The Effect of Soft Pre-Annealing of Differently Stacked Cu-Sn-Zn Precursors on the Quality of Cu₂ZnSnSe₄ Absorbers

Abstract

Cu2ZnSnSe4 p-type semiconductors currently investigated for use in thin film solar cells can be synthesized by firstly depositing a metallic precursor and secondly annealing the precursor in selenium vapor. Differently stacked Cu-Sn-Zn metallic precursors were characterized after a soft annealing at 350°C under nitrogen atmosphere. For the stack where the Sn and Zn were in direct contact with sufficient Cu to form a stable alloy, a bi-layered structure consisting of Cu-Sn on the bottom and Cu-Zn on the top was formed. Contrarily, when Zn was not in direct contact with Cu, the metals diffused to form a stable alloy and the system segregates horizontally, forming a mixed columnar structure. These two types of precursors were selenized under exactly the same conditions to form kesterite absorbers for solar cell devices. Using this approach the improvement from 0.44% power conversion efficiency for the bi-layered precursor to 4.5% for the mixed precursor was achieved.

Understanding quaternary compound Cu2ZnSnSe4 synthesis by microscopic scale analyses at an identical location

“…In terms of phase composition, these results are consistent with previous literature 11 . However, the low magnification micrograph reveals that the segregated alloys reach a size of 10-20 m, which is one order of magnitude larger than previously reported.…”

“…However, since the metal deposition is sequential, the films do have a strong vertical compositional gradient (the stack is composed of 200 nm Cu, 240 nm Sn, 180 nm Zn). When such a stack is used, it was shown by Ahmed et al 10 and explained by Arasimowicz et al 11 that a soft annealing step under inert atmosphere leads to the formation of micron sized columnar Cu-Zn and Cu-Sn alloys.…”

“…deposition of a metal precursor film, followed by a reactive thermal treatment to incorporate sulfur and/or selenium 2,[7][8][9] . For the fabrication of Cu 2 ZnSnSe 4 Kesterite, our modified version of this process involves (i) the sequential electrodeposition of Cu, Sn and Zn on Mo-coated glass substrates, followed by (ii) a soft-annealing step at 350°C in inert atmosphere 10,11 , followed by (iii) a gas-phase selenization step at 525°C to allow Kesterite formation.…”

A Temporary Barrier Effect of the Alloy Layer During Selenization: Tailoring the Thickness of MoSe2 for Efficient Cu2ZnSnSe4 Solar Cells

“…Because the stacking sequence of the precursor and the alloying process are different from those used in refs. [20,21], only Cu 6 Zn 5 is preferred alloy phase in sample B-2.…”

Electrodeposition of kesterite thin films for photovoltaic applications: Quo vadis?

“…The recent refinement of the duration of the pre‐alloying step by Matsumura's group in Japan has allowed the achievement of the current 8% world record efficiency for a pure sulfide kesterite electrodeposited device . These results highlight the importance of the precursor microstructure for the formation of kesterite absorber films with reduced secondary phase segregation . Likewise, the choice of electrochemical deposition sequence itself has crucial implications in the determination of the precursor microstructure .…”