Electronically Coupled Photon Upconversion Solar Cells via Molecular Self-Assembled Bilayers

Photon upconversion (UC) is a unique process that has garnered increased interest in the past decade as a way to harness lower energy photons and convert them to higher energy for use in a host of applications. In molecular UC, a sensitizer molecule absorbs low energy light and undergoes intersystem crossing to generate a triplet excited state. Subsequent sensitizer-to-acceptor triplet energy transfer (TET) yields acceptor molecules in the triplet excited state. Two triplet acceptor molecules in proximity can then undergo triplet−triplet annihilation (TTA), generating one excited singlet acceptor and one ground state acceptor. Emission from the resulting singlet excited state is hypsochromically shifted relative to the excitation light and thus the photon energy is upconverted during the process. Application of molecular TTA-UC into solar cell devices, where the conversion of photons of energy below the bandgap of typical absorbing materials to those of higher energy could yield higher efficiencies, is theoretically predicted to circumvent the theoretical efficiency limit of a standard solar cell device from ~33% to more than ~43%. One approach is the use of self-assembled bilayers of sensitizer and acceptor molecules electronically coupled into a dye-sensitized solar cell (DSSC) type device to facilitate TTA-UC and direct charge separation at the organic-inorganic hybrid interface of the self-assembled bilayer and the mesoporous nanoparticle semiconductor substrate. This dissertation explores the assembly of these bilayer structures onto films of zirconium dioxide (ZrO2) and titanium dioxide (TiO2) mesoporous nanoparticle substrates in order to study the enablement of photon upconverted emission and photocurrent generation, respectively. Herein it is experimentally outlined how self-assembled bilayers can be formed via a simple, stepwise soaking procedure to afford films with surface bound acceptors followed by linking of sensitizers to acceptors via metal ion coordination. These films are shown to facilitate TTA-UC on ZrO2 as well as allow direct charge separation of the upconverted singlet state from acceptor molecules on TiO2, even under solar intensity. These self-assembled bilayer films are compared to alternative solution based systems to show they are not limited by some of the common loss pathways. Finally, the role of the redox mediator in TTA-UC solar cell devices is explored in order to understand the impact of adding an additional component relative to the TTA-UC system and how they affect excited state species. Commentary on problems faced in the system will be given along with insights into optimizing key processes to maximize efficiency.