What makes a particular material efficient at converting sunlight to electrical or chemical energy? Conversely, what makes a material a poor energy converter? The Grimmgroup is motivated by quantifying and controlling the bulk and surface properties of solar energy conversion materials. As a research group in the Department of Chemistry and Biochemistry at Worcester Polytechnic Institute, we seek an atom- and bond-level understanding of material properties.
Our primary research goals are to:
- Identify the chemical species that contribute to deleterious surface states
- Etch, passivate, or react such chemical species for improved photovoltaic properties
The principal thesis of the Grimmgroup is that a fundamental atom- and bond-level understanding of material surfaces will ultimately lead to enhanced photovoltaic performance. Below is a broad overview of our motivation. The sidebar will direct you to our specific projects, as well as a description of the tools and instrumentation that enable our research.
Identifiying deleterious surface states
We could grow a semiconductor with excellent bulk material properties, but it wouldn’t matter if we can’t control the chemical species at the surface. Silicon is a perfect example: industry can produce human-sized, single-crystal boules of silicon with zero structural defects but the surfaces are a different story. Silicon surfaces oxidize in air, and that “native” oxide of silicon has a large concentration of dangling silicon bonds. Dangling silicon bonds are highly efficient at enabling the recombination of free electrons and holes, thus wasting the light that went into producing that free electron – hole pair. Minimizing such chemical states (surface states) drives a significant portion of academic and industrial efforts into silicon photovoltaics.
Cuprous oxide is an energy conversion material with good bulk properties that suffers from surface breakdown in aqueous photoelectrochemical cells:
In water, photoreduction reactions at cuprous oxide surfaces are more efficient at reducing interfacial cuprous cations to copper metal than reducing aqueous-phase species. Can chemical passivation shut down this deleterious reaction?
Cuprous oxide is natively a p-type material, so it performs reduction electrochemistry under illumination. Electrons freed from the bulk migrate to the surface to do some reduction reaction and they’ll reduce whatever is the easiest to reduce without prejudice. In non-aqueous solutions, that is likely some dissolved oxidized species, however, in water, it’s usually easier to reduce interfacial cuprous cations to copper metal. Copper metal makes a bad photovoltaic junction to cuprous oxide, so wouldn’t it be great if we could passivate that cuprous oxide surface?
Passivating deleterious surface states
Once we have identified the nature of deleterious surface states, our next job as chemists is to try to eliminate those states, either through an etch, or perhaps chemical reaction. Different surfaces require different passivation schemes. In the case of silicon, growing an oxide at high temperatures produces an interface with a low concentration of dangling bonds. In the case of cuprous oxide, a self-assembled monolayer blocks water from reaching the surface.
Every material is different, so every surface is different. Lots of work is required to tackle these problems. Are you up to the challenge?