Researchers Make Case for Gallium-doped p-type Heterojunction Solar Cells

Researchers of an Austrian-Russian research group have come out with a technique to make Silicon Heterojunction Solar Cells based on P-type wafers more efficient and stable with Gallium-doping.

These p-type, Gallium-doped wafers have proved to be 22.6% efficient, more stable, and cheaper than the usual n-type Phosphorus-doped Czochralski–grown silicon (Cz–Si) wafers. 

The scientists of the research group from the University of New South Wales (UNSW) in Australia and Russian heterojunction solar module manufacturer Hevel Solar believe that these wafers may emerge as a primary solution for the SHJ segment in the coming decade. 

According to the scientists, the current n-type phosphorus-doped Czochralski–grown silicon (Cz–Si) wafers ensure no susceptibility to the boron-oxygen light-induced degradation (B-O LID) that is typical for p-type boron-doped wafers and severely affects the performance of SHJ cells over time. 

Although n-type wafers ensure more stability usually, they are currently more expensive to produce than p-type wafers, which are the mainstream solution for the manufacturing of PERC (passivated emitter and rear contact) cells. So, using p-type wafers may potentially reduce the costs for the heterojunction technology, since wafer costs still represent 40% of a cell’s total cost. 

How Was the New technology Developed?

The expiration of Shin Etsu‘s gallium doping patent (US6815605B1) has encouraged the solar industry to adopt p-type gallium-doped Cz-Si wafers, according to the research group.

Researchers developed two solar cells with a new advanced hydrogenation process (AHP) at an existing SHJ line operated by Hevel using 156.75×156.75 mm p-type wafers doped with boron (B) and gallium (Ga), respectively. The first products were provided by Chinese manufacturer Longi and the second by Taiwanese wafer maker Sino-American Silicon (SAS). For reference, researchers also developed the standard n-type SHJ device. 

All the wafers initially treated with a potassium hydroxide (KOH) saw-damage etch and KOH anisotropic texturing. B-doped hydrogenated amorphous silicon (a-Si:H) layers were deposited in the rear side of the wafers and intrinsic and P-doped (a-Si:H) layers were then deposited in the front of the wafers using a plasma-enhanced chemical vapor deposition (PECVD). In the final step, indium-tin tin-oxide (ITO) transparent conductive oxide (TCO) layers were deposited on both sides through physical vapor deposition.

According to the scientists, the cell built with B-doped wafers exhibited an efficiency of 20.5% and an open-circuit voltage of 719.6 mV, while the Ga-doped device was found to have an efficiency of 22.6%, a fill factor of 78.2%, an open-circuit voltage of 730 mV and no degradation during light-soaking. 

The research group indicated that the conversion efficiency of the gallium-doped SHJ solar cells is still lower than the n-type reference cells, which was high due to a reduced fill factor (FF). Further work is required to overcome this FF limitation to facilitate high-efficiency gallium-doped SHJ solar cells.

The next steps would require understanding the exact requirements for different SHJ solar cell manufacturers with different toolsets and processing conditions. The costs would be the same as for methods solving LID/LeTID in p-type PERC cells and when advanced hydrogenation tools are implemented for industrial n-type SHJ solar cells and TOPCon solar cells, the UNSW researcher informed. 

So, the researcher group thinks p-type wafers have the potential to take on the n-type SHJ solar cell wafers. The research group published the complete process of solar cells development and solar cell and the related hydrogenation process in a paper entitled, “Stability Study of Silicon Heterojunction Solar cells fabricated with Gallium- and Boron-doped Silicon Wafers”, published in RRL Solar.