Influence of Ge Incorporation from GeSe2 Vapor on the Properties of Cu2ZnSn(S,Se)4 Material and Solar Cells

Chao Gao,Yali Sun,Wei Yu

doi:10.3390/coatings8090304

Abstract

Cu2ZnSn(S,Se)4 (CZTSSe) and Cu2Zn(Sn,Ge)(S,Se)4 (CZTGSSe) thin films were prepared based on a non-vacuum solution method. The CZTSSe films were obtained by annealing the solution-deposited precursor films with Se, while the CZTGSSe films were obtained by annealing the similar precursor films with Se and GeSe2. We found that Ge could be incorporated into the annealed films when GeSe2 was present during the annealing process. The Ge incorporation obviously enlarged the sizes of the crystalline grains in the annealed films. However, the energy dispersive spectrometry (EDS) measurements revealed that the element distribution was not uniform in the CZTGSSe films. We fabricated solar cells based on the CZTSSe and CZTGSSe films. It was found the Ge incorporation decreases the Eu energy of the absorber material. The solar cell efficiency was increased from 5.61% (CZTSSe solar cell) to 7.14% (CZTGSSe solar cell) by the Ge incorporation. Compared to CZTSSe solar cells, the CZTGSSe solar cells exhibited a lower diode ideality factor and lower reverse saturation current density.

Highlights

The Cu2 ZnSn(S,Se)4 (CZTSSe) material has gained a great deal of attention as a promising photovoltaic material due to its properties, including suitable optical bandgap (1.0–1.5 eV), high absorption coefficient (>10−4 cm−1 in the visible light region), and Earth-abundant nature [1]
The precursor film looked smooth on the surface, and no obvious crystalline grains were found in the film (Figure 1a,b)
The Ge incorporation greatly increased the sizes of the crystalline grains in the annealed films

Summary

Introduction

The Cu2 ZnSn(S,Se) (CZTSSe) material has gained a great deal of attention as a promising photovoltaic material due to its properties, including suitable optical bandgap (1.0–1.5 eV), high absorption coefficient (>10−4 cm−1 in the visible light region), and Earth-abundant nature [1]. To-date, the efficiencies of CZTSSe solar cells have increased to 12.6% [2]. The efficiencies of CZTSSe solar cells are mainly limited by the low open-circuit voltage, which could be related with the poor properties (e.g., unfavorable electrical defects, secondary phases, large band-tail energy) of the CZTSSe material [3]. The Ge incorporation in CZTSSe material can increase the sizes of the crystalline grains, increase the carrier lifetime, decrease the band-tail energy, prevent the formation of deep-level defects, and more [4]. Several groups have incorporated Ge into CZTSSe, and they found the Ge incorporation can increase the open-circuit voltage and efficiency of the solar cell [6,7]. For most of the Ge-incorporated CZTSSe materials produced by the two-step method, the Ge element was introduced during the precursor preparation. Umehara et al reported on the preparation of

Methods

Results

Conclusion