Designing simple non-fused terthiophene-based electron acceptors for efficient organic solar cells

Low-cost photovoltaic materials are essential for realizing large-scale commercial applications of organic solar cells(OSCs).However,highly efficient OSCs based on low-cost photovoltaic materials are scarce due to a deficiency in understanding the structure-property relationship.Herein,we investigated two low-cost terthiophene-based electron acceptors,namely,3TC8 and 3TEH,with 3,4-bis(octan-3-yloxy)thiophene,differing only in the alkylated thiophene-bridges.Both acceptors exhibit low optical gaps(∼1.43 eV)and possess deep highest occupied molecular orbital(HOMO)levels(∼−5.8 eV).Notably,the single-crystal structure of 3TEH demonstrates highly planar conjugated backbone and strongπ-πstacking between intermolecular terminal groups,attributed to the presence of the bulky alkylated noncovalently conformational locks.Upon utilizing both acceptors to fabricate OSCs,the 3TC8-based device exhibited a power conversion efficiency(PCE)of 11.1%,while the 3TEH-based OSC demonstrated an excellent PCE of 14.4%.This PCE is the highest among OSCs based on terthiophene-containing electron acceptors.These results offer a new strategy for designing low-cost electron acceptors for highly efficient OSCs.

Significant influence of doping effect on photovoltaic performance of efficient fullerene-free polymer solar cells

The modification mechanism of the water/alcohol cathode interlayer is one of the most complicated problems in the field of organic photovoltaics,which has not been clearly elucidated yet;this greatly restricts the further enhancement of the PCE for polymer solar cells.Herein,we clarified the different effects of PFN and its derivatives,namely,poly[(9,9-bis(3’-((N,N-dimethyl)-N-ethylammonium)-propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)](PFN-Br) in modifying fullerene-free PSCs.It is found for the first time that doping on IT-4F by the amino group of PFN leads to the unfavorable charge accumulation,and hence,forms a dense layer of electronegative molecule due to the poor electron transport capacity of the non-fullerene acceptor IT-4F.The electronegative molecular layer can block the electron transfer from the active layer to the interlayer and cause serious charge recombination at the active layer/cathode interface.This mechanism could be verified by the ESR measurement and electron-only devices.By replacing PFN with PFN-Br,the excessive doping effect between the cathode interlayer and IT-4F is eliminated,by which the charge transport and collection can be greatly improved.As a result,a high PCE of 13.5%was achieved in the fullerene-free PSCs.

Optimizing polymer aggregation and blend morphology for boosting the photovoltaic performance of polymer solar cells via a random terpolymerization strategy

Compared to regular conjugated polymers,the random conjugated terpolymers are usually not beneficial to achieve highly efficient non-fullerene(NF)-based polymer solar cells(PSCs)due to their disordered chemical structures.In this work,we report two random terpolymer donors(PBNB80 and PBNB50)by tuning the molar ratio of electron-accepting units of 1,3-di(thiophen-2-yl)naphtho[2,3-c]thiophene-4,9-dione(NTD)and 1,3-bis(4-chlorothiophen-2-yl)-4 H,8 H-benzo[1,2-c:4,5-c’]dithiophene-4,8-dione(ClBDD),at the same time,the parent polymers(PBNB100 and PBNB00)are also compared to study.These four polymer donors exhibit similar optical bandgaps and gradually deepen highest occupied molecular orbital levels.Importantly,aggregation and self-organization properties of the random terpolymer donors are optimized,which result in the better morphology and crystal coherence length after blending with NF acceptor of BO-4 Cl.Particularly,a PBNB80:BO-4 Cl blend forms an optimal nanoscale phase-separation morphology,thereby producing an outstanding power conversion efficiency of 16.0%,which is much higher than those(12.8%and 10.7%)of their parent binary polymer donor-based devices.This work demonstrates that rational using terpolymerization strategy to prepare random terpolymer is a very important method to achieve highly efficient NF-PSCs.

Manipulating Film Formation Kinetics Enables Organic Photovoltaic Cells with 19.5%Efficiency

Organic photovoltaic(OPV)cells have demonstrated remarkable performance in small,spin-coated areas.Nevertheless significant challenges persist in the form of large efficiency losses due to the fact that the ideal morphology cannot be preserved in the transition of small-area cells to large-scale panels.Herein,the ternary strategy of incorporating the third component FTCC-Br into the active layer of PB2:BTP-eC9 is employed to improve absorption response,optimize morphology,and reduce charge recombination,leading to a power conversion efficiency(PCE)of 19.5%(certified as 19.1%by the National Institute of Metrology,China).Moreover,the addition of FTCC-Br can control the aggregation kinetics of the active layer during the film formation process,transferring the optimal morphology to the blade-coated large-area films.Based on the highly efficient ternary bulk heterojunction,the 50 cm^(2) OPVmodules exhibited a PCE of 15.2%with respect to the active area.Importantly,the ternary OPV cells retain 80%of its initial PCE after 4000 h under continuous illumination.Our work demonstrates that the addition of a third component has the potential to improve the efficiency and stability of large-area organic solar cells.

15.3% efficiency all-small-molecule organic solar cells enabled by symmetric phenyl substitution

Synergistic optimization of donor-acceptor blend morphologyis a hurdle in the path of realizing efficient non-fullerene small-molecule organic solar cells(NFSMOSCs)due to the anisotropic conjugated backbones of both donor and acceptor.Therefore,developing a facile molecular design strategy to effectively regulate the crystalline properties of photoactive materials,and thus,enable the optimization of blend morphology is of vital importance.In this study,a new donor molecule B1,comprising phenyl-substituted benzodithiophene(BDT)central unit,exhibits strong interaction with the non-fullerene acceptor BO-4 Cl in comparison with its corresponding thiophene-substituted BDT-based material,BTR.As a result,the B1 is affected and induced from an edgeon to a face-on orientation by the acceptor,while the BTR and the acceptor behave individually for the similar molecular orientation in pristine and blend films according to grazing incidence wide angle X-ray scattering results.It means the donor-acceptor blend morphology is synergistically optimized in the B1 system,and the B1:BO-4 Cl-based devices achieve an outstanding power conversion efficiency(PCE)of 15.3%,further certified to be 15.1%by the National Institute of Metrology,China.Our results demonstrate a simple and effective strategy to improve the crystalline properties of the donor molecule as well as synergistically optimize the morphology of the all-small-molecule system,leading to the high-performance NFSM-OSCs.

A ternary organic solar cell with 300 nm thick active layer shows over 14% efficiency

In order to meet the requirements for making organic solar cells(OSCs)through solution printing techniques,great efforts have been devoted into developing high performance OSCs with relatively thicker active layers.In this work,a thick-film(300 nm)ternary OSC with a power conversion efficiency of 14.3%is fabricated by introducing phenyl-C61-butyric-acid-methyl ester(PC61BM)into a PBDB-T-2Cl:BTP-4 F host blend.The addition of PC61BM is found to be helpful for improving the hole and electron mobilities,and thus facilitates charge transport as well as suppresses charge recombination in the active layers,leading to the improved efficiencies of OSCs with relatively thicker active layers.Our results demonstrate the feasibility of employing fullerene derivative PC61BM to construct a high-efficiency thick-film ternary device,which would promote the development of thick layer ternary OSCs to fulfill the requirements of future roll to roll production.

Enhanced intermolecular interactions to improve twisted polymer photovoltaic performance

In polymer solar cells(PSCs), twisted polymer donors usually have low photovoltaic efficiencies due to their poor photoactive layer morphologies. Herein, we successfully improved twisted polymer(PBDT-3T) photovoltaic efficiency by employing C=O groups(PBDT-3TCO) to enhance intermolecular interactions. The maximum power conversion efficiency(PCE) of PBDT-3T is only 1.05%, while the PCE of PBDT-3TCO reaches 11.77% in non-fullerene(NF) PSCs. Both polymers-based PSCs show very similar open-circuit voltages but remarkable differences in their short-circuit currents and fill factors. The single crystals of both functionalized terthiophenes with methyl substituents demonstrate that the terthiophene with C=O units changes molecular pattern by forming intra/inter molecular S???O and O???H interactions but its molecular planarity does not significantly improve.Our comparative studies show that PBDT-3TCO with C=O units possesses a strong aggregation property and optimal photoactive layer morphology in NF PSCs. This study provides important insight into the design of high-performance twisted polymer donors for NF PSCs.

Benzo[1,2‑b:4,5‑b′]dithiophene-Based Conjugated Polymers for Highly Efficient Organic Photovoltaics

In the past several years,power conversion efficiencies(PCEs)of bulk heterojunction(BHJ)single-junction organic photovoltaic(OPV)cells have increased rapidly because of the innovation of photovoltaic materials,including polymer donors and nonfullerene acceptors(NFAs),device engineering,and morphology optimization.The development of photovoltaic materials has been deemed as the core to imporve the PCEs of OPV cells.Regarding the diversification of NFAs,the rational design of polymer donors is becoming more and more challenging,because an ideal polymer donor is required not only to meet matched molecular energy levels and complementary absoprtion spectra with NFAs but also to improve BHJ morphology and electroluminescence quantum efficiency of OPV cells.Among tens of thousands of polymer donors,benzo[1,2-b:4,5-b′]dithiophene(BDT)-containing conjugated polymers have become dominant donor materials because they are closely related to the frequent breakthroughs in the PCEs of OPV cells.A comprehensive understanding of the correlation among their chemical structures,optical properties,aggregation behaviors,and photovoltaic efficiencies is urgently required to develop nextgeneration outstanding polymer donors.In this Account,we focus on the molecular design strategies of BDT-containing polymer donors with the goal of developing new photovoltaic materials for fabricating the state-of-the-art OPV cells.First,we summarize our recent achievements in developing highefficiency BDT-containing polymer donors.In the meantime,the role of surface electrostatic potentials of active layer materials on exciton dissociation of BHJ layer is briefly discussed;the influence of fluorine and chlorine atoms in polymer donors on molecular energy levels,molecular torsion,and photovoltaic efficiencies is analyzed in detail.Second,to pursue higher PCEs of OPV cells,we highlight the following three aspects for in-depth discussion of the BDT-based polymer donor design.(i)The BHJ morphologies are regulated by changing the steric hindrance of the flexible chains,intermolecular interactions,and side chain orientations of polymers to modulate their aggregation in solution.(ii)The optical gaps of polymer donors are fine-controlled by combining the theories of the frontier orbitals hybrid and electron delocalization to develop the ideal polymer materials for versatile applications of OPV cells.(iii)The reduction of nonradiative recombination energy losses of OPV cells is discussed in detail by adding a third component,the reduction of energetic offsets of active layer materials,and the development of polymer donor with strong electroluminescence quantum efficiency.On the basis of the aforementioned molecular design considerations,we achieved PCEs up to 19.0%,20.2%,and 28.4%for single-junction,double-junction,and indoor light OPV cells,respectively.Lastly,we briefly discuss the opportunities and challenges to further improve the PCEs of OPV cells.

Efficient Small-Molecule Organic Solar Cells by Modulating Fluorine Substitution Position of Donor Material

Comprehensive Summary The fluorine substitution position in organic semiconductors is critical in improving device performance for organic solar cells(OSCs).Herein,two similar small-molecule donors,B3T-PoF and B3T-PmF,are designed and synthesized,which only differ on the fluorine substitution position on the pendent benzene unit.Although both small-molecule donors exhibit similar absorption profiles and molecular energy levels,B3T-PmF has stronger crystallinity and lower energetic disorder than B3T-PoF.After blending with the non-fullerene acceptor of BO-4Cl,B3T-PmF shows better phase separation and more ordered molecular packing in blend film.As a result,the B3T-PoF:BO-4Cl-based OSC shows a power conversion efficiency(PCE)of 12.3%.In contrast,the B3T-PmF:BO-4Cl-based cell demonstrates obviously increased JSC and FF values,thus yielding an excellent PCE of 14.7%.This study indicates that reasonable selection of fluorine atom substitution position in conjugated side chains is one of the promising strategies for achieving high-performance SM-DSCs.

Modulation of terminal alkyl chain length enables over 15% efficiency in small-molecule organic solar cells

In small-molecule organic solar cells(SM-OSCs),it remains a big challenge to obtain favorable bulk heterojunction morphology by donor material design.Herein,we design and synthesize three small-molecule donors BPF3T-C4,BPF3T-C6 and BPF3T-C8,with different terminal alkyl chains.Although they possess similar absorption profiles and molecular energy levels,their crystallinity gradually decreases with the chain length of the terminal alkyl chains.After blending with an electron acceptor of BO-4Cl,the crystallinity is suppressed and the packing orientations of these donors changed from edge-on to face-on.Simultaneously,the crystallinity of BO-4Cl is gradually weakened with the chain length of the terminal alkyl chain of donor materials.Finally,The BPF3T-C6 with moderate crystallinity exhibits the best phase-separation morphology among these blend films.As a result,the BPF3T-C6:BO-4Cl-based SM-OSC shows an impressive power conversion efficiency of 15.1%.

Organic polaritonic light-emitting diodes with high luminance and color purity toward laser displays

Achieving high-luminescence organic light-emitting devices(OLEDs)with narrowband emission and high color purity is important in various optoelectronic fields.Laser displays exhibit outstanding advantages in next-generation display technologies owing to their ultimate visual experience,but this remains a great challenge.Here,we develop a novel OLED based organic single crystals.By strongly coupling the organic exciton state to an optical microcavity,we obtain polariton electroluminescent(EL)emission from the polariton OLEDs(OPLEDs)with high luminance,narrow-band emission,high color purity,high polarization as well as excellent optically pumped polariton laser.Further,we evaluate the potential for electrically pumped polariton laser through theoretical analysis and provide possible solutions.This work provides a powerful strategy with a material–device combination that paves the way for electrically driven organic single-crystal-based polariton luminescent devices and possibly lasers.