Current transport studies of amorphous n/p junctions and its application in a-Si:H/HIT-type tandem cells

This paper presents an understanding of the fundamental carrier transport mechanism in hydrogenated amorphous silicon (a-Si:H)-based n/p junctions. These n/p junctions are, then, used as tunneling and recombination junctions (TRJ) in tandem solar cells, which were constructed by stacking the a-Si:H-based solar cell on the heterojunction with intrinsic thin layer (HIT) cell. First, the effect of activation energy (E_a) and Urbach parameter (E_u) of n-type hydrogenated amorphous silicon (a-Si:H(n)) on current transport in an a-Si:H-based n/p TRJ has been investigated. The photoluminescence spectra and temperature-dependent current-voltage characteristics in dark condition indicates that the tunneling is the dominant carrier transport mechanism in our a-Si:H-based n/p-type TRJ. The fabrication of a tandem cell structure consists of an a-Si:H-based top cell and an HIT-type bottom cell with the a-Si:H-based n/p junction developed as a TRJ in between. The development of a-Si:H-based n/p junction as a TRJ leads to an improved a-Si:H/HIT-type tandem cell with a better open circuit voltage (V_oc), fill factor (FF), and efficiency. The improvements in the cell performance was attributed to the wider band-tail states in the a-Si:H(n) layer that helps to an enhanced tunneling and recombination process in the TRJ. The best photovoltage parameters of the tandem cell were found to be V_oc = 1430 mV, short circuit current density = 10.51 mA/cm², FF = 0.65, and efficiency = 9.75%.