Theoretical Design Of Counter Electrode In Dye-sensitized Solar Cells

In recent years,in order to solve the growing global energy demands and the environmental issues,substantial research efforts have been devoted to enhance the power conversion efficiency of solar cells.Compared with the conventional silicon-based solar cells,dye-sensitizedsolarcells（DSSCs）arelow-cost,easily-fabricated,and environmentally-friendly,which offer new opportunities for solar cells.The counter electrode（CE）is an important component of DSSCs,and a classical CE material is Pt due to its excellent catalytic activity,good electrical conductivity,and high stability.However,Pt is relatively expensive,which limits the potential large-scale application of DSSCs.Therefore,the development of low-cost CE materials with high catalytic activity is very important for DSSCs.In this thesis,the density functional theory（DFT）calculations have been performed to investigate the adsorption process of I₂ at the classical Pt CE surface,and a series of Pt-free catalytic alternatives have been designed.The main contents are summarized as following:（1）By means of the DFT calculations,the adsorption process of I₂ at Pt（111）surface has been investigated.The obtained adsorption energies and stable structures depending on the adsorption sites of I₂ at the Pt（111）surface show that the dissociative chemisorption and the non-dissociative chemisorption are competitive for the adsorption of I₂ on the Pt（111）surface,and the former is more energetically favorable.This study is expected to enrich the understanding of the origin of the excellent catalytic performance of Pt for triiodide reduction in DSSCs.（2）The DFT calculations were performed to investigate the reduction of triiodide ion catalyzed by metal atom embedded in graphene.It is shown that the binding energy of a single Pt atom embedded into the divacancy of graphene（Pt@DV）is about-11.0 eV,larger than that of Pt atom at single vacancy（Pt@SV）.In the Pt@DV with a Pt loading content of25 wt%,the adsorption energy of iodine atom and the dissociation energy of iodine anion are respectively-1.54 and 0.60 eV,indicating a potentially high catalytic activity.Further investigations of diverse transition metal embedded graphenes ranging from Sc to Zn imply that the Co embedded graphene（in divacancy）may be a good candidate to be utilized as counter electrode in DSSCs.（3）The reactions of I₃^-/I^-catalyzed by Cu₂ZnSnS₄（CZTS）and its derivatives have been investigated using DFT.By studying the geometrical and electronic properties of iodine atom adsorbed on CZTS,Cu₂ZnSnS_xSe_4-x（CZTSSe）,and Cu₂ZnSnSe₄（CZTSe）,it was found that the charge transport performance of CZTS can be significantly enhanced when the S in CZTS was replaced by Se.Further investigation on K,V,Ir,or Pt substituted CZTS reveals that the formation of K and V substituted CZTS is energetically favorable.By comparing adsorption energies of iodine atom and desorption energies of iodine anion on various surfaces,it has been demonstrated that the designed material shows a high catalytic activity for the reaction of I₃^-/I^-when the Cu2 atom in Cu₂ZnSnS₄ is replaced by V(V_Cu2).The V_Cu2u2 is thus proposed to be a promising candidate as CE in DSSCs.（4）The reactions of I₃^-/I^-catalyzed by two Pt atoms embedded into graphene have been investigated using DFT.It was shown that the binding energy of two Pt atoms embedded into the opposite side of the graphene（Pt-O）is larger than that in the same side（Pt-S）,and the cluster of Pt-O has a higher catalytic activity than Pt-S.With the further analysis of diverse transition metal-embedded graphenes ranging from Sc to Zn,it was proposed that the Mn-S,i.e.,two Mn atoms embedded into the same side of graphene plane,may be a good candidate to be utilized as counter electrode in DSSCs.