From the Laser-doped Semiconductor Fingers to the Advanced Semiconductor Fingers Silicon Solar Cell

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Copyright: Wang, Kee Soon
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Abstract
To reach the goal of grid parity for photovoltaic-generated power, the efficiency of conventional screen-printed p-type silicon solar cells should be increased without significant increase in the manufacturing cost. The semiconductor fingers (SCF) screen-printed silicon solar cell technology fabricated by laser-doping the SCF can potentially achieve this aim. Previous laser-doped SCF p-type silicon solar cell efficiencies were too low, limited by the achievable SCF doping and therefore sheet resistance levels using available laser technology. Recently, the new Spectra Physics Millennia Prime laser introduced new laser technology apparently with the potential to produce laser-doped features with sheet resistances low enough for the SCF cell. The objective of this thesis is to design and develop an n-type SCF with this laser so as to demonstrate high efficiency laser-doped SCF solar cells on p-type Czochralski (Cz) silicon wafers. Since the sheet resistance of the laser-doped SCF (lines) is important to the efficiency of the SCF solar cell, appropriate methods to measure the sheet resistance of these laser-doped lines were investigated. A method widely-used to measure the sheet resistance of a laser-doped line was demonstrated here to produce unreliable results and thus not used. Instead, a new measurement method was presented along with a new upper sheet resistance limit concept. A theory was also presented that relates these two different measurement methods, and was experimentally-supported within an error of 10 %. The Spectra Physics laser was also demonstrated to produce laserdoped lines that can be as conductive as 2 Ω/□. Thus, this laser is suitable for high efficiency SCF solar cells. Beside the benefits that highly-conductive SCF can bring to a SCF cell, there are drawbacks that appear mainly in the form of SCF effective shading losses. To account for them, a model was built to simulate the efficiencies of laser-doped SCF solar cells with different SCF sheet resistance and junction depths. The cell efficiency potential of SCF laser-doped by the Spectra Physics laser was then assessed. With the most optimistic assumptions for contact resistance, and with experimentally-derived SCF sheet resistances and junction depths, the highest efficiency was predicted to be 18.81 % and was only 1.13 % relatively higher than that of an optimised screen-printed silicon solar cell. High SCF effective shading loss was the limiting factor. Subsequently, laser-doped SCF solar cells screen-printed with appropriate lowreactivity silver pastes were fabricated. From these cells, the contact resistance was determined to be too high and not uniform enough for high efficiency laser-doped SCF solar cells from being demonstrated in the duration of this thesis work. Plating the SCF with metal was then proposed as a solution that can overcome both challenges of high effective shading loss and low contact quality. This new metalplated SCF solar cell is known as the advanced SCF solar cell. A laser-doping and metallisation sequence in the order of screen-printing, laser-doping, and nickel/copper stack plating was analysed to be a practical sequence to fabricate the advanced SCF cell. During the development of the recipe, nickel was found to not uniformly plate across the cell and these nickel voids resulted in higher series resistance levels. A modified dopant dispense method for the laser-doping step was developed to significantly reduce these nickel voids. By applying this solution to a batch of six advanced SCF solar cells, an average batch and highest efficiency of 18.40 % and 18.82 % respectively were achieved on p-type Cz 1 Ωcm textured silicon wafers. Modelling and simulation of the advanced SCF solar cell show that this new cell design can have a direct ≈ 0.7 % absolute and ≈ 3.8 % relative efficiency gain over the laser-doped SCF solar cell. This is mainly due to the ability of the advanced SCF solar cell to space the screen-printed silver fingers much wider apart and to use narrower SCF. The predicted efficiency potential of the advanced SCF solar cell with a full area back surface field exceeds 20 %. Of secondary importance and it was discovered while developing techniques to minimise the contact resistance between a screen-printed metal and a high sheet resistance diffused emitter, that using dilute hydrofluoric acid to improve the contact resistance can increase the cell s recombination impact and reduce its pseudo-fill factor. It was also demonstrated that by treating the cell in phosphoric acid, this impact can be significantly reduced or eliminated. Chemical analyses suggest lead to be the likely recombination source.
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Author(s)
Wang, Kee Soon
Supervisor(s)
Wenham, Stuart
Uddin, Ashraf
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Publication Year
2012
Resource Type
Thesis
Degree Type
PhD Doctorate
UNSW Faculty
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