Electronically asymmetric poly(1,4-phenylenevinylene)s for photovoltaic cells

We report a study on the feasibility of using an intramolecular dipole to enhance charge separation for organic photovoltaic applications. We have developed a poly(1,4-phenylenevinylene) derivative, poly{2-[(9,9-di-n-propyl-9H-7-nitrofluoren-2-yl)]-5-(2-ethylhexyloxy)-1,4-phenylenevinylene} 12, which has electronically asymmetric chromophores as a result of the para attachment of alkoxy and nitrofluorenyl groups on the phenyl units of the polymer backbone. The luminescence of the polymer with the electronically asymmetric chromophores was found to be smaller by a factor of eight when compared to the equivalent polymer but without the nitro group attached to the fluorenyl unit. Light-induced electron spin resonance experiments showed that the polymer with the electronically asymmetric chromophores had more photo-induced spins, showing that the quenching of the luminescence was due to intramolecular charge separation. Single layer photovoltaic devices containing neat films of 12 showed efficiencies similar to other single layer devices and this was ascribed to poor transport of the separated charges. This was confirmed by a strong improvement in device power efficiency to 0.52% at AM1.5 by blending the polymer with an electron transporting material.