Solar cells assemblies

In general, the cells for concentration are assembled on supporting substrates, treated similarly to bare dies in electronic technology. So, the process is completely different to that for standard PV assembling, but can take advantages by the huge progresses, standardizations and experiences collected during the last decades by the electronic devices industry.

Depending on the cells nature (materials, sizes and manufacturing technologies) and on the operative working conditions, different mounting technologies are used. Generally, the surface mounting technologies (SMT) directly derived from power electronics are applied. Even in this particular subset of components there’s plenty of different solutions. A good assembling is fundamental for the performances of the systems; thermal properties, reliability and optical matching are strongly dependent on the assembling solutions. Generally, thermal substrates are used, in order to drain out the high heat flux generated by the concentrated beam on the small cells; as every PV devices, the cells for concentration decrease their performances, as previously described, with the temperature. A substrate able to efficiently drain the heat out from the cells and spreads it onto a large area for heat exchange with the external air or with other cooling means is required. For this purpose, ceramic materials like alumina (Al2O3) or aluminium nitride (AlN) are often use, as in hybrid electronics, when the thermal flux are very high, because of their properties of thermal conductivity; when the thermal budget is lower, cheaper material can be employed as, for example, insulated metal substrate (IMS), i. e. an electronic support fabricated laminating an insulator between a massive mechanical substrate of aluminium and a foil of copper used as electrically conductive layer. Depending on the material and thickness adopted for the insulator layer, the circuit will have consequent thermal properties as well as dielectric capabilities. These insulating materials have usually a thermal conductivity in the range of 0.8 – 3 W/mK. In table (1) a summary of thermal conductivity of useful materials employed in CPV receivers assembling is reported.

The cells are electrically connected at the circuitry on the substrate; the rear contacts are attached using electrically conductive adhesives or soldering, while the frontal contact is
connected with soldered ribbons or bonded wires; in fig. (12) two different solutions using soldered leads and wire bonding, with chip on board technology (CoB), are shown.

a)

Fig. 12. CPV solar cells assembled on substrates: a) soldered silicon solar cell (Courtesy of CPower Srl); b) solar cell of 1mm2 assembled with chip on board technology (Courtesy of CRP – Centro Ricerche Plast-ottica)

Because of the technology used is derived from the electronic industry, the reliability issue related to the assembling with these approaches have been evaluated for long time; the CPV receivers in working condition can suffer different stresses respect to many electronic applications; however, many standards are already defined to verify the level of quality of the assembling processes and some possible defects leading to probable reliability problems can be identified even prior to carry out accelerated aging tests. In fig.(13a) a X-ray picture of a solar cell soldered onto a substrate using a correct surface mounting technology is shown, while in fig. (13b) a cell with an excess of voids in the soldering of the rear cell’s surface is sketched. The voids can produce cracking and failures during thermal cycling, as known in electronic technology. (Yunus et al., 2003).

a)

Fig. 13. X-ray image of soldered solar cells – a) acceptable soldering with <5% of voids area; b) unacceptable soldering, with high fraction of voids under the cell

Bypass diodes are often mounted on the same substrate of the cells; for multijunctions solar cells these component assumes great importance due to the high sensitivity to reverse bias of these cells, protecting the devices against destructive reverse loads. Currently, each individual cell has its own bypass diode, which can be an integral diode or an external, more standard, Si-diode. Basically, the integral concept consists in separating small area of the multijunction cell via mesa etching, and using the p-n junctions of the cell as protective diode.

Secondary optics, wherever used, are components of the receiver. These components require a high level of precision for their assembling in the module; indeed, the higher is the concentration level to menage, the higher is the precision of positioning, in order to avoid magnified losses; these secondary optics usually have to work under beams already highly concentrated. In high concentration photovoltaic modules, positioning errors higher than 100 microns can produce not negligible power losses (Diaz et al. 2005); however, this level of precision is usually achieved by high speed pick & place equipments for SMT in electronics, which are employed for the receivers assembly (Jaus et al., 2009).