- Two basic structures of thin film solar cells
Thin-film solar cells can be divided into two types according to the type of substrate. One uses a transparent substrate, which is a front wall type, as shown in Figure 1(a); the other uses an opaque substrate, which is a back wall type. As shown in Fig. 1(b).
In a “front wall” design, sunlight enters the p-n junction through a transparent substrate, typically glass or clear plastic. The substrate is insulated, and a transparent conductive film (TCO), such as SnO2, needs to be deposited on it, and then the corresponding silicon or compound material is deposited on the TCO to form a p-n junction, and finally a back reflection layer is deposited on it. The reflective layer also functions as a back electrode.
In the “back wall” design, where sunlight directly enters the p-n junction, the order of deposition is reversed from the “front wall” design: the back reflector is deposited first, then the p-n junction, and finally the layer The transparent conductive film is used as the upper electrode and the window layer of the p-n junction. CdTe cells and most amorphous silicon cells adopt a “front wall type” design, and CIS cells and some amorphous silicon cells adopt a “back wall type” design. Would you like to learn more about batteries? click here to open.
- Manufacturing process of amorphous silicon thin film solar cells
Taking the “front wall type” amorphous silicon thin film solar cell as an example, the structure, working principle and preparation process of the thin film cell are introduced.
(1) Structure and working principle of amorphous silicon thin film solar cells Rigid amorphous silicon solar cells consist of a transparent conductive film (TCO) on water white glass, a p-type layer of amorphous silicon p(a-Si:H), an intrinsic layer Amorphous silicon i (a-Si: H), n-type layer of amorphous silicon n (a-Si: H) and aluminum thin film. After the photons in sunlight enter the i-layer amorphous silicon through the glass, they generate hole-electron pairs, which are immediately separated by the pin junction, forming a photo-generated voltage between the TCO and the aluminum film, and a photo-generated current flows after the load is connected (Fig. 2)
(2) Preparation of transparent conductive film Transparent conductive film (TCO) is used as the front contact electrode of a-Si:H solar cells, which is usually composed of suede SnO2:F. The main requirements for it are: ①High light transmission rate; ② low resistivity; ③ good uniformity. Before preparing the transparent conductive film, the glass substrate should be cleaned with neutral detergent, then rinsed with deionized water and dried.
SnO2:F is usually prepared by atmospheric pressure chemical vapor deposition (APCVD). In order to improve the light transmittance and electrical conductivity of SnO2, C2H2F2 is introduced during the deposition process to form SnO2:F. SnO2:F is granular, and most of its particle sizes are in the range of 0.2~0.3μm. The experiment also found that the size of the grains is related to the deposition temperature and film thickness. Generally, the deposition temperature increases, the film thickness increases, and the grains become larger. Put the glass substrate of the prepared transparent conductive film on the platform, and use the laser beam of the YAG laser to scribe the transparent conductive film into sections to divide the unit cells.
(3) Fabrication of p(a-Si:H)/i(a-Si:H)/n(a-Si:H) layer
① Deposition of p(a-Si:H) type film Before depositing amorphous silicon alloy, the scribed conductive glass is rinsed with cleaning agent and deionized water, dried and placed in a PECVD discharge chamber to start deposition. The gas source for depositing p(a-Si:H) is a mixed gas of SiH4, B2H6 and He, B2H. To achieve material doping, He is used as a diluent gas, which is incorporated to improve the optical properties of the window layer of a-Si:H solar cells. Typical process conditions for preparing a-Si:H thin film P-type layer: temperature is 200~250℃, air pressure is 400500mTorr (1mTorr=0.133322Pa, the same below), radio frequency current is RF(I)=5A, time is 40s, The thickness is 80A.
② Preparation of i(a-Si:H) intrinsic layer The gas source for depositing the i(a-Si:H) layer is a mixed gas of SiH4 and H2. Before depositing the intrinsic layer, a large amount of electronic grade SiH is used. Flush the deposition chamber and related gas paths to avoid contamination of various impurity atoms as much as possible.
The intrinsic layer is the generation area of photogenerated carriers. a-Si:H solar cells have two requirements for the intrinsic layer, which is to minimize the space charge density in the intrinsic layer and improve the lifetime and mobility of photogenerated carriers. , under the determined intrinsic layer thickness, the electric field in the junction is increased, thereby improving the collection efficiency of photogenerated carriers and the stability of the cell; the second is to have a suitable thickness to maximize the light absorption in the intrinsic layer.
The properties of the a-Si:H intrinsic layer mainly depend on the RF power, substrate temperature, reaction pressure and gas flow rate during preparation. In addition, when SiH4 is diluted with H2, the grown a-Si:H film has large grain size and high electrical conductivity. Typical process conditions for preparing the intrinsic layer of a-Si:H thin films: the temperature is 200~250°C, the pressure is 600~700mTorr, the radio frequency current is RF(I)=3.5A, the time is 2500s, and the thickness is 5000 Å.
③ Preparation of n(a-Si:H) type layer film The gas source for depositing n(a-Si:H) type layer film is a mixed gas of SiH4, PH3, H2 and He, among which PH3 is used to realize material doping, The structure and optoelectronic properties of n(a-Si:H) thin films are closely related to the substrate temperature, gas source ratio, reaction pressure, RF discharge power, gas flow rate and other factors. Typical n-type preparation of a-Si:H solar cells The process conditions of the thin film are as follows: the temperature is 200~300°C, the pressure is 800~1000mTorr, the radio frequency current is RF(I)=6A, the time is 90~100s, and the thickness is 300 Å.