Organic Solar Cell materials are conjugated polymers
Charge Transfer States
Charge transfer (CT) states occur when an electron moves from and electron-rich molecular motif (donor) to an electron-deficient molecular motif (acceptor) in an excited state. This inherently unstable excited state must be controlled to lead to successful operation of organic semiconductor-based devices such as solar cells and LEDs.
Why are charge transfer states important in organic semiconductor based devices?
Charge transfer (CT) states are crucial to interconversion of charges and light in all organic optoelectronics. The CT state energy sets the maximum achievable voltage for organic solar cells and any nonradiative losses from these states further drops the maximum achievable efficiency. Organic light emitting diodes (OLED), which have already achieved widespread adoption in displays, invert solar cell operation to generate light from injected charges. Unlike organic solar cells where the CT state is an intermolecular state at donor-acceptor interfaces, current champion OLEDs feature an intramolecular CT state with a covalently bonded donor and acceptor. The OLED CT state is the emissive state itself, with any nonradiative losses directly impacting overall radiative efficiency. For inorganic solar cells such as GaAs, it is well established that a ‘good’ solar cell is also a ‘good’ LED from the reciprocity relationship in the detailed balance thermodynamic treatment of solar cell efficiency. We will seek to understand and control manmade CT state properties in both organic LEDs and solar cells to provide concerted design guidelines towards reducing nonradiative losses across all organic optoelectronics.
Tuning organic semiconductor charge transfer states with solid-state solvation
Using time-resolved photoluminescence techniques we will probe the mechanism of solid state solvation and it’s impact on current state-of-the-art OLED emitter and model dyes. Solid state solvation is the solid-state analog of solvatochromism where polar solute molecules can stabilize polar CT excited states, thereby red-shifting their emission.
This project will involve creating samples using both spin-coating and drop-casting techniques, measuring these samples with our time-resolved fluorimeter, and analyzing that data with python scripts. We will also model the properties of solid-state solvation active dopant molecules to understand their ability to move around within films.