Abstract
A series of novel electron transport (ET) polymers composed of different conjugated main chains (fluorene, thiophene, and 2,7-carbazole) and crown ether side chain (crown ether, aza-crown ether and amine) is presented for bulk-heterojunction polymer solar cells with poly(3-hexylthiophene) (P3HT) or 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) as the active polymer and aluminum metal as the cathode. Unexpectedly, it is found that the main chain of ET polymers has a greater effect on the interfacial dipole than the side chain, even when attaching a high polarity group. The electron-rich bridge atom of the main chain may also contribute appreciably to the interfacial dipole. When used as the ET layer, all of these polymers can generate an optical interference effect for redistribution of the optical electric field as an optical spacer and, therefore, allow more light to be absorbed by the active layer, thus leading to an increase in short-circuit current density. They can also block hole diffusion to the cathode and prevent electron-hole recombination during the ET process. Among the five ET polymers investigated, PCCn6 is the most effective one, providing a remarkable improvement in the power conversion efficiency (measured in air) of the device to 8.13% compared to 5.20% for PTB7:[6,6]-phenyl-C 71 -butyric acid methyl ester (PC 71 BM).