Photovoltaic power generation is considered as an alternative for conventional power generation, for example from fossil fuels and nuclear energy. It is particularly vied for its minimal environmental impact as a ‘clean energy’ source. Photovoltaic power is generated in a photovoltaic device, popularly known as a solar cell. In order for photovoltaic power generation to be viable as a future source, reliability and cost of photovoltaic energy conversion must be comparable with the conventional power generation sources. Currently available technologies, especially the low cost photovoltaic energy conversion devices are lagging significantly, in both these areas.
Typically, a photovoltaic energy conversion apparatus comprises a plurality of photovoltaic devices arranged in modules, wherein one or more modules are connected together in a support structure, depending upon power requirement for a given application. A major fraction of cost of generating photovoltaic power lies in manufacturing photovoltaic devices and modules. Therefore, in order to lower the cost of photovoltaic power generation, it is imperative to reduce the cost of photovoltaic devices.
Basic principles of photovoltaic energy conversion can be described in reference with FIG. 1A, representing a cross section view 100A of the most widely understood generic prior-art planar, surface illuminated photovoltaic device. The photovoltaic device as represented by 100A comprises a silicon substrate 102, having a p-type doping, a first layer (103) having p-type doping higher than the substrate (102) doping, and the second layer (105) having a n-type doping that is relatively higher than the p-type doing level of the first layer (103). The interface between the first layer (103) and the second layer (105) forms a p-n junction having a built-in electric field. The bottom surface of the substrate has a metal layer coating 101 forming a first electrode of the photovoltaic device. The top surface of the layer 105 is coated with a second metal layer 106 forming a second electrode of the photovoltaic device. The first and second electrodes have opposite polarities.
In operation, the silicon photovoltaic device absorbs energy from the incident photon flux represented by a plurality of arrows 107, and generates a plurality of photo-carriers 108 and 109 (only one pair labeled for clarity) within the photovoltaic device. In general, photo-carriers are polar meaning that each photo-carrier has either a positive charge or a negative charge associated with it (for simplicity only one carrier of each polarity is shown).
In the silicon photovoltaic device shown in 100A for example, photo-carriers in the first layer (103) are predominantly holes (each one having a positive charge), and in the second layer (105) the photo-carriers are predominantly electrons (each having a negative charge). The photo-carriers that reach the p-n junction get separated by the built-in electric field and are subsequently collected by the electrodes (101 and 106) having the like polarity, respectively, thereby generating a photocurrent.
Those skilled in the art will appreciate that the generation of photo-carriers in the silicon layers (103 and 105) is not uniform. For example, and as shown in FIG. 1B, 100B schematically represents a photon flux profile 111 incident on the metal layer 106, as it penetrates down the photovoltaic device in the vertical direction. Therefore, maximum photo-carrier generation is near the surface and it reduces as lower and lower photon flux reaches to the layers 105 and 103 below the surface. Therefore, a thin photovoltaic device having the p-n junction closer to the surface is advantageous, such that the maximum number of photo-carriers separate in the junction and contribute to the photocurrent generation.
In another type of a prior art device, thin films of materials having higher absorption coefficient are utilized, where the p-n junction region may be extended deeper into the device for better efficiency. One example of such a prior art a-Si device is schematically shown in FIG. 2, wherein 200 is a multi-layer planar a-Si photovoltaic device. It is important to note that a thin film of about 1-5 micrometer of a-Si can absorb about 90% or more of the incident light.
The prior art photovoltaic device 200 is a multilayered structure comprising (in vertical order) a transparent substrate (201), a layer of a transparent conductor (202), a layer of doped a-Si (203), a layer of intrinsic (undoped) a-Si (204), a second layer of doped a-Si (205) followed by a layer of metal (206). In this example the photovoltaic device receives the photon flux represented by a plurality of arrows (207) from the bottom surface of the transparent substrate. The doping in the two doped a-Si layers is of opposite kind thereby creating p-i-n junction (between the layers 203-205) instead of a p-n junction described in reference with FIG. 1A.
In operation, the photovoltaic device 200 upon receiving the incident photon flux (207) at the bottom surface of the transparent substrate (201), generates a plurality of photo-carriers (208 and 209, only one pair labeled for clarity) in all three a-Si layers (203-205) wherein each photo-carrier has either a positive or a negative charge associated with it. The photo-carriers are separated in a built-in field of the p-i-n junction and subsequently diffuse towards respective electrodes (the transparent conductor layer 202, and the metal layer 206) according to the charge on the photo-carrier. The efficiency of the photovoltaic device 200 is seriously affected by a partial loss of the plurality of photo-carriers (208 and 209) due to their small diffusion length within the a-Si layers (203-205).
It is well known to those skilled in the art that the efficiency of absorption of light, and in particular absorption of the sunlight in different materials are different for the reasons outlined below—a) the photon flux in each section of the solar spectrum is not the same, and b) different materials absorb light from different sections of the solar spectrum depending upon their bandgap ‘Eg’ and absorption coefficient ‘α’. For example, while several micrometer thick films or sheets of c-Si or poly-Si are necessary for absorbing sufficient sunlight from the solar spectrum, only about one micrometer thick film of a-Si is sufficient for absorbing about 90% or more light from the solar spectrum.
Accordingly, performance of different photovoltaic devices are compared in terms of photovoltaic conversion efficiency (or efficiency), which depends on several factors including but not limited to, a) incident photon flux, b) absorption of photon flux within the photovoltaic device, c) photo-carrier generation, d) separation of photo-carriers in the p-n junction, e) efficient diffusion of carriers to the respective electrodes, and f) efficient collection of photo-carriers by the respective electrodes.
For example, efficiency of a heterojunction photovoltaic device made from III-V alloy semiconductors is about 25-28%, efficiency for a conventional homojunction c-Si or poly-Si photovoltaic device is between 12-17%, and efficiency of a multi-layer a-Si photovoltaic device is between 8-10%, respectively. Module efficiencies tend to be 0.5% to 2% lower, based on the total area of the module.
The cost of manufacturing photovoltaic devices mainly depends on the materials as well as processing cost. For example, high efficiency photovoltaic devices that mainly utilize single crystalline materials such as, crystalline silicon (c-Si), poly-crystalline silicon (poly-Si), III-V alloy semiconductors including but not limited to, GaAs, InP, GaAlAs, GaInAsP, GaInAs, and their combinations thereof in single or multiple homojunctions, heterojunctions, or tandem photovoltaic devices are the most expensive ones.
However, these types of photovoltaic devices are not cost effective in power generation for terrestrial applications such as residential, industrial, and commercial power generation that require better reliability and higher efficiency than applications in consumer electronics. Therefore, some low cost materials and technologies that are being pursued for providing photovoltaic devices at a reasonable cost are, thin films of c-Si including microcrystalline silicon (micro-Si), poly-Si, and amorphous silicon (a-Si). These materials are also adaptable for low cost processing technologies that are suitable for mass production and considerable progress has been made in this area as well to lower the overall cost of manufacturing photovoltaic devices.
While other low cost materials, such as II-VI semiconductors including but not limited to, cadmium sulfide (CdS)/cadmium telluride (CdTe) and copper indium diselenide (CIS) are useful for photovoltaic devices, they have adverse environmental impact, and are therefore not a viable choice for ‘clean energy’. These materials may be used alone or, in combination with other materials including but not limited to, other semiconductors, as well as photosensitive polymers. As a matter of design choice and compatibility with the materials utilized in constructing a device, the device structure may include but is not limited to, single and multiple homojunctions, heterojunctions, or a suitable combination thereof.
In this application a design of a hybrid photovoltaic device utilizing nanostructures embedded in a matrix of another material exhibiting photovoltaic effect is disclosed. Those skilled in the art will appreciate that the principles of the invention disclosed herein, and further defined by the scope of claims to follow, are merely illustrative and are not construed to be limited to specific examples of structure and materials used to explain the principles in this document.