Theoretical Insights Into The Electron Transfer Mechanism In Dye-sensitized Solar Cells

Dye-sensitized solar cells(DSSC)have attracted widely attetions due to its low cost and high photoelectric conversion efficiency.Deeply understanding of its working mechanism is important for further improving the performance.The key part of DSSC is the dye molecule,which is responsible for the photo-absorption,interface charge transfer,and eventually the device performance.Quantum chemistry calculation can reveal the working mechanism and characterize the contribution of each component to the DSSC efficiency at the atomistic details,has been an important complement to the experimental investigation.This article studied a series of metalfree organic dyes,with particular attention paid to the modification of the π-spacer and the mechanism of the interface charge transfer,with the aim of understanding how the dye’s molecular structure affects the conversion efficiency.The details are described as following:[1] We present a comprehensive investigation of how the elongation of π-spacer of D-π-A organic dyes affects the cell performance.Based on the calculated results,detailed discussions were paid to the absorption spectrum,total and projected density of states for the isolated and adsorption systems,we further explicitly analyzed the impact of π-spacer modification on the photo-absorption,interface electron transfer,and the shift of Ti O2 conduction band edge,which are in close connection to DSSC efficiency.We demonstrated that the elongation of π-spacer enhances the extent of π-conjugation,subsequently red-shifts,intensifies,and broadens the absorption spectrum,which is favorable for the improvement of photocurrent.Finally we design a high-efficiency organic dye contains the bithiophene π-spacer for DSSC application.[2] We reported a systematically theoretical study concerning the impact of rigidifying dithiophene π-spacer of D-π-A organic dyes on the performance of DSSC.By using the state-of-the-art theoretical calculations,the general influences of fastening atoms(C,N,and O)for π-spacer rigidification in planar amine-based organic dyes are firstly investigated and elucidated.Three key parameters,such as charge transfer distance,charge transfer amount,and overlap between dye excited and ground states which are based on electron density grid,were used to evaluate the extent of intramolecular charge transfer.The dye-I2 interaction which is responsible for charge recombination was discussed in detail.The calculated results demonstrated that rigidification of dithiophene π-spacer can efficiently regulate the DSSC performance,and the dye with the incorporation of O-bridged dithiophene π-spacer is promising for DSSC application since it has better photo-absorption,faster interface electron transfer rate,and slower charge recombination.Furthermore,benzothiadiazole(BTD)and 3,4-ethylenedioxythiophene(EDOT)moieties are the well-known π-skeletons that can effectively tune the electronic structure properties and the light-harvesting ability.Subsequently,a series of dyes are designed through introducing the BTD and EDOT groups into π-spacer.The calculated results reveal that the dye with the incorporation of EDOT moiety would be more beneficial for photocurrent and photovoltage performance.[3] The anchoring group plays important roles in determining the overall efficiency of DSSC device since it anchors the dye onto the Ti O2 surface and also guides the interface electronic coupling,and thus the interface electron transfer rate(IET).We report,from a theoretical point of view,the first comparative study between the highly water-stable hydroxamate and the widely used carboxylate,in addition to the robust phosphate anchors.Theoretical calculations reveal that hydroxamate would be better for photo-absorption.A quantum dynamics description of the IET,including the underlying nuclear motion effect,is presented.We find that both hydroxamate and carboxylate would have efficient IET character;for phosphate the injection time is significantly longer(several hundreds femtoseconds).We also verified that the symmetry of the geometry of the anchoring group plays important roles in the electronic charge delocalization.We conclude that hydroxamate can be a promising anchoring group,as compared to carboxylate and phosphate,due to its better photoabsorption and comparable IET time scale as well as the experimental advantage of water stability.We expect the implications of these findings to be relevant for the design of more efficient anchoring groups for DSSC application.[4] We present a time-domain ab initio study of electron-hole recombination in pristine MAPb I3,and compare it to the trap mediated recombination in MAPb I3 with the iodine interstitial defect.Non-adiabatic molecular dynamics combined with time-domain density functional theory show that the iodine interstitial defect creates a sub-gap state capable of trapping both electrons and holes.Hole trapping occurs much faster than electron trapping or electron-hole recombination.The trapped hole survives for hundreds of nanoseconds,since,rather surprisingly,recombination of electrons with the trapped hole takes several times longer than recombination of electrons with holes in the valence band.Since the hole trap is relatively shallow,the hole can escape into the conduction band prior to recombining with the electron.The differences are rationalized by variation in non-adiabatic electron-phonon couplings,phonon-induced pure-dephasing times and electronic energy gaps.The timedomain atomistic simulations contribute to understanding of the experimentally known defect-tolerance of perovskite solar cells,which is of great importance to the solar cell performance.