**Fabrication of Crystalline Silicon Solar Cell with Emitter Diffusion, SiNx Surface Passivation and Screen Printing of Electrode**

S. M. Iftiquar, Youngwoo Lee, Minkyu Ju, Nagarajan Balaji, Suresh Kumar Dhungel and Junsin Yi

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/51065

**1. Introduction**

The amount of solar energy incident on the earth surface every second (1650 TW) is high‐ er than the combined power consumption by using oil, fossil fuel, and other sources of en‐ ergy by the entire world community (< 20 TW) in 2005. The solar photovoltaic power generation are ever increasing in capacity, yet at a lower scale. Thus there is a scope of fur‐ ther use of solar energy to produce more electricity. For this purpose a demand for a large scale commercial production of solar cells have emerged. There is a large variety of solar cell structures proposed with various types of materials, of which p-type c-Si solar cell has been one of the most popular and widely used in commercial production with screen print‐ ing technique.

Looking back to the history of solar cell, one can find that, in 1839 Becquerel observed a light dependant voltage between two electrodes, that were immersed in an electrolyte. In 1941, first silicon based solar cell was demonstrated and 1954 is the beginning of modern solar cell research. Since then there has been several proposals for solar cell design, that can lead to various photovoltaic (PV) conversion efficiencies (η) of the solar cells. A conventional Si so‐ lar cell gives 14.7% PV efficiency[1], whereas other designs, for example, back surface field (BSF) 15.5% [2], rear local contact (RLC) solar cell efficiency ~20%, as reported by NREL. However these values are not the theoretical or experimental limit, and there is a continuous effort in improving the efficiency.

© 2012 Iftiquar et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Iftiquar et al.; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The c-Si solar cells fabricated on the high quality silicon wafers, having selective emitter on the front and local contact on the rear surface [3] shows higher η, but the required additional measures to be taken for the production of such solar cells may substantially increase the production cost.

ponents of the solution and its concentration. With a dilute NaOH solution containing isopropyl alcohol (IPA) and DI-W, the Si(100) oriented smooth wafers can grow pyramidal surface texture at 80°C temperature [6]. The surface texturing was performed by asymmetric etching of front surface of the wafers, in a dilute alkaline solution, as against the concentrat‐ ed solution used for saw damage removal. The loss in mass of each wafer were estimated from the mass of the wafer measured with a microbalance before and after texturing, which subsequently led to the estimation of the etched thickness of the wafer and hence etch rate. Optical microscopic observations, SEM images, and laser scanning were the tools that were used for the characterization of the textured surface morphology. Ultraviolet visible (UV-Vis) spectrophotometry was used to estimate the retro-reflectivity of the textured surface. The etching depends mainly on two processes. One is the rate of the reaction at the surface, and the other is the rate at which reactants diffuse into the surface. These two processes con‐ trol the overall rate of the micro structural growth during the etching. The anisotropic etch‐ ants is expected to etch (110) plane at a faster rate than the (100) plane while the (111) plane etches at a slowest rate [7]. However if chemical composition of the etchant is such that some insoluble residue is formed during etching process (like oxides etc.) then diffusion of

Fabrication of Crystalline Silicon Solar Cell with Emitter Diffusion, SiNx Surface Passivation and Screen Printing of

etchant into the Si will be hindered and hence etching will not happen as expected.

tion that takes place is as follows,

of 54.7° with the (100) surface.

pyramidal surface texture.

**2.3. Phosphorus Diffusion for p-n Junction Formation**

sion coefficient (D) can be approximated as D~0.0013μm2

deposition. At 80o

IPA enhances surface diffusion, so a rapid etching can take place in presence of IPA in the solution [8]. The NaOH etches silicon crystal planes differently, mostly because of different atomic concentration in different crystallographic planes. So, at a lower NaOH concentration the selective etching process helps to create textured surface of the wafer. The chemical reac‐

The sodium silicate (Na2SiO3) is soluble in water and thus Si surface remains devoid of any

than (111) planes [9]. For a (100) silicon wafer, a solution of NaOH, IPA, DI-W creates square based four sided pyramids consisting of sections of (111) planes which form internal angles

The degree of isotropy is sensitive to the concentration of the solution. While a 8% NaOH solution at 80°C temperature etches silicon isotropically to achieve a polished wafer surface, a 2% NaOH, 8% IPA solution at 80°C temperature etches anisotropically to a square based

The thermal diffusion of phosphorus is necessary to create an n-type emitter to the p-type wafer. The diffusion depends on various factors, of which temperature and gaseous envi‐ ronment is most important [10]. In oxygen environment and at 850°C temperature, the diffu‐

Si 2NaOH H O Na SiO 2H + +® + 2 23 2 (1)

/hr. The phosphorus diffusion

Electrode

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http://dx.doi.org/10.5772/51065

C temperature, (100) planes etch about two orders of magnitude faster

Presently the cost of the silicon wafer alone covers >20% of the total cost of solar cell produc‐ tion, so there may be a technology available in future, by which a large scale production of silicon solar cells from a thin wafer ( < 200μm) will be possible
