Atomic layer deposition of materials for applications to photovoltaics [electronic resource]
Keywords: Chemical Engineering

The world currently consumes over 16 TW of energy which is derived primarily from carbon-based sources including natural gas, oil, and coal, and energy use is expected to double by the year 2050. As concerns about energy security and carbon emissions have increased over the past decade, the search for alternative and renewable energy sources has garnered much attention. Photovoltaic (PV) technology is a leading candidate to be a major contributor to future electricity production since sunlight is a vast resource of energy and can be directly converted into usable electricity. As research into photovoltaics has rapidly progressed, interfacial effects on the nanoscale have increasingly come into focus; thus, the requirements for deposition techniques of PV materials have become more stringent. Atomic layer deposition (ALD) has emerged as a promising tool for studying and improving PV technology because of its unique capabilities to coat nanoporous substrates, to controllably deposit films at sub-Ångstrom thicknesses, and to manipulate compositions of very thin films. Understanding ALD processes and the quality of deposited films is an important step in developing systems with applications to PV manufacturing. The II-VI semiconductor system is particularly interesting for its use in transparent conducting oxides and in buffer layers for thin film PV. Of particular relevance, the bandgap, crystal structure, growth rate, index of refraction, conductivity, and resistivity of these materials can be tuned over large ranges by controllably depositing tertiary alloys. ALD is one of the premier techniques for achieving this control since it is a surface reaction rate-limited process in which a sub-monolayer of material is deposited per ALD cycle. Thus, ALD allows for control of material deposition at the Ångstrom level. The equipment utilized for ALD material deposition is an important consideration for any process and application. We have developed two ALD reactors: one has been optimized for the deposition of II-VI alloy materials, and the other has been designed to efficiently vaporize low vapor pressure precursors for relevant ALD processes. With the first reactor, we have demonstrated a method for in situ generation of small quantities of H2S for sulfide films, and we have expanded the knowledge of the II-VI system by ALD. The processes of ZnS, CdS, CdxZn1-xS, and ZnOyS1-y were developed for testing as buffer layers in thin film photovoltaics, and we analyzed the surface reactions that affect deposition of tertiary ALD films. Finally, we developed and characterized the ALD process for CdO and CdxZn1-xO, which is the first step in developing low resistivity transparent conducting oxides by ALD. The metalorganic precursors utilized for each of the depositions affected the ALD growth properties, and we performed experiments to show that the size of the ligand was an important consideration for these processes. The growth and material properties of these films were studied by spectroscopic ellipsometry, ultraviolet-visible spectroscopy, transmission electron microscopy, atomic force microscopy, X-ray diffraction, scanning electron microscopy. The II-VI semiconductor project was concluded with a study of interfacial engineering of CuIn1-xGax(S1-ySey)2 (CIGS) thin film photovoltaics in which the pn heterojunction was formed via ALD of CdxZn1-xOyS1-y. Using these ALD materials, the effect of thickness, surface treatment with solutions, alloy composition, and grading of materials was analyzed. The devices were characterized by current-voltage (I-V) and external quantum efficiency (EQE) measurements, which indicated that device performance is strongly related to the treatment and to the composition of the film. This thesis concludes with thoughts and perspectives of the future of ALD in PV manufacturing.