A. Laskarakis

On the optical anisotropy of poly(ethylene terephthalate) and poly(ethylene naphthalate) polymeric films by spectroscopic ellipsometry from visible-far ultraviolet to infrared spectral regions

The optical anisotropy of biaxially stretched poly(ethylene terephthalate) and poly(ethylene naphthalate) films has been investigated, by visible-far ultraviolet to IR spectroscopic ellipsometry (SE). The consistency of the SE spectra analysis results between the two energy regions, in a full rotational scan of the angle θ between the plane of incidence and the stretching direction, justified the approximation of the films as uniaxial materials with their optic axis parallel to the surface.The optical anisotropy of biaxially stretched poly(ethylene terephthalate) and poly(ethylene naphthalate) films has been investigated, by visible-far ultraviolet to IR spectroscopic ellipsometry (SE). The consistency of the SE spectra analysis results between the two energy regions, in a full rotational scan of the angle θ between the plane of incidence and the stretching direction, justified the approximation of the films as uniaxial materials with their optic axis parallel to the surface.

Study of the electronic and vibrational properties of poly(ethylene terephthalate) and poly(ethylene naphthalate) films

We investigate the optical properties of biaxially stretched poly(ethylene terephthalate) (PET) and poly(ethylene naphthalate) (PEN) polymer films by spectroscopic ellipsometry (SE) in a wide spectral region, from the infrared (IR) (900−3500 cm−1) to the vis-far UV (vis-fUV) (1.5−6.5 eV), in terms of their optical, electronic, and vibrational response. The stretching procedure during the fabrication of the films leads to the rearrangement of the macromolecular chains parallel to the stretching direction (or machine direction), resulting in an optical anisotropy of the films. For the deduction of valuable and accurate information about the films’ electronic and vibrational response, the analysis of the measured SE spectra has been realized by approximating the PET and PEN films as uniaxial materials with their optic axes parallel to the surface. In the vis-fUV spectral region, the characteristic features corresponding to the n→π* electronic transitions of the carbonyl −C=O and the A1g1→B1u1 electronic tran...

Effects of buffer layer properties and annealing process on bulk heterojunction morphology and organic solar cell performance

The performance of polymer–fullerene bulk heterojunction (BHJ) solar cells is strongly dependent on the vertical distribution of the donor and acceptor regions within the BHJ layer. In this work, we investigate in detail the effect of the hole transport layer (HTL) physical properties and the thermal annealing on the BHJ morphology and the solar cell performance. For this purpose, we have prepared solar cells with four distinct formulations of poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) buffer layers. The samples were subjected to thermal annealing, applied either before (pre-annealing) or after (post-annealing) the cathode metal deposition. The effect of the HTL and the annealing process on the BHJ ingredient distribution – namely, poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) – has been studied by spectroscopic ellipsometry and atomic force microscopy. The results revealed P3HT segregation at the top region of the films, which had a detrimental effect on all pre-annealed devices, whereas PCBM was found to accumulate at the bottom interface. This demixing process depends on the PEDOT:PSS surface energy; the more hydrophilic the surface the more profound is the vertical phase separation within the BHJ. At the same time those samples suffer from high recombination losses as evident from the analysis of the J–V measurements obtained in the dark. Our results underline the significant effect of the HTL–active and active–ETL (electron transport layer) interfacial composition that should be taken into account during the optimization of all polymer–fullerene solar cells.

A study on the bonding structure and mechanical properties of magnetron sputtered CNx thin films

Abstract Carbon nitride (CN x ) films have been deposited by reactive (RF) magnetron sputtering, in order to investigate the effect of the energetic ion bombardment during deposition (IBD), in terms of applied V b , on their bonding structure. Fourier Transform IR Ellipsometry (FTIRE) and X-ray photoelectron spectroscopy (XPS) were used for the investigation of the films bonding structure, while their mechanical properties were evaluated by nanoindentation measurements. At films grown with low negative V b , (low energy IBD) the N atoms are distributed homogeneously in substitutional sites in graphitic rings through both sp 2 and sp 3 bonds and in linear chains, through sp 2 bonds. In contrast, the high negative V b (high energy IBD) has been suggested to promote the non-homogeneous N distribution at localized regions in the films where the formation of sp 3 CN bonds is favored. This behavior was also evidenced by the C1s and N1s XPS peak components, assigned to the sp 3 and sp 2 carbon–nitrogen bonds. Also, high energy IBD films revealed increased values of hardness and elasticity, while hardness values up to 45 GPa were measured at localized regions.

Variation of nitrogen incorporation and bonding configuration of carbon nitride films studied by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FT-IR) spectroscopic ellipsometry

Abstract A detailed study of the bonding structure and chemical composition of carbon nitride (CN x ) thin films prepared on c-Si substrates by RF reactive magnetron sputtering is presented. The nitrogen content in 19×10 −3 and 4×10 −3 Ar/N 2 mixtures was varied between 0 and 100% while the bias voltage ( V b ) between +10 and −200 V. The films’ elemental composition and chemical bonding were determined by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopic ellipsometry (FTIRSE). The N/C ratio decreased substantially for films grown with high negative V b . The N1s spectral region was fitted with multiple peaks corresponding to various carbon–nitrogen bonding configurations (sp 3 , sp 2 and sp 1 ), and oxygen-bonded N. At films grown with V b =+10 V, the N atoms distribution among the possible bonding structures with C was unaffected by the P N 2 changes. These films exhibit a mixed sp 1 /sp 2 /sp 3 character with an enhanced contribution of sp 2 CN and sp 1 CN and NC bonds, as evidenced by FTIRSE. A substantial increase of the sp 3 CN bond contribution in films grown with −40≤ V b ≤−200 V was evidenced by both XPS and FTIRSE.

Thin film and interface properties during ZnO deposition onto high-barrier hybrid/PET flexible substrates

The combination of transparent conductive oxides with high-barrier films deposited onto flexible polymeric substrates is of considerable importance in order to improve the efficiency, lifetime and stability of flexible electronic devices. In this work, ZnO thin films have been deposited onto high-barrier hybrid/PET flexible substrates by pulsed DC magnetron sputtering, at room temperature and by applying different power values on the target. The employment of in situ and real-time Vis-fUV (1.5-6.5 eV) spectroscopic ellipsometry allowed the investigation of the growth mechanisms of ZnO thin films as well as the modification procedure in the hybrids surface. Island growth is dominant during the initial stages of deposition concerning low target power regime, whereas layer-by-layer deposition prevails at the high target power regime. The hybrids modified layer of approximately 10nm was confirmed by the transmission electron microscopy measurements which additionally revealed a columnar structure of the film with a nanocrystalline morphology. The estimated size of the nanocrystals ( approximately 15 nm and above) was compatible with atomic force microscopy (AFM) measurements. Finally, the evolution of the optical parameters (energy gap and absorption peaks) of the ZnO films during the deposition was similar.

In situ and real-time ellipsometry diagnostic techniques towards the monitoring of the bonding structure and growth kinetics: silicon oxide coatings

Abstract We present a modern methodology concerning the applicability of in situ and real-time optical diagnostic techniques for the study of the optical properties, bonding structure evolution during film growth, in the case of silicon oxide films deposited by e-beam evaporation. Fourier Transform IR Spectroscopic Ellipsometry (SE) was used for in situ and real-time monitoring of the film optical properties during growth. This technique enables the investigation of the films bonding structure and stoichiometry at different growth stages that were associated with the partial pressures of the evaporated species. In addition, films’ optical properties, density and surface and interface quality were studied by multi-wavelength SE in UV–vis region. This methodology reveals the potential of in situ and real-time diagnostic techniques, for the optimization of process control by the preparation of the appropriate transparent oxide coatings for various applications.

Spectroscopic ellipsometry studies on BN films from IR to vacuum UV energy region

Abstract The dielectric function e(ω) of fully sp 3 or sp 2 amorphous BN materials, was calculated in the range 0.3–20 eV by applying the appropriate modeling, using one or two Tauc–Lorentz (TL) oscillators, respectively, in the analysis of the measured e(ω) from 1.5 to 9.5 eV, combining conventional spectroscopic ellipsometry (SE) with synchrotron radiation (SR) SE. The basic characteristics of the electronic transitions occurring in the different types of BN hybridized bonds, such as energy of absorption, broadening, and the fundamental optical gap were deduced. Based on these results we studied composite BN films prepared by magnetron sputtering, and measured by SRSE, and in situ SE in the range 1.5–5.5 eV. The dominant sp 2 or sp 3 BN character can easily be revealed when the upper limit of the e(ω) spectrum is confined to 5.5 eV. However, the extension of this limit to 9.5 eV can provide more valuable information on the bonding configuration in BN materials, which are well-correlated to the Fourier Transform IR SE measurements, also presented in this work.

High mobility transistors based on electrospray-printed small-molecule/polymer semiconducting blends

Spray-coating techniques have recently emerged as especially effective approaches for the deposition of small semiconducting molecules toward the fabrication of organic field-effect transistors (OFETs). Despite the promising mobility values and the industrial implementation capability of such techniques, the resultant devices still face challenges in terms of morphology control and performance variation. In this work, the efficient process control of electrostatic spraying deposition (ESD) and the excellent film forming properties of polymer:small molecule blends were successfully combined to develop reliable and high performance transistors. Specifically, a highly efficient blended system of 2,8-difluoro-5,11-bis(triethylsilylethynyl)-anthradithiophene (diF-TES-ADT) and poly(triarylamine) (PTAA) was employed in order to realize top-gate OFETs under ambient conditions, both on rigid and on flexible substrates. The films revealed extensive crystallization and microstructural organization implying distinct phase separation in the electrosprayed blend. Furthermore, we investigated the effect of processing temperature on film continuity and the presence of grain boundaries. Remarkably, the electrosprayed OFETs exhibited field-effect mobilities as high as 1.7 cm2 V−1 s−1 and enhanced performance consistency when compared to conventional gas-sprayed transistors. Additionally, the transistors showed excellent electrical and environmental stability, indicative of the good interface quality and the self-encapsulation capability of the top-gate structure. These results highlight the great potential of electrohydrodynamic atomization techniques for implementation in large-area processing for OFET fabrication.

Organic transistors based on airbrushed small molecule-insulating polymer blends with mobilities exceeding 1 cm2 V−1 s−1

Spray-coating, has recently fueled scientific interest as a versatile solution-processing technique for the realization of organic electronic devices, such as organic field-effect transistors (OFETs). In the present work, air-brush method was used for the deposition of semiconducting blends of triisopropylsilylethynyl-pentacene (TIPS-PEN) and common insulating polymers of polystyrene or polymethylmethacrylate. The use of such blend systems not only resulted in an improved wet film formation but also enabled efficient control over the crystallization process. A systematic study on the effect of different composition ratio on the morphology and crystallinity of the sprayed films as well as their macroscopic uniformity, was carried out. Both blend systems revealed well-ordered TIPS-PEN crystalline domains on the top surface, indicative of the pronounced phase separation phenomena. The optimized airbrushed OFETs exhibited excellent electrical characteristics with a maximum hole mobility value of 1.3 cm2 V−1 s−1, negligible hysteresis, near-zero turn-on voltages and on/off current ratio greater than 105. Additionally, the transistors revealed good long-term environmental stability, with no significant degradation after a period of 13 months. These results represent an important step for present and future applications of spaying techniques toward the controlled growth of high performance and environmentally stable OFETs.