WO2008061586A1

WO2008061586A1 - Photovoltaic cells and methods for production thereof

Info

Publication number: WO2008061586A1
Application number: PCT/EP2007/008401
Authority: WO
Inventors: Kristofer Tvingstedt; Olle Inganäs
Original assignee: Epipolysteme Ab
Priority date: 2006-11-23
Filing date: 2007-09-27
Publication date: 2008-05-29

Abstract

A photovoltaic device comprising a light trapping and light diluting structure has a first reflective surface portion (4a), at least partially provided with a first active material portion (3a), and a second reflective surface portion (4b), at least partially provided with a second active material portion (3b). At least one of the first and second active material portions (3a, 3b) is a printed photovoltaic active material. The reflective surface portions (4a, 4b) make an angle (a) of less than or equal to 90 degrees relative to each other, such that light reflected from one of the reflective surface portions (4a, 4b) is directed to the other one of the reflective surface portions (4b, 4a). Methods for producing such structures are also disclosed.

Description

PHOTOVOLTAIC CELLS AND METHODS FOR PRODUCTION THEREOF

Technical Field

The present disclosure generally relates to thin-film solar cells and methods of enhancing the optical absorption in such devices, and to improve the energy conversion efficiency. More specifically, the present disclosure is related to photovoltaic devices, with a structured design such that the optical absorption and hence the efficiency in thin film photovoltaic devices with otherwise limited absorption is enhanced by absorbing light in a multitude of different absorbers via a set of tilted mirrors.

Background

Conversion of solar energy to electrical power by means of photovoltaic cells is well known. Various arrangements for enhancing the energy conversion efficiency from photovoltaic devices, by means of multiple band gap cells and by light concentration or light trapping geometries are also well demonstrated. Thermodynamic limits for solar cell in light trapping structures and for multiple bandgap solar cells are deduced, see Green, M.: Third Generation Photovoltaics: Advanced Solar Energy Conversion (Springer Series in Photonics), 2003. These are valid for any type of material, but most considerations have been focused on crystalline or thin film inorganic materials.

A single planar photovoltaic cell 10 generally comprises a charge carrier generating active layer 3 sandwiched between two charge collecting electrodes 2a, 2b situated on a substrate 1 , as is illustrated in Fig. 1. A general drawback with photovoltaic cells is the cost of the material forming the cell. Hence, efforts are being made to reduce the amount of material of the cells.

Organic and printed photovoltaic cells are also known, e.g. from US 2005/0164425A1. For organic and polymer based solar cells, processing via printing of active materials from solvents leads to the possibility of coating large areas with very thin layers of active material. This is not an option with crystalline inorganic solar cells, which are typically thick (>100 μm), compared to the thin organic photovoltaic elements (<0.2 μm). Neither can they be produced over large areas, though polycrystalline thin film inorganic photovoltaic elements can be. The smaii amount of active materials used in the thin film devices allows a very large area, with a different focus on materials and device economics. Therefore, alternative geometries which are compatible with the goals of multiple bandgap devices, and of light confinement, may be relevant, if they can be prepared via low cost and large area deposition routes. A drawback with organic photovoltaic cell technology is its low conversion efficiency. A particular problem to be solved in thin film solar cells with organic materials, is the conflict between optical absorption and electrical transport. Thick devices do not give sufficient electrical collection of photogenerated charge carriers; thin devices do not absorb sufficient amount of light.

Furthermore, in order to provide efficient printing of large cell arrays, using conventional-type printing equipment, a planar carrier substrate is needed.

DE 41 41 083 A1 discloses a solar cell on a metallically shining saw tooth substrate, wherein adjacent flanks are coated with solar cells for different wavelength coverages. The device of DE 41 41 083 A1 is to be manufactured by evaporation of materials onto a sawtooth-shaped substrate, or by mechanical assembly. Hence, the substrate upon which the solar cell is produced is not planar, and so the device of DE 41 41 083 A1 is difficult to produce, rendering it expensive, and thus unsuitable for large scale production.

Hence, there is a general need for improvements to photovoltaic cell technology, in order to reduce cost and improve conversion efficiency. Summary

It is a general object of the present disclosure to eliminate or at least alleviate the problems of the prior art. It is a specific object of the present disclosure to provide a photovoltaic device, which can be produced in large volumes at a low cost, having improved conversion efficiency and/or improved spectral coverage.

The invention is defined by the appended independent claims, with non-limiting embodiments being set forth in the appended dependent claims, and in the following description and drawings.

The invention is based on the understanding that it is possible to produce organic photovoltaic cells by printing on a planar substrate, and then folding or creasing the substrate in a controlled manner to provide reflective surfaces presenting an angle relative to each other.

Hence, there is provided a photovoltaic device comprising a light trapping and light diluting structure, having a first reflective surface portion, at least partially provided with a first active material portion, and a second reflective surface portion, at least partially provided with a second active material portion, wherein at least one of the first and second active material portions is a printed photovoltaic active material. The reflective surface portions make an angle of less than or equal to 90 degrees relative to each other, such that light reflected from one of the reflective surface portions is directed to the other one of the reflective surface portions. Thus, there is provided a photovoltaic device, which may be produced at a low cost and provide an improved conversion efficiency and/or improved spectral coverage. It would not have been possible to provide such a photovoltaic device using e.g. wafer based (silicon etc.) solar cells, nor would it be economically feasible to produce using a method requiring assembly onto an already shaped substrate.

This concept is easily generalized to e.g. more than two active materials. More efficient use of solar energy for photovoltaic conversion entails the use of multiple bandgap devices. By directing higher energy photons to devices comprising absorbers with larger band gap that hence generates larger voltage, a better overall efficiency may be obtained. An improvement of the thermodynamic limits for energy conversion is obtained with a second (optimal) bandgap material. With more bandgaps available, this limit is further increased. Organic polymers and molecules allow very simple modification of bandgaps of the active materials, and therefore are well suited for the construction of multiple bandgap devices. By making a higher bandgap cell with higher voltage, and a lower bandgap cell with a lower voltage at the opposing sides in a reflective element, there is a serial optical connection between the cells, and a possibility of using electrical parallel or serial connections from the two individual devices. It is also conceivable to cover the sides in the reflective cell with multiple devices from different bandgap solar cells, distributing the light between them. By combining absorption in two solar cells in a reflective tandem arrangement, the compromise between optical absorption and electrical transport is less difficult. It has yet another consequence. With the observation that higher photocurrents lead to lower fill factors in organic solar cells, it follows that by distributing the optical power between at least two cells, the collected photocurrent density is reduced within each cell, leading to a higher fill factor, and therefore a higher power conversion efficiency. This principle of optical dilution is the opposite of the optical concentration mostly relevant with high performance, high cost inorganic crystalline solar cells. The non-normal incidence of direct light is further enhancing the optical path length of photons through the active material; this leads to a better collection of optical energy, and to extension to wavelengths of lower absorption coefficients. We consider that our invention adresses all the issues of light confinement, light dilution and multibandgap devices, in different combinations. An arbitrary number of tandem arrangements may be provided on both reflective surfaces.

Active materials that may, as non-limiting examples, be printed, may be e.g. organic materials, but also inorganic ones, such as e.g. CIGS, Si or modified Si particles, or nanoparticles.

The term "printing" is understood as transfer of a liquid phase onto the substrate. The liquid phase may, but does not need to, have gel-like or viscous properties. The printing may be followed by evaporation and/or solidification. The reflective surfaces may be planar, or curved. At least one of first and second active material portions may be formed as a layer on the first or second reflective surface portion, respectively. In a particular embodiment, active material layers are formed on both reflective surface portions. The first and second reflective surface portions may be arranged adjacent to each other, thus maximizing surface usage.

Both first and second reflective surface portions may be covered with the first and second active material portion, respectively, thus maximising the surface usage. The reflective surfaces may be provided with the same, or different, active materials, having different thicknesses. Thus, a balance between electrical collection of photogenerated charge carriers and sufficient light absorption may be provided.

The first and second reflective surfaces may be provided with active materials having different bandgaps.

At least one of the reflective surface portions may be provided with at least two active material portions having different bandgaps and/or different thickness.

At least two active material portions on at least one of the reflective surface portions may be arranged substantially in the same plane.

In the alternative, or as a complement, at least two active material portions on at least one of the reflective surface portions may be arranged in stacked tandem.

The first and second active material portions may form part of a respective photovoltaic element.

At least one of the photovoltaic elements may comprise a transparent electrode and a reflective electrode.

At least one of the photovoltaic elements may comprise a transparent top electrode and a reflective bottom electrode and be arranged to be illuminated through the top electrode.

At least one of the photovoltaic elements may comprise a transparent bottom electrode and a reflective metal top electrode and be arranged to be illuminated through the transparent bottom electrode. The reflective surfaces may be provided with at least two photovoltaic elements, which are connected in an electrical series or parallel connection.

According to a second aspect, there is provided a photovoltaic module comprising at least two photovoltaic devices as set forth above. In such a module, the angle a may be variable.

The reflective surfaces may be continuous with a plurality of identical photovoltaic devices .

The reflective surfaces may be provided with at least two photovoltaic cells, which are connected in an electrical series or parallel connection. According to a third aspect, there is provided a method for producing a photovoltaic device according to any one of the preceding claims. The method comprises providing a planar substrate, forming at least two reflective surface portions on the substrate, and printing at least two active material portions, on the substrate. The forming and printing are performed such that one of the reflective surface portions and one of the active material portions at least partially overlap. Subsequently the substrate is shaped such that the reflective surface portions make an angle of less than or equal to 90 degrees relative to each other, such that light reflected from one of the reflective surface portions is directed to the other one of the reflective surface portions . The order in which the reflective surface portions and the active material portions are formed is arbitrary.

The substrate may be shaped by folding, buckling and/or lateral compression of the substrate.

The photovoltaic element may be formed with a cathode, an anode and a photovoltaic active material in a respective pattern.

According to a fourth aspect, there is provided a method for producing a photovoltaic device according to any one of the preceding claims, wherein at least one such photovoltaic device is formed with a cathode, an anode and an organic active material in a pattern on a thin flexible carrier substrate, the active material is printed from a solvent on top of the anode or cathode, and the cathode or anode, respectively, is subsequently deposited on the photovoltaic material. Brief Description of the Drawings

Fig. 1. is a schematic cross sectional view of a prior art single planar photovoltaic cell.

Fig. 2. is a schematic cross sectional view of a pair of photovoltaic cells placed next to each other in the shape of a V.

Figs 3a-3c schematically illustrates, in cross section, how a pair of different photovoltaic cells may be arranged.

Fig. 4 schematically illustrates, in cross section, a method for producing a photovoltaic device according to the present disclosure. Fig. 5 illustrates absorption results for a planar photovoltaic cell, and for photovoltaic cells folded 45 and 60 degrees, respectively.

Fig. 6 illustrates absorption results for planar photovoltaic cells and for a photovoltaic reflective tandem cell folded 45 and 60 degrees, respectively.

Description of Embodiments

Fig. 2. is a schematic cross sectional view of a pair of photovoltaic cells 10a, 10b placed next to each other in the shape of a V, with the angle a in between the two photovoltaic cells. This geometry generates a reflective tandem cell, where light is first reflected on one cell, then hitting the other. The length L may be anything from micrometer to meter; this is entirely a matter of production and assembly methods. If the photovoltaic cells 10a, 10b are built from the same organic photovoltaic element, we have a simple light trapping element combined with a single bandgap device. In this case the two different sides may be put in electrical series or in parallel connection, as they should have substantially identical current-voltage characteristics, from symmetry considerations. Using this light trapping geometry, more photovoltaic material is necessary, as the two sides of the V will be longer than the corresponding sides of the projected area over which sun light is being trapped. This increase of area is however of small influence on the cost of the device, as the active materials, which are organic, are very thin and very cheap.

Referring to Figs 3a and 3b, there is illustrated different arrangements of photovoltaic cells 10a-1 , 10a-2, 10a-3, 10b-1 , 10b-2, 10b-3 having different characteristics: two active materials (symbolized by different shading) are combined in a V geometry. In the illustrated example, the cell 10a-1 is identical with the cells 10b-2 and 10a-3, whereas the cell 10a-2 is identical with the cells 10b-1 and 10b-3. It is recognized that any combination of materials may be provided within a reflective surface portion and/or between a pair of adjacent reflective surfaces forming an angle of less than or equal to 90 degrees relative to each other.

For clarity, the electrodes and substrate are not indicated in Figs 3a- 3c. Instead, reference is made to Fig. 2, where different active materials are indicated by 3a and 3b, and reflective surfaces are indicated by 4a, 4b.

In the embodiment of Fig. 3a, both cell types 10a-1 , 10a-2, 10b-1 , 10b- 2 may be found on both wings, whereas in the embodiment of Fig. 3b, one cell type 10a-1 , 10a-3 is found only on one of the wings, whereas the other cell type 10b-1 , 10b-2 is found only on the other one of the wings of the V. This concept is easily generalized to more than two active materials.

The art of stacked tandem absorbers is not prevented, but rather complemented, by the invention. It is hence further possible to have two or more materials stacked on top of each other on at least one of the reflective surface portions in the V- geometry. Fig 3c illustrates such a transmitting, or stacked, tandem arrangement of solar cells placed in the reflecting V- geometry, with a respective transmissive recombination layer 10a-1', 10b-1'. Thus, in this configuration, at least one side of the V has a transmissive recombination layer. In this configuration at least one side, but possibly both, of the V comprise at least two absorbing materials. Hence, by combining two or more transparent polymer solar cells 10a-1 ', 10a-2'; 10b-1 ', 10b-2' on top of each other, the V geometry may be combined with a conventional tandem solar cell, for even further enhanced photon management.

Production of all above described combinations of structures is feasible with combinations of thin film coating and printing. Common among the different solar cells is their construction (Fig. 1) as a series of thin layers with anode/active layer/cathode, where anode, cathode or both must be transparent for light to excite the active material in between anode and cathode. There may be added layers for electrical control and exciton control in such structures. Devices may be manufactured in a set of different sequences. Two active layer absorbers with different bandgaps may be put on top of each other with a transparent thin recombination layer between, all to be sandwiched between a top and bottom electrode. The top and bottom electrode may further comprise both transparent and reflective materials.

In one particular case devices are built in what is named an inverted fashion, where a metal layer is first deposited on a surface to act as the cathode, where an active layer is later deposited by f.i. printing methods, and where the upper anode is a transparent polymer layer deposited by different coating methods. This can be done by methods of printing, depositing different active materials on a surface to build independent solar cells with a small separation in between individual planar devices. These devices are considered to be printed on a flexible layer, which will enable the construction of a multitude of V shaped solar cell next to each other. Referring to Fig. 4, by similar means as an accordion, such multiple device pattern may form the multitude V-geometry via a lateral D1 compression to incur an expansion in the vertical direction D2.

Another implementation may be to use pre-shaped elements, which are free to undergo rotation to form the V structures. Variation of the entrance angle a may be used to vary the light dilution factor; with smaller angles, the light dilution is increasing. As illustrated in Fig. 4, the substrate may be laterally expanded or compressed, whereby the angle a may be altered. The description will now be directed to an example of photovoltaic cells. In the example, polymer solar cells based on alternating copolymers of fluorene (APFOs) combined with acceptors such as the fullerene PCBM were used. The polymer blends were spin coated into films of thickness 50-60 nm and sandwiched between a transparent electrode (ITO/PEDOT:PSS) and LiF/AI. This polymer class can generate high photovoltage, typically 1 V and power conversion efficiency at AM1.5 of 3.5% from the high bandgap material APFO3 with absorption out to 650 nm. Two different folded photovoltaic cells were manufactured: one using an ITO coated glass substrate (550 μm thick from Naranjo Substrates) and another one on ITO covered plastic sheets (80 μm from CPFilms).

Prior to deposition of the photoactive blends, the ITO was covered with a 40 nm thick film of PEDOTPSS. The polymer/PCBM blend was prepared in ratios of 1 :4 and diluted to 12.5 mg/ml in chloroform.

The blends were deposited via spincoating at 2000 rpm to provide the thin films required for obtaining better charge carrier collection. The samples with ~0.7 cm² active areas were subsequently finalized by thermal evaporation of <1 nm LiF and ~70 nm of Al.

To minimize the small open area at the bottom of the folded cell, the glass substrate end was provided with a V-groove with an angle to fit the folding conditions.

The plastic cells were manufactured by folding along a small generated indentation in the plastic substrate.

Referring to Fig. 5, the two APFO3/PCBM cells in a folded geometry (45 and 60 degrees, respectively) and a planar cell (dotted curve) show optical reflectance indicating an almost complete absorption of light within the spectral range of the material. There was further noted a more pronounced increase of absorption in the spectral region where the individual planar cells have lower absorption.

Referring to Fig. 6, there is illustrated measured absorptance from a reflective tandem cell with APFO3/PCBM on one side and APFO- Green9/PCBM on the other side (without PEDOT), as compared to planar cells having the respective active material type.

As mentioned above, folded structures can be used with cells of different bandgap on each of the two sides of a V, or with several cells of different bandgap located on one of the two sides, forming a reflective tandem or multi junction cell. With one high and one low bandgap part, optical reflectance from the combined cells in a folded configuration demonstrates broadening of optical absorption (Fig. 6), and the overall absorptance is strongly influenced by the fold angle. In a specific embodiment, there is provided a photovoltaic device comprising a light trapping and light diluting structure, in which a layer of active material is found at a reflective surface, with another adjacent reflecting surface being covered with another active material, and where the two surfaces make an angle less than or equal to 90 degrees, so the light reflected from one photovoltaic cell thus formed is directed to the next photovoltaic cell. The two reflective surfaces may be covered with the same type of thin film solar cell in different thickness. At least one of the two reflective surfaces may be covered with a thin film solar cell. The two reflective surfaces may be covered with solar cells with different bandgaps. At least one of the reflective surfaces may be covered with several thin film solar cells with different bandgaps. The two adjacent reflective surfaces may be contiguous with identical elements in an indefinite number. The two reflective surfaces covered with thin film solar cell may be connected in an electrical series or parallel connection or any combination thereof. The two solar cells may utilize a transparent top electrodes and reflective metal bottom electrodes and are illuminated through the top electrode. The two solar cells may utilize a transparent bottom electrode and a reflective metal top electrode and are illuminated through the transparent substrate. Different photovoltaic devices may be created with cathode, anode and photovoltaic active material in patterns on a thin flexible carrier, and the active material may be printed from solvents on top of anode or cathode, with subsequent deposition of cathode or anode. The structures may be produced such that different photovoltaic devices are created with cathode, anode and photovoltaic active material in patterns on a planar surface, which is buckled under lateral compression to give a V shaped geometry, with one or more devices on each wing of the V.

Light trapping can be done with the help of microstructured surfaces enhancing reflection within the surface layer, incorporating a photovoltaic material. Multiple reflection can also be accomplished with the simple arrangement of planar reflective elements at an angle of a<= 90 degrees. With organic photovoltaic elements created as thin organic layers on top of a thin metallic layer, the combined organic solar cell can also be used as a reflective element, forming one or two sides of a profile in the shape of a V, combining two sides of length L.

The above-mentioned maximum angle a of 90 degrees is deemed to be the preferred maximum angle, however, the invention may be applied with angles of e.g. less than or equal to 100 degrees, less than or equal to 110 degrees, less than or equal to 120 degrees or less than or equal to 130 degrees.

Claims

1. A photovoltaic device comprising a light trapping and light diluting structure, having: a first reflective surface portion (4a), at least partially provided with a first active material portion (3a), and a second reflective surface portion (4b), at least partially provided with a second active material portion (3b), wherein at least one of the first and second active material portions (3a, 3b) is a printed photovoltaic active material, c h a r a c t e r i s e d in that the reflective surface portions (4a, 4b) make an angle (a) of less than or equal to 90 degrees relative to each other, such that light reflected from one of the reflective surface portions (4a, 4b) is directed to the other one of the reflective surface portions (4b, 4a).

2. The photovoltaic device as claimed in claim 1, wherein at least one of first and second active material portions (3a, 3b) is formed as a layer on the first or second reflective surface portion (4a, 4b), respectively.

3. The photovoltaic device as claimed in claim 1 or 2, wherein the first and second reflective surface portions (4a, 4b) are arranged adjacent to each other.

4. The photovoltaic device as claimed in any one of claims 1-3, wherein both first and second reflective surface portions (4a, 4b) are covered with the first and second active material portion (3a, 3b), respectively.

5. The photovoltaic device as claimed in any one of the preceding claims, wherein the reflective surfaces (4a, 4b) are provided with active material (3a, 3b) with different thicknesses.

6. The photovoltaic device as claimed in any one of the preceding claims, wherein the first and second reflective surfaces (4a, 4b) are provided with active materials (3a, 3b) having different bandgaps.

7. The photovoltaic device as claimed in any one of the preceding claims, wherein at least one of the reflective surface portions (4a, 4b) is provided with at least two active material portions having different bandgaps or different thickness.

8. The photovoltaic device as claimed in claim 7, wherein the at least two active material portions on said at least one of the reflective surface portions (4a, 4b) are arranged substantially in the same plane.

9. The photovoltaic device as claimed in claim 7, wherein the at least two active material portions on said at least one of the reflective surface portions (4a, 4b) are arranged in stacked tandem.

10. The photovoltaic device as claimed in any one of the preceding claims, wherein the first and second active material portions (3a, 3b) form part of a respective first and second photovoltaic element (10, 10a, 10b, 10a-1 , 10a-2, 10b-1 , 10b-2, 10a-1', 10b-1\ 10a-2', 10b-2").

11. The photovoltaic device as claimed in claim 10, wherein at least one of the photovoltaic elements comprises a transparent electrode and a reflective electrode.

12. The photovoltaic device as claimed in claim 11 , wherein said at least one of the photovoltaic elements comprises a transparent top electrode (2b) and a reflective bottom electrode (2a) and is arranged to be illuminated through the top electrode (2b).

13. The photovoltaic device as claimed in claim 11 , wherein said at least one of the photovoltaic elements comprises a transparent bottom electrode (2b) and a reflective metal top electrode (2b) and is arranged to be illuminated through the transparent bottom electrode (2a).

14. The photovoltaic device as claimed in claim 12 or 13, wherein the reflective surfaces (4a, 4b) are provided with at least two photovoltaic elements, which are connected in an electrical series or parallel connection.

15. The photovoltaic device as claimed in any one of the preceding claims, wherein the active material is an organic material.

16. A photovoltaic module comprising at least two photovoltaic devices as claimed in any one of the preceding claims.

17. The photovoltaic module as claimed in claim 16, wherein the angle (a) is variable.

18. The photovoltaic module as claimed in claim 16 or 17, wherein the reflective surfaces (4a, 4b) are continuous with a plurality of identical photovoltaic devices.

19. A method for producing a photovoltaic device according to any one of the preceding claims, comprising: providing a planar substrate (1), forming at least two reflective surface portions (4a, 4b) on the substrate (1), printing at least two active material portions (3a, 3b), on the substrate, wherein said forming and printing are performed such that one of the reflective surface portions (4a, 4b) and one of the active material portions (3a, 3b) at least partially overlap, and subsequently shaping the substrate (1), such that the reflective surface portions make an angle of less than or equal to 90 degrees relative to each other, such that light reflected from one of the reflective surface portions is directed to the other one of the reflective surface portions.

20. The method as claimed in claim 19, wherein the substrate (1) is shaped by folding, buckling and/or under lateral compression of the substrate.

21. The method as claimed in claim 19 or 20, wherein the photovoltaic element (10, 10a, 10b, 10a-1 , 10a-2, 10b-1 , 10b-2, 10a-1', 10b-1\ 10a-2', 10b-2') is formed with a cathode (2a, 2b), an anode (2b, 2a) and a photovoltaic active material (3, 3a, 3b) in a respective pattern.

22. A method for producing a photovoltaic device according to any one of the preceding claims, wherein at least one such photovoltaic device is formed with a cathode (2a, 2b), an anode (2b, 2a) and an organic active material (3a, 3b) in a pattern on a thin flexible carrier substrate (1), the active material (3a, 3b) is printed from a solvent on top of the anode or cathode, and the cathode or anode, respectively, is subsequently deposited on the active material.