Ultrathin Ammonium Heptamolybdate Films as Efficient Room-Temperature Hole Transport Layers for Organic Solar Cells

Ammonium heptamolybdate (NH4)6Mo7O24·4H2O (AHM) and its peroxo derivatives are analyzed as solution-processed room temperature hole transport layer (HTL) in organic solar cells. Such AHM based HTLs are investigated in devices with three different types of active layers, i.e., solution-processed poly­(3-hexylthiophene)/[6,6]-phenyl C61-butyric acid methyl ester­(P3HT/PC60BM), poly­[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)]/[6,6]-phenyl C70-butyric acid methyl ester­(PCDTBT/PC70BM) and evaporated small molecule chloro­(subphthalocyaninato)­boron­(III) (SubPc)/C60. By virtue of their high work functions, AHM based HTLs outperform the commonly used poly­(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) HTL for devices employing deep HOMO level active materials. Moreover, devices using AHM based HTLs can achieve higher short circuit current (Jsc) than the ones with evaporated molybdenum oxide­(eMoO3), and thus better power conversion efficiency (PCE). In addition, P3HT/PC60BM devices with AHM based HTLs show air stability comparable to those with eMoO3, and much better than the ones with PEDOT:PSS.