Energy level alignment and morphology of Ag and Au nanoparticle recombination contacts in tandem planar heterojunction solar cells

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 (V_OC) 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.