Low band-gap donor-acceptor polymers and heterolyptic ruthenium complex containing polymers for photovoltaic applications

Abstract

A series of low band-gap conjugated polymers with intramolecular charge

transfer properties were synthesized and bulk heterojunction devices based on

these polymers were fabricated. The electrochemical and photophysical properties

of the polymers were tuned by using different electron withdrawing molecules or

ruthenium complexes as the comonomer. Preliminary results suggested that the

electronic structures of the polymer were significantly altered by the incorporation

of different acceptor units. The polymers also demonstrated intense absorption

bands in the visible region, indicating that they are suitable photoactive materials

in bulk heterojunction devices.

The synthesis and characterization of a series of organic donor-acceptor

copolymers were studied. All of the polymers contained alternating

cyclopenta[2,1- b:3,4-b’]dithiophene (CPDT) units. The effects of the different

acceptor monomers were evaluated. The electron-withdrawing carboxylate and

amide functional groups of the acceptors were found to effectively stabilize the

HOMO levels of the polymer, and the optical band-gaps were significantly

reduced. Bulk heterojunction devices were fabricated using the polymers and

6,6-phenyl C61 butyric acid methyl ester (PCBM) as donors and acceptors

respectively. These devices exhibited high open circuit voltage (Voc) up to 0.86 V.

The extended photosensitizing range was confirmed by the external quantum

efficiency (EQE) spectra. The device performance was further improved by

optimizing the active layer thickness and applying 1,8-diiodooctane in the blend

solution.

A new synthetic route to novel ruthenium containing polymers was also

reported. [Ru(L)(L’)Cl2] complexes (L and L’ = bidentate N^N ligands) with a

dibromo-substituted ligand were found to be polymerizable by Stille cross-coupling

reaction. The subsequent displacement of the chloride ligands by

thiocyanate was highly effective and the structures of the target polymers were

fully characterized. The main chain absorption showed a significant red-shift upon

metal coordination and the metal-to-ligand charge transfer (MLCT) of the

complex strengthens the photon harvesting ability of the polymer. The dual

function of these Ru(II) complexes demonstrated a new avenue to develop new

classes of optoelectronic materials. The extent of _-delocalization of the ancillary

ligands also showed interesting effects on the electronic properties of the

polymers. Bulk heterojunction devices were fabricated. Photovoltaic response was

observed in these devices, and the device performance can be improved by further

modifying the surface morphology of the blend films.