Organic solar cells are thin films that usually contain a blend of (photoexcited) electron donor and acceptor small molecules and/or polymers. One of the species absorbs light, which is followed by rapid electron transfer to generate an electron and hole (absence of electron) pair across a donor/acceptor interface. These charges then drift away from the interface and eventually find their way to metal electrodes; once charges leaves the organic layer, a photocurrent is generated in the external circuit.
In a partially disordered donor/acceptor blend, charges moving through the organic photoactive layer must traverse a tortuous pathway riddled with energetic and topological traps. When the blend thin film is first formed, nonequilibrium thermodynamics drives the formation of phase-separated pure and partially mixed phases. This leads to the development of charge transport pathways making up the complex transport network that percolates through the bulk of the thin film. How does the molecular structure of the components dictate the quality and microstructure of the charge transport network? Can we predict how the nonequilibrium microstructure will evolve without first synthesizing the molecules and using a frustratingly long trial-and-error approach of making numerous solar cells with a plethora of variations in processing conditions? These are some of the questions that we are currently working to address. To do this, we combine temperature-dependent charge transport measurements with structural information obtained via synchrotron X-ray scattering.