HOPV12 - Blogs

4 th Hybrid and Organic Photovoltaic Conference -Uppsala 2012 183
C64 - In situ reflectance imaging of organic thin film formation from solution
Jonas Bergqvist a , Hans Arwin b , Olle Inganäs a
a, 1Biomolecular and Organic Electronics, IFM, and Center of Organic Electronics, Linköpings Universitet, Linköping, 581 83, SE
b, Laboratory of Applied Optics, IFM, Linköpings Universitet, Linköping, 581 83, SE
The rapid progress of organic photovoltaic devices during the last decade, with power
conversion efficiencies now exceeding 8%, has brought the technology close to an industrial
breakthrough. For polymer solar cells, roll to roll printing is desired to gain the production
advantage. The formation of the photoactive material from solutions needs to be controlled
and optimized. Therefore a suitable method to monitor the deposition process is needed as
deviations of drying times 1 and drying rates 2 during the coating process have proven to
generate morphology variations causing variations in photocurrent generation.
Here we demonstrate how reflectance imaging can be used to monitor the drying process,
both for spin coating and blade coating deposition. A blue LED is used as light source to
generate specular reflections imaged by a CMOS camera. The thinning of the wet film can then
be observed by thin film interference, and can be recorded for each pixel. This enables an
estimation of the evaporation rate for each pixel mapped over the substrate. For spin coating
the evaporation rate is shown to increase with the distance from the rotation center, whereas
the air flow is the determining parameter during blade coating.
Figure 1 Snapshot during the spin coating process of a polymer:PCBM blend dissolved in orthodichlorbenzene on a
1.5x2 cm silicon substrate. The shifting light intensities are due to thin film interference.
By mapping the times when interference ceases, lateral variations in drying time are
visualized. Furthermore the quenching of polymer photoluminescence during the drying
process can be visualized, thus creating a possibility to estimate morphological variations.
Moreover lateral thickness variations of the dry film can be visualized by scanning
ellipsometry. After depositing a top electrode photocurrent images can be generated by a
laser scanning method. This allows for a direct comparison of drying conditions and
photocurrent generation. The possibility to monitor the thin film formation as well as lateral
variations in thickness in-situ by a non-invasive method, is an important step for future large
scale applications where stable high performing generating morphologies have to be formed
over large areas.
© SEFIN 2012