, be reflected back through the absorbing active layer for another two passes.

Following this the ray is bent to an angle (180-8α) to the vertical (or 8α-90 to the horizontal as shown). At this point the ray may again be reflected, but with typical parameters the ray is likely to no longer satisfy the conditions for total internal reflection and will exit the transparent top substrate at this point.

Typical design parameters require calculation based on the materials used, but essentially having chosen the materials for the transparent top substrate and the absorbing layer, the condition that must be met is that the first reflected ray undergoes total internal reflection.

This requires that:

e, = 2 'α > θ_m,

Where the critical angle is given by Snell's law as:

θ_cr, = arcsin(«_exr /«₁ )

Where ni and rw are the refractive indices of the transparent top substrate and surrounding (air) media respectively.

Typical materials for the transparent top substrate are glass and many polymers, which have refractive indices of approximately 1.5. This gives a critical angle of ctcπt = 41.8°. In order to satisfy the condition for α above, a suitable choice is therefore α = 22.5°. Following this choice, the ray diagram in Fig. 3a exits the transparent top substrate perpendicular to its surface after four encounters with the active absorbing layer.

With the structure shown in Fig. 3a and 3b, not all optical rays have the same path. The paths of rays that originate further to the right of the diagram in the optical structure shown in Fig. 3b undergo less encounters with the active absorbing layer. In the case shown these have only two encounters with the active absorbing layer, and therefore undergo less net absorption. Clearly, however there is still an advantage for these rays as compared to the case where there is only one double-pass through the active absorbing layer. The distance L is the period of the periodic non flat profile, and the height h is the smallest thickness of the top substrate. The ratio (IVh) determines the example optical paths of propagating rays in Figs. 3a and 3b, but simulations in Table 1 below show that this ratio has little effect on the benefit provided by the structure. Typically, (IVh) in the range of 2 to 20 is suitable. In terms of the absolute value of h, assuming that the design is outside the region where diffraction effects take place (h » λ_incident of for example 750 nm), it is then the mechanical properties of the thin film solar cell layers, the EPLaR stack, which determine the minimum value of h. The solar cell layers must withstand the strain encountered by shaping the substrate into the sawtooth pattern. Specifically, calculations for the EPLaR solar cell permit a minimum radius of curvature of approximately 200 μm. In practise h = 1 mm is taken as a guideline.

The optical confinement advantage offered by the optical structure described in Fig. 3 is now compared with a reference situation. The reference situation is shown in Fig. 4, in which a flat solar cell is simply sandwiched between the same transparent top substrate and the lower substrate, including the mirror. Simulations have been performed using optical design software to determine the benefit of the proposed design.

The assumptions are that the incidence is normal, propagation is incoherent, polarisation is random and that the ITO (which typically will have a thickness of 200 nm and practically 99 % transmission) is absent from all simulations. Owing to the difference in refractive index with wavelength in the absorbing layer, calculations were made at several wavelengths in the visible region of the sun's emission spectrum.

The optical constants for the absorbing layer (a: Si-H) were taken from standard reference materials, although in practice it is well known that these vary considerably with process conditions. Nevertheless the same general trends are expected across a broad range of deposition parameters. It is expected that the greatest benefit will be at the longer wavelengths where the reduced absorption coefficient means that for the wavelengths that pass through the optical film during the first double pass there is more to be gained from the multiple reflections.

The model calculates the fraction of incident light at a particular wavelength, under normal incidence, that is absorbed by the entire absorbing active a:Si layer.

The chosen model parameters are given in Table 1, wherein:

Complex Refractive index = n-ik , where i is V(-l) and k is the extinction coefficient.

Absorption coefficient α = 4πk/λ Thickness of a: Si layer = 1000 nm

Calculations are given for three optical confinement structures that typify examples of the invention. Firstly in the reference case shown in Fig. 4. Secondly for the design shown in Fig. 3a and 3b, and thirdly the design shown in Fig. 5. Fig. 5 shows a nonsymmetrical sawtooth design.

Fig. 5 was simulated with L/h = 2 and α = 22.5° and was conceived as an improvement to the design in Fig. 3. It reduces the fraction of rays that undergo fewer encounters with the absorbing layer, which is those that fall into the category shown in Fig. 3b.

The simulation results for the designs in Figs. 3 and 5 are shown in Table 1.

Table 1 : Fraction of incident power that is absorbed by the entire absorbing active layer

Table 1 shows that in both designs there is an improvement in the fraction of absorbed light over the reference case. The benefit is due to the second and subsequent reflections of light rays through the absorbing a: Si layer that were not completely absorbed during their first pass through the a: Si layer. These second and subsequent reflections are caused by the angled reflective surfaces and the total internal reflection that occurs as a consequence of the incidence of the rays with the air-(glass) transparent top substrate interface. The main benefit is at the longer wavelengths in the simulation since it is these rays that are less strongly absorbed by the absorbing active layer, and therefore stand to gain the greatest benefit from being passed through the absorbing active layer several times. It is seen that the asymmetric sawtooth is the preferred design amongst the two structures examined as it has the strongest total absorption.

In the reference case at the short wavelength of 510 nm, 78 % of the incident light is absorbed. By moving to the symmetric sawtooth structure, the absorption increases to 94 %, thus a reflectance of 6 %. This is nearly the ideal situation since the 4 % reflectance of the air-glass interface contributes nearly all of this (6 %) reflectance. Nearly all the short wavelength light (94 %) that propagates into the glass is subsequently absorbed.

At the longer wavelength of say 598 nm, in the reference case only 22 % of the incident light is absorbed. Only 22 % of the incident light is absorbed because much of the light passes through the glass, through the weakly absorbing a: Si stack, and then back out of the glass toward the sun again where it is of course wasted. By moving to the symmetric sawtooth structure the absorption increases to 41 %. This is because the light that was poorly absorbed in its first pass through the a: Si is subsequently passed back through the a: Si many times due to its confinement by the total internal reflection and the symmetric sawtooth mirrors. At this longer wavelength the absorption is therefore nearly double that in the reference case.

The net benefit offered by the proposed structures in Figs. 3 and 5 is determined by multiplication of the solar spectrum at sea level by the results in Table 1. Specifically, the number of photons at each wavelength is multiplied by the percentage of incident light absorbed, and the result integrated over the above bandgap wavelengths in a: Si. The gives a relative increase in efficiency of 17 % for the Symmetric sawtooth (h=L/2), 16 % for the Symmetric sawtooth (h=L/20) and 21 % for the Asymmetric sawtooth (h=L/2) over the reference case. This means that for example if the reference structure in Fig. 4 had an efficiency of 15 % then the Asymmetric sawtooth structure in Fig. 5 would have an efficiency of 18 %. One aspect of the invention is based on the implementation of an antireflection method without the need for additional layers. This is provided primarily by the shaping of the solar cell into the zigzag topography, as is shown below. A minor contribution is also provided by the sequence of refractive indices, between air (n = 1), the glass or polymer layer (n = 1.5), the ITO (n = 2) and amorphous silicon (n = 4) materials. Although in the invention the benefit provided by this sequence of refractive indices is relatively minor, it would be good design practice to make the ITO layer act as a simple quarter-wavelength antireflection coating with an optical thickness equal to an odd integer of quarter wavelengths. The simulations results in Table 1 , do not include the ITO layer. An antireflection coating could be added to the prior art design of Fig. 1 to further improve efficiency by increasing the amount of light that penetrates into the structure. This would typically be added as an additional layer, requiring additional processing. It is noted however that the surface of known prior art structures is ITO, and through correct selection of its thickness, the ITO could (using the insight gained by this invention) be made to operate as an antireflection coating. The analysis below shows how the non-flat profile provides further improvements compared to the prior art electrode design, even if the prior art is improved by implementing an antireflection coating by the appropriate ITO thickness selection.

Fresnel's equation below gives the average reflectance of a ray propagating from a medium with refractive index Ti₁ to one with refractive index n_t for the first-reflection alone. In practise, the reflectance has a wavelength-dependence due to the addition of the reflections from the front and back surfaces of each layer but this wavelength dependence is neglected in the calculation below. The reflectance calculated below is the average across a large number of fringes.

^R = (n_t - njn, + n_t f

A modified version of the prior art case of Fig. 1 is considered in order to determine the maximum benefit achievable if the ITO thickness were designed optimally. The optical layers encountered by an incident ray are air (n = 1) then ITO (n = 2) then a: Si (ureal = 3.6 at 550 nm). For the air - ITO interface (transition between n = 1 and n = 2) this gives R = 11.1 % for the first reflection. 89 % of the original ray then propagates toward the ITO - a:Si interface (transition between n = 2 and n = 3.6). This interface gives R = 8.2 % of the incident 89 %. This means that, ignoring subsequent propagations, the total reflectance is 11.1 % + 0.89x8.2 % = 18.4 %. That is to say that this light never reaches the absorbing a: Si layer. In order to properly act as a quarter- wavelength antireflection coating over a broad range of wavelengths, the thickness of the ITO (tno) should be chosen such that: ^ ITO ^~ ^centre ' ^ ^{' n}iTO

For a central design wavelength in middle of the visible region of 550 nm this means making the ITO 69 nm thick. This thickness gives a broad fringe at the design wavelength, giving antireflection over a broad range of wavelengths. In practise the ITO must also have a low electrical resistance in order to efficiently extract electrical power from the solar cell, and this demands a thickness of approximately 200 nm. Providing the coating is still an odd integer number of quarter wavelengths thick it still acts as an antireflection coating but the fringes become more closely spaced, effectively reducing the bandwidth of its operation. An optimal thickness for this ITO layer is therefore 3^χ69 nm = 207 nm, and this gives the power reflectance spectrum shown in Fig. 6.

The simple antireflection coating provided by the ITO has reduced the reflectance at the design wavelength of 550 nm to practically zero, but owing to it being several quarter-wavelengths thick, its bandwidth is somewhat reduced. The average reflectance over the passband is calculated through integration from 380 nm to 780 nm and is 21 %. This agrees well to the estimation of 18.4 % above. Slight errors are due to the integration not being made over a large number of fringes. This shows the minimum reflectance that could be offered by modifying the design of the prior art by additionally providing an antireflection property at the ITO surface. The same calculation is now made for the optical stack in the reference design in Fig. 4 in order to show the benefit of the stack order (sequence of materials) of the invention. Using the same reflectance equation above, in this case the optical layers encountered by the incident ray in Figs. 3, 4 and 5 are air (n=l) then glass (or a polymer) (n=1.5) then a:Si (n=3.6 at 550 nm). Again calculating for just the first reflected rays, the first air-glass interface has a reflectance of 4 %, and then 96 % of the transmitted light meets the glass-ITO interface where it has a reflectance of 2 %. 98 % of this ray is then transmitted to the ITO-a:Si interface where it has a reflectance of 8.2 %. These values sum to give a net reflectance for the reference case in Fig. 4 of 13.6 %. Since this reflectance is lower than the 18.4 % achievable with the prior art, less light is reflected back toward the sun by the reference structure in Figs. 4. The prior art of course does have an additional benefit in recycling some of the 81.6 % (=100 %-18.4 %) of transmitted light, but the reference design reflects less of the initially- incident light than the prior art.

The effect of the zigzag structure in Figs. 3 and 5 is now considered on the optical stack in the invention. This differs from Fig. 4 in that the invention recycles the light that is incident on the structured surfaces. That is to say, all light apart from that incident on the front air-glass interface has a chance of being recycled. This means that in the invention, in Figs. 3 and 5, only the 4 % reflection at the initial air-glass interface has no chance of being recycled. The remaining 96 % has a chance of undergoing multiple reflections providing the conditions are met for total internal reflection. In the prior art, even with the improvement of using an ITO antireflection coating made to the prior art, only 81.6 % of the incident light has a chance of being recycled. This shows the clear improvement of the design of the invention.

The operation of the structure as an effective antireflection method is confirmed by the simulation results in Table 1. Table 1 shows that in the short wavelength region where the a:Si is strongly absorbing, up to 95 % of the light is absorbed by the a:Si in Figs. 3 and 5. This is very nearly the ideal case, in which theoretically only 4 % of the incident light is reflected.

The invention is based on the use of a solar cell which is sufficiently flexible in order to be able to conform to the sawtooth pattern shown in Figs. 3 and 5. EPLaR is only one of many suitable technologies.

The upper substrate can be made from any one of a number of optically transparent substrates, such as glass, hard polymers such as polycarbonate, or for example sapphire, and which typically have a refractive index of between 1.4 and 1.6. The refractive index should not be too low or this leads to a small angle within which total internal reflection occurs. The refractive index should not be too high or this leads to high reflectance at the first air- substrate interface. The lower substrate is preferably made from the same material in order to match the thermal expansion coefficients, but could be made from another material in which this condition is met. The lower substrate does not need to be transparent but it will not significantly affect the operation if it is.

To obtain the desired patterning, grooves can be machined into both the upper and lower substrates using milling or abrasive techniques that are currently used to make optical lenses. Alternatively, for some materials these grooves can be moulded from a liquid starting material. The entire structure is then adhesively or compressively sandwiched together in order to ensure the surfaces are in intimate contact. In the event of adhesive attachment, glues with a refractive-index lying between the values of the bonded layers are used to minimise additional reflections. External connection to the electric circuit is achieved by mechanical removal or etching to create vias through the superstrate and substrate to the ITO and Aluminium contact regions. The approximate thickness of the optical absorbing layer is in the order of 1 μm. It is preferable that the thickness of the transparent ITO contact layer be not too thick (which increases the optical absorption) and not too thin (or it adds significant series resistance to the solar cell). As explained above, although it provides small additional benefit to the invention the ideal thickness from an optical point of view is when the ITO thickness also acts as an antireflection coating at the central wavelength of the absorption, requiring:

¹ITO

The structures shown in Figs. 3 and 5 are examples of possible design. Other pattern shapes, such as sine wave, steeper sawtooth, and cone-type structures are also possible.

The solar cell of the invention can be used to power a huge range of rechargeable equipment. In the examples above, the lower substrate forms part of the finished design,

However, the lower substrate can be removed after the flexible film structure has been deformed.

The invention relates particularly to thin film solar cells, in which the deposition processes (such as sputtering and vapour deposition) typically produce a characteristically disordered atomic structure in the active layer. This is in contrast to bulk solar cells in which the active layer is formed from a crystalline material and is typically grown from a molten phase with its inherently ordered structure. In bulk solar cell materials there is an emphasis on making high purity bulk material, which involves high cost refining processes. In contrast, the approach taken with thin film solar cells is to use a thin absorbing layer, accept the consequently lower proportion of the incident light that is absorbed, especially in the longer wavelengths, and to reflect the light multiple times through the same absorbing layer in order to absorb more of the incident flux from these longer wavelengths. The thin films are typically of less ordered material and have an inherently shorter bulk carrier lifetime. However, the requirement that the carriers still diffuse to the respective p and n regions is still met because the material is thinner. Thin film solar cells require less active material so are cheaper to produce and are flexible which has certain advantages. It is the flexibility of the solar cells that is exploited in the manufacturing process of this invention. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims

CLAIMS:

1. A method of fabricating a solar cell, comprising: forming a flat flexible film structure (20,22,24,26) including an optical absorbing layer (26) and a reflective conductive electrode (22) beneath it; forming a lower substrate (30) having a non-flat upper surface; - forming an upper substrate (28) having a non-flat lower surface corresponding in profile with the upper surface of the lower substrate (30); and sandwiching the flexible film structure (20,22,24,26) between the upper and lower substrates (30,28), thereby deforming the flexible film to have a profile corresponding to the non-flat upper and lower surfaces.

2. A method as claimed in claim 1, wherein the lower substrate (30) is removed after the flexible film structure (20,22,24,26) has been deformed.

3. A method as claimed in claim 1, wherein the film structure (20,22,24,26) comprises: a plastic substrate (24); the reflecting conducting lower electrode (22); the optical absorbing layer (26); and a top electrode (20).

4. A method as claimed in claim 3, wherein the top electrode (20) is transparent, and the bottom electrode (22) is made from a metal.

5. A method as claimed in claim 4, wherein the top electrode (20) comprises ITO and/or the bottom electrode (22) comprises aluminium.

6. A method as claimed in claim 3, wherein the top electrode (20) has a thickness t, wherein t is equal to an odd integer multiple of the value λ/4n, where n is the refractive index of the material of the top electrode and λ is an optical wavelength close to the peak of the sun's emission spectrum.

7. A method as claimed in claim 6, wherein λ is equal to 550 nm.

8. A method as claimed in claim 3, wherein the optical absorbing layer (26) comprises amorphous silicon.

9. A method as claimed in claim 8, wherein the optical absorbing layer (26) is made using an EPLaR process, and the plastic substrate comprises polyimide (24).

10. A method as claimed in claim 3, wherein the film structure (20,22,24,26) has a thickness of less than 20 μm.

11. A method as claimed in claim 1 , wherein the non-flat surfaces have a symmetric sawtooth profile.

12. A solar cell comprising: a lower substrate (30) having a non-flat upper surface; - an upper substrate (28) having a non-flat lower surface corresponding in profile with the upper reflective surface of the lower substrate; and a flexible film structure (20,22,24,26) including an optical absorbing layer (26) sandwiched between the upper and lower substrates, wherein the flexible film structure comprises: - a plastic substrate (24); a reflective conducting lower electrode (22); the optical absorbing layer (26); and a top electrode (20).

13. A solar cell as claimed in claim 12, wherein the top electrode (20) has a thickness t, wherein t is substantially equal to an odd integer multiple of the value λ/4n, where n is the refractive index of the material of the top electrode (20) and λ is an optical wavelength close to the peak of the sun's emission spectrum.

14. A solar cell as claimed in claim 13, wherein λ is equal to 550 nm.

15. A solar cell as claimed in claim 12, wherein the material of the top electrode

(20) is ITO.