Rodrigo Martins

The charge transfer dynamics at interfaces are fundamental to know the mechanism of photovoltaic processes. The internal potential in solar cell devices depends on the basic processes of photovoltaic effect such as charge carrier generation, separation, transport, recombination, etc. Here we report the direct observation of the surface potential depth profile over the cross-section of the ZnO nanorods/Cu2O based solar cell for two different layer thicknesses at different wavelengths of light using Kelvin probe force microscopy. The topography and phase images across the cross-section of the solar cell are also observed, where the interfaces are well-defined on the nanoscale. The potential profiling results demonstrate that under white light illumination, the photoinduced electrons in Cu2O inject into ZnO due to the interfacial electric field, which results in the large difference in surface potential between two active layers. However, under a single wavelength illumination, the charge carrier generation, separation, and transport processes between two active layers are limited, which affect the surface potential images and corresponding potential depth profile. Because of changes in the active layer thicknesses, small variations have been observed in the charge carrier transport mechanism inside the device. These results provide the clear idea about the charge carrier distribution inside the solar cell in different conditions and show the perfect illumination condition for large carrier transport in a high performance solar cell. © 2016 American Chemical Society.

Electrochemically active bacteria (EAB) have the capability to transfer electrons to cell exterior, a feature that is currently explored for important applications in bioremediation and biotechnology fields. However, the number of isolated and characterized EAB species is still very limited regarding their abundance in nature. Colorimetric detection has emerged recently as an attractive mean for fast identification and characterization of analytes based on the use of electrochromic materials. In this work, WO 3 nanoparticles were synthesized by microwave assisted hydrothermal synthesis and used to impregnate non-treated regular office paper substrates. This allowed the production of a paper-based colorimetric sensor able to detect EAB in a simple, rapid, reliable, inexpensive and eco-friendly method. The developed platform was then tested with Geobacter sulfurreducens, as a proof of concept. G. sulfurreducens cells were detected at latent phase with an RGB ratio of 1.10 ± 0.04, and a response time of two hours.

Results are presented which are geared towards an understanding of the influence of powder formation during film growth. Plasma chemistry is correlated with the morphology, structure (inferred through infrared spectroscopy, scanning electron microscopy and X-ray diffraction) electro-optical and density of states of intrinsic films deposited under continuous and power modulated operation. Results show that for modulation frequencies where no powder formation occurs and low substrate temperatures T (150°C), silane decomposition gives rise to the growth of inhomogeneous films while in the high modulation frequency regime, at the same temperature, the anions and powder are trapped resulting in films with high deposition rates and low defect density.

P- and n-type weakly absorbing highly conductive (σ>0·1Ω-1 cm-1) SiC thin films with similar structural and optoelectronic properties have been prepared in a two-consecutive-decomposition-deposition-chamber reactor. These films are composed of Si microcrystals (δ = 50-100 Å) embedded in an amorphous Si:0:C:H matrix, with concentrations up to 25at.%O and 20at.%C. From diffraction studies there is no evidence for the presence of SiC crystallites. Electrical conduction appears to be in extended states via percolation channels through Si crystallites of sufficient volume fraction. © 1991 Taylor & Francis Ltd.

ZnO/CNT composites were prepared using ZnO nanoparticles and tetrapods synthesized by the Laser Assisted Flow Deposition method. The co-operative behaviour between these two materials may give rise to the production of advanced functional materials with a wide range of applications in electronics and optoelectronics. Despite some degree of aggregation in the case of the nanoparticles, scanning electron microscopy images evidence that the produced ZnO structures are well dispersed in the CNT buckypapers. Independent of the ZnO morphology the samples resistivity was shown to be of the order of ∼10-1 Ω cm while in the case of the electron mobility, the composite with tetrapods reveals a lower value than the ones obtained for the remaining samples. Well-structured ZnO luminescence was observed mainly in ultraviolet highlighting the high optical quality of the produced structures. The temperature dependence of the luminescence reveals a distinct trend for the composites with ZnO tetrapods and ZnO nanoparticles. © 2015 Elsevier B.V.

In this study, we report solution-processed amorphous zinc tin oxide transistors exhibiting high operational stability under positive gate bias stress, translated by a recoverable threshold voltage shift of about 20% of total applied stress voltage. Under vacuum condition, the threshold voltage shift saturates showing that the gate-bias stress is limited by trap exhaustion or balance between trap filling and emptying mechanism. In ambient atmosphere, the threshold voltage shift no longer saturates, stability is degraded and the recovering process is impeded. We suggest that the trapping time during the stress and detrapping time in recovering are affected by oxygen adsorption/desorption processes. The time constants extracted from stretched exponential fitting curves are ∼106 s and 105 s in vacuum and air, respectively. © 2015 Author(s).

In the present work, the spectroscopic ellipsometry (1.5 - 5.5 eV) was used to investigate the effects of current density induced microstructural variations and their influence on the electronic states of as-prepared and a-Si:H coated porous silicon (PS). The pseudodielectric responses of the low and high current densities (5 and 40 mA/cm2) were analyzed using a multilayer model within the effective medium approximation. The FTIR investigation reveals me enhancement of surface oxide (Si-Ox) layer with current density and the improvement of the Si-Hx band after a-Si:H coating.

PbZrxTi1-xO3 (PZT) films are currently being investigated in view of their large switching polarization and piezoelectric coefficients useful for various applications. Besides, PZT films possess large photosensitivity, which, in combination with the above listed properties, can be a base for future microelectronic applications including photostrictive actuators and optical storage devices. In this work, PZT thin films of several compositions (x=0.2 and 0.45) were deposited on Pt-coated Si and ITO/glass substrates via modified sol-gel technique. Microstructures of the films were evaluated using XRD, SEM and AFM. The optical transmission measurements on PZT films deposited on ITO/glass revealed a high transparency over 80% and a band gap of about 3.4 eV. The observed photocurrent exhibited a maximum and was attributed to band-to-band optical transitions.

In the production of porous silicon (PS) to optoelectronic application one of the most significant constrains is the surface defects passivation. In the present work we investigate, gallium-doped zinc oxide (GZO) thin films deposited by rf magnetron sputtering at room temperature on PS obtained with different etching times. The X-ray diffraction (XRD), Fourier transform infrared (FTIR) and atomic force microscopy (AFM) analysis have been carried out to understand the effect of GZO films coating on PS. Further, the XRD analysis suggests the formation of a good crystalline quality of the GZO films on PS. From AFM investigation we observe that the surface roughness increases after GZO film coating. The photoluminescence (PL) measurements on PS and GZO films deposited PS shows three emission peaks at around 1.9 eV (red-band), 2.78 eV (blue-band) and 3.2 eV (UV-band). PL enhancement in the blue and ultraviolet (UV) region has been achieved after GZO films deposition, which might be originated from a contribution of the near-band-edge recombination from GZO. © 2007 Elsevier B.V. All rights reserved.

Aqueous suspensions of cordierite-based glass ceramics were prepared by using four types of dispersants and binders and different solids loading. The experiments were designed according to the Taguchi method, which shows great advantages in optimising more than two factors that need to be considered in an experimental design. Different parameters such as the type and concentration of the dispersants and the binders, and the solids loading were optimised to obtain homogeneous and crack-free green tapes. Dolapix CE 64 (1.0 wt.%) and Duramax B-1080 or Duramax B-1070 (10 wt.%) with 65 wt.% solids loading represent an optimal selection of the parameters to obtain low viscosity suspension, and crack-free green tapes with the highest green and sintered density. Microstructural differences between crack-free and cracked samples were observed by scanning electron microscopy (SEM). The crack-free green tapes show homogenous microstructures from top to bottom with organic additives uniformly surrounding the powders, whereas cracked samples exhibit heterogeneous microstructures and non-uniform distribution of the organics. © 2002 Elsevier Science B.V. All rights reserved.

The aim of this work is to optimize the metal/silicon ratio on nickel metal induced crystallization of silicon. For this purpose amorphous silicon layers with 80, 125 and 220 nm thick were used on the top of which 0.5 nm of Ni was deposited and annealed during the required time to full crystallize the a-Si. The data show that the 80 nm a-Si layer reaches a crystalline fraction of 95.7% (as detected by spectroscopic ellipsometry) after annealed for only 2 hours. No significant structural improvement is detected by ellipsometry neither by XRD when annealing the films for longer times. However, on 125 nm thick samples, after annealing for 2 hours the crystalline fraction is only 59.7%, reaching a similar value to the one with 80 nm only after 5 hours, with a crystalline fraction of 92.2%. Here again no significant improvements were achieved by using longer annealing times. Finally, the 220 nm thick a-Si sample is completely crystallized only after 10 hours annealing. These data clear suggest that the crystallization of thicker a-Si layers requires thicker Ni films to be effective for short annealing times. A direct dependence of the crystallization time on the metal/silicon ratio was observed and estimated. © 2005 Materials Research Society.

Transparent electronics is today one of the most advanced topics for a wide range of device applications. The key components are wide bandgap semiconductors, where oxides of different origins play an important role, not only as passive component but also as active component, similar to what is observed in conventional semiconductors like silicon. Transparent electronics has gained special attention during the last few years and is today established as one of the most promising technologies for leading the next generation of flat panel display due to its excellent electronic performance. In this paper the recent progress in n- and p-type oxide based thin-film transistors (TFT) is reviewed, with special emphasis on solution-processed and p-type, and the major milestones already achieved with this emerging and very promising technology are summarizeed. After a short introduction where the main advantages of these semiconductors are presented, as well as the industry expectations, the beautiful history of TFTs is revisited, including the main landmarks in the last 80 years, finishing by referring to some papers that have played an important role in shaping transparent electronics. Then, an overview is presented of state of the art n-type TFTs processed by physical vapour deposition methods, and finally one of the most exciting, promising, and low cost but powerful technologies is discussed: solution-processed oxide TFTs. Moreover, a more detailed focus analysis will be given concerning p-type oxide TFTs, mainly centred on two of the most promising semiconductor candidates: copper oxide and tin oxide. The most recent data related to the production of complementary metal oxide semiconductor (CMOS) devices based on n- and p-type oxide TFT is also be presented. The last topic of this review is devoted to some emerging applications, finalizing with the main conclusions. Related work that originated at CENIMAT|I3N during the last six years is included in more detail, which has led to the fabrication of high performance n- and p-type oxide transistors as well as the fabrication of CMOS devices with and on paper. Transparent electronics is one of the most advanced science topics for a broad range of device applications. In this article an overview is presented of state-of-the-art n- and p-type oxides for TFTs and their integration, processed by physical vapor deposition and by solution processing techniques. Some of the most relevant emerging applications are also presented, including CMOS devices based on oxide TFTs fabricated on glass and even on paper. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

The effect is considered of order and disorder on the electrical and optical performance of ionic oxide semiconductors used to produce optoelectronic devices such as p-n heterojunction solar cells and thin-film transistors (TFTs). The results obtained show that p-type c-Si/a-IZO/poly-ZGO solar cells exhibit efficiencies above 14% in device areas of about 2.34 cm2, whereas amorphous oxide TFTs based on the Ga-Zn-Sn-O system demonstrate superior performance to the polycrystalline ZnO TFTs, with ION/I OFF ratio exceeding 107, turn-on voltage below 1-2 V and saturation mobility above 25 cm2 V-1 s-1. In addition, preliminary data on a p-type oxide TFT based on the Zn-Cu-O system are presented. © 2009 Taylor & Francis.