The work reported here explores the impact of polymer morphology on the physics and performance of perylene benzimidazole/poly(3-hexylthiophene) bilayer photovoltaic devices. By varying both the annealing temperature and the solvent used for polymer deposition, we demonstrate control of the polymer chain morphology. An increase in the relative ordering of the polymer chain conformation is observed through a shift in the absorption onset and absorption spectral shape, and results in improved photovoltaic performance.
We have explored the use of polymer / small molecule organic composites in the form of a polymer / perylene diimide heterojunction bilayer in order to combine the advantageous properties of both materials. Using the electron transporting perylene benzimidazole (PBI) and the hole conducting polymer poly[2,5-dimethoxy-1,4-phenylene-1,2-ethenylene-2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene-1,2-ethenylene (M3EH-PPV), we have achieved increased power conversion efficiencies for a planar device of up to 0.71% under 80 mW/cm2 white illumination. By varying the order of the photoactive layers, we have probed the mechanisms creating the photovoltage and found that the photovoltage is not determined by the difference in electrode work functions alone. In addition to the internal field, the interfacial chemical potential gradient, caused by exciton dissociation at the polymer / perylene diimide interface, appears to contribute to the photovoltage. We also discuss why, contrary to some expectations, the polymer / perylene diimide devices are more efficient than the analogous pure small molecule perylene diimide / phthalocyanine cells.
The main factors inhibiting higher conversion efficiencies in plain polymer layer sandwich photovoltaic devices are the low exciton dissociation efficiency and the low carrier mobilities in the polymer. We consider two different blend approaches for increasing these qualities. NiO (or LiNiO) hole transporting nanoparticles are blended into the photoactive polymer MEH-DOO-PPV in an attempt to increase hole mobility across the device. Improvements to device performance were not significant at these blend concentrations. Devices made using blends of hole and electron transporting polymers M3EH-PPV and CN-ether-PPV showed increased dissociation efficiency and gave power conversion efficiencies of up to 0.6% with stable electrodes.
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