Dual Functionality of BODIPY Chromophore in Porphyrin-Sensitized Nanocrystalline Solar Cells

A new organic dye (BET) was synthesized and coadsorbed on TiO2 nanoparticles to make mixed BET/porphyrirf-sensitized solar cells (DSCs). The BET is a boron dipyrromethene compound with one benzoic acid group attached to the meso position for its binding to the TiO2 nanoparticles and two ethyl groups in the 3 and 3' positions of pyrrolic units to broaden its absorption. Two ethyl groups are in the cis position, as revealed by its single-crystal X-ray diffraction analysis. The BET exhibits strong absorption in the green light region with an absorption maximum at 528 nm in CH2Cl2, which is complementary to the absorption spectrum of porphyrin dyes. When the BET coadsorbs on the TiO2 nanoparticles with porphyrin dyes (TMPZn and LD12), the power conversion efficiencies increase from 1.09% to 2.90% for TMPZn-sensitized solar cells and from 6.65% to 7.60% for LD12-sensitized solar cells, respectively. The IPCE of the devices in the green light region increases dramatically due to the cosensitizing effect of BET. The fluorescence of BET in solution is partially quenched and that of porphyrin is enhanced in the presence of BET dye, indicating an intermolecular energy transfer from BET to the porphyrin dyes. The direct electron injection from BET to the TiO2 conduction band was rather poor; only negligible photocurrent was observed. Comparative studies of absorption spectra on the TiO2 nanoparticle films and electrochemical impedance at the dye/TiO2 interface also indicate that the BET is an excellent coadsorber to prevent the aggregation of porphyrin dyes. An intermolecular energy transfer model is proposed to account for the observed photovoltaic enhancement of the cosensitization system.