N-type solar cell with aluminum-titanium passivating contact achieves 21.9% efficiency

As single-junction silicon solar cells push closer to their practical efficiency limit (somewhere around the 30% mark), it becomes more and more difficult to find further improvements to performance. And while plenty of PV research is already looking to multijunction devices and other alternatives to keep the numbers going up, there’s still plenty of work going into optimizing silicon.

Passivating contacts are still relatively new in manufacturing and have plenty of room for improvement. Currently, doped hydrogenated amorphous silicon (a-Si:H) or polycrystalline silicon (Poly-Si) are the most commonly used, and have achieved the best results in passivation quality and low resistivity to date.

However, the limited transparency of these materials means they can only go so far in improving cell performance. Several material classes are investigated as possible replacements, and transition metal oxides are among the leading candidates for a replacement, and are the subject of newly published research led by scientists at Australia National University (ANU).

Transition metal oxides

Transition metal oxides – in particular, titanium oxide (TiO₂), offer many of the ideal qualities for a passivating contact layer. But attempts to use them so far have struggled with passivation quality, thermal stability and other practical issues.

“A general trend apparent in these previous studies is that improving the surface passivation of pure or doped TiO₂ single layers or Al₂O₃/TiO₂ stacks generally comes at the expense of increasing contact resistivity, and vice versa,” the ANU researchers said. “We demonstrate an innovative method for obtaining an outstanding silicon surface passivation combined with a low contact resistivity.”

They described their method in “Outstanding surface passivation for highly efficient silicon solar cells enabled by innovative Al_yTiO_x/TiO_x electron-selective contact stack,” which was recently published in RRL Solar. The group combined aluminum alloyed TiO₂ and pure TiO₂ in a stack formation, deposited using atomic layer deposition. By testing various film thicknesses and deposition temperatures they were able to optimize the passivation quality and electrical performance of the layers, which was observed and characterized using various imaging techniques.

Proof of concept

Crystalline silicon cells fabricated with this layer achieved a top efficiency of 21.9%, which the group says is already comparable with other passivating contact materials, and offers more room for further optimization.

“The electron-selective passivating  contact  enabled  by  our  novel stack (Al_yTiO_x/TiO_x) at 75 C appears to be highly competitive with state-of-the-art poly-Si electron-selective  passivating  contacts  (SiO_x/n+-poly-Si),  which  are  currently  expected  to enable the  next generation of c-Si  solar cells (while  having  better optical performance),” they said.

To achieve this potential for higher performance, the group says it will now focus on further improving passivation stability after metallization.

“We believe that by definitively addressing these challenges, our novel stack has the potential to match, and possibly outperform, Si-based contacts (a-Si:H and poly-Si) in terms of fabrication cost, simplicity, and performance,” they said.