Stability of Chemically Modified Indium–Tin–Oxide Surfaces against Water

Abstract

We studied stability of indium-tin-oxide (ITO) surfaces chemically modified with -SO 2 Cl and -PO 2 Cl 2 binding groups of para substituted benzene derivatives as well as those oxidized with UV ozone by measuring time dependence of the work function and water contact angle after their surface modifications. It seems most likely from the effect of water contents in environments upon their time changes that water is the primary reactant for degradation of the chemically modified and oxidized ITO surfaces.Organic light-emitting diodes (OLEDs), 1 organic photovoltaics, 2 and dye-sensitized solar cells 3 have attracted attention owing to their ease of fabrication, potential for low-cost production, and compatibility with flexible substrates. 4 As optically transparent electrode materials for these optoelectronic devices, indium-tin-oxide (ITO) or SnO 2 has been used widely.5 However, for enhanced hole injection and open-circuit voltages, matching between the Fermi level of the oxide electrode and the highest occupied molecular orbital level of the organic layer is required. [4][5][6] Therefore, changing the work function () of these oxide electrodes has been attempted by surface treatment with adsorption of acids and bases, 7 self-assembled monolayer (SAM) formation with polar adsorbate molecules, 8 and exposure to O 2 plasma 9 or UV ozone. 10 Among them, we have demonstrated tuning by chemical modification of ITO surfaces with various acid chloride (i.e., -SO 2 Cl, -COCl, and -PO 2 Cl 2 ) binding groups of para-substituted benzene derivatives. 8c Similar chemical modification methods were used for the first time by us for enhanced dye sensitization current on SnO 2 . 5,11However, acid treatment was not reliable and an abrupt decrease of was observed.7c So far, silanized surfaces are considered to exhibit improved durability because of their adherence to the oxide surfaces via covalent bonds. 4a,8b To the best of our knowledge, none have studied stability of the chemically modified surfaces with acid chlorides in detail. 5 In this work, the degradation processes of the chemically modified as well as oxidized ITO surfaces with UV ozone (ITO(uv)) were studied by measuring the time change in or water contact angle (CA) after exposure of freshly prepared samples to various environments. In particular, we focused our attention on the effect of water in the environments.Three kinds of modified ITO electrodes (i.e., Cl-S, CF 3 -S, and Cl-P in Figure 1) as well as ITO(uv) (12 Â 15 mm 2 ) 12 were prepared as previously. 8c Values of were measured more carefully than in the previous study, 8c because tip and time dependence of observed values were not negligible. 13Averaged initial and CA values 13 observed on the three kinds of modified and oxidized samples over the number (n) of each sample (n > 10) are listed in Table 1. In Figure 2a are shown the time change in after exposure of the samples to environments with two different relative humidities (RHs) at room temperature. Because the large shifts in from those of clean...

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“…However, such performances might deteriorate after the device is immersed in the solution for more than 1 month because σ max appeared to decrease slightly from around day 21. This may be because the deterioration rate of the ITO films in solutions is basically larger than that in air owing to their slight solubility, 47 and then the concentration of oxygen vacancies gradually changes.…”