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Heterojunction Metal Wrap Through Solar Cells

Fig. 1: Schematic drawing of a novel silicon heterojunction metal wrap through solar cell, where the front side contact is connected to the back side through via interconnects.

Silicon heterojunction (SHJ) solar cells are based on hydrogenated amorphous and crystalline silicon and achieve very high efficiencies of above 25 % in a simple low-temperature process scheme. However, the potential of this concept is still not fully exploited. To reduce shading of the front side metallization, new back contact cells are investigated. Several research groups pursue the combination of SHJ cells with the interdigitated back contact (IBC) concept, in which the emitter is shifted to the wafer back side. Within a consortium of industry and research institutes, NaMLab pursues another concept to significantly reduce the front side shading. The metal wrap through (MWT) technology is integrated into the SHJ solar cell. In this technology, the front side metallization is contacted through via interconnects to the back side (Fig. 1).

Fig. 2: Lateral distribution of carrier lifetime in the vicinity of a via hole processed in ‘via last’ sequence. The surface recombination at the via hole is low.

The main advantage of the MWT technology is that it can be integrated by the introduction of a single additional process step, which is the via hole formation. Within the MWT-plus research project different process sequences were evaluated including the via hole formation before (via first) chemical cleaning and wafer texturing and after (via last) the thin film deposition, i.e. the deposition of a-Si:H and TCO. Standard diffused emitter MWT solar cells are usually based on the ‘via first’ approach. However, it was found that in heterojunction solar cells the ‘via last’ sequence gave significantly better results, after the parameters of the laser process for via hole formation were optimized. Fig. 2 shows the measured carrier lifetime in the vicinity of a via hole. The recombination losses at the via hole surface hardly effected the carrier lifetime and thus, low via hole related losses are expected.

Fig. 3: Top-down view of the SHJ-MWT solar cell. The integration of MWT technology made it possible to reduce the width of the front side bus bar.

Based on this ‘via last” process flow, complete SHJ-MWT solar cells were realized. This development also involved innovative laser processes for via hole formation and novel low-temperature pastes for filling the via interconnects. The demonstrator solar cell employed an H-pattern grid layout (Fig. 3) where the front side layout was very similar to the reference solar cell produced in the Meyer Burger pilot line. However, the front side bus bars were connected to a rear side through interconnects. Because of the electrical current is mainly transported at the rear side of the cell, the front side bus bar width could be reduced to 500 μm in the demonstrator cell. Simulation shows that even narrower bus bars are feasible, resulting in more than 30 % reduction of front side shading.


This integration scheme proves that novel SHJ-MWT solar cells are feasible and that this concept is a promising route to reach low reflection losses in a simple and low-cost process flow.

 

Contact: Dr. Ingo Dirnstorfer

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