Grain-Boundary Structural Relaxation in Sb2Se3 Thin-Film Photovoltaics

Grain boundaries play an important role in the efficiency of thin-film photovoltaics, where the absorber layer is invariably polycrystalline. Density-functional-theory simulations have previously identified a “self-healing” mechanism in <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><a:msub><a:mi>Sb</a:mi><a:mn>2</a:mn></a:msub><a:msub><a:mi>Se</a:mi><a:mn>3</a:mn></a:msub></a:math> that passivates the grain boundaries. During “self-healing,” extensive structural relaxation at the grain boundary removes the band-gap electronic defect states that give rise to high carrier recombination rates. In this work, lattice imaging in a transmission electron microscope is used to uncover evidence for the theoretically proposed structural relaxation in <d:math xmlns:d="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><d:msub><d:mi>Sb</d:mi><d:mn>2</d:mn></d:msub><d:msub><d:mi>Se</d:mi><d:mn>3</d:mn></d:msub></d:math>. The strain measured along the [010] crystal direction is found to be dependent on the nature of the grain-boundary plane. For a (010) grain boundary, the strain and structural relaxation is minimal, since no covalent bonds are broken by termination of the grain. On the other hand, strains of up to approximately 4% extending approximately 2 nm into the grain interior are observed for a (041) grain boundary, where grain termination results in significant structural relaxation due to the ideal atomic coordination being disrupted. These results are consistent with theory and suggest that <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><g:msub><g:mi>Sb</g:mi><g:mn>2</g:mn></g:msub><g:msub><g:mi>Se</g:mi><g:mn>3</g:mn></g:msub></g:math> may have a high level of grain-boundary-defect tolerance.