Narrow-bandgap diketo-pyrrolo-pyrrole polymer solar cells : the effect of processing on the performance

A study has reported on the application of a novel narrow bandgap polymer, poly[3,6-bis-(4'-dodecyl-[2,2']bithiophenyl-5-yl)-2,5-bis-(2-ethyl-hexyl)-2, 5-dihydropyrrolo[3,4-]pyrrole-1,4-dione] (pBBTDPP2) in photovoltaic cells. The conjugated polymer is based on alternating quaterthiophene (BBT) and diketopyrrolo-pyrrole (DPP) units with a bandgap of1.4eV, which is close to the optimum for single, bandgap polymer solar cells. After careful optimization of the processing parameters and morphology, the donor polymer in combination with C60 and C70 PCBM derivatives affords external quantum efficiencies of ~0.4 over a broad spectral range and power conversion efficiencies of 3.2 and 4.0%, respectively under simulated AM1.5G solar light conditions. Significant molecular weights of Mn=20000 and M w=67000g/mol (DPI)=3.35) also were obtained, as determined by gel permeation chromatography (GPC) using 1,2,4-trichlorobenzene as eluen