UV-ozone Treatment Research Articles

Device and morphological engineering of organic solar cells for enhanced charge transport and photovoltaic performance

Conjugated polymers are potential materials for photovoltaic applications due to their high absorption coefficient, mechanical flexibility, and solution-based processing for low-cost solar cells. A bulk heterojunction (BHJ) structure made of donor–acceptor composite can lead to high charge transfer and power conversion efficiency. Active layer morphology is a key factor for device performance. Film formation processes (e.g., spray-coating, spin-coating, and dip-coating), post-treatment (e.g., annealing and UV ozone treatment), and use of additives are typically used to engineer the morphology, which optimizes physical properties, such as molecular configuration, miscibility, lateral and vertical phase separation. We will review electronic donor–acceptor interactions in conjugated polymer composites, the effect of processing parameters and morphology on solar cell performance, and charge carrier transport in polymer solar cells. This review provides the basis for selection of different processing conditions for optimized nanomorphology of active layers and reduced bimolecular recombination to enhance open-circuit voltage, short-circuit current density, and fill factor of BHJ solar cells.

Improvement of adhesion and paint ability of EVA copolymers with different vinyl acetate contents by treatment with UV-ozone

The surface modifications produced by UV-ozone treatment of two ethylene-vinyl acetate (EVA) copolymers containing 12 and 20wt% vinyl acetate (EVA12 and EVA20 respectively) were studied. The treatment with UV-ozone improved the wettability of both EVAs due to the creation of new carbon–oxygen moieties. The extent of these modifications increased with increasing length of the treatment and the modifications produced in EVA20 were produced for shorter lengths of treatment. The UV-ozone treatment also created roughness and heterogeneities on the EVA surfaces. Whereas roughness formation prevailed on the UV-ozone treated EVA12, important ablation was dominant on the treated EVA20. T-peel strength values in joints made with polychloroprene adhesive increased when the EVAs were treated with UV-ozone. Short length of UV-ozone treatment (1min) produced higher T-peel strength in joints made with EVA20 whereas higher T-peel strength values in joints made with EVA12 were obtained after treatment for 5–7.5min in which a cohesive failure into a weak boundary layer on the treated EVA surface was found. Furthermore, the adhesion of UV-ozone treated EVA20 to acrylic paint increased. Finally, the ageing resistance of the treated EVA/polychloroprene adhesive joints was good and the surface modifications on the UV-ozone treated EVAs lasted for 24h after treatment at least.

Improved functionality of graphene and carbon nanotube hybrid foam architecture by UV-ozone treatment.

Optimization of the electrode/electrolyte double-layer interface is a key factor for improving electrode performance of aqueous electrolyte based supercapacitors (SCs). Here, we report the improved functionality of carbon materials via a non-invasive, high-throughput, and inexpensive UV generated ozone (UV-ozone) treatment. This process allows precise tuning of the graphene and carbon nanotube hybrid foam (GM) transitionally from ultrahydrophobic to hydrophilic within 60 s. The continuous tuning of surface energy can be controlled by simply varying the UV-ozone exposure time, while the ozone-oxidized carbon nanostructure maintains its integrity. Symmetric SCs based on the UV-ozone treated GM foam demonstrated enhanced rate performance. This technique can be readily applied to other CVD-grown carbonaceous materials by taking advantage of its ease of processing, low cost, scalability, and controllability.

The Mechanical Aspects of Formation and Application of PDMS Bilayers Rolled into a Cylindrical Structure

A polydimethylsiloxane (PDMS) film with its surface being oxidized by a plasma treatment or a UV-ozone (UVO) treatment, that is, a bilayer made of PDMS and its oxidized surface layer, is known to roll into a cylindrical structure upon exposure to the chloroform vapor due to the mismatch in the swelling ratio between PDMS and the oxidized layer by the chloroform vapor. Here we analyzed the formation of the rolled bilayer with the mechanical aspects: how the mismatch in the swelling ratio of the bilayer induces rolling of the bilayer, why any form of trigger that breaks the symmetry in the in-plane stress level is needed to roll the bilayer uniaxially, why the rolled bilayer does not unroll in the dry state when there is no more mismatch in the swelling ratio, and how the measured curvature of rolled bilayer matches well with the prediction by the theory. Moreover, for the use of the rolled bilayer as the channel of the microfluidic device, we examined whether the rolled bilayer deforms or unrolls by the flow of the aqueous solution that exerts the circumferential stress on the rolled bilayer.

Solution-processed nickel compound as hole collection layer for efficient polymer solar cells

We demonstrated efficient bulk heterojunction polymer solar cells (PSCs) by inserting a solution-processable hole collection layer (HCL) between the indium tin oxide (ITO) electrode and photoactive layer. The HCL was prepared by spin-coating nickel acetylacetonate (Ni(acac)2) isopropanol solution on ITO, and then baking in air at 180 °C for 10 min followed by UV ozone treatment, which was marked as a-Ni(acac)2. The a-Ni(acac)2 HCL shows suitable energy levels, high hole mobility of 4.09 × 10−3 cm2 V−1·s−1, and high transparency with light transmittance better than poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) in the wavelength range 550–800 nm. The PSCs with a-Ni(acac)2 HCL showed improved performance compared with the PSCs without or with traditional PEDOT:PSS HCL. The power conversion efficiency of the PSC based on PBDTTT-C-T:PC70BM with a-Ni(acac)2 HCL reached 7.84% under the illumination of AM 1.5 G, 100 mW cm−2.

Triple-junction hybrid tandem solar cells with amorphous silicon and polymer-fullerene blends.

Organic-inorganic hybrid tandem solar cells attract a considerable amount of attention due to their potential for realizing high efficiency photovoltaic devices at a low cost. Here, highly efficient triple-junction (TJ) hybrid tandem solar cells consisting of a double-junction (DJ) amorphous silicon (a-Si) cell and an organic photovoltaic (OPV) rear cell were developed. In order to design the TJ device in a logical manner, a simulation was carried out based on optical absorption and internal quantum efficiency. In the TJ architecture, the high-energy photons were utilized in a more efficient way than in the previously reported a-Si/OPV DJ devices, leading to a significant improvement in the overall efficiency by means of a voltage gain. The interface engineering such as tin-doped In2O3 deposition as an interlayer and its UV-ozone treatment resulted in the further improvement in the performance of the TJ solar cells. As a result, a power conversion efficiency of 7.81% was achieved with an open-circuit voltage of 2.35 V. The wavelength-resolved absorption profile provides deeper insight into the detailed optical response of the TJ hybrid solar cells.

Low-temperature UV ozone-treated high efficiency radial p-n junction solar cells: N-Si NW arrays embedded in a p-Si matrix

High-efficiency radial p-n junction SiNW solar cells have been realized using UV ozone treatment of n-Si NW arrays embedded in a p-Si matrix structure. This structure provides the advantage of homogeneous and continuous front electrode patterning, which results in a lower series resistance than for core-shell morphology. Surface and bulk recombination is chiefly responsible for degrading cell performance with increased SiNW length in spite of enhanced junction area and improved light-trapping capability. In this study we estimated the critical NW length necessary to obtain optimal cell performance with minimum carrier loss in a simple radial p-n junction solar cell. Further low-temperature UV ozone treatments offer the potential to enhance solar cell properties by passivating, by oxidation, the large number of surface defect states.

Slip behavior in polymethylmethacrylate films in dependence of self-assembled monolayer wettability

The slip behavior of polymethylmethacrylate (PMMA) films prepared on self-assembled monolayers (SAM) supported by Si substrate was investigated according to the wettability of the SAM. To this end, the dewetting of thermally annealed PMMA films was measured varying the surface energies of underlyed SAM. Total surface energy (γt) of SAM increased with increasing UV exposure time. At surface energy of <31.3 mJ/m2, the PMMA films showed slip behavior; radius of holes (R) ∝ t0.68−0.72, where t is dewetting time. The slip behavior seems to be dominated by the viscous friction between the PMMA films and the SAM. As t increased, slip behavior disappeared, R ∝ t0.34−0.39. At surface energy of >42.0 mJ/m2, the PMMA films did not show slip behavior, R ∝ t0.45. It means that interface effects between PMMA films and SAM layers increase with increasing of γ of SAM layers, and then the molecular interaction in the PMMA films increases. The spreading coefficient (S) of the PMMA films approached positive number for UV exposure times between 0 s and 300 s. We concluded that the stability of the PMMA films can be simply controlled by UV-ozone treatment on the SAM layers.

Guided wrinkling with nanoimprinted SU-8 surfaces

The wrinkling of SU-8 may be useful for microelectromechanical systems and optical applications, but highly regular and periodic wrinkles are the most convenient for such applications. This paper reports wrinkles generated on SU-8 by exposure to 172 nm-wavelength ultraviolet under ozone. To identify the relevant processing parameters, the impact of the SU-8 layer thickness and the effects of the postexposure bake temperature and UV-ozone treatment time are investigated. With 5 μm-thick SU-8 layers, distinct wrinkles are obtained with an ultraviolet-ozone treatment of 3 min and at a postexposure bake temperature of at least 140 °C. To guide the wrinkling, topography is induced on the SU-8 surface via capillary force lithography. By using a stamp with equidistant cavities possessing different widths, the structure size dependence is investigated. Owing to local stress relaxation, single or double wrinkles form along the elevated lines depending upon the line width. Within the gaps between the lines, wrinkles oriented perpendicular to the lines develop induced by the nonrelaxed compressive stress. Wrinkling represents a simple alternative for covering the surface of SU-8 with largely self-forming regular structures that may act as transmissive optical elements.

Controlling the dispersion of supported polyoxometalate heterogeneous catalysts: impact of hybridization and the role of hydrophilicity-hydrophobicity balance and supramolecularity.

The hybridization of polyoxometalates (POMs) through an organic–inorganic association offers several processing advantages in the design of heterogeneous catalysts. A clear understanding of the organization of these hybrid materials on solid surfaces is necessary to optimise their properties. Herein, we report for the first time the organization of Keggin phosphotungstic [PW12O40]3− and Wells–Dawson (WD) phosphomolybdic [P2Mo18O62]6− anions deposited on mica (hydrophilic), and highly oriented pyrolytic graphite (HOPG) (hydrophobic) surfaces. Next, the supramolecular organization of the organic–inorganic hybrid materials formed from the association of POM anions and dimethyldioctadecylammonium bromide (DODA) is investigated as a function of the hydrophilic or hydrophobic nature of the surfaces. The height of the Keggin-POM anions, measured with tapping mode (TM-AFM) is always in good agreement with the molecular dimension of symmetric Keggin-POM anions (ca. 1 nm). However, the asymmetric WD-POM anions form monolayer assemblies on the surfaces with the orientation of their long molecular axis (ca. 1.6 nm) depending on the hydrophilic or hydrophobic properties of the substrate. Namely, the long axis is parallel on mica, and perpendicular on HOPG. When hybridized with DODA, the organization of the hybrid material is dictated by the interaction of the alkyl side chains of DODA with the substrate surface. On HOPG, the DODA–POM hybrid forms small domains of epitaxially arranged straight nanorod structures with their orientation parallel to each other. Conversely, randomly distributed nanospheres are formed when the hybrid material is deposited on freshly cleaved mica. Finally, a UV–ozone treatment of the hybrid material allows one to obtain highly dispersed isolated POM entities on both hydrophilic and hydrophobic surfaces. The hybridization strategy to prevent the clustering of POMs on various supports would enable to develop highly dispersed POM-based heterogeneous catalysts with enhanced functionalities.

Evolution of interfacial intercalation chemistry on epitaxial graphene/SiC by surface enhanced Raman spectroscopy

A rapid and facile evaluation of the effects of physical and chemical processes on the interfacial layer between epitaxial graphene monolayers on SiC(0001) surfaces is essential for applications in electronics, photonics, and optoelectronics. Here, the evolution of the atomic scale epitaxial graphene-buffer-layer–SiC interface through hydrogen intercalation, thermal annealings, UV-ozone etching and oxygen exposure is studied by means of single microparticle mediated surface enhanced Raman spectroscopy (smSERS). The evolution of the interfacial chemistry in the buffer layer is monitored through the Raman band at 2132cm−1 corresponding to the SiH stretch mode. Graphene quality is monitored directly by the selectively enhanced Raman signal of graphene compared to the SiC substrate signal. Through smSERS, a simultaneous correlation between optimized hydrogen intercalation in epitaxial graphene/SiC and an increase in graphene quality is uncovered. Following UV-ozone treatment, a fully hydrogen passivated interface is retained, while a moderate degradation in the quality of the hydrogen intercalated quasi-freestanding graphene is observed. While hydrogen intercalated defect free quasi-freestanding graphene is expected to be robust upon UV-ozone, thermal annealing, and oxygen exposure, ozonolytic reactivity at the edges of H-intercalated defected graphene results in enhanced amorphization of the quasi-freestanding (compared to non-intercalated) graphene, leading ultimately to its complete etching.

High performance unipolar inverters by utilizing organic field-effect transistors with ultraviolet/ozone treated polystyrene dielectric

High performance unipolar inverters based on a significant variation of threshold voltage (Vth) of organic field-effect transistors (OFETs), which was realized by introducing UV/ozone (UVO) treatment to polystyrene (PS) dielectric, were fabricated. A controllable Vth shift of more than 10 V was obtained in the OFETs by adjusting the UVO treating time, and the unipolar inverters exhibited inverting voltage near 1/2 driving voltage and a noise margin of more than 70% of ideal value. From the analysis of scanning electron microscopy, atom force microscopy, and X-ray photoelectron spectroscopy, the dramatic controllable Vth of OFETs, which played a key role in high performance unipolar inverters, was attributed to the newly generated oxygen functional groups in the PS dielectric induced by UVO treatment.

Schwann cell interactions with polymer films are affected by groove geometry and film hydrophilicity

We have developed a biodegradable polymer scaffold made of a polycaprolactone/polylactic acid (PCL/PLA) film. Surface properties such as topography and chemistry have a vital influence on cell–material interactions. Surface modifications of PCL/PLA films were performed using topographical cues and UV–ozone treatment to improve Schwann cell organisation and behaviour. Schwann cell attachment, alignment and proliferation were evaluated on the grooved UV–ozone treated and non-treated films. Solvent casting of the polymer solution on patterned silicon substrates resulted in films with different groove shapes: V (V), sloped (SL) and square (SQ) shapes. Pitted films, with no grooves, were prepared as a negative control. The UV–ozone treatment was performed to increase hydrophilicity. The process specifications for UV–ozone treatment were evaluated and 5 min radiation time and 6 cm distance to the UV source were suggested as the optimal practise. When cultured on grooved films, Schwann cells elongated on the V and SL shape grooves without crossing over, and grew in the direction of the grooves. However, there was less elongation with more crossing over on the SQ shape grooves. The maximum cell length (511 μm) was observed on the treated V-grooved films. The cells cultured on pitted UV–ozone treated surfaces showed random arrangements with no increase in length. We have demonstrated that the synergic effects of physical cues combined with UV–ozone treatment have the potential to enhance Schwann cell morphology and alignment.

Selective-Area Atomic Layer Deposition of Copper Nanostructures for Direct Electro-Optical Solar Energy Conversion

We investigate selective-area atomic layer deposition (ALD) of copper onto nanofabricated palladium structures for plasmonic applications in electro-optic conversion devices. We examine arrays of nanofabricated tip shaped electrodes for the formation of geometrically asymmetric tunnel junctions. We find ALD growth to be sensitive to sample pretreatment, and demonstrate that UV-ozone treatment is effective for initiating growth. We show that nucleation density and grain evolution are important properties of growth because ALD deposited topological features are on the same scale as the nanostructures. We predict electric-field intensities within the gap region between electrodes to scale exponentially with ALD growth. We establish the basis for ALD tuning of plasmonic nanostructures.

Surface treatment with UV-ozone to improve adhesion of vulcanized rubber formulated with an excess of processing oil

An excess of processing oil was intentionally added in the formulation of vulcanized styrene-butadiene rubber for analyzing the effect on the treatment with UV radiation combined with ozone (UV-ozone) in improving the adhesion to waterborne polyurethane adhesive in footwear. Due to the excess of processing oil in the rubber, poor adhesion was expected. Both the length of the treatment and the distance between the UV radiation source to the rubber surface were studied, and the effects of the treatment on the surface chemistry, wettability and surface energy, and topography of the rubber were analyzed. The treatment of the rubber with UV-ozone removed hydrocarbon moieties and zinc stearate from the surface, and surface oxidation (C-O, C=O and COO- groups formation) occurred. As a consequence, improved wettability and increased surface energy (mainly due to the polar component of the surface energy) were obtained. The increase in the length of treatment and for a distance of 3–5 cm from the rubber to the UV radiation source favored the effectiveness of the UV-ozone treatment. Besides, whereas the UV-ozone treatment for 3min produced the ablation of surface contaminants mainly, longer lengths of treatment (i.e., 6–9min) caused roughness and cracks on the rubber surface. The UV-ozone treatment also caused heating of the surface. Finally, the treatment with UV-ozone increased the adhesion of the rubber to waterborne polyurethane adhesive, the highest adhesion was obtained in the joints made with UV-ozone treated rubber for 3 and 6 min at a UV radiation source-rubber surface distance of 5cm.

Nickel Oxide Hole Injection/Transport Layers for Efficient Solution-Processed Organic Light-Emitting Diodes

Solution-processed nickel oxides (s-NiOx) are used as hole injection and transport layers in solution-processed organic light-emitting diodes (OLEDs). By increasing the annealing temperature, the nickel acetate precursor fully decomposes and the s-NiOx film shows larger crystalline grain sizes, which lead to better hole injection and transport properties. UV–ozone treatment on the s-NiOx surface is carried out to further modify its surface chemistry, improving the hole injection efficiency. The introduction of more dipolar species of nickel oxy-hydroxide (NiO(OH)) is evidenced after the treatment. Dark injection–space charge limited (DI–SCL) transient measurement was carried out to compare the hole injection efficiency of s-NiOx and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) hole injection layers (HIL). The UV–ozone treated s-NiOx shows significantly better hole injection, with a high injection efficiency of 0.8. With a p-type thin film transistor (TFT) configuration, the high-tem...

Do Surface Wetting Properties Affect Calcium Carbonate Heterogeneous Nucleation and Adhesion?

We study the effect of wetting properties on the propensity of a surface to heterogeneously nucleate or adsorb calcium carbonate from a saturated aqueous solution. Glass, silanized glass, and polyethylene surfaces are considered. UV-ozone is used to tune the wetting behavior from hydrophobic to hydrophilic by forming oxidized carbon groups (alcohol, aldehyde, carboxylic). For all substrates that do not promote any specific orientation of CaCO3 crystals, increasing hydrophilicity inhibits CaCO3 nucleation. Complete ⟨104⟩ and ⟨001⟩ crystal orientations relative to the substrate plane are obtained for silanized glass exposed to prolonged UV-ozone treatment; nucleation densities are then also considerably higher. Our results highlight the role of interfacial surface energies and orientation in heterogeneous crystal nucleation and adsorption phenomena and contribute to the rational design of antiscaling surface treatments.

Highly transparent and flexible triboelectric nanogenerators: performance improvements and fundamental mechanisms

Transparent, flexible and highly efficient power sources are essential components of mobile electronics and optoelectronic devices. Here, based on the first generation of the transparent triboelectric nanogenerator (TENG), we demonstrate a simple and innovative design that can simultaneously improve the output performance and transmittance of the TENG. The improved TENG gives a maximum output up to 200 V and 7 μA at a current density of ∼0.78 μA cm−2. The TENG shows a high transmittance of ∼78%. To deeply understand the nature of the triboelectric effect, we investigated the influence of the UV–ozone treatment, surface properties, and surrounding environment on the output performance. Integrating the characterization results, we conclude that the tribocharge generation of the PDMS surface is probably due to the bond breaking of Si–O–Si groups, and is closely related to the surface properties and surrounding environment.

UV/ozone surface treatment increases hydrophilicity and enhances functionality of SU-8 photoresist polymer

Abstract SU-8 photoresist polymer is widely used in the fabrication of microdevices. However, for biological applications, the problem of efficiently modifying SU-8 surfaces without perturbing roughness has not been successfully resolved. We present UV/ozone (UVO) surface pre-treatment as an effective method to increase the hydrophilicity of SU-8 films without affecting surface roughness, thus improving specific covalent binding of bio-molecules. We demonstrate that 30 s UVO treatment suffices to create carboxyl groups at the surface that can then be used for high density binding of molecules via amide bond formation. We further demonstrate that a two-step surface modification where the surface is first protected with an ethylene glycol monolayer leads to an increase in binding specificity. Finally, to illustrate the controlled binding and accessibility of immobilized molecules, we show three cycles of reversible interactions between anti-tamra antibody and tamra-cadaverine immobilized on the surface of SU-8.

Finding the lost open-circuit voltage in polymer solar cells by UV-ozone treatment of the nickel acetate anode buffer layer.

Efficient polymer solar cells (PSCs) with enhanced open-circuit voltage (Voc) are fabricated by introducing solution-processed and UV-ozone (UVO)-treated nickel acetate (O-NiAc) as an anode buffer layer. According to X-ray photoelectron spectroscopy data, NiAc partially decomposed to NiOOH during the UVO treatment. NiOOH is a dipole species, which leads to an increase in the work function (as confirmed by ultraviolet photoemission spectroscopy), thus benefitting the formation of ohmic contact between the anode and photoactive layer and leading to increased Voc. In addition, the UVO treatment improves the wettability between the substrate and solvent of the active layer, which facilitates the formation of an upper photoactive layer with better morphology. Further, the O-NiAc layer can decrease the series resistance (Rs) and increase the parallel resistance (Rp) of the devices, inducing enhanced Voc in comparison with the as-prepared NiAc-buffered control devices without UVO treatment. For PSCs based on the P3HT:PCBM system, Voc increases from 0.50 to 0.60 V after the NiAc buffer layer undergoes UVO treatment. Similarly, in the P3HT:ICBA system, the Voc value of the device with a UVO-treated NiAc buffer layer increases from 0.78 to 0.88 V, showing an enhanced power conversion efficiency of 6.64%.