In this work is reported an analysis of the external and internal quantum efficiency (EQE and IQE) and, its correlation with the power conversion efficiency (PCE) of organic photovoltaic (OPV) cells, as a function of the active layer thickness. It was used the bulk heterojunction architecture under the configuration ITO/PEDOT:PSS/PTB7- Th:PC71BM/PFN/Field’s Metal (FM) and the active film thickness range was 40-165 nm. FM is a eutectic alloy, composed by 32.5% Bi, 51% In and 16.5% Sn, that melt at 65°C and is easily deposited on top of the electron transport layer (ETL) at low temperature (~ 90 °C). EQE set up was a home-made and the IQE spectra for the active film thickness range were determined from EQE and net internal absorption spectra; net absorption was estimated through the transfer matrix method (TMM). It was observed a significant reduction of IQE with the increasing of the active layer thickness up to 120 nm. IQE decreases and consequently EQE and PCE too because of the reduction in charge carriers collection. It was calculated the short-circuit current density (Jsc) from these measurements and compared these values with those achieved by the J-V plots to verify EQE results. It is noteworthy that EQE takes into account the effect of optical losses by reflection and transmission, while IQE is related with the efficiency of the photons that are not reflected or transmitted out of the cell.
We have numerically evaluated the complex valued eigenfrequencies of infinite silver nanotubes as a function of the ratio of internal and external radius, by solving the surface plasmon dispersion relations for the TE polarization. The results are shown for tubes of external radius of 20 nm, 50 nm, 100 nm, 150 nm, 200 nm and 300 nm with the internal radius varying from 0.1 to 0.9 fractions of the external radius. We have observed that their quality factor Q is substantially enhanced as compared to solid cylinders or wires.
Point sources of polarized light generate inhomogeneous fields at the surface of a metallic slab
localized at its vicinity. It is found that the associated k-spectrum contains wave vectors larger than
those corresponding to free oscillations. Consequently evanescent fields are available and surface
plasmons can be excited. With FDTD calculations we prove in detail this effect using a point source
of cylindrical symmetry placed near a silver film. In addition, the interaction of the cylindrical wave
and the metallic surface is analytically described by writing the incident wave in terms of a plane
wave basis. Images of the surface plasmon excited by this source and the field distribution in the
vicinity of the surface are shown.
Resonant cavities based on whispering gallery modes (WGM) have high quality factor, Q. This property is highly
desirable for the design of a variety of devices. The quality factor is larger for larger systems, but, at the same time the
density of modes increases, and not as regularly spaced frequencies, because modes of different kind superpose. For a
dielectric cylinder, the modes are characterized by their azimuthal and radial behavior. The complex frequencies
corresponding to the modes have a strong dependence on the radial order. We use the finite difference time domain
method, to study the transmission of light by two prisms with a dielectric cylinder between them. The system acts as a
filter with sharp peaks of modes of high Q. The coupling to the modes can be controlled by changing the distance of
separation between the prisms. For large coupling, the full width at half maximum of the transmission peaks increases,
being more noticeable for the high Q modes.
Experimental results of reflection and light scattering measurements using the technique of attenuated total reflection (ATR) in the Kretschmann configuration of a system when it is excited an electromagnetic guided mode are reported. The system used is BK7 glass-prism/Dielectric1/Dielectric2/Dielectric1/air, where the refractive index of dielectric1 is less than the refractive index of the dielectric2. It was found a dip in the specular reflection as a function of the incident angle due to the excitation of a guided mode in the dielectric2 film. The guided mode was found for s polarization of the incident light. The angular dependence of the scattered light displays a peak caused by single-scattering and located approximately at the angles of excitation of the guided modes whose normalized wave numbers are less than the refractive index of the glass. Values of thickness, refractive index and absorption index obtained from Lorentz dispersion model of the film are reported.
Optical characterization in commercial paints helps to improve and optimize the fabrication process and to minimize the cost of the final product. The paint under study is elaborated with a polymer matrix and some amount of particulate inclusions, its main characteistics are good quality, homogeneity of white appearance and brightness. When dried it is transparent. We prepare echelled layers with different paint thickness deposited onto transparent and metallic substrates and measure its reflection and transmission for both colorimetric and angular variation of the incident light. The experimental results allow the determination of the refractive index and thickness. Correlation with the granular structure is treated with different models, the size and filling factor of the inclusions playing an important role.
We reported experimental results of the resonant scattering of light from a system prism-glass/Ag/MgF2/air in the ATR-Kretschmann configuration, for p-polarized light incident by the glass side. The thickness of the dielectric film is chosen in such a way that in the absence of roughness the system supports 3 transverse magnetic (TM) guided modes, at a wavelength λ = 632.8 nm of the incident light. The scattering is due to the natural roughness of each interface of the system, while the resonant character of the scattering is due to the excitation of the guided modes and their interaction with the interfaces roughness. The scattered light shows six peaks at angles given by θ1 = ± 61.65 0, θ2 = ± 53.69 0, and θ3 = ± 43.40 0, for any angle of incidence. These angles correspond to the excitation of the guided modes. The scattering response is enhanced when the angle of incidence is equal to one of the angles of excitation of the guided modes.
We study the resonant excitation of the electromagnetic modes in a planar waveguide of metallic walls - light incident on the guide from the air can transfer energy through the walls exciting normal modes of propagation. It is found theoretically that radiation propagates along the guide while the reflectivity presents a minimum. The energy of the incident radiation can be transferred to the guide almost completely when the thickness dm of the metallic wall is around two times the skin depth. Experimental evidence of the injection of light is presented for the system Ag/Al2O3/Ag that was grown by pulsed laser deposition.