Material development and surface evaluation of the Cu-P-Se system for solution-processed optoelectronic thin films

Abstract

The work in this dissertation focuses on the challenges of directing surface passivation in a complex, unestablished phase space for solution processing of semiconducting nanoparticles for deposition of optoelectronic thin films. Chapter 1 provides fundamental literature background and a review of characterization techniques used as the foundation for research and development of nanoparticle interfaces. Therein, the importance of ligands for semiconducting colloidal nanoparticles is established from synthetic phase and morphological targeting to colloidal stability and transport properties. Following, Chapter 2 provides an experimental basis for nanoparticle passivation and deposition within the established model system CZTS and early studies of Cu3PSe4 films, illuminating the future challenges and directions for material development. The differences from CZTS suggest that the nanoparticle interfaces are not well passivated, which concurrently limits the synthetic understanding and property evaluation of this nanomaterial. To explore further, Chapter 3 establishes thermogravimetric analysis as a significant tool in the identification and quantification of surface coverage to support interfacial studies. As characterization methods singularly intended for bulk or molecular chemistry often present challenges for nanomaterials, several complementary techniques must be used in order to observe the full ligand-surface interface. One underutilized technique that complements surface studies is thermogravimetric analysis (TGA), where thermal degradation can inform on the identity and quantity of ligand coverage. A range of factors are explored for impact on the resulting weight loss and interpretation for nanoparticle systems, including sample mass, thermal scan rate, and particle purification. The technique provides the support for published manuscripts by Neisius and MacHale, as well as furthering the studies detailed herein. Next, Chapter 4 explores the development of large-scale synthesis of Cu-P-Se colloidal nanoparticles for property measurements and synthetic development. While nanoparticles are promising for their scalability and processability for future applications, novel synthesis is often developed on a small scale to improve phase control. By increasing the synthetic yield of both Cu3P and Cu3PSe4 syntheses can improve material development and expedite future studies. The findings herein also reinforce the importance of establishing atom-economy for nanoparticle syntheses. Further, Chapter 5 aims to direct surface passivation of Cu3PSe4 in order to improve colloidal stability and particle quality for deposition of optoelectronic films. The addition of coordinating ligands to nanoparticle synthesis is complicated by binary impurities which often occur with the introduction of coordinating surfactants to complex nanoparticle phases. Following all of the developments detailed in previous chapters, the first transport measurements of the colloidal materials were enabled through the stable particle passivation of near phase-pure Cu3PSe4. Lastly, Chapter 6 restates conclusions on the work presented herein and provides conceptualization of future research paths to direct material development. This dissertation shows the difficulty of nanoparticle development for property evaluation after establishing a novel synthesis. The process and techniques described within can provide a fundamental framework for future researchers and developers of nanoparticles for solution processed thin films.