TECHNOLOGY I MANUFACTURING
Figure 2: The subsequent steps in making Ni/Cu contacts on the front side of the n-PERT solar cells.
(no BSF). In the case of PERL (Passivated Emitter and Rear Locally diffused) cells the BSF is locally present while for PERT (Passivated Emitter and Rear Totally diffused) cells it covers the whole rear side of the cell. The advantage of this type of rear structure compared to an Al-BSF cell, is that there is less free carrier recombination (improved passivation) as well as an improved internal reflection (> 90%) at long wavelengths. This explains the higher conversion efficiency that can be realized as compared to Al-BSF cells.
Many research groups worldwide, including imec, are studying PERx-type cells. Imec’s PERT cells, which reached world-leading efficiencies up to 21.5%, showcase several innovations such as the use of monocrystalline n-type material (instead of the p-type silicon most commonly used by manufacturers); Ni/Cu plated contacts on the front side (instead of screen-printed Ag contacts) and an emitter applied at the rear side.
n-PERT cells with Ni/Cu contacts
The PERT cells made at imec have some specific characteristics:
A first one is the use of Ni/Cu plated contacts at the front side. Ag screen- printed contacts, which are normally used in mainstream production lines, are rather expensive. This problem will only get worse, as the PV production capacity increases worldwide, because of the scarcity of this noble metal. Cu on the other hand is 100 times less expensive than Ag (per kilogram) and faces no availability issues. Other advantages of Ni/Cu contacts over Ag screen-printed contacts are the lower series resistance, the lower contact resistance and the largely reduced process temperatures.
The low contact resistance is achieved through the use of Ni as contact layer to the underlying silicon while it also serves as diffusion barrier for Cu. Finally, the Ni/Cu contacts are sealed with a very thin
Ag ‘flash’ layer to prevent oxidation of Cu. All three materials (Ni/Cu/Ag) are applied in one integrated plating sequence. The developed plating process allows to form much narrower metal lines or ‘fingers’ which results in less shadow losses and thus more light entering the cell. All these advantages make that this process is clearly a very promising metallization technology for the solar cell industry. In the end, its mass application will depend on solving the last issues which are mainly related to long term reliability.
The PERT cells of imec are built on an n-type monocrystalline silicon substrate. Monocrystalline silicon in general has a higher quality than multicrystalline and thus leads to solar cells with a higher conversion efficiency.
The expected increase in use of this type of material is clearly predicted by the International Technology Roadmap for PhotoVoltaics (ITRPV). Next, n-type monocrystalline silicon in particular is
than multicrystalline and thus leads to solar cells with a higher conversion efficiency. The expected increase in use of this type of material is clearly predicted by the International Technology Roadmap for PhotoVoltaics (ITRPV).
“ Issue IV 2014 I
www.solar-international.net 67 ”
Monocrystalline silicon in general has a higher quality
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