Electronic Properties of Ultrathin a-Si:H Layers and the a-Si:H/c-Si Interface

Abstract

The a-Si:H/c-Si heterojunction constitutes the core building block of a-Si:H/c-Si solar cells. In these cells, a key issue to obtain high efficiencies is the minimization of recombination losses at the a-Si:H/c-Si interfaces: The a-Si:H layers induce the band bending at the p/n-junction, but also passivate the surface of the c-Si, by saturation of dangling Si bonds. This is essential to realize the V oc potential > 700 mV of this cell type. High defect densities at the a-Si:H/c-Si interfaces lead to a pronounced decrease of the cell efficiency, by ~4% absolute at defect densities of 1012 cm− 2 (~ 1 dangling bond per 1000 interface atoms). Thus, it is important to obtain information on recombination-active defects in the ultra-thin a-Si:H layer and at the a-Si:H/c-Si interface. After introducing the basic electronic properties of a-Si:H, this chapter discusses the density of occupied valence band and defect states N occ(E) and the position of the Fermi level in the band gap of undoped (so called intrinsic) and of doped ultra-thin a-Si:H layers. The measured a-Si:H properties are correlated to the band bending in the c-Si absorber and to charge carrier recombination at the a-Si:H/c-Si interface. The connection to solar cell open circuit voltage V oc is made, and the current state-of-the-art of c-Si surface passivation by (i)a-Si:H is reviewed. Furthermore, the use of temperaturedependent current-voltage measurements on complete a-Si:H/c-Si solar cells to extract information on recombination and transport is discussed. Finally, the influence of band bending at the TCO/a-Si:H interface on cell performance is outlined briefly.