Design And Absorption Performance Investigation Of Thin Film Solar Cells Based On Two Dimensional Light Trapping Structures

Photovoltaic technology is a promising,green,and sustainable source of energy,which will play an increasingly important role in future energy application.Thin film solar cells have been recognized as the very promising future direction of development of solar cells,due to saving the material,reducing energy consumption,portability,benefitting the production of flexible and semi-transparent devices etc.To date,low-cost ultrathin-film solar cells,including OSCs（organic solar cells）,PSCs（perovskite solar cells）and so on,have been fabricated using various active materials.However,the ultrathin thickness of the active layer（s）in turn lead to poor solar light absorption,resulting in low power conversion efficiency（PCE）.To compensate for the inefficient light absorption,trapping structures,which can excite plasmonic and/or photonic mode,including cavity modes and Bloch modes,have been widely used for harvesting light.This thesis theoretically designs the thin-film OSCs and the thin-film PSCs based on two-dimensional light-trapping structures through FDTD（Finite Difference Time Domain）and FEM（Finite Element Method）.Through scanning and optimizing the main parameters of the designed thin-film cell structures,the effect of device parameters on the light absorption performance is clarified.In addition,the mechanism of light absorption enhancement is discussed by the spatial distribution of electromagnetic field,the propagation characteristics in time domain and the analysis of resonance modes.This paper will provide scientific basis and theoretical guidance for the development of high-efficiency thin film solar cells.The main results are as follows:Firstly,Metal Nanospheres（MNPs）were incorporated into organic solar cells and the effects of two different spherical MNPs（Ag and Au nanospheres）on absorption enhancement in the active layer with the optimal thickness were analyzed using FDTD（Finite Difference Time Domain）simulation.The results reveal clearly that the absorption enhancement in the OSCs is dependent on both the properties of MNPs（Metal Nanospheres）and the types of the donor/acceptor blend systems;For active layers with optimized thickness,these calculated absorption enhancement values induced by MNPs are much lower than the reported experimental PCE improvement results.It is suggested that the effect of MNPs on performance enhancement in OSCs is not only from the light absorption enhancement induced by MNPs and the OSCs electrical properties improvement induced by MNPs are more important.Then,the light trapping capability of the active layer incorporating Ag@SiO₂ core-shell nanospheres array in the organic solar cells is investigated.Ag@SiO₂ nanoparticles array incorporating in OSC can bring obvious absorption enhancement,slightly less than Ag nanospheres but higher than SiO₂ nanospheres.In addition,the detailed analysis on field distributions in the PSBTBT:PC71BM based OSCs with Ag nanospheres shows that the apparent absorption enhancement is due to the excitation of magnetic and electric resonances of surface plasmon polaritons.Secondly,a patterned OSC based on two-dimensional arrayed dielectric nanospheres loaded on the indium tin oxide anode is proposed,whose other functional layers conformally embossed in the same profile as the nanosphere array.This design shows an integrated light absorption efficiency of 76.6% in PSBTBT:PC71BM active layer over the wavelength range from 400 to 900 nm at normal incidence,outperforming the structurally identical planar control cell by 21.6%.Moreover,the broadband absorption enhancement of our proposal is insensitive to the incident angle,favoring the practical application.Detailed investigations reveal that the excitation of photonic modes including the the hybridization of cavity modes and Bloch modes,plasmonic modes and their mutual coupling are responsible for the observed broadband enhancement in light absorption.Among them,the photonic modes are confirmed to be main contributor at most of the absorption band of PSBTBT:PC71BM.Thirdly,for the insufficient light absorption in PSCs with a 100 nm thick active perovskite especially in the red to infrared range,two dimensional photonic-structured lead halide perovskite solar cells（PSCs）,induced by a 2-D photonic structured TiO₂ nanobowl or TiO₂ semi-nanosphere electron extraction layer,are proposed.Then,the pattered PSCs are analyzed using FEM（Finite Element Method）simulation.The numerical results indicate that for optimal 2-D photonic structured PSCs with a 100 nm thick perovskite layer based on TiO₂ nanobowl or TiO₂ nano-semisphere,the integrated absorption efficiency reaches 65.7% and 64.8% over the wavelength range from 350 to 800 nm at normal incidence considering AM 1.5G solar irradiation,with enhancement of 18.4% and 16.8% with respect to the control cell.And both patterned cells are nearly 90% of that of the planar PSC with 300 nm-thick perovskite layer by only 1/3 perovskite material consumption.In addition,the proposed PSCs own almost omnidirectional feature,being beneficial for the practical applications.The detailed field distribution investigations reveal that the great enhancement of light absorption in the long wavelength region is attributed to the collective impact of the different photon and plasmonic modes and their mutual coupling.