Fully depleted emitter layers: a novel method to improve band alignment in thin-film solar cells

Abstract

The interface between the emitter and absorber layers in a thin-film solar cell must satisfy two important criteria, namely a small lattice mismatch and electron barrier height. It is shown that the barrier height is lowered when the emitter is fully depleted of free electron carriers by making the layer thinner than its space charge region, thereby enhancing thermionic emission of the photocurrent across the interface. Lattice matching is therefore the only requirement for a fully depleted emitter. The concept is applied to a lattice matched ZnS-Cu2ZnSnS4 (CZTS) interface which has a large intrinsic barrier height. Recent experimental evidence however suggest that ZnS becomes current unblocking when sufficiently thin. The theoretical efficiency for fully depleted ZnS is as high as 16.1%, due to the combination of a large open circuit voltage (1.0 V) from lattice matching and reasonable short circuit current density (24 mA/cm2). Lattice matched GaP and AlP are also potential CZTS emitter layers in the fully depleted configuration. The possibilities for exploring new materials combinations are therefore greater with fully depleted emitters. Furthermore, the concept can in principle be applied to any thin-film solar cell, making it highly versatile.