High-performance polymer solar cells with a conjugated zwitterion by solution processing or thermal deposition as the electron-collection interlayer

We report highly efficient polymer solar cells (PSCs) with rhodamine 101, a conjugated zwitterion with positive and negative charges on the same molecule, as the electron-collection interlayer. Rhodamine 101 can be processed by solution processing techniques or thermal deposition. The rhodamine 101 interlayer simultaneously improves the short-circuit current, open-circuit voltage, fill factor so as to improve the photovoltaic efficiency of the PSCs with poly[[9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl] (PCDTBT) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as the active materials in comparison with control PSCs with Al or Ca/Al as the cathode. The photovoltaic efficiency reaches 6.15% for the PSCs with rhodamine 101/Al as the cathode under AM1.5G illumination, whereas the efficiencies are only 3.81% and 4.57% for the control PSCs with Al and Ca/Al as the cathodes, respectively. On the other hand, the improvement on the photovoltaic performance by the rhodamine 101 interlayer is less remarkable for the PSCs with poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) or PC71BM as the active materials. The photovoltaic efficiency is 4.25% for the PSCs with rhodamine 101/Al as the cathode, almost the same as that (4.20%) of the control PSCs with Ca/Al as the cathode. The high efficiency of the PSCs with the rhodamine 101 interlayer is ascribed to the lowering of the work function of metals by rhodamine 101 that has a strong dipole moment. The different effects of the rhodamine 101 interlayer on the two PSCs with PCDTBT and P3HT as the donor materials are attributed to different reactivities of the two polymers with active metals like Ca and Al. PCDTBT consisting of both electron-donating and electron-withdrawing units is more reactive with the active metals. The reaction between PCDTBT and Ca leads to low photovoltaic efficiency of the PSCs with Ca/Al as the cathode.