Growth and characterization of Cu2BaSn(S1-xSex)4 thin films for solar cell application

Abstract

In recent years, Cu2ZnSnS4 (CZTS) has been focus of study as a promising absorber layer in thin film solar cells due to its high absorption coefficient, appropriate bandgap, and environment-friendly earth-abundant constituents. Despite global efforts the efficiency of the CZTS based devices is stagnated at 12.6% since 2014. This stagnated performance is attributed to a large Voc deficit caused by recombinations due to charged point defects and defect clusters. The dominant defect in CZTS is CuZn antisite defect, formed due to similar ionic radii of Cu and Zn (0.74 Å) ions. A potential approach to curb the formation of these antisite defects is isoelectronic substitution of one of the cation by cation of larger or smaller ionic radii. Among various substitutions proposed in literature (e.g. Ag for Cu, Cd for Zn, Ge for Sn, etc.), substitution of Zn by Ba is considered to significantly change the cationic disorder in CZTS. In Cu2BaSnS4 (CBTS), Ba has a larger ionic radii (1.56 Å) and the dominant defect in CBTS is VCu which results in p-type conductivity of CBTS, similar to the more matured absorber material CuInGaS2. Owing to the large size difference, significant structural changes in CBTS, and hence, opto-electronic properties compared to CZTS are expected. This work deals with the growth and characterization of CBTS and Se-alloyed CBTS (CBTSSe) thin films. In view of the differences in the optical and electrical properties of CBTS films with those of CZTS, we have numerically investigated and compared the performance of both devices using the SCAPS software. Simulations were carried out by considering the typical solar cell structure of glass/Mo/CZTS/CdS/i-ZnO/ITO. For a more realistic approach, a thin MoS2 layer is considered between Mo and CZTS. Simulations revealed an efficiency of about 17.68%, which is much higher than the experimentally obtained record efficiency of 11%. This suggest that the simulation should include an appropriate amount of defects in the bulk and at interfaces (i.e., back interface MoS2/CZTS and front interface CZTS/CdS). The experimental champion device parameters could be successfully simulated only when bulk defect density of 5.5 × 1015 cm-3, defect density of ~1×1015 cm-2 and ~1 × 1014 cm-2 at back and front interfaces was introduced. A possible route – by inserting a back surface field (BSF) layer - to improve the efficiency of the devices with CBTS films having these amounts of defect density has been demonstrated. It is shown that the CZTS solar cell efficiency can be increased up to 14.7% and 15.7% by optimizing Cu2O and SnS films as BSF layers, respectively. On the other hand, for CBTS films with similar defect density that resulted 11% efficiency for CZTS (experimentally obtained champion cell) simulations yielded an efficiency of only 4.55%. This is because of larger bandgap (2.0 eV for CBTS vs 1.5 eV for CZTS) and different nature of defects. Performance of the CBTS devices could be increased to 6.9% (reported experimental value) only when the defect densities were considerably reduced (interface defect density NMoS2/CBTS ~ 1015 cm-2, NCdS/CBTS ~ 1010 cm-2 and bulk density NCBTS ~ 1014 cm-3). The results suggest that the performance improvement of CBTS solar cells is more challenging than that for CZTS cells and hence, experimental conditions for the fabrication of CBTS films are expected to be more stringent. The CBTS films were synthesized by a solution based approach. A precursor film was prepared by spin coating of a non-toxic 2-methoxy ethanol based molecular precursor solution and was heat treated in presence of sulphur powder to obtain the eventual film. Since the formation of the secondary phases must be suppressed during the growth of the films as they degrade the performance of the solar cells, the process parameters were carefully optimized and the reaction pathway leading to the formation of single phase CBTS was established. We have systematically varied the molar concentration ratio in the solution and the sulfurization parameters (temperature, dwelling time and sulphur amount) and studied the impact thereof on the evolution of single phase CBTS. It was found that ideal molar concentration ratio [Ba]/[Sn] =1.0 always yielded secondary phases in spite of a large variation in the sulfurization parameters. Single phase CBTS thin films are obtained only for [Ba]/[Sn] = 1.4 in the precursor solution and sulfurization at 575 °C for 45 min with 1.0 g of powder S. UV–visible and room temperature PL measurements revealed a band gap of ~2.0 eV for these films. A symmetric PL peaks suggests reduced cationic disorder in the films compared to CZTS. The films showed white light sensitivity (~30%) for illumination of 24 mW/cm2. Detailed electrical and electro-impedance measurements showed p-type conductivity with a carrier concentration of 1.7×1014 cm-3 for the films. The CBTSSe films were obtained by heat treating the as-prepared precursor films in the presence of 1.0 g of sulphur (that yielded CBTS films) and varying amounts of selenium. It was found that the process parameters that produced CBTS films yielded various secondary phases that necessitated further optimization of the parameters including Ba/Sn ratio in the precursor solution, sulpho-selenization temperature and dwelling time, etc. Phase pure CBTSSe thin films were obtained for Ba/Sn=1.7 and annealing at 550 °C for 45 min with 1.0 g of S and 0.1 g of Se. By placing varying amounts of Se in the furnace during the sulpho-selenization process step, the concentration of Se in the films was systematically varied and the impact thereof was investigated. It was observed that by varying the Se amount from 0.1 to 0.4 g during sulpho-selenization, the Se/(Se+S) ratio in the resulted films increased from 0.05 to 0.22. With increase in the Se amount in the films, the bandgap of the films decreased gradually from 1.93 to 1.55 eV. The electro-impedance spectroscopy measurements on the film grown with 0.4 g of Se during sulpho-selenization revealed its p-type conductivity with an acceptor concentration of 1.58 × 1017 cm-3. The results indicate that these films can be potentially used as photocathode for hydrogen evolution.