Application Of Cu??/?? Redox Couple And Cu₉S₅ Counter Electrode In Dye-sensitized Solar Cells

In this dissertation,we focus on the investigation of the redox couple and counter electrode in dye-sensitized solar cell?DSSC?.Our aims is to improve the long-term stability of DSSC devices continued light irradiation and to reduce the cost of counter electrode?CE?matericals by designing now redox couple and developing new CE materials.As one of the most promising next-generation solar cell technologies,DSSC have been intensively studied over the past two decades,the power-conversion efficiency?PCE?have been significantly increased especially based on copper redox couples.However,the long-term stability of copper complex-based redox couples seemingly constitutes a problematic issue and so far the research in terms of this subject has been rather scarce.Thus,searching for efficient and stable redox couples is an important endeavor for commercialization of DSSCs.Herein,we report a new class of copper complexes containing diamine-dipyridine tetradentate ligands?L1=N,N?-dibenzyl-N,N?-bis?pyridin-2-ylmethyl?ethylenediamine;L2=N,N?-dibenzyl-N,N?-bis?6-methylpyridin-2-ylmethyl?ethylenediamine?as redox mediators in DSSCs.Notably,by the introduction of two methyl groups to the ligand,the redox potential of[Cu?L2?]⁺is positively shifted by 300 mV relative to that of[Cu?L1?]⁺.Since there is no macrocyclic aromatic conjugation system in the complexes studied in this work,the absorptions of of[Cu?L1?]^2+/+and[Cu?L2?]^2+/+are mainly loacted in the UV-light region.Therefore,they display almost negligible visible-light absorption competition with the sensitizer Y123.Devices constructed with[Cu?L2?]^2+/+redox couple afford an impressive PCE of 9.2%measured under simulated one sun irradiation(100mW cm^–2,AM 1.5G),which is among the top efficiencies reported thus far for DSCs with copper complex-based redox mediators.Remarkably,the excellent air,photo,and electrochemical stability of the[Cu?L2?]^2+/+complexes renders an outstanding long-term stability of the whole DSSC device,maintaining?90%of the initial efficiency over 500 h under continuous full sun irradiation.To the best of our knowledge,this is the first example of copper complex-based redox couple in DSSCs that simultaneously combines high efficiency and high long-term stability.This work unfolds a new platform for developing highly efficient and stable redox mediators for large-scale application of DSSCs.The platinum?Pt?is the most commonly counter electrode materials used in DSSC system,however,Pt is a rarely reserved metal resouse and the preparation process typically contains450?heating,which limit its commercialization application in the future.The development of low-cost and easy fabrication materials to the commonly used but expensive Pt counter elecrode in DSSCs is important from a commercial point of view.In this work,Cu₉S₅nanocrystalline film is fabricated directly onto F-doped SnO₂?FTO?substrate by a solution-processed spin-coating method with low temperature post-treatment at 250?and it is further explored as a counter electrode?CE?material in DSSCs.The results from cyclic voltammetry?CV?and electrochemical impedance spectroscopy?EIS?disclose that Cu₉S₅ film exhibits a higher catalytic ability for the state-of-the-art cobalt?III/II?tris?bipyridyl?([Co?bpy?₃]^3+/2+)redox system as compared to the widely used iodine-based electrolyte.Consequently,the DSSC devices based on the cobalt complex redox shuttles show a PCE of 5.7%measured at 100 mW cm^–2 illumination?AM 1.5G?,which is substantially higher than that of the iodine-based counterpart?3.9%?.This has been the first presentation for the application of digenite copper sulfides as an electrocatalyst for the[Co?bpy?₃]^3+/2+redox system in DSSCs.The present finding represents a promising solution for the development of alternative cost-effective CE materials for DSSCs in the future.