ORGANICSOLAR
Fig 2: Schematic diagram of
photoconductive AFM (pcAFM) setup with the MFP-3D AFM (Asylum Research). PcAFM works in contact mode, using a
conductive AFM probe such as gold or
platinum-coated silicon tip.
layer in organic solar cells is often less than 200nm thick.
Nanoscale Morphology 40
directly produce free electrons and holes for electric current generation. Due to high charge carrier mobilities, the photoactive layer in inorganic solar cells can be made in the micrometer range.
Fig 3: Images of topography (a), current collected at +1V (b) and current collected at –1V (c) of 30:70 DPPBFu:PC71
BM films.
By contrast, in organic solar cells, the light absorption forms a bound electron-hole pair – a so-called exciton. To generate the electric power, the exciton must dissociate into free electrons and holes. To dissociate the exciton, an interface of electron donor and electron acceptor counterparts with the right energy level alignments needs to be utilized. For this reason, the photoactive layer made from a mixture of donor and acceptor molecules has been developed, so-called bulk heterojunction structure (bi-continuous networks of donor and acceptor).
Under solar irradiation, the photogenerated carriers travel along donor and acceptor phases toward the anode and cathode electrodes, respectively, where they are collected to generate power. Due to much lower charge carrier mobilities, the photoactive
In organic solar cells, the charge generation and charge transport depend strongly on the nanoscale morphology, defined as the arrangement of the donor and acceptor networks throughout the bulk. At the heart of improving solar cell efficiency is engineering the photoactive morphology to get large interface areas for exciton dissociation and, at the same time, to form continuous donor and acceptor networks for charge transport. Fig 1 provides a schematic drawing of device structure and typical solar cell morphology. The optimal phase of each component should have a domain size between 10nm and 20nm, similar to the exciton diffusion length.
To better understand how nanoscale morphology affects charge generation and transport, one needs powerful tools to visualize the phase separation of the two components, as well as to understand the optoelectronic processes occurring in the devices at the nanometer scale. For this purpose, many techniques such as high resolution transmission electron microscopy (TEM) and scanning probe microscopy (SPM) have been deployed.
With its wide variety of scanning and measurement modes such as atomic force microscopy (AFM), conductive AFM, transient-resolved electrostatic force microscopy (trEFM), and Kevin probe microscopy (SKPM), SPM can enable simultaneous local probing of morphology, electrical and optoelectronic properties of solar cell materials, establishing a direct correlation of local heterogeneity in nanostructure and photocurrent generation with bulk device performance.
www.solar-pv-management.com Issue VI 2010
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