WO2008155130A1

WO2008155130A1 - Photovoltaic device

Info

Publication number: WO2008155130A1
Application number: PCT/EP2008/005003
Authority: WO
Inventors: Paul Van Steenberghe
Original assignee: Solar Evolution Systems Gmbh
Priority date: 2007-06-21
Filing date: 2008-06-20
Publication date: 2008-12-24

Abstract

The invention discloses a photovoltaic multi layer device comprising a support layer having at least two electrodes, a light absorbing polarization layer having a light absorbing element for absorbing photons and for transmitting polarized light, wherein a wave turning layer turning the transmitted light and a reflecting layer reflecting the turned polarized light to the light absorbing polarized layer which are laminated to the light absorbing polarization layer.

Description

Photovoltaic device

The present invention relates to photovoltaic multilayer device comprising a support layer with at least two electrodes and a light absorbing polarization layer having light absorbing elements for absorbing photons and transmitting polarized light, a method and an apparatus for producing such a device.

From the prior art, the American patent US 5,229,624, a photovoltaic polarizing film is known, where the light absorbing elements are formed by molecular polymer based antennas. The molecular polymer based antennas are interspersed by molecular diode structures which are added and oriented to create a so-called one-dimensional rectenna array. The one-dimensional rectenna array is a current rectifying antenna array with all antennas lying parallel to each other and a defined electrical current direction, hence only absorbing one of the two components of an electro magnetic waves electric vector. The current rectifying is performed by the diodes which are electrical diodes of molecular dimensions in which an electron can travel in only one direction. The diode comprises a substantially linear complex containing groups of atoms specialised as an electron donor group, an insulator-spacer group and an electron-acceptor group. An electron from the donor can tunnel across the insulating spacer to the acceptor but not in reverse direction. Diodes can also be called diode dipoles because they can hold a potential difference between its donors and acceptor ends.

The formed one-dimensional rectenna array absorbs UV (ultraviolet), visible and IR (infrared) portions of the electro magnetic wave spectrum and a support layer comprises electrodes which carry the photon induced electrical current to be summed on bus bars. The polarizing properties of the known film make that one component of the electromagnetic wave of light is absorbed by the antenna array and the second component is transmitted, whereby polarized light is produced. In order not to absorb only roughly fifty per cent of the incident light a second light absorbing film is placed orthogonally to the first film in order to form a two dimensional rectenna, whereby the polarized light can almost be fully absorbed.

The problem with the known state of the art is that the use of two layers of one- dimensional rectifying film to create a two dimensional rectenna is very expensive and the two films have to be arranged with high precision in exactly 90° to each other for providing high efficiency. Particularly the arrangement in exactly 90° to each other is very difficult to achieve, and in case the films are not arranged orthogonally to each other, the efficiency of the photovoltaic element is reduced.

It is therefore the object of the present invention to provide a photovoltaic element which has a high efficiency and is easy and inexpensive to produce.

This object is solved by a photovoltaic multilayer device according to claim 1 , a method to produce such a photovoltaic element according to claim 9 and an apparatus for producing such a photovoltaic device according to claim 14.

The invention is based on the idea that, instead of using two light absorbing films being arranged orthogonally to each other, a photovoltaic element having a high efficiency can be achieved by placing a light absorbing polarization layer onto a so- called reflux layer comprising of a wave-turning layer and a reflecting layer.

The wave-turning layer turns the polarized light transmitted through the light absorbing polarization layer by 45°. This can easily be achieved by using a A/4-layer (quarter-wave turner). Such a layer provides a precise turn of the incident light of around 45°.

The turned light incidents on the reflecting layer from which it is reflected back through the wave-turning layer to the light absorbing polarization layer. By being transmitted through the wave turning layer a second time the light is again turned by 45°. This results in a total turn of the polarized light by exactly 90°. The 90° turned light incidents on the light absorbing polarization layer where the polarized, turned light is almost fully absorbed. Thereby a photovoltaic element absorbing almost 100 % of the incident light is provided having a very high efficiency.

In a further preferred embodiment, the light absorbing polarization layer comprises light absorbing elements which are molecular polymer based antennas, preferably interspaced with a molecular diode structure, whereby the one dimensional rectenna array is created. The polymer based antennas are preferably in parallel to each other. This parallel alignment can be achieved by stretch-orienting a so-called polymer precursor film from which the molecular antennas are formed.

In a further preferred embodiment, the molecular antennas or light absorbing elements are interspersed by molecular diads (= donor insulator acceptor device i.e. a electrical diode of molecular dimensions) which are placed between the ends of the antenna and allow oscillating electrons, which are stimulated by photons captured by the antenna, to exit the antenna in only one direction. Thereby, the electric current is rectified, since the movements of the electrons create an electric current from antenna to antenna through the diads. The diads additionally provide an insulating barrier for the potential differences between adjacent antennas thereby working as diad dipoles. A diad is, as explained above, a donor insulator acceptor device, i. e. an electrical diode of molecular dimensions in which an electron can travel in only one direction. It comprises of a substantial linear molecular complex containing groups of atoms, particularly an electron-donor group, an insulator-spacer group and an electron-acceptor group. An electron from the donor can tunnel across the insulating spacer to the acceptor but not in a reverse direction. Diads can also be called diad dipoles, since they can hold a potential difference between a donor and acceptor ends.

Preferably, the support layer comprises at least two electrodes and a bus bar which connects the electrodes electrically. The electrodes are adapted to pick up the current and transfer it to the bus bar, where it is summed up and transmitted to a consumer. Preferably, the electrodes are made from a conductive polymer material, particularly polyacetylene, polyamylene, PEDOT and/or Baytron.

In a further preferred embodiment, the electrodes are interdigitated electrodes which is an efficient electrode structure for transferring charges to bus bars.

Interdigitated electrodes can be optimally used for continuous web-based roll-to-roll processing of photovoltaic firms. The one dimensional rectenna array can also be regarded as one dimensional quantum well which is a potential well that confines particles (in this case electrons) that were free to move in three dimension to one dimension, forcing them to occupy a linear region. The effects of the quantum confinement lead to carriers having only discrete energy values.

In a further preferred embodiment, the molecular antennas have different lengths for absorbing light from the UV to the IR spectrum. Thereby it is advantageous to have a length distribution of antenna's lengths with its centre at approximately 250 nanometers. In a preferred embodiment, the molecular antennas are made from polyacetylene doped with iodine, whereby the diads are formed by placing and orienting electron-donor and electron-acceptor molecules at the ends of the insulating polyvinyl alcohol segments by applying an electrical potential across the electrodes while the layer is in a anhydrous alcohol solution comprising donor and acceptor molecules.

It is further advantageous to produce the photovoltaic multilayer following at least one of the following steps:

1. Prepare polymer-based casting solution, preferably, with the following materials. a. Polyvinyl Alcohol b. Hydriodic Acid c. Iodine d. Water e. 2-Propanol 2. Cast the polymer solution onto a moving casting belt.

3. Partially dry the polymer solution to form a precursor film on the casting belt and stretch-orient the film onto the interdigitaded electrode structure. This can be preferably done by the following steps: a. Approximately 5 minutes in about 90% relative humidity, 90⁰F air. b. Approximately 10 minutes in about a 90°C dry oven. c. Approximately 5 minutes in about 90% relative humidity, 90°F air. d. Stretch across an air gap at about 90% relative humidity, 90⁰F to a stretching belt, containing the electrode structure, moving approximately 8 times as fast as the casting belt.

4. Place stretched film in about a 9O⁰C dry oven to dehydrate polyvinyl alcohol to form iodine doped polyacetylene and polyvinyl alcohol interspersed on the same strands. The doped polyacetylene sections act as one-dimensional quantum wells or molecular antennas. 5. Add donor and acceptor components to the film by placing it in an anhydrous solution of 2-propanol with donor and acceptor components. 6. Subject film to an electric potential to position donor and acceptor components around the insulating polyvinyl alcohol sections, creating oriented diads. 7. Heat film to around 90⁰C to eliminate residual solvents and to the stabilize crystalline structure of the now formed one-dimensional rectenna.

8. Laminate film to reflux system to form a sealed multilayer film, protecting crystalline structure from water and oxygen infiltration.

9. Cover the multilayer film with the lenticular lens system to redirect light around the bus bars.

For producing the inventive photovoltaic multilayer device, it is preferred to use an apparatus or an apparatus arrangement which is adapted to perform at least one of the pre-described steps. Thereby, the apparatus can be a single apparatus being adapted to perform all steps, but it is also possible to use an assembly line comprising a plurality of apparatus, where e.g. a moving belt is provided for transporting the photovoltaic device from apparatus to apparatus, whereby each apparatus is adapted to perform at least one of the pre-described steps.

With the inventive method it is also possible to overcome another disadvantage of the known method, namely that the connections needed between the antenna and the diads do not always occur in a proper sequence, locations or numbers due to the random nature of the 2-component mixtures (antenna and diads). This results in improper connections substantially reducing the current flow expected in the device. This disadvantage is overcome by the inventive idea to create molecular polymer based antennas and diads from the same strand of pre-cursor polymer. This eliminates the random nature sequencing, location and numbers of diads among the antennas by building both antennas and diads on the same polymer strand. Therefore, each polymer strand becomes an already connected, properly sequenced circuit with the right number of diads in the correct locations.

In a further preferred embodiment of the invention, a lenticular lens layer is provided on the light absorbing layer. The use of lenticular lenses has the advantage that solar energy is directed away from the bus bars and toward the rectenna. A lenticular lens is an array of parallel linear optical elements or lenses, which can direct and focus light into parallel beams away from certain areas onto others. This gives the obvious advantage of not loosing energy to the bus bars in the form of heat. Also, if lenses were to be fitted over the photovoltaic film known from the state of the art, they would be in a two dimensional square array with each lens focusing light between 4 bus bars, above, below, right and left of the absorbing film. The inventive approach, because of its one-dimensional nature and its linear lenticules directing light between only 2 bus bars (right and left) will obtain more energy over an entire season. That is, since the sun is at various angles of elevation above the horizon for a fixed position array, the rectangular lenses needed for photovoltaic films known from the state of the art would loose some energy to heat on the upper and lower bus bars, whereas the inventive setup has no upper and lower bus bars to loose energy to. Further advantages and preferred embodiments are defined in the pendent claims, the description and the figures.

In the following the present invention will be described more detailed with reference to the embodiments shown in the figures.

The figures show:

Fig. 1 : a schematic illustration of the inventive principle using a reflux layer; Fig. 2: an explosive illustration of a preferred embodiment of the photovoltaic multilayer device according to the invention

Fig. 3: a schematic illustration of a small portion of the one dimensional rectanna array with electrodes and bus bars.

In the following the invention will be described for a preferred embodiment where the light absorbing elements are molecular antennas interspersed with diads, whereby an inventive photovoltaic device and particularly the inventive light absorbing layer can be thought of as an array of incredibly small antennae which pick up light (photon) energy in much the same way as a radio stick antennae picks up radio waves. Radio waves and light are electromagnetic waves. The difference is, visible light has short wavelengths which are easily picked up by short antennae and radio waves have long wavelengths and are easily picked up by a long antennae.

When a radio wave is picked up by a radio antennae the electrons in the antennae are excited and oscillate back and forth in the antenna, the signal (oscillation) is then amplified and the information transmitted by the waves can be sensed. In much the same way, the visible light excites the electrons in the nano-scale antennae within the light absorbing polarization layer and they oscillate. However, within the light absorbing polarization layer there are extremely small molecular diodes in the structure between the ends of the antennae and they allow the oscillating electrons stimulated by photons to exit the antennae in only one direction hence rectifying the current. This movement of electrons creates electric current from antenna to antenna through the diads. The diad provides an insulating barrier for the potential difference between adjacent antennas, that is, it works as a diad-dipole. Finally, electrodes contacting the strings of alternating antennae and diads pick up the current and transfer it to the bus bars.

The absorbing layer can be imagined as very many extremely small antenna-diode strings (rectenna strings) lying side-by-side (parallel to each other) to create a flat sheet. The antennae strings are so thin, straight and parallel that they form a polarizing film. The polarizing property of the layer gives the inventive photovoltaic device the advantage of organizing the light energy into two perpendicular components. One component is absorbed by the antenna array when the light first strikes the array and the second energy component will at first pass through the antenna array and then be absorbed on the backside of the array. The second component passes through a wave- turning film, strikes a reflector, goes back through the wave-turning film and onto the back surface of the rectenna array where the energy is absorbed. This is schematically shown in fig. 1.

Figure 1a shows six exemplary photons 2 of the incident light 4 with different electric vector directions passing through the light absorbing polarization layer 6 and the wave-turning layer 8 and incidenting on the reflecting layer 10. The electric vectors are each orthogonal to the path of the photon. That is, each electric vector is parallel to the xy-plane. The light absorbing polarization layer 6 containing the one-dimensional rectenna (polarizing film with antennas and diads) absorbs the y-component of the electric vector of each photon and allows the x-component to pass through. The wave- turning layer or retarder effectively rotates the electric vector by 45°. The reflecting layer reverses the path of the photons, but leaves the direction of the electric vector unchanged.

Since the 2nd photon from the left in Fig. 1a has its electric vector purely in the x- direction before passing through polarizer (1 D rectenna) 6, it is not absorbed and so retains the full magnitude of its electric vector when exiting the light absorbing polarization layer 6. Also, since the 5th photon has its electric vector purely in the y- direction before passing through the first layer 6, it is completely absorbed by the polarizing layer 6.

Figure 1b shows the reflected path of the photons and corresponding electric vector components of the second stage of the reflux system. The electric vectors of the photons are shown to rotate another 45° as they pass through the wave turner by layer 8. The electric vectors are now aligned with the antennas in the light absorbing polarization layer 6 and are fully absorbed.

The polarizing rectenna array forming the absorbing polarization layer 6 of inventive photovoltaic device contains a distribution of antenna lengths to allow for easy absorption of electromagnetic waves of various wavelengths. This broad-spectrum absorption property gives the inventive photovoltaic device the advantage that it will absorb most of the infrared, visible and ultra-violet light, which is the range of the electromagnetic spectra that passes through the atmosphere as sunlight. That is, unlike other photovoltaic devices, the inventive device absorbs virtually all of the available light energy reaching the earth's surface, not just selected frequencies. Thereby, it had proven advantageous to use a distribution with its centre at approximately 250 nm.

Figure 2 shows a small portion of the five major layers of Nanopolytech. The layers are as follows: a. Lenticule layer 12 - A single lenticule of the lenticular lens system - used to focus incident light between the bus bars and onto the one-dimensional rectenna. b. lnterdigitated Electrode Structure and Bus Bars on a polyethylene terephthalate I layer which is also known as Mylar (brandnahme for polyethylene terephthalate from DuPont, Wilmington, USA) 14 - used to collect induced photocurrent. c. One-dimensional Rectenna 6 - polarizing absorber with antennas and diodes

- used to absorb two components of the incident photon's electric vector. d. Wave Turner 8 - used to rotate a photon's electric vector component. Two passes through the wave turner will rotate a photon's electric vector component 90° e. Reflector 10 on Mylar - used to reverse the path of a photon without affecting the direction of the electric vector component.

Figure 3 shows a small portion of the light absorbing polarization layer 6, particularly the one-dimensional rectenna layer with strands of polymeric antennas 16 (quantum wells) and oriented diodes 18 (diads) spanning the gap between adjacent electrodes 20, 22. The electrodes 20, 22 can carry the photon induced electrical current to be summed at the bus bars 26, 28.

In the following the creating of the rectenna array comprising of the molecular antennas 16 and the diads 18 will be described in more detail.

As explained above the light absorbing polarization layer 6 is made from a casting solution which is casted onto a moving casting belt, where it is particularly dried and stretch-oriented onto the interdigitated electrodes 20, 22 being composed on the support layer.

The casting solution used to produce the polymer film that becomes the inventive light absorbing polarization layer which contains three primary ingredients, poly-vinyl- alcohol (PVA, (-HCOH-)), hydriodic acid (HI) and iodine (I). The casting solution is processed and stretched to approximately an 8x stretch ratio for proper alignment of PVA strands that will form strings of interspersed antennas and diodes. The inventive light absorbing elements, i. e. the molecular antennas 16, have as their base molecule the conductive polymer trans-polyacetylene, a conjugated polymer (PA, (-CH=)) formed by dehydrating PVA. The diads 18 have as a base small sections of undehydrated PVA. PVA is converted to PA after stretch orientation by dehydration, but the process is stopped when about 90% of the PVA are converted to PA, since long antennas 16 and short diodes 18 are desirable. The desired distribution of lengths of the antennas 16 is centered at about 250 nanometers for the best absorption light, i. e. sunlight. The diode length 18 is preferably between 5 and 25 Angstroms for proper tunneling of electrons.

The molecular polymer antennas 16 are made highly conductive by doping polyacetylene with iodine in the form of singly ionized linear polyiodide (Z₃ ^" or /₅ ^"). The polyiodide molecules are held by the conjugated PA chains (PA is doped with I) while the PA is being formed from PVA during the dehydration process according to the reaction

(HCOH)_x + xH⁺ + xi; → (CH)_x + xl ^~ + xH₂0 , where y = 3 or 5. The hydrogen ions and the polyiodide are provided by HI and I in the casting solution. Doping PA with iodine is known to make it highly conductive, on the order of 10^Λ6 Siemens per centimetre, similar to the conductivity of copper. The diads 18 are produced by placing and orienting electron-donor and electron-acceptor molecules at the ends of the insulating PVA segments by applying an electric potential across the electrodes while in an anhydrous alcohol solution of donor and acceptor molecules.

The voltage build-up between electrodes 20, 22 is a dynamic process. An incident photon causes an electron in the antenna 16 (quantum well) between the diads 18 to increase its voltage potential. This happens because the absorbed photon increases the orbital energy of the linear conductor (the antenna) by about 2.5 eV. As a result, an electron is excited and moves from the valence band (the PA backbone, π orbital) to the conduction band (the iodine "column", π^* orbital) forming an exciton (a paired electron and hole) that travels down the polymer chain. The distribution of the sizes of the quantum wells (equals lengths of antennas 16) determines a distribution of band gaps (energy levels between the valence and conduction bands) making the inventive photovoltaic device a multiple band gap device. The excited electrons can be transmitted only in one direction through an adjacent diad to the next antenna. Charge separation occurs at the diad/antenna interface where the electron tunnels through the diad 18 and the hole is left behind in the antenna 16. The inventive photovoltaic device is many chains thick and therefore is a multi-layered, multi-gap, photovoltaic device that absorbs most of the solar spectrum from IR to UV.

The voltage potentials in adjacent antenna-diads 16, 18 are additive in voltage steps. Hence the potential difference between electrodes 20, 22 depends on the photon flux time-rate of charge production on the series of antenna-diads, which induces a voltage between the electrodes 20, 22. The voltage generated, as measured by a voltmeter in parallel with the load is the sum of voltages generated by the series of antenna-diads between the electrodes. With a saturation photo flux the voltage generated between the electrodes is proportional to the number of diads 18 between the electrodes times the voltages generated per photon absorbed by each antenna- diad.

The interdigitated electrode structure used to conduct current to the bus bars 26, 28 from the rectenna film has a further advantageous property. Since, the resistance of the overall inventive photovoltaic device as seen at the bus bars 26, 28 per square centimetre is inversely proportional to the number of electrodes used to pick up the current from the rectenna layer per centimetre length, the conductivity of the layer is increased by approximately n-squared times, if the number of electrodes per centimetre are increased by n-times.

Disclosed is a new and inventive photovoltaic multilayer device comprising a stretch-oriented electrically conducting light-polarizing polymer film containing molecular diodes (diads) and molecular antennas (one-dimensional quantum wells) interspersed along the same polymer strands. Light energy is captured by the quantum wells and the induced electrical current is rectified by the diads, forming a one-dimensional rectenna that absorbs UV₁ visible and IR portions of the electromagnetic spectrum. The film is laminated to interdigitated electrodes, a lenticular lens system and a reflux system. Electrodes carry the photon induced electrical current to be summed on the bus bars. The lenticular lens system redirects light away from the bus bars and onto the polymer film for increased efficiency. The reflux system, comprised of a wave turner and reflector, reorient the light energy transmitted on the first pass through the front side of the rectenna film to be reflected and absorbed on the back side, eliminating the need for two such films to be arranged orthogonally.

Claims

1. A photovoltaic multilayer device comprising: - a support layer having at least two electrodes;

- a light absorbing polarization layer having light absorbing elements for absorbing photons and transmitting polarized light characterized by

- a wave turning layer turning the transmitted light; and - a reflecting layer back-reflecting the turned polarized light to the light absorbing polarization layer.

2. Photovoltaic multilayer device according to claim 1 , wherein the light absorbing elements are molecular, preferably polymer based, antennas, which are aligned in parallel.

3. Photovoltaic multilayer device according to claim 1 or 2, wherein the light absorbing layer further comprises current rectifying elements, particularly molecular diads, interspersing the light absorbing elements.

4. Photovoltaic multilayer device according to any of the preceding claims, further comprising a light directing layer, particularly comprising lenticular lenses, for directing light to the light absorbing polarization layer.

5. Photovoltaic multilayer device according to any of the preceding claims, wherein the support layer further comprises at least two bus bars for connecting the at least two electrodes and providing an electric current to a consumer.

6. Photovoltaic multilayer device according to any of the claims 2 to 5, wherein the molecular antennas have a different length for absorbing light from UV to IR spectrum, particularly having a length distribution with its centre at approximately 250 nm.

7. Photovoltaic multilayer device according to any of the claims 3 to 6, wherein a length of the current rectifying element is between 2 and 30 Angstrom, particularly 5 and 25 Angstrom.

8. Photovoltaic multilayer device according to any of the preceding claims, wherein the wave-turning a layer is adapted to turn the incident light by 45°.

9. Method for producing a photo voltaic multilayer device according to any of the claims 1 to 8 comprising the steps of:

- stretch-orienting a light absorbing polarization layer to a multiple of its original length, particularly to 8-times of its original length. - placing and laminating the stretched light absorbing layer onto a support layer comprising at least two electrodes;

- laminating a wave-turning layer to the stretched light absorbing polarization layer; and

- laminating a reflecting layer to the wave turning layer.

10. Method according to claim 9 further comprising the step of: covering the support layer with a light directing layer, preferably comprising lenticular lenses.

11. Method according to claim 9 or 10 further comprising the step of: providing the light absorbing polarization layer by at least one of the following steps: - preparing a polymer base casting solution, preferably containing at least one of the following materials: Polyvinylalcohol, hydriotic acid, iodine, water, 2- propanol; - casting the solution on a casting belt, preferably a moving casting belt; - partially drying the polymer solution to form a stretchable light absorbing polarization layer.

12. Method according to any of the claim 9 to 11 , further comprising the step of forming molecular antennas, particularly by placing the stretched film in a dry oven to dehydrate polyvinyl alcohol to form iodine doped polyacetylene and polyvinyl alcohol.

13. Method according to any of the claims 9 to 12, further comprising the step of forming diads, particularly by adding donor and acceptor components to the stretched layer and placing the layer in an anhydrous solution of 2-propanol with donor and acceptor components, and subjecting the layer to an electric potential.

14. Apparatus for producing a photovoltaic multilayer device according to any of the claims 1 to 8.

15. Apparatus according to claim 14 which is further adapted to perform at least one of the steps of the method according to any of the claims 9 to 13.