In recent years, the energy crisis and environmental pollution have greatly promoted the development of the photovoltaic industry. Silicon solar cells have a wide range of sources and have always dominated the solar cell market. Reducing costs and improving photoelectric conversion efficiency are the focus of solar cell research.
Thin-film solar cells are second-generation solar cells. Their consumption of raw materials is very small, and the thickness is usually 1-2 μm. However, the optical thickness of silicon that absorbs sunlight sufficiently is 180 μm. Therefore, the absorption layer of thin-film solar cells cannot absorb all the light. , resulting in a lower photoelectric conversion rate of the battery. Thin-film solar cells are not suitable for surface texturing due to their own thickness problems, so it is considered to apply a hybrid light-trapping structure on their surface.
The hybrid light trapping structure is to enhance the light absorption of the solar cell by using the combination of the front light trapping structure and the back light trapping structure. The metal particles on the front side of the battery will absorb light, but because the absorption coefficient of dielectric particles is very small, the absorption of light is weak and almost negligible; the scattering of metal particles on the backside is better than that of dielectric particles, so the dielectric on the front of the battery The particles do the light trapping structure, and the back side uses the metal particles as the light trapping structure, and the optimal mixed light trapping structure is obtained by optimizing the duty ratios of the front dielectric particles and the back silver particles, respectively.
1, analysis
Figure 1 shows the structure of a thin-film silicon solar cell. The front of the cell is a radiused hemispherical TiO2 particle, the front electrode is an ITO conductive layer, the absorption layer is monocrystalline silicon, and the back of the cell is a ZnO:Al back electrode with a hemispherical silver particle and a silver mirror. The sunlight is incident from the front and the wavelength range is 400-1100 nm.
FIG. 3 shows the absorption enhancement ratios of the batteries with different light trapping structures relative to the light-receiving-free structure. It can be clearly seen that the absorption-enhanced wavelength ranges of the batteries with different light trapping structures for the reference battery. Figure 4 shows the short-circuit current density (Jsc) plots of various photo-concentrating cells. The Jsc values ​​of Structure I, II, III, and IV cells were 13.0 mA/cm2, 14.5 mA/cm2, 15.2 mA/cm2, and 15.5 mA/cm2, respectively. Relative to the reference battery (Structure I), the increase in the circuit current density of the other batteries was 1.5 mA/cm2, 2.2 mA/cm2, and 2.5 mA/cm2, respectively.
However, the strong scattering of TiO2 particles allows the incident light to penetrate deeper into the absorption layer than 300 nm below the surface, much larger than the absorption of (a) (c) cells, thereby enhancing absorption of short-wavelength light by the absorber layer. For long-wavelength light, a 1 μm silicon absorber layer is not sufficient for partial absorption and some will transmit out of the cell. The TiO2 particles are equivalent to a film of oil refractive index with respect to the long wavelength band and do not affect the propagation of light. The metallic silver particles can reflect the light transmitted through the cell and absorb the layer, thereby enhancing the absorption of long-wavelength light.
Figure (g) (h) shows the reflection of transmitted light by metallic silver particles, forming a periodic Bloch diffraction image. Figure (e)(f) shows a battery with no metallic silver particles on the back and its image is typical of Fabry-Perot oscillations.
It can be seen from the electric field diagram that TiO2 particles can form a kind of dielectric grating to reduce the reflection of light because of scattering; while metallic silver particles can not only form a kind of metal grating due to scattering, but also the near-field enhancement of plasma on the surface of silver particles. The edge is still valid.
2. Results and Discussion
Figure 6 shows the short-circuit current density of TiO2, SiO2, and Si3N4 particles as a function of particle radius. It can be seen from the figure that although the front dielectric particles of the battery are different, the change trend of the short-circuit current density is similar. The short-circuit current density increases with the increase of the radius of the particles. When the radius increases to a certain extent, the short-circuit current density decreases with the increase of the radius.
3, the conclusion
The light trapping structure significantly enhances the light absorption of thin film silicon solar cells. Light is mainly transmitted in the medium in the radiation mode and guided wave mode. In the radiation mode, the lifetime of the light wave is very short, and the distance traveled in the medium is limited. In the guided wave mode, the light wave has a long life and can travel a long distance in the medium, allowing the medium to fully absorb light.
Light does not form a guided wave mode when it enters the slab waveguide. The incident light can be coupled into the guided wave mode only if it is scattered so that its propagation angle is greater than the total reflection angle of the medium and the air. The light trapping structure is to couple the incident light into the guided wave mode while scattering the light, increase the optical path of the light in the absorption layer, and enhance the light absorption.
The guided wave mode can be coupled with the plane wave to form the guided wave resonance, which corresponds to an absorption peak on the spectral response curve of the battery. Changing the period and duty cycle of the trap structure can increase the number of guided wave modes and increase the absorption peak on the battery spectral response curve. Enhance the broad spectrum of light absorption.
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