Screen printed metal contacts to Si solar cells — Formation and synergistic improvements
Silicon solar cell technology has advanced by improvements to all aspects of cell design including metallization. Improvements to metal contacts have been synergistic with other cell improvements such as passivation, lightly doped emitters, and boron doped emitters. This paper briefly recounts the history of c-Si PV metallization technology and its synergy with other cell technologies. It concludes with a perspective on various metal contact mechanisms.
Front-side Ag contacts enabling superior recombination and fine-line performance
The standard silicon solar cell process continues on an evolutionary improvement path. High quality monocrystalline cells are now able to reach 19.2 % conversion efficiencies in industrial production. A key enabler for these high efficiencies has been the front-side Ag contact. This paper will discuss recent developments in this technology on two parallel fronts: reduced recombination and fine line printing. Front-side Ag can reduce solar cell recombination currents directly through reduced metal contact saturation current. In addition front-side Ag can indirectly lower recombination through improved contact formation to low saturation current emitters (lightly doped emitters, or LDE). Through improvements in the frit chemistry a superior recombination performance was enabled, yielding a 3 mV Voc gain and 0.1 % efficiency gain over the control. Improvements in the Ag particle dimensions and paste rheology reduced the optimum finger width approximately 10 μm, increasing Jsc by 0.3 mA/cm2 improving the efficiency gain another 0.1 % over the incumbent technology. In net we are able to demonstrate a next generation front-side Ag paste that can improve efficiency 0.2 %, from 18.8 % to 19.0 %.
Formation of high quality screen-printed contacts to homogeneous high sheet resistance emitters (HHSE)
In this paper we report on the formation of high quality contacts to HHSE with a sheet resistance variation of ≥30 Ω/sq using a commercial front-side Ag paste, PV16A from DuPont. We fabricated and characterized solar cells with emitter sheet resistances of 65, 75, 85, 95 and 105 Ω/sq. We found that emitter sheet resistances in the range of 65–95 Ω/sq can be contacted with low average series resistance of 0.63–0.77 Ω-cm<sup>2</sup> and high fill factor (FF) of 77.4–78.8%. The 75 Ω/sq emitter gave the best average efficiency of 18.3%, followed by 18.2% for the 65 Ω/sq, 18.0% for the 85 Ω/sq, and finally 17.7% for the 95 Ω/sq. The 105 Ω/sq emitter gave a low FF due to high series resistance, but the shunt resistance and ideality factor were excellent, which suggests the paste was not encroaching the shallow emitter junction.