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Junctionless Solar Cell for Enabling Third-Generation Photovoltaics

Krishna Saraswat, Electrical Engineering

This project seeks to increase the efficiency and decrease the manufacturing costs of solar photovoltaic panels by developing a solar cell without the conventional p-n junction, instead using transparent electrodes on commonly available semiconductor materials, such as silicon, germanium, or gallium arsenide. Krishna Saraswat and his research associates are trying to eliminate the need for doped positive/negative junctions by using conducting oxides, which form a selective contact for either electrons or electron holes. Such cells could also be stacked on top of each other to enable  efficiencies greater than 50%, while avoiding the limitations of the lattice matching and current matching associated with conventional multi-junction cells.

The project has shown that the junctionless solar cell has the potential of providing very high efficiency at low cost. The simplified fabrication with no thermal processing results in improved carrier lifetime and lower cost. A junction can be replaced by a metal/insulator/semiconductor structure for collection of carriers such that it enables a selective contact for either electron/hole while reflecting the other carrier away from the contact, thus minimizing recombination. TiO2 and ZnO have nearly zero conduction-band offset with most semiconductors and large valance-band offset. This makes them capable of working as electron-selective contacts. NiO has nearly zero valance-band offset with most semiconductors and large conduction-band offset, and thus works as hole-selective contact.

This project’s proposed tri-terminal, multi-material solar cell does not require current and lattice matching as in traditional multijunction solar cells. Simulations show 39.3% efficiency for the design in ideal conditions. Germanium growth on silicon eliminates a need for the expensive germanium wafers used in traditional multijunction solar cells.

Based on these findings, the researchers changed the direction of a previously started project and are now using a junctionless approach. They have applied for additional research funding to advance development. They have also collaborated with a California startup company specializing in new solar cell designs and are in talks with a nanofabrication plant.

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Publications and media:

"Si heterojunction solar cells: A simulation study of the design issues" IEEE Transactions on Electron Devices 63 (2016): 4788-4795.

"Lateral overgrowth of germanium for monolithic integration of germanium-on-insulator on silicon" Journal of Crystal Growth 416 (2015): 21-27.

"Nickel oxide carrier selective contacts for silicon solar cells" 42nd IEEE Photovoltaic Specialist Conference (2015).

"Schottky barrier height reduction for holes by Fermi level depinning using metal/nickel oxide/silicon contactsApplied Physics Letters 105 (2014): 182103.

"Metal/insulator/semiconductor carrier selective contacts for photovoltaic cells" 40th IEEEPhotovoltaic Specialists Conference (2014): 0285-0289.

"Germanium on insulator (GOI) structure locally grown on silicon using hetero epitaxial lateral overgrowth" IEEE Subthreshold Microelectronics Technology Conference (2013).


Awarded 2013