Electrical Property Heterogeneity at Transparent Conductive Oxide/Organic Semiconductor Interfaces: Mapping Contact Ohmicity Using Conducting-Tip Atomic Force Microscopy

Abstract

We demonstrate mapping of electrical properties of heterojunctions of a molecular semiconductor (copper phthalocyanine, CuPc) and a transparent conducting oxide (indium-tin oxide, ITO), on 20-500 nm length scales, using a conductive-probe atomic force microscopy technique, scanning current spectroscopy (SCS). SCS maps are generated for CuPc/ITO heterojunctions as a function of ITO activation procedures and modification with variable chain length alkyl-phosphonic acids (PAs). We correlate differences in small length scale electrical properties with the performance of organic photovoltaic cells (OPVs) based on CuPc/C(60) heterojunctions, built on these same ITO substrates. SCS maps the "ohmicity" of ITO/CuPc heterojunctions, creating arrays of spatially resolved current-voltage (J-V) curves. Each J-V curve is fit with modified Mott-Gurney expressions, mapping a fitted exponent (γ), where deviations from γ = 2.0 suggest nonohmic behavior. ITO/CuPc/C(60)/BCP/Al OPVs built on nonactivated ITO show mainly nonohmic SCS maps and dark J-V curves with increased series resistance (R(S)), lowered fill-factors (FF), and diminished device performance, especially near the open-circuit voltage. Nearly optimal behavior is seen for OPVs built on oxygen-plasma-treated ITO contacts, which showed SCS maps comparable to heterojunctions of CuPc on clean Au. For ITO electrodes modified with PAs there is a strong correlation between PA chain length and the degree of ohmicity and uniformity of electrical response in ITO/CuPc heterojunctions. ITO electrodes modified with 6-8 carbon alkyl-PAs show uniform and nearly ohmic SCS maps, coupled with acceptable CuPc/C(60)OPV performance. ITO modified with C14 and C18 alkyl-PAs shows dramatic decreases in FF, increases in R(S), and greatly enhanced recombination losses.

Characterization of Charge-Transfer Kinetics at Organic/Electrode Interfaces Using Potential-modulated Attenuated Total Reflectance (PM-ATR) Spectroscopy

“…Conductive-tip atomic force microscopy has been used to map the electrical properties of ITO; the results show that the conductivity of the electrode surface is spatially heterogeneous on the nm length scale, and this heterogeneity has been correlated with electrochemical properties and OPV performance. [35][36][37] We hypothesize that PDIPAs bound to the more electrically conductive regions ("hot spots") 37 on ITO undergo more rapid ET than PDI-PAs bound to less conductive regions. In the latter case, the dominant mechanism for ET is likely to involve both lateral self-exchange and exchange between a PDI and ITO at a "hot spot".…”

Direct Imaging of Space‐Charge Accumulation and Work Function Characteristics of Functional Organic Interfaces

“…The increase of its ionization potential and electron affinity leads to an important n-type acceptor behavior. [34][35][36] We reported previously that thin films of pure MnPc present a typical diode-like transport behavior, with a large voltage drop of about 1.5 V. [7] Such transport behavior has been extensively reported and is characteristic for this and structurally similar organic systems such as porphyrins, [37] other phthalocyanine molecules with metallic centers like Cu, Ni, Al, and Mg, [38][39][40][41] and even fluorinated phthalocyanine systems such as the ones investigated in this work. [7] For MnPc, the orbital alignment around the Fermi level presents a significant metal 3d contribution, rather than a pure π character.…”

Mechanically exfoliated ultra-thin WO3 nanostructures: study of their enhanced electrical properties

“…CSFS-AFM evaluations were performed to establish and identify the surface profile and simultaneously obtain typical topographical, tunnelling and current/voltage properties of the developed and exfoliated ultra-thin Q2D WO 3 nanoflakes [26][27][28][29]. This procedure was done using Bruker MultiMode-8 Atomic Force System with installed Peak Force TUNA module (model: MM8-PFTUNA for MultiMode8 AFM system, Germany).…”