| It is an important subject of scientific research for people to maximize utilization of solarenergy wherein the enhancement of photoelectric conversion efficiency of thin silicon solarcells is of particular significance. Many ways exist to improve photoelectric conversionefficiency of solar cells and these include depositing anti-reflection coatings on their surfaces,structuring the surfaces with different patterning techniques, and introducing a light-trappingstructure into the cells, etc., by which one may reduce the fraction of reflected power andenhance the absorption of light in the active silicon layer. Due to the broadband, diffusive,and fickle nature of sunshine, optimization of the antireflection coatings (or a textured surfacestructure) and the light-trapping structures becomes a problem and solution to the problemmay be of help to the design of multilayer anti-reflection coatings and light-trappingstructures. The basic principles, the output properties as well as those factors that have impacton the photoelectric conversion efficiency for thin silicon solar cells are analyzed bynumerical simulation. On this basis, the light trapping ability for the common light trappingstructure with omnidirectional front transmission characteristics and random back-reflectionis investigated. As a concrete example, the optical absorption properties of a glass-basedall-in-one thin silicon solar cell with dual anti-reflection coatings are studied. Based on broadanalysis of the anti-reflection properties of a variety of textured structures, the opticalbehaviors for periodic subwavelength textured dielectric surfaces are invetigated using therigorous coupled-wave analysis method. The main results are given in the following.The built-in fields and the space-charge distributions in doped thin silicon are accuratelycalculated using the numerical shooting method, and the inverse-problem method is proposedto construct the doping profile with given field or potential distribution within thesemiconductor. To produce the right results, certain conditions must be satisfied for the valuesof the doping concentrations. Numerical analysis shows that this may limit the integratingfactor for the electric potential to a very small scope, and some other field profiles may notgive the right results.Influences of the intensity of light, the surface recombination and the dopingconfigurations on the photoelectric conversion efficiency are studied and the optimizedparameter in configuration is given.A probability method is presented to calculate the light absorption properties of a siliconlayer with a flat front anti-reflection coating and a random-reflective back reflector. Theangle-averaged overall absorptance (AAOA) is chosen as the figure of merit for light at agiven wavelength. We find that, although the AAOA is a functional of the angle-dependenttransmittance at a given wavelength, the functional has no extreme value. So the maximumlight-trapping ability for this design can not be determined by this method. But our furtheranalyses reveal that both the angle-dependent aborption curves and the angle-dependenttransmission curves show the same asymptotic behavior consistently, thus we are able todetermine the maximum light-trapping ability by the upper limit of the light absorptioncurves.We propose a light-trapping structure with dual anti-reflection design—a piece of glasswith both sides coated with anti-reflection coatings. On the anti-reflection coating for one side of the glass a silicon layer is deposited with its back surface roughened and covered witha reflective medium. Combining the transfer matrix method and the probability method, wecalculate and optimize the light-trapping properties for this structure and find that theoptimized absorptance is close to the maximum value of its kind derived from theaforementioned general light-trapping theory. With a5-μm thick silicon, thespectrum-averaged absorptance in the0.3–1.2μm wavelength range (under the AM1.5G sun)decreases only by2.58%when the incident angle varies from0°to75°, clearly showing theomnidirectional characteristics of the dual anti-reflection design. Particularly, it is found thatthe angle-dependent absorption curves in the longer wavelength region tend to converge to asmall neighborhood of the upper limit irrespective of significant differences between thetransmission profiles. This indicates the anti-reflection requirement in the longer wavelengthregion can be significantly relaxed.The current work resolves the problem of maximum light-trapping ability for thin siliconsolar cells with a light-trapping structure which exhibits complex angle-dependent andspectrum-dependent absorption characteristics. The results can be directly employed to helpdesign and optimize a solar cell structure. The theoretical approach adopted here alsoprovides a reference for the optimization of other light-trapping structure.Using the rigorous coupled-wave analysis method, we come up with the doublecoordinate systems on the reciprocal space of the periodic surface texture and analyze theoptical behaviors for these structures. The conditions are given quantitatively for thesubwavelength surface texture to be equivalent to a multilayer dielectric coating. Also therange of the parameters for the subwavelength surface texture as an antireflection structureare presented for solar glass and the silicon cell. The anti-reflection characteristics for theair-glass and air-silicon periodic interface indicate that on the whole an optimizedsubwavelength surface texture can generally give quite small reflectance in a wide spectraland angle range, but for the current texture design its performance at higher incident angles isusually poorer than an optimized multilayer AR coating with graded index along thethickness direction. So further work is needed to improve the performance at higher incidentangles by searching for other surface design. |
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