Starting from the year 2006 electrical and electronic devices sold in Europe may not contain any lead [EU02]. For the replacement of lead containing solder two procedures can be taken: First of all soldering with lead free solder at an increased temperature and secondly gluing with conductive adhesives. In the following section we focus on conductive adhesives since the exchange of lead containing by lead free solder seems to be unproblematic because the solder temperature has only to be increases by 40°C.

In the literature some publications can be found which deal with the contacting of solar cells by adhesives. Mainly with very thin solar cells ([Frisson01], [Eikelboom02]), with solar cells which for manufacture-technological reasons have no bus bar and thus can not be soldered ([Beier01], [MAU02], [Mau03]) and with back contact solar cells [Eikelboom01].

Fig. 2 shows the measured values for different pairs of materials connected with four different contacting materials.

120

100

80

60

40

20

0

on silver

Fig. 2: Transition resistance of different material pairs. The error bars indicate the standard deviation. Meaning of the letters: a) soldering paste, b) one component silver filled acrylate adhesive, c) two component silver filled acrylate adhesive, d) one component silver filled polyimide adhesive.

it has to be recognised that the contacting on aluminium leads to insufficient transition resistance, which is due to the oxide existing on the aluminium. Furthermore one can see that adhesives are hardly suitable for the contacting of tinned copper ([Hennemann91], [Verbundprojekt99]).

The material combination silver/copper results in a very good transition resistance compared to soldered contacts. it was shown that this resistance remains extremely stable after aging by temperature changes ([Beier01], [Eikelboom01]).

Connecting the solar cells

For interconnecting high efficient solar cells in shingle technology the two material combinations silver/Ti, Pd, AG or silver/aluminium had to be contacted with each another. Regarding a process simplification during the solar cell production it would make sense to contact an aluminium layer, since the back metal of the solar cells is aluminium. Today on this aluminium a layer consisting of titanium, palladium and silver is vapour-deposited, to ensure a good electrical and mechanical contact. However if one regards the measured
values in Fig. 2 the material combination silver/aluminium has a very bad transition resistance no matter which contacting material is used. With solder this material combination could not be contacted. If good electrical module characteristics should be achieved, it’s necessary, to evaporate a titanium, palladium and silver layer on the aluminium back side of the solar cells. This back side can be glued or soldered with the silver front side of the next solar cell.

In the following part of this work the contacting of solar cells by conductive adhesives is examined. It turned out that adhesives have the advantage of the higher flexibility opposite solder. They are used if mechanical stress is feared by thermal expansion and no other element (e. g. tabs) can take up the tension. In addition adhesives are used where contacting is not possible by solder (unsolderable materials) or it appears technologically advantageous (back contact cells).

In summary adhesives for special applications are definitely an alternative to solder. They won’t achieve for the manufacturing of high efficient solar modules, since the curing time is too long.