Study Of Enhancement Of Optical Absorption Efficiency In Organic Solar Cells Based On Guided Mode Resonance In Asymmetric Grating

Organic solar cells(OSCs)have been a concerned research topic in recent years because of the characters such as the low costs,flexibility and better film-forming.Meanwhile,OSCs device can work at low temperature and can be modified according to demand of molecular features in specific applications.Although the organic material is easy to fabricate,however,suffering from the short carriers' diffusion length,the collecting efficiency of carriers could be drastically decreased when the OSCs device utilize a thick layer(>200 nm)of organic material as the active layer.This contradiction restricts the enhancement of absorption of OSCs,and the conversion efficiency of organic solar cells is relatively low.In recent years,numerous researches paid attention in modifying the internal structure of OSCs on nano-scale by introducing periodical structures or embedding metallic nanoparticles in order to improve the ability of capturing incident light in the active layer.By introducing periodical metallic gratings or embedding metallic nanoparticles into active layer,a surface plasmons polariton(SPP)could be excited on the interface of active layer and metal,and a strong electromagnetic field enhancement can occur surround the gratings or nanoparticles,leading to the improvement of the absorption of active layer.However,the photon-induced carriers could be absorbed with certain probability by the noble metal material and hot electron radiation can occur when the active layer material and metal material are directly contacted,which will lead to a nagtive affection on the absorption efficiency of active layer.The other aspect,in OSCs devices containing one dimensional nanometer metal grating,the surface plasmons can be excited only under the TM(transverse magnetic)polarization of the incident light.However,for TE(transverse electric)polarization of the incident light,absorption efficiency have no obvious enhancement.In this thesis,we proposed a type of a asymmetric rectangular grating structure composed of P3HT:PC60BM material as active layer and PEDOT:PSS material as a hole transport layer(HTL),which can avoid the absorption in non-electrode metallic nanostructures.By choosing proper parameters of diffraction element and thin film structure,the guided-mode resonance can occur for mode-coupling between the incident light field and the waveguide leaky mode,leading to energy redistribution in the device.The enhanced near electric field in the active layer can improve the absorption of active layer and also have a better performance for two polarizations in real condition.Besides,the guided-mode resonance in symmetric gratings can be degenerated in asymmetric structures and the number of resonance mode can be increased,leading to the improvement of broadband absorption spectrum of OSCs device.The main contents of this thesis are as follows:(1)Chapter one describes the development of OSCs and introduces the absorption principle of OSCs,and the factors influencing the absorption efficiency of active layer in OSCs.(2)Chapter two introduces the characteristics of subwavelength grating and the excitation principle of guided-mode resonance.The electromagnetic analysis method in TE and TM polarizations of incident light are also introduced.(3)Based on the guided-mode resonance principle,a kind of T-shape asymmetric grating in OSCs structure is proposed in chapter three.Through analysis,it was found that the period of asymmetric nano gratings plays an important role in modulating the shape and resonance locations of absorption spectrum.The results shows that the integrated absorption efficiency could be enlarged by 4.2% in 2-ridges asymmetric grating structure compared to the planar device,and the integrated absorption efficiency of asymmetric grating structure with 3 ridges could be further more enhanced by ~52%.(4)On the basis of the principle in the third chapter,we proposed a type of core-shell-ITO asymmetric gratings structure by active layer and ITO material with a HTL with reduced thickness of 10 nm as a spacer in between.This composite structure has a better performance for enhancing absorption efficiency resulting from the localized near field in the active layer compared to the T-shape structure.