Screen Printed Metallisation for Silicon Heterojunction Solar Cells

Abstract

Screen printing for metallisation of conventional silicon wafer solar cells typically includes a hightemperature (~800 °C) firing step for contact formation to heavily doped silicon regions. However, amorphous/ crystalline silicon (a-Si:H/c-Si) heterojunction (HET) solar cells have two unique metallisation requirements: (1) the metal needs to contact a transparent conductive oxide layer and (2) the annealing step for contact formation/metal sintering needs to be restricted to moderate temperatures (typically ≤ 200 °C) to avoid degradation of the cell’s thin a-Si layers. Screen printing using silver based polymer pastes can meet these requirements. In this paper, we present a detailed characterisation of three silver based polymer pastes (PV412, L9281 and L9282 from DuPont) in terms of contact formation to aluminium doped zinc oxide (AZO) layers and the annealing behaviour of the pastes. Average specific contact resistance for all three pastes on AZO is measured to be in the range 3 to 5 mΩ.cm2. The sheet resistance of printed fingers for L9281 and L9282 is measured to be ~5 mΩ/sq for typical single prints after annealing in air at 200 °C for 60 minutes. Based on our results both L9281 and L9282 appear to be promising candidates for HET metallisation.