  The active layer microstructure of organic solar cells is critical to
efficiency. By studying the photovoltaic properties of organic solar cell's
microstructure, it is possible to increase the efficiency of the solar cell. A
graph-based microstructure model was employed to approximate the efficiency,
measured as short circuit current, of a solar cell given a microstructure.
Through probabilistic graph-based optimization, a class of microstructures were
found with an efficiency surpassing that of more conventional morphologies.
These optimized solar cells surpass the efficiency of more conventional
photovoltaic devices as they better facilitate charge transport, generation,
and dissociation. A device was designed with a 40.29% increase in short circuit
current from the solar cells with the currently believed optimal morphology.
The designed morphologies feature two dendritic clusters of the donor material
poly(3-hexylthiophene-2,5-diyl) (P3HT) and the acceptor material
phenyl-C61-Butyric-Acid-Methyl Ester (PCBM). The designed microstructure's
increase in performance contrasts with more conventional structures featuring
interdigitated or bilayer strands of P3HT and PCBM. The change of
microstructure morphology through graph-based evolution obtains an organic
solar cell with an efficiency significantly greater than conventional organic
solar cells, proves the validity of graph-based microstructure models for
simulation in materials science, and advances the vision of an inexpensive,
efficient form of renewable energy.
