Effect Of Optical Interference Research Articles

Investigation of nature of excitons in PPDT2FBT and effect of optical interference

Electroabsorption (EA) measurements can be used to identify the type of excitons contributing to the absorption spectra of semiconductors. However, the shape of the EA spectrum may vary depending on the mode of measurement due to the optical interference effects. Analysis without considering these effects may lead to erroneous conclusions. In this work, we present EA measurements and analysis for reflection mode measurements considering optical interference effects. We compared the inferences with transmission mode measurements and discuss the limitations. We identified the nature of excitons associated with each transition in the absorption spectrum of poly[(2,5-bis(2-hexyldecyloxy)-phenylene)-alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c]-[1,2,5]-thiadiazole)] thin film from EA measurements. The bands at 1.89, 2.05, and 2.27 eV had a mixed nature consisting of charge transfer and Frenkel characteristics. Of these, the band at 2.05 eV showed the strongest charge transfer characteristic. From thickness dependent measurements, we showed that the interference effects increase with the thickness of the semiconductor layer. The nature of excitons, however, could still be deduced qualitatively from reflection mode EA measurements.

Revealing defective interfaces in perovskite solar cells from highly sensitive sub-bandgap photocurrent spectroscopy using optical cavities

Defects in perovskite solar cells are known to affect the performance, but their precise nature, location, and role remain to be firmly established. Here, we present highly sensitive measurements of the sub-bandgap photocurrent to investigate defect states in perovskite solar cells. At least two defect states can be identified in p-i-n perovskite solar cells that employ a polytriarylamine hole transport layer and a fullerene electron transport layer. By comparing devices with opaque and semi-transparent back contacts, we demonstrate the large effect of optical interference on the magnitude and peak position in the sub-bandgap external quantum efficiency (EQE) in perovskite solar cells. Optical simulations reveal that defects localized near the interfaces are responsible for the measured photocurrents. Using optical spacers of different lengths and a mirror on top of a semi-transparent device, allows for the precise manipulation of the optical interference. By comparing experimental and simulated EQE spectra, we show that sub-bandgap defects in p-i-n devices are located near the perovskite-fullerene interface.

Influence of Al-doped ZnO transparent electrodes on thin-film interference in Cu2ZnSn(S,Se)4 thin-film solar cells prepared via a sputtering method

Transparent electrodes are often regarded as a trade-off between transmittance and sheet resistance, although the effect of thin-film interference on the performance of an optoelectronic device is significant. In this paper, we report the effect of optical thin-film interference of an Al-doped ZnO (AZO) transparent electrode on the performance of Cu2ZnSn(S,Se)4 (CZTSSe) solar cells. First, we distinguished the transmittance in the overall wavelength region (450–800 nm) of the AZO transparent electrode from the transmittance in the maximum absorption region for CZTSSe solar cells (550–700 nm). From the results, a thicker AZO transparent electrode showed higher transmittance (87.1%) than that of a thinner AZO transparent electrode (82.9%) in the maximum absorption region. These electrodes were applied in CZTSSe solar cells, which showed a difference of up to 26% in power conversion efficiency. In addition, the colors generated by the optical interference between the AZO transparent electrodes. We anticipate that our results will be useful for improving the performance and/or color tuning of thin-film solar cells for application in building-integrated photovoltaics.

Efficiency Limit of Colloidal Quantum Dot Solar Cells: Effect of Optical Interference on Active Layer Absorption

Efficiency Limit of Colloidal Quantum Dot Solar Cells: Effect of Optical Interference on Active Layer Absorption

Micro-MOKE with optical interference in the study of 2D Cr2Ge2Te6 nanoflake based magnetic heterostructures

Due to its high sensitivity and sub-micrometer spatial resolution, the microscopic magneto-optical Kerr effect (micro-MOKE) technique has been widely utilized in the study of two-dimensional (2D) magnetic materials and heterostructures. Here, by taking 2D Cr2Ge2Te6 nanoflakes on a silicon wafer substrate as a model system, we present the effect of optical multilayer interference induced “optical artifacts” on the effective micro-MOKE measurements in such a system. It is found that not only the magnitude but also the sign of the micro-MOKE signal could be modulated by the “optical artifacts” with the variation of either the Cr2Ge2Te6 nanoflakes’ thickness or probe light wavelength. The detailed analysis based on the multilayer interference model reveals that there are two kinds of MOKE signals, sign reversal and magnitude modulation behavior, and the interference effect on the MOKE response can be easily predicted from unpolarized optical images. Our findings provide instructional principles on the promotion of micro-MOKE characterization as well as the optical studies in van der Waals magnets.

Multirate Spectral Phase-Encoded Time-Spreading O-CDMA System Using Orthogonal Variable Spreading Factor Code Sequences

We propose a multirate spectral phase-encoded time- spreading optical code-division multiple-access system using orthogonal variable spreading factor code sequences. The proposed multirate access networking method can support differentiation on more bit-rate/bit-error-rate (BER) requirements. The proposed method can also improve the BER performances of higher rate users compared to the conventional single-rate system due to mitigation of the effect of optical beat interference.

Passive mode-locking of a Nd:YAG ceramic laser by optical interference modulation in a GaAs wafer

We demonstrated passive mode-locking of a Nd:YAG ceramic laser by optical interference modulation in a GaAs wafer. The combined effect of Kerr nonlinearity and optical interference in GaAs acted as an artificial saturable absorber and resulted in mode-locking. Maximum average output power of the mode-locked laser was as high as 2.84W, with a slope efficiency of 48%. The mode-locked pulse duration was as short as 4.1 ps. To our knowledge, this is the shortest pulse obtained from all Nd:YAG lasers without dispersion compensation.

Investigation of the spectra of phosphorescent organic light-emitting devices in relation to emission zone

The dependence of the electroluminescent spectra on the emission zone of blue electrophosphorscent light emitting diodes (PHOLEDs) was investigated. The light emission of a PHOLED was tuned from blue to greenish blue by adjusting the position of the emission zone in the PHOLED. Experimental results agreed well with the numerical simulation based on the effect of the wide-angle optical interference by the metal cathode. The comparison of the numerical results and the electroluminescent spectra of the PHOLED was then extended to serve as the basis of another method to determine the location of the emission zones of PHOLEDs.

Performance Enhanced Asynchronous Optical CDMA Systems using Double Hard-limiters and FEC Codes

Optical code division multiple access (OCDMA) systems are generally interference limited. To alleviate the adverse effect of optical multiple access interference (OMAI), use of forward error correcting (FEC) codes and optical double hard-limiters (DHLs) are explored. The system analysis is carried out under chip synchronous case among different users with PIN+OA as the receiver. The analysis incorporates the effect of amplifier noise, thermal noise and OMAI. It is observed that optical CDMA systems with FEC and double hard-limiters give rise to satisfactory performance even when the number of simultaneous users is large. It has been observed that with the usage of DHLs there is a significant improvement in the performance. When FEC is also used along with DHLs, there is slight enhancement in the performance.

Effect of Optical Interference on Borophosphosilicate Glass Profiles Obtained with a Glow Discharge Optical Emission Spectrometer

We have studied the effect of optical interference on element profiles obtained with a glow discharge optical emission spectrometer (GDS). The specimens were borophosphosilicate glass films deposited on polished silicon wafers. We found that in some cases the interference is accompanied with a beat in the profile: a damping of the sinusoidal wave followed by a recovery. Profilometry showed that the depth resolution is better when the interference continues continuously without a beat. We showed by simulation that the cause of the beat came from variation of the penetration rate throughout the sputtered area.

Modelling of interference effects in anisotropic conjugated polymer devices

In this paper, we study the effect of optical interference on the emission from conjugated polymer devices. We extend the current models to include the effects of birefringence in the polymer layers. We also include the effect of the variation of radiative and non-radiative rate with position on the quantum efficiency of emission of an excited state. We then model the overall angular distribution, spectrum and efficiency of emission from a device structure. Experimental measurements of photoluminescence emission from PPV light-emitting diode structures are obtained. Comparison between the experimental and calculated angular distribution of emission is made, and good agreement is found. This demonstrates the importance of birefringence in determining the emission from polymer devices. We also demonstrate how to use the interference model to deduce the position of the recombination zone in electroluminescent devices.

Effect of optical beat interference on the dynamic range of a subcarrier multiple access passive optical network using Fabry-Perot lasers

Fiber-in-the loop systems that use subcarrier multiple access can have error floors from optical beat interference (OBI). We show that, when Fabry-Perot lasers are used, OBI can limit the allowable variation in received power from the lasers in the multiple access system. We demonstrate a 7 dB increase in the allowable power variation by using a very large optical modulation index.

Characterization of the defect density and band tail of an a-Si:H i-layer for solar cells by improved CPM measurements

Abstract A new CPM (Constant Photocurrent Method) system, which improves the accuracy of measurement by suppressing the effect of optical interference, has been developed. This system enables determination of the defect density (Nd) and the slope of the Urbach tail (Ech) of a-Si:H films for solar cells with improved accuracy. The system has shown that Nd and Ech are minimized when Eopt = 1.55–1.61 eV. Here, Eopt is the optical gap determined from ( αhv 1 3 versus hv plots. The conversion efficiency of a-Si:H solar cells can be optimized by using the i-layer in this range of Eopt.

Transmission of pulse sequences through monomode fibers

The transmission of sequences of pulses of finite linewidth through monomode fibers is investigated using methods of optical coherence theory. It is shown that, in the absence of third-order dispersion, this is mathematically analogous to spatial Fresnel diffraction. The effect of optical interference of overlapping pulses on the transmission capacity is discussed.