(109b) Exciton Engineering - Using Transport and Reaction Engineering to Solve Problems in Solar Energy

The commercial successes of conventional solar cells (SCs) have emboldened researchers to raise efficiency, reduce cost, and develop new functionalities for next-generation photovoltaics (PVs). Several parallel fields, e.g., bulk crystal SCs, dye-sensitized SCs, organic PVs, are approaching these challenges from different angles. Nanostructured PVs, such as nanowire or semiconducting single-walled nanotube (SWNT), offer tremendous potential through reduced materials costs, scale-able fabrication, and unique and tunable properties.  In this presentation, I will argue that chemical engineers are uniquely positioned to use their foundational skills in understanding population balances, transport, and reactivity to analyze solar energy harvesting systems.  The exciton is a quasiparticle electron-hole pair whose diffusion and “reaction” to other entities is at the heart of photocatalysis and photoconduction in many systems.  Excitons are produced when photons interact with some forms of matter, and their diffusion can be described using recognizable variants of Ficks law as they seek to minimize their chemical potential gradient.  Excitons “chemically react” at surfaces to produce electrons and holes in a process that is central to photoconduction.  They decay to phonons as they heat the surrounding lattice and radiative recombine to often emit a detectable photon, allowing precision measurement of their kinetics.  This presentation will introduce the concept of “Exciton Engineering” – a term that we have used in our laboratory for the past 5 years.  I will show that excitons have a Thiele modulus for the first time, and that reaction engineering concepts can be utilized to optimize the design of solar energy devices.  Validation of this approach with several experimental systems will be presented, including an exciton antenna functioning as a solar funnel for photon concentration.  I will also present our work on the all carbon photovoltaic cell using single walled carbon nanotubes and no other matrix as the photoabsorber for the first time.  We have mathematically analyzed current and future nanotube based photovoltatic devices to produce important scaling laws that point towards their optimal design for maximal efficiency.  Lastly, I will conclude with a perspective on how Exciton Engineering may lead to next generation solar harvesting devices.