Fabrication of high band gap kesterite solar cell absorber materials for tandem applications
Introduction
The total amount of energy generated by solar energy has been very rapidly increasing these past few years. The global capacity has been multiplied by 4 in the last five years [1] and is expected to keep increasing in the coming years with the emergence of renewable energy favorable policies in many countries on the planet. It will be necessary, in the years to come, to be able to consistently produce large amounts of highly efficient solar cells to keep up with the demand. Traditional Si-based photovoltaic (PV) setups might still occupy an important place on the global market but could very well be replaced by Thin Film (TF) PV technologies with the emergence of building-integrated (BI) PV solutions. Indeed, TFPV can be implemented on a wide variety of solid or flexible substrates, which makes them interesting for BIPV applications. Classical TFPV materials like Cu(In,Ga)(S,Se)2 (CIGS) and CdTe, on the other hand, could be limited in the long run, due to scarcity of elements like In and Ga or the toxicity of Cd.
Due to this, in recent years, kesterite absorber materials, most prominently represented by Cu2ZnSn(SxSe1-x)4 (CZTS) [[2], [3], [4], [5], [6]], have been largely studied as a replacement thin film absorber for solar cell applications. The main advantage this type of materials provides is the non-toxic and earth abundant nature of the elements involved in their production. However, no matter how good a certain material might be, the efficiency of the subsequently produced solar cell is intrinsically limited by what is commonly known as the Shockley-Queiser limit, which predicts the theoretical maximal values of the various solar cell parameters [7]. One way to outperform this theoretical limit is to create cells using multiple p-n junctions, commonly known as tandem cells. In order for this type of architecture to work, it is necessary to have a top-cell with a higher band gap than the one of the regular bottom cell. In addition to the commonly used S/Se replacement [8, 9], elemental replacement of Sn by Ge in the CZTS structure has been investigated as a possible solution to increase the band gap of the absorber material [8]. In the case of the selenium phase of the tin kesterite, Cu2ZnSnSe4, the band gap has been shown to vary from 1.1 eV to 1.4 eV when varying the Ge/(Sn + Ge) ratio from 0 to 1 [8]. Subsequently, Cu2ZnGe(SxSe1-x)4 (CZGS(e) [10], with a band gap of 1.4 eV to 2.0 eV [8, 11] is a promising candidate to produce a high band gap kesterite material to be used in the top cell of a tandem thin film solar cell. The ideal band gap for the top cell is situated between 1.6 eV and 1.8 eV [12].
The most predominantly used methods to produce CZGS are wet processes (e.g. [13, 14]) or annealing of metallic precursors using elemental Se and S sources [[15], [16], [17], [18]]. These methods allow for a high throughput. In the present case, it was decided to produce samples through annealing of metallic precursors, but to perform the annealing in gaseous atmosphere (H2Se and H2S). This approach was chosen in the hope that it would allow for an optimal control of the concentration of elements in the annealing reactor.
Section snippets
Experimental methodology
The metallic precursors are deposited on a 5 cm × 5 cm Soda-Lime Glass substrates coated with a 150 nm Si(O,N) diffusion barrier and a 400 nm Mo back contact. The deposition is performed by e-beam evaporation in a Pfeiffer E-Gun PLS 580. A stack of a 200 nm layer of Ge, a 125 nm layer of Zn, a 170 nm thick layer of Cu is deposited by evaporation from solid, 99.999% pure, metal pellet crucibles in vacuum.
Before the samples are processed any further, they are split into two 2.5 cm × 5 cm samples.
Results and discussion
Before developing the recipe used to obtain the samples presented in this work and the results that are linked to them, an annealing recipe with one single filling of the reaction chamber with H2S gas was used. It appeared that reducing the sulfur inclusion after a 120 min annealing period was possible by reducing the amount of H2S pressure injected into the reactor prior to annealing, as shown in (Fig. 1a). In this figure, the sulfur inclusion, as determined from XRD measurements, is plotted
Conclusions
Using an annealing process in which evaporated metallic precursors are successively submitted to a short selenization step and a long sulfurization step, it was possible to create CZGSSe absorber layers with sulfur inclusion going from 30% to 100% that have respective band gap of 1.45 eV to 2.0 eV. The results of these annealing procedures were reproducible and brought forth an almost pure kesterite with a controllable band gap. In addition to the linear dependence between sulfurization time
Acknowledgements
This work was performed within the framework of the SWInG project funded by the European Union's Horizon 2020 research and innovation program, grant agreement No. 640868.
M. Neuwirth gratefully acknowledges financial support by the German Federal Ministry of Education and Research (BMBF, FKZ: 03SF0530B) and the Karlsruhe School of Optics and Photonics (KSOP).
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