Understanding the Charge Transport Mechanisms through Ultrathin SiOx Layers in Passivated Contacts for High-Efficiency Silicon Solar Cells

We report on the microscopic structure of the SiO _x layer and the transport mechanism in polycrystalline Si (poly-Si) passivated contacts, which enable high-efficiency crystalline Si (c-Si) solar cells. Using electron beam induced current (EBIC) measurements, we accurately map nanoscale conduction-enabling pinholes in 2.2 nm thick SiO _x layers in a poly-Si/SiO _x /c-Si stack. These conduction enabling pinholes appear as bright spots in EBIC maps due to carrier transport and collection limitations introduced by the insulating 2.2 nm SiO _x layer. Performing high-resolution transmission electron microscopy at a bright spot identified with EBIC reveals that conduction pinholes in SiO _x can be regions of thin tunneling SiO _x rather than a geometric pinhole. Additionally, selectively etching the underlying poly-Si layer in contacts with 1.5 and 2.2 nm thick SiO _x layers using tetramethylammonium hydroxide results in pinhole-like etch features in both contacts. However, EBIC measurements for a contact with a thinner, 1.5 nm SiO _x layer do not reveal pinholes, which is consistent with uniform tunneling transport through the 1.5 nm SiO _x layer. Finally, we theoretically show that reducing the metal to the c-Si contact size from microns, like in the p-type passivated emitter rear contact, to tens of nanometers, like in poly-Si contacts, allows lowering of the unpassivated contact area by several orders of magnitude, thus resulting in excellent passivation, as has been demonstrated for these contacts.