Photovoltaic Devices

Technical Field

The present invention relates to the field of photovoltaic devices, for example organic photovoltaic devices.

Background Art

Whilst 99% of today's solar cells are based on silicon, there is also considerable research ongoing in the field of solar cells using organic semiconductors.

In the 1970's, conducting and semi-conducting polymers were discovered, which combined the mechanical flexibility of polymers with electrical and optical properties of inorganic semi-conductor materials. These polymers are able to conduct charge due to their conjugated structure. This ability to conduct has led to these polymers being used in organic photovoltaic devices. Photoconductivity in organic molecules was first observed in anthracene and this discovery has led to the development of molecular organic semiconductors.

Organic solar cells offer the prospect of many attractive features and advantages over inorganic devices, such as flexibility, transparency, potential for continuous processing and possibility of formation of large area devices, as well as ecological and economic advantages. Further, they are cheaper to manufacture than inorganic solar cells.

The most important consideration for photovoltaic devices is their conversion efficiency. Typically, commercial silicon based devices have a conversion efficiency of about 15%. Organic devices currently operate with a maximum efficiency of about 5%. In order to create an organic photovoltaic device that is able to compete with silicon based devices, an improvement by a factor of two would be desirable.

The basic design of an organic photovoltaic (PV) cell consists of an organic layer sandwiched between two electrodes. In order to allow light into the cell, typically one electrode must be transparent and conductive, for example consisting of a thin film coating of indium tin oxide (ITO) or SnO ₂ on a glass substrate.

Early work was based on single molecular organic layers, typically made of phthalocyanines (Pc) or polyacenes, positioned between two electrodes. In the

1980s a device comprising a bilayer (planar) heteroj unction was developed, using copper phthalocyanine (CuPc) and a perylene derivative, resulting in an order of magnitude improvement in power conversion efficiency. The discovery of C ₆ o in 1985 and its use in a CuPc/C ₆₀ planar heterojunction device further increased the power conversion efficiency achievable.

The majority of organic PV devices operate by combining organic materials which have donor and acceptor properties and providing a heterojunction between two such organic layers, where one layer is an electron transporter (acceptor) and the other is a hole transporter (donor). In particular, known organic solar cells are based on thin films of organic semiconducting materials, such as phthalocyanines and fullerenes, or conjugated polymers and fullerenes. The donor-acceptor films are typically lOOnm in thickness.

Upon absorption of light into an organic PV device, an exciton, that is, a bound electron hole pair is generated. The electron and hole are bound together by electrostatic attraction and are strongly localised. The exciton is able to migrate or diffuse to a lower energy state. This exciton must reach a donor- acceptor interface in order to dissociate efficiently into free charge carriers. This dissociation is essential in solar cells such that when an exciton reaches an interface between the donor material and acceptor material, the electron of the electron hole pair (exciton) may be transferred to the acceptor material. The electron in the acceptor material is transported to the cathode, and the hole, remaining in the donor material, is transported to the anode. The diffusion length of an exciton is of the order of 10 to 50nm; for example in copper phthalocyanine (CuPc) it has been found experimentally to be about 30nm. Beyond this length the probability of the electron and hole recombining increases. It may therefore appear desirable to reduce the film thickness to less than 30nm in order that the exciton reaches a donor-acceptor interface and dissociates. However, in order to absorb light efficiently and hence create excitons, film thicknesses of typically lOOnm are required.

Figure 2 shows a typical planar heteroj unction between a layer of donor material 40 and a layer of acceptor material 42. The organic layer is sandwiched between a transparent electrode 44 and a conductor electrode 46. The efficiency of devices with such a structure is limited by the diffusion length of the exciton, which is typically much shorter than the thickness of layers needed for useful light absorption.

To effect a compromise between the short diffusion length of excitons, and still achieve good light absorption, organic devices have been developed having mixed blend layers. A typical example of such a mixed blend device is shown in Figure 3. A transparent electrode 48 and a conductor electrode 50 are situated on opposite sides of the mixed blend layer 52 made up of donor

material 54 and acceptor material 56. The donor and acceptor materials are co- deposited to form a random distributed heterojunction.

By increasing the number of interfaces between the donor and acceptor material, efficiency is increased despite the relatively short exciton diffusion length in these materials. Whereas this arrangement may achieve efficient dissociation, it is inefficient for subsequent free charge transport. This is because the electrons must be transported to the appropriate electrode by the acceptor material, and the holes must diffuse towards the other electrode in the donor material. However, the blended nature of the donor and acceptor materials leads to discontinuity of each of these respective materials, making charge transfer more difficult. Charges can be trapped in isolated domains, which reduces the total charge collection and therefore the overall power conversion efficiency of the device.

A further variation of such a mixed blend organic device has been developed, having multiple mixed blend layers. In such a multilayer device, the composite is arranged in planar layers which are composed of blended donor material and acceptor material. The composition of the layers is graded, the composition of the layer nearest one electrode being made of 100 percent acceptor material, the proportion of acceptor materials then decreasing to zero percent acceptor material and 100 percent donor material in the layer adjacent to the second electrode. This type of device is, however, extremely difficult to manufacture.

To overcome this limitation, a 3D corrugated interface structure, as shown in a cross sectional view in Figure 4, is desirable. Two electrodes 60, 62 are situated on opposite faces of the organic layer 64. The organic layer 64 is

made up of corrugated 'fingers' of acceptor material 66 and donor material 68. The maximum thickness of these fingers should be about two times the exciton diffusion length in that material. All excitons will thus be formed within the diffusion distance of the acceptor-donor material interface and charge transfer to the electrodes is efficient.

In order to improve the efficiency of organic solar cells, it is desirable to increase yet further the interfacial area between the acceptor and donor phases, so as to minimise the exciton diffusion path, while having a structure thick enough to absorb light efficiently and ensuring that there is continuity between each phase and its respective electrode.

By maximising the interfacial area, the exciton diffusion path is minimised. This leads to greater probability that the excitons dissociate at a heteroj unction, resulting in a current being generated by the cell, rather than the excitons recombining and the current being lost.

Summary of the Invention

The invention is set out in the claims. In particular, creation of continuous interpenetrating lattices for each of the donor and acceptor materials maximises the interfacial area between the materials and hence the exciton dissociation efficiency, minimising the exciton diffusion path.

By employing organic materials, room temperature processing may be used, allowing low cost and high throughput production of devices according to the invention.

Such a composite may for example be used in a photovoltaic device or any other semiconductor device, for example a sensor, a photodetector, or a light emitting diode.

Brief Description of the Figures

Embodiments of the invention will now be described, by way of example with reference to Figure 1 which shows the various stages of preparation of a device according to the invention.

Figures 2, 3 and 4 show schematic cross-sectional views of a planar heterojunction, a distributed heterojunction and an idealised 3D corrugated interface structure, in known types of organic semiconductor devices.

The invention can be understood referring to Figure 1. In overview using nanosphere lithography, a nanocomposite organic film comprising a donor phase and acceptor phase in the form of interpenetrating lattices has been engineered, wherein the interfacial area between the phases is maximised. Continuous electrically conductive pathways formed by the respective lattices run between each phase and either the top or bottom electrode (substrate).

In addition to the donor and acceptor materials both being organic materials, in a further embodiment of this invention, one of the two phases is an inorganic material, i.e. a device could comprise an organic donor material and an inorganic acceptor material, or vice versa, the donor phase could be an inorganic material and the acceptor phase could be an organic material.

The fabrication process involves a series of fabrication steps as follows:

At steps 1 and 2 an electrode 1, which may consist of any conducting material for example a flexible plastic material, for example an ITO coated transparent material, e.g. ITO coated glass, is coated with a layer of donor or acceptor material 3. The donor material might for example be a phthalocyanine, e.g. a metal phthalocyanine, such as copper phthalocyanine, and the acceptor material could for example be a fullerene or a perylene. Alternatively, the layer 3 may be an appropriate inorganic material, such as titanium oxide or zinc oxide. This layer of material enables continuous contact with the electrode.

To form an active layer for the device more than one layer of monodisperse particles such as colloidal spheres 5 is then deposited at step 3 on the layer of material 3, for example by controlled self-assembly deposition from a colloid suspension. Typically, several layers of spheres 5 are deposited, according to the device thickness required. The spheres may be of any monodisperse, removable material, which is able to produce an ordered, hexagonally or cubic or other geometry close-packed structure. As this structure is ordered or structured and not random, each sphere will be in contact with all of its neighbours, and hence be interconnected. The removable particles may for example be polystyrene.

If used, any carrier solvent is then removed, leaving the layers of spheres in contact with each other but with the interstitial space 7 empty. The interstitial space 7 is then infiltrated at step 4 with a further amount of the donor or acceptor material 3 used to coat the electrode 1. This may be done for example by solution infiltration, by dipping or by deposition from the vapour phase.

At step 5 a sufficient top layer of the material 3 is then removed to allow access to the uppermost layer of spheres 5, if necessary. Due to the controlled nature of the self-assembly of the layers of spheres, this step will usually not be

required. The spheres 5 are then removed from the structure by a suitable means so as to leave no residue, resulting in empty space where the spheres were. This might be by combustion, or by a low temperature process, preferably room temperature solution processing, for example the spheres may be removed by solvent extraction, or sonication.

This leaves a skeleton of the donor or acceptor material 3, which is a lattice corresponding to and shaped as the interstitial spaces 7 and which is continuous and connected to the electrode 1. The voids created by removal of the spheres 5 are interconnected because the spheres were originally in contact with each other. The voids form a continuous lattice 9 that interpenetrates the skeleton 3.

The interconnected lattice 9 comprising the empty space previously occupied by the spheres in the composite structure is then infiltrated at step 6 by the second phase material 11, which will be an acceptor material if material 3 was a donor material, or vice versa.

The spheres need only be removable if they are 'sacrificial', as described above. In an alternative embodiment of this invention, the spheres may be 'non-sacrificial' in which case removability is not a desirable characteristic of the spheres. As shown as an alternative route (ii) in Figure 1, the steps of removing the spheres and filling the resulting empty lattice with either donor or acceptor material as appropriate (step 5 and 6) are omitted.

Non-sacrificial particles may for example be used in inorganic-organic hybrid devices, wherein one of either the donor or the acceptor material is an inorganic material. In such a device, the deposited particles could for example be titanium oxide or zinc oxide spheres, which are able to conduct. Alternatively

the particles could be an organic material, examples of which are given above, which would not be removed.

After such non-sacrificial particles are deposited at step 3, the interstitial space 7 is infiltrated at step 4 with a further amount of the donor or acceptor material 3 used to coat the electrode 1.

At step 7 a continuous layer 13 of the second phase material 11 is then formed at the upper face of the composite structure and a second electrode 15, which may be of any appropriate material, for example a metal such as aluminium, gold or copper, is then applied to the layer 13 of the composite structure.

The electrodes are then furnished with suitable electrical connections, by means known in the art. Protective coatings and anti-reflective layers may also be applied, as required. An additional exciton-, electron-, or hole-blocking layer, depending on the electrode material used, may be deposited to form layers at either or both electrodes to optimise the performance of the device. Such a layer would preferably have a thickness of approximately 10 to lOOnm.

The composite device thus formed should have sufficient thickness to ensure efficient absorption of light, whilst the interlaced or interpenetrated nature of the structure provided by the topologically interconnected lattices or networks will ensure that excitons are only required to diffuse a short distance before encountering a donor-acceptor heteroj unction (i.e. at the boundary previously defined by a sphere), the interfacial area of which is maximised. By varying the sphere/particle dimension length scale tuneability is achievable. In addition a simple, low cost and high throughput process is provided.

Such a fabrication method can be used to form, for example, a photovoltaic device (solar cell).

When incorporated into a photovoltaic device, in operation light shines on to the solar cell, and enters the cell via the transparent electrode. When a photon is absorbed by the organic material(s), an exciton may be created. When the exciton reaches the interface between the acceptor and donor material, the electron is transferred from the donor material to the acceptor material and transported to the appropriate electrode (the cathode). The positively charged hole diffuses through the donor material towards the anode. By this method it is possible to draw current from the photovoltaic device with improved absorption efficiency resulting from the thickness of the device achievable whilst minimising recombination losses because of interfacial area and providing efficient charge transport as a result of the physical continuity of the interpenetrating lattices.

The volume ratio in the composite is approximately 74% spheres to 26% spaces for hexagonally packed structures. The volume ratio in the composite depends on the order of infiltration and on the processing methods and conditions, such as temperature. It can be optimised for any particular combination of donor and acceptor materials. If one of the donor or acceptor materials is the more efficient at light absorption for exciton generation, this more efficient material should preferably form the dominant phase, occupying the greater volume. Typically, three to four layers of spheres may be deposited, depending on the absorption coefficient and the exciton diffusion length of the materials selected. Preferably the array of spheres will be hexagonally or cubic close packed.

Different size spheres may be used in different devices. However, within a device, all spheres should be of an identical size, i.e. monodisperse. Sphere size should relate to the exciton diffusion length in the material selected. Spheres may typically be approximately between 10 and 500nm in diameter. The smaller the diameter of the spheres used, the more layers may be required to achieve adequate light absorption. Preferably, for currently known materials, spheres may typically be about 50nm in diameter, in which case at least two layers of spheres are required at least for useful light absorption.

The total film thickness of the composite structure of a typical organic PV device will usually be less than 150nm, in order to accommodate the short diffusion lengths of excitons, as discussed above. The optimum thickness of the composite structure will depend on the properties of the specific organic materials used.

In such a device, the diameter of the spheres would typically be in the range of 10 to 50nm, and the number of layers of spheres comprised in the composite structure will depend on the thickness of the composite structure.

For example, taking the close packed arrangement of spheres into account, a 160nm thick composite structure would require 20 layers of spheres with a IOnm diameter. Similarly, if spheres having a diameter of 50nm are used, then four layers of spheres would be required in a 160nm thick composite structure.

All process steps in fabrication may be carried out at low temperatures, preferably about room temperature, as required by the organic materials used. It will be understood by a person skilled in the art that the initial substrate applied to the first electrode and the subsequent infiltration may use an

acceptor material in place of the donor material referred to, and the second infiltration would then be by a donor material

A composite, as described herein may for example be used in a photovoltaic device, as well as any other electronic device which incorporates heterojunctions between electron donating and electron accepting material (i.e. electron and hole transporting materials), for example photo detection devices.

In an alternative embodiment of the composite, one of the acceptor and donor materials may be an organic material and one may be an inorganic material. Alternatively, both the acceptor and donor materials may be inorganic materials.

It will be appreciated that the monodisperse particles may be any geometric shape which will pack to form a continuous interpenetrable lattice for example non-tessellating shapes or tessellating shapes laid out in a non-tessellating orientation and that any packing geometry may be adopted. A further alternative could comprise both larger and smaller, respectively monodisperse particles, where several of the smaller particles would pack together, for example hexagonally close packed, between larger particles, occupying the space that could alternatively be occupied by a larger particle. The resulting lattices would still be completely interconnected, as when particles of only one size are used.

Furthermore, in the case of a photovoltaic cell the electrodes may be reversed, that is, the transparent electrode may form the upper substrate as appropriate in the fabrication approach of Figure 1.

It will be appreciated that the term lattice encompasses both regular and non- regular interconnected structures or arrays. It will also be understood that in practice the term monodisperse encompasses spheres or particles whose variation in diameter is extremely small. Further, the term close packing will be understood to comprise both perfect close packing and nearly close packing.