The effects of bifacial deposition of ALD AlOx on the contact properties of screen-printed contacts for p-type PERC solar cells

Abstract

Most c-Si solar cells adopt monofacial thin film structures and consequently much effort, particularly at the equipment design level, is devoted to ensure monofacial thin film processing. This often leads to more complex production equipment with lower throughput. However, in some cases, bifacial processing can be tolerated, and it is shown that a bifacial deposition of aluminium oxide (AlOx) using atomic layer deposition (ALD) can even result in improved performance of the solar cell. The merits of ALD AlOx at the rear of p-type PERC solar cells are well established and this work shows that a thin AlOx layer on the front of a PERC solar cell can also significantly reduce the contact resistance of screen printed Ag without affecting the optical properties of the solar cell. In this work, a equidistant linear transmission line method (TLM) pattern is used to characterise the contact resistance and specific contact resistivity. This technique has the advantage of being able to measure the fingers of completed cells, and this also explores the error introduced by measuring the resistance across interjacent fingers. A 3 or 5 nm AlOx film reduces the contact resistivity to 0.06 mΩ.cm2 and 0.11 mΩ.cm2, respectively, significantly lower than a value of 0.25 mΩcm2 achieved with an uncapped reference. Also, the peak temperatures required to achieve a good contact was lower, and the optimum firing temperature window was found to be wider, in comparison to the reference. A 10 nm ALD AlOx film is found to have a detrimental effect on the contact resistance which cannot be mitigated by a higher firing temperature.