Abstract
A new method for fabricating crystalline silicon solar cells with selective emitters is presented. In this method, shallow trenches corresponding to metal contact area are first formed by screen printing and chemical etching, followed by heavy doping over the whole front surface of the silicon wafer. After a polymer mask is pasted by aligned screen-printing to cover the shallow trenches, the silicon wafer is etched such that the heavy doping remains at the shallow trench area, while other areas become lightly doped. With the presented method, two screening printing steps are required for obtaining a selective emitter structure on a solar wafer. Compared with existing etch-back methods, the presented one is believed to be able to easily conform with present industrial process. Experimental results show that optical responses at the short and long wavelengths were both improved by applying the proposed selective emitter technique to fabricate solar cells with an a-Si:H film deposited on the back surface. The selective emitter cell with a-Si:H back surface deposition had improvements of 1.66 mA/cm2and 1.23% absolute inJscand conversion efficiency, respectively, compared to the reference cell that had a homogeneous emitter and no a-Si:H on the back surface.
Highlights
In the field of silicon wafer based solar cells, selective emitter structures are attracting attention to a great extent because of enhanced light-to-electricity conversion, especially at the short-wavelength range
The suppressed carrier recombination leads to increases in both the open-circuit voltage and short-circuit current, while the reduced contact resistance improves the fill factor
After being deposited with an antireflection coating (ARC) layer, this sample shows no difference in optical reflectivity (as shown in Figure 6(b)) from the sample without etching, indicating that the above etching-back step for forming lightly doped regions is of no harm to the surface morphology as well as the optical reflectivity
Summary
In the field of silicon wafer based solar cells, selective emitter structures are attracting attention to a great extent because of enhanced light-to-electricity conversion, especially at the short-wavelength range. An alternative process of using doped silicon ink for selective emitter formation developed by Innovalight has proved to be viable in a mass production test [4] Another industrial selective emitter processing sequence may involve lightly doping the entire front surface of a p-type substrate with phosphorus followed by the standard antireflection application process and completed by screen printing a silver paste containing phosphorus through the front contacts [7, 8]. A dopant is doped into the substrate through high-temperature diffusion to form a heavily doped region and a lightly doped region at one time The disadvantage of this method is the requirement for a mask layer, which is usually coated by a high-priced facility. There are shallow trenches with a depth of several μm or more for metal contacts in the presented selective emitter structure, while no trenches are dug by using Schmid’s and other methods
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