Reduced Passivation of Silicon Surfaces at Low Injection Densities Caused by H-Induced Defects

Surface recombination of Si solar cells is effectively reduced by deposition of charged dielectrics, such as positively charged SiNx or negatively charged Al2O3. However, surface passivation of Si by positively charged SiNx layers is well known to deteriorate strongly at low illumination conditions for acceptor densities below 10 17 cm. Thus, the surfaces of p-Si cells are usually diffused with dopants. A qualitatively similar effect has recently been observed for Al2O3 layers on n-Si. This phenomenon is not well understood and, accordingly, cannot be avoided up to date. We quantify this effect for all relevant dopant and injection densities by means of a physical model that includes a damaged region in a thin layer underneath the passivating layer. The thickness of this layer is 0.1 – 1 μm in case of SiNx and only 10 – 100 nm for Al2O3. The damage does not necessarily originate from the deposition conditions alone, but possibly also from the wafer pretreatment. A tenfold improvement of the defect density within the surface damage region is shown to eliminate the problem.