Enhanced charge separation by interchain hole delocalization in nonfullerene acceptor-based bulk heterojunction materials

Bulk heterojunction(BHJ)composites show improved power conversion efficiencies when optimized in terms of morphology using various film processing methods.A reduced carrier recombination loss in an optimized BHJ was characterized previously.However,the driving force that leads to this reduction was not clearly understood.In this study,we focus on the decreased carrier recombination loss and its driving force in optimized nonfullerene acceptor-based PTB7-Th:IEICO-4F BHJ composites.We demonstrate that the optimized BHJ shows deactivation in the sub-nanosecond nongeminate carrier recombination process.The driving force for this deactivation was determined to be the improved interchain hole delocalization between the polymers.An enhanced interchain hole delocalization was observed using steady-state photoinduced absorption(PIA)spectroscopy.In particular,increased splitting between the polaron PIA bands was noted.Moreover,improved interchain hole delocalization was observed for other state-of-the-art BHJ materials,including D18:Y6 with optimized morphologies.

Efficient nonfullerene organic solar cells with active layers fabricated by water transfer printing

Preparation of high-quality films plays an important role to achieve high-performance nonfullerene (NF) organic solar cells. NF active layer films are typically fabricated by spin coating. Novel fabrication methods to process the NF active layer are desirable to be compatible with large-area production. Herein, we report on the fabrication of NF active layer films via a water transfer printing method.This method delivers a uniform film with controllable film thicknesses. NF active layers of PDBD-T:ITIC and PBDB-T-2F:IT-4F were fabricated via the method to validate its effectiveness. Solar cells with the water transfer-printed active layers show comparable performance (up to 11.7%) to the cells with spin-coated active layers. Furthermore, NF solar modules containing 4-sub cells with the active area of 3.2 cm2 are also fabricated via the method. The module shows VOC of up to 3.4 V and a power conversion efficiency of 8.1% with the PBDB-T-2F:IT-4F active layer.

Asymmetric molecular engineering in recent nonfullerene acceptors for efficient organic solar cells

Nonfullerene acceptors(NFAs),which usually possess symmetric skeletons,have drawn great attention in recent years due to their pronounced advantages over the fullerene counterparts.Moreover,breaking the symmetry of NFAs could fine tune the molecular dipole,solubility,energy level,intermolecular interaction,molecular packing,crystallinity,etc.,and give rise to improved photovoltaic performance.Currently,there are three main strategies for the design of asymmetric NFAs.This review highlights the recent advances of high-performance asymmetric NFAs and briefly outlooks the materials exploration for the future.

Near-Infrared All-Fused-Ring Nonfullerene Acceptors Achieving an Optimal Efficiency-Cost-Stability Balance in Organic Solar Cells

Synergistically achieving stability,cost,and efficiency is crucial for the commercialization of organic solar cells(OSCs).Despite the rapid development of 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malo nonitriletypenon fullerene acceptors(NFAs),they areinherently unstable due to the vulnerable exocyclic double bond and possess high synthesis complexity(SC).Based on the“all-fused-ring electron acceptor(AFAR)”concept,we report two new near-infrared NFAs,F11 and F13,featuring all fused dodecacyclic rings.By developing a whole set of synthetic procedures,F11 and F13 can be conveniently prepared at a 10 g scale within a notably short period,displaying both the low SC and the lowest costs among reported NFAs,even comparable to the classical photovoltaic material,P3HT.In comparison with the one-dimensional stacking of ITYM(ITYM=2,2′-(7,7,15,15-tetrahexyl-7,15-dihydro-s-indaceno[1,2-b:5,6-b′]diindeno[1,2-d]thiophene-2,10(2H)-diylidene)dimalononitrile),the first AFRA,and mixed J-and H-aggregations in Y6,F-acceptors show a compact honeycomb-type three-dimensional stacking with exclusive J-aggregations,favoring multichannel charge transport.By matching a medium-bandgap polymer donor,F13 delivers greater than 13%power conversion efficiencies,which is the highest performance among non-INCN acceptors,and shows device stability superior to the typical ITIC-and Y6-based OSCs as evidenced by the negligible burn-in losses.This work presents a first and successful example of NFAs achieving an optimal efficiency-cost-stability balance in OSCs.

Enhancing the Intermolecular Interactions of Ladder-Type Heteroheptacene-Based Nonfullerene Acceptors for Efficient Polymer Solar Cells by Incorporating Asymmetric Side Chains

Asymmetric nonfullerene acceptors(NFAs)possess larger dipole moments and stronger intermolecular bonding energy than their symmetric counterparts thereby making them promising candidates for high-performance polymer solar cells(PSCs).Herein,we report twoefficient acceptor–donor–acceptor(A–D–A)type NFAs(M14 and M18)with asymmetric side chains that show enhanced intermolecular interactions compared with their corresponding counterparts(M17 and M19)based on symmetric side chains.Furthermore,M14 and M18 exhibit elevated lowest unoccupiedmolecular orbitals and smallerπ–πstacking distances in comparison with M17 and M19,respectively.In combination with the benchmark polymer donor of PM6,the PM6:M14 blend affords superior charge transport properties,and more importantly,an increased power conversion efficiency(PCE)of 15.49%in comparison with the M17-based counterpart(13.01%PCE).Similarly,the asymmetric M18-based blend also shows a higher PCE of 13.00%than the M19-based blend(11.55%).Through further interface engineering,the bestperforming M14-based device delivers an enhanced PCE of 16.46%,which represents a record value among all asymmetric A–D–A type NFAs.Our results provide new insights into the design of asymmetric NFAs with enhanced intermolecular interactions for highperformance PSCs.

26mA cm^(-2) J_(sc) from organic solar cells with a low-bandgap nonfullerene acceptor

Nonfullerene-based organic solar cells(NFOSCs)have received great interest recently due to their higher performance and greater potential compared with fullerene-based solar cells[1].Power conversion efficiencies(PCEs)over 13% have been realized in single-junction NFOSCs[2].Compared with traditional fullerene acceptors,the greatest advantage of nonfullerene acceptors is their stronger light-harvesting capability in the visible and

An A-D-A′-D-A type unfused nonfullerene acceptor for organic solar cells with approaching 14%efficiency

In recent years,power conversion efficiency(PCE)of organic solar cells(OSCs)has made significant improvement.A large number of studies were reported to achieve high PCEs through exploring new active layer materials,especially the high efficiency fused ring acceptors(FRAs).Compared with FRAs,another type of so-called unfused-ring acceptors(UFAs),possessing some advantages such as simple synthesis and low cost,have attracted a lot of attention.Herein,a new UFA BTzO-4F,incorporating with a benzotriazole moiety and S···O intramolecular noncovalent interactions,has been successfully synthesized.The photovoltaic device based on PBDB-T:BTzO-4F achieved a record PCE of 13.8%for UFAs,which indicates that introducing the benzotriazole moiety is an effective strategy for high quality acceptors.Thus,these findings of this work demonstrate the great potential of UFAs for high performance OSCs.

High-efficiency quaternary polymer solar cells enabled with binary fullerene additives to reduce nonfullerene acceptor optical band gap and improve carriers transport

The polymer/small-molecule electron donor and nonfullerene organic electron acceptor are of structural similarity with both donor and acceptor molecules consisting of polycyclic fused-ring backbone and being decorated with alkyl-chains.In this study,we report that the introduction of binary fullerenes(C_(60)-/C_(70)-PCBM and C_(60)-/C_(70)-ICBA)into a nonfullerene binary system PBDB-T:ITIC reduces the polymer-nonfullerene acceptor intermixing,obtaining higher crystallinity with(100)crystal coherence length from 28 to 29–33 nm for the ITIC,and from 14 to 20–24 nm for the PBDB-T,and improved electron and hole mobilities both.Unprecedentedly,such a protocol reduces the ITIC optical band gap from 1.59 to 1.55 eV.As consequences,higher short-circuit current-density(17.8–18.4 vs.15.8 m A/cm^2),open-circuit voltage(0.92 vs.0.90 V)and fill-factor(0.72–0.73 vs.0.68)are simultaneously obtained,which ultimately afford higher efficient quaternary polymer solar cells with power conversion efficiencies(PCEs)up to 12.0%–12.8%comparing to the host binary device with 9.9%efficiency.For the polymer,ITIC,and ICBA/PCBM ternary blends,11%PCEs were recorded.The use of PCBM leads to larger red-shifting in thin film absorption and external quantum efficiency(EQE)response.Such effect is more pronounced when ICBA:PCBM mixture is used.These results indicate the size and shape of C_(60)and C_(70)as well as the substituent position of the second indene unit on C_(60)-/C_(70)-ICBA affect not only the blend morphology but also the electronic coupling in BHJ mixtures:the quaternary device performance increased in sequences of C_(70)-PCBM:C_(70)-ICBA→C_(70)-PCBM:C_(60)-ICBA→C_(60)-PCBM:C_(70)-ICBA→C_(60)-PCBM:C_(60)-ICBA.The resonant soft X-ray scattering(RSoXS)data indicated the most refined phase separation in the C_(60)-PCBM:C_(60)-ICBA based blend,corresponding to its best device function among the quaternary devices.These results indicate that the using of binary fullerenes as the acceptor additives allows for tuning nonfullerene blended film’s optical properties and filmmorphologies,shedding light on the designing high-performance multi-acceptor polymer solar cells.

Calculation aided miscibility manipulation enables highly efficient polythiophene:nonfullerene photovoltaic cells

Polythiophenes,with merits of low cost and high scalability of synthesis,have received growing interest in organic solar cells.To date,the best-performing polythiophene:nonfullerene solar cells exhibit typical power conversion efficiencies (PCEs) of 10%–12%,which is much lower than those employing PM6-and D18-type polymers.This inferior performance is mostly limited by the improper miscibility between polythiophene and acceptors.Efforts on engineering the molecular structure to systematically tune the miscibility are required.With the aid of group contribution calculations,the miscibility of polythiophene:nonfullerene blend system was finely tuned by varying the ratios of siloxane-terminated chains and alkyl chains in ester-substituted polythiophenes through random copolymerization.Based on a series of the polythiophene and nonfullerene acceptors,the detailed analysis of blend miscibility and performance reveals a surprising anticorrelation between the Flory-Huggins interaction parameter (χ_(aa)) and the optimal time of solvent vapor annealing for device performance across these systems.Primarily due to the slightly higher χ_(aa),the blend of PDCBT-Cl-Si5 and ITIC-Th1 results in a record-high PCE of 12.85%in polythiophene:nonfullerene solar cells.The results not only provide a calculation-guided approach for molecular design but also prove that precise control of the miscibility is an effective way to design high-performance polythiophene:nonfullerene blends and beyond.

Sulfur vs. tellurium: the heteroatom effects on the nonfullerene acceptors

The effect of chalcogen heteroatom variation on donor materials has been systematically investigated. However, this effect on acceptors has rarely been explored. Herein, nonfullerene acceptors BFPSP and BFPTP were reported by simply changing the chalcogen atoms from S to Te. The differences between BFPSP and BFPTP in light absorption, energy levels, excited-state lifetimes, energy loss, charge mobilities, morphology, and photovoltaic properties were systematically investigated to understand the heteroatom effects. More importantly, the electroluminescence spectra, external quantum efficiency of photovoltaics and TDDFTcalculations revealed that the triplet excited state(T1) in energy of BFPTP equals to the charge transfer(CT) state in PBDBT:BFPTP, which allows T1 excitons, generated by intersystem crossing, to split into free charges to contribute to the efficiency.This contribution provides a strategy for tuning the photophysical properties of nonfullerene acceptors and designing high performance triplet materials for OSCs.

Recent progress on all-small molecule organic solar cells using small-molecule nonfullerene acceptors

Recently,solution-processed organic solar cells combining small-molecule donor and nonfullerene acceptor have achieved breakthrough results with the certified efficiency over 15%.These impressive progresses are driven by the concerted efforts of modifying the donor and acceptor materials and optimizing the morphology.Considering the defined chemical structures and easily tuned properties of small-molecule materials,it is of great necessity and importance to pay more attentions on the topic of all-small molecule organic solar cells.Here,we summarize the recent progress of all-small molecule organic solar cells from the prospect of materials'evolutions and expect to provide some hints for its future developments.The involved small-molecule donors including oligothiophene-,benzodithiophene-,naphthodithiophene-,and porphyrin-based materials are discussed to illustrate the relationship of chemical structures,properties,and device performance.Then,the small-molecule nonfullerene acceptors in all-small molecules organic solar cells are discussed to highlight their vital role.Finally,we will present the challenges and future of this research area.

Surfactant-Encapsulated Polyoxometalate Complex as a Cathode Interlayer for Nonfullerene Polymer Solar Cells

An alcohol-soluble,environmentally friendly,and low-cost surfactant-encapsulated polyoxometalate complex[(C8H17)4N]4[SiW_(12)O40](TOASiW_(12))as a cathode interlayer(CIL)has exhibited excellent universality for various active layers and cathodes in nonfullerene polymer solar cells(NF-PSCs).In particular,incorporating TOASiW_(12) as the CIL enhanced power conversion efficiencies(PCEs)of the PM6:Y6-based NF-PSCs with Al or Ag cathode to 16.14%and 15.89%,respectively,and the PCEs of PM6:BTP-BO4Cl-based NF-PSCs with Al or Ag cathode to 17.04%and 17.00%,respectively.More importantly,the performances of the devices with TOASiW_(12) were insensitive to the TOASiW_(12) thickness from 3 to 33 nm.Furthermore,the NF-PSCs with TOASiW_(12) exhibited better device stability.Combined characterization of the photocurrent density versus effective voltage,capacitance versus voltage and electron mobility demonstrated that TOASiW_(12) as the CIL effectively promoted exciton dissociation,charge-carrier extraction,built-in potential,charge-carrier density,and electron mobility in the NF-PSCs.These findings suggest that TOASiW_(12) is a promising,competitive CIL for NF-PSCs fabricated by roll-to-roll processing.

Achieving high short-circuit current and fill-factor via increasing quinoidal character on nonfullerene small molecule acceptor

Recently, the fused-ring based low band gap （LBG） small molecule acceptors （SMAs） have emerged as efficient nonfullerene acceptors. So far, these LBG SMAs are mainly designed with IC （2-methylene-（3- （1,1 -dicyanomethylene）indanone）） or its analogs, the benzo-type electron-accepting （A） units. Compared to benzene, thiophene is less aromatic and thus the thiophene-involving semiconducting molecule has more quinoidal character, which effectively reduces the energy gap between the highest occupied molecular orbit （HOMO） and the lowest unoccupied molecular orbit （LUMO）. Herein, we show that replacing the IC units in ITIC with the CT （cyclopenta[c]thiophen-4-one-5-methylene-6-（1,1-dicyano- methylene））, a thiophene-fused A unit, the quinoidal character is enhanced from 0.0353 on ITIC to 0.0349 on ITCT, the CT-ended SMA. The increase in the quinoidal character reduces the optical band gap and enhances the near IR absorptivity. When blended with the wide band gap （WBG） polymer donor, PBDB-T, an average power conversion efficiency of 10.99% is obtained with a short-circuit current-density （Jso） of 17.88 mA/cm2 and a fill-factor （FF） of 0.723. For comparisons, theJsc is of 16.92 mA/cm2, FF is of 0.655 and PCE is of 9.94% obtained from the ITIC：PBDB-T device. This case indicates that the replacement of the benzene ring on the IC unit with a more polarizable five-member ring such as thiophene is an effective way to enhance the absorption of the near IR solar photons towards designing high-performance nonfullerene polymer solar cells.

Molecular engineering of Y-series acceptors for nonfullerene organic solar cells

The power conversion efficiencies(PCEs)of single-junction organic solar cells(OSCs)have surpassed 19%,owing to the emerging Y-series nonfullerene acceptors(NFAs).Undoubtedly,the power and flexibility of chemical design has been a strong driver for this rapid efficiency improvement in the OSC field.Over the course of the past 3 years,a variety of modifications have been made to the structure of the Y6 acceptor,and a large number of Y-series NFAs have been reported to further improve performance.Herein,we present our insights into the rationale behind the Y6 acceptor and discuss the design principles toward high-performance Y-series NFAs.It is clear that structural modifications through choice of heteroatom,soluble chains,πspacers,central cores,and end groups alter the material characteristics and properties,contributing to distinctive photovoltaic performance.Subsequently,we analyze various design strategies of Y-series-containing materials,including polymerized small-molecule acceptors(PSMA),non-fused-ring acceptors(NFRA),and all-fused-ring acceptors(AFRA).This review is expected to be of value in providing effective molecular design strategies for high-performance NFAs in future innovations.

Banana-shaped electron acceptors with an electron-rich core fragment and 3D packing capability

The emergence of Y6-type nonfullerene acceptors has greatly enhanced the power conversion efficiency(PCE)of organic solar cells(OSCs).However,which structural feature is responsible for the excellent photovoltaic performance is still under debate.In this study,two Y6-like acceptors BDOTP-1 and BDOTP-2 were designed.Different from previous Y6-type acceptors featuring an A–D–Aʹ–D–A structure,BDOTP-1,and BDOTP-2 have no electron-deficient Aʹfragment in the core unit.Instead,there is an electron-rich dibenzodioxine fragment in the core.Although this modification leads to a marked change in the molecular dipole moment,electrostatic potential,frontier orbitals,and energy levels,BDOTP acceptors retain similar three-dimensional packing capability as Y6-type acceptors due to the similar banana-shaped molecular configuration.BDOTP acceptors show good performance in OSCs.High PCEs of up to 18.51%(certified 17.9%)are achieved.This study suggests that the banana-shaped configuration instead of the A–D–Aʹ–D–A structure is likely to be the determining factor in realizing high photovoltaic performance.

Highly crystalline acceptor materials based on benzodithiophene with different amount of fluorine substitution on alkoxyphenyl conjugated side chains for organic photovoltaics

Organic solar cells based on narrow bandgap small-molecule acceptors(SMAs)with highly crystalline characteristics have attracted great attentions for their superiority in obtaining high photovoltaic efficiency.Employing highly crystalline SMAs to enhance power conversion efficiencies(PCEs)by regulating and controlling morphology and compatibility of donor and acceptor materials has turned out to be an effective approach.In this study,we synthesized three different crystalline SMAs by using fluorine substitution on alkoxyphenyl conjugated side chains to modulate the relationship of crystallinity and morphologies,namely ZY1(zero F atoms),ZY2(two F atoms),and ZY3(four F atoms).The three SMAs show the broad absorption edges and similar frontier orbital energy levels,generating the analogical(over 0.9 V)open circuit voltage(VOC)of the polymer solar cells(PSCs).As a result,the PM6:ZY2-based PSCs yield a PCE of 10.81%with a VOC of 0.95 V,a short-circuit current density(JSC)of 16.154 mA cm^(-2),and a fill factor(FF)of 0.71,which is higher than that of 9.17%(PM6:ZY1)and 6.37%(PM6:ZY3).And the PCE(17.23%)of the PM6:Y6:ZY2 based ternary PSCs is also higher than that of 16.32%PM6:Y6 based binary device.Obviously,the results demonstrate that adding fluorine atoms on the conjugated side chains to construct high crystalline materials is a positive strategy to effectively increase the efficiencies of binary and ternary PSCs.

Effect of Hyperconjugation on Photovoltaic Performance in Pseduo-2D Perylene Diimide Derivatives

Theπ-πinteraction is acknowledged as the predominant factor to determine the molecular packing in organic photovoltaic materials,while other non-covalent intermolecular interactions especially theσ-πhyperconjugation are often ignored.Herein,a perylene diimide(PDI)derivative named FIDT-PDI is designed and synthesized to shed light into the effect of hyperconjugation on the molecular packing and further the photovoltaic performance.Dynamic NMR and 2D NOE NMR demonstrate the formation of intermolecularσ-πhyperconjugation between the C—H bond of the PDI moiety in one molecule and the phenyl sidechain in another molecule of FIDT-PDI.Benefiting from theσ-πhyperconjugation,FIDT-PDI with twisted backbone reversely exhibits more ordered packing and stronger crystallinity compared with another PDI derivative FIDTT-PDI which has better planarity,consequently achieving superior PCE and higher carrier mobility.This contribution is the first paradigm to unravel the structure-property relationship betweenσ-πhyper-conjugation of conjugated materials and corresponding photovoltaic performance.

Porphyrinic Acceptors forFullerene-Free MolecularPhotovoltaics

Over the past few years,the development of nonfullerene acceptors(NFAs)has become a prominent focus in both organic and perovskite solar cell(OSCs and PSCs,respectively)research fields.In this context,porphyrinoids,compounds structurally related to porphyrins,have emerged as promising solar cell candidates.In contrast to the widely used fullerene acceptors,porphyrinoids exhibit strong,broad absorption properties across the UV–vis/NIR spectrum,which can be easily tuned through chemical modifications.Furthermore,they can be prepared and derivatized using cost-effective and straightforward methodologies,allowing for convenient adjustments in thin-film morphology,processability,supramolecular organization,and energy levels.Additionally,these compounds offer higher thermal and photochemical stability,resulting in longer device lifetimes compared to their fullerene-based counterparts.In this review,we outline the utilization of porphyrinoids as NFAs in OSCs and PSCs,discussing essential aspects such as design guidelines,molecular properties,and device configuration.Our goal is to inspire and further promote the development of n-type porphyrinoids,which have not yet fully unleashed their potential.

A Simple Nonfused Ring Electron Acceptor with a Power Conversion Efficiency over 16%

By simplifying the r-bridge unit,a nonfused ring electron acceptor(NFREA)BM-2F was designed and synthesized with several high-yield steps.The specific molecular structure features of BM-2F are planar molecular backbone and out-of-plane side chain,which is favorable for charge transport and can suppress the over-aggregation.BM-2F based neat and blend films display obvious face-on molecular orientation.Specially,D18:BM-2F based blend film can form good bicontinuous interpenetrating network.More excitingly,a power conversion efficiency of 16.15%was achieved with D18:BM-2F based photovoltaic devices,which is the highest one based on NFREAs.Our researches manifest that NFREA is a promising direction for low-cost and high-performance organic solar cells.

Chlorinated Perylene Monoimide Monoanhydrate Synthesized via Hydrolysis and Its Application in Organic Solar Cells

Perylene-3,4-(dicarboxylic monoimide)-9,10-(dicarboxylic monoanhydrate)(PIA)is one key intermediate to construct functionalized perylene diimides(PDIs)for various applications.However,the difficulty in synthesizing chlorinated PIA hinders the study of chlorinated PDIbased materials.Although chlorination has been widely used to modify the properties of organic semiconductors.We successfully synthesize chlorinated PIA via a simple hydrolysis reaction using LiOH as the base,then a PDI dimer connected at the imide position,N-di-PDI-4Cl,is synthesized as an application example of chlorinated PIA.The heavily chlorinated PDI dimer exhibits deeper energy levels,slightly blue-shifted UV-Vis absorption compared to the non-chlorinated analogue.In addition,the photovoltaic performance of N-di-PDI-4Cl is characterized.This study paves one easy way to synthesize chlorinated PIA and its more delicate derivatives.