Development of Commercially Viable Silicon Solar Cells with Open Circuit Voltages Exceeding 760 mV

Achievement date: 
2016
Outcome/accomplishment: 

Researchers at the National Science Foundation (NSF) Engineering Research Center (ERC) for Quantum Energy and Sustainable Solar Technologies (QESST) have exceeded the world-record for open-circuit voltage of a silicon solar cell under one sun illumination. By enabling dual layer stacks of silicon solar cells, QESST achieved an open circuit voltage of 762 millivolts (mV) in a silicon solar device, effectively reducing recombination losses to zero.

Impact/benefits: 

The open circuit voltage of a solar cell is a measure of how well the cell collects carriers before they recombine. An open circuit voltage of 762 mV allows for even thinner wafers and higher efficiencies. A decade ago the practical limit on cell open circuit voltage was considered to be 720 mV and the voltage of a typical commercial solar cell is currently under 650 mV. But because QESST achieved the high lifetimes on a commercially available Czochralski (Cz) wafer, the low defect levels observed are possible in current production.

Explanation/Background: 

In order to achieve the high open circuit voltage of 762 mV, QESST researchers first developed an amorphous silicon passivation process to reduce parasitic recombination processes at the surfaces of silicon wafers. This improved passivation process was then applied to the buffer layer of silicon heterostructure solar cells in combination with silicon nitride to form the dual layer stacks. The highest values measured were 764 mV on non-metalized and 759 mV on metalized 50-µm-thick solar cells.

According to the QESST model, Auger recombination is responsible for over 80 percent of the recombination losses. A positive effect of the hydrogen plasma treatment of a-Si layers and a lower power indium tin oxide (ITO) deposition on a-SI surface passivation contributed to the record high Voc. Using these two methods increased the open-circuit voltage and the pseudo fill factor of 50-micron-thick silicon heterojunction solar cells from previously reported values of 750 mV and 84 percent to 760 mV and 86 percent respectively. Additional funding for this work was provided by the Department of Energy.