Abstract
The efficiency of kesterite-based solar cells is limited by various non-ideal recombination paths, amongst others by a high density of defect states and by the presence of binary or ternary secondary phases within the absorber layer. Pronounced compositional variations and secondary phase segregation are indeed typical features of non-stoichiometric kesterite materials. Certainly kesterite-based thin film solar cells with an off-stoichiometric absorber layer composition, especially Cu-poor/Zn-rich, achieved the highest efficiencies, but deviations from the stoichiometric composition lead to the formation of intrinsic point defects (vacancies, anti-sites, and interstitials) in the kesterite-type material. In addition, a non-stoichiometric composition is usually associated with the formation of an undesirable side phase (secondary phases). Thus the correlation between off-stoichiometry and intrinsic point defects as well as the identification and quantification of secondary phases and compositional fluctuations in non-stoichiometric kesterite materials is of great importance for the understanding and rational design of solar cell devices. This paper summarizes the latest achievements in the investigation of identification and quantification of intrinsic point defects, compositional fluctuations, and secondary phases in non-stoichiometric kesterite-type materials.
Highlights
This work focuses on structural variations in kesterite-type compound semiconductors, in particular Cu/Zn disorder and intrinsic point defects, as well as on compositional variations, in particular stoichiometry deviations in the kesterite-type phase and the segregation of related binary and ternary phases.This review provides the vital approaches by discussing results and trends concerning intrinsic point defects and structural disorder, compositional fluctuations, and secondary phases on a macroscopic and microscopic scale and even on the nano-scale
The systematic investigations by neutron diffraction revealed the structural origin of point defects in offstoichiometric CZTS, CZTSe, CZTSSe, and CZGeSe kesterite phases
Detection of very light elements may be precluded with nanoXRF if the sample and detector are set up in air rather than in a vacuum chamber. (ii) Since the investigated length scale amounts to several micrometers, the whole solar cell layer stack including all interfaces and multiple grains can be analyzed in one single measurement different to TEM-energy dispersive x-ray analysis (EDX) or atom probe tomography (APT). (iii) In comparison to Raman spectroscopy, nanoXRF allows the direct observation of the spatial distribution of the different elements, revealing the number, size, morphology, and localization of secondary phase segregation
Summary
Original content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Susan Schorr1,2 , Galina Gurieva1 , Maxim Guc3,4 , Mirjana Dimitrievska3,5,6, Alejandro Pérez-Rodríguez3,7, Victor Izquierdo-Roca3, Claudia S Schnohr8 , Juran Kim9, William Jo9 and José Manuel Merino10 Keywords: kesterites, point defects, stoichiometry deviations, secondary phases, diffraction, Raman spectroscopy, Kelvin probe force microscopy
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