António F. da Cunha

Incorporation of Ga in CIGS Absorber Layers Formed by RF-Magnetron Sputtering in Se Vapours

Cu(In,Ga)Se2 (CIGS) thin film semiconductors are among the most attractive materials for thin film solar cell applications. Conversion efficiency exceeding 19% has been achieved for CIGS absorber layers deposited by three-stage co-evaporation technique. From a technological point of view the sputtering deposition process is more attractive than thermal co-evaporation, however, solar cell parameters obtained so far are worse. The highest efficiency value reported for co-sputtered CIS thin films is less than 8% and there is no data found for CIGS layers produced by a similar technique. We have developed a hybrid RF-magnetron sputtering/evaporation method for the deposition of the CIGS absorber layer. In this method Cu and In are sequentially sputtered from metallic targets in the presence of Se vapour. Ga depth profiling leads to a band gap grading which is known to play an important role in cell performance. Here, we report the results of our work on three different ways of Ga incorporation into the CIGS thin films. They consisted of sputtering from In-GaSe, Cu-GaSe composite targets and Ga evaporation. The Ga content and distribution across the layer thickness was investigated by AES measurements. The CIGS formation kinetics, structural and compositional studies were performed by SEM, XRD and AES measurements.

Synthesis, characterization and electrochemical properties of meso-thiocarboxylate-substituted porphyrin derivatives

Considering the versatility of 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin (TPPF20) to react with nucleophiles we highlight here the synthesis and characterization of several mono- and tetra-thiocarboxylate derivatives. The selective displacement of the para-fluorine groups in TPPF20 by thiocarboxylic acids demonstrates that TPPF20 is an ideal platform for the rapid formation of thiocarboxylate porphyrins. The optical and electrochemical features of the thiocarboxylate derivatives were also examined thinking on their potential use in photovoltaic devices. From their electrochemical characterization the following parameters were taken into account: (i) electronegative induced effect of the thiocarboxylate dyes owing the presence of the fluorine and sulfur atoms on the molecular structure of the porphyrin; and (ii) the free rotation and flexibility features that such S atom gives to the porphyrin relatively to the semiconductor.

Cu(In,Ga)Se2 Prepared by a 2 and 3-Stage Hybrid RF-Magnetron Sputtering and Se Evaporation Method: Properties and Solar Cell Performance

The potential of RF-magnetron sputtering to achieve high quality Cu(In,Ga)Se2 (CIGS) thin films and efficient solar cells with the goal of using a single technique for all solar cell processing steps is explored. The end point detection method was adapted to RF-magnetron deposition of CIGS in two- or three stages sputtering process. It allows the control of the final composition of the deposited layers in a reproducible way. The influence of substrate temperature and Ar pressure during the deposition on the surface and crossectional morphology of CIGS films was studied for two and three-stage sputtering process sequence. The solar cells prepared with films deposited by two-stage sputtering nave showed a better performance with maximum efficiency above 8 %.

Continuous frequency control from 5 to 10 GHz of an optical oscillator

Continuous frequency control from 5 to 10 GHz of an optical oscillator, can be induced in a single cavity DFB-MQW laser by simple change the laser bias current using the injection locking mechanism. This system is very attractive for optical communications, namely for clock recovery in high bit rate optical transmission systems.

A Deep Learning Approach for Red Lesions Detection in Video Capsule Endoscopies.

The wireless capsule endoscopy has revolutionized early diagnosis of small bowel diseases. However, a single examination has up to 10 h of video and requires between 30–120 min to read. Computational methods are needed to increase both efficiency and accuracy of the diagnosis. In this paper, an evaluation of deep learning U-Net architecture is presented, to detect and segment red lesions in the small bowel. Its results were compared with those obtained from the literature review. To make the evaluation closer to those used in clinical environments, the U-Net was also evaluated in an annotated sequence by using the Suspected Blood Indicator tool (SBI). Results found that detection and segmentation using U-Net outperformed both the algorithms used in the literature review and the SBI tool.

P–N Junction Passivation in Kesterite Solar Cells by Use of Solution-Processed TiO2 Layer

In this work, we used a solution-processed TiO₂ layer between Cu₂ZnSnSe₄ and CdS buffer layer to reduce the recombination at the p–n junction. Introducing the TiO₂ layer showed a positive impact on <italic>V</italic>_OC but fill factor and efficiency decreased. Using a KCN treatment, we could create openings in the TiO₂ layer, as confirmed by transmission electron microscopy measurements. Formation of these openings in the TiO₂ layer led to the improvement of the short-circuit current, fill factor, and the efficiency of the modified solar cells.

Detection of ZnS phases in CZTS thin-films by EXAFS

Copper zinc tin sulfide (CZTS) is a promising Earth-abundant thin-film solar cell material; it has an appropriate band gap of ∼1.45 eV and a high absorption coefficient. The most efficient CZTS cells tend to be slightly Zn-rich and Cu-poor. However, growing Zn-rich CZTS films can sometimes result in phase decomposition of CZTS into ZnS and Cu2SnS3, which is generally deleterious to solar cell performance. Cubic ZnS is difficult to detect by XRD, due to a similar diffraction pattern. We hypothesize that synchrotron-based extended X-ray absorption fine structure (EXAFS), which is sensitive to local chemical environment, may be able to determine the quantity of ZnS phase in CZTS films by detecting differences in the second-nearest neighbor shell of the Zn atoms. Films of varying stoichiometries, from Zn-rich to Cu-rich (Zn-poor) were examined using the EXAFS technique. Differences in the spectra as a function of Cu/Zn ratio are detected. Linear combination analysis suggests increasing ZnS signal as the CZTS films become more Zn-rich. We demonstrate that the sensitive technique of EXAFS could be used to quantify the amount of ZnS present and provide a guide to crystal growth of highly phase pure films.

Comparative study of the structural and optical properties of CIS films prepared by RFmagnetron sputtering and selenization of elemental layers

We report on a study where the properties of films obtained by RF-magnetron sputtering and by Selenization of elemental precursor layers are analysed by Raman scattering, x-ray diffraction and optical measurements. Three routes were followed to prepare CIS films on glass. CIS type-I was prepared by selenization at various temperatures, CIS type-II was prepared by RF-magnetron sputtering on room temperature substrate followed by annealing at 450°C in air for 10 min and CIS type-III was prepared by RF-magnetron sputtering on a substrate at a temperature ranging from 200°C up to 500°C with a post-deposition annealing in the same conditions as for CIS type-II. Correlating the results from x-ray diffraction with the Raman scattering and optical measurements it was possible to establish unequivocally the formation of CIS with the chalcopyrite structure for CIS type-I at 400°C. Through the same procedure it was possible to establish a way to produce CIS type-II with the chalcopyrite structure. A high density of defects was inferred from the transmission results. Finally the growth dynamics of CIS type-III was studied. It was observed a structural/compositional transition around substrate temperature of 300°C. It was observed that all the films had a sphalerite structure even for the highest substrate temperatures. It was establish the need for a post-deposition annealing to obtain CIS type-III with the chalcopyrite structure. The Raman scattering was found to be a very sensitive technique that allowed us to uncover a difference in the CIS type-I and II with the chalcopyrite structure.

Experimental results on high bit rate optical synchronization of RZ soliton type signals

We report the development of a lab prototype for all optical synchronization at 10 GHz. Adjusting the distance between the AR coated facet of the MQW semiconductor laser and the ferrule it was possible to induce the self-pulsation needed for correct operation. In the presence of optical RZ signals, the device leaves is natural pulsation frequency synchronizing with the input data.

High-bit-rate optical synchronization of RZ signals using external-cavity DFB lasers

We report the development of two lab prototypes for all optical clock recovery and optical synchronization, composed by a DFB semiconductor laser driven by a ultra-fast switching current circuit. It was verified that a fast switch of the laser bias current accomplished by the strong feedback in the external cavity induce the self-pulsation needed for correct operation. In the presence of optical RZ signals, the device leaves in natural pulsation frequency synchronizing with the input data.