Resumen de Solution-Processing of Chalcogenide Nanoparticles and Thin Films for Photovoltaic Applications
Alex Carrete Bello
Thin film solar cells based on direct band gap semiconductors have attracted much research during last decades. Thin film technologies are currently commercial and display record power conversion efficiencies up to 20% at the laboratory scale. However, typical direct band gap semiconductors, CdTe and CIGS, content scarce and toxic elements such as In, Ga or Cd. An alternative to these materials is CZTS, formed by abundant and non toxic elements. CZTS is a quaternary p-type semiconductor which presents great absorption coefficient, has similar crystalline structure and optical properties to CIGS, and a suitable and tunable band gap (1-1.5 eV). An interesting strategy to develop thin film solar cells is the solution processing. Solution based approaches are especially interesting for their potential low production costs and their easy scalability. Among different solution processing techniques, the spraying of nanoparticles or metal salts is an especially interesting approach. The easy scalability of spraying techniques to prepare large-area panels in a non vacuum atmosphere, which is translated in a significant reduction of the production costs, renders the spraying very attractive for industrial implantation. A pulsed spray deposition system, which was custom made and operates in open air, is here used to produce CIGS and CZTS films from colloidal CIGS and CZTS nanoparticles. This work is divided in 5 chapters. The 1st chapter is an introduction to the photovoltaic technology and in particular to thin film PV technology, with special focus to chalcopyrite CIGS and kesterite CZTS. In the 2nd chapter I review the work done towards solving one of the major challenges associated to nanoparticle-based photovoltaic technologies: the complexity to transform nanoparticles into highly crystalline thin films by sintering processes. The 3rd chapter describes the experimental procedures used to prepare all the required materials and thin films and to fabricate solar cells. This chapter also describes the techniques used to characterize the morphological, compositional, structural and optoelectronical properties of the materials and films. Chapter 4 and 5 describe the work done regarding CIGS and CZTS technologies, respectively. Both chapters describe: NP colloidal synthesis, ligand exchange strategies to remove organic carbon surrounding the NP, subsequent thin film deposition techniques used, thermal treatments performed and final hetero-junction formation and device completion. Chapter 5 also describes a scale up method to produce large quantities of nanoparticles using a continuous flow reactor. In summary, the goal of this thesis is to establish a non vacuum technology to produce CIGS and CZTS photovoltaic devices prepared by solution process of the absorber. Additionally, drawbacks involved in the solution processing of nanoparticle-based films, such as elimination of organic carbon present in nanoparticle and film crystallization are addressed.
