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Addressing the light-soaking issue in inverted organic solar cells using chemical bath deposited fluorinated TiOx electron transport layer

: Lim, F.J.; Set, Y.T.; Krishnamoorthy, A.; Ouyang, J.Y.; Luther, J.; Ho, G.W.


Journal of materials chemistry. A, Materials for energy and sustainability 3 (2015), Nr.1, S.314-322
ISSN: 2050-7488
ISSN: 2050-7496
Fraunhofer ISE ()

The device lifetime of an inverted organic solar cell (IOSC) is significantly better compared to standard-architecture OSC under ambient conditions. However, various studies have shown that when an n-type oxide is used as a selective electron transport layer (ETL) in the IOSC, a reversible light-soaking treatment is required. This reversible treatment largely hampers the practicality of the device, especially in outdoor applications, in which the light-soaking time may take hours every morning. In this work, fluorinated TiOx (F-TiOx), prepared by low-temperature solution-processed chemical bath deposition technique, was used as the ETL to significantly reduce the light-soaking time for a P3HT:PCBM based IOSC. Without affecting the device efficiency, more than ten-fold reduction in light-soaking time was observed for fluorinated TiOx (F-TiOx) when compared with conventional sol-gel TiOx. Ultraviolet photoelectron spectroscopy (UPS) and UV photoconductivity measurements were used to understand the light-soaking time reducing mechanism. From the perspective of ITO/TiOx interface, shift in work function was observed in F-TiOx due to the partial filling of its defective sites by fluorine atoms. Consequently, this process reduces its intrinsic trap state density compared to sol-gel TiOx even before the light-soaking treatment. As a result, the trap filling action can be completed in a shorter time upon illumination, and thus significantly reduce the duration of the necessary light-soaking.