Joana Farinhas

Enhanced Efficiency of PTB7 : PC61BM Organic Solar Cells by Adding a Low Efficient Polymer Donor

Ternary blend polymer solar cells combining two electron-donor polymers, poly[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl] (PTB7) and poly[2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene] (pBTTT) and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM), as electron-acceptor, were fabricated. The power conversion efficiency of the ternary cells was enhanced by 18%, with respect to the reference binary cells, for a blend composition with 25% (wt%) of pBTTT in the polymers content. The optimized device performance was related to the blend morphology, nonrevealing pBTTT aggregates, and improved charge extraction within the device.

Improved stability of organic solar cells by cross-linking of the electron-donor polymer

We fabricated organic photovoltaic cells with enhanced performance stability measured after temperature annealing at 100 °C and over a period of 24 days at room temperature. This improvement was achieved upon chemical cross-linking of the photo-active layer of the devices, combining a cross-linkable polymer and a fullerene, PC61BM. The cells efficiency decayed by up to 14 % upon heating at 100 °C and by 37 % after the 24 days period, while the efficiency of control devices based on noncross-linked layers was decreased by 58 % and 55 %, respectively.

Organic photovoltaic cells with structured interfaces: Columnar-grain active layers made of cross-linked semiconducting polymers

We report on organic photovoltaic cells (OPVs) with columnar-grain active layers consisting in cross-linked semiconducting (conjugated) polymers as electron-donors and PCBM as the electron-acceptor and compare with OPVs based either on blends or on planar bilayers made with the same components. Although the columnar-grain layers have morphologic characteristics still far from ≪optimal≫ for PV operation, we found a significant improvement in the cell performances relative to planar bilayer architectures whereas cells with blends exhibited the highest efficiencies. Differences in cell performances with analogous layer architectures using two different polymers were attributed to the distinct charge mobility of the polymers. In view of the several benefits posed by the use of cross-linkable conjugated polymers in OPVs, ranging from higher morphologic stability to facilitated processing, our results provide insights for rational approaches when employing such materials.