Composition variations in Cu(In,Ga)(S,Se)2 solar cells: not a gradient, but an interlaced network of two phases

TL;DR

This study reveals that Cu(In,Ga)(S,Se)2 solar cell absorbers contain interlaced phases with distinct band gaps rather than a smooth gradient, impacting efficiency and recombination, with implications for improving device performance.

Contribution

It demonstrates that the band gap variation in these solar cells is due to interlaced phases, not a gradient, and links phase composition to recombination and efficiency losses.

Findings

Two main radiative recombination paths identified at 1.04 eV and 1.5-1.6 eV.

Removing the back part increases PL intensity and quasi Fermi level splitting.

Reducing high gap phase could increase open circuit voltage by up to 180 mV.

Abstract

Record efficiency in chalcopyrite-based solar cells Cu(In,Ga)(S,Se)2 is achieved using a gallium gradient to increase the band gap of the absorber towards the back side. Although this structure has successfully reduced recombination at the back contact, we demonstrate that in industrial absorbers grown in the pilot line of Avancis, the back part is a source of non-radiative recombination. Depth-resolved photoluminescence (PL) measurements reveal two main radiative recombination paths at 1.04 eV and 1.5-1.6 eV, attributed to two phases of low and high band gap material, respectively. Instead of a continuous change in the band gap throughout the thickness of the absorber, we propose a model where discrete band gap phases interlace, creating an apparent gradient. Cathodoluminescence and Raman scattering spectroscopy confirm this result. Additionally, deep defects associated to the high gap phase reduce the absorber performance. Etching away the back part of the absorber leads to an increase of one order of magnitude in the PL intensity, i.e., 60 meV in quasi Fermi level splitting. Non-radiative voltage losses correlate linearly with the relative contribution of the high energy PL peak, suggesting that reducing the high gap phase could increase the open circuit voltage by up to 180 mV.