Conductive Polymer Junctions in the UV and

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Photoconductance of ITO/Conductive Polymer Junctions in the UV and Visible Ranges Yulia Furmansky, Shlomi Sergani, Nurit Ashkenasy, and Iris Visoly-Fisher J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00826 • Publication Date (Web): 15 Mar 2018 Downloaded from http://pubs.acs.org on March 15, 2018

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The Journal of Physical Chemistry

Photoconductance of ITO/Conductive Polymer Junctions in the UV and Visible Ranges Yulia Furmanskya,b,c, Shlomi Serganid, Nurit Ashkenasyb,c, and Iris Visoly-Fishera,c,* a

Department of Solar Energy and Environmental Physics, Swiss Institute for Dryland Environmental and Energy Research, Jacob Blaustein Institutes for Desert Research, b Department of Materials Engineering, cIlse Katz Institute for Nanoscale Science and Technology, dDepartment of Chemistry, Ben-Gurion University of the Negev, Be’er Sheva 84105, Israel

Abstract Controlling charge transfer at indium-doped tin oxide (ITO)/conductive polymer junctions is of special importance for organic photovoltaic (OPV) devices and organic light emitting diodes (OLEDs), where ITO is often the transparent electrode of choice. Light induced conductance enhancement, i.e., photoconductance, can allow such control. ITO/ conductive polymer junctions are shown herein to exhibit photoconductance under UV illumination mostly due to photo-induced decrease of an electron barrier at the ITO-polymer interface by discharging of ITO extrinsic surface states, related to the adsorption of oxygen species. Furthermore, we show that ITO surface modification by photo-active porphyrin adsorption can sensitize the ITO/conductive polymer junctions extending the photoconductance to the visible range, to which ITO is transparent. This process is ascribed mostly to discharging of ITO adsorbate states by recombination with photo-generated holes in the photo-excited molecules. Such sensitization is highly relevant for organic optoelectronic devices utilizing ITO interfaced with photoactive organic species and operating in the visible range, such as OPV and OLED devices, and might be applicable also to other UV-photoconductive metal oxide electrodes.

*

Corresponding author. Email: [email protected]

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Introduction Indium-tin oxide (ITO, tin-doped indium oxide In2O3:Sn) is one of the most commonly used conductive oxides in organic light emitting diodes (OLEDs) and organic photovoltaic (OPV) devices due to its high transparency (optical gap ≥ 3.5eV)1-3 and low resistivity.4 ITO surface electronic properties, such as the work function, are known to depend strongly on surface treatments.5-9 Oxidative treatments, and oxygen and water adsorption, were shown to increase ITO work function,3, 6, 10, 11 possibly via negatively charged surface adsorbed species,3, 6, 12, 13 and/ or a surface dipole due to partial charge transfer upon adsorption6,

14

. ITO surface

charging can be associated with a thin surface space charge layer/ band bending.12, 15, 16 This surface modification can pose a barrier for charge transfer across its interface with organic layers. Controlling these charge transfer processes is one of the central issues towards optimization of organic and hybrid organic-inorganic optoelectronic devices' performance.1720

UV photoconductance was observed in conductive metal oxides including SnO2, In2O3 and ZnO. Desorption of negatively charged oxygen species, such as O2- and OH-, as a result of recombination of the electron trapped at the adsorbate state with a hole photo-generated by super-bandgap absorption, was used to explain this phenomenon.12, 21-27 A similar mechanism was suggested to explain ITO work function photosensitivity to UV light.6, 28 In the first part of this work we show that ITO/ conductive polymer junctions exhibit photoconductance under UV illumination due to photo-induced decrease of an electron barrier at the ITOpolymer interface, by discharging of adsorbate states related to oxygen species on the ITO surface. Chemisorption of molecular monolayers or thin polymeric layers at ITO/ organic semiconductor interfaces is a well-known approach to control the interfacial electronic energy level alignment and in return the interfacial charge transfer.29-32 Such molecular interfacial modifiers can affect the ITO’s work function by changing the ITO surface charging and by introducing interfacial dipoles. A molecular layer may also change the ITO surface energy and wetting properties, affecting the morphology of organic films deposited on it.30,

31

Accordingly, molecular interfacial modifications have been widely used in

OLEDs,16 and were shown to improve light harvesting, enhance interfacial charge transfer and reduce recombination rates in OPV devices, by this improving the device performance.33, 34

Molecular adsorption has been further used to stabilize the ITO surface work function and 2 ACS Paragon Plus Environment

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increase the device photostability.35 In addition, the dipole of such molecular layers has been shown to induce charge selectivity,34, 36 eliminating the need for a separate charge selective layer in OPV devices. In most cases molecular modification of ITO/ organic interfaces was done using non-photoactive molecules, however, photo-activity in the visible range of an adsorbed molecular layer can grant further advantages by affectively changing the ITO work function upon illumination. In a few examples where photoactive molecules were used to modify ITO/organic semiconductor interfaces and improve performance of OPV devices,37-39 molecular aggregates were used rather than monolayers, and the effect was related to improved electron transport in the molecular stacks. Several works have incorporated interfacial photoactive molecular monolayers at TiO2/ organic semiconductor interfaces and detected improved device characteristics due to enhanced light harvesting and reduced recombination.40-44 However, no surface space charge layer is expected in anatase TiO2. Zuppiroli et al. studied the photovoltage in ITO/ organic semiconductor junctions incorporating photoactive molecular monolayers, but the analysis referred to the molecular dipoles and the effect of molecular photo-excitation was ignored.31 While UV-photoconductance was previously shown for several metal oxide electrodes,12, 21-27 visible light photoconductance was rarely demonstrated in such systems,37,

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and neither

effects were shown for ITO. We and others have previously shown that visible-range illumination of photoactive molecules adsorbed on ITO surface resulted in long-lived charge separated states due to charge transfer between the excited molecules and the ITO surface.4649

However, the effect of these charge-separated states on interfacial charge transfer was not

studied. Herein we show that such photo-excitation of porphyrin layers at the interface between ITO and poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) can induce photoconductance at the visible range. Prophyrin adsorption therefore extends the spectral range of ITO photconductance to the visible wavelength range, in addition to photoconductance observed in the UV range. We assign this photoconductance to photoinduced discharging of ITO adsorbate states by the excited molecules.

Results and Discussion Figures 1a, b show the absorption spectra of the photoactive molecules used in this study, Meso-tetra(4-carboxyphenyl)porphyrin, TCPP (Figure 1a, inset), in solution and adsorbed on the ITO surface, respectively, compared to bare ITO (Figure 1c). The TCPP spectra show the 3 ACS Paragon Plus Environment


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absorption peaks typical of porphyrins, with the Soret band absorption near 430 nm and four Q-band absorption smaller peaks between 500-670 nm. The ITO absorption spectrum shows an onset of significant absorption near 380 nm corresponding to the ITO's optical gap (~3.5 eV), i.e., the onset of allowed/ direct interband optical transitions in ITO. A small rise in the absorption around 450 nm might be related to the fundamental energy gap (~2.7 eV).50, 51 Finite-sized, metal-free junctions were fabricated on ITO substrates with PEDOT:PSS as a conductive polymeric layer according to an in-house developed method (see Experimental Details, Figure 1d).52 Current-voltage measurements were performed in the dark and under laser illumination at wavelengths of 376, 405 and 532 nm in order to selectively photo-excite different parts of the junctions: 376 nm - the ITO absorption edge,53, 54 405 nm - ITO tail states- and/ or weakly absorbing indirect interband optical transitions (adsorbate state-to-band transitions can also contribute to this absorption), as well as the porphyrin Soret band absorption,54, 55 and 532 nm - TCPP-only absorption at its first Q-band (Figure 1a, b). The change in conductance (i.e., the photoconductance) was quantified as the normalized change in current, ∆ % ≡ / (%), at 0.1 V after 3 min illumination.

Figure 1: Absorption spectra of: (a) 18 µM TCPP solution in ethanol (inset: TCPP schematic structure); (b) TCPP adsorbed on ITO (after subtraction of the ITO absorption); and (c) bare ITO. The dashed lines indicate the illuminating wavelengths (376 nm, 405 nm and 532 nm) used for photoexcitation experiments. (d) Schematic structure of the ITO/molecules/PEDOT:PSS junctions.


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UV-Photoconductance of ITO/ Conductive Polymer Junctions Figure 2a shows the conductance photoresponse of ITO/PEDOT:PSS junctions (no adsorbed molecules) upon illumination at the different wavelengths. The photoresponse is shown to saturate at higher illumination intensities, indicating that the photoconductance does not result from heating but rather from absorption involving a finite number of electron energy states. A pronounced photoresponse is observed under UV (376 nm) illumination corresponding to efficient light absorption by the ITO. A smaller photoresponse is observed upon illumination at 405 nm, and the photoresponse at 532 nm is negligible, as expected. These results are in accordance with the surface photovoltage (SPV) response of bare ITO (Figure 2b). The SPV was measured using the Kelvin probe technique, which measures the contact potential difference (CPD) between the sample and a reference gold electrode ( = !"#$%&# !#$ ' !"#$%&# ). The SPV expresses the change in the CPD under illumination (() = * = ).56 The positive SPV responses at 405 nm and 376 nm, in agreement with the ITO absorption spectrum (Fig. 1c), may indicate a reduction in surface band bending in n-type materials such as ITO.56 We note that a decrease in the ITO work function upon UV illumination was previously ascribed to oxygen desorption.6 The magnitude of ∆I and SPV at different wavelengths can be qualitatively correlated with the differences in absorption (Figure 1c): a significant signal was recorded upon illumination at 405 nm and 376 nm, while a much smaller photoresponse was detected at 532 nm illumination to which ITO is transparent. We conclude that ITO/PEDOT:PSS junctions are photoconductive due to ITO absorption in the UV range and light induced changes in the ITO surface charging. It is noted that changes in the surface charging, due to adsorption/ desorption, can be viewed as surface structural changes. However, such changes are profoundly different from those causing photo-induced hydrophilicity in TiO2 and other metal oxides, which are caused by defect formation due to photo-generated holes oxidizing surface lattice oxygen atoms on exposure to UV light. It is noted that photo-induced hydrophilicity was not found in In2O3 surface,57 and is not typically observed under visible illumination58.



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Figure 2: (a) Photoconductance of ITO/PEDOT:PSS junctions at various wavelengths as a function of intensity. (b) SPV of bare ITO upon illumination at 376 nm (2300 mW/cm2), 405 nm (6000 W/cm2) and 532 nm (10000 mW/cm2) in nitrogen atmosphere.

The kinetics of the SPV photoresponse of bare ITO and conductance photoresponse of ITO/PEDOT:PSS junctions are similar (Figure 3). Upon light turn-on, at dry nitrogen atmosphere, a gradual increase in conductance/ SPV was observed, reaching saturation with time. We note that the magnitude of the photoconductance is expected to decrease at longer time scales of hours, in accordance with previous studies of the photo-stability of the interface between PEDOT:PSS and electrodes, which indicated photo-induced degradation of the charge transfer efficiency.35,

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Upon light turn-off the photoresponse was found to be

persistent, but exposure to room atmosphere significantly increased the photoresponse decay rate (Figure 3a, b). This behavior indicates that the photoresponse decay is related to slow changes in the charge distribution at the ITO surface, which are accelerated by exposure to air. Such slow kinetics were previously attributed to the involvement of ITO surface states60 or unknown traps61 in ITO/ organic semiconductor interfacial charge transfer. A similar UV 6 ACS Paragon Plus Environment

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photoresponse was previously reported for In2O3, SnO2 and ZnO.12, 21-25 In accordance with the model suggested in these works and for the UV photoresponse of the ITO's work function,6, 28 we postulate that the desorption of negatively charged oxygen species under UV light is responsible for a decrease in the work function that is observed as an increase in the SPV. It is therefore responsible also for the observed photoconductance. According to this model, oxygen species adsorb to the surface by trapping free electrons from the metal oxide conduction band, resulting in an upward band bending. This space charge layer, though very thin, may act as a barrier for current transport across the ITO surface. Upon UV/ superbandgap excitation photo-generated holes recombine with electron trapped at the adsorbed oxygen-related states resulting in release of molecular oxygen and water. This can result in a decreased barrier for interfacial charge transfer hence enhanced junction conductance, which is persistent until oxygen-related species re-adsorb. An additional conductance enhancement may be afforded by reduced recombination of photogenerated electrons in the conduction band, however this effect is expected to be much smaller due to the limited number of adsorbate states, and does not explain the photoconductance persistence. A near-UV (405 nm) tail states-to-band and/ or adsorbate states-to-band and/ or weak interband absorption induces similar phenomena. Further support to the involvement of adsorbed oxygen species in the observed photoconductance is offered by the dependence of the photoresponse on the surface cleaning procedure: ITO cleaned by a UV-ozone treatment showed a significantly smaller photo-response compared to piranha-treated ITO (Figure S1). Piranha treatment was previously shown to enrich the surface with hydroxide species,62 which are desorbed under illumination, explaining the larger photoresponse.



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Figure 3: (a) SPV transients of bare ITO upon illumination at 376 nm (2300 mW/cm2), 405 nm (6000 W/cm2) and 532 nm (10000 mW/cm2) in nitrogen atmosphere. (b) and (c): Photoconductance transients of ITO/PEDOT:PSS junctions and their ambient dependence at λ =405 nm (650 mW/cm2), and λ = 376 nm (350 mW/cm2), respectively. Light turn -on and –off times are indicated on the graphs. We note that PEDOT:PSS exposure to air is expected to reduce its conductance and may contribute to the accelerated photoconductance decay shown in 3b.

Visible-Range Photoconductance Induced by a Photoactive Interfacial Layer Porphyrins are highly conjugated heterocyclic, macrocycle compounds, with remarkable light harvesting properties in the visible range, hence they participate in natural (and artificial) photosynthesis. Carboxylic acid substituents allow their chemisorption to metal oxide surfaces. ITO surface was modified by adsorption of Meso-tetra(4-carboxyphenyl)porphyrin (TCPP, inset of Figure 1a) via carboxylic acid substituents as a photoactive interface modifier. The adsorption was verified by UV-vis absorption spectrometry (Figure 1b). A surface molecular density of ca. 6 , 10/0 $/$1 was calculated from the Soret


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absorption peak height and the calibrated TCPP extinction coefficient (22340 M-1cm-1 at 417 nm, see Experimental Details). This value is slightly larger than that expected for a monolayer according to TCPP geometry (0.4÷1.6 ×1014 molec/cm2, depending on the adsorption geometry: flat or stacked).63 This discrepancy might be explained by the roughness of ITO surface (Figure S2), and by an extinction coefficient of the adsorbed layer being different than that in the solutions used for calibration. We estimate that this density indicates a single or few (

Conductive Polymer Junctions in the UV and

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