Browsing by Author "Sazonov, Andrei"
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- Amorphous silicon photovoltaic modules on flexible plastic substratesPublication . Vygranenko, Yuri; Fernandes, Miguel; Louro, Paula; Vieira, Manuela; Khosropour, Alireza; Yang, Ruifeng; Sazonov, AndreiThis paper reports on a monolithic 10 cm x 10 cm area PV module integrating an array of 72 a-Si:H n-i-p cells on a 100 mu m thick polyethylene-naphtalate substrate. The n-i-p stack is deposited using a PECVD system at 150 degrees C substrate temperature. The design optimization and device performance analysis are performed using a two-dimensional distributed circuit model of the photovoltaic cell. The circuit simulator SPICE is used to calculate current and potential distributions in a network of sub-cell circuits, and also to map Joule losses in the front TCO electrode and the metal grid. Experimental results show that the shunt leakage is one of the factors reducing the device performance. Current-voltage characteristics of individual a-Si: H p-i-n cells were analyzed to estimate a variation of shunt resistances. Using the LBIC technique, the presence of multiple shunts in the n-i-p cell was detected. To understand the nature of electrical shunts, the change in the surface roughness of all device layers was analyzed throughout fabrication process. It is found that surface defects in plastic foils, which are thermally induced during the device fabrication, form microscopic pinholes filled with highly conductive top electrode material.
- Characterization of a-Si:H solar cell modules on plastic substrates by high resolution LBIC techniquePublication . Fernandes, Miguel; Vygranenko, Yuri; Vieira, Manuela; Sazonov, Andrei; Yang, R.; Khosropour, A.This article reports on characterization of hydrogenated amorphous silicon (a-Si:H) photovoltaic modules fabricated on 100 mu m thick PEN plastic films. Experimental results show that the shunt leakage is one of the factors reducing the device performance. Current-voltage characteristics of individual a-Si:H p-i-n cells were analysed to estimate a variation of shunt resistances. A SPICE model of the a-Si:H p-i-n cell with local shunt leakage was also developed to analyse the impact of leakage currents on the device performance. Using the LBIC technique, the presence of multiple shunts in the cell was detected. They are attributed to surface defects in plastic foils, which are thermally induced during the device fabrication. (C) 2015 Published by Elsevier Ltd.
- Driving scheme using MIS photosensor for luminance control of AMOLED pixelPublication . Vygranenko, Yuri; Fernandes, Miguel; Sazonov, Andrei; Vieira, ManuelaThis paper presents a new driving scheme utilizing an in-pixel metal-insulator-semiconductor (MIS) photosensor for luminance control of active-matrix organic light-emitting diode (AMOLED) pixel. The proposed 3-TFT circuit is controlled by an external driver performing the signal readout, processing, and programming operations according to a luminance adjusting algorithm. To maintain the fabrication simplicity, the embedded MIS photosensor shares the same layer stack with pixel TFTs. Performance characteristics of the MIS structure with a nc-Si : H/a-Si : H bilayer absorber were measured and analyzed to prove the concept. The observed transient dark current is associated with charge trapping at the insulator-semiconductor interface that can be largely eliminated by adjusting the bias voltage during the refresh cycle. Other factors limiting the dynamic range and external quantum efficiency are also determined and verified using a small-signal model of the device. Experimental results demonstrate the feasibility of the MIS photosensor for the discussed driving scheme.
- Optoelectronic properties of a-Si(1-x)C(x)H films grown in hydrogen diluted silane-methane plasmaPublication . Vygranenko, Yuri; Fernandes, Miguel; Louro, Paula; Vieira, Manuela; Sazonov, AndreiThis work reports on the optoelectronic properties and device application of hydrogenated amorphous silicon carbide (a-Si(1-x)C(x):H) films grown by plasma-enhanced chemical vapour deposition (PECVD). The films with an optical bandgap ranging from about 1.8 to 2.0 eV were deposited in hydrogen diluted silane-methane plasma by varying the radio frequency power. Several n-i-p structures with an intrinsic a-Si(1-x)C(x):H layer of different optical gaps were also fabricated. The optimized devices exhibited a diode ideality factor of 1.4-1.8, and a leakage current of 190-470 pA/cm(2) at -5 V. The density of deep defect states in a-Si(1-x)C(x):H was estimated from the transient dark current measurements and correlated with the optical bandgap and carbon content. Urbach energies for the valence band tail were also determined by analyzing the spectral response within sub-bandgap energy range. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
- Photo-induced instability of nanocrystalline silicon TFTsPublication . Bauza, Marius; Ahnood, Arman; Li, Flora; Vygranenko, Yuri; Esmaeili-Rad, Mohammad R.; Chaji, G.; Sazonov, Andrei; Robertson, John; Milne, William; Nathan, ArokiaWe examine the instability behavior of nanocrystalline silicon (nc-Si) thin-film transistors (TFTs) in the presence of electrical and optical stress. The change in threshold voltage and sub-threshold slope is more significant under combined bias-and-light stress when compared to bias stress alone. The threshold voltage shift (Delta V-T) after 6 h of bias stress is about 7 times larger in the case with illumination than in the dark. Under bias stress alone, the primary instability mechanism is charge trapping at the semiconductor/insulator interface. In contrast, under combined bias-and-light stress, the prevailing mechanism appears to be the creation of defect states in the channel, and believed to take place in the amorphous phase, where the increase in the electron density induced by electrical bias enhances the non-radiative recombination of photo-excited electron-hole pairs. The results reported here are consistent with observations of photo-induced efficiency degradation in solar cells.
- Phototransistor with nanocrystalline Si/amorphous Si bilayer channelPublication . Vygranenko, Yuri; Nathan, Arokia; Vieira, Manuela; Sazonov, AndreiWe report a field-effect phototransistor with a channel comprising a thin nanocrystalline silicon transport layer and a thicker hydrogenated amorphous silicon absorption layer. The semiconductor and dielectric layers were deposited by radio-frequency plasma enhanced chemical vapor deposition. The phototransistor with channel length of 24 microns and photosensitive area of 1.4 mm(2) shows an off-current of about 1 pA, and high photoconductive gain in the subthreshold region. Measurements of the quantum efficiency at different incident light intensities and biasing conditions, along with spectral-response characteristics, and threshold voltage stability characterization demonstrate the feasibility of the phototransistor for low light level detection.
- Preparation and characterization of a-SiC:H absorber layer for semi-transparent solar cellsPublication . Vygranenko, Yuri; Fernandes, Miguel; Louro, Paula; Vieira, Manuela; Sazonov, AndreiThis paper reports on device-quality silicon-carbon alloy (a-SiC:H) application as an absorber material in semi-transparent solar cells. Films with an optical bandgap ranging from 2 to 2.3 eV were prepared by plasma enhanced chemical vapour deposition (PECVD). The n-i-p structures with undoped SiC:H layers deposited under the same experimental conditions were also fabricated and characterized. The optimized devices showed forward current-voltage characteristics with a diode ideality factor in the range from 1.4 to 1.8, and an open circuit voltage up to 0.92 V. The density of deep defect states in a SiC:H was estimated from the transient current measurements and correlated with the optical bandgap.