A morphology control layer of a pyrene dimer enhances the efficiency in small molecule organic photovoltaic cells

Abstract

Improved efficiency in organic photovoltaic (OPV) devices induced by a morphology control donor layer as an anode buffer layer (ABL) was achieved. We synthesized the pyrene dimer 1,1′-dipyrene (1DPy) and applied it in vacuum-evaporated small-molecule-type OPV devices based on pentacene. N,N′-Di(naphthalen-1-yl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine (NPD) was used for comparison in the ABLs. While NPD led to a decreased fill factor, the insertion of a 1DPy layer with only 1 nm thickness improved the power conversion efficiency (PCE) 1.5-fold compared with that of the device without 1DPy. Moreover, 1 nm thick 1DPy layers were also incorporated into devices with different donor materials: copper-phthalocyanine and tetraphenyldibenzoperiflanthene. All of the devices showed enhanced short-circuit current densities, which resulted in improvements in the power conversion efficiency. The 1DPy layer prevented hole accumulation owing to its island-like grain structure. The morphologies of the donor layers on top of the 1DPy layers were characterized using atomic force microscopy images and X-ray diffraction patterns. The roughness of the pentacene layer was decreased from 4.41 nm to 2.15 nm when a 1DPy layer was used, and the pentacene layer on 1DPy showed a tilted crystal phase. The tilted pentacene orientation may facilitate perpendicular carrier transport or exciton diffusion through an increase in molecular orbital overlap. 1DPy inserted as an ABL can be useful in controlling the morphology of the donor layer, thereby resulting in an improvement in the efficiency of the device.