Work Function Of Indium Tin Oxide Research Articles

Surface Tuning of Transparent Electrodes with Atmospheric Pressure Plasma for Third Generation Photovoltaics

Transparent semiconductor thin films such as indium tin oxide (ITO) and fluorine-doped tin oxide (FTO) are commonly used in optoelectronic devices due to their favorable properties such as high conductivity and transparency and suitable work function [1]. Their use ranges from light-emitting diodes through flat panel displays up to solar cells. In this context, surface stoichiometry plays a crucial role in influencing the final properties of the final device. Therefore, efficient surface treatment has been a topic in research for years, covering a wide range of approaches from wet and dry treatments. Among these possibilities, plasma treatment remains today the most effective. In several studies [2, 3], atmospheric pressure plasma shown an increase of surface energy, decrease the contamination while enhancing the work function of both ITO and FTO. Low temperature, fast processing times, and no special requirements for working gas are the main advantages of this approach.In this study, the authors compare the effects of cold (<100°C) atmospheric pressure plasma on ITO and FTO after air and nitrogen treatment to provide deeper insight into the interaction of discharge with semiconductive materials during treatment. A coplanar dielectric barrier discharge has been used for the surface modification (more information available in [4]). For this configuration both electrodes lay in one plane, and the discharge occurs on the top of the barrier. Chemical changes in the surface were also evaluated as a function of plasma parameters without any pre-treatment of the substrates. For both studied materials, similar trends were observed at fast treatment times (<60s of plasma exposure). The plasma treatment resulted in a decrease of water contact angle from the initial 90° for ITO to 10° after 30 seconds of exposure. For FTO, the initial angle 60° was lowered to 10° after just 5 seconds of exposure, and complete hydrophilicity was obtained after 30 seconds of plasma treatment. An interesting trend in oxygen concentration measured by XPS was observed for ITO but not FTO samples. From a physical point of view, the interaction with the conductive sample affects the orientation of filaments during the surface modification. Additionally, the results suggest that for small working distances, the physical regime can vary from coplanar to volume discharge as shown in Figure 1. All changes were observed with ICCD imaging (Pi-Max 4, Princeton Instruments). Regarding the physical changes over the surface, the plasma treatment resulted in decreased roughness with no significant effect on conductivity.In conclusion, the results obtained in this study can be utilized in the manufacturing process of microelectronic devices and provide a deeper insight into how the treatment process influences discharge parameters.[1] Armstrong et al. https://doi.org/10.1016/j.tsf.2003.08.067[2] Šimor et al. https://doi.org/10.1063/1.1513185[3] Donley et al. https://doi.org/10.1021/la011101t[3] Chaney et al. https://doi.org/10.1016/S0169-4332(03)00617-2 Figure 1

Phosphonic acid anchored tripodal molecular films on indium tin oxide.

Whereas monopodal self-assembling monolayers (SAMs) are most frequently used for surface and interface engineering, tripodal SAMs are less popular due to the difficulty in achieving a reliable and homogeneous bonding configuration. In this context, in the present study, the potential of phosphonic acid (PA) decorated triptycene (TripPA) for formation of SAMs on oxide substrates was studied, using indium tin oxide (ITO) as a representative and application-relevant test support. A combination of several complementary experimental techniques was applied and a suitable monopodal reference system, benzylphosphonic acid (PPA), was used. The resulting data consistently show that TripPA forms well-defined, densely packed, and nearly contamination-free tripodal SAMs on ITO, with the similar parameters and properties as the monopodal reference system. Modification of wetting properties and work function of ITO by non-substituted and cyano-decorated TripPA SAMs was demonstrated, showing a potential of this tripodal system for surface engineering of oxide substrates.

Modification of ITO with Composite Self-Assembled Monolayer Enables Hole Injection and Transport Layer-Free TADF-OLED with Preferable Performance

A complex multilayer device structure severely obstructs the commercial development of thermally activated delayed fluorescence material-based organic light-emitting diodes (TADF-OLEDs) due to high preparation costs. Herein, simplified TADF-OLED without both hole injection layer (HIL) and hole transport layer (HTL) was obtained by embellishing indium tin oxide (ITO) with a composite self-assembled monolayer modification strategy. Perfluorobenzyl phosphoric acid with perfluoroalkyl carboxylic acids formed a denser composite monolayer on the surface of ITO. This increases the work function of ITO, improves the quality of emitting layer film, and facilitates hole injection, which realized direct hole injection without the HIL and HTL. The resultant HIL/HTL-free TADF-OLED gains favorable luminescence performance including a current efficiency (CE) of 76.9 cd A–1 and an external quantum efficiency (EQE) of 22.0% for green TADF-OLED and a CE of 21.4 cd A–1 and EQE of 13.4% for blue TADF-OLED. This work provides an effective method in simplifying TADF-OLED structures for its further commercial application.

Improved contact quality for silver-free silicon heterojunction solar cells by phosphoric acid treatment

Electroplating technology is considered as a promising alternative to low-temperature sintered silver pastes for the purpose of significantly reducing the cost of silicon heterojunction (SHJ) solar cells (SCs). However, achieving excellent contact quality between the plated grid and indium tin oxide (ITO) remains a big challenge to satisfy the requirements for fabrication and high performance. In this work, phosphoric acid (H3PO4) is used to modify the ITO surface to improve the contact quality of the plated grid/ITO and enhance the performance of SHJ SCs. It is found that the phosphate groups are adsorbed on the ITO surface to form a dipole layer after H3PO4 treatment. The wettability of H3PO4-treated ITO is substantially improved, resulting in a pinhole-free electroplated nickel (Ni) seed layer. Moreover, the work function of ITO rises by approximately 0.2 eV, which reduces the contact potential barrier between the ITO and the metal electrode. The contact resistivity (ρc) between the plated grid and H3PO4-treated ITO is reduced to approximately 0.4 mΩ·cm2, compared to 1.5 mΩ·cm2 of the control sample. In addition, the metal grid formed on the H3PO4-treated ITO exhibits a better uniformity. Finally, the SHJ cell with H3PO4 treatment process receives a remarkable power conversion efficiency (PCE) of 23.19%, which is approximately 1% (absolute) higher than that of the control device. Therefore, we demonstrate that modifying ITO by H3PO4 is a promising way to improve the contact quality for the electroplating metallization in SHJ SCs.

Enhancement of electronic effects at a biomolecule-inorganic interface by multivalent interactions.

The binding of peptides and proteins through multiple weak interactions is ubiquitous in nature. Biopanning has been used to "hijack" this multivalent binding for the functionalization of surfaces. For practical applications it is important to understand how multivalency influences the binding interactions and the resulting behaviour of the surface. Considering the importance of optimization of the electronic properties of surfaces in diverse electronic and optoelectronic applications, we study here the relation between the multivalency effect and the resulting modulation of the surface work function. We use 12-mer peptides, which were found to strongly bind to oxide surfaces, to functionalize indium tin oxide (ITO) surfaces. We show that the affinity of the peptides for the ITO surface, and concurrently the effect on the ITO work function, are linearly affected by the number of basic residues in the sequence. The multivalent binding interactions lead to a peptide crowding effect, and a stronger modulation of the work function for adodecapeptide than for a single basic amino acid functionalization. The bioderived molecular platform presented herein can pave the way to a novel approach to improve the performance of optoelectronic devices in an eco-friendly manner.

A Multifunctional Self‐Assembled Monolayer for Highly Luminescent Pure‐Blue Quasi‐2D Perovskite Light‐Emitting Diodes

AbstractQuasi‐2D perovskite materials have promise to unlock the full potential of blue perovskite light‐emitting diodes (PeLEDs). However, the efficiency of blue emissive PeLEDs still lags behind the green‐ and red‐emitting counterparts. Here, a multifunctional passivating molecule of (2‐(3,6‐dichloro‐9H‐carbazol‐9‐yl)ethyl)phosphonic acid (36ClCzEPA) that can form a self‐assembled monolayer (SAM) on the indium tin oxide (ITO) electrode is reported. The 36ClCzEPA SAM facilitates hole injection by increasing the work function of ITO through the strong interfacial dipole layer formation at the interface between the perovskite emitter and the ITO electrode. Moreover, it allows a pure‐blue emission and reduces the exciton quenching of luminescence in the perovskite emitter considerably because of its neutral nature, compared to the commonly used acidic PEDOT:PSS. Furthermore, chlorine atoms in the 36ClCzEPA promote well‐ordered crystalline 2D perovskite phases and decrease interfacial trap‐assisted deactivation channels by interfacial passivation. These beneficial characteristics of the 36ClCzEPA SAM yield the excellent luminescence property of PeLEDs with a maximum luminance of 1253 cd m−2 and a peak external quantum efficiency of 4.80% at 473 nm. This work demonstrates that a well‐designed molecule forming an interfacial SAM can be an important component for enhancing the luminescence property of pure‐blue PeLEDs.

Silk Fibroin‐Based Flexible Organic Light‐Emitting Diode with High Light Extraction Efficiency

AbstractIt is an essential issue for the efficiency of flexible organic light‐emitting diodes (OLEDs) to be improved in display and lighting applications. Herein, the use of silk fibroin (SF) as a flexible OLED substrate with enhanced light extraction efficiency (LEE) is reported. Regenerated SF is prepared via a casting method and then coated with conductive indium tin oxide (ITO) via magnetron sputtering. SF‐based flexible OLED outperforms its reference device in terms of turn‐on voltage, peak brightness, and LEE, where glass and polyethylene terephthalate are used as rigid and flexible substrates, respectively. The improved device performance can be ascribed to the low refractive index of SF and its lower surface roughness and more matched energy levels compared to those of other substrates, resulting in a high LEE. Notably, a molecular dipole layer can be formed between the interface of ITO and SF. The dipole moment pointing to the ITO surface effectively changes the ITO work function and reduces the potential barrier height for hole transfer. The work provides new ideas for constructing high‐performance OLEDs on biomass‐based substrates with excellent flexibilities and high LEEs.

Revival of Insulating Polyethylenimine by Creatively Carbonizing with Perylene into Highly Crystallized Carbon Dots as the Cathode Interlayer for High-Performance Organic Solar Cells.

The development of new electron transporting layer (ETL) materials to improve the charge carrier extraction and collection ability between cathode and the active layer has been demonstrated to be an effective approach to enhance the photovoltaic performance of organic solar cells (OSCs). Herein, water-soluble carbon dots (CDs) as ETL material have been creatively synthesized by a vigorous chemical reaction between polyethylenimine (PEI) and 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) via a simple one-step hydrothermal method. Taking full advantage of the high electron transfer property of PTCDA and the work function (WF) reduction ability of PEI, CD gained high electron mobility due to its large π-conjugated area and reduced the WF of indium tin oxide (ITO) by 0.75 eV. As for the photovoltaic performance of devices, inverted OSCs based on CDs have achieved a high power conversion efficiency (PCE) of 17.35%, exhibiting no burn-in effect with no reduction in PCE after more than 4000 h of storage. The successful application of CDs in OPV has developed a new avenue for designing efficient ETL materials that benefits the photovoltaic performance of OSCs.

β-cyclodextrin–polyacryloyl hydrazide-based surface modification for efficient electron-collecting electrodes of indoor organic photovoltaics

Indoor organic photovoltaics (OPVs) show immense potential as a reliable energy harvester for powering emerging Internet of Things devices because of their unique optoelectrical properties. The extremely low number of charge carriers under indoor lighting conditions in comparison to 1-sun conditions necessitates different techniques to optimize the performance of indoor OPVs. In this study, an indium tin oxide (ITO) surface was modified using a water-soluble β-cyclodextrin–polyacryloyl hydrazide (CD–PAH). The abundant amine functional groups on the polyacryloyl hydrazide arms induce a vacuum-level shift owing to their excellent electron-withdrawing ability. Consequently, the work function (WF) of ITO decreased from 4.5 to 4.1 eV, providing a suitable energy-level alignment between ITO and the photoactive layer. The photovoltaic performance of inverted poly(3-hexylthiophene):indene-C60 bisadduct-based OPVs with the surface-treated ITO was evaluated under various lighting conditions. The average power conversion efficiency of the optimized OPV increased substantially from 1.2 ± 0.1% to 3.5 ± 0.1% under 1 sun illumination and 2.4 ± 0.2% to 8.1 ± 0.4% under light-emitting diode illumination. This remarkable performance improvement can be attributed to the excellent transmittance, smooth surface morphology, and suitable WF of the surface-modified ITO.

Interfacial Electrostatic-Interaction-Enhanced Photomultiplication for Ultrahigh External Quantum Efficiency of Organic Photodiodes.

A photomultiplication-type organic photodiode (PM-OPD), where an electric double layer (EDL) is strategically embedded, is demonstrated, with an exceptionally high external quantum efficiency (EQE) of 2210000%, responsivity of 11200 A W-1 , specific detectivity of 2.11 × 1014 Jones, and gain-bandwidth product of 1.92 × 107 Hz, as well as high reproducibility. A polymer electrolyte, poly(9,9-bis(3'-(N,N-dimethyl)-N-ethylammoinium-propyl-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene))dibromide is employed as a work-function-modifying layer of indium tin oxide (ITO) to construct an EDL-embedded Schottky junction with p-type polymer semiconductor, poly(3-hexylthiophene-diyl), resulting in not only advantageous tuning of the work function of ITO but also an enhancement of the electron-trapping efficiency due to electrostatic interaction between exposed cations and trapped electrons within isolated acceptor domains. The effects of the EDL on the energetics of the trapped electron states and thus on the gain generation mechanism are confirmed by numerical simulations based on the drift-diffusion approximation of charge carriers. The feasibility of the fabricated high-EQE PM-OPD especially for weak light detection is demonstrated via a pixelated prototype image sensor. It is believed that this new OPD platform opens up the possibility for the ultrahigh-sensitivity organic image sensors, while maintaining the advantageous properties of organics.

Effect of waiting time of indium tin oxide electrode in entry chamber on organic light-emitting diodes

To obtain efficient organic light-emitting diodes (OLEDs), the degradation in the surface properties of the indium tin oxide (ITO) electrode during the device fabrication should be prevented. In this study, we investigated the effect of the waiting time of ITO in the entry chamber before the transfer to the deposition chamber on the device performances of OLEDs. The current density and luminance of the OLED significantly decreased owing to the reduced hole injection with the increase in the waiting time. The origin of the reduced hole injection was estimated to be (1) the formation of trap sites at the interface between ITO and hole-transport layer and (2) decreased work function of ITO by an interface dipole. These phenomena could be attributed to the contamination of the ITO surface by the waiting under a relatively low vacuum and room light illumination.

Engineering of P3CT-Na through diprophylline treatment to realize efficient and stable inverted perovskite solar cells

Poly[3-(4-carboxybutyl) thiophene-2,5-diyl] (P3CT) is extensively used as hole transport layers (HTL) for efficient inverted perovskite solar cells (PSCs). However, some drawbacks of P3CT-Na such as strong aggregation tendency and unwell-matched energy alignment restrict the application of P3CT-Na-based devices. Here, diprophylline is first used to modify the P3CT-Na HTL in inverted PSCs. Diprophylline modifies work function of ITO (indium tin oxide) by self-assembling onto the ITO surface. Meanwhile, it could interact with P3CT-Na through hydrogen bonding interaction, thus inducing ordered arrangement of P3CT-Na molecules, and increasing transmittance of the HTL. In addition, diprophylline could partially dissolve into the upper perovskite layer, and facilitate the perovskite crystallization. As a result, inverted PSCs utilizing diprophylline added P3CT-Na as HTL yield a remarkable efficiency of 20.87% and excellent long-term stability. Notably, the fill factor (FF) is approaching 84%, representing one of the highest results in inverted PSCs. More interestingly, when the diprophylline treated P3CT-Na layer undergoes ultrasonic cleaning in deionized water for 10 mins, the as-fabricated PSCs still show a high efficiency of 18.77%, manifesting good durability of the HTL. Thus, the recyclability and cost-effectiveness of the HTL makes it more applicable in future commercialization. This work provides a new strategy of improving efficiency and long-term stability of inverted PSCs through HTL engineering.

Influence of end groups variation of self assembled monolayers on performance of planar perovskite solar cells by interface regulation

We present the synthesis and application of new class of self-assembled monolayer molecules (SAMs) for acquiring feasible interfacial engineering in inverted type perovskite solar cells (PSCs). The proposed SAMs bearing different electron donating terminal groups have been utilized to tune the work function of indium tin oxide (ITO) electrodes. Fine-tuning of terminal groups of the SAMs allows us to compare relationship between molecular structures and device parameters. Moreover, ionic and hybrid nature of perovskite enables forming various chemical interactions with terminal groups of SAMs. Employed SAMs has resulted in permanent increase in work function of ITO, increase power conversion efficiency (PCE) of the cells and passivation of trap states at the interface between electrode and perovskite layer. The present study provides new insights into correlation between molecular engineering and solar cell performance through treating holistic comparison of synthesized molecules at the interface of ITO and perovskite layer.

Synthesis and Characterization of New Imidazole Phthalocyanine for Photodegradation of Micro-Organic Pollutants from Sea Water

In this study, a series of new metal phthalocyanines with imidazole function MPc(Imz) (M: Cd, Hg, Zn and Pd) were synthesized to improve the photocatalyst performances. All physical properties such as total energy, HOMO, LUMO energies of MPc(Imz), as well as their vibrational frequencies have been determined by DFT method using B3LYP theory level at 6-311G (d, p) and sdd basis set. The gap of energy level between work function (WF) of ITO and LUMO of PdPc(Imdz) was 1.53 eV and represents the highest barrier beneficial to electron injection compared to WF of ZnPc(Imz), HgPc(Imz), and CdPc(Imz). Furthermore, the PdPc(Imdz) thin films on indium tin oxide (ITO) glass were prepared by spin coating and vacuum evaporation technique, and were characterized by X-ray diffraction (XRD), surface electron morphology (SEM), atomic force microscopy (AFM), and UV–Vis spectroscopy. The photocatalytic activity of the ITO/glass supported thin films and degradation rates of chlorinated phenols in synthetic seawater, under visible light irradiation were optimized to achieve conversions of 80–90%. Experiments on synthetic seawater samples showed that the chloride-specific increase in photodegradation could be attributed to photochemically generated chloride radicals rather than other photoproduced reactive intermediates [e.g., excited-state triplet PdPc(Imz) (3PdPc(Imz)*), reactive oxygen species]. The major 2,3,4,5-Tetrachlorophenol degradation intermediates identified by gas chromatography-mass spectrometry (GC/MS) were 2,3,5-Trichlorophenol, 3,5-dichlorophenol, dichlorodihydroxy-benzene and 3,4,5-trichlorocatechol.

Enhanced efficiency and stability of organic light-emitting diodes via binary self-assembled monolayers of aromatic and aliphatic compounds on indium tin oxide

Abstract Regulating hole injection via self-assembled monolayers (SAMs) modification on indium tin oxide (ITO) is significantly favourable for achieving high efficiency and stability of organic light-emitting diodes (OLEDs). In this work, ITO was modified by pentafluorobenzylphosphonic acid (F5BnPA) and heneicosafluorododecylphosphonic acid (HF21DPA) in a binary SAMs way. By altering mole ratios of these two materials, ITO work function can be achieved from 5.20 to 5.82 eV and the roles of F5BnPA and HF21DPA in affecting hole injection and device performances were also explored. The results demonstrate a synergistic effect that HF21DPA improves ITO work function efficiently for its fluorinated alkyl chain and F5BnPA enhances the charge transfer directly due to its aromatic group gathering HOMO level. When α-naphthylphenylbiphenyldiamine (NPB) and 4,4′,4″-tris(carbazol-9-yl) triphenylamine (TCTA) were used as hole transport layers (HTLs) in OLEDs with a structure of ITO/SAM/HTL/tris(8-hydroxyquinolino) aluminum (III) (Alq3)/lithium fluoride (LiF)/Al, brightness up to 36839 and 58189 cd/m2 and current efficiency up to 5.75 and 7.35 cd/A were obtained, respectively. Furthermore, the influence of binary SAMs on the stability of NPB-based OLEDs was studied. The maximum half-lifetime at the initial brightness of 2200 cd/m2 reaches 368.0 h, which is 75.1 times that of the poly(3, 4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)-containing device. The high durability is attributed to higher hydrophobicity and lower hole injection barrier caused by SAMs. This is the first report on the synergistic effect of aromatic and aliphatic compounds in binary SAMs on regulating hole injection, showing a new approach of obtaining high efficiency and stability of OLEDs.

Synergistic Interface Energy Band Alignment Optimization and Defect Passivation toward Efficient and Simple‐Structured Perovskite Solar Cell

Efficient electron transport layer–free perovskite solar cells (ETL‐free PSCs) with cost‐effective and simplified design can greatly promote the large area flexible application of PSCs. However, the absence of ETL usually leads to the mismatched indium tin oxide (ITO)/perovskite interface energy levels, which limits charge transfer and collection, and results in severe energy loss and poor device performance. To address this, a polar nonconjugated small‐molecule modifier is introduced to lower the work function of ITO and optimize interface energy level alignment by virtue of an inherent dipole, as verified by photoemission spectroscopy and Kelvin probe force microscopy measurements. The resultant barrier‐free ITO/perovskite contact favors efficient charge transfer and suppresses nonradiative recombination, endowing the device with enhanced open circuit voltage, short circuit current density, and fill factor, simultaneously. Accordingly, power conversion efficiency increases greatly from 12.81% to a record breaking 20.55%, comparable to state‐of‐the‐art PSCs with a sophisticated ETL. Also, the stability is enhanced with decreased hysteresis effect due to interface defect passivation and inhibited interface charge accumulation. This work facilitates the further development of highly efficient, flexible, and recyclable ETL‐free PSCs with simplified design and low cost by interface electronic structure engineering through facile electrode modification.

Energy Level Modification with Carbon Dot Interlayers Enables Efficient Perovskite Solar Cells and Quantum Dot Based Light‐Emitting Diodes

AbstractControlling the transport and minimizing charge carrier trapping at interfaces is crucial for the performance of various optoelectronic devices. Here, how electronic properties of stable, abundant, and easy‐to‐synthesized carbon dots (CDs) are controlled via the surface chemistry through a chosen ratio of their precursors citric acid and ethylenediamine are demonstrated. This allows to adjust the work function of indium tin oxide (ITO) films over the broad range of 1.57 eV, through deposition of thin CD layers. CD modifiers with abundant amine groups reduce the ITO work function from 4.64 to 3.42 eV, while those with abundant carboxyl groups increase it to 4.99 eV. Using CDs to modify interfaces between metal oxide (SnO2 and ZnO) films and active layers of solar cells and light‐emitting diodes (LEDs) allows to significantly improve their performance. Power conversion efficiency of CH3NH3PbI3 perovskite solar cells increases from 17.3% to 19.5%; the external quantum efficiency of CsPbI3 perovskite quantum dot LEDs increases from 4.8% to 10.3%; and that of CdSe/ZnS quantum dot LEDs increases from 8.1% to 21.9%. As CD films are easily fabricated in air by solution processing, the approach paves the way to a simplified manufacturing of large‐area and low‐cost optoelectronic devices.

Construction of Effective Polymer Solar Cell Using 1,7-Disubstituted Perylene Diimide Derivatives as Electron Transport Layer.

The poor compatibility of an inorganic electron transport layer with the active layer and an ultrathin film organic material becomes a great obstacle in producing high-quality polymer solar cells with high-throughput roll-to-roll (R2R) method. Novel effective polymer solar cells had been fabricated by introducing 1, 7-disubstituted perylene diimide derivatives PDIH, PDIC, and PDIN as an electron transporting layer. It was noteworthy that PDIN could obviously improve the power conversion efficiency of solar cells that incorporated a photoactive layer composed of poly[(3-hexylthiophene)-2, 5-diyl] (P3HT) and the fullerene acceptor [6, 6-phenyl-C71-butyric acid methyl ester] (PC71BM). The power conversion efficiency varies from 1.5% for ZnO transparent cathode-based solar cells to 2.1% for PDIN-based electron transport layer-free solar cells. This improved performance could be attributed to the following reasons: the interaction between N atom in PDIN and O atom in indium tin oxide (ITO) reduced the work function of ITO, increased the built-in electric field, and thus lowered the electron transport barrier and improved the electron extraction ability of cathode, the appropriate roughness of the active layer increased the contact area with anode interfacial layer and enhanced the hole transport efficiency. These experimental results revealed that PDIN can be a promising novel effective material with a simplified synthesis process and lower cost as an electron transporting layer.

Effects of damages induced by indium-tin-oxide reactive plasma deposition on minority carrier lifetime in silicon crystal

A reactive-plasma deposition (RPD) process damage is evaluated for an indium-tin-oxide (ITO)/SiO2/Si structure in terms of the recombination properties at the interface. To calculate the surface recombination velocity (SRV), the defect density at the SiO2/Si interface (Dit), the effective total charge (Qtot) which is the sum of the charges in the SiO2 and the trapped charge at the SiO2/Si interface, and the workfunction of ITO at the ITO/SiO2 interface (ϕITO), are extracted by capacitance-voltage (C-V) analysis as a function of a postdeposition annealing (PDA) temperature. The Dit and Qtot significantly increase by the RPD process to 1.4×1012 cm-2eV-1 and 1.5×1012/q cm-2, respectively, and decrease by the PDA at 200 °C in N2 ambient to 1.4×1011 cm-2eV-1 and 1.1×1011/q cm-2, respectively. The ϕITO also varies from 4.80 eV to 4.35 eV by the PDA. The correlation is examined between the SRV values calculated based on the extended SRH model using the extracted Dit, Qtot, and ϕITO values [SRV(CV)], and the SRV ones obtained from the minority carrier lifetime measurements [SRV(LT)]. It is found that multiple sets of capture cross-sections were required for SRV(CV) values to coincide with SRV(LT) ones. The effect of variation in the Qtot and ϕITO on the band bending is almost canceled out in the present PDA condition. It is thus considered that the increase in Dit is the main cause to decrease the minority carrier lifetime.

The role of Dawson Polyoxometalates as interfacial layers on the energy band alignment between indium tin oxide and poly(3-hexylthiophene) films

In this work a series of Dawson Polyoxometalates (POMs) with different cations and addenda atoms were grown as interfacial layers between indium tin oxide (ITO) and regio-regular poly(3-hexylthiophene) (rr-P3HT) in order determine the effect of different POMs films on the energy band alignment at the heterojunction rr-P3HT/ITO. The POMs/ITO and rr-P3HT/POMs/ITO interfaces were investigated by X-ray Photoelectron and Ultra-Violet Photoelectron Spectroscopies (XPS, UPS) to gain insight into their chemical morphology, the energetics of the formed interfaces and the charge carrier barriers. We show that POMs are reduced upon deposition on ITO and the extent of their reduction is controlled by their cation and addenda atom. The deposition of POMs on ITO results to the formation of interface dipoles and the work function of ITO is increased up to 5.9 eV. When, a thin layer of rr-P3HT is formed on top of POMs modified ITO, electron transfer from rr-P3HT to the substrate occurs when the work function of POM/ITO substrates is >4.5 eV, resulting to a pinning of the highest occupied molecular orbital (HOMO) of rr-P3HT and the formation of ohmic contacts. The results were interpreted by the integer charge transfer model (ICT).