Simulation And Design Of Light Trapping Structure For Amorphous Silicon Solar Cell

The amorphous silicon thin film solar cells have some clear advantagescompared with the first generation crystalline silicon solar cells, such as saved rawmaterials, low cost, the preparation technology was simple, and so on. However, dueto the defect density of amorphous silicon thin film materials is very high, the thickamorphous silicon layer can absorb incident light better, but it exacerbated the carrierscomposite, the carrier collection efficiency would decline. So the thickness of theactive layer should be several times shorter than the effective diffusion length. Theultrathin active layer apparently leads to a poor absorption incident light, especially tothe photons near the band gap, and also limits the short-circuit current andphotoelectric conversion efficiency. In order to resolve these contradictions, we mustdesign some effective light-trapping structures, which could greatly increasing its"optical thickness" while "physical thickness" are remain unchanged. Such alight-trapping structure can efficiency collect the carriers, improve the opticalabsorption of the cell.This dissertation first introduced the light-trapping technologies of amorphoussilicon thin film, then summarized the applications of surface plasmon (SP)light-trapping technologies in solar cells which are excited on the surfaces of metalnano-structures. On this basis, the one-dimensional or two-dimensional periodicdistribution of metal nano structure (i.e., nano-metal grating) were introduced to thefront or back surface of the amorphous silicon thin-film solar cells respectively, andcombined with the traditionly light-trapping techniques, such as anti-reflectioncoatings, surface texture, etc, we have designed varieties of light-trapping structures.This dissertation using COMSOL simulation software which are based on the finiteelement method, simulated the optical absorption of solar cells of differentlight-trapping structures. Through analysis the photon absorption rate and theabsorption spectrum of the solar cell, the electromagnetic field distribution of theabsorbing layer and the scattering cross section of the metal nanoparticles underdifferent wavelengths, we have optimized these above-mentioned light-trappingstructures and elaborated the light-trapping mechanism. The main results of this paperare as follows:Design one-dimensional Ag nano-grating on the front surface of amorphous silicon thin film solar cell: In the case of TM wave vertical incidence, the photonabsorption rate of the Ag nano-grating on the front of the amorphous silicon thin filmsolar cell has a decline in the short wavelength compared with the reference cell, but ithas a large increase in the long wavelength. The total optical absorption of the cellwere improved29.5%when the grating section radius R=50nm, period P=350nmcompared with the reference cell. However, in the TE wave incidence, for the Agnano-grating surface can not excited surface plasmon and the Ag nano-grating have acertain absorption and reflection of the incident light, the total absorption of thephoton numbers is only92%of the reference cell when the hybrid wave incidenceunder AM1.5illumination in the wavelength range of300~800nm. In other words,the optical absorption of amorphous silicon thin film solar cells have a reduce whenwe put Ag nano-grating on the front surface of the cell compared with the referencecell. So, such a structure is unsuitable for light-trapping.The design and optimization of the combined light-trapping structure: Prepared aone-dimensional metal nano-grating between the Ag back electrode and α-Si film, anddeposited antireflective coatings on the front surface of the solar cell throughconformal growth. This combined light-trapping structure can get a good performancefor light trapping. when the cross section of the one-dimensional metal nano grating isa triangle, the light-trapping effect of the grating which fill factor FF=0.5is bettercompared with the grating which fill factor FF=1, compared with the Al grating, Aggrating have a better performance for light-trapping. When the grating cross-sectionApex angle θ=80°, area S=18750nm2, the Eabswhich is the integrated absorption rateimprovement compared with the reference cell can reached96%under AM1.5illumination at normal incidence. When the cross section of the one-dimensionalmetal nano-grating is a rectangular, the Eabsreached its maximum value at103%when the rectangular s height H=90nm, width W=180nm and the grating periodP=600nm. The combined light-trapping structure can improve the optical absorptionin a wide spectral range. While, in the short-wavelength the antireflection film and thesurface texture have an important contribution to the optical absorption, and in thelong-wavelength the waveguide mode and the excited surface Plasmon form the Agnano-grating play a key role. In addition, the solar cell with the combined structure ismuch less sensitive to the angle of incident light.The influence of the amorphous silicon thin film solar cells light absorption byperiodic distribution of Ag nano-particles: When we put the Ag nanoparticals on the front of the cell surface, and under the plane wave vertical incidence, the cells photonabsorption rate in the long wavelength have significantly improved compared with thereference cell. The bigger Ag nanoparticles light trapping effect is better under thebigger distribution period. When the Ag nanoparticles radius R=110nm, gratingperiod P=500nm, the Eabsreached its maximum value at49%. When the Agnanoparticles were placed in the same position above and under the solar cell, the Eabscan be enhanced to55%. But this light-trapping structure was sensitive to the angle ofincident light, if we want to obtain a good performance of light-trapping, we mustkeep the angle of incident light within30degrees.