ORGANICSOLAR
Photoconductive AFM Recently, photoconductive AFM (pcAFM using the MFP-3D AFM from Asylum Research) has been implemented for analyzing solar cell materials. , The pcAFM is based on a conductive AFM setup equipped with a light source (Fig 2). The light is focused on the device through the ITO (indium tin oxide) using an inverted optical microscope and a sample (e.g. a film or device) is loaded in a closed air-tight cell flowed with dry nitrogen. The AFM probe can either sit on a specific point on a sample surface to record the current as a function of an applied bias or the probe can be scanned with a fixed applied bias to provide a current map.
Metal-coated silicon probes with varying work functions can be employed as the top nanoelectrode for either hole or electron collection. Due to the small radius of the probe used in pcAFM measurements, solar cell performance at the nanoscale can be examined and correlated to the bulk measurements, thus providing a comprehensive picture of phase separation, charge generation, charge transport and collection. When the pcAFM is equipped with a tunable monochromatic light source, it can reveal not only spatially, but also spectrally the complexity of the morphology and photocurrent generation.
Nanoscale phase separation of donor and acceptor molecules in the photoactive layer is elucidated by imaging electron and hole collection networks at the same location. Due to the high work function of the gold-coated silicon probe (~5.1 eV), photogenerated holes are collected by the AFM probe and electrons are collected by the ITO electrode when a bias above open-circuit voltage is applied.
This process is reversed when a bias below the open-circuit voltage is applied to the substrate; the photogenerated holes then travel toward the cathode, while the probe tip collects electrons. The applied bias must be small enough so that no charge is injected from the electrodes. Therefore, the photocurrent structure collected at positive and negative biases reveals hole and electron collection networks corresponding to the donor and acceptor phases at the film surface, respectively. An example is shown in Fig 3.
The separation of donor and acceptor phases is not apparent in the topography image (Fig 3a). When a bias of +1V is applied to the substrate, the donor domains at the film surface can be
visualized at 200nm diameter (Fig 3b). At the same location, the electron collection pathways of 20nm in diameter are imaged at an applied bias of –1V (Fig 3c).
Contributing to the low efficiency of the bulk device is the large phase separation of donor and acceptor materials in the blend films which leads to the reduction of interface areas for exciton dissociation and interruption of charge collection pathways.
The pcAFM technique also offers a great opportunity to study nanoscale photophysics using light intensity dependence measurements as described in Fig 4. The heterogeneity in nanostructure and optoelectronics of the photovoltaic materials can be a reason for low performance of bulk devices.
Analyzing short-circuit current (Isc ) as a function of
light intensity sheds light on local variation in the photocurrent generation and recombination. A better understanding of the relationship between nanostructure and optoelectronic properties will help improve the device efficiency. The Isc increases with incident light intensity (P), according to a power-law behavior Isc
~ Pa .
This process is reversed when a bias below the open-circuit voltage is applied to the substrate; the photogenerated holes then travel toward the cathode, while the probe tip collects electrons. The applied bias must be small enough so that no charge is injected from the electrodes
Fig 4. (a) Short-circuit photocurrent image and (b) light intensity dependence of Isc collected at three locations marked in (a) in DPPBFu:PC71
BM
films using a gold- coated silicon probe. The data was obtained using the MFP-3D AFM (Asylum Research).
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www.solar-pv-management.com Issue VI 2010
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