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

摘要

To reach the goal of grid parity for photovoltaic-generated power, the efficiency ofconventional screen-printed p-type silicon solar cells should be increased withoutsignificant increase in the manufacturing cost. The semiconductor fingers (SCF)screen-printed silicon solar cell technology fabricated by laser-doping the SCF canpotentially achieve this aim. Previous laser-doped SCF p-type silicon solar cellefficiencies were too low, limited by the achievable SCF doping and therefore sheetresistance levels using available laser technology. Recently, the new Spectra PhysicsMillennia Prime laser introduced new laser technology apparently with the potentialto 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 soas 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 efficiencyof the SCF solar cell, appropriate methods to measure the sheet resistance of theselaser-doped lines were investigated. A method widely-used to measure the sheetresistance of a laser-doped line was demonstrated here to produce unreliable resultsand thus not used. Instead, a new measurement method was presented along with anew upper sheet resistance limit concept. A theory was also presented that relatesthese two different measurement methods, and was experimentally-supported withinan error of 10 %. The Spectra Physics laser was also demonstrated to produce laserdopedlines that can be as conductive as 2 Ω/□. Thus, this laser is suitable for highefficiency SCF solar cells.Beside the benefits that highly-conductive SCF can bring to a SCF cell, there aredrawbacks that appear mainly in the form of SCF effective shading losses. To accountfor them, a model was built to simulate the efficiencies of laser-doped SCF solar cellswith different SCF sheet resistance and junction depths. The cell efficiency potentialof SCF laser-doped by the Spectra Physics laser was then assessed. With the most optimistic assumptions for contact resistance, and with experimentally-derived SCFsheet 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-printedsilicon solar cell. High SCF effective shading loss was the limiting factor.Subsequently, laser-doped SCF solar cells screen-printed with appropriate lowreactivitysilver pastes were fabricated. From these cells, the contact resistance wasdetermined to be too high and not uniform enough for high efficiency laser-dopedSCF 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 bothchallenges of high effective shading loss and low contact quality. This new metalplatedSCF solar cell is known as the advanced SCF solar cell. A laser-doping andmetallisation sequence in the order of screen-printing, laser-doping, and nickel/copperstack plating was analysed to be a practical sequence to fabricate the advanced SCFcell. During the development of the recipe, nickel was found to not uniformly plateacross the cell and these nickel voids resulted in higher series resistance levels. Amodified dopant dispense method for the laser-doping step was developed tosignificantly reduce these nickel voids. By applying this solution to a batch of sixadvanced SCF solar cells, an average batch and highest efficiency of 18.40 % and18.82 % 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 celldesign can have a direct ≈ 0.7 % absolute and ≈ 3.8 % relative efficiency gain over thelaser-doped SCF solar cell. This is mainly due to the ability of the advanced SCF solarcell to space the screen-printed silver fingers much wider apart and to use narrowerSCF. The predicted efficiency potential of the advanced SCF solar cell with a full areaback surface field exceeds 20 %.Of secondary importance and it was discovered while developing techniques tominimise the contact resistance between a screen-printed metal and a high sheetresistance diffused emitter, that using dilute hydrofluoric acid to improve the contactresistance can increase the cell’s recombination impact and reduce its pseudo-fillfactor. It was also demonstrated that by treating the cell in phosphoric acid, this impactcan be significantly reduced or eliminated. Chemical analyses suggest lead to be thelikely recombination source.