Influence of Fluorine Plasma Treatment of TiO₂ Films on the Behavior of Dye Solar Cells Employing the Co(II)/(III) Redox Couple

Abstract

Fluorine plasma treatment was investigated as an appropriate means for the surface modification of TiO 2 thin film electrodes and the optimization of their performance as photoanodes in dye solar cells (DSCs) employing the Co(II)/ (III) redox shuttle and the organic D35 sensitizer. Detailed surface and structural characterization of the titania films by contact angle measurements, atomic force microscopy, profilometry, and Raman and UV−vis spectroscopy showed that high density SF 6 plasma provoked severe film densification and thus an increase of the nanoparticles packing density, leaving intact the crystallinity, particle size, and optical bandgap. Surface fluorination of the TiO 2 films was also identified by X-ray photoelectron spectroscopy. The combination of the above effects resulted in the enhancement of both photocurrent and power conversion efficiency of the corresponding DSCs at moderate plasma treatment durations, while the photovoltage decreased continuously as a function of the fluorine processing time. Electrochemical impedance spectroscopy analysis revealed a marked increase of the density and distribution of trap states due to fluorine induced surface states along with a systematic downward shift of the TiO 2 conduction band, probably attributed to the electrostatic coupling of intercalated Li + cations with the polar Ti−F species at the TiO 2 surface, in agreement with the V oc drop. In contrast, enhanced electron injection was inferred to underlie the observed J sc and DSC performance improvements, as surface fluorination and the concomitant film densification slightly increased electron transport while hardly affecting dye loading capacity, light harvesting efficiency, and recombination kinetics, except for the case of prolonged plasma treatment. Effective control of the detrimental side effects of fluorine species can render this kind of plasma treatment a powerful method to tune the surface and electrical properties of TiO 2 films and optimize the behavior and performance of the resulting DSC devices.

Interface Engineering of Perovskite Solar Cells with Air Plasma Treatment for Improved Performance

“…However, inserting an extra interlayer, which should be simultaneously ultrathin and uniform, is sophisticated and hard to realize in terms of scale‐up fabrication, and it also could leave some traces of by‐products on the TiO 2 surface that may restrain the electron transfer. Considering the superior advantages of plasma treatment on surface modification: 1) the surface organic contaminants can be removed; 2) the surface modification process is controllable; 3) the process does not involve additional layer; this facile technique has been widely used on interface engineering of solar cells including polymer solar cells, dye‐sensitized solar cells, organic solar cells and heterojunction with intrinsic thin layer (HIT) solar cells …”

Formation of oxygen vacancies and Ti3+ state in TiO2 thin film and enhanced optical properties by air plasma treatment

“…On the other hand, surface modification methods including surface hydrogenation33, vacuum activation32 and plasma treatment34 are also practiced. In the hydrogenation method, the surface of TiO 2 is terminated with hydrogen leading to an enhanced photocatalytic activity35 in visible region; however, it is still unknown that how does the hydrogenation modify a surface to enhance its optical performance (photocatalytic activity)36.…”

“…The drawback of the hydrogenation method is that it requires high temperature and the obtained TiO 2 sample/film are black35, which makes the films unable for many optoelectronic applications, such as a transparent electrode in optoelectronic devices. Both the vacuum activation and plasma treatment methods create highly stable Ti 3+ and oxygen vacancies3234. In vacuum activation method, the sample may exhibit higher absorption intensity but it appears brown in color35, that makes it unable for transparent electrode applications.…”

Transition metal complex redox shuttles for dye-sensitized solar cells

“…38 The overall efficiency increment is more than double, reaching from 0.94% (TiO 2 ) to 2.48% (TiO 2 + Al 2 O 3 ) under 300 W m −2 AM 1.5 illumination. 25 Additionally, the surface modification of TiO 2 thin film electrodes by fluorine plasma treatment 41 and utilization of different low cost carbon materials as counter electrodes 42 were investigated with employing the Co(II/III) redox shuttle and the organic D35 sensitizer. 39 In order to determine the reason of remarkable variations between photovoltaic parameters, the impedance measurements were performed on DSSCs based on these cobalt bipyridyl redox couples.…”