专利汇可以提供Thin Film Solar Cell专利检索,专利查询,专利分析的服务。并且A solar cell that has a thin film of active material in which is defined a series or array of diffraction cells, wherein each diffraction unit-cell has a non-periodic series of diffraction features and is adapted to suppress at least one low order diffraction mode.,下面是Thin Film Solar Cell专利的具体信息内容。
The present invention relates to a thin film solar cell. In particular, the present invention relates to a thin film solar cell that has one or more diffractive optical supercell structures with the purpose of enhancing light trapping.
Solar panels are made of a series of solar cells. These are used to capture sunlight and convert it into electrical power. With the on-going move towards renewable energy, solar panels are becoming of increasing interest to consumers and businesses alike. However, the cost of solar panels is still relatively high and the efficiency relatively low.
To make solar panels more accessible, there has been a drive to reduce costs and improve efficiency. To this end, thin film solar cells are increasingly being used. These devices have an active layer that has a reduced thickness. This means that processing and material costs are reduced, while the overall device efficiency can be maintained. To maintain the efficiency of thin film devices, light trapping structures are used to effectively couple light into guided modes of the thin film, thus facilitating efficient absorption.
Recent work has shown that the use of diffractive optical structures provides a promising light trapping technique for thin-film solar cells. Diffractive structures or gratings can be used to excite quasi-guided modes in the absorbing film, providing strong absorption enhancement at the resonant wavelength. As an example, a simple grating has been used in a solar cell to act as a surface coupler. The grating has a period corresponding to the wavelength of light, i.e. 500-1000 nm for silicon-based solar cell applications. Such a grating relies on few diffraction orders, mainly the first, to couple light into the thin film. This gives a very narrow-band and strongly angle-dependent enhancement, so only a limited scope for light trapping.
To increase the bandwidth and the number of angles that can be coupled into guided modes, a larger period grating can be used. In this case, higher orders of the grating are used to excite multiple modes in the thin film. Simple gratings diffract most of the energy into the first diffracted order. However, this cannot excite a guided mode if the period is larger than the wavelength. Hence, most of the incoming power is lost and not successfully diffracted into a guided mode.
According to the present invention, there is provided a solar cell that has a thin film of active material on which is defined a periodic series of diffraction cells, wherein each diffraction cell has a non-periodic series of diffraction features and is adapted to suppress at least one low order diffraction mode.
By designing each diffraction cell to suppress lower order diffraction modes, which generally cannot be coupled into the thin film, more energy is transferred into the higher order modes, which can couple into the thin film. This allows multi-mode coupling, and maximises the excitation of guided modes into the thin film.
Typically, the period of the diffraction cells is two or more optical wavelengths, at the wavelength of interest. Preferably, the period is three to five optical wavelengths.
Preferably, the first order mode is suppressed. In addition, the second order mode may be suppressed. Further higher order modes may be suppressed, for example the third and optionally the fourth mode.
The diffraction cells may be provided in a one dimensional array. Alternatively, the diffraction cells may be provided in a two dimensional array.
The diffraction cells may all be the same.
The thin film of active material may have a thickness in the range of 100 nm to 100 μm, the range between 100 nm and 1 μm being the most promising, depending on material.
The thin film of active material may comprise silicon, but the method can also be successfully applied to other materials such CdTe, CIGS and the different forms of silicon, such as amorphous, microcrystalline and single crystalline silicon.
According to a second aspect of the present invention, there is provided a solar panel that includes a plurality of solar cells according to the first aspect of the invention.
According to yet another aspect of the invention, there is provided a solar panel system that includes a plurality of solar panels according to the second aspect of the invention.
Various aspects of the invention will now be described by way of example only and with reference to the accompanying drawings, of which:
The supercell solar cell of
The solar cell of
The supercell grating is designed to couple higher diffracted orders into guided modes and to suppress modes that cannot be coupled, typically the first and second order modes. As a rule of thumb, if the supercell is m wavelengths long, then the m-th and higher diffracted orders couple into a guided mode while the m−1st and lower order modes are diffracted into air, so need to be suppressed. For example, for a wavelength of λ=600 nm and m=3, the supercell would be a=1800 nm long, and the fine structure would be designed to suppress the 1st and 2nd diffracted order while enhancing the third and higher orders.
The supercell of
For the sake of completeness, it should be noted that in the example of
Suppression of the first and second orders is caused by interference. The supercell Fourier series comprises multiple top-hat functions Fourier series. Because each top-hat function is displaced in space from each order, their Fourier series are phase shifted by a factor of exp(i2πna/ ). Here, i is the pure complex number, n is the order number, a is the displacement length and is the period. Since this phase shift depends on the order number, it is possible to get constructive interference for higher orders and destructive interference for the lower orders when multiple top-hat functions are added together. By suitably designing the supercell, it is possible to ensure that the lower diffraction orders destructively interfere and so are suppressed.
To design the cell the Fourier spectrum of the grating is tailored. This process is illustrated in
where F(m) is the Fourier series of f(x), m is an integer corresponding to the diffraction order, a is the period, and x0 is the spatial translation. Since the phase shift depends on x0 and on m, the spatial shift can be chosen to provide destructive interference for lower orders and constructive interference for higher orders. In this example, the aim is to couple light mainly in the wavelength region above 600 nm, because this is the spectral region where the absorption in silicon is very low.
Once the wavelength region of interest is defined, the highest diffractive order is given by the ratio between the period and the wavelength. As an example, the period is selected to be 1800 nm, which gives a ratio of 3; i.e., the m=3 order couples into air for λ<600 nm, and only into the slab for λ>600 nm, while orders 1 and 2 always diffract into air and must therefore be suppressed. The next step is to superimpose multiple gratings in order to reduce orders 1 and 2. The selection of the required spatial shifts is done using a simple binary search. In the binary search, an array of pixels is defined. Pixels defined as 1 represent ridges and pixels defined as 0 represent grooves. The pixel size is arbitrary, and so the fabrication limitations were chosen as the constraint criteria, i.e. 112 nm. Therefore, there are 32 pixels for the chosen period of 1800 nm, with a minimum block size of 2 pixels.
A binary search is performed by swapping the value of each pixel and calculating the Fourier transform of the total pixel array each time a pixel is changed. The resulting supercell consists of 10 blocks, with the following sequence of number of pixels: 4, 2, 3, 4, 2, 2, 3, 5, 4, 3. This sequence, with bold representing ridges, is shown schematically in
The performance of the supercell can be compared to conventional sub-wavelength gratings using the Rigorous Coupled Wave Analysis (RCWA), which is a standard semi-analytical method to calculate diffraction problems. As an example, the performance of an optimised simple sub-wavelength grating (with 600 nm period) was compared to the supercell shown in
The supercell concept can be extended to 2-D structures. An example of a fabricated 2D supercell is shown in
The present invention provides a simple and effective solution to the technical problem of how to maximise coupling efficiency in thin film solar cells. Advantageously, the thin film solar cells of the present invention can be manufactured using conventional semiconductor processing techniques, for example photolithography, nanoimprint litography and etching to form the diffractive optical structures.
A skilled person will appreciate that variations of the disclosed arrangements are possible without departing from the invention. For example, although the solar cells described above have periodic arrays of supercells, this is not essential, and the supercells may be non-periodically spaced, provided the overall arrangement and interaction of the supercells is such as to suppress at least one low order diffraction mode. Accordingly, the above description of the specific embodiment is made by way of example only and not for the purposes of limitation. It will be clear to the skilled person that minor modifications may be made without significant changes to the operation described.
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