Photovoltaics

The Schlenker group aims to develop a better fundamental understanding of energy materials for informed materials design.

Excited state carrier dynamics in perovskite solar cells

Transient absorption spectroscopy has been used to elucidate many of the excited state properties in hybrid organic metal halide perovskite films including the nature of photoexcited carriers, charge mobilities, and interfacial charge transfer kinetics. Numerous studies have been performed to characterize spectral signatures in the visible range, but relatively few studies have focused on the nIR region.

We have observed that by utilizing alternating current spectroelectrochemistry, oxidizing and reducing conditions result in spectra which we posit correlates to charge carriers relevant to photocurrent under device operating conditions.

Heavy-atom-free Upconversion for photovoltaic applications

Photon upconversion is a photophysical process that generates high energy photons through the absorption of low energy light. In single-junction photovoltaics, upconversion could be use as an effective way to exceed the Shockley-Queissler limit, as it provides a means to utilize sub-bandgap photons. In TTA-UC, the wavelength of excitation and emission can be easily tuned through selection of the appropriate sensitizer/emitter combination, and upconversion can be achieved without the need for coherent light or high excitation densities.

The Schlenker group has developed a metal- and heavy atom-free TTA-UC system, utilizing a thionated squaraine dye as the triplet sensitizer. Importantly, the inclusion of sulfur in the squaraine core introduces non-bonding orbitals that reorder the energy levels in the singlet landscape, opening a channel for intersystem crossing without reliance on the heavy atom effect.

Kinetics of Charge Transfer in OPV Materials

In the past, the Schlenker group has explored charge transfer in organic photovoltaics. Using transient absorption, time-resolved photoluminescence, and device measurements, it was shown that fullerene aggregation in small-molecule organic photovoltaic blends correlates with photocurrent enhancement due to kinetically avoided recombination to thermodynamically favored triplet states. The electron donor chloroboron subphthalocyanine (SubPc) blended with a C60 fullerene electron acceptor shows photocurrent generation that nearly doubles for SubPc:C60 blends with a higher C60 ratio (1:2 versus 1:1) and enhanced fullerene aggregation. Our spectroscopic results suggest that aggregation at the higher C60 loading ratio aids in sustaining the free charge population by inhibiting recombination to form SubPc triplets. Our results provide new insight into the role that aggregation plays in promoting charge separation and photocurrent collection in small-molecule organic photovoltaics. Our findings suggest new avenues for improving device performance by kinetically avoiding recombination to triplet states, despite the presence of multiple thermodynamically accessible pathways for triplet formation in these blended films.