Epitaxially grown p-type wafers with TOPCon rear emitter solar cells

Germany's Fraunhofer Institute for Solar Energy Systems (ISE) has proposed the use of epitaxially grown p-type silicon wafers in TOPCon rear emitter (TOPCore) solar cells, which combine tunnel oxide passivated contact (TOPCon) technology with a rear emitter instead of a front-side pn junction.

Epitaxially grown p-type wafers, also called “EpiWafers,” offer low-cost and low-carbon material for solar cells, providing higher voltages and fill factors compared to cells with a front-side collecting emitter.

Epitaxially grown p-type wafers, also called “EpiWafers,” offer low-cost and low-carbon material for solar cells, providing higher voltages and fill factors compared to cells with a front-side collecting emitter.

The scientists grew p-type Cz-Si wafers with a resistivity of 2.5 Ω cm and a diameter of 150 mm through a chemical vapor deposition (CVD) batch reactor at atmospheric pressure. They then chemically cleaned and passivated the wafers with a surface passivation layer made of aluminum oxide (Al2O3) deposited via plasma-assisted atomic layer deposition (ALD) at 180 C for 78 cycles, which resulted in 10 nm thick layers on both sides.

“The resulting sample set consists of EpiRef wafers with a final thickness of 110 μm and three different base resistivities of 3, 14, and 100 Ω cm,” they said.

Applying a front finger grid design to all EpiRef wafers, the team observed that each of the three devices could potentially achieve a power conversion efficiency exceeding 25%. They also noted that the presence of iron (Fe), chromium monoboride (CrB), and Boron (BO) defects in the EpiWafers did not adversely affect cell performance, demonstrating the high quality and purity of the epitaxially grown wafers.

“In our case, these structural defects, originating from epitaxial growth, can be avoided by using another susceptor design,” the academics said. “Structural defect-free areas preserve their excellent material quality during high-temperature processing.”

They presented the novel manufacturing technique in “Epitaxially Grown p-type Silicon Wafers Ready for Cell Efficiencies Exceeding 25%,” which was recently published in RRL Solar.

In April 2021, Fraunhofer ISE said it had achieved an efficiency of 26.1% for a TOPCore solar cell.

“Based on a systematic simulation-based analysis, we were able to derive some fundamental design rules for future high-efficiency silicon solar cells above 26% efficiency,” Stefan Glunz, division director of photovoltaics research at Fraunhofer ISE, said at the time. “Both-sides-contacted solar cells have the potential to reach efficiencies up to 27 percent and thus surpass the previous world record for silicon solar cells.”