Ink Jet Printing Approaches to Solar Cell Contacts

ABSTRACT We are developing inkjet printing as a low cost, high throughput approach to the deposition of front contacts for Si solar cells. High deposition rates of 1µm per printing pass were achieved with a new metalorganic ink composed of silver(trifluoroacetate) in ethylene glycol. The printing conditions were optimized to achieve a relatively high line resolution of 120 µm. The optimal parameters for the piezoelectric inkjet were a pulse frequency of 50 Hz and pulse amplitude of 25 V. The best resolution and the line quality were achieved at a substrate temperature of 180 °C and drop separation of 40 µm. 1. Introduction Inkjet-printing is an attractive alternative to screen-printing or vacuum evaporation for fabrication of the front contacts to solar cells[1]. Vacuum evaporation is effective but requires patterning and is capital intensive. Screen-printing is a low-cost, atmospheric printing technique, but has resolution and throughput problems. Inkjet printing is an inherently suitable approach to mass manufacturing and conveyor processing of cells and modules. Inkjet printing should also provide better line resolution and improved aspect ratios for the conducting grid lines, which would lead to improved solar cell performance. A significant advantage of inkjet printing over screen-printing is that it is a non-contact, conformal deposition technique and therefore it is especially suitable for processing fragile and uneven poly-Si wafers. An essential element in the development of inkjet-based contacts is the formulation of the ink. For contact grids the optimization of the ink components is focused on achieving high deposition rates, good line resolution, high conductivity of the printed lines, and high quality ohmic contact to the substrate. Such inks should have a long storage and use life and be suitable for the chosen inkjet. In previous work, we have demonstrated highly conductive inkjet and spray-printed silver layers [2-4] deposited at elevated temperatures (300-400 °C ) from a metalorganic (MO) silver ink. The deposition rate reported for these inks was rather low 300-500 A per printing pass. The lines that were inkjet-printed from these inks were either broad and thin or composed of isolated islands [2]. We report here on improved deposition rates and line qualities for inkjet-printed silver contact patterns. We have achieved resolution and aspect ratios comparable to those for screen-printed lines. The ink composition and the processing parameters were optimized to achieve higher deposition rates and to control the spreading of the drops to achieve better continuity and resolution. The details of the optimization study will be presented below.