Small molecule tandem organic photovoltaic cells incorporating an α-NPD optical spacer layer

We report an improvement in power conversion efficiency in a small molecule tandem organic photovoltaic (OPV) device by the optimisation of current balancing of the sub-cells using an optical spacer layer. A co-deposited layer of N,N’-bis(1-naphthyl)-N,N′-diphenyl-1,1’-biphenyl-4,4’-diamine (α-NPD) and molybdenum oxide was used as the optical spacer layer and provided a highly transparent and conductive layer. Optical simulations showed the addition of the optical spacer in a boron subphthalocyanine (SubPc)/C60 based tandem OPV device increased the SubPc absorption in the front sub-cell and resulted in current balancing through the device. Fabricated tandem OPV devices showed similar trends, with the power conversion efficiency increasing from 2.3% to 4.2% with the addition of an optimised optical spacer thickness. External quantum efficiency and total absorption efficiency measurements back up the optical model data which attribute the increased performance to improved SubPc absorption in the front sub-cell, balancing the photocurrents of the two sub-cells.