Solar cells need conductors for electrical connection of electrodes in the substrate to terminals. Typically, the electrodes are defined as highly doped regions in the semiconductor substrate. After passivation of the semiconductor substrate, such passivation layer is locally opened and a conductor is applied locally.
Generally, two methods of conductor deposition on solar cells are available: the use of a screen printed metal paste and the use of electroplating, particularly with copper. A metal paste has a composition for local removal of any underlying insulating and/or passivation layer, such as silicon oxide and silicon nitride. Disadvantages of the use of such metal paste include its limited conductivity, the large area needed and a high price. Copper-containing conductors are well-known in the art of semiconductor device manufacturing and printed circuit board manufacturing. Such copper-containing conductors may contain pure copper or a copper alloy and are suitably manufactured by means of an electrochemical process, more particularly a plating process. The advantage of such copper-containing conductors in comparison to other conductors is the low resistivity of copper.
A disadvantage of such copper-containing conductors is that copper has a tendency to migrate, which may cause malfunctioning. The migration is particularly problematic in the presence of semiconductor substrates, such as silicon substrates. Therefore, copper-containing conductors need to be isolated through so-called diffusion barriers. The application of diffusion barriers and copper-containing conductors has been developed for interconnect structures of integrated circuits and is known as damascene and double damascene processes. These damascene processes require plurality of lithographical steps, which result in a high cost price, and are therefore specific to the application in integrated circuits.
In solar cells the use of copper containing conductors is less widespread, even though the low cost price of electrochemical processes fits such applications. There is however a second disadvantage in using copper that needs to be overcome, which is a protection of copper-containing conductors against corrosion and oxidation, leading f.i. to increased resistance of the conductors, failures due to open circuits and poor contacts to any underlying substrate, such as a semiconductor substrate.
It is known to protect the copper-containing conductor in solar cells with a protective cover in the form of either an organic surface protection (known as OSP), a tin (Sn) cover layer, or an additional Ni-based layer and a thin silver (Ag) or gold (Au) cover layer. These known protective covers suffer from either only short-term protection (OSP) or high material and processing costs (Sn, Ni—Au).