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NCPRE c-Si solar cells: Its insightful progress

With the increase in the development of new technologies in photovoltaics, it is equally important to understand the vulnerabilities of such technologies, as it is expected to have a consistently good performance and ability to maintain trust among the consumers. Since eventually, the energy generated by the solar plants matters more than simply the installed capacity, it is important to pay attention to the long term reliability of PV power plants.

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Novel techniques have been implemented in significant fabrication steps like texturization, emitter diffusion, emitter passivation and an anti-reflection coating, edge-isolation, screen printing and co-firing. Let's have a look at some of these techniques at a deeper level on how it is carried out.

Innovative Texturization Method: A novel single-step low-cost saw damage removal (SDR) solution of potassium hydroxide (KOH) has been prepared. In essence, sodium hypochlorite (NaOCl) has been introduced, followed by pyramidal texturing using KOH, potassium silicate (K2 SiO3 ) and isopropyl alcohol (IPA) solution resulting in a weighted average reflectance of 13.4% for mono-crystalline wafers.

Better Diffusion and Passivation techniques: Improved diffusion recipes using POCl3 , resulting in better uniformity in sheet resistance has been incorporated. Further, the sheet resistance has been increased from 65 ohm/sq. to 90 ohm/ sq. resulting in better passivation and improvement in quantum efficiency at a lower wavelength range of 300-450 nm. The emitter surface passivation quality has been improved using a low-cost, low-temperature (40°C), non-acidic and safe chemical oxide passivation process (named as NCPRE-oxide) was grown with the help of sodium hypochlorite solution. In comparison with other existing oxide growth or deposition processes such as dry thermal oxide, this process has the thermal budget, easy waste disposal, and single component nature, which makes it viable for industrial-scale implementation.

Uniform and Non-absorbing Anti-Reflection Coating: Deposition recipes for anti-reflection coating have been optimized, resulting in non-absorbing and a uniform thickness (<5%) and refractive index (<1%). Uniformity was improved by increasing pressure, and reduction in absorption was achieved by decreasing silane -ammonia ratio.

Improved screen printing and co-firing techniques: With the availability of new and advanced screen printers, NCPRE can now print fingers on the top side of solar cells with 45 micro-meter widths maintaining a high aspect ratio. This has resulted in the reduction of current loss due to metal shading without compromising the electrical properties. Further, a recently installed industrial-grade belt furnace has been used for co-firing of front and back metal contacts which have improved the fill factor from 80.1% to 81.2%.

Prof. Anil
Hemanta Ghosh