A study of CZTS thin films for solar cell applications [electronic resource]
Keywords: Materials Science and Engineering

Recently, a lot of attention has been given to the link between the rise in fossil fuel consumption, CO2 emissions, and the average temperature of the planet. Multiple models have shown that if this trend is allowed to continue, the consequences for the environment could be quite significant. Furthermore, even if we are able to reduce our consumption of fossil fuels and reduce the CO2 levels, it will not change the fact that fossil fuels are a limited resource and are likely to be exhausted within the next 100-150 years. Addressing this problem will require the development of a renewable source of energy that is carbon neutral. Solar radiation is one of the most abundant forms of energy available on the planet, but harnessing this energy has been hampered by the high costs of current solar technologies. These costs stem from the use of non-optimal (silicon), toxic (cadmium) and expensive (indium) materials. To further reduce the costs of solar energy, a novel, earth abundant, non-toxic and inexpensive material is required that can make this energy source competitive with fossil fuels. Previous work on Cu2ZnSn(S, Se)4 (CZTSSe) has shown that it is an excellent candidate for use in thin film solar cells due to its earth abundant, inexpensive, non-toxic constituents and optimal material properties. The focus of this work was to use sputter deposition to synthesize this material as a thin film, incorporate it into a device and develop techniques to improve efficiency. Reactive sputtering was used to grow thin films of the pure sulfide Cu2ZnSnS4(CZTS) material while compound sputtering was used to incorporate selenium into the film and grow the sulfo-selenide hybrid material Cu2ZnSn(S, Se)4 (CZTSSe). The thin films were incorporated into devices based on the the widely accepted Cu(InGa)Se2 (CIGS) device stack. The composition of the films was analyzed using inductively coupled plasma optical emission spectroscopy and Auger electron spectroscopy. Morphology and interfaces between device layers were imaged using scanning electron microscopy. Phase analysis was conducted using x-ray diffraction and Raman spectroscopy. The devices were characterized using current-voltage and external quantum efficiency measurements. Reactive sputtering of CZTS films results in a unique morphology that can be controlled by varying the deposition parameters. Short circuit current (Isc) and efficiency were highly dependent on film composition while open circuit voltage (Voc) was not. The films phase separated when grown off stoichiometry but the secondary phases detected did not always agree with the established phase diagram. This phase separation was also controlled using deposition parameters and used to form nano-structured CZTS-ZnS films. Compound sputtering of CZTSSe films resulted in significantly larger grains as compared with CZTS films. The device performance was also significantly higher with all device parameters (Isc, Voc etc.) showing improvement. The secondary phases detected in this material did agree with the established phase diagram and it was shown that copper sulfide phase (Cu2S) is very detrimental to device performance while SnS2 and ZnS are not. Thus, the ideal composition regime for growth of CZTSSe would be copper poor and zinc/tin rich. The best efficiencies achieved during this work were 3.4% for CZTS and 9.3% for CZTSSe.