Atomic Force Microscopy Based Electric Modes in Characterization of Organic Photovoltaics
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Atomic Force Microscopy Based Electric Modes in Characterization of Organic Photovoltaics Craig Wall, Sergei Magonov, Sergey Belikov and John Alexander NT-MDT Development Inc., 430 W. Warner Rd., Tempe, AZ 85284, U.S.A. ABSTRACT Capabilities of Atomic Force Microscopy in different modes including Electric Force Microscopy and Kelvin Force Microscopy are reviewed and illustrated on several samples including organic photovoltaics (P3HT/PCBM, PEDOT:PSS). Compositional mapping of these blends is enhanced with a combined use of the modes, and variations of local electric properties are detected down to the nanometer scale. The revealed morphology will assist in development of comprehensive models accounting for the structure-property relationship in solar cells and related devices. INTRODUCTION Atomic Force Microscopy (AFM) - based electric methods such as Electric Force Microscopy (EFM) and Kelvin Force Microscopy (KFM) have substantially enriched the characterization capabilities of scanning probe microscopy at small scales. Compositional mapping of heterogeneous materials employed on differences in their local conductivity, work function, molecular dipole strength, capacitance, etc. complement the differentiation of the components based on their mechanical properties. In addition, quantitative measurements of local electric properties (surface potential, dielectric permittivity, capacitance, etc.) can be also achieved. The capabilities of EFM and KFM in single and double-pass approaches will be explained and demonstrated on a number of organic materials including those used for photovoltaic applications. BACKGROUND OF ELECTROSTATIC FORCE MODES AFM electric methods are based either on the detection of electrostatic force interactions between a conducting probe and sample locations or on the use of additional sensors (current, capacitance, microwave radiation, etc.). We shall focus on EFM and KFM that employ electrostatic force interactions [1-2]. An extraction of the electrostatic force from the overall force experienced by the probe during scanning is essential for practical realization of EFM and KFM. This problem has been solved in two ways. In the double-pass approach, which is based on making separate measurements of mechanical and electrostatic forces at a single frequency (usually, the flexural resonance of the probe, ωmech), the long-distant electrostatic interactions are detected with the probe being removed from the surface at some distance (10-40 nm). The electrostatic force can be also stimulated by a DC bias applied to the probe or sample. Sensitive force measurements are realized by recording the change of the phase and amplitude of the probe, which is driven at ωmech, or by measuring the frequency and amplitude changes when the phase of the oscillating probe is kept constant. The relation between the electrostatic force FE and the probe phase (θ) and amplitude (A) are described by the equations [3-4]:
sin θ = A A0
cos θ = −
(1)
π
2 FE (Z c + A cos y ) cos ydy N ∫0
(2)
– free amplitude of the prob
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