Optimization of deposition parameters for thin silicon films on flexible substrates in a hot-wire chemical vapor deposition reactor

This paper studies the deposition of thin silicon films from silane on plastic substrates in a recently build hot-wire chemical vapor deposition reactor. Hydrogen dilution of silane was used to induce amorphous-to-nanocrystalline phase transition. Thin-film deposition rate, r(d), is roughly proportional to silane concentration during deposition but the proportionality factor depends on filament temperature, T-fil. At T-fil similar to 2500 degrees C (1900 degrees C), r(d) increases from 2.1 angstrom/s (1.2 angstrom/s) at 97% H-2 dilution to 14.5 angstrom/s (10.7 angstrom/s) for films deposited from pure silane. At T-fil similar to 2500 degrees C, films deposited under 80% H-2 dilution were amorphous, under 90% H-2 dilution the crystalline fraction was X-C = 49.4% and under 95% H-2 dilution, X-C = 52.8%. At T-fil similar to 1900 degrees C, samples were amorphous up to similar to 95% H-2 dilution where a crystalline fraction of 22.3% was measured. Films with amorphous structure have sigma(d) similar to 10(-10)-10(-9) Omega(-1.)cm(-1) while those with a measured crystalline fraction have sigma(d)similar to 10(-7)-10(-5) Omega(-1)cm(-1), depending on the amount of crystalline fraction and grain size. Films with lower sigma(d) have optical band gap in the range similar to 1.85-1.9 eV, typical of hydrogenated amorphous silicon, while those with higher sigma(d) have larger optical band gap (similar to 2 eV), typical of hydrogenated nanocrystalline silicon. Adhesion of the films to the plastic substrate was good, as they survived bending to small radius of curvature (< 1 mm) without peeling. Structural, optical and transport properties were similar on films deposited both on PEN and on glass under the same deposition conditions.Fundação para a Ciência e a Tecnologia (FCT

Cascade up-conversion of photoluminescence in quantum dot ensembles

We present a theoretical model and Monte Carlo simulation results that naturally explain all the features of the thermally activated photoluminescence upconversion effect also known as anti-Stokes photoluminescence ASPL observed in ensembles of colloidal semiconductor nanocrystal quantum dots QDs . The proposed ASPL mechanism includes the following principal ingredients: i optical-phonon-assisted absorption of an incident photon in a relatively large dot in the ensemble, ii emission of a higher-energy photon from the zero-phonon exciton-polaron state, with an upconversion equal to one optical-phonon energy, and iii cascade reabsorption and re-emission processes involving QDs of successively smaller sizes within the sample, rendering the experimentally observed large anti-Stokes shift of the energy of the photon that finally leaves the sample. The results obtained by the Monte Carlo modeling based on the proposed mechanism reproduce all the experimentally observed ASPL trends in colloidal QD solutions.Fundação para a Ciência e a Tecnologia (FCT

Study on excimer laser irradiation for controlled dehydrogenation and crystallization of boron doped hydrogenated amorphous/nanocrystalline silicon multilayers

We report on the excimer laser annealing (ELA) induced temperature gradients, allowing controlled crystal-lization and dehydrogenation of boron-doped a-Si:H/nc-Si:H multilayers. Depth of the dehydrogenation and crystallization process has been studied numerically and experimentally, showing that temperatures below the monohydride decomposition can be used and that significant changes of the doping profile can be avoided. Calculation of temperature profiles has been achieved through numerical modeling of the heat conduction differential equation. Increase in the amount of nano-crystals, but not in their size, has been demonstrated by Raman spectroscopy. Effective dehydrogenation and shape of the boron profile have been studied by time of flight secondary ion mass spectroscopy. The relatively low temperature threshold for dehydrogenation, below the monohydride decomposition temperature, has been attributed to both, the large hydrogen content of the original films and the partial crystallization during the ELA process. The results of this study show that UV-laser irradiation is an effective tool to improve crystallinity and dopant activation in p+-nc-Si:H films without damaging the substrate.Fundação para a Ciência e Tecnologia (FCT)CRUP Spanish–Portuguese bilateral agreement HP2006- 0122Spanish national and regional research contracts: MAT-2000-1050, MAT-2003-04908MAT-2011-24077, PGIDIT03-04908, PGIDT-01PX130301PN, XUGA- Infra 93, XUGA-Infra 94-58, SB93-A0742819D and INFRA 99-PR 405a-46

Fullerene-modified polyamide 6 by in situ anionic polymerization in the presence of PCBM

Activated anionic ring-opening polymerization of e-caprolactam (ECL) was carried out for the first time in the presence of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) to prepare polyamide 6 (PA6)-based composites comprising up to 3 wt% of this fullerene derivative. This in situ polymerization process produced high molecular weight composites containing 52–80 % of gel fraction at PCBM concentration C0.5 wt%. Spectral, thermomechanical, synchrotron X-ray, and scanning electron microscopy data were used to elucidate the structure and morphology of the PA6/PCBM composites. A mechanism of the chemical structure evolution was proposed starting with incipient complexation between ECL and PCBM, via subsequent chemical linking of ECL moieties on the C60 spheroid and final formation of star-burst and cross-linked morphologies. PCBM amounts of 0.1 wt% and more decreased the volume resistivity from 10E12 Ω.cm (neat PA6) to 10E9–10E7 Ω.cm, thus opening the way for new applications of anionic PA6.Fundação para a Ciência e a Tecnologia (FCT

Impact of composition and morphology on the optical properties of Si-NC/P3HT thin films processed from solution

Blends of Si nanocrystals (Si-NCs) and organic semiconductors are promising materials for new optical and electronic devices processed from solutions. Here, we study how the optical properties of composite films containing Si-NCs and the organic semiconductor poly(3-hexylthiophene) (P3HT) are influenced by the composition and morphology resulting from different solution-processing parameters and different solvents used dichlorobenzene vs. chloroform). The optical spectra of the hybrid films are described using a simple phenomenological model, with which we can discern the contribution of each material in the films to the optical properties. From this analysis, we obtain quantitative information about the composition and morphology of the hybrid nanostructured films, which otherwise would be obtained from more demanding microscopy and spectroscopy techniques. For the case of the Si-NC/P3HT blend, we find that in films deposited from dichlorobenzene solutions the Si-NCs contribute sizably to light absorption.This work was funded by FCT/I3N via the HybridSolar project, by the DFG via Teilprojekt B2 and by the European Union and the Ministry of Innovation, Science and Research of the German State of North Rhine-Westphalia in the framework of an Objective 2 Programme (European Regional Development Fund, ERDF). Funding is also acknowledged from the Portuguese Foundation for Science and Technology (FCT) through the Projects Nos. PTDC/FIS/112885/2009 and PEst-C/CTM/LA0025/2011 and via the Strategic Project LA 25:2011-2012

Broadband photocurrent enhancement in a-Si:H solar cells with plasmonic back reflectors

The authors acknowledge Francesco Ruffino for the AFM measurements. This work was funded by the EU FP7 Marie Curie Action FP7-PEOPLE-2010-ITN through the PROPHET project (Grant No. 264687), the bilateral CNR/AVCR project "Photoresponse of nanostructures for advanced photovoltaic applications", the MIUR project Energetic (Grant no. PON02_00355_3391233) and by the Portuguese Science Foundation (FCT-MEC) through the Strategic Project PEst-C/CTM/LA0025/2013-14 and the research project PTDC/CTM-ENE/2514/2012.Plasmonic light trapping in thin film silicon solar cells is a promising route to achieve high efficiency with reduced volumes of semiconductor material. In this paper, we study the enhancement in the opto-electronic performance of thin a-Si:H solar cells due to the light scattering effects of plasmonic back reflectors (PBRs), composed of self-assembled silver nanoparticles (NPs), incorporated on the cells' rear contact. The optical properties of the PBRs are investigated according to the morphology of the NPs, which can be tuned by the fabrication parameters. By analyzing sets of solar cells built on distinct PBRs we show that the photocurrent enhancement achieved in the a-Si:H light trapping window (600 - 800 nm) stays in linear relation with the PBRs diffuse reflection. The best-performing PBRs allow a pronounced broadband photocurrent enhancement in the cells which is attributed not only to the plasmon-assisted light scattering from the NPs but also to the front surface texture originated from the conformal growth of the cell material over the particles. As a result, remarkably high values of J(sc) and V-oc are achieved in comparison to those previously reported in the literature for the same type of devices. (C)2014 Optical Society of Americapublishersversionpublishe

Phosphorous and boron doping of nc-Si:H thin films deposited on plastic

The authors thank Dr. S. Chiussi (University of Vigo) for SIMS measurements. One of the authors (S.A.F.) acknowledges Fundação para a Ciência e Tecnologia (FCT) for a post-doctorate grant.Gas-phase phosphorous and boron doping of hydrogenated nanocrystalline thin films deposited by HWCVD at a substrate temperature of 150 °C on flexible-plastic (polyethylene naphthalate, polyimide) and rigid-glass substrates is reported. The influence of the substrate, hydrogen dilution, dopant concentration and film thickness on the structural and electrical properties of the films was investigated. The dark conductivity of B- and P-doped films (σd=2.8 S/cm and 4.7 S/cm, respectively) deposited on plastic was found to be somewhat higher than that found in similar films deposited on glass. n- and p-type films with thickness below ~50 nm have values of crystalline fraction, activation energy and dark conductivity typical of doped hydrogenated amorphous silicon. This effect is observed both on glass and on plastic substrates

Maskless interdigitated a-Si:H PECVD process on full M0 c-Si wafer: Homogeneity and passivation assessment

International audienc

Area selective deposition of silicon by plasma enhanced chemical vapor deposition using a fluorinated precursor

International audienceAn Area Selective Deposition (ASD) process using Plasma-Enhanced Chemical Vapour Deposition (PECVD) is demonstrated. Using a plasma chemistry containing a fluorinated silicon precursor (SiF 4), no deposition is observed on an aluminum oxide (AlO X) surface area, whereas a thin film of silicon is deposited on a silicon nitride (SiN X) surface area, while both areas are located on the same crystalline silicon substrate. The thin film deposition is characterized using spectroscopic ellipsometry, scanning electron microscopy, and atomic force microscopy, showing that 10 nm of silicon is deposited on the SiN x in 4 min. The growth on the SiN X is characterized by small grains and a rough surface, consistent with microcrystalline silicon, while no deposition or etching is observed for the AlO X surface