Mutiari, A. (2021). Cu2ZnSn(S,Se)4 Solar ink for earth-abundant thin films photovoltaic application [Dissertation, Technische Universität Wien]. reposiTUm. https://doi.org/10.34726/hss.2021.83288
E164 - Institut für Chemische Technologien und Analytik
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Date (published):
2021
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Number of Pages:
127
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Keywords:
Kesterit; dünne Filme; Photovoltaik; Solarzellen
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Kesterite; thin films; photovoltaics; solar cells
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Abstract:
The quaternary semi-conductive compound Cu2ZnSn(S,Se4) (CZTSSe) has shown to be an excellent candidate as an abundant, inexpensive and non-toxic material with beneficial properties for the thin-film photovoltaic (PV) absorber. The vacuum-based processes have been largely used for preparing absorber layers. However, the process complexes and the production cost high. The composition of the absorber layer also needs to be carefully considered to achieve a high-performance PV device. CZTSSe has a narrow stability region to form a single phase. The presence of an unwanted phase will be detrimental to the PV device performance.A working CZTSSe PV device requires other layers to create the p-n junction. Cadmium sulfide (CdS) is the standard n-type buffer layer widely used in the thin-film PV device. Since it has a toxic metal element of Cd, a non-toxic buffer such as Zn(O, S) need to be promoted. Another consideration is to choose the front contact layer. Indium tin oxide (ITO) is expensive due to the lack of indium. Nevertheless, the aluminium doped zinc oxide (AZO) used as an alternative electrode has a higher sheet resistance than ITO. The potential front contact electrode that accommodates this challenge needs to be addressed.This work aims to demonstrate the facile route of CZTSSe PV device fabrication to solve the abovementioned challenges. The scope of the work including the preparation of absorber, buffer and front contact electrode layers.There are two proposed solution-based methods to reach the work aims; molecular-based and powder-based solutions. Both approaches were prepared the CZTSSe absorber in two stages. First was depositing the solutions into the substrate, followed by an air-annealing. Second, to improve the crystallinity, the absorbers were sintered in a high-temperature furnace. Based on the structural characterization, both CZTSSe thin-film preparation approaches were found to yield the main phase with the kesterite structure in good quality.However, the CZTSSe films prepared by the powder-based approach showed smaller grains and less dense films. Attempts were made by varied binder and alkali addition to the CZTSSe precursors to enhance the film morphology properties. The sodium and potassium addition deliberately alter the morphology to bigger grain. The porous were still found yet limiting the PV device efficiency by 2.2%.The CZTSSe absorber prepared from the molecular-based solution was having homogenous, dense and bigger grains. Further, the study was focusing on absorber production using this method. First, the precursor compositions were adjusted to avoid the secondary phase formation. After this, the CZTSSe absorbers were sintered at different temperatures to obtain the optimum morphology. To have a better junction with the buffer layer, the Cu2ZnSn(Sx,Se1-x)4 absorbers were prepared in the range of 0 ≤ x ≤ 1 (x=S/(S+Se)). The incorporation of selenium into the absorber tuned the bandgap energy of the CZTSSe material between 1.1 to 1.45 eV.The heterojunction was established by depositing an n-type CdS layer on top of the p-type CZTSSe absorber film via a low-cost chemical bath deposition (CBD) technique. Another buffer layer was also deposited using a non-toxic n-type Zn(O,S) layer by radio frequency (RF) sputtering. Parameter process optimization for both buffer layer materials was investigated to achieve good structural and optical properties. Further, at the top-most layer of the CZTSSe PVdevice, an ultrathin dielectric/metal/dielectric (DMD) layer of AZO/Ag/AZO as a transparent electrode was employed. The sputtered AZO/Ag/AZO was used to replace the conventional standard AZO layer.In the end, using a molecular-based solution method, the PV performances of the fabricated cells were evaluated under simulated AM1.5G (100 mW/cm2) solar radiation. The champion cell was achieved with the set-up of Mo/MoS2/CZTS/CdS/i-ZnO/AZO/Ag/AZO. Based on an active area of 0.07cm2, it exhibited an open-circuit voltage (Voc) of 580 mV, a short circuit current density (Jsc) of 24.1 mA/cm2, and a fill-factor (FF) of 58.2 %, resulting in a power conversion efficiency (η) of 8.1% without any antireflection coating.Lastly, to visualize the band diagram model, the band structures of CZTSSe PV devices were simulated using a numerical model developed on SCAPS. The model explained the different conduction bands at the junction of the absorber/buffer layer interface. The simulation demonstrates the different performances of the CZTSSe device by different buffer layers of CdS and Zn(O,S).The dissertation results demonstrated the route of CZTSSe PV device fabrication with a solution-based method and employing a DMD structure as a promising transparent electrode for an earth-abundant CZTSSe PV device.
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