Neutron scattering: an experimental approach to defects in semiconductors
Helmholtz‐Zentrum Berlin fuer Materialien und Energie, Department Structure and Dynamics of Energy Materials, Berlin, Germany
Freie Universitaet, Berlin Institute of Geological Sciences, Berlin, Germany
Photovoltaic thin film solar cells based on kesterite Cu2ZnSn(SxSe1‐x)4 compounds (CZTSSe) have reached >12% sunlight‐to‐electricity conversion efficiencies . This is still far away from the >20% record of chalcopyrite‐type Cu(In,Ga)Se2‐based (CIGSe) devices. The performance of CZTSSe thin film solar cells to date has been limited by a low open circuit voltage (VOC), or large VOC deficit, the origin of which has been the subject of intense debate. A leading explanation is that CZTSSe suffers from extreme band tailing due to a high density of bulk defects and disorder which could account for a significant part of the VOC loss [2, 3]. As the most likely origin of the band tailing the disorder in the Cu‐ Zn plane in the kesterite structure is discussed . This effect does not exist in chalcopyrite‐type CIGSe. CZTS/Se undergoes a reversible temperature dependent Cu‐Zn order‐disorder transition going along with a reversible band gap change . Moreover kesterite is susceptible to all types of point defects, which are vacancies, anti‐sites and interstitials. The formation of such defects is driven thermodynamically by minimizing the Gibbs free energy of the crystal. Because the best performing kesterite‐based thin film solar cells  were obtained with a material quite different from the stoichiometric compound, especially with a Cu‐poor/Zn‐rich composition, a classification of point defect clusters (off‐stoichiometry types) has been developed considering the off‐stoichiometry [6,7]. We have demonstrated, that kesterite type CZTS/Se can self‐adapt to Cu‐poor and Cu‐rich compositions without any structural change except the cation distribution . The ability to accept deviations from stoichiometry is correlated to the formation of intrinsic point defects.
We were able to show for the first time quantitatively that the Cu/Zn disorder (formation of ZnCu and CuZn anti sites) in kesterites causes shifts in the energy band gap giving raise to band tailing , a possible performance limiting parameter for thin‐film solar cell devices based on kesterite‐type absorber layers.
A potential way to avoid Cu/Zn disorder and optimize in this way the kesterite type material, could be
an alloying with Ag, forming (Ag,Cu)2ZnSnSe4. By systematic investigations of this solid solution series, we were able to show that already small amounts of silver stabilize the stannite type structure in the material . In this structure type no Cu/Zn disorder is present, which can be correlated to a significant decrease of the energy band tailing determined experimentally.
Neutron scattering – in this case elastic coherent scattering (diffraction) was applied – allows a non‐ destructive analysis of the crystal structure of photovoltaic absorber materials like kesterites from the surface deep into the volume of the sample. Only with the use of neutrons a differentiation between the electronic similar elements copper and zinc in the crystal structure is possible. In a systematic study based on neutron powder diffraction, applying the average neutron scattering length analysis method , we were able to evaluate experimentally the off‐stoichiometry type and intrinsic point defect concentrations as well as the Cu‐Zn disorder in kesterite‐type semiconductors.
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