Atomic Layer Deposition of Materials for Applications to Photovoltaics

Jonathan Bakke
Abstract & Cover

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  vi  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.  

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Stanford Libraries
Stanford University
(Stanford, USA)
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