Advanced Front-Side Technology in Crystalline Silicon Solar Cells

Name: Victor Prajapati

• Prof. dr. ir. Robert Pierre Mertens (promotor)
• Prof. dr. ir. Jozef Poortmans (mede-promotor)

"The whole is greater than the sum of its parts" 
-Aristotle 

In order to increase the conversion efficiency of silicon solar cells each individual processing step must be optimized. However, if one losses sight of the interplay between subsequent processing steps and process integration, the results can be counterproductive. Losses inconventional full Al-BSF silicon solar cell's conversion efficiency are dominated by the front-side and can be categorized into three parts: resistance, optical, recombination. Improving any individual category typically leads to degradation in one or more of the others, especially considering the ultimate constraint of the PV industry, cost. Thus, this dissertation's aim is to improve the overall solar cell conversion efficiency, whilst having industrial applicability. In this dissertation three patented processing technologies are presented, each attributed to one oft he loss categories. 

The advanced texturing process presented utilizes one etchant solution to perform two actions, texture the front surface of the silicon while polishing the rear. Rough as-cut wafers are subject to the application of an acrylic adhesive micro-masking layer. Upon removal, remnants of the adhesive layer adhere only to the peaks of the as-cut wafer. The wafer is then subject to a 4 minute 12.5 wt% heated NaOH etch, where the front-side of the wafer is textured and the rear-side of the wafer is polished and only 15 µm of silicon is lost. Equivalent or better performance has been demonstrated in both reflectance as well as cell efficiency results with a process that saves time, silicon and only uses one etchant solution and therefore can have a significant cost impact. 

The advanced emitter formation technology presented replaces traditional diffusion with a process that incorporates ion implantation, passivation and annealing in a firing furnace. After ion implantation, wafers are deposited with SiNy:H nitride and then subjected to a rapid thermal anneal in a firing furnace. During the firing procedure (950°C for 90 seconds) three processes occur: First, hydrogen is released from the SiNy:H layer which can passivate defects. Second, the top-most silicon surface that has been amorphized due to sustaining ion implantation damage is re-crystallized by means of solid phase epitaxy. Lastly, phosphorus dopants take a substitutional position in the siliconlattice, thus becoming electrically active. There are several advantages of the proposed process flow over conventional diffusion: the technique described can reduce cycle time, consumables (water, acids, silicon), energy consumption and relax contamination constraints in manufacturing. 

The final technology improvement involves silicon oxide, which has been well known to passivate the interface of silicon. However, in this dissertation it is presented that silicon oxide containing phosphorus can significantly improve contact resistance of Ag screen-printed contacts on high (>90 Ω/□) sheet resistance emitters. The developed oxide has been extensively studied to demonstrate potential improvements on the front and rear-side of i-PERC cells. With the addition of the developed step, both open circuit voltage and fill factor increase, improving cell efficiency by 0.5-1.0% absolute. 

The developed processes presented in this thesis have been characterized using a plethora of methods such as: SEM, SIMS, XPS, TEM, XPS, EDX, AFM, C-AFM, IQE, EQE, lifetime and the most relevant, solar cell results. Integrating all the knowledge gained in each of the categories, a confirmed efficiency above 20% on screen printed large area (125 x 125 mm) Cz siliconhas been achieved "the whole is> greater than the sum of its parts".

https://lirias.kuleuven.be/handle/123456789/361062

• Prof. dr. ir. Robert Pierre Mertens (promotor)
• Prof. dr. ir. Jozef Poortmans (mede-promotor)
• Prof. dr. ir. Joseph Vandewalle (voorzitter/chairman)
• Prof. dr. ir. Johan Driesen (secretaris/secretary)
• Prof. dr. Corneel Claeys
• Dr. Giso Hahn , University of Konstanz
• Dr. Joachim John , IMEC
• Dr. Arthur Weeber , ECN
• Dr. Martin Hermle , Fraunhofer-Institute for Solar Energy Systems