Sulfide-based ink formulation for earth-abundant photovoltaic applications

Abstract

With increasing output figures for solar cell modules in the past years, which could potentially lead to supply bottlenecks of high-purity grade silicon, thin film solar cells are an interesting alternative to the polycrystalline silicon cell (c-Si) technology that currently dominates the photovoltaic-module market. Among the broad variety of light-absorbing materials for thin films solar cells, Cu2ZnSnS4 (CZTS) has been in the focus of many researchers. CZTS is related to the material Cu2InGaS4 (CIGS), which has achieved high energy conversion efficiencies of 20 – 24% in solar cells, but CZTS includes elements that are more abundant in the earth’s crust compared to CIGS. However, the best efficiency for laboratory CZTS solar cells amounts to just 12.6% so far and the fabrication of CZTS thin films still involves the use of hazardous reagents and/or expensive equipment. In order to commercialize the CZTS solar cell, a high-through put method, which uses cheap and safe chemicals, but which produces CZTS thin films of high crystallinity with low impurities is required.In the present thesis, the production of CZTS thin films as part of an alternative fabrication method was investigated. For this purpose, a solution-based production route, which utilizes pre-fabricated CZTS powder, was established. Firstly, porous CZTS films of 1 – 10 μm thickness were deposited on molybdenum-coated soda-lime-glass (SLG) substrates with the aid of an automated doctor-blade device. Then, these porous films were sintered under an inert gas atmosphere (N2 + S) in order to be converted into crystalline CZTS thin films. For the deposition step, the CZTS powder was dispersed in water to create the coating solution, which is referred to as ‘ink’ in this thesis. The basic ink formulation included only water, CZTS powder and an organic binder component (gum arabic). Different CZTS powder and binder concentrations were selected and the effect of these modifications on the deposited and sintered CZTS thin films was studied. Therefore, layer thickness and porosity, as well as surface and particle morphology, were investigated thoroughly.After the ideal concentrations of CZTS powder and gum arabic have been found, the ink formulation was extended with additives, such as gum xanthan or sodium acetate in order to(a) improve the handling of the ink during deposition by adjusting viscosity and surface tension.(b) improve the crystallinity and reduce the porosity of the CZTS thin films after the sintering.It was crucial that any additive must not leave residues behind upon decomposition during sintering. Thus, the presence of residues or secondary phases in the sintered CZTS thin films was detected visually with scanning electron microscopy (SEM) from prepared cross and surface sections of the CZTS thin films. Furthermore, the chemical identity of the phases was revealed with grazing incidence X-ray diffraction (GI-XRD). In addition to that, the particle size and chemical composition of the CZTS powder was studied to determine, if the powder is suitable for the production of CZTS thin films.Despite the large-sized particles of the CZTS powder (𝑑𝐶𝑍𝑇𝑆 = 100 – 1100 nm), the fabrication of CZTS thin films down to 2 μm thickness was demonstrated in this work. Two principle types of ink formulations were created: one type includes only gum arabic and a high amount of dispersed CZTS powder (> 1.60 g CZTS / mL H2O) and the other type contains gum xanthan in addition to gum arabic but also a lower amount of dispersed CZTS powder (0.71 g CZTS / mL H2O). Still, the sintered CZTS thin films created from those basic ink formulations exhibited a high porosity and lacked crystallinity. During the sintering, the diffusion of alkali ions (Na, K), which are incorporated in the SLG substrate, into the CZTS material was found to greatly improve the crystallinity of CZTS thin films. It wasdemonstrated that not only deposition on bare SLG but also simple contact (e.g. placing a SLG plate on the films) was sufficient to achieve higher film densities. In other instances when a sputtered Mo-layer on the SLG blocks the diffusion, this substrate effect can be mimicked by adding Na- and K-salts directly into the inks in order to fabricate Na- and K-doped CZTS thin films.Nevertheless, none of the crystalline and densified CZTS films discussed in this work were found to be sufficiently conductive in order to be utilized in the fabrication of a complete CZTS solar cell. With that said, it is possible that carbon or oxygen residues from the gum binder and the other ink additives passivate the grain boundaries in the sintered CZTS thin films. Alternatively, the selected doping concentration of Na and/or K may be too high and either isolating additive salt residue is left behind, or the CZTS material is decomposed into secondary phases due to Na/K in the CZTS thin film as a result. In order to further develop the inks utilized for the production of CZTS thin films, alternative water-soluble Na- and K-sources should reduce or even prevent detrimental residues. However, any further modification must allow only non-toxic chemicals to be added (e.g. not NaF) so that the current production routine remains easily scalable and safe.