MICRO METROLOGY | ARTICLE How it works: the organic solar cell
Te 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 (figure 1). Photons hiting the active layer 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, each travels toward their respective electrodes, creating the charge separation necessary to form an electrical circuit.
Dr Schiek’s research looks at using a transparent silver nanowire (AgNW) mesh electrode to replace the britle and rare ITO, in addition to forming the active layer from organic semiconductor materials as an alternative to environmentally damaging chemicals – creating flexible, sustainable and affordable thin-film solar cells for consumer applications.
Surface metrology techniques — contact versus optical
With its complex multi-layered structure, surface analysis techniques provide vital insights into the workings of a solar cell. While tactile profilometry and Atomic Force Microscopy (AFM) have been the mainstays of surface metrology for a number of years, 3D confocal laser scanning microscopy (CLSM) is becoming an ever more popular tool.
Combining the ability to generate detailed, true-colour optical images with the non-contact capabilities of laser scanning technology, the confocal laser scanning microscope really comes into its own as an optical profilometer. Faster and more efficient than stylus-based techniques, 3D CLSM is able to measure soſt or adhesive surfaces and offers a resolution of 0.2 µm. As such, the recent introduction of an Olympus LEXT OLS4100 3D confocal laser scanning microscope into Dr Schiek’s laboratory has greatly enhanced research into alternative means of photovoltaic manufacture.
Accurate measurement of active layer thickness
Te thickness of the active layer is crucial: 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 vital.
Within Dr Schiek’s laboratory, once a scratch is made through the active layer surface with a fine needle, the step edges of this ‘valley’ are measured using profilometry. Tactile profilometry was previously relied upon, but the soſtness of the organic material hampered accurate measurement. As the needle steps up from the valley, it scratches into the surface and thus underestimates the height – oſten by around 20 nm. Considering the average thickness of the active layer is 100 nm, this level of error is highly significant.
<< Figure 2: Accurate metrology of soſt material. Formed from soſt organic material, the active layer is easily damaged by contact. Non-contact profilometry is achieved here with the Olympus LEXT OLS4100 (A). Images courtesy of Dr M. Schiek, University of Oldenburg. >>
20 | commercial micro manufacturing international Vol 7 No.6
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