Metallization of Si heterojunction solar cell by Cu electroplating

In response to recent climate change, many renewable energy solutions have acquired great importance; in fact, the need of sustainable development is increasing and the use of renewable energy, such as solar energy, can be applied to many industrial and consumer applications. Photovoltaic systems, which are made up of solar cells, are used for electrical vehicle charging stations and to supply remote areas not connected to the power distribution network; moreover, solar energy is widely used as a primary or secondary source of domestic electric power. In this scenario, the increasing in efficiency, and the cost reduction of solar cells, becomes a priority in the diffusion of PV systems for energy supply. One of the most recent and promising technology in the field of solar cells is the silicon based HJT (heterojunction technology); in this type of cells, a monocrystalline Si layer is covered by two thin films of hydrogenated amorphous silicon (a-Si:H) on both side; the inner passivate the surface, allowing very large carrier lifetimes as well as record efficiency over 25% [1]; the other a-Si layers, instead, are doped to create the p-n device structure (Fig. 1). Since the lateral conductivity of the doped amorphous silicon films is quite low, a transparent conductive oxide film (TCO) is used to enhance the lateral current flow to the contacts; this layer, made by sputtering of indium-tin oxide (ITO), acts also as an anti reflection material. Last step in the technological process involves the contact realization, which is generally made up by silver paste; the metallization is the second most expensive process of the solar cell manufacture and affects the performance of the solar cell [2]. A limitation of silicon heterojunction solar cells is the requirement of a low-temperature process after the a-Si:H layer deposition; otherwise a degradation of the passivation properties of the material would occur. This constraint imposes the use, in the metallization process, of low temperature cured Ag pastes. The use of this material limits the performance of the cell, as the conductivity of silver pastes is minor than bulk Ag [3]. To overcome this problem and to reduce the cost of the cell, authors intend to create, as a proof of concept, a set of samples where silver contacts are replaced with a low-cost material i.e. copper, which presents a conductivity comparable to bulk Ag (Tab. 1). These samples, consisting of an ITO film on a glass substrate, emulate the top section of the HJT solar cell. The metallization is drawn on the ITO film using photoresist as a negative mask; afterwards, a thick copper layer is grown up on the uncovered areas using an electroplating process (Fig. 2). The track of the metallization reproduces a common used pattern in solar cells which is called H-grid (Fig. 3). The result expected is an improvement of the quality of the metallization in term of contact resistance respect printing process paste based Ag.