Highly-tunable nickel cobalt oxide as a low-temperature P-type contact in organic photovoltaic devices

We report on the investigation of nickel cobalt oxide (Ni _xCo_3-xO₄ ) thin fi lms grown by pulsed laser deposition as hole-transport interlayers (HTL) in organic photovoltaic (OPV) devices. Films of 7 nm thickness were grown under various oxygen deposition pressures (pO₂ ) in the range of 2-200 mTorr. We explore both bulk and surface properties of these thin fi lms. The workfunction ( φ ) for each of the fi lms was statistically similar ( ~ 4.7 eV), regardless of pO ₂ . There was not a strong dependence of the power conversion effi ciency ( η ) on the conductivities of the HTLs varying between 0.009 -10 S/cm. The observed differences in OPV effi ciencies (ranging from 1.16 tO ₂.46%) were correlated to the near surface chemical composition of the Ni_xCo_3-xO₄ HTL, as observed by differences in the relative surface hydroxyl concentration. The critical role of the near-surface composition of the HTL at the HTL/organic interface was further explored by modifying the hydroxyl concentration using an oxygen plasma treatment. This treatment mitigated the impact of surface hydroxyl coverage, demonstrating either identical or increased values for phi; and η , regardless of initial pO₂ in the creation of the Ni _xCo_3-xO₄ HTL. To further explore this we also employed a phosphonic acid surface modifi cation agent on the HTL, increasing φ to 5.2 eV producing the best ? value of 3.4%, equivalent to the PEDOT:PSS control devices. These results indicate that nickel cobalt oxide is a promising p-type oxide for carrier-selective interlayers in organic solar cells; however, for this to be fully realized the specifi c surface chemistry at the oxide/polymer interface must be controlled to increase φ and optimize device performance.