Secondary phase formation in Zn‐rich Cu₂ZnSnSe₄‐based solar cells annealed in low pressure and temperature conditions

Abstract

Zn‐rich Cu2ZnSnSe4 (CZTSe) films were prepared by a two‐step process consisting in the DC‐magnetron sputtering deposition of a metallic stack precursor followed by a reactive anneal under a Se + Sn containing atmosphere. Precursor composition and annealing temperature were varied in order to analyze their effects on the morphological, structural, and optoelectronic properties of the films and solar cell devices. Raman scattering measurements show the presence of ZnSe as the main secondary phase in the films, as well as the presence of SnSe at the back absorber region of the films processed with lower Zn‐excess values and annealing temperatures. The ZnSe phase is found to accumulate more towards the surface of the absorber in samples with lower Zn‐excess and lower temperature annealing, while increasing Zn‐excess and annealing temperature promote its aggregation towards the back absorber region of the devices. These measurements indicate a strong dependence of these process variables in secondary phase formation and accumulation. In a preliminary optimization of both the composition and reactive annealing process, a solar cell with 4.8% efficiency has been fabricated, and potential mechanisms limiting device efficiency in these devices are discussed. Copyright © 2014 John Wiley & Sons, Ltd.

High efficiency bifacial Cu2ZnSnSe4 thin-film solar cells on transparent conducting oxide glass substrates

“…Many studies have also reported void formation in CZTSe due to Sn reaction and movement. 19 The obtained result is in accord with reports from Ge et al, 11 which showed that CZTS was converted to CZTIS, and a SnO 2 interlayer between CZTIS and ITO was observed as a result of In diffusion. However, in this work, the ITO was completely converted to SnO 2 .…”

The importance of back contact modification in Cu2ZnSnSe4 solar cells: The role of a thin MoO2 layer

“…[19,34]. The effects of the back contact degradation after selenization can be clearly observed in Figure S1 of the Supporting information (S.I.…”

Fill Factor Losses in Cu2ZnSn(SxSe1−x)4 Solar Cells: Insights from Physical and Electrical Characterization of Devices and Exfoliated Films

“…Nevertheless, secondary phases with overlapping peaks such as Cu 2 Sn(S,Se) 3 and Zn(S,Se) could occur too. [ 19 ] The main CZTSSe peaks are attributed to the A 1 vibration mode of S (Z2′-Z6′: 191, 202, 213, 216, 233 cm −1 ) and Se atoms (Z2′-Z6′: 322, 324, 325, 325, 329 cm −1 ) surrounding the cations. [ 38 ] The peak shift is due to the increased vibrational frequency resulting from lighter S atoms and CZTSSe with intermediate S/(S + Se) ratio exhibits a bimodal behavior.…”

“…[ 16 ] On the formation of CZTSe, SnSe was found at the back interface while ZnSe was found both on the surface (Zn/ Sn = 1.24 and annealing temperature < 500 °C) and back interface (Zn/Sn = 1.73) depending on the processing condition. [ 19 ] Therefore, secondary phase formation and segregation might be expected to be process dependent. The above reviews show that secondary phases typically coexist and impact the electrical behavior, especially in pure-sulfi de CZTS.…”