Thionated Squaraine Molecules as Heavy-Atom-Free Sensitizers in NIR Upconversion Systems

Abstract

Solar energy conversion has the largest energy potential of all accessible energy resources. However, it is important to consider that the extractable amount of power from a solar cell is limited by the maximum cell efficiency. According to the detail-balance limit outlined by Shockley and Queisser, the maximum theoretical power conversion efficiency of a conventional solar cell is 33%, largely due to spectral mismatch between the solar spectrum and the cell absorption profile. One strategy for mitigating the loss of subgap photons is photon upconversion, which is a photophysical process that converts photons of lower energy to photons of higher energy in a 2-to-1 efficiency ratio. Triplet-triplet annihilation photon upconversion is a mechanism of upconversion that shows great promise due to its low incident power density requirements and highly tunable excitation and emission wavelengths. However, the triplet sensitizers used in the TTA-UC scheme typically involve precious metal complexes to induce the heavy atom effect for efficient intersystem crossing, which provides a costly scalability barrier to widespread adoption of these systems. Work enclosed in this dissertation outlines a novel heavy-atom-free TTA-UC sensitizer in the form of a thionated squaraine molecule. Importantly, the thionation of the squaraine inverts the lowest energy singlet states of the molecule, opening a channel for intersystem crossing as expected via El-Sayed’s rule. Spectroscopic characterization of the thiosquaraine triplet reveals intersystem crossing in the thionated molecule within 4.5 ps of excitation, resulting triplet lifetime of 20 µs. This lifetime is sufficiently long for the necessary triplet-triplet energy transfer process to occur. A proof-of-concept upconversion system using the thiosquaraine as sensitizer and rubrene as the emitter molecule demonstrated successful upconversion with a quenching constant on the order of the diffusion rate of molecules in solution. Further exploration of the thiosquaraine sensitizer revealed that it is able to convert deep red excitation to upconverted emission across the visible range. The highest peak-to-peak emission margin achieved with the thiosquaraine sensitizer is 0.8 eV, a record anti-Stokes shift for a heavy-atom-free system. The squaraine platform provides a tunable template for additional heavy-atom-free sensitizers with absorption in the application-relevant near-infrared regime, and we explore the trade-offs between lowering the energy gap and the resulting triplet yield with a first-principles analysis.