Optimization of Cu2ZnSnS4 Absorber Layers via Sulfurization of Electrodeposited Cu-Zn-Sn Precursors for Photovoltaic Applications

Solar cells based on thin film absorbers provide an inexpensive and versatile technology to directly convert solar energy to electricity. Recent manufacturing developments have led to a significant decrease in the cost of solar energy, which has recently become lower than that of electricity from the grid, thus making this renewable energy option much more attractive. Kesterite solar cells comprised of a Cu2ZnSnS4 (CZTS) absorber layer are unique among thin film solar cell architectures since they are made up of earth-abundant, non-toxic elements, potentially enabling a widespread adoption of this technology, in contrast to current CdTe or Cu(In,Ga)Se2 which contain toxic elements. Unfortunately, CdTe and Cu(In,Ga)Se2 achieve efficiencies above 20%, while the record efficiency of thin film solar cells based on CZTS absorber films is only 12%. A simple, inexpensive, and scalable method for growing CZTS absorbers is electrodeposition of Cu-Zn-Sn metallic precursor and sulfurization method; so far CZTS grown with this method however is still only around 5%, and the origin of this limitation is not understood. On the basis of this knowledge gap, this dissertation investigates in detail the electrochemical synthesis and characterization of CZTS absorber layers with the aim to achieve a highly uniform, single phase material with a high conversion efficiency and good stability.
Electrodeposition of Cu-Zn-Sn (CZT) films followed by vapor phase sulfurization is an environmentally friendly and practical method to grow kesterite Cu2ZnSnS4 (CZTS) absorber layers on Mo substrates. In this work we discuss the influence of CZT alloy composition, its compositional spread, and sulfurization parameters on the formation of secondary phases and structural inhomogeneity, which negatively affect the performance of CZTS layers.
The stoichiometric composition for Cu2ZnSnS4 is Cu: 25 at%, Zn: 12.5 at%, Sn: 12.5 % and S: 50 at%, however slightly off stoichiometric Zn-rich and Cu-poor CZTS films have been reported to give higher efficiency than the stoichiometric ones. The compositional ratios that best correlate with performance in CZTS materials are the Zn/Sn and Cu/(Zn+Sn) ratio. Zn-rich films have Zn/Sn > 1.0 ratio and Cu-rich films have Cu/(Zn+Sn) < 1 ratio. Therefore, in our studies we target Zn-rich and Cu-poor CZTS films and for this purpose, we deposited Cu-poor CZT metallic precursors with different Zn/Sn ratios before sulfurization of the layers. Electrodeposition of CZT was carried out using two distinct chemistries: an acidic citrate solution and an alkaline pyrophosphate solution. The main difference between these two electrolytes was the reduction potential of zinc, which was deposited only after the hydrogen evolution reaction, significantly influencing the morphology of the CZT metallic precursors. In addition, the alkaline solution contained potassium, which acted as a growth modifier during the growth of CZTS films, increasing the size of the grains.
CZT alloys obtained from the acidic electrolyte were sulfurized at various temperatures and the influence of Zn/Sn ratio of the resulting CZTS layers on the film quality was studied. We found that Zn-rich (27-29%) and Cu-poor (47-49 at %) CZT metallic precursors formed fewer amounts of secondary phases and showed better crystallinity after sulfurization. Furthermore, CZTS layers with a higher Zn/Sn ratio showed improved photoelectrochemical (pec) response, up to 0.5 mA/cm2 and 0.99 mA/cm2 for sulfurization temperatures of 500 °C and 550 °C, respectively. The photoelectrochemical response was correlated to the phase purity and crystallinity of the CZTS, as determined from XRD and Raman Spectroscopy analyses.
In order to understand the influence of the electrodeposition current distribution on the standard deviation of the alloy composition, we deposited the CZT metallic precursors with a basic electrolyte, using vertical and horizontal deposition experimental set ups; the latter configuration avoided natural convection and thus enhanced flow uniformity. The compositional standard deviation of CZT metallic precursors did indeed decrease by using the horizontal deposition set up, and higher pec response was observed owing to the improved phase purity of the CZTS layers, as seen from compositional mapping of the films. Despite the improved uniformity however, the highest pec response (0.4 mA/cm2) of CZTS films from the basic solution was lower than that of CZTS films grown from an acidic solution. Even though Raman spectra results showed similar crystallographic properties at the surface of the CZTS layers, XRD analyses suggested the presence of slightly more secondary phases for the bulk of the CZTS layers grown from the alkaline solution in comparison to the acidic solution, which was found to be the reason for lower pec performance.
In a parallel effort, we deposited CZT metallic precursors with the addition of small amounts of bismuth (< 1 at%, CZT-Bi) from an acidic citrate solution to investigate the dopant role of bismuth and its influence on the performance of CZTS based materials. These films showed the highest pec, 2.65 mA/cm2, with the optimum amount of Bi (~0.5 at%) in Zn-rich CZTS layers. The increased pec response was attributed to the enhanced growth of the CZTS grains and improved phase purity with respect to CZTS films without any dopant addition.
Even though reasonably good pec responses were obtained with CZTS based films, the solar cell efficiency of the CZTS layers was less than 1%. The highest solar cell efficiency was 0.64% for CZTS films formed via sulfurization of CZT precursors grown from the acidic electrolyte. We found that the solar cell performance was compromised in presence of secondary phases, particularly the conductive phase CuxSy , which lowered the short circuit current by acting as a shunt path. Time resolved photoluminescence and measurements of the external quantum efficiency also suggested the presence of a high density of recombination centers, which lowered the open circuit voltage, hence the efficiency of the CZTS based cells. Various possible solutions towards improved material and higher performance are also outlined.