23 research outputs found
A combined experimental and simulation study on thickness dependence of the emission characteristics in multicolor single layer organic light-emitting diodes
No full textThe impact of the active layer thickness on the emission characteristics of multicolor single layer organic light-emitting diodes based on poly(9-vinylcarbazole) is examined by combining experimental results with model simulations. We compare experimental electroluminescence spectra with simulations using photon-emitting point dipoles and find a very good agreement. We also simulate the location of the recombination zone, considering that the emission probability distribution has a peak located 25 nm from the cathode, which decays exponentially above and below that point. Simulated radiation patterns show that microcavity effects dominate the thickness dependent emitting properties of these devices.</p
Tungsten oxides as interfacial layers for improved performance in hybrid optoelectronic devices
No full textTungsten oxide (WO3) films with thicknesses ranging from 30 to 100 nm were grown by Hot Filament Vapor Deposition (HFVD). Films were studied by X-Ray Photoemission Spectroscopy (XPS) and were found to be stoichiometric. The surface morphology of the films was characterized by Atomic Force Microscopy (AFM). Samples had a granular form with grains in the order of 100 nm. The surface roughness was found to increase with film thickness. HFVD WO3 films were used as conducting interfacial layers in advanced hybrid organic-inorganic optoelectronic devices. Hybrid-Organic Light Emitting Diodes (Hy-OLEDs) and Organic Photovoltaics (Hy-OPVs) were fabricated with these films as anode and/or as cathode interfacial conducting layers. The Hy-OLEDs showed significantly higher current density and a lower turn-on voltage when a thin WO3 layer was inserted at the anode/polymer interface, while when inserted at the cathode/polymer interface the device performance was found to deteriorate. The improvement was attributed to a more efficient hole injection and transport from the Fermi level of the anode to the Highest Occupied Molecular Orbital (HOMO) of a yellow emitting copolymer (YEP). On the other hand, the insertion of a thin WO3 layer at the cathode/polymer interface of Hy-OPV devices based on a polythiophene-fullerene bulk-heterojunction blend photoactive layer resulted in an increase of the produced photogenerated current, more likely due to improved electron extraction at the Al cathode.</p
Passivation and process engineering approaches of halide perovskite films for high efficiency and stability perovskite solar cells
No full textThe surface, interfaces and grain boundaries of a halide perovskite film carry critical tasks in achieving as well as maintaining high solar cell performance due to the inherently defective nature across their regime. Passivating materials and felicitous process engineering approaches have significant ramifications in the resultant perovskite film, and the solar cell s overall macroscale properties as they dictate structural and optoelectronic properties. Herein, we exploit a vast number of defect engineering approaches aiming to increase the performance and the stability of perovskite solar cells, especially against humidity, continuous illumination, and heat. This review begins with the perovskite materials fundamental structural properties followed by the advances made to induce higher stabilization in perovskite solar cells by fine tuning materials chemistry design parameters. We continue by summarizing defect passivation strategies based on molecular entities application, including suitable functional groups that enable sufficient surface, bulk and grain boundary passivation, morphology, and crystallinity control. We also present methods to control the density of defects through the variation of processing conditions, solvent annealing and solvent engineering approaches, gas assisted deposition methods, and use of self assembled monolayers, as well as colloidal engineering and coordination surface chemistry. Finally, we give our perspective on how a combined understanding of materials chemistry aspects and passivation mechanisms will further develop high efficiency and stability perovskite solar cell
Optical modeling of hybrid polymer solar cells using a transmission-line model and comparison with experimental results
No full textOrganic photovoltaic (OPV) cells provide an efficient alternative solution compared to traditional PV devices. Their conversion efficiency is directly dependent to the light intensity in the photoactive layer. The present work proposes a fast, accurate, and easy-to-implement transmission-line model, which evaluates the light intensity in any layer inside a multilayer OPV cell. In particular, a plane optical wave impinging normally on a planar multilayer OPV configuration is considered and next the optical field distribution is determined. Numerical results and optimization guidelines for a hybrid PV configuration are presented and discussed. © 2006 IEEE
Electrical transport and EPR properties of the α, β, and γ phases of Fe2WO6
No full textThe polymorphic modifications α, β, and γ phases of Fe2WO6 of the iron (III) tungstate have been investigated by resistivity and electron paramagnetic resonance (EPR) measurements. The high-electrical resistivity for all phases complies with the dominant antiferromagnetic component of the compounds. The EPR spectra comprise a broad exchange narrowed EPR line due to Fe3+ ions. The broadening of the EPR linewidth is explained in terms of the critical spin fluctuations near the antiferromagnetic phase transition. The shift of the resonance field and the temperature dependence of the EPR intensity imply an extended region of short-range order correlations. © 1999 The American Physical Society
Core-shell carbon-polymer quantum dot passivation for near infrared perovskite light emitting diodes
No full textHigh-performance perovskite light-emitting diodes (PeLEDs) require a high quality perovskite emitter and appropriate charge transport layers to facilitate charge injection and transport within the device. Solution-processed n-type metal oxides represent a judicious choice for the electron transport layer (ETL); however, they do not always present surface properties and energetics compatible with the perovskite emitter. Moreover, the emitter itself exhibits poor nanomorphology and defect traps that compromise the device performance. Here, we modulate the surface properties and interface energetics between the tin oxide (SnO2) ETL with the perovskite emitter by using an amino functionalized difluoro{2-[1-(3,5-dimethyl-2H-pyrrol-2-ylidene-N)ethyl]-3,5-dimethyl-1H-pyrrolato-N}boron compound and passivate the defects present in the perovskite matrix with carbon-polymer core-shell quantum dots inserted into the perovskite precursor. Both these approaches synergistically improve the perovskite layer nanomorphology and enhance the radiative recombination. These properties resulted in the fabrication of near-infrared PeLEDs based on formamidinium lead iodide (FAPbI3) with a high radiance of 92 W sr-1 m-2, an external quantum efficiency (EQE) of 14%, reduced efficiency roll-off and prolonged lifetime. In particular, the modified device retained 80% of the initial EQE (T80) for 33 h compared to 6 h of the reference cell. © 2022 The Author(s). Published by IOP Publishing Ltd
Avoiding ambient air and light induced degradation in high-efficiency polymer solar cells by the use of hydrogen-doped zinc oxide as electron extraction material
No full textPolymer solar cells have undergone rapid development in recent years. Their limited stability to environmental influence and during illumination, however, still remains a major stumbling block to the commercial application of this technology. Several attempts have been made to address the instability issue, mostly concentrated on the insertion of charge transport interlayers in the device stack. Although zinc oxide (ZnO) is one of the most common electron transport materials in those cells, the presence of defects at the surface and grain boundaries significantly affects the efficiency and stability of the working devices. To address these issues, we herein employ hydrogen-doping of ZnO electron extraction material. It is found that devices based on photoactive layers composed of blends of poly(3-hexylthiophene) (P3HT) with electron acceptors possessing different energy levels, such as [6,6]-phenyl-C70butyric acid methyl ester (PC70BM) or indene-C60 bisadduct (IC60BA) essentially enhanced their photovoltaic performance when using the hydrogen-doped ZnO with maximum power conversion efficiency (PCE) reaching values of 4.62% and 6.65%, respectively, which are much higher than those of the cells with the pristine ZnO (3.08% and 4.51%). Most significantly, the degradation of non-encapsulated solar cells when exposed to ambient or under prolonged illumination is studied and it is found that devices based on un-doped ZnO showed poor environmental stability and significant photo-degradation while those using hydrogen-doped ZnO interlayers exhibited high long-term ambient stability and maintained nearly 80–90% of their initial PCE values after 40 h of 1.5 AM illumination. All mechanisms responsible for this enhanced stability are elucidated and corresponding models are proposed. This work successfully addresses and tackles the instability problem of polymer solar cells and the key findings pave the way for the upscaling of these and, perhaps, of related devices such as perovskite solar cells. © 2017 Elsevier Lt
Surface passivation effect by fluorine plasma treatment on ZnO for efficiency and lifetime improvement of inverted polymer solar cells
No full textZinc oxide (ZnO) is an important material for polymer solar cells (PSCs) where the characteristics of the interface can dominate both the efficiency and lifetime of the device. In this work we study the effect of fluorine (SF6) plasma surface treatment of ZnO films on the performance of PSCs with an inverted structure. The interaction between fluorine species present in the SF6 plasma and the ZnO surface is also investigated in detail. We provide fundamental insights into the passivation effect of fluorine by analyzing our experimental results and theoretical calculations and we propose a mechanism according to which a fluorine atom substitutes an oxygen atom or occupies an oxygen vacancy site eliminating an electron trap while it may also attract hydrogen atoms thus favoring hydrogen doping. These multiple fluorine roles can reduce both the recombination losses and the electron extraction barrier at the ZnO/fullerene interface improving the selectivity of the cathode contact. Therefore, the fabricated devices using the fluorine plasma treated ZnO show high efficiency and stable characteristics, irrespective of the donor:acceptor combinations in the photoactive blend. Inverted polymer solar cells, consisting of the P3HT:PC71BM blend, exhibited increased lifetime and high power conversion efficiency (PCE) of 4.6%, while the ones with the PCDTBT:PC71BM blend exhibited a PCE of 6.9%. Our champion devices with the PTB7:PC71BM blends reached a high PCE of 8.0% and simultaneously showed exceptional environmental stability when using the fluorine passivated ZnO cathode interlayers. © The Royal Society of Chemistry 2016
Synthesis of novel pyrene discotics for potential electronic applications
No full textNovel pyrene discotics, 6,7,15,16-tetrakis(alkylthio)quinoxalino[2′,3′:9,10]phenanthro[4,5-abc]phenazines, TQPP-[SR]4, were synthesized efficiently. The HOMO and LUMO energy levels of TQPP-[SR]4 were estimated to be 5.57 eV and 2.97 eV, respectively. The average saturation hole mobility of TQPP-[C12H25]4 was ∼10-3 cm2 V-1 s-1. © 2007 Elsevier Ltd. All rights reserved.Boden N, 1999, J MATER CHEM, V9, P2081, DOI 10.1039-a903005k; Cornil J, 2002, ADV MATER, V14, P726, DOI 10.1002-1521-4095(20020517)14:10726::AID-ADMA7263.0.CO;2-D; Deibel C, 2006, ORG ELECTRON, V7, P495, DOI 10.1016-j.orgel.2006.07.002; Dimitrakopoulos CD, 1999, SCIENCE, V283, P822, DOI 10.1126-science.283.5403.822; DODABALAPUR A, 1995, SCIENCE, V268, P270, DOI 10.1126-science.268.5208.270; Gamier F., 1994, SCIENCE, V265, P1864; Hu J, 2005, J ORG CHEM, V70, P707, DOI 10.1021-jo048509q; Hu J, 2004, CHEM MATER, V16, P4912, DOI 10.1021-cm0492179; Kaafarani BR, 2005, J AM CHEM SOC, V127, P16358, DOI 10.1021-ja0553147; Kestemont G, 2001, CHEM COMMUN, P2074, DOI 10.1039-b107135c; Kumar S, 2006, CHEM SOC REV, V35, P83, DOI 10.1039-b506619k; Lehmann M, 2005, CHEM-EUR J, V11, P3349, DOI 10.1002-chem.200400586; Li ADQ, 2004, J PHYS CHEM B, V108, P12842, DOI 10.1021-jp0380576; Palilis LC, 2003, ORG ELECTRON, V4, P113, DOI 10.1016-j.orgel.2003.08.006; POMMEREHNE J, 1995, ADV MATER, V7, P551, DOI 10.1002-adma.19950070608; Schmidt-Mende L, 2001, SCIENCE, V293, P1119, DOI 10.1126-science.293.5532.1119; Simpson CD, 2004, J MATER CHEM, V14, P494, DOI 10.1039-b312789c; Sirringhaus H, 2005, ADV MATER, V17, P2411, DOI 10.1002-adma.200501152; TANG CW, 1986, APPL PHYS LETT, V48, P183, DOI 10.1063-1.96937; TANG CW, 1987, APPL PHYS LETT, V51, P913, DOI 10.1063-1.98799; van de Craats AM, 1999, ADV MATER, V11, P1469, DOI 10.1002-(SICI)1521-4095(199912)11:171469::AID-ADMA14693.0.CO;2-K; Vollmann H, 1937, LIEBIGS ANN CHEM, V531, P1; Warman JM, 2003, MOL CRYST LIQ CRYST, V396, P41, DOI 10.1080-15421400390213186; Young ERR, 1998, J ORG CHEM, V63, P9995, DOI 10.1021-jo970344g35353
