Solution-Processing of Chalcogenide Nanoparticles and Thin Films for Photovoltaic Applications

Tesi realitzada a l'Instut de Recerca en Energia de Catalunya – IREC 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 CuIn1-xGaxS2 (CIGS), content scarce and/or toxic elements such as In, Ga or Cd. An alternative to these materials is Cu2ZnSnS4 (CZTS), formed by abundant and non toxic elements. CZTS is a quaternary p-type semiconductor which presents great absorption coefficient (> 104 cm-1), has similar crystalline structure and optical properties to CIGS, and a suitable and tunable band gap (1.00-1.5 eV) by varying the S/Se ratio. 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 nanocrystals 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 NPs. This work is divided in 5 chapters. The 1st chapter is an introduction to the photovoltaic (PV) 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 NP-based PV technologies: the complexity to transform NPs 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 fil