Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

Conductive probe based atomic force microscopy techniques like conductive atomic-force microscopy (C-AFM) [1,2], photoconductive AFM (PC-AFM) [3, 4], and photo-assisted Kelvin probe force microscopy (PA-KPFM) [5,6] provide an opportunity to study electronic and optoelectronic properties of surfaces and interfaces with nanometer resolution.
Here, we present our results on investigations of: 1) photoconductivity of single upright standing ZnO nanorods (NRs) grown by thermal evaporation [7], and 2) photoresponse in thin organic semiconductor films, namely AnE-PVstat:PCBM blends. The measurements have been performed in ambient or under N2 atmosphere using MFP3D from Asylum Research and in UHV with room temperature AFM/STM from Omicron extended with an external illumination system employing lock-in detection.
First, a novel PC-AFM setup which has been implemented to study the optoelectronic properties of individual upright standing ZnO NRs under illumination from the top will be presented. Using this setup we investigated transient photocurrent behavior and recorded photocurrent spectra from single upright standing ZnO NRs. A persistent photoconductivity from single ZnO NRs for the time intervals up to 1800 s has been observed. Simultaneously, the photoconductivity spectra (Fig. 1) revealed that the minimum photon energy sufficient for photocurrent excitation is 3.1 eV, which is at least 100 mV less than the energy sufficient for the band-to-band excitation. The mechanism of the persistent photoconductivity in ZnO is discussed in the frame of existing theoretical models.
In the second part, the results on investigations of the photoresponse in AnE-PVstat:PCBM blends spin-coated on PEDOT:PSS/ITO/ will be presented. The AnE-PVstat:PCBM blend acts as an active layer where the charge generation (by illumination) and charge separation processes take place. The charge separation in this structure occurs due to the presence of the effective electric field between AnE-PVstat, which serves as a donor, and PCBM which serves as an acceptor. The heterogeneity of the films, in turn, impacts the charge carrier generation, separation, and transport. Here, we analyzed the correlation between the heterogeneity and efficiency of the light-to-electricity conversion for (1:1), (1:2) and (1:3) AnE-PVstat to PCBM blend ratios. The 2D current maps recorded under illumination reveal the regions of high photoresponse for the case of (1:3) blend ratio, which is also confirmed by PA-KPFM and local current-to-voltage characteristics. The results on local photoresponse characterization of the samples with different blend ratios are correlated with the data on the light-to-electricity conversion efficiency obtained macroscopically.

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