Zwitterionic ammonium and neutral amino molecules as cathode interlayer for inverted polymer solar cells

Cathode interlayer is essential to inverted bulk heterojunction polymer solar cells (PSCs). A series of zwitterionic ammonium and neutral amino organic molecules are introduced into inverted PSCs as cathode interlayer and power conversion efficiency (PCE) as high as 8.07% is demonstrated. Compared to the devices without interlayer, all the devices exhibit significant improvements of the device parameters by reducing the work function of indium tin oxide (ITO) cathode. It is striking that the devices with neutral amino molecules as interlayer exhibit remarkably higher PCEs than the devices with zwitterionic ammonium molecules as interlayer. We attribute the improved performance to the better photoactive morphology induced by the hydrophobic properties of the neutral amino derivatives through research of ultraviolet photoelectron spectroscopy, atomic force microscopy, and contact angle measurements. Interestingly, the PCEs of the inverted PSCs with cathode interlayer are positively correlated with the hydrophobic properties of the interlayer materials, since devices with neutral amino molecules or molecules with a more hydrophobic alkyl pendant (piperidine) as interlayer exhibit higher PCEs. These results pave the way to the design of effective cathode interlayer materials.