Advancing the Understanding of Reverse Breakdown in Cu(In,Ga)Se2 Solar Cells

Abstract

Reverse breakdown is investigated in multiple Cu(In,Ga)Se2 solar cells with varying buffer layer thicknesses. A method to extract transition voltage, which marks the change of conduction mechanism that leads to electrical breakdown, is described as an alternative to the often less-meaningful breakdown voltage. Transition voltages for samples with CdS and Zn x Sn1- x O y buffers are extracted from breakdown measurements performed in darkness and under illumination. The electric field is calculated for ZTO-based samples measured in darkness, and its implications for the energy band structure are examined. Fowler–Nordheim tunneling and Poole–Frenkel conduction are considered as candidates for the main breakdown mechanism in darkness. A model combining the two conduction mechanisms is proposed, and fits for experimental data are presented and discussed. Involvement of defects is debated, and defect-and-breakdown-related phenomena are showcased.