KEYWORDS: Organic photovoltaics, Solar cells, Dielectric spectroscopy, Dielectrics, Heterojunctions, Renewable energy, Chemical engineering, Physics, Current controlled current source
Organic photovoltaics (OPV) show strong potential for a number of renewable energy applications because of some specifically appealing features (light weight, flexibility, color, …). Over the past decade, the power conversion efficiencies of organic solar cells have strongly risen to values surpassing the 10% threshold, mainly due to strong efforts in chemical engineering of the photoactive components, architectural device optimization and acquisition of fundamental insights in the underlying device physics. As part of the device optimization, the use of conjugated polyelectrolyte (CPE) interfacial layers has been introduced as a popular and powerful way to boost the inherent I-V characteristics. In the presented work, we applied impedance spectroscopy to probe the dielectric permittivity of a series of polythiophene-based CPE interlayer materials as a means to postulate design rules toward novel generation interfacial layers. The presence of ionic pendant groups grants the formation of a capacitive double layer, boosting the charge extraction and device efficiency. A counteracting effect is that the material’s affinity with respect to the underlying photoactive layer diminishes. To enhance the interlayer-photoactive layer compatibility, copolymer structures containing a certain amount of non-ionic side chains are found to be beneficial.
We report on dye sensitized solar cells with PEDOT-PSS coated directly on flexible polyester substrate as counter
electrode. The behavior of such plastic counter electrode in the presence of I - /I3
redox electrolyte has been
investigated with X-ray photoelectron spectroscopy. We have found that some of iodine species are "trapped" within
the PEDOT-PSS layer. The presence of I3-
, I2 and PEDOT charge transfer complexes with iodine species may block
the surface of the electrode. Furthermore, the PEDOT may be further oxidized (p-doped) during cell operation, which
in turn may cause over oxidation and loss of conductivity in the PEDOT-PSS film. The interactions between PEDOT
and iodine species may be enlarged because of the partial loss of PSS protective counter ion. The result is a decrease of
PEDOT-PSS catalytic activity for reduction of I3-
to I - in the redox electrolyte and worse cell performance than in the
case of DSSC with Pt counter electrode.
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