Research On Antireflection Photovoltaic Glass With Gradient Refractive Indexes

Nowadays, the utilization of new energies with merits on clean, environmental-friendly,and efficient properties has met great opportunities and challenges under the dual difficultiesof solving global energy crisis and sustainable social development. Solar energy has been paidgrowing interests because it is a kind of unexhausted energy. The most efficient approachingof utilizing solar energy is photovoltaic conversion. In each solar cell, photovoltaic glass, aglass substrate with high optical transparency, is crutial for the enhancement oflight-to-electric conversion efficiency. In the traditional photovoltaic glass, at least8%ofincident light will be reflected by the polished surface (the reflectivity is27%at an incidentangle of70o). Therefore, it is of great significance to design a kind of photovoltaic glass withantireflection functions in order to avoid the incident light loss. Among current antireflectionapproaches, magnetron sputtering technique has a merit of controllability, but the fabricationexpense is high along with the limit in coating multilayer wide-band antireflection film.Althogh sol-gel method can be used to realize the preparation of antireflection films in a largerscale, the homogeneously distributed refractive index makes it unrealizable of wide-bandantiflection. After a careful survey in optical antiflection theories, methods, and techniques, weare inspired by the optical films with gradient refractive index (GRIN) to demonstrate thecharacteristics in wide-band, high-angle reflection et al. Among them, etching method is anefficient antireflection technique in realizing the above merits and commercialization.In order to simulate the antireflection films with GRIN by the current design softwares onoptical films, we have successfully designed a kind of GRIN films by replacing ahomogeneous film by multilayer film piles, where there is a single refractive index in eachlayer. The simulation computation can be divided into two steps by adjusting refractiveindexes, number of layer, and film thickness:(i) For visible-light region, film thickness of110-880nm, three layers (35,70, and140), and five types of flectivity (linear, cosine,arccosine, and double quadratic functions) are employed to simulate the resultantantireflection films, indicating that the film thickness of higher than200nm is required toobtain the purpose of wide-band antireflection.(ii) Another films with six thicknesses, fivelayers ranging from100to300, and four antireflection curves are employed to investigate the antireflection properties in the full solar spectrum, suggesting a low effect beyond100layers.The results indicate that cosine function and linear function have maximum opticaltransparency in visible-light and infrared-light regions, respectively, whereas cosine functionis better than linear function in assessing the visible light transmittance ratio and solarspectrum direct transmission ratio.Additionally, the relationships between density and refractivity are generated based onDrude, Lorentz-Lorenz, and Macleod theories. We launch an etching approach by controllingthe density of surface antireflection layer in order to obtain GRIN films and simulate theantiflection properties by sofewares.The proposed etching route comprises two steps: in the first step, calcium and sodium etal metal ions are removed by low-concentrated aqueous solution; In the second step, thedeep-positioned metal ions are dissolved using high-concentrated aqueous solution to obtain alayer of porous SiO2structure with thickness of200-300nm after rarefaction treatments on thephotovoltaic glass substrate. The resultant SiO2structure exhibit gradiently distributedrefractive index and excellent antireflection properties, referring an optical transparency ofabove96%in the wavelength ranging from ultraviolet to near infrared (1200nm) regions. Thebifacial reflectivity is lower than1%in350-1084nm, and it is lower than0.2%in thewavelength range of624-922nm. Notably, an average optical transparency of99.22%isobtained in the wavelength ranging from390to1000nm.The results on the dependence of optical transparency on incident angle indicate thetransparency is95.43%at an incident angle of70o, which is increased by23%in comparisonwith that for photovoltaic glass without antireflection. The antireflection spectra are finallysimulated by softwares and compared with experimental results, revealing a good agreement.The combination of patterned photovoltaic glass with sol-gel technique can fill thepatterns and therefore decrease the antireflection functions, whereas etching method willremain good antireflection effect becaust it will not destroy the microstructures of patternedphotovoltaic glass. An optical transparency of nearly100%has been measured after dubleetching treatments. To the best of our knowledge, it is the highest record for photovoltaic glass.The hardness measurements reveal the resultant antireflection layer on photovoltaic glass has ahardness of higher2H, which can meet the requirement for the applications of antireflectionphotovoltaic glass.