TY - JOUR
T1 - Electrical property heterogeneity at transparent conductive oxide/organic semiconductor interfaces
T2 - Mapping contact ohmicity using conducting-tip atomic force microscopy
AU - MacDonald, Gordon A.
AU - Veneman, P. Alexander
AU - Placencia, Diogenes
AU - Armstrong, Neal R.
PY - 2012/11/27
Y1 - 2012/11/27
N2 - 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/C60 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/C60/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/C60OPV performance. ITO modified with C14 and C18 alkyl-PAs shows dramatic decreases in FF, increases in R S, and greatly enhanced recombination losses.
AB - 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/C60 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/C60/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/C60OPV performance. ITO modified with C14 and C18 alkyl-PAs shows dramatic decreases in FF, increases in R S, and greatly enhanced recombination losses.
KW - conducting-tip atomic force microscopy
KW - injection efficiency
KW - ohmic contacts
KW - organic semiconductor
KW - organic solar cell
UR - http://www.scopus.com/inward/record.url?scp=84870441716&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=84870441716&partnerID=8YFLogxK
U2 - 10.1021/nn303043y
DO - 10.1021/nn303043y
M3 - Article
C2 - 23030667
AN - SCOPUS:84870441716
SN - 1936-0851
VL - 6
SP - 9623
EP - 9636
JO - ACS Nano
JF - ACS Nano
IS - 11
ER -