Interface band gap narrowing behind open circuit voltage losses in Cu$_2$ZnSnS$_4$ solar cells

Andrea Crovetto; Mattias Palsgaard; Tue Gunst; Troels Markussen; Kurt Stokbro; Mads Brandbyge; Ole Hansen

doi:10.1063/1.4976830

paper

Interface band gap narrowing behind open circuit voltage losses in Cu $_{2}$ ZnSnS $_{4}$ solar cells

arXiv:1702.04229 · doi:10.1063/1.4976830

Abstract

We present evidence that band gap narrowing at the heterointerface may be a major cause of the large open circuit voltage deficit of Cu $_{2}$ ZnSnS $_{4}$ /CdS solar cells. Band gap narrowing is caused by surface states that extend the Cu $_{2}$ ZnSnS $_{4}$ valence band into the forbidden gap. Those surface states are consistently found in Cu $_{2}$ ZnSnS $_{4}$ , but not in Cu $_{2}$ ZnSnSe $_{4}$ , by first-principles calculations. They do not simply arise from defects at surfaces but are an intrinsic feature of Cu $_{2}$ ZnSnS $_{4}$ surfaces. By including those states in a device model, the outcome of previously published temperature-dependent open circuit voltage measurements on Cu $_{2}$ ZnSnS $_{4}$ solar cells can be reproduced quantitatively without necessarily assuming a cliff-like conduction band offset with the CdS buffer layer. Our first-principles calculations indicate that Zn-based alternative buffer layers are advantageous due to the ability of Zn to passivate those surface states. Focusing future research on Zn-based buffers is expected to significantly improve the open circuit voltage and efficiency of pure-sulfide Cu $_{2}$ ZnSnS $_{4}$ solar cells.

Accepted at Applied Physics Letters

Interface band gap narrowing behind open circuit voltage losses in Cu2​ZnSnS4​ solar cells

Abstract

Cited by in corpus (7)

Interface band gap narrowing behind open circuit voltage losses in Cu $_{2}$ ZnSnS $_{4}$ solar cells