The structure and fabrication of large-area single-junction integrated a-Si:H solar cells

1 Introduction

As we all know, there are many advantages in using solar energy. Photovoltaic power generation will provide the main energy for mankind. However, at present, in order to make solar power generation have a larger market and be accepted by the majority of consumers, improving the photoelectric conversion efficiency of solar cells and reducing production costs should be our biggest goal. From the current development process of international solar cells, we can see that its development trend is single crystal silicon, polycrystalline silicon, ribbon silicon, and thin film materials (including microcrystalline silicon-based thin films, compound-based thin films and dye-based thin films). From the perspective of industrial development, the focus has shifted from single crystal to polycrystalline. The main reasons are: [1] The head and tail materials that can be supplied to solar cells are getting less and less; [2] For solar cells, square substrates are more cost-effective. Polycrystalline silicon obtained by casting and direct solidification can directly obtain square materials; [3] The production process of polycrystalline silicon has made continuous progress. The fully automatic casting furnace can produce more than 200 kilograms of silicon ingots per production cycle (50 hours), and the grain size reaches the centimeter level; [4] Due to the rapid research and development of single crystal silicon technology in the past decade, the technology has also been applied to the production of polycrystalline silicon cells.

The emergence of a-Si:H solar cells is like a ray of dawn, illuminating the road to large-scale application of solar cells. After more than ten years of development, its preparation process has become increasingly stable and mature. Its ingenious structural design and cheap preparation process show people that if solar energy is to move from supplementary energy to alternative energy, this structural design technology and preparation method must be adopted, otherwise the high price of solar cells will become a bottleneck for its development. This article describes the structural design and preparation analysis of a-Si:H solar cells, and discusses the process parameters that affect its performance.

2 Structural design of large-area single-junction integrated a-Si:H solar cells

2.1 Structure of a-Si:H solar cells

The a-Si:H solar cell is a flat-plate photoelectric transducer device with an amorphous silicon PIN structure formed by glow discharge deposition on a glass substrate. The structure of a single cell is shown in Figure 1. When sunlight shines on the cell, the cell absorbs light energy to generate electron-hole pairs. Under the action of the built-in electric field of the photocell, the photogenerated electrons and holes are separated, and opposite charges accumulate at both ends of the photocell, that is, photogenerated voltage is generated. If electrodes are drawn out on both sides and connected to a load, a photogenerated current will flow through the load, thereby obtaining power output. Figure 2 shows the equivalent circuit of the a-Si:H solar cell. IL is the photogenerated current, Id is the dark current of the diode, Rsh is the parallel resistance, Rs is the series resistance, and RL is the load resistance.

At present, the open circuit voltage Uoc of a-Si:H single cell is about 0.8V, the operating voltage Um is about 0.55V, the short circuit current density Jsc is about 13.4mA/cm2, and the operating current density Jm is about 11mA/cm2. Such a small power output is basically useless. If higher voltage and larger current are to be output, effective series and parallel measures must be taken in the structure.

Figure 1a-Si:H single cell structureFigure 2a-Equivalent circuit of Si:H solar cell

2.2 Structural design of integrated a-Si:H solar cells

In order to obtain a certain power output, a large number of a-Si:H solar cells must be effectively connected in series and parallel. However, a-Si:H solar cells are thin-film devices, and it is neither reliable nor convenient to connect them in series with external leads. Therefore, internal integration must be considered during preparation. Figure 3 shows its internal integration structure. In the figure, 1, 2, 3, and 4 are four single cells, which are connected in series, and the current flow direction is shown in the figure.

Figure 3 Internal structure of an integrated a-Si:H solar cell

If you want to design an a-Si:H solar cell module to charge a 6V, 4Ah VRLA battery, the required operating voltage of the solar cell module is:

Um≥3×UT+Ud=8V

Where: UT is the overcharge threshold voltage of the VRLA battery, and Ud is the forward voltage drop of the reverse charging protection diode.

Since the working voltage of a single a-Si:H solar cell is about 0.55V, at least 15 cells are required to be connected in series. If the current is charged at C/10, the output current of the a-Si:H solar cell module should be at least greater than 400mA, so the area of ​​a single solar cell should be at least greater than 37cm2. Considering the area occupied by the internal series wiring, the area of ​​a single solar cell is finally taken as 1×51cm2.

Therefore, the required a-Si:H solar cell module is 15 solar cells with a single cell area of ​​1×51cm2 connected in series.

3. Preparation and analysis of large-area single-junction integrated a-Si:H solar cells

The a-Si:H solar cell is made of multiple layers of thin films organically combined, and is manufactured according to the process sequence shown in FIG. 4 .

Figure 4a-Si:H solar cell process sequence

3.1 Preparation of TCO

TCO glass refers to a component formed by uniformly coating a layer of transparent conductive oxide film (Transparent CONductive Oxide) on the surface of flat glass by physical or chemical coating methods. For thin-film solar cells, since the intermediate semiconductor layer has almost no lateral conductivity, TCO glass must be used to effectively collect the current of the battery. At the same time, the TCO film has the functions of high transmittance and anti-reflection, allowing most of the light to enter the absorption layer. The production process of TCO glass is mainly divided into ultra-clear float glass production and TCO coating. The production process of ultra-clear float glass is relatively difficult. At present, the main suppliers in the world include Asahi Glass of Japan, PPG of the United States, Saint-Gobain of France, etc. There are limited domestic suppliers. Currently, only Jinjing Technology, CSG, and Xinyi can supply.

TCO is a velvet SnO2:F film, which can be prepared by chemical vapor deposition (CVD) process. The preparation uses float glass with good flatness, high transmittance, freshness, no pollution, and no water corrosion as the substrate. It is cut into the area size calculated above, washed and dried, and then sent into the CVD furnace to start deposition. The chemical reaction that occurs is as follows:

SnCl4+O2=SnO2+2Cl2

SnCl4+2H2O=SnO2+4HCl

After deposition, it is placed on the platform of a yttrium neodymium garnet laser for laser scribing. The amount of scribing is determined by the required number of series-connected cells.

3.2 Preparation of P layer

The composition of the P layer is a-Si:H:B:C, and the preparation process is plasma enhanced chemical vapor deposition (PECVD), which is a technology that combines high-frequency (13.56MHz) glow discharge physical process and chemical reaction. The advantages of this method are fast deposition rate, good film quality, few pinholes, and not easy to crack. The deposition gas source is a mixed gas of SiH4, CH4, B2H6 and He. B2H6 is used to achieve material doping, He is used as a dilution gas, and the addition of CH4 is to improve the optical properties of the P layer. By changing the gas partial pressure ratio during the deposition process, P layer (a-Si:H:B:C) films with different C contents can be obtained, and different C contents have different photoelectric properties.

3.3 Preparation of I layer

The composition of the I layer is a-Si:H, and the preparation process is still PECVD, and the deposition gas source is SiH4 and H2. The intrinsic layer is the generation area of ​​photocurrent, so its film quality directly affects the performance of a-Si:H solar cells, and its performance is mainly determined by the discharge power, substrate temperature, reaction pressure and gas flow rate during preparation. During the film formation process, while maintaining a certain film formation rate, try to use low discharge power to improve the optoelectronic performance of the film.

3.4 Preparation of N layer

The N layer is a-Si:H:P, and the deposition gas source is a mixed gas of SiH4, PH3, H2 and He, among which PH3 is used to achieve material doping. The structure and photoelectric properties of a-Si:H:P film are closely related to factors such as substrate temperature, gas source ratio, reaction pressure, discharge power and gas flow rate.

In the process of preparing the above-mentioned thin films, the reaction pressure, discharge time, gas flow rate and reaction chamber temperature are all automatically monitored and controlled by a computer, and the required control parameters are realized by software.

After the preparation of each layer of film is completed, the component is placed on a mechanical comb table. The width of the engraved line should be less than 0.2mm, the silicon engraved line should be close to the laser engraved line, and the tolerance between the two is 0.2~0.7mm. The engraving transmittance should be greater than 80%. The purpose is to form the amorphous silicon layer of each single cell and make Al and TCO in good contact.

3.5 Aluminum Steaming

The Al electrode is made by vacuum evaporation. In the integrated a-Si:H solar cell, aluminum is not only used as the negative electrode of each sub-cell, but also connects each single cell in series in structure. In addition, the aluminum film can also reflect the long-wavelength limited photons that are not absorbed by the amorphous silicon alloy layer, increasing the utilization rate of light by the solar cell.

The integrated a-Si:H solar cell module designed and prepared according to the above requirements has its cell output characteristics measured on the solar cell test bench of CHRONAR Corporation in the United States as shown in Figure 5. The test conditions are: standard light intensity, AM1.5, 100mW/cm2, 25℃. From the results, it can be seen that the design requirements have been met.

Figure 5 Output characteristics of experimental integrated a-Si:H solar cells

4 Conclusion

Integrated a-Si:H solar cells have a simple structure, low manufacturing process cost, and can be easily designed into different forms to meet different user needs. Its emergence has greatly promoted the development of the entire solar cell industry.