Pathways And Spectral Signatures Of Upper Excited Singlet Fission

Humans have been using solar energy for thousands of years,but the amount of solar energy used on Earth is far less than the amount received.Although solar cells seem to be ubiquitous,solar power accounts for a very small proportion of the total electricity generated in countries.Therefore,the solar cell industry still has enormous potential for development,providing clean and renewable energy for mankind.In any traditional silicon based solar cell,there is an absolute limit to the total efficiency,in part because each photon can only release one electron,even if the photon carries twice the energy required.Einzinger and colleagues demonstrated a method to improve battery efficiency using the principle of singlet fission（SF）,opening the door to the advent of a new type of solar cells.SF is a spin allowed process in which a singlet molecule in an excited state shares energy with adjacent ground state molecules,producing two triplet excited state molecules.This process allows the exciton yield to reach 200%,thereby improving the power generation efficiency of solar cells.SF is a photophysical process discovered more than 50 years ago.In recent years,SF has received widespread attention because of its great potential in improving light harvesting and photovoltaic efficiency and bypassing the Shockley-Queisser limit.In this thesis,I theoretically study SF induced by the excitation to upper states S_N（N>1）,and develop method for the simulation of femtosecond nonlinear signals of S_N→TT₁SF systems.The results show that compared with the traditional S₁→TT₁SF pathway,the S_N→TT₁SF is faster and more efficient.We believe that S_N→TT₁SF is promising for building new SF systems and improving SF efficiency.This paper has two tasks.First of all,by establishing a theoretical framework for simultaneous description of S₁→TT₁and S_N→TT₁SF,as well as simulating the electronic populations,transient absorption（TA）pump probe（PP）signals and time and frequency resolved fluorescence（TFRF）spectra,this paper provides a fairly comprehensive view of SF process.So far,the research methods for S₁→TT₁SF system have been relatively comprehensive,but the S_N→TT₁SF pathways were not simulated.Secondly,the spectral characteristics of S_N→TT₁SF were explored.By eliminating the interference of other energy states（only the coupled S_N--TT₁pair is considered）and monitoring the total population of the triplet state P_TT1（t）after the instantaneous S₀→S_Nexcitation,the main microscopic mechanisms controlling the S_N→TT₁SF are clarified and four factors affecting this process are established.Then the main pathways of S_N→TT₁SF are studied.The S_N-dominated excitation channel and its spectral characteristics are discussed.The developed method is used to simulate the short time part of the TA PP signal of the rubrene film experiment[Ma et al,J.Chem.Phys.2013,138,184508]monitoring S₁→TT₁and S_N→TT₁SF with TT₁→TT_Mexcited state absorption（ESA）。Based on this,the new strategies for optimizing S_N→TT₁SF are proposed which are not achievable for traditional S₁→TT₁SF.