Efficient planar heterojunction based on α-sexithiophene/fullerene through the use of MoO3/CuI anode buffer layer

Abstract

In this work, planar heterojunction organic photovoltaic cells (PHJ-OPVs) based on alpha-sexithiophene (α-6T) and fullerene active materials were investigated. We have studied the effect of the hole transporting layer (HTL) on the performances of the PHJ-OPVs. The studied HTLs are molybdenum trioxide (MoO3), cuprous iodide CuI and MoO3 coupled with CuI (MoO3/CuI). The PHJ-OPVs were fabricated using the thermal evaporation/sublimation technique. The experimental study shows that double HTL MoO3/CuI, using 1.5 nm of CuI thickness, improves significantly the efficiency of the PHJ-OPVs compared with those with single HTLs. This enhancement is due to the fact that MoO3 contributes to the optimization of the holes collection and CuI causes significant influence on the nucleation and the growth of α-6T. CuI has led to spectacular modification of the structural properties of α-6T. When α-6T is deposited onto ITO/MoO3, the α-6T layers are amorphous, while they are crystallized into monoclinic crystalline structure with a laying down molecular orientation when they are deposited onto ITO/MoO3/CuI. The use of the double HTL and the optimization of the α-6T deposition parameters have led to an important improvement in the α-6T light absorption, roughness and the holes mobility. These findings participated in the significant enhancement of the PHJ-OPVs performances and their life time.