Screen-printed emitter-wrap-through solar cell with single step side selective emitter with 18.8% efficiency

A fabrication process for Emitter-Wrap-Through solar cells on monocrystalline material with high quality gap passivation by wet thermal silicon dioxide is investigated. Masking and structuring steps are performed by screen-printing technology. Via-holes are created by an industrially applicable high-speed laser drilling process. The cell structure features a selective emitter structure fabricated in a single high temperature step: a highly doped emitter at the via-holes and the rear side, allowing for a low via-hole resistivity as well as a low resistivity contact to screen-printed pastes, and a moderately doped front side emitter exhibiting high quantum efficiency in the low wavelength range. Therefore a novel approach is applied depositing either doped or undoped PECVD silicon dioxide layers on the front side. It is shown that doping profiles advantageous for the EWT-cell structure can be achieved. The screen-printed aluminum paste is found to penetrate the underlying thermal dioxide layer at appropriate contact firing conditions leading to a zone of high recombination in the overlap region of aluminum and silicon dioxide. It is shown that conventional PECVD-anti-reflection silicon nitride acts as effective protection layer reducing the recombination in this region. Designated area conversion efficiencies up to 18.8% on FZ material are obtained applying the single step side selective emitter fabrication technique.