Fraunhofer ISE researchers have sought to utilize epitaxially grown p-type silicon wafers in TOPCo rear emitter (TOPCore) solar cells with the goal of reducing production costs and carbon footprint. They claim this combination paves the way for TOPCore devices with efficiencies exceeding 25 percent.
Epitaxially grown p-type wafers, also called “EpiWafers”, provide low-cost and low-carbon materials for solar cells, offering higher voltages and fill factors compared to cells with front-side collecting emitters.
Epitaxially grown p-type wafers, also called “EpiWafers”, provide low-cost and low-carbon materials for solar cells, offering higher voltages and fill factors compared to cells with front-side collecting emitters.
Scientists grew p-type Cz-Si wafers with a resistivity of 2.5 Ω cm and a diameter of 150 mm in a chemical vapor deposition (CVD) reactor at atmospheric pressure. They then chemically cleaned and passivated the wafers with a surface passivation layer made of aluminum oxide (Al2O3) deposited by plasma-assisted atomic layer deposition (ALD) at 180 C for 78 cycles, resulting 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 resistances of 3, 14 and 100 Ω cm,” they said.
When a finger mesh was applied to all EpiRef wafers, the team found that each of the three devices could potentially achieve power conversion efficiencies greater than 25%. They also found that iron (Fe), chromium monoboride (CrB), and boron (BO) defects in the EpiWafers did not adversely affect cell performance, indicating the high quality and purity of the epitaxially grown wafers.
“In our case, these structural defects due to epitaxial growth can be avoided by using another susceptor design,” the researchers said. “Structural defect free areas retain their excellent material quality under high temperature treatment.”
They presented the new fabrication technique in “Epitaxially grown p-type silicon wafers ready for greater than 25% cell efficiency,” published recently at RRL Solar.
In April 2021, Fraunhofer ISE reported that it had achieved an efficiency of 26.1 percent for the 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 with efficiencies above 26%,” Stefan Glunz, head of Fraunhofer ISE’s Photovoltaic Research Division, said at the time. “Solar cells touched on both sides have the potential to achieve an efficiency of up to 27 percent, thereby surpassing previous world records for silicon solar cells.”
