Finger Metallization Using Pattern Transfer Printing Technology for c-Si Solar Cell

Front metallization of crystalline silicon solar cells requires smaller finger lines in order to collect the generated current from the active area efficiently. Pattern transfer printing (PTP) technology enables finger widths down to 20 μm and aspect ratios above 0.6 using thick film silver paste. This work investigates the influence of PTP process parameters, i.e., laser power and trench geometry, on the finger geometry and the print quality. Silver fingers obtained from four different trench geometries were analyzed and the electrical parameters of corresponding printed solar cells were compared with standard screen-printed solar cells. We observed a strong dependence of the finger aspect ratio and the amount of debris on the laser power that was used during the transfer process. Each trench geometry requires a different laser power, which is related to the ratio between the circumference and the horizontal interface of the trench. Despite a modest FF loss due to higher series resistance, a significant gain in Jsc and a smaller gain in Voc increased the cell efficiency by 0.12%abs for the best group of PTP-printed cells compared with the reference screen-printed cells. Importantly, the amount of silver required was reduced by 54% compared with the screen-printed reference.

I. Introduction

The world market of photovoltaics is dominated by crystalline silicon (c-Si) technology. In 2018, the market share of c-Si reached about 95%, while the remaining 5% was accounted for by thin-film technologies [1]. The worldwide production volume of photovoltaic modules has steadily increased as energy policies in many countries have stimulated the transition to renewable energy generation [2]. This has led to an increasing demand for silver by the photovoltaics industry since 2014 [3]. In order to curb silver consumption and further reduce the cost of photovoltaics, it is necessary to reduce the finger width for the front side metallization of c-Si cells, which simultaneously increases the cell efficiency due to reduced shading. Even though the median silver paste consumption for one solar cell has decreased from 0.3 g/cell in 2010 [4] to 0.1 g/cell in 2018 [5], the silver for metallization is still the costliest nonsilicon material in the c-Si cell and contributes about 14% to the non-Si cell price [5]. On the other hand, a reduced finger cross section will reduce the electrical conductivity of the finger, requiring a higher number of busbars or more fingers to overcome this series resistance loss. Fig. 1 shows the finger width and the amount of silver paste consumption since 2015 for a monofacial p-type cell concept, and the estimated reduction trend for the next 10 years. The mainstream technology for metallization in the photovoltaic industry over the last decades has been screen printing, and in 2019 an average finger width of  = 39.5 μm was achieved [6]. Currently, the silver paste consumption of a monofacial p-type cell for a finger grid on the front side and solder pads on the rear side is around 95 mg/cell [6]. It is anticipated that over the next 10 years the silver paste consumption can be reduced by 50% with a finger width of  = 20 μm. Fig. 1.

Development of finger width using screen printing (SP) and PTP technology, as well as expenditure of silver for finger grid on the front side and solder pads on the rear side of monofacial p-type cells since 2015 and estimated reduction for the next 10 years [6].