Rapid thermal processing and screen-printing for low cost silicon solar cells

Rapid and potentially low-cost process techniques are analyzed and applied toward the fabrication of high-efficiency Si solar cells. First, a high quality screen-printed (SP) contact methodology is developed that results in fill factors of 0.785-0.790 on monocrystalline Si. Nest, aluminum back surface field (Al-BSF) formation is studied in detail to establish the process conditions that result in optimal BSF action. Screen-printing of Al conductor paste and rapid thermal processing (RTP) are integrated into the BSF procedure, and effective recombination velocities (S-eff) as low as 200 cm/s are demonstrated on 2.3 Omega-cm Si with this RTP SP Al-BSF process. A novel passivation scheme consisting of a dielectric stack (plasma silicon nitride on top of a rapid thermal oxide) is developed to reduce the surface recombination velocity (S) to approximate to 10 cm/s at the 1.3 Omega-cm Si surface. The important feature of this stack passivation scheme is its ability to withstand a high-temperature anneal (700-850 degrees C) without degradation in S. This feature is critical for most current commercial processes that utilize SP contact firing. Finally, the individual processes are integrated to form high-efficiency, manufacturable devices. Solar cell efficiencies of 17% and >19% are achieved on FZ Si with SP and evaporated (photolithography) contacts, respectively.