Ph.D Propositions Exam
1. Bulk-limited open-circuit voltages values of alloys based on Cu(In,Ga)(S,Se)2
Thin film photovoltaic devices based on Cu(In,Ga)(S,Se)2 (CIGS) alloys have reached 20% solar energy conversion efficiency and represent the highest achieved performance for thin film polycrystalline solar cells. The CIGS alloy system supports band gaps from 1.0 to 2.5 eV, but to date the most efficient cells have come exclusively from the <1.2 eV band gap range. Wide band gap alloys have exhibited lower open-circuit voltage (Voc) values with respect to the band gap than narrow gap alloys, and determining the nature of this loss mechanism is critical to expanding the utility of the CIGS alloy system. Nonaqueous photoelectrochemical experiments are ideally suited for exploring the fundamental limits of Voc values and device performance of the CIGS alloy system. The effects of bulk diffusion lengths, dopant density, conduction band offsets, and interface recombination on Voc values will be investigated for absorber layers encompassing the CIGS material system.
2. Thio sol-gel synthesis of metal sulfides: Thiolysis and condensation of tin thiolates
Metal sulfides show promise for application as absorbers in solar cells and there is significant opportunity to develop synthetic techniques for the low temperature, non-vacuum processing of metal sulfide thin films. Although high vacuum processing of the elements has successfully produced metal sulfide thin films, low temperature processing under ambient pressure has not yielded good quality photovoltaic devices. We propose an extension of metal oxide sol-gel chemistry to metal sulfides that can be used to prepare dense films with reduced sintering temperatures. The thiolysis and condensation of metal precursors such as heteroleptic tin(IV) thiolate and dithiocarbamate complexes by H2S will be investigated under ambient pressure and temperature. Then we will examine the sintering properties of condensed tin thiolate polymers as precursors to dense tin sulfide (SnS) thin films.
3. Absorption enhancement of black carbon incorporated into cloud droplets
The effect of anthropogenic aerosol emissions on global climate constitutes the most significant source of uncertainty in quantifying anthropogenic climate forcing. One challenge is understanding the factors that affect the visible light absorption of black carbon aerosols, the dominant visible light-absorbing species in the atmosphere. Although several groups have measured absorption enhancements for black carbon aerosols coated with organic and inorganic species, there is a lack of experimental information about the optical absorption enhancement of black carbon incorporated in water droplets. We propose to use modern continuous-flow cloud nucleation chambers in conjunction with laser-induced photoacoustic spectroscopy to monitor the mass specific absorption of black carbon incorporated into cloud droplets. A quantitative understanding of how black carbon absorption evolves under atmospherically relevant conditions would enable climate models with more sophisticated treatment of anthropogenic aerosols and improved predictive capability.
Ammonia as a Fuel?
I wrote the following paper in 2007 as part of an energy sciences class. Compressed ammonia is carbon-free, it doesn’t have the risk of explosion of compressed hydrogen, and it can be burned in internal combustion engines. The only problem is… well… have you ever smelled ammonia?!!
Sure, huge amounts of ammonia are piped around the world as fertilizer – more than 100 millions tons every year – but ammonia is nasty stuff. I wonder if ammonia-based life forms say the same thing about water.