Ethylenediamine-assisted Hydrothermal Synthesis Of Metastable Orthorhombic Cu₂ZnSnS₄for Photovoltaic Application

There are several important demands for further photovoltaic materials. Firstly, materials should be environmentally benign; secondly, materials should be comprised of abundant and inexpensive elements to reduce the cost of solar cells; thirdly, tunable band gaps are also required for materials to maximize solar absorption. Among the numerous potential materials, Cu2ZnSnS4(CZTS), which is naturally abundant in the environment, has a suitable optical band gap energy and absorption coefficient, has the potential application in the files of thin film solar cells. The theoretical limit of power conversion efficiency for CZTS is as high as32.2%, according to Shockley-Queisser photon balance calculations. Experimentally, thin film solar cells based on CZTS have achieved a power conversion efficiency of nearly10%, which promote the research on CZTS and its application in the field of thin film solar cells.Results reported in references show that CZTS particles are usually synthesized via hot-rejection methods, and are kesterite phase. Herein, novel orthorhombic CZTS has been synthesized via an ethylenediamine-assisted hydrothermal method. Compared with the traditional CZTS, the orthorhombic structure is based on a double wurtzite cell, and has a Pmn21space group. The structure was confirmed by XRD, XPS, TEM, HRTEM and Raman spectrum. Band gap of the orthorhombic CZTS is about1.45eV obtained by room temperature UV-Visible absorption spectroscopy, which approaches to the optimum value for solar photoelectric conversion. To analyze the electrical properties of as-synthesized orthorhombic CZTS, films were fabricated. The photoelectric response of as-synthesized film demonstrated its potential in the application of photovoltaic devices.Ethylenediamine plays an important part in the construction of orthorhombic phase. Kesterite CZTS was also obtained in ultrapure water system without ethylenediamine. What’s more, we found that this orthorhombic CZTS is metastable. Annealed at500℃, phase transformation from orthorhombic structure to tetragonal kesterite structure would be achieved.