P. K. Bhatnagar

Study of optical absorption and photoluminescence of quantum dots of CdS formed in borosilicate glass matrix

Optical absorption and photoluminescence (PL) measurements have been made on the quantum dots (QDs) of CdS grown in a borosilicate glass matrix using a two-step annealing technique. The absorption measurements, made in the energy range of 1.3?3.2?eV, indicate the presence of nonradiative trap centers located in the forbidden gap at an energy level near 1.5?eV. The origin of these traps is attributed to the impurities present in the glass matrix. The PL measurements have been made at an excitation energy of 2.75?eV and it is concluded that the origin of PL is not due to either direct recombination of electrons and holes or deep traps, but that it is the shallow traps which are responsible for the observed PL. The shallow traps are attributed to sulfur vacancies formed at the glass-QD interface. The reason for the observed decrease in PL peak intensity with the increase of annealing time is due to the decrease of surface to volume ratio for QDs of higher size.

Role of donor–acceptor domain formation and interface states in initial degradation of P3HT:PCBM-based solar cells

This work is devoted to identifying the degradation mechanism in various structures of a poly(3-hexylthiophene) (P3HT):6,6-phenyl C61-butyric acid methyl ester (PCBM)-based solar cell. We have tried to identify the dominant initial degradation mechanism on the basis of experimental studies carried out on different structures of an organic solar cell. It is known that many of these problems can be solved by means of the following: using proper electrodes and a suitable annealing temperature and duration, improving the morphology of the active film and maintaining a donor–acceptor phase-segregated ordered network as far as possible. The present studies have been carried out both in the dark and under illumination, and it is suggested that initial degradation plays a key role in device performance. The dominant degradation mechanism is the growth of the donor–acceptor complex with time, which not only reduces the effective surface area but also hampers the charge separation. The little change in VOC and the significant change in JSC suggest that once the LiF/Al electrode is improved, one must modify the structure either by introducing a thin layer of high-molecular-weight P3HT between PEDOT:PSS (poly(3,4-ethylenedioxythiopene) poly(styrenesulfonate)) and photoactive P3HT:PCBM layers or by introducing an optimized content of P3HT nanofibrils/nanoparticles into the P3HT:PCBM blend. The best structure was found to be ITO/PEDOT:PSS/P3HT:PCBM/LiF/Al and it can be improved by the above two methods.

Study of CdTe quantum dots grown using a two-step annealing method

High size dispersion, large average radius of quantum dot and low-volume ratio has been a major hurdle in the development of quantum dot based devices. In the present paper, we have grown CdTe quantum dots in a borosilicate glass matrix using a two-step annealing method. Results of optical characterization and the theoretical model of absorption spectra have shown that quantum dots grown using two-step annealing have lower average radius, lesser size dispersion, higher volume ratio and higher decrease in bulk free energy as compared to quantum dots grown conventionally.

Optical absorption spectrum of single-walled carbon nanotubes dispersed in sodium cholate and sodium dodecyl sulfate

Commercially procured single-walled carbon nanotubes were dispersed in 2 wt% solution of sodium cholate and also in 1 wt% solution of sodium dodecyl sulfate. The absorption spectrum of the suspensions was studied in ultraviolet–visible–near-infrared (UV–vis–NIR) range. Two distinct bands, each containing three peaks, were observed in NIR range for both the suspensions. These peaks correspond to transitions between van Hove singularities E11 and E22 in the density of states of the semiconducting nanotubes. Comparing positions of the observed peaks with the empirical Kataura plot, the diameters and chiralities of the nanotubes were estimated. Using tight binding approximations, the diameter of the nanotubes was also estimated theoretically. Discrepancies between the theoretically calculated diameters and those obtained by empirical Kataura plots are found to be higher for E11 peaks. It has been suggested that the reason for this discrepancy is that the observed E11 peaks are blue-shifted due to Coulomb interactions and exciton formation.

Enhancement of efficiency of a conducting polymer P3HT:CdSe/ZnS quantum dots hybrid solar cell by adding single walled carbon nanotube for transporting photogenerated electrons

Hybrid solar cells consisting of a composite of poly (3-hexylthiophene) (P3HT), single walled carbon nanotube (SWCNT), and cadmium selenide/zinc sulphide (CdSe/ZnS) coreshell quantum dots (QDs) have been fabricated in the present work. The bulk hetrojunction has been formed from the bilayer of P3HT:SWCNT composite and QDs using inter-diffusion process. Due to low percolation limit and high conductivity of SWCNT, the photo-generated electrons are collected at the electrode very fast (within few femto-seconds) enhancing the efficiency of the solar cell. The absorption measurements on the composite film show that the addition of SWCNT in the hybrid structure increases the absorption coefficient in the near infrared region and also makes the spectrum wider as compared to that of P3HT. The photoluminescence (PL) measurements show that the PL of hybrid P3HT, SWCNT, and QDs is quenched about ∼15 times as compared to that of P3HT film. This shows that a significant charge transfer of electrons occurs through SWCN...

Study of the crystallization behavior of Ag-In-Sb-Te phase change optical recording film

Recently the demand for high-speed and high-density optical recording media using a direct overwrite scheme is very high. Among some of the potential candidates, the AgInSbTe alloy appears to be one of the promising materials that has drawn worldwide attention. It can give direct overwrite capability within a short period of time and is reported to give a well-defined shape with sharp edges, leading to intrinsically lower jitter values, thereby increasing the linear density. Hence, considerable interest has been focused on the study of the crystallization behavior of AgInSbTe alloy. Different crystalline phases are observed due to thermal annealing of AgInSbTe four-element alloy films. Results of the x-ray diffraction analysis of amorphous and crystalline (AgSbTe) x (In 1–y Sb y ) 1–x films with x = 0.2, 0.4 and y = 0.7, deposited by thermal evaporation technique are presented. The difference in crystallization behavior of the crystalline phases formed after 1 h of isothermal annealing at temperature between 200 and 400°C are studied through x-ray diffraction analysis. The experimental results are presented for a composition close to eutectic Sb 69 Te 31 in which some of the Te is replaced by Ag and In. The results indicate that the growth of the crystalline phases depends on the annealing temperature and it is also affected by the change in the composition.

Optical properties and average flow of energy in AgInSbTe films used for phase change optical recording

Optical constants of amorphous (AgSbTe) x (In 1– y Sb y ) 1– x films (AIST) for a typical composition ( x = 0.4 and y = 0.7) deposited by the thermal evaporation technique, are calculated from transmittance spectra. They are also been calculated during the crystallization process induced by 1-h thermal annealing at different temperatures. The effect of heat treatment on the absorption coefficient, the extinction coefficient, and the refractive index of the films are studied. The optical gap for as deposited amorphous films is found to lie in near-IR region. The bandgap shifts resulting from annealing are also calculated for the crystalline films. Further, these results are used for a multilayer structure for AIST as the recording layer, illuminated by a laser beam. The flow of optical energy in the absorptive layers and hence the diffusion of the resultant heat throughout the structure are studied.

Image force effects in metal‐oxide‐semiconductor solar cells

Image force effects have been shown to play an important role in deciding the current‐voltage characteristics of the Metal‐Oxide‐Semiconductor (MOS) Solar Cell. Because of reduction in barrier height and the barrier width, the dark saturation current is increased, and thereby the open circuit voltage is greatly affected.

Effects of aging on the mobility and lifetime of carriers in organic bulk heterojunction solar cells

Mobility and lifetime of the carriers are two very crucial parameters, which can account for the aging of a photovoltaic device, because changes in morphology, electrode/polymer interfaces, etc., will finally result in decrease of mobility and lifetime of carriers. In the present work, we have tried to explain our experimental data with the help of theoretical analysis based on our earlier model in which we have incorporated the activity at electrode interfaces in terms of density of surface states, thickness of interface layer, and tunneling probability. Calculations show that decrease in mobility for a cell under illumination is much faster than in a dark cell. Photoinduced oxidation plays a dominant role in fast degradation of the mobility. The lifetime is also seen to decrease because of modification of parameters such as interface thickness, density of interface states, and roughness.

The effect of cadmium vacancies on the optical properties of chemically prepared CdS quantum dots

CdS quantum dots (QDs) in a polyvinyl alcohol (PVA) matrix have been grown by a chemical method and are characterized by transmission electron microscopy (TEM), UV?vis absorption, photoluminescence (PL) and energy dispersive x-ray diffraction (EDX). TEM studies of CdS films show that a nearly spherical cluster of CdS QDs with an average radius of 10?15?nm is formed. From absorption measurements, it is observed that with increasing the PVA concentration from 5 to 8?wt.%, the absorption edge shifts from 3.1 to 3.6?eV, which is attributed to an increase in quantum confinement with decreasing the QD size. PL studies in an energy range of 1.8?3.3?eV show two distinct peaks. The higher-energy peak corresponds to band edge emission, whereas the lower-energy peak corresponds to defect emission. EDX results revealed that the atomic concentration of cadmium is much lower than that of sulfur, indicating that cadmium vacancies are predominant. It was concluded that cadmium vacancies are mainly responsible for defect emission in the PL spectrum.