Improved performance and stability of inverted polymer solar cells with ammonium heptamolybdate acted as hole extraction layers via thermal annealing method

High-performance Inverted polymer solar cells (PSC) were fabricated with ammonium heptamolybdate (NH4)6Mo7O24−4H2O as hole extraction layer (HEL), and the blend of the poly(3-hexylthiophene) (P3HT):indene-C60 bisadduct (ICBA) as photoactive layer, using the structure of ITO/ZnO/P3HT:ICBA/(NH4)6Mo7O24/Ag. Correspondingly, the influence of different annealing time of ammonium heptamolybdate on carrier mobility and thus device performance were investigated. The power conversion efficiency (PCE) has been optimized from 4.12% to 6.24% after thermal annealing treatment of (NH4)6Mo7O24 layer at 130 °C for 15 min. The results of ultraviolet photoelectron spectroscopy (UPS) curve clearly proven the increase of conduction-band minimum (CBM) and valence-band maximum (VBM) energy level of (NH4)6Mo7O24 layer upon thermal annealing, which is caused by the decrease of barrier potential between HOMO of P3HT and HEL. This obviously facilitate hole transport hole transport between HOMO levels of P3HT and HEL block electron transfer between LUMO levels of P3HT and Ag electrode. The space charge-limited current (SCLC) test has further confirmed the enhanced hole collection and transport capability of (NH4)6Mo7O24 after thermal annealing treatment.