Near surface defects: Cause of deficit between internal and external open-circuit voltage in solar cells

TL;DR

This paper investigates how near-surface defects at the absorber/buffer interface in copper indium diselenide solar cells cause a discrepancy between internal and external open-circuit voltages, highlighting the importance of interface recombination analysis.

Contribution

A deep near-surface defect model is developed and validated through experimental techniques to explain VOC deficit caused by interface recombination in CIGS solar cells.

Findings

Deep acceptor defects are identified at the absorber/buffer interface.

Temperature-dependent measurements confirm interface defects as the cause of VOC loss.

VOC, in measurements alone can be misleading without complementary analysis.

Abstract

The presence of interface recombination in a complex multilayered thin-film solar structure causes a disparity between the internal open-circuit voltage (VOC,in), measured by photoluminescence, and the external open-circuit voltage (VOC,ex) i.e. an additional VOC deficit. Higher VOC,ex value aim require a comprehensive understanding of connection between VOC deficit and interface recombination. Here, a deep near-surface defect model at the absorber/buffer interface is developed for copper indium di-selenide solar cells grown under Cu excess conditions to explain the disparity between VOC,in and VOC,ex.. The model is based on experimental analysis of admittance spectroscopy and deep-level transient spectroscopy, which show the signature of deep acceptor defect. Further, temperature-dependent current-voltage measurements confirm the presence of near surface defects as the cause of interface recombination. The numerical simulations show strong decrease in the local VOC,in near the absorber/buffer interface leading to a VOC deficit in the device. This loss mechanism leads to interface recombination without a reduced interface bandgap or Fermi level pinning. Further, these findings demonstrate that the VOC,in measurements alone can be inconclusive and might conceal the information on interface recombination pathways, establishing the need for complementary techniques like temperature dependent current voltage measurements to identify the cause of interface recombination in the devices.