Effect of germanium incorporation on the properties of kesterite Cu2ZnSn(S,Se)4 monograins
Introduction
The earth abundant kesterites Cu2ZnSn(S,Se)4 (CZTSSe) have gained much attention in materials research for solar cell application due to their suitable characteristics such as a high absorption coefficient and band gap (Eg) tunability between 1.0 eV and 1.5 eV [1]. The highest efficiency of kesterite-based solar cells of 12.6% was reached in 2013 by hydrazine-based solution produced thin film cells [2]. One of the main reasons for the lack of improvement in efficiency of CZTSSe solar cells compared to the closely related Cu(In,Ga)Se2 solar cells is the recombination losses based on large open-circuit voltage (VOC) deficit (Eg/q − VOC, where q is the elementary charge) [3]. The optimization of the bandgap of the absorber by cation or anion substitution can improve the VOC of the devices. One interesting approach is to substitute tin (Sn) by other cations such as germanium (Ge) leading to an increase in the bandgap [4]. The influence of Ge incorporation on the CZTSSe thin film solar cell device performance has been studied by several research groups [[5], [6], [7]]. Previous studies claim that the partial substitution of Sn by Ge can significantly reduce the VOC deficit and lead to higher efficiency compared to the Ge-free sample [5]. Moreover, improved charge carrier collection [5] and longer minority carrier lifetime [6,7] have been reported for Ge containing CZTSSe thin film solar cells. Till now, only small quantities of Ge have led to significant improvements in the kesterite solar cell performance [8]. The beneficial effect of Ge is particularly related to processes at the absorber surface and not associated with any bulk improvement [5,8]. Giraldo et al. implemented a Ge superficial nanolayer leading to efficiency improvement from 7% to 10.1% only substituting 4% of Sn by Ge in the kesterite compound [5]. The VOC increase was attributed to the formation of a liquid Ge-related phase, the possible reduction of Sn multicharge states, and the formation of GeOx nanoinclusions. Although, Ge substitution by larger amounts (30 to 40%) has also led to small (< 1%) increase in the solar cell performance, the VOC deficit has actually increased [1,6]. Those conflicting results on the effect of Ge on the performance of kesterite solar cells was the driving force for this study of the mechanisms by which the Ge could improve or deteriorate the solar cell properties.
There are only few reports in which the radiative recombination mechanisms in Cu2Zn(Sn1−xGex)(SSe)4 (CZGTSSe) absorber layer were investigated [1,4]. Usually, a broad asymmetric PL band is detected at low temperature and several recombination models are proposed to explain its origin. Our contribution provides an investigation of the impact of Ge incorporation into CZTSSe monograins, and its influence on the material properties and solar cell performance. A detailed photoluminescence (PL) study was carried out in order to understand the role of Ge in the CZGTSSe absorber layer.
Section snippets
Experiment description
Cu2ZnGexSn1–x(S0.8Se0.2)4 solid solution monograins with different Ge content were prepared and studied. The amount of Sn substituted by Ge is presented as %-values of the [Ge]/([Ge] + [Sn]) atomic ratio i.e. [Ge]/([Ge] + [Sn]) atomic ratio of 0.02 is 2% of nominal Sn substitution by Ge. The contents investigated here are 0%, 2%, 4%, 6%, 8% and 10%. The Ge-free sample is the reference material. CZGTSSe materials with slightly Cu-poor ([Cu]/([Zn] + [Sn] + [Ge]) =0.94), Zn-rich
Results and discussion
According to the EDX analysis of the synthesized CZGTSSe solid solution monograins, Sn is substituted with Ge by the chosen amount. Representative Raman spectra of the CZTSSe and CZGTSSe monograins are presented in Fig. 2. The Raman peaks of CZTSSe and CZGTSSe monograins are in good agreement with literature confirming the kesterite crystal structure of the formed grains [4].
Current-voltage characterization of the solar cells was carried out for the reference powder and powders with different
Conclusions
We show that one way to increase the band gap and the p-type conductivity of CZTSSe kesterite material is through the partial substitution of Sn by Ge. The higher p-type conductivity of CZGTSSe could explain the slower decrease of the performance of the Ge-substituted devices at low temperatures compared to the reference device. The partial substitution of Sn by Ge in the solid solution case increases the degree of cationic disorder and leads to higher defect concentration and band gap
Acknowledgements
This work was supported by Institutional research funding IUT19-28 of the Estonian Ministry of Education and Research and by the European Union through the European Regional Development Fund, Projects TK141 and MOBJD308.
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