CN101246917A - Method for increasing light absorption of thin film solar cell - Google Patents

Abstract

The invention discloses a method for increasing light absorption of a thin film solar cell. Glass beads with a diameter of less than 1 micrometer are first deposited on a glass substrate to give a surface with a suitable micrometer-sized relief structure, and then a transparent front electrode is deposited to give a similar surface structure with a pronounced light scattering effect. The thin film photovoltaic device formed on the transparent front electrode has higher light absorption capacity and conversion efficiency.

Description

Method to increase light absorption in thin-film solar cells

technical field

The invention belongs to the field of solar photovoltaic devices, in particular to the manufacturing technology of thin-film silicon-based photovoltaic devices.

Background technique

In recent years, the development of photovoltaic cells and large-area photovoltaic modules has attracted widespread attention in the world. Hydrogenated amorphous silicon and nanocrystalline silicon, in particular, have shown great potential with the widespread adoption of photovoltaic devices in commercial and residential installations. A notable feature of producing thin-film silicon photovoltaic devices at such a low temperature below 260°C is that the large-area deposited silicon-related semiconductor film layers and electrical contact film layers have excellent performance. At the same time, using well-established coating equipment and procedures, low-cost stencils can be produced industrially. The laser patterning of different thin films applied on the same glass substrate allows multiple solar cell elements to directly form an integrated large-area photovoltaic module during the thin film deposition process, reducing processing steps and improving product reliability .

For photovoltaic devices, especially thin-film photovoltaic devices, the key to their excellent performance is to optimize the absorption of light energy by the semiconductor photoelectric conversion layer and reduce the light loss in the device at the same time. The ability to absorb light energy to the maximum in a very thin absorbing layer is a necessary condition for high conversion efficiency. Solar cells made of hydrogenated thin-film silicon usually have a p-i-n structure, in which the p-layer and n-layer are inactive "dead layers", which establish a built-in electric field in the non-doped i-layer, so that photo-induced carriers are effective collection. The thickness of the absorbing layer is generally only a few hundred micrometers, at most not more than about 2000 micrometers. Moreover, the absorption coefficient of red light and infrared light of silicon hydrogenated film is relatively low, so a large part of sunlight cannot be effectively used. The p-i-n structure based on the hydrogenated silicon film is sandwiched between the front and rear electrodes (electrical contact layers) to form a complete photovoltaic element. The commonly used front electrode must have good transparency and conductivity, and it is usually made of transparent conductive oxide (TCO), such as a doped tin oxide or zinc oxide film with a thickness of 600-900 nm. The rear electrode is usually composed of a TCO and a metal film, and one of its important functions is to reflect unabsorbed light back into the p-i-n structure. Various approaches have been tried to improve light absorption, including the use of rough transparent front electrodes. In addition, a highly reflective back electrode is also used, so that the unabsorbed light is thrown back into the cell again. For amorphous silicon cells, the i-layer of the absorption layer cannot be made very thick, because the material has the defect of light quality attenuation. So excellent optical design plays a decisive role in the conversion efficiency of thin-film solar cells like hydrogenated silicon.

The commonly used front electrode TCO, such as tin oxide, is difficult to make with a high degree of surface texture or roughness when its thickness does not exceed 1000 nm, that is, its ability to scatter light is often unsatisfactory . TCOs with rough surfaces often have poor electrical conductivity and high light loss, which limits the further improvement of the photoelectric efficiency of thin-film photovoltaic devices. There have been various attempts to make the surface texture of TCO more obvious, such as chemically or mechanically treating the deposited TCO film to make the surface rougher, but the roughness obtained by this method is not as good as Very good control and repeatability, which often lead to defects in thin-film photovoltaic devices. Another approach is to increase the thickness of the TCO, so that the surface roughness increases with the thickness, but the thickened TCO increases the absorption of incident light, and also prolongs the production cycle of photovoltaic devices. Therefore, it is necessary to find a way to make TCO with a moderate thickness have a high and controllable surface structure. The surface structure that facilitates light scattering is preferably provided by the substrate itself, rather than relying on the formation process of the TCO film.

Contents of the invention

Based on the above considerations, the applicant formulated the primary purpose of the present invention: to improve the conversion efficiency of hydrogenated silicon-based thin-film solar photovoltaic devices.

A further object of the present invention is to improve the manufacturing process of thin-film solar cells, thereby enhancing the optical properties of the device, especially the response to long-wave light.

In order to achieve the above object, the present invention adopts a method for increasing the light absorption of a thin film solar cell. First, glass beads with a diameter of less than 1 micron are deposited on the glass substrate to make the surface have a suitable micron-sized undulating structure, and then the transparent front electrode TCO is deposited to obtain a surface structure or particle state with obvious light scattering effect ( roughness). Then on top of this TCO, a p-i-n type photovoltaic unit and a reflective back electrode based on hydrogenated silicon film are formed. These subsequently deposited semiconductor and back electrode films largely preserve the rough surface structure of the TCO. Therefore, the incident sunlight is scattered at the two interfaces of glass and TCO and TCO and silicon hydrogenated film before entering the p-i-n photovoltaic unit. The long-wave light that is not absorbed by the photovoltaic element is also reflected twice by scattering on the two interfaces of the back electrode, and returns to the photoelectric conversion area at a relatively large angle. Therefore, the thin film photovoltaic device manufactured according to the present invention has a good optical design, which is very effective for capturing weakly absorbed light and improving photoelectric conversion efficiency.

Before the glass substrate is plated with a transparent front electrode, it has an undulating surface, which makes it easier for the transparent conductive layer plated below to have a higher undulation, and greatly enhances its ability to refract light. It is more feasible to use this method to obtain a higher light refraction than the method of plating metal oxide film at high temperature, and it also greatly reduces the requirement for the thickness of the metal oxide film. Our experiments have demonstrated the feasibility of this concept.

The invention is also applicable to a single-junction photovoltaic device composed of a single p-i-n photovoltaic unit, and a multi-junction photovoltaic device formed by stacking a plurality of p-i-n photovoltaic units.

Description of drawings

The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

Figure 1 shows the process of making a transparent front electrode with a rough surface.

Figure 2 shows the layered structure of a thin film solar cell manufactured according to the present invention.

Detailed ways

As shown in Figure 1, the first step of the present invention is to clean the flat glass plate 1, and then use sol-gel deposition method (sol-gel) to coat glass balls on the surface of the glass substrate. The specific method is to immerse the glass plate in a sol containing glass beads and adhesives, and constantly stir the sol or move the substrate itself, so that the glass beads evenly adhere to the surface of the glass substrate, and then the glass The substrate is taken out from the sol stagnant body, and heat-treated at a temperature not higher than 500° C. to solidify the adhesive, so that the glass beads are firmly attached to the glass substrate, forming a film layer 11 with an undulating structure. Thereafter, a transparent front electrode 2 (TCO), such as tin oxide or zinc oxide, is deposited on the surface layer 11 so that its surface 27 has a surface roughness similar to that of the glass surface layer 11 or to a greater extent. The average undulation of the surface structure should be between 30-80 nanometers, preferably close to 50 nanometers.

The thin-film photovoltaic device structure formed after this is shown in Figure 2, and the structure includes: a glass substrate 1, which has a non-specular surface layer 11; a transparent front electrode 2; one or more thin films based on hydrogenated silicon The p-i-n type photovoltaic unit 8; the second transparent conductive oxide 7 and one or more metal thin films 45. Since the front electrode 2 has a good surface structure, the hydrogenated silicon thin film 8 and the transparent conductive oxide 7 grown thereafter generally maintain this surface structure. That is to say, before the incident light enters the p-i-n photovoltaic unit, it is scattered twice at the interface 17 between the glass substrate and the transparent front electrode and the interface 27 between the front electrode and the thin-film silicon, so that the light enters the semiconductor at a larger angle. Photoelectric conversion region 8. The long-wave light not absorbed by the p-i-n photovoltaic unit is also subjected to large-angle scattering reflection twice at the interface 87 between the thin film silicon and the second transparent conductive oxide and the interface 77 between the second transparent conductive oxide and the metal film. Therefore, most of the weakly absorbed light re-enters the p-i-n photovoltaic unit in a direction exceeding the critical angle of total internal reflection. Thereby greatly improving their chances of being absorbed. Therefore, the increased light scattering effect described in the present invention is more pronounced as the surface roughness of the substrate glass increases, because it enhances the light scattering effect at the interface of all subsequent film layers.

Claims (3)

1. p-i-n type photovoltaic device, its structure comprises successively: a glass substrate; A transparent preceding electrode; One or more p-i-n type photovoltaic cells that constitute by film based on silane; Back electrode with light reflective properties, it can comprise a transparent conductive oxide and one or more metallic film.It is characterized in that: be coated with the glass beads of one deck diameter between the 0.5-1 micron on the glass substrate, make one surface have relief fabric, electrode before the deposit transparent on the glass beads top layer then, comprise tin oxide and zinc oxide, electrode also has similar or more tangible relief fabric before making this transparent, thereby increase sunlight in photovoltaic device scattering and to weak light absorbing capture ability, improve photoelectric conversion efficiency simultaneously.

2. p-i-n type photovoltaic device according to claim 1 is characterized in that: described transparent preceding electrode is the zinc oxide that is no more than 800 nanometers with the thickness that magnetically controlled sputter method forms.

3. p-i-n type photovoltaic device according to claim 1 is characterized in that: described back electrode with light reflective properties is made of a transparent conductive oxide and a non-conductive ultrawhite reverberation.

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