TY - THES

T1 - Photoconductive Atomic Force Microscopy and Kelvin Probe Force Microscopy Measurements of Organic Semiconductor Nanostructures

AU - Wachauer, Astrid

N1 - embargoed until null

PY - 2011

Y1 - 2011

N2 - Organic semiconductors are of increasing importance for electronic and optoelectronic device applications. They are inexpensive and easy to process on large flexible areas. Organic semiconductors are already used in organic light emitting diodes (OLED), organic field effect transistors (OFET), and organic solar cells. The electronic properties of thin layers of organic semiconductors are known to be morphology dependent. The heterogeneity of the films impacts their optical and electrical transport properties. The main focus of the present work is the investigation of the electronic and optoelectronic properties of organic solar cells on the nanometer scale by means of photoconductive atomic force microscopy (PC-AFM) and photoassisted Kelvin probe force microscopy (PA-KPFM). Both, PC-AFM and PA-KPFM techniques allow the simultaneous detection of the electrical and optoelectrical properties along with the topography. Here, the correlation between the heterogeneity and efficiency of the light-to-electricity conversion for blends with varying ratios of AnE-PVstat and 1-(3-methoxycarbonyl)propyl-1-phenyl[6,6]C61 (PCBM) have been investigated. The AnE-PVstat : PCBM samples showed a photoresponse under illumination with white light (150 W halogen lamp) at room temperature which is also confirmed by lTocal current-to-voltage characterization. It has been revealed that measurements under ambient conditions result in degradation of the electrical properties. The photoresponse was partially recovered by annealing of the samples in nitrogen flow at 100°C. In the current maps, a higher conductivity of the matrix compared to the grains is observed. In some cases, small grains of 100 nm - 150 nm diameter with strongly reduced conductivity can be found. Contact potential difference (CPD) maps measured with PA-KPFM in general revealed a smaller CPD under illumination. Additionally, a smaller CPD of the matrix can be found, which is consistent with the higher conductivity observed with PC-AFM. Other systems under investigation were polyvinylidene fluoride (PVDF) - poly(3-methylthiophene) (P3MT) core-shell nanoparticles with poly(3-hexylthiophene-2,5-diyl) (P3HT) binder and P3HT : PCBM blends on PVDF. The photoresponse in these systems was observed at room temperature and did not exhibit any signs of degradation when measured under ambient conditions. However, the topography images recorded using PC-AFM appeared blurry due to the weak mechanical stability of the samples. Further, PC-AFM was also applied to investigate the temperature dependence of the photocurrent in C60 thin films under different degrees of illumination in order to verify the so called Meyer-Neldel rule. As expected, an Arrhenius dependence on the temperature was observed on films with different morphologies. However, the influence of the illumination by a 150 W xenon lamp on the measured current density appeared below the detection limits, which is due to fast degradation of the C60 thin films under ambient conditions. The amorphous film has a thickness of 280 nm and consists of particles with 50 nm - 200 nm in diameter. The polycrystalline film consists of crystallites with sizes up to 700 nm, which is in the same size range as the film thickness of about 800 nm. Current images of the polycrystalline C60 film revealed the existence of smaller crystallites of up to 200 nm size that were non-conductive, indicating bad electrical contact to adjacent grains.

AB - Organic semiconductors are of increasing importance for electronic and optoelectronic device applications. They are inexpensive and easy to process on large flexible areas. Organic semiconductors are already used in organic light emitting diodes (OLED), organic field effect transistors (OFET), and organic solar cells. The electronic properties of thin layers of organic semiconductors are known to be morphology dependent. The heterogeneity of the films impacts their optical and electrical transport properties. The main focus of the present work is the investigation of the electronic and optoelectronic properties of organic solar cells on the nanometer scale by means of photoconductive atomic force microscopy (PC-AFM) and photoassisted Kelvin probe force microscopy (PA-KPFM). Both, PC-AFM and PA-KPFM techniques allow the simultaneous detection of the electrical and optoelectrical properties along with the topography. Here, the correlation between the heterogeneity and efficiency of the light-to-electricity conversion for blends with varying ratios of AnE-PVstat and 1-(3-methoxycarbonyl)propyl-1-phenyl[6,6]C61 (PCBM) have been investigated. The AnE-PVstat : PCBM samples showed a photoresponse under illumination with white light (150 W halogen lamp) at room temperature which is also confirmed by lTocal current-to-voltage characterization. It has been revealed that measurements under ambient conditions result in degradation of the electrical properties. The photoresponse was partially recovered by annealing of the samples in nitrogen flow at 100°C. In the current maps, a higher conductivity of the matrix compared to the grains is observed. In some cases, small grains of 100 nm - 150 nm diameter with strongly reduced conductivity can be found. Contact potential difference (CPD) maps measured with PA-KPFM in general revealed a smaller CPD under illumination. Additionally, a smaller CPD of the matrix can be found, which is consistent with the higher conductivity observed with PC-AFM. Other systems under investigation were polyvinylidene fluoride (PVDF) - poly(3-methylthiophene) (P3MT) core-shell nanoparticles with poly(3-hexylthiophene-2,5-diyl) (P3HT) binder and P3HT : PCBM blends on PVDF. The photoresponse in these systems was observed at room temperature and did not exhibit any signs of degradation when measured under ambient conditions. However, the topography images recorded using PC-AFM appeared blurry due to the weak mechanical stability of the samples. Further, PC-AFM was also applied to investigate the temperature dependence of the photocurrent in C60 thin films under different degrees of illumination in order to verify the so called Meyer-Neldel rule. As expected, an Arrhenius dependence on the temperature was observed on films with different morphologies. However, the influence of the illumination by a 150 W xenon lamp on the measured current density appeared below the detection limits, which is due to fast degradation of the C60 thin films under ambient conditions. The amorphous film has a thickness of 280 nm and consists of particles with 50 nm - 200 nm in diameter. The polycrystalline film consists of crystallites with sizes up to 700 nm, which is in the same size range as the film thickness of about 800 nm. Current images of the polycrystalline C60 film revealed the existence of smaller crystallites of up to 200 nm size that were non-conductive, indicating bad electrical contact to adjacent grains.

KW - AFM

KW - PC-AFM

KW - KPFM

KW - organic solar cells

KW - P3HT

KW - P3MT

KW - PCBM

KW - AnE-PVstat

KW - C60

KW - Meyer-Neldel rule

KW - AFM

KW - PC-AFM

KW - KPFM

KW - organische Solarzellen

KW - P3HT

KW - P3MT

KW - PCBM

KW - AnE-PVstat

KW - C60

KW - Meyer-Neldel Regel

M3 - Diploma Thesis

ER -