Influence of electrode surface composition and energetics on small-molecule organic solar cell performance: Polar versus nonpolar donors on indium tin oxide contacts

We present a systematic study of the band-edge energy offsets and shifts in local vacuum levels for indium tin oxide (ITO)/donor heterojunctions, using vacuum-deposited chloroindium phthalocyanine (ClInPc), titanyl phthalocyanine (TiOPc), pentacene (PEN), and copper phthalocyanine (CuPc) donor layers. We include a comparison of the performance of ITO/donor/C₆₀ planar heterojunction (PHJ) organic solar cells (OPVs) as a function of activation and modification of the ITO contact. UV photoemission spectroscopy (UPS) was used to characterize the interfacial region between these donors and ITO to infer the ordering of these first deposited 1-2 monolayers as a function of ITO activation and modification. For the polar donors ClInPc and TiOPc, deposited on air plasma (AP) treated, high work function ITO (φ_eff ≈ 5.1-5.2 eV), shifts in local vacuum level observed during deposition of the first two monolayers of donor molecules suggest that the halo-metal or oxo-metal bond points (on average) toward the oxide surface in the first monolayer. Inversion of this orientation is inferred during formation of the second monolayer. Thin-film absorbance data for the Q-band spectra of ClInPc or TiOPc 1-2 monolayers on AP-ITO confirmed the organization of the first deposited layers. Use of nonactivated ITO contacts, or contacts modified with pentafluorobenzylphosphonic acid (F₅BPA), disrupts this order in the first two monolayers as revealed in the UPS and absorbance data, leading to decreases in OPV open-circuit photopotential (V_OC), fill factor (FF), and efficiency (η). For nonpolar donors like PEN and CuPc, shifts in local vacuum level as a function of donor coverage were less sensitive to ITO surface composition, and PEN-based OPVs show relatively invariant V_OC, J _SC, FF, and η values regardless of ITO pretreatment or F ₅BPA modification.The interaction of ClInPc donor layers with AP-ITO is strong enough to generate photovoltaic activity in the absence of a C ₆₀ acceptor layer (ITO/ClInPc/Al device configurations), where photocurrent appears to be generated at the ITO/ClInPc interface. Such an effect is not seen for ITO/ClInPc/Al devices on nonactivated ITO substrates, highlighting the importance of the interactions between the donor layer and the hole-harvesting interface in PHJ OPVs. These results suggest that the organization of a dipolar donor layer at a hole-harvesting oxide contact can affect OPV performance where charge harvesting efficiency is sensitive to molecular organization in the interfacial region near the contact.