Reverse Bias Induced Worm-like Defects in Cu(In,Ga)Se2 Thin-Film Solar Cells

Abstract

Environmental factors in the field cause partial shading of Cu(In,Ga)Se2 (CIGS) thin-film photovoltaic modules. Partial shading forces shaded cells into reverse bias leading to irreversible efficiency loss. The origin of the irreversible efficiency loss is the formation of defects in the shaded cell. The defects are going by the name of ’worm-like defects’. Understanding the mechanisms involved during the formation of worm-like defects can result in preventive measures to mitigate negative effects of partial shading.

The formation of worm-like defects involves a moving hot-spot across the cell. The exact mechanisms involved in the movement of a hot-spot is unknown. The problem in studying the mechanisms present in these hot-spots is an unknown current density and temperature. After rapid formation of worm-like defects severe changes in the solar cell are observed.

This work expands the knowledge on changes in geometry, composition and crystal structure after worm-like defect formation. X-ray diffraction results shows crystal structure of CIGS below worm-like defects has not changed. Energy dispersive X-ray results showCu rich areas near the back contact and areaswith high Ga and O content near the transparent conductive oxide. A band outside the worm-like defects is observed where Se is exchanged with S while Cu, In or Ga is exchanged with Cd.

Furthermore, a new experimental method is presented that allows control over locally applied conditions. The new method shows applied conditions can be controlled inducing features while resistive heating is suppressed.This enables future research on involved mechanisms.

Based on experimental results multiple possible mechanisms are proposed. In the authors view a plausible and major mechanism is electromigration of Cu towards the back contact. This migration is enhanced by the electromagnetic field and elevated temperature. Cu migration initiates a phase transformation of CIGS into a Cu rich and Cu poor phase. Subsequent to this phase transformation is a reaction of Ga and O. Further testing with this method is needed to investigate in which order mechanisms occur during worm-like defect formation.