We present the interface characterization of vacuum-deposited metal nanoparticle recombination layers (Ag, Au; 1 nm equivalent thickness) at donor/acceptor heterojunctions comprising copper phthalocyanine (CuPc) and C60 as model interfaces for tandem planar heterojunction organic photovoltaics (TOPVs). We compare the extent to which voltage doubling occurs using these two metal recombination contacts (RC) in TOPVs (ITO/CuPc/C 60/(Ag,Au)/CuPc/C60/BCP/Al) and correlate the differences with energetic dissimilarities revealed by UV-photoemission (UPS) and inverse photoemission (IPES) spectroscopies, and morphology as revealed by atomic force microscopy (AFM) and field-emission scanning electron microscopy (FE-SEM). Ag interlayer RCs produce the expected voltage doubling in the open-circuit voltage (VOC) for the TOPV, whereas Au RCs showed poor voltage addition. Significant shifts in ionization potential and electron affinity and shifts in local work function were observed for C60/metal heterojunctions and for heterojunctions based on C60/metal/C60 and for C 60/metal/CuPc, with clear evidence for partial charge redistribution between C60 and Ag nanoparticles. AFM and FE-SEM images revealed discrete Ag nanoparticles at the C60 interface, whereas Au/C 60 heterojunctions consisted of more uniform Au thin films that wet the C60 surface and penetrated below the surface. These studies point to the need for careful control of both electronic and morphological properties of thin RCs in emerging tandem organic solar cell technologies.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films