Photovoltaic systems are implemented to convert light energy into electricity for a variety of applications. Power production by photovoltaic systems may offer a number of advantages over conventional systems. These advantages may include, but are not limited to, low operating costs, high reliability, modularity, low fabrication costs, and environmental benefits. As can be appreciated, photovoltaic systems include a number of photovoltaic devices, commonly known as “solar cells,” so named for their ability to produce electricity from sunlight.
Conventional solar cells convert light into electricity by exploiting the photovoltaic effect that exists at semiconductor junctions. Accordingly, conventional solar cells generally implement active semiconductor layers to produce electron current. The semiconductor layers generally absorb incoming light to produce excited charge carriers. In addition to the semiconductor layers, solar cells generally include front and back electrodes. The front electrode provides a destination for electrons excited by exposure to the incident light source. The back electrode or “counter electrode” completes the electrical circuit with the front electrode by providing a destination for the holes produced during the exposure to incident light.
One particular type of solar cell is a dye-sensitized solar cell (DSSC). A dye-sensitized solar cell generally uses an organic dye to absorb incoming light to produce excited electrons. The dye sensitized solar cell generally includes two planar conducting electrodes arranged in a sandwich configuration, at least one of which is substantially transparent. A dye-coated semiconductor film separates the two electrodes. The electrodes may comprise a transparent conducting oxide (TCO) film disposed on a substrate, for example. The semiconductor layer is porous and has a high surface area thereby allowing sufficient dye to be attached as a molecular monolayer on its surface to facilitate efficient light absorption. The remaining intervening space between the electrodes and the pores in the semiconductor film (which acts as a sponge) is filled with an organic electrolyte solution containing an oxidation/reduction couple, such as triiodide/iodide, for example.
One exemplary technique for fabricating a dye-sensitized solar cell is to coat a flexible substrate having a conductive layer (e.g., TCO) disposed thereon with a semiconductor film such as titanium oxide (TiO2) or zinc oxide (ZnO), for example. As will be appreciated, the substrate having the conductive layer disposed thereon forms a first electrode. The semiconductor film is saturated with a dye and a single layer of dye molecules self-assembles on each of the particles in the semiconductor film, thereby “sensitizing” the film. A liquid electrolyte solution containing triiodide/iodide is introduced into the semiconductor film. The electrolyte fills the pores and openings left in the dye-sensitized semiconductor film. To complete the solar cell a second flexible substrate having a conductive layer disposed thereon to form the second electrode having low overpotential for triiodide reduction is employed to provide a structure having a dye-sensitized semiconductor and electrolyte composite sandwiched between the two electrodes.
One problem associated with solar cells is electrical shorting between electrodes. Shorting between electrodes is of particular concern when implementing flexible substrates. One technique for mitigating electrical shorting between electrodes is to increase the thickness of the active material between the electrodes. However, the thicker the semiconductor film, the greater the distance the electrons and ions may travel to reach an electrode. Although longer light paths may be desirable to facilitate greater light absorption, the current losses due to the increased recombination of the charge carriers, as well as limits to current caused by slow ion diffusion through the electrolyte in the semiconductor pores, make the increased thickness in the semiconductor film disadvantageous since it may produce a less efficient solar cell.
Embodiments of the present invention may address one or more of the problems set forth above.
In accordance with one aspect of the present technique, there is provided a solar cell comprising a first electrode, a hole blocking layer disposed on the first electrode, an active layer disposed on the hole blocking layer an electron blocking layer disposed on the active layer, and a second electrode disposed on the electron blocking layer.
In accordance with another aspect of the present technique, there is provided a solar cell comprising a first electrode, a first hole blocking layer disposed on the first electrode, a first active layer disposed on the first hole blocking layer, a first electron blocking layer disposed on the first active layer, a second electrode disposed on the electron blocking layer, wherein the second electrode comprises a substrate having conductive layers disposed on each the front surface and the back surface of the substrate, a second electron blocking layer disposed on the back surface of the second electrode, a second active layer disposed on the second electron blocking layer, a second hole blocking layer disposed on the second active layer, and a third electrode disposed on the second hole blocking layer.
In accordance with a yet another aspect of the present technique, there is provided a method of forming a solar cell comprising forming an anode component comprising a first electrode, a hole blocking layer disposed on the first electrode and an active layer disposed on the hole blocking layer, forming a cathode component comprising a second electrode and an electron blocking layer disposed on the active layer, and laminating the anode component and the cathode component together.
In accordance with a still another aspect of the present technique, there is provided a method of forming a solar cell comprising forming an anode component comprising a first electrode, a hole blocking layer disposed on the first electrode, an active layer disposed on the hole blocking layer and an electron blocking layer disposed on the active layer, forming a cathode component comprising a second electrode, and laminating the anode component and the cathode component together.
Advantages and features of the invention may become apparent upon reading the following detailed description and upon reference to the drawings in which:
Further, in accordance with one exemplary embodiment, the substrate 14 is generally thin and flexible, having a thickness in the range of approximately 0.25-50.0 mils, and preferably in the range of approximately 0.5-10.0 mils. As used herein, the term “flexible” generally means being capable of being bent into a shape having a radius of curvature of less than approximately 100 cm. The flexible substrate 14 may be dispensed from a roll, for example. Advantageously, implementing a roll of transparent film for the flexible substrate 14 enables the use of high-volume, low cost, reel-to-reel processing and fabrication of the solar cell 10. The roll of transparent film may have a width of 1 foot, for example, on which a number of solar cells may be fabricated and excised. The substrate 14 may comprise a single layer or may comprise a structure having a plurality of adjacent layers of different materials. The substrate 14 has an index of refraction in the range of approximately 1.05-2.5, and preferably in the range of approximately 1.1-1.6. Further, the substrate 14 generally comprises any flexibly suitable polymeric material. For instance, the substrate 14 may comprise polycarbonates, polyarylates, polyetherimides, polyethersulfones, polyimides, such as Kapton H or Kapton E (made by Dupont) or Upilex (made by UBE Industries, Ltd.), polynorbornenes, such as cyclic-olefins (COC), liquid crystal polymers (LCP), such as polyetheretherketone (PEEK), polyethylene terephthalate (PET), and polyethylene naphtalate (PEN).
The substrate 14 is coated with a transparent conductive layer, such as a transparent conducting oxide (TCO) layer 16. The first substrate 14 and the TCO layer 16 form the front or solar electrode of the solar cell 10. As previously described, the front electrode provides a destination for electrons excited by exposure to the incident light source.
The second substrate 18 also comprises a flexible material. The second substrate 18 may comprise the same material as described above with reference to the substrate 14. The second substrate 18 may or may not be transparent. The second substrate 18 may be coated with a platinized or carbonized TCO layer 20. As will be appreciated, the second substrate 18 and the platinized TCO layer 20 form the back or counter electrode. As will be appreciated, each electrode includes a conductive material. Thus, as described above, each electrode may include a non-conductive substrate having a conductive material (e.g., metal) disposed thereon. Alternatively, the first substrate 14 or second substrate 18 may comprise a flexible metal, such that the substrate 14 or 18 is conductive and forms an electrode without an additional metal layer disposed thereon. Regardless, those skilled in the art will appreciate that at least one of the electrodes is substantially transparent.
The active layer 12 is a composite layer comprising materials configured to convert solar light into electron/hole current. As used herein, “adapted to,” “configured to,” and the like refer to elements that are sized, arranged or manufactured to form a specified structure or to achieve a specified result. The active layer 12 has three basic functions: absorbing light, transporting electrons to the front electrode, and transporting holes to the back electrode. As will be described further with reference to
In accordance with embodiments of the present invention, the active layer 12 may be disposed at thickness in the range of approximately 5 to 20 microns, for example. As will be appreciated, there are competing reactions to be considered in solar cell design and fabrication. The minimum thickness of the active layer 12 should be sufficient to facilitate efficient light absorption. While the thickness of the active layer 12 should be sufficient to facilitate desired light absorption, to achieve high efficiency solar cell, the cell thickness should be minimized. As will be appreciated, a thinner solar cell 10 provides a shorter charge carrier transport distance which is desirable in solar cells because organic charge transporters have low mobility constants. Further, a thinner solar cell is advantageous in that it leads to low light loss through the electrolyte. As will be appreciated, the absorbance and scattering of the light increases as the thickness of the solar cell increases. However, as previously described thinner solar cells create design challenges and increase the risk of electrical shorting between the electrodes. This is especially true for solar cells implementing flexible substrates. While gaskets may be employed to mitigate the risk of shorting, misalignment or surface roughness of the electrode may still enhance the probability of electrical shorting between electrodes to an undesirable level.
To address these concerns without substantially increasing the thickness of the solar cell 10, the solar cell 10 also employs blocking layers 22 and 24 disposed directly adjacent to the front and back electrodes. More specifically, the blocking layer 22 comprises a hole blocking layer 22 and the blocking layer 24 comprises an electron blocking layer 24. The thickness of the each of the hole blocking layers 22 and 24 is generally in the range of approximately 10 nm to about 1000 nm. The hole blocking layer 22 is configured to block hole transport to the TCO layer 16 of the solar electrode while having little or no effect on the electron transport to the platinized TCO layer 20 or the counter electrode. The hole blocking layer 22 may comprise a dense titanium oxide (TiO2), for example. As will be appreciated, because of the dense nature of the TiO2 layer, it will block holes from migrating to the TCO layer 16 of the solar electrode thereby preventing electrons from being taken away from the solar electrode. Other materials suitable for the hole blocking layer 22 may include, but are not limited to, dense oxides of zirconium, hafnium, strontium, zinc, indium, tin, antimony, yttrium, lanthanum, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, and mixtures of such oxides. The material for hole blocking layer 22 should provide appropriate electronic conduction energy level, that is in between that of the TCO and that of the porous semiconductor material. Also it will be appreciated that the terms “electron blocking,” or “hole blocking” refer to relative conductivity, so that the material that is a significantly better electron conductor than it is a hole conductor will be referred to as “hole blocking,” and vice versa. The electron blocking layer 24 is configured to block electron transport to the platinized TCO layer 20 of the counter electrode while having little or no effect on the hole transport. The electron blocking layer 24 may comprise a porous silicon dioxide (SiO2), for example. Other materials suitable for the electron blocking layer 24 may include, but are not limited to, porous oxides of aluminum, boron, phosphorus, and mixtures of such oxides. Also, it may include organic polymers such as polymethylmethacrylate, polyethylene, polystyrene and polypropylene, for example. Electron blocking materials in the form of beads (having a bead size larger than the TiO2 pore size, but preferably not exceeding several μm) can be also dispersed in electrolyte, and thus introduced into the device with the electrolyte. Advantageously, by employing the blocking layers 22 and 24 in the solar cell 10 in accordance with embodiments of the present invention, the undesirable back reactions (i.e., htm with the solar electrode, and etm with the counter electrode), can be eliminated. Further, the two blocking layers 22 and 24 advantageously prevent shorting between the solar electrode and the counter electrode.
Turning now to
As will be appreciated, various other embodiments of the active layer 12 incorporating various configurations of the htm 26 and etm 28 may be employed in the solar cell 10, in accordance with embodiments of the present invention.
Referring now to
Turning now to
While the invention may be susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and have been described in detail herein. However, it should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the following appended claims. Specifically, it will be understood that “anode component” and “cathode component” and the various exemplary embodiments of fabricating those components are provided by way of example. Those skilled in the art will appreciate that the fabrication techniques described herein may be modified such that layers are disposed on one or the other of the components before lamination. For example, the active layer(s) may be disposed on one or both of the anode component and the cathode component in accordance with the spirit and scope of the present invention.