Point defects, compositional fluctuations, and secondary phases in non-stoichiometric kesterites

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 offstoichiometric 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.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. Various analytical methods have been used in these studies.The review comprises five parts:A. Crystal structure, structural disorder, and intrinsic point defects in kesterites B. Raman spectroscopy investigations on kesterites OPEN ACCESS RECEIVED

Upper limit to the photovoltaic efficiency of imperfect crystals from first principles

“…The voltage-dependent nonradiative recombination rate R SRH is obtained by combining eqn (4), (8), (11), (12), and (15). Finally, we evaluate the photovoltaic maximum efficiency:…”

“…9 One of the biggest questions in the field is if there is an intrinsic problem with kesterite semiconductors that prevent them approaching the radiative limit. [10][11][12] The discrepancy between the SQ limit and efficiencies of real solar cells results from the extra irreversible processes such as electron-hole nonradiative recombination. While Shockley and Queisser studied the effect of the nonradiative recombination, it has been treated as a parameter of radiative efficiency and often a radiative efficiency of 100% is assumed, which is unrealistic for real materials.…”

Interplay of Performance‐Limiting Nanoscale Features in Cu2ZnSn(S,Se)4 Solar Cells

“…In fact, various types of off‐stoichiometric CZTS and CZTSe powders were grown by solid‐state reaction, including Cu‐poor, Zn‐rich material. [ 10,19,20 ] In the present case, the occurrence of ZnS(Se) secondary phase segregations leads to a less Cu‐poor, Zn‐rich average CZTSSe composition than expected, although the absorber material is still, on average, Cu‐poor, Zn‐rich, and thus off‐stoichiometric. With respect to the 20 individual spots, the variation of the data points is much larger than that for the stoichiometric reference (Figure 2a).…”

“…So far, the state‐of‐the‐art absorbers are typically synthesized under Cu‐poor and Zn‐rich conditions, [ 3–6 ] which are associated with favorable Cu vacancies and intrinsic p‐type doping. However, these growth conditions also promote the formation of binary secondary phases, [ 7–10 ] which can have detrimental effects on the device performance. [ 11,12 ] In general, a discrepancy among the intended integral composition, typically probed via spatial global averaging, and the local elemental distribution in the actual CZTSSe absorber is produced by the presence of secondary phases.…”

“…[ 11,12 ] In general, a discrepancy among the intended integral composition, typically probed via spatial global averaging, and the local elemental distribution in the actual CZTSSe absorber is produced by the presence of secondary phases. [ 10,13 ] Such local compositional fluctuations are also inherently related to different intrinsic point defects and defect clusters that may significantly affect the device performance. [ 11,14–18 ] It is, therefore, of particular interest to obtain detailed information about the nature, quantity, size, and spatial distribution of the secondary phases and the local composition of the quaternary material to further enhance the conversion efficiency.…”