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News

Transparent silver nanowire (AgNW) mesh electrode could replace brittle, rare ITO in silver cells

Olympus Europa Holding : 12 September, 2014  (Application Story)
Olympus has released a new application note detailing how the quick and non-destructive nature of the Olympus LEXT OLS4100 high-resolution analysis and stitching function capabilities has enhanced research into organic semiconductors and transparent electrodes.
Current solar cell manufacture processes are quite hazardous and energy consuming in contrast to the ‘green’ intentions of the end products. Dr Manuela Schiek’s research at the University of Oldenburg, Germany, focuses on alternative materials for solar cell manufacture that are non-hazardous and readily available, including the use of organic semiconductors and a transparent electrode system formed from a silver nanowire mesh.
 
The most popular organic solar cell architecture is based on a photon-harvesting active layer, sandwiched between two electrodes – one of which must be transparent in order to allow light to penetrate. Photons hitting the organic semiconductor generate the charge-carrying excitons, which by the use of two materials – an electron donor and an electron acceptor - are then separated into their separate electron and holes. Driven by an electrical field,the electrons and holes travel toward their respective electrodes, creating the charge separation necessary to form an electrical circuit. 
 
Schiek’s research looks at using a transparent silver nanowire (AgNW) mesh electrode to replace the brittle and rare ITO, in addition to forming the active layer from organic materials as an alternative to environmentally damaging chemicals – creating flexible, sustainable and affordable thin-film solar cells for consumer applications.
 
Organic materials in the active layer
 
The active layer is where energy is captured from photons, and within organic solar cells this is often formed from a discontinuous blend of two materials – a polymer and a fullerene. With the polymer acting as an electron donor and the fullerene as an electron acceptor, this bulk heterojunction structure leads to enhanced charge separation of electrons and holes, and hence improved solar cell function. But polymers are often roughly defined mixtures of material with differing chain lengths, and with properties that are highly batch-specific. Molecular semiconductors, on the other hand, are defined building blocks with properties that can be adjusted by small changes to their structure, which can therefore be optimised for improved solar cell function. An interesting class of such molecules are the squaraine dyes, whose structure gives a broad absorption in the red region of the light spectrum. Dr Schiek’s research is investigating a bulk heterojunction active layer formed from squaraines mixed with a fullerene acceptor. The thickness of the active layer is crucial for this application: too thin and the mobility of charge carriers is restricted, but too thick and both light absorbance and flexibility are significantly reduced.
 
Accurate measurement of layer thickness is therefore equally important. Within Schiek’s laboratory, once a scratch is made through the active layer surface with a fine needle, the step edges of this ‘valley’ are subsequently measured using profilometry. Tactile profilometry was previously relied upon, but the softness of the organic material hampered accurate measurement. In fact, a height discrepancy of around 20 nm was frequently observed between the two step edges, which is significant considering the average thickness of the active layer is 100 nm. As the needle steps up from the valley, it scratches into the surface and results in the false lower height reading. 
 
Moving towards the sustainable manufacture of photovoltaics, Schiek’s research group has discovered how the quick and non-destructive nature of the Olympus LEXT OLS4100 high-resolution analysis and stitching function capabilities have enhanced their research.
 
With its complex multi-layered structure, surface analysis techniques provide vital insights into the workings of a solar cell. Combining the ability to generate detailed, true-colour optical images with the non-contact capabilities of laser scanning technology, the LEXT 3D CLSM enables exceptional high precision optical profilometry and metrology. In Schiek’s work, the LEXT was employed to accurately measure the thickness of organic semiconductor materials in the energy-capturing active layer of solar cells as this can critically affect their efficiency. The LEXT was found to be faster, more efficient and more accurate than stylus-based techniques through its non-contact capability to measure soft or adhesive surfaces at high-resolution.
 
The expanded field of view offered by the rapid and intuitive stitching function of the LEXT also markedly improved the efficiency of surface roughness evaluation in comparison to Atomic Force Microscopy (AFM). The LEXT enabled viewing of a more representative sample of silver wire mesh on the transparent electrode surface and this combined with the wide magnification range helped to identify regions of unwanted aggregation that would otherwise have been missed. Schiek commented, “The LEXT has been a great asset in our work. As well as being much faster than AFM, the area that you can inspect is a lot larger, with true colours and a 3D height profile for more insightful analyses."
 
 
 
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