DESIGN AND SIMULATION OF DIELECTRIC NANOPARTICLES AND GRATING EMBEDDED MULTILAYER STRUCTURES FOR ORGANIC LIGHT EMITTING DIODES AND SOLAR CELLS

Abstract

Optoelectronics is the field that deals with devices based on light matter interaction. A device which converts electrical energy into light or vice-versa is usually referred to as an optoelectronic device. Major optoelectronics devices which show direct conversion between photons and electrons include light emitting diodes, laser diodes, photodiodes and solar cells. Light emitting diodes and laser diodes convert the electricity into light energy whereas photodiodes and solar cells convert light energy into electrical energy. In this thesis, we have mainly focused on light emitting diodes and solar cells due to their significant contribution in general purpose lighting and sustainable renewable green energy. Organic light emitting diodes (OLEDs) have drawn much interest than that of inorganic or semiconductor LEDs due to their advantage such as low power consumption, high internal quantum efficiency, high contrast, high brightness, and low cost. Despite achieving almost 100% internal quantum efficiency, the external quantum efficiency of OLED is limited to 20% in conventional cases. A large fraction of light (~80%) is lost due to the different mechanisms that take place inside the device. Use of various techniques such as microlens array, textured surface, gratings, nanoparticles, nanostructures has been reported in the literature to increase the light extraction efficiency of OLEDs. Organic solar cells (OSCs) have been of great significance over the past several years because of their low-cost roll-to-roll manufacturing, suitability with flexible substrates and high absorption coefficient. However, when compared with silicon based conventional solar cells, the power conversion efficiency of OSCs is still low. Due to the low diffusion length of the charge carriers in organic materials, regarding the choice of the thickness of the organic active layer there has always been a trade-off between light absorption and charge carrier extraction. The optimal thickness for the active layer of bulk heterojunction solar cells based on polymer and fullerene derivative is found to be in the range of 100 nm to 200 nm by taking into account the recombination losses. Many photons are left unharvested due to the thin active layer. To overcome the issue of light absorption in the active layer of the device to improve its efficiency, interest has been aroused in light trapping techniques and optical designs of the organic solar cells. Light trapping ii techniques are designed to redirect the incoming light. These techniques help in increasing the optical path length or photon absorption in the active layer of the cell. Initially these light trapping techniques were developed for thick active layer silicon solar cells but now they have attracted attention for thin film solar cells. Various techniques such as textured grooves, anti reflection coating, photonic crystals, diffraction gratings, and nanostructures have been numerically and experimentally studied to enhance the light absorption in active layer and hence the efficiency of the organic solar cells. This thesis presents designs and simulations of novel multilayer structures for organic light emitting diodes and organic solar cells. In particular, we have used dielectric nanoparticles and dielectric diffraction grating as light trapping techniques to reduce the optical losses and to increase the efficiency of the devices. Nanoparticles will act as scattering medium and diffraction gratings will split and diffracts the light into different directions. In organic light emitting diodes, we have proposed two multilayer designs based on dielectric nanoparticles. In first design, the dielectric nanoparticles layer is placed at glass substrate and in second design; the nanoparticles layer is incorporated at anode layer. Optical properties of single nanoparticle have been studied by Mie theory and the full OLED structure has been simulated and analyzed using Lumerical software based on finite difference time domain (FDTD) method. We have optimized the size, interparticle separation and refractive index of the nanoparticles. Enhancement of 1.7 times in light extraction efficiency is achieved with optimized parameters in both the OLED designs. We have further investigated the role of dielectric nanoparticles and diffraction grating in the enhancement of light absorption in the active layer and short circuit current density of OSC. Optical properties of single nanoparticle have been studied by Mie theory and the full OSC structure has been designed and studied by Full Wave Utility based on FDTD method. In the first design, the dielectric nanoparticles are placed at the glass substrate of the OSC structure. Optimization of diameter and interparticle separation has been carried out to achieve the maximum enhancement. Using this design, enhancement of 20.4% in light absorption and 19.8 % in short circuit current density (Jsc) with SiC nanoparticles has been achieved. Using TiO2 nanoparticles, absorption could be enhanced by 19% and Jsc by 18.5%. iii In the next design, the periodic dielectric diffraction grating has been placed at the glass substrate. We have examined the effects and optimized the width, period and height of the grating. Improvement in the light absorption by 24.2% and short circuit current density by 18.6% is obtained with optimized parameters. Further, the dielectric nanoparticles are introduced at anode (ITO) layer. By considering this design we were able to achieve enhancement of 40% in absorption and 33.9% in Jsc using SiC nanoparticles. Also, the light absorption improved by 29% and Jsc by 20.2% with ZrO2 material whereas for TiO2 material, the absorption improved by 37% and Jsc by 32.7%. The proposed studies will be useful for developing high efficiency OLEDs and OSCs. We have also discussed in brief the future scopes coming out of this thesis.