Fine Tuning of Donor-Acceptor Structures in Fused-Carbazole Containing Thermally Activated Delayed Fluorescence Materials towards High-Efficiency OLEDs

Comprehensive Summary Conjugated fused-ring structures have attracted extensive attention due to their high molecular rigidity to restrain excited-state relaxation and non-radiative decay,and further to enhance the luminance efficiency for emissive materials.Herein,we develop a series of donor-acceptor type thermally activated delayed fluorescence(TADF)emitters by introducing fused-ring 5H-benzofuro[3,2-c]carbazole(32BFCz)as electron donating unit.Through optimizing the numbers and structure of donor and acceptor moieties,three compounds named 32BFCzA,mCF3BFCzOXD and dCF3BFCzOXD are designed,which are composed by mono-32BFCz/trifluoromethylpicolinonitrile,penta-BFCz/3-(trifluoromethyl)phenyl)-1,3,4-oxadiazol-2-yl)benzene and penta-32BFCz/3,5-bis(trifluoromethyl)phenyl)-1,3,4-oxadiazol-2-yl)benzene as donor/acceptor groups,respectively.

Conjugated Random Tpeoilymer Donors towards High-Efficiency Polymer Solar Cells

Over the past decades,polymer solar cells(PSCs)which contain conjugated polymers as electron donor and/or acceptor materials in active layers have achieved the power con version efficiency(PCE)over 17%.Among them,tremendous alternative donor-acceptor(D-A)type conjugated copolymers have been reported as donor materials.Nevertheless,plenty of rooms still exist to further improve the photovoltaic performs nee for practical applications.Besides the exploration of the increasingly challenging novel D and/or A monomers to construct new D-A copolymer donors,conjugated random terpolymer donors which involve a third existing monomer(D or A)provide an extra simple promising strategy to promote the photovoltaic performance to a higher level.Herein,recent progress on random terpolymer donors for efficient PSCs was reviewed.Firstly,random terpolymer donors were classified by several typical molecular building blocks.Then,the influences of the third monomer on various random terpolymers were highlighted according to the enhancement of light-harvesting ability,modification of energy levels and optimization of the bulk-heterojunction(BHJ)morphology.Finally,several issues which might be concerned in future research on random terpolymer donors were proposed.This review may be helpful for providing guidelines to design efficient random terpolymer donors as well as better-understanding of the structure-property-performance correlations towards high performance PSCs via random terpolymer approach.

Enhanced power conversion efficiency in iridium complexbased terpolymers for polymer solar cells

By introducing various low concentrations of Iridium complexes to the famous donor polymer of PTB7-Th backbone,new heavy metal containing terpolymers have been demonstrated.When blended with PC71BM,an obvious increase of power conversion efficiency(PCE)is obtained in 1 mol%Ir containing polymer for different photovoltaic devices either using Ca or PDIN as cathode interface layers.The impact of molecular weight on the photovoltaic performance has been particularly considered by using three batches of control polymer PTB7-Th to ensure a fair and more convincing comparison.At similar molecular weight conditions(Mn:~60 kg mol−1,M_(w):100-110 kg mol^(−1)),the 1 mol%Ir containing PTB7-ThIr1/PC71BM blends exhibits enhanced PCE to 9.19%compared with 7.92%of the control PTB7-Th.Through a combination of physical measurement,such as optoelectrical characterization,GIWAXS and pico-second time-resolved photoluminescence,the enhancement are contributed from comprehensive factors of higher hole mobility,less bimolecular recombination and more efficient slow process of charge separation.

Molecular engineering on all ortho-linked carbazole/oxadiazole hybrids toward highly-efficient thermally activated delayed fluorescence materials in OLEDs

The highest efficiency thermally activated delayed fluorescence(TADF)emitters in OLEDs are mostly based on twisted donor/acceptor(D/A)type organic molecules.Herein,we report the rational molecular design on twisted all ortho-linked carbazole/oxadiazole(Cz/OXD)hybrids with tunable D-A interactions by adjusting the numbers of donor/acceptor units and electron-donating abilities.Singlet-triplet energy bandgaps(ΔEST)are facilely tuned from^0.4,0.15 to^0 eV in D-A,D-A-D to A-D-A type compounds.This variation correlates well with triplet-excited-state frontier orbital spatial separation efficiency.NonTADF feature with solid state photoluminescence quantum yield(PLQY)<10%is observed in D-A type 2CzOXD and D-A-D type 4CzOXD.Owing to the extremely lowΔEST for efficient reverse intersystem crossing,strong TADF with PLQY of 71%-92%is achieved in A-D-A type 4CzDOXD and 4tCzDOXD.High external quantum efficiency from 19.4%to 22.6%is achieved in A-D-A typed 4CzDOXD and 4tCzDOXD.

Twisted penta-Carbazole/Benzophenone Hybrid Compound as Multifunctional Organic Host, Dopant or Non-doped Emitter for Highly Efficient Solution-Processed Delayed Fluorescence OLEDs

A new multi-functional penta-carbazole/benzophenone hybrid compound 5CzBP was designed and synthesized through a simple one-step catalyst-free C--N coupling reaction by using 2,3,4,5,6-pentafluorobenzophenone and carbazole as starting materials. 5CzBP is very soluble in tetrahydro- furan （THF）, which brings an environmentally friendly device fabrication for solution-processed OLEDs instead of most widely used chlorinated solvents when 5CzBP is employed as the bulk-phase of organic host or non-doped emitter in the emissive layer. 5CzBP exhibits thermally activated delayed fluo- rescence （TADF） characteristic with relatively high triplet energy of 2.60 eV and a low bEsT of 0.01 eV. By using the new TADF material as organic host for another green TADF emitter, maximum external quantum efficiency （EQE） of 12.5% has been achieved in simple solution-processed OLED device. Besides, a maximum EQE of 8.9% and 5.7% was further obtained in TADF devices based on 5CzBP as dopant and non-doped emitter, respectively. The simultane- ously acting as efficient TADF host and non-doped TADF emitter provides the potential guidance of the future simple single-layer two-color white OLEDs based on low-cost pure organic TADF materials.

Engineering of Energy Levels for Fully Conjugated D-A Block Copolymers via Tuning the Ratios of Donor P3HT and Acceptor PNDIT

Fully conjugated donor-acceptor （D-A） block copolymers, P3HT-b-PNDIT, containing p-type poly（3-hexylthio- phene） （P3HT） and n-type poly（naphthalene bismide） （PNDIT） segments are synthesized in a one-pot reaction via Stille coupling polycondensation. Various D-A block copolymers with low polydispersities （1.42--1.56） are ob- tained through further separation via preparative GPC. The structural and molecular features of block copolymers are verified by 1H NMR, FTIR spectra, UV-Vis absorption, differential scanning calorimetry （DSC） and cyclic voltammetry （CV）. It is found that optical and electrochemical properties of D-A block copolymers are strongly de- pendent on the combination ratio of the donor P3HT and acceptor PNDIT segment. By reducing the acceptor block lengths, the highest occupied molecular orbital （HOMO） energy levels of D-A block copolymers sequentially rise, leading to the sequential decrease of energy gaps. To the best of our knowledge, this is the first report on engineer- ing the energy levels of fully conjugated D-A block copolymers by tuning donor and acceptor chain lengths. More- over, compared to D/A polymer blends, the all-conjugated D-A block copolymer films show significant fine struc- tures and much smoother film morphologies.