Jon A Bender

In an effort to increase societal solar energy harvesting by lowering the cost/power of current commercial solar panels, an alluring strategy is to improve upon materials and architectures already optimized and engrained in our solar industry.  Single-junction silicon (Si) solar cells are by far and large make up this market. A cost effective approach is to stymie thermalization losses and photon transmission in these devices by mindful incorporation of well-designed organic semiconductors (OSCs) for down-conversion and up-conversion of excitations. Singlet exciton fission (SF) and triplet-triplet annihilation can be thought of as two sides of the same coin, each showing promise for suppressing respective losses due to the aforementioned efficiency loss channels.

SF is a down-conversion process unique to OSCs wherein a spin-singlet excited state repartitions its energy into two spin-triplet excited states of roughly half the energy. A particularly appealing architecture for incorporating OCSs designed for efficient SF into existing devices involves interfacing a SF capable polycrystalline OSC thin film with a Si p-n junction to absorb photons in large excess of the Si bandgap. These photons will then have newly available pathways for creating two triplet excitations that may be coupled into Si across the OSC:Si buried interface, circumventing losses that would otherwise arise if absorbed directly by Si.

Realizing net efficiency increases with this strategy will however require commercially viable, high-yield SF capable OSCs and well-designed OSC:Si interfaces for effective triplet excitation energy transfer into Si. The respective mechanisms through which these processes proceed is still either hotly debated or entirely unknown. Though, their proposed electron/hole transfer nature suggest high sensitivity to spatial overlap, phase, and energetics between participating states. As such, the myself and Roberts group as a whole are interested in exploring structure-function design of SF in photostable polycrystalline OSC thin films with time-resolved electronic and vibrational spectroscopic techniques. Additionally, we utilize electronic sum-frequency generation (ESFG) to explore structure-function relationships governing triplet excitation transfer into Si across buried OSC:Si interfaces.