scientific research summary

In my doctoral studies I attempted to understand the fundamental physics of a new kind of solar cell -- photovoltaics made with conducting plastics. I received my PhD in Condensed Matter Physics in the laboratories of Dr. Sue Carter at the University of California at Santa Cruz in December 2005.

My Work

I worked with Dr. Sue Carter on polymer photovoltaics (solar cells).  My dissertation was on time-resolved photoluminescence studies of heterojunctions of electron- and hole-transporting materials for photovoltaics. My goal was to understand how certain canonical electron-transporters affect the excited state dynamics in a popular hole-transporting PPV polymer.

If you're interested in more about me, please take a look at my bio.

You want proof?

You can download my dissertation here: .

Or, for those who are short of attention span, here is my abstract.

Research Publications

	Ph.D. Dissertation. Exciton Dynamics in Conjugated Polymer Photovoltaics: Steady-State and Time-Resolved Optical Spectroscopy. December 2005.
	Toward optimization of device performance in conjugated polymer photovoltaics: Charge generation, transfer and transport in poly(p-phenylene-vinylene) polymer heterojunctions. Sol. Energy Mat. & Sol. Cells (92), 651-659, 2008.
	The effect of broken conjugation on the excited state due to ether-linkage in a cyano-substituted poly(p-phenylene vinylene) conjugated polymer: CN-PPV vs. CN-ether-PPV, S. V. Chasteen, G. Rumbles, S. A. Carter, J. Chem. Phys. (24), 214704, 2006.
	Blended versus Layered Structures in Polymer Photovoltaics. S. V. Chasteen, J. O. Haerter, G. Rumbles, C. Scott, S. A. Carter. J. Appl. Phys. (99), 033709, 2006.
	Exciton Dynamics and Device Performance in Polythiophene Heterojunctions for Photovoltaics. S. V. Chasteen, S. A. Carter, G. Rumbles, Proc. of SPIE (5938), 59380J-1, 2005.
	Numerical Simulations of Layered and Blended Organic Photovoltaic Cells, J. O. Haerter, S.V. Chasteen, S. A. Carter, J. C. Scott, Applied Physics Letters (86), 164101, 2005.

More about polymer photovoltaics

Since my BA is in Social Psychology, I spent my first years at UCSC getting a crash course in upper division physics. It was brutal. It was important to me that my graduate work was directed towards a worthwhile goal -- clean energy. It would have been difficult to finish my PhD otherwise.

My graduate advisor, Dr. Sue Carter, supports research both in photovoltaics and light emitting diodes (LED's). Photovoltaics take in light and give back electricity, whereas light emitting diodes (LED's) do just the opposite, taking in electricity and emitting light. "Standard" semiconductors used in such applications are Silicon and Gallium Arsenide, which have high efficiencies (up to 30%). However, they are quite expensive to produce. A relatively new field of research has focussed on using conjugated polymers (basically, special plastics) in such applications. Polymers are much less efficient at producing energy than silicon photovoltaics, though recent research has obtained efficiencies up to 6%. The goal is to produce devices that can make up in economic savings what they lose in efficiency. A picture of polymers, not in solution, is to the left.

The physics of LED's and photovoltaics is very similar. Basically, an LED is a photovoltaic in reverse. A picture of a polymer LED is to the right.

When light enters a photovoltaic, in the form of photons, it has enough energy to knock an electron loose from a bond in the molecule. In Silicon, there are electrons loosely held in the lattice, which conduct upon being bumped free. In polymers, there are electrons loosely held (in pi-bonds) in the molecular chain. When one of these electrons is knocked free, it can travel out of the device and conduct electricity.

Polymer photovoltaics will never be as efficient as silicon. In Silicon, when an electron is knocked free (by an incoming photon) it shoots out of the device like a bullet. In polymers, the electrons are bound to the winding molecular chain of the polymer and they move really slowly to get out of the device. In a sense, it's a 1973 VW bus trying to get up a curvy mountain road. That means we're more likely to lose those electrons before we can get them out of the device as current (they recombine to emit light). So, because of this limitation, polymer photovoltaics won't be as efficient as silicon, but because they're cheaper, we hope to make the cost per watt cheaper than silicon. The relative cheapness of polymers, plus the fact that they can be processed to be flexible and bendy, makes them ideal candidates for new solar technology. A picture of polymers in solution is to the left. You can read an article I co-wrote for NOVA on the operation of a traditional solar cell.

 

Links

Saved by the Sun: a web interactive I co-wrote for NOVA on the operation of a traditional solar cell.

Solar on the Cheap (PDF). A good (but old) popular press article on polymer photovoltaics.

Conjugated Polymers: New Materials for Photovoltaics. A more technical paper, but a good summary of the field.

Electricity through Plastic. Physical Review Focus article on the Nobel prize winning discovery

Solar Energy Research Heats Up. An article I wrote for the local paper. (Not one of my best)