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 %.
Nano-Ag colloids assisted tunneling mechanism for current conduction in front contact of crystalline Si solar cells
It has long been known that performance of industrial silicon solar cells depends critically on the firing conditions (e.g., temperature-time profile) of the screen-printed thick film Ag conductors (for front-side contact). Using dual beam (focused ion and electron) microscopy and transmission electron microscopy (TEM), we have investigated the microstructure of the front-side contact/solar cell emitter interface fired at temperatures from below to above optimal. We found that as we swept through optimal firing condition, the interfacial glass layer separating the emitter and the Ag-bulk evolved from one richly decorated with nanometer-size Ag colloids into one with progressively more Ag crystallites attached directly to the silicon emitter surface. To improve the statistical relevance of our microscopy work, we used contact resistance maps to guide our sampling. This allowed us to further relate the appearance and disappearance of microstructures (e.g. nano-Ag colloids, Ag crystallites, residual SiNx:H layer… etc.) to contact resistance. Finally, we have used selective acid etching technique to investigate the two-dimension distribution of Ag crystallites in cells fired at different temperatures. These microstructural observations offered new insights supporting the existence of a tunneling mechanism of current flow (“nano-Ag colloid assisted tunneling” model), in the absence of Ag-crystallites, in screen-printed industrial multicrystalline-Si solar cells.