Analysis of Cu(In,Ga)Se₂ heterojunction solar cells in terms of their balance of thermodynamic potentials

This paper analyses theoretically electronic loss mechanisms in Cu(In,Ga)Se-2 heterojunction solar cells. The analysis is based on the balance of thermodynamic potentials (BoTPot), a novel approach that considers the complete set of thermodynamic potentials: electrical, chemical, and electrochemical potentials for electrons and holes. Here we concentrate on loss mechanisms that are specifically important for ZnO/CdS/Cu(In,Ga)Se-2 heterojunction solar cells, like interface recombination and carrier transport across barriers at heterointerfaces. We attribute the recombination losses at interfaces to losses in chemical potential leading to a reduction of the open-circuit voltage. In contrast, carrier transport across energy barriers may result either in losses of the electrostatic potential (for majority carriers) or in losses of the chemical potential (for minority carriers). This different attribution of minority and majority carrier losses sheds new light on the question of how to describe the selectivity of contacts in solar cells.