Control of doped layers in p-i-n microcrystalline solar cells fully deposited with HWCVD

In this paper, the influence of the deposition conditions on the performance of p-i-n microcrystalline silicon solar cells completely deposited by hot-wire chemical vapor deposition is studied. With this aim, the role of the doping concentration, the substrate temperature of the p-type layer and of amorphous silicon buffer layers between the p/i and i/n microcrystalline layers is investigated. Best results are found when the p-type layer is deposited at a substrate temperature of 125 °C. The dependence seen of the cell performance on the thickness of the i layer evidenced that the efficiency of our devices is still limited by the recombination within this layer, which is probably due to the charge of donor centers most likely related to oxygen.

Compositional influence on the electrical performance of zinc indium tin oxide transparent thin-film transistors

In this work, zinc indium tin oxide layers with different compositions are used as the active layer of thin film transistors. This multicomponent transparent conductive oxide is gaining great interest due to its reduced content of the scarce indium element. Experimental data indicate that the incorporation of zinc promotes the creation of oxygen vacancies. In thin-film transistors this effect leads to a higher threshold voltage values. The field-effect mobility is also strongly degraded, probably due to coulomb scattering by ionized defects. A post deposition annealing in air reduces the density of oxygen vacancies and improves the fieldeffect mobility by orders of magnitude. Finally, the electrical characteristics of the fabricated thin-film transistors have been analyzed to estimate the density of states in the gap of the active layers. These measurements reveal a clear peak located at 0.3 eV from the conduction band edge that could be attributed to oxygen vacancies.

Amorphous silicon thin film solar cells deposited entirely by Hot-Wire Chemical Vapour Deposition at low temperature (<150 ºC)

Amorphous silicon n-i-p solar cells have been fabricated entirely by Hot-Wire Chemical Vapour Deposition (HW-CVD) at low process temperature < 150 °C. A textured-Ag/ZnO back reflector deposited on Corning 1737F by rf magnetron sputtering was used as the substrate. Doped layers with very good conductivity and a very less defective intrinsic a-Si:H layer were used for the cell fabrication. A double n-layer (μc-Si:H/a-Si:H) and μc-Si:H p-layer were used for the cell. In this paper, we report the characterization of these layers and the integration of these layers in a solar cell fabricated at low temperature. An initial efficiency of 4.62% has been achieved for the n-i-p cell deposited at temperatures below 150 °C over glass/Ag/ZnO textured back reflector.

The Electronic Structure of Co-Sputtered Zinc Indium Tin Oxide Thin Films

Zinc indium tin oxide (ZITO) transparent conductive oxide layers were deposited via radio frequency (RF) magnetron co-sputtering at room temperature. A series of samples with gradually varying zinc content was investigated. The samples were characterized with x-ray and ultraviolet photoemission spectroscopy (XPS, UPS) to determine the electronic structure of the surface. Valence and conduction bands maxima (VBM, CBM), and work function were determined. The experiments indicate that increasing Zn content results in films with a higher defect rate at the surface leading to the formation of a degenerately doped surface layer if the Zn content surpasses 50%. Furthermore, the experiments demonstrate that ZITO is susceptible to ultraviolet light induced work function reduction, similar to what was earlier observed on ITO and TiO2 films.

Involvement of a transforming-growth-factor-beta-like molecule in tumor-cell-derived inhibition of nitric-oxide synthesis in cerebral endothelial cells.

Nitric oxide (NO) has been shown to exert cytotoxic effects on tumor cells. We have reported that EC219 cells, a rat-brain-microvessel-derived endothelial cell line, produced NO through cytokine-inducible NO synthase (iNOS), the induction of which was significantly decreased by (a) soluble factor(s) secreted by DHD/PROb, an invasive sub-clone of a rat colon-carcinoma cell line. In this study, the DHD/PROb cell-derived NO-inhibitory factor was characterized. Northern-blot analysis demonstrated that the induction of iNOS mRNA in cytokine-activated EC219 cells was decreased by PROb-cell-conditioned medium. When DHD/PROb cell supernatant was fractionated by affinity chromatography using Con A-Sepharose or heparin-Sepharose, the NO-inhibitory activity was found only in Con A-unbound or heparin-unbound fractions, respectively, indicating that the PROb-derived inhibitory factor was likely to be a non-glycosylated and non-heparin-binding molecule. Pre-incubation of DHD/PROb-cell supernatant with anti-TGF-beta neutralizing antibody completely blocked the DHD/PROb-derived inhibition of NO production by EC219 cells. Addition of exogenous TGF-beta 1 dose-dependently inhibited NO release by EC219 cells. The presence of active TGF-beta in the DHD/PROb cell supernatant was demonstrated using a growth-inhibition assay. Moreover, heat treatment of medium conditioned by the less invasive DHD/REGb cells, which constitutively secreted very low levels of active TGF-beta, increased both TGF-beta activity and the ability to inhibit NO production in EC219 cells. Thus, DHD/PROb colon-carcinoma cells inhibited NO production in EC219 cells by secreting a factor identical or very similar to TGF-beta.

Antigenic stimulation is more efficient than LPS in inducing nitric oxide production by human mononuclear cells on the in vitro granuloma reaction in schistosomiasis

Nitric oxide (NO) is an extremely important and versatile messenger in biological systems. It has been identified as a cytotoxic factor in the immune system, presenting anti- or pro-inflammatory properties under different circumstances. In murine monocytes and macrophages, stimuli by cytokines or lipopolysaccharide (LPS) are necessary for inducing the immunologic isoform of the enzyme responsible for the high-output production of NO, nitric oxide synthase (iNOS). With respect to human cells, however, LPS seems not to stimulate NO production in the same way. Addressing this issue, we demonstrate here that peripheral blood mononuclear cells (PBMC) obtained from schistosomiasis-infected patients and cultivated with parasite antigens in the in vitro granuloma (IVG) reaction produced more nitrite in the absence of LPS. Thus, LPS-induced nitrite levels are easily detectable, although lower than those detected only with antigenic stimulation. Concomitant addition of LPS and L-N-arginine methyl ester (L-NAME) restored the ability to produce detectable levels of nitrite, which had been lost with L-NAME treatment. In addition, LPS caused a mild decrease of the IVG reaction and its association with L-NAME was responsible for reversal of the L-NAME-exacerbating effect on the IVG reaction. These results show that LPS alone is not as good an NO inducer in human cells as it is in rodent cells or cell lines. Moreover, they provide evidence for interactions between LPS and NO inhibitors that require further investigation.

Interfacial effects in organic solar cells

Solceller baserade på organiska halvledare erbjuder en möjlighet till storskalig och billig solenergiproduktion. Organiska halvledare har den fördelen att de är lösningsprocesserbara vilket gör att solceller och andra elektroniska komponenter baserade på dessa halvledare kan tillverkas vid låga temperaturer och med liten energiförbrukning. Nackdelen med dessa material är deras strukturella och energetiska oordning som leder till lägre effektivitet. För att organiska solceller ska kunna kommersialiseras krävs grundläggande insikter i de olika processer som begränsar effektiviteten. En stor del av forskningen om dessa processer har varit fokuserad kring egenskaperna av solcellens olika komponenter (de aktiva materialen) som sådana, medan gränsytorna mellan olika material har fått mindre uppmärksamhet. Gränsytor mellan olika material har distinkt olika egenskaper jämfört med ett rent material, och gränsytors olika egenskaper kan ha en väldigt stor inverkan på hur solcellerna fungerar. Syftet med denna avhandling är att klargöra några olika gränsyterelaterade effekter i organiska dioder och solceller. De gränsytor som behandlas är gränsytan mellan kontakten och det aktiva lagret (metall-organisk) och gränsytan mellan donor och acceptor (organisk-organisk). Resultaten visar att metall-organiska gränsytor måste designas noggrant för att begränsa förlust av effektivitet. En icke-idealisk kontakt leder till starkt reducerad effektivitet på grund av att elektronerna extraheras ineffektivt. Även till synes idealiska kontakter kan orsaka förluster genom spontan laddningsöverföring från metallen till det organiska lagret som effektivt sett minskar på den spänning som cellen kan alstra. Den organisk-organiska gränsytan påverkar hur mycket ström cellen kan alstra och beroende på gränsytans beskaffenhet kan de negativa rekombinationsprocesserna i materialet kontrolleras. ------------------------------------------------- Orgaanisille puolijohteille perustuvat aurinkokennot mahdollistavat suurimuotoisen ja edullisen aurinkoenergiatuotannon. Orgaanisten puolijohteiden etu on että ne voidaan liuottaa, jolloin aurinkokennot ja muut näille johteille perustuvat elektroniset komponentit voidaan valmistaa alhaisessa lämpötilassa kuluttaen vähän energiaa. Materiaalien huonona puolena on kuitenkin niiden rakenteellinen ja energeettinen epäjärjestys, jonka seurauksena niiden tehokkuus on huonompi. Orgaanisten aurinkokennojen kaupallistaminen edellyttää perustavanlaatuista ymmärystä tehokkuutta rajoittavista prosesseista. Aurinkokennotutkimus on pääosin keskittynyt aurinkokennon eri komponenttien (aktiivisten materiaalien) ominaisuuksiin, kun taas eri materiaalien rajapinnat ovat jääneet vähemmälle huomiolle. Eri materiaalien välisillä rajapinnoilla on huomattavan erilaisia ominaisuuksia verrattuna puhtaisiin materiaaleihin. Rajapintojen ominaisuudet voivat kuitenkin vaikuttaa merkittävästi aurinkokennojen toimintaan. Tämän väitöstutkimuksen tarkoituksena on selventää joitain rajapintoihin liittyviä toimintoja orgaanisissa diodeissa ja aurinkokennoissa. Käsiteltävät rajapinnat ovat rajapinta kontaktin ja aktiivisen kerroksen välillä (metallis-orgaaninen) ja rajapinta donorin ja akseptorin välillä (orgaanis-orgaaninen). Tutkimustulokset osoittavat, että metallis-orgaaniset rajapinnat tulee suunnitella huolellisesti, jotta tehokkuuden alenemista voidaan rajoittaa. Mikäli kontakti ei ole ideaalisti suunniteltu, vähenee tehokkuus huomattavasti, mikä johtuu elektronien tehottomasta ekstrahoinnista. Jopa ideaalisilta vaikuttavat kontaktit voivat johtaa tehokkuuden alenemiseen, mikäli varaus siirtyy spontaanisti metallista orgaaniseen kerrokseen, sillä tämä alentaa jännitettä jonka kenno voi tuottaa. Kennon orgaanis-orgaaninen rajapinta vaikuttaa siihen, kuinka paljon virtaa kenno pystyy tuottamaan. Rajapinnan ominaisuuksista riippuen materiaalin rekombinaatio on hallittavissa.

First-principles study on electronic and structural properties of Cu(In/Ga)Se alloys for solar cells

Thin-film photovoltaic solar cells based on the Cu(In1−xGax)Se2 (CIGS) alloys have attracted more and more attention due to their large optical absorption coefficient, long term stability, low cost, and high efficiency. Modern theoretical studies of this material with first-principles calculations can provide accurate description of the electronic structure and yield results in close agreement with experimental values, but takes a large amount of calculation time. In this work, we use first-principles calculations based on the computationally affordable meta- generalized gradient approximation of the density-functional theory to investigate electronic and structural properties of the CIGS alloys. We report on the simulation of the lattice parameters and band gaps, as a function of chemical composition. The obtained results were found to be in a good agreement with the available experimental data.

Modification of a quantum efficiency measurement system for multi-junction solar cells

In this thesis the basic structure and operational principals of single- and multi-junction solar cells are considered and discussed. Main properties and characteristics of solar cells are briefly described. Modified equipment for measuring the quantum efficiency for multi-junction solar cell is presented. Results of experimental research single- and multi-junction solar cells are described.

Two-dimensional drift-diffusion simulation of organic solar cells

The aim of this master's thesis is to develop a two-dimensional drift-di usion model, which describes charge transport in organic solar cells. The main bene t of a two-dimensional model compared to a one-dimensional one is the inclusion of the nanoscale morphology of the active layer of a bulk heterojunction solar cell. The developed model was used to study recombination dynamics at the donor-acceptor interface. In some cases, it was possible to determine e ective parameters, which reproduce the results of the two-dimensional model in the one-dimensional case. A summary of the theory of charge transport in semiconductors was presented and discussed in the context of organic materials. Additionally, the normalization and discretization procedures required to nd a numerical solution to the charge transport problem were outlined. The charge transport problem was solved by implementing an iterative scheme called successive over-relaxation. The obtained solution is given as position-dependent electric potential, free charge carrier concentrations and current densities in the active layer. An interfacial layer, separating the pure phases, was introduced in order to describe charge dynamics occurring at the interface between the donor and acceptor. For simplicity, an e ective generation of free charge carriers in the interfacial layer was implemented. The pure phases simply act as transport layers for the photogenerated charges. Langevin recombination was assumed in the two-dimensional model and an analysis of the apparent recombination rate in the one-dimensional case is presented. The recombination rate in a two-dimensional model is seen to e ectively look like reduced Langevin recombination at open circuit. Replicating the J-U curves obtained in the two-dimensional model is, however, not possible by introducing a constant reduction factor in the Langevin recombination rate. The impact of an acceptor domain in the pure donor phase was investigated. Two cases were considered, one where the acceptor domain is isolated and another where it is connected to the bulk of the acceptor. A comparison to the case where no isolated domains exist was done in order to quantify the observed reduction in the photocurrent. The results show that all charges generated at the isolated domain are lost to recombination, but the domain does not have a major impact on charge transport. Trap-assisted recombination at interfacial trap states was investigated, as well as the surface dipole caused by the trapped charges. A theoretical expression for the ideality factor n_id as a function of generation was derived and shown to agree with simulation data. When the theoretical expression was fitted to simulation data, no interface dipole was observed.

Fabrication and Characterization of Dye Sensitized Polymer Films for Holographic Applications

Holographic technology is at the dawn of quick evolution in various new areas including holographic data storage, holographic optical elements, artificial intelligence, optical interconnects, optical correlators, commerce, medical practice, holographic weapon sight, night vision goggles and games etc. One of the major obstacles for the success of holographic technology to a large extent is the lack of suitable recording medium. Compared with other holographic materials such as dichromated gelatin and silver halide emulsions, photopolymers have the great advantage of recording and reading holograms in real time and the spectral sensitivity could be easily shifted to the type of recording laser used by simply changing the sensitizing dye. Also these materials possess characteristics such as good light sensitivity, real time image development, large dynamic range, good optical properties, format flexibility, and low cost. This thesis describes the attempts made to fabricate highly economic photopolymer films for various holographic applications. In the present work, Poly (vinyl alcohol) (PVA) and poly (vinyl chloride) (PVC) are selected as the host polymer matrices and methylene blue (MB) is used as the photosensitizing dye. The films were fabricated using gravity settling method. No chemical treatment or pre/post exposures were applied to the films. As the outcome of the work, photopolymer films with more than 70% efficiency, a permanent recording material which required no fixing process, a reusable recording material etc. were fabricated.

Optical and electrical properties of amorphous zinc tin oxide thin films examined for thin film transistor application

Structural, electronic, and optical properties of amorphous and transparent zinc tin oxide films deposited on glass substrates by pulsed laser deposition (PLD) were examined for two chemical compositions of Zn:Sn=1:1 and 2:1 as a function of oxygen partial pressure PO2 used for the film deposition and annealing temperature. Different from a previous report on sputter-deposited films Chiang et al., Appl. Phys. Lett. 86, 013503 2005 , the PLD-deposited films crystallized at a lower temperature 450 °C to give crystalline ZnO and SnO2 phases. The optical band gaps Tauc gaps were 2.80−2.85 eV and almost independent of oxygen PO2 , which are smaller than those of the corresponding crystals 3.35−3.89 eV . Films deposited at low PO2 showed significant subgap absorptions, which were reduced by postthermal annealing. Hall mobility showed steep increases when carrier concentration exceeded threshold values and the threshold value depended on the film chemical composition. The films deposited at low PO2 2 Pa had low carrier concentrations. It is thought that the low PO2 produced high-density oxygen deficiencies and generated electrons, but these electrons were trapped in localized states, which would be observed as the subgap absorptions. Similar effects were observed for 600 °C crystallized films and their resistivities were increased by formation of subgap states due to the reducing high-temperature condition. High carrier concentrations and large mobilities were obtained in an intermediate PO2 region for the as-deposited films.

Optical and electrical properties of co-sputtered amorphous transparent conducting zinc indium tin oxide thin films

Highly conductive and transparent thin films of amorphous zinc indium tin oxide are prepared at room temperature by co-sputtering of zinc 10 oxide and indium tin oxide. Cationic contents in the films are varied by adjusting the power to the sputtering targets. Optical transmission study of 11 films showed an average transmission greater than 85% across the visible region. Maximum conductivity of 6×102 S cm−1 is obtained for Zn/In/ 12 Sn atomic ratio 0.4/0.4/0.2 in the film. Hall mobility strongly depends on carrier concentration and maximum mobility obtained is 18 cm2 V−1 s−1 13 at a carrier concentration of 2.1×1020 cm−3. Optical band gap of films varied from 3.44 eV to 3 eV with the increase of zinc content in the film 14 while the refractive index of the films at 600 nm is about 2.0.