International Journal of

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ZnS/metal/ZnS (ZMZ) nanomultilayer films with Au, Ag and Cu as a metal layer have been deposited on a glass substrate by thermal evaporation and then, were annealed in air at different temperatures from 100 to 300 ºC for one hour. Several analytical tools such as X-ray diffraction, four point probe and spectrophotometer were used to study the changes in structural, electrical and optical properties of the samples. XRD patterns show that the crystallinity of structures and also grain size of particles increases with increasing the annealing temperature. Improved electrical property (a sheet resistance of 7 Ω/sq for ZnS/Au/ZnS) and considerable improvement in the transmittance curves (86% maximum transmittance for ZnS/Au/ZnS) of the samples after heat treatment at 200ºC was observed. Also, the optical constants of the ZMZ multilayer samples were calculated from transmittance and reflectance measurements. The figure of merit was applied on the ZMZ coatings and the most suitable films and annealing temperature for the application as transparent conductive electrodes were determined.

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In this research, the lifetime of green organic light emitting diodes (OLEDs) is studied using four passivation layers. To encapsulate the OLEDs, MgF2, YF3, composed of alternating MgF2/ZnS and YF3/ZnS layers were grown by thermal vacuum deposition. Measurements show that the device lifetime is significantly improved by using YF3 and ZnS as passivation layers. However, diodes encapsulated by MgF2/ZnS and YF3/ZnS nano-structures show a highly efficient gas diffusion barrier that results in a longer lifetime of the devices. The half lifetime of the green OLEDs reached 1200 minutes using YF3/ZnS layers. The electroluminescence (EL) and current-voltage characteristics of the devices were also examined to compare the electrical and the emissivity properties of the devices before and after encapsulation. This simple and inexpensive thin-film encapsulation method would be potentially employed to capsulate top emitting OLEDs and flexible OLEDs due to their good performance and easy fabrication.

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By introducing a thin ZnO layer as an optical spacer, we have demonstrated that inserting this layer between an active layer and a reflective electrode results in a re-distribution of the optical electric field inside bulk heterojunction solar cells. A theoretical analysis by optical modeling showed that the thin ZnO layer could shift the position of the maximum of the electric field into the absorbing layer. Theoretical calculations were compared with experimental results for devices with and without an optical spacer. By using a ZnO optical spacer layer, a significant increase was observed in the short circuit current density of J-V curves. This increase might be due to harvesting more lights and also hole-blocking by the ZnO layer. Both electrical and optical characteristics of the device provided improved results in the power conversion efficiency of the bulk heterojunction solar cell up to 3.49%.

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Planar superstrate CuInS2 (CIS) solar cell devices are fabricated using totally solution-processed deposition methods. A titanium dioxide blocking layer and an In2S3 buffer layer are deposited by the spray pyrolysis method. A CIS2 absorber layer is deposited by the spin coating method using CIS ink prepared by a 1-butylamine solvent-based solution at room temperature. To obtain optimum annealing temperature, these layers are first annealed at 150°C and then annealed at 210°C, 250°C and 350°C respectively. The optimum annealing temperature of the layer is found to be 250°C, where 23 mA current density and 505 mV open circuit voltage are measured for the best fabricated solar cell sample.

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Nanostructures of noble metal materials have been used in organic solar cells for enhancement of performance and light trapping. In this study, we have introduced branched silver cauliflower-like nanopatterns as sub-wavelength structured metal grating in organic solar cells. Self-assembled fabrication process of branched nanopatterns was carried out on a bio-template of cicada wing nanonipple arrays using a gas aggregation dc magnetron sputtering nanocluster source without size filtration. The branched nanostructures provide surface gaps with dimensions near the organic exciton diffusion length, which prevents recombination of charge carriers. An increased power conversion efficiency of 14.8% compared to that of the planar device was achieved mainly due to the enhancement in the short-circuit current density. Besides, these branched cauliflower-like nanopatterns had enhanced optical light absorption in the solar cell as a result of enhancing the optical path length of the reflected light in the active layer and plasmonic effects of the noble metal material.

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To investigate the plasmonic effect in perovskite solar cells, the effect of depositing Au@SiO₂ nanoparticles on the top and the bottom of mesoporous TiO₂ layers was studied. First, Au@SiO₂ nanoparticles were synthesized. The particles were then deposited at the different interfaces of mesoporous TiO₂ layers. Although the two structures show approximately similar optical absorption, only cells with Au@SiO₂ nanoparticles deposited at the bottom of the mesoporous TiO₂ layers demonstrated an improved photocurrent performance compared to the reference cells. This structure shows a short-circuit current density (JSC) of 20.7 mA/cm² and open circuit voltage of 1081 mV. This enhancement may be attributed either to the interface surface engineering or plasmonic resonance of Au@SiO₂ nanoparticles depends to the NPs size and position.  

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Owing amongst other to its high electron mobility, fullerene C70, has been widely used as an electron transporting layer in organic solar cells. In this research, we report the use of C70 thin films as electron transport layers of planar perovskite solar cells (PSCs) using a conventional device structure. The thickness of the C70 layer has been optimized to achieve the best efficiency of 12%. It is demonstrated that ultra-thin C70 films can effectively block holes and thus become selective to the transport of electrons in PSC devices.