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

Ammonium heptamolybdate (NH<sub>4</sub>)<sub>6</sub>Mo<sub>7</sub>O<sub>24</sub>·4H<sub>2</sub>O (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 C<sub>61</sub>-butyric acid methyl ester­(P3HT/PC<sub>60</sub>BM), poly­[<i>N</i>-9′-heptadecanyl-2,7-carbazole-<i>alt</i>-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)]/[6,6]-phenyl C<sub>70</sub>-butyric acid methyl ester­(PCDTBT/PC<sub>70</sub>BM) and evaporated small molecule chloro­(subphthalocyaninato)­boron­(III) (SubPc)/C<sub>60</sub>. 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 (<i>J</i><sub>sc</sub>) than the ones with evaporated molybdenum oxide­(eMoO<sub>3</sub>), and thus better power conversion efficiency (PCE). In addition, P3HT/PC<sub>60</sub>BM devices with AHM based HTLs show air stability comparable to those with eMoO<sub>3</sub>, and much better than the ones with PEDOT:PSS.