End-group modulation of phenazine based non-fullerene acceptors for efficient organic solar cells with high open-circuit voltage

Phenazine-based non-fullerene acceptors(NFAs)have demonstrated great potential in improving the power conversion efficiency(PCE)of organic solar cells(OSCs).Halogenation is known to be an effective strategy for increasing optical absorption,refining energy levels,and improving molecular packing in organic semiconductors.Herein,a series of NFAs(Pz IC-4H,Pz IC-4F,Pz IC-4Cl,Pz IC-2Br)with phenazine as the central core and with/without halogen-substituted(dicyanomethylidene)-indan-1-one(IC)as the electron-accepting end group were synthesized,and the effect of end group matched phenazine central unit on the photovoltaic performance was systematically studied.Synergetic photophysical and morphological analyses revealed that the PM6:Pz IC-4F blend involves efficient exciton dissociation,higher charge collection and transfer rates,better crystallinity,and optimal phase separation.Therefore,OSCs based on PM6:Pz IC-4F as the active layer exhibited a PCE of 16.48%with an open circuit voltage(Voc)and energy loss of 0.880 V and 0.53 e V,respectively.Accordingly,this work demonstrated a promising approach by designing phenazine-based NFAs for achieving high-performance OSCs.

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.

π-Extension and chlorination of non-fullerene acceptors enable more readily processable and sustainable high-performance organic solar cells

Organic solar cells(OSCs)processed without halogenated solvents and complex treatments are essential for future commercialization.Herein,we report three novel small molecule acceptors(NFAs)consisting of a Y6-like core but withπ-extended naphthalene with progressively more chlorinated end-capping groups and a longer branched chain on the Nitrogen atom.These NFAs exhibit good solubilities in nonchlorinated organic solvents,broad optical absorptions,closeπ-πstacking distances(3.63–3.84A),and high electron mobilities(~10^(-3)cm^(2)V^(-1)s^(-1)).The o-xylene processed and as-cast binary devices using PM6 as the donor polymer exhibit a PCE increasing upon progressive chlorination of the naphthalene end-capping group from 8.93%for YN to 14.38%for YN-Cl to 15.00%for YN-2Cl.Furthermore similarly processed ternary OSCs were fabricated by employing YN-Cl and YN-2Cl as the third component of PM6:CH1007 blends(PCE=15.75%).Compared to all binary devices,the ternary PM6:CH1007:YN-Cl(1:1:0.2)and PM6:CH1007:YN-2Cl(1:1:0.2)cells exhibit significantly improved PCEs of 16.49%and15.88%,respectively,which are among the highest values reported to date for non-halogenated solvent processed OSCs without using any additives and blend post-deposition treatments.

Simple non-fullerene electron acceptors with unfused core for organic solar cells

Two simple electron acceptors based on unfused bithiophene core and 1,1-dicyanomethylene-3-indanone end group were easily prepared via three synthetic steps. These acceptors exhibited broad absorption in the range of 300 nm to 800 nm, aligned energy levels and high crystallinity. When combined with a wide band gap donor polymer in non-fullerene solar cells, an initial power conversion efficiency of 2.4% was achieved. The relatively low efficiencies were due to the large phase separation in blended thin films, which is originated from their high aggregation tendency in thin films. Our results suggest that these electron acceptors with unfused core are promising candidates for commercial application of solar cells due to the low cost starting materials and facile synthesis.

Small molecular non-fullerene electron acceptors for P3HT-based bulk-heterojunction solar cells

Three small molecules with the same arms and different cores of perylene diimide(PDI)or indaceno[2,1-b:6,5-b']dithiophene(IDT)were designed and synthesized as the acceptor materials for P3HT-based bulk-heterojunction(BHJ)solar cells.The impacts of the different cores on the optical absorption,electrochemical properties,electron mobility,film morphology,photoluminescene characteristics,and solar cell performance were thoroughly studied.The three compounds possess a broad absorption covering the wavelength range of 400–700 nm and relatively low lowest unoccupied molecular orbital(LUMO)energy levels of?3.86,?3.81 and?3.99 eV.The highest power conversion efficiency of 0.82%was achieved for the BHJ solar cells based on SM3 as the acceptor material,the compound with a PDI core.

Non-fullerene small molecule electron acceptors for high-performance organic solar cells

Fullerenes and their derivatives are important types of electron acceptor materials and play a vital role in organic solar cell devices. However, the fullerene acceptor material has some difficulties to overcome the intrinsic shortcomings, such as weak absorption in the visible range, difficulty in modification and high cost, which limit the performance of the device and the large-scale application of this type of acceptors. In recent years, non-fullerene electron acceptor material has attracted the attention of scientists due to the advantages of adjustable energy level, wide absorption, simple synthesis, low processing cost and good solubility. Researchers can use the rich chemical means to design and synthesize organic small molecules and their oligomers with specific aggregation morphology and excellent optoelectronic prop- erties. Great advances in the field of synthesis, device engineering, and device physics of non-fullerene acceptors have been achieved in the last few years. At present, non-fullerene small molecules based photovoltaic devices achieve the highest efficiency more than 13% and the efficiency gap between fullerenetype and non-fullerene-type photovoltaic devices is gradually narrowing. In this review, we explore recent progress of non-fullerene small molecule electron acceptors that have been developed and led to highefficiency photovoltaic devices and put forward the prospect of development in the future.

Molecular design towards two-dimensional electron acceptors for efficient non-fullerene solar cells

Non-fullerene polymer solar cells(NF-PSCs) have gained wide attention recently. Molecular design of non-fullerene electron acceptors effectively promotes the photovoltaic performance of NF-PSCs. However,molecular electron acceptors with 2-dimensional(2 D) configuration and conjugation are seldom reported.Herein, we designed and synthesized a series of novel 2 D electron acceptors for efficient NF-PSCs. With rational optimization on the conjugated moieties in both vertical and horizontal direction, these 2 D electron acceptors showed appealing properties, such as good planarity, full-spectrum absorption, high absorption extinction coefficient, and proper blend morphology with donor polymer. A high PCE of 9.76%was achieved for photovoltaic devices with PBDB-T as the donor and these 2 D electron acceptors. It was also found the charge transfer between the conjugated moieties in two directions of these 2 D molecules contributes to the utilization of absorbed photos, resulting in an exceptional EQE of 87% at 730 nm. This work presents rational design guidelines of 2 D electron acceptors, which showed great promise to achieve high-performance non-fullerene polymer solar cells.

Photophysics, morphology and device performances correlation on non-fullerene acceptor based binary and ternary solar cells

Non-fullerene acceptor(NFA) based organic solar cells(OSCs) are of high efficiency and low energy loss and low recombination features, which is owing to the advantage of non-fullerene acceptors. The photophysics investigation of non-fullerene solar cells, in comparing to fullerene based analogue as well as mixed acceptor ternary blends could help to understand the working mechanism of NFA functioning mechanism. We choose PBDB-T donor, the fullerene derivative PC71 BM acceptor, and the non-fullerene acceptor ITIC as the model system, to construct binary and ternary solar cells, which then subject to ultrafast spectroscopy investigation. The charge transfer pathway in binary and ternary blends is revealed.And it is seen that ITIC leads to a faster exciton separation and exciton diffusion. ITIC in blends suppresses the geminate recombination and shows smaller amount of charge transfer states, which is beneficial for the device performance. And the addition of ITIC enhances the crystallinity for both donor and acceptor leads to a morphology change of forming bicontinuous crystalline networks and phase separation. In a consequence, fill factor and JSC, increase dramatically for the related OSC.

A new non-fullerene acceptor based on the heptacyclic benzotriazole unit for efficient organic solar cells

Non-fullerene acceptors(NFAs)become an interesting family of organic photovoltaic materials,and have attracted considerable interest for their great potential in manufacturing large-area flexible solar panels by low cost coating methods[1–5].Recently,our group proposed in the first time an A-DA’D-A molecular strategy and synthesized a new class of non-fullerene acceptor Y6 with a record efficiency above 15%with single junction organic solar cells(OSCs)[6].To further improve the photovoltaic performance of OSCs,many effective strategies have been successfully explored,such as side-chain engineering and extension of fused core and terminal group engineering[7–12].As well-known,PCE of devices is determined by the open circuit voltage(Voc),short-circuit current density(Jsc)and fill factor(FF)[13].Among them,Voc is associated with low-lying highest occupied molecular orbital(HOMO)of donor and lowest unoccupied molecular orbital(LUMO)of acceptor of the active layer[14–16].Side-chain engineering is an effective strategy for manipulating energy levels and improving photovoltaic performance of devices[17–19].For example,introducing the alkyl/alkoxy chains can effectively tune the HOMO/LUMO energy levels[20–22].Tang et al.have reported a novel non-fullerene acceptor ITC6-IC.ITC6-IC has relatively high LUMO level and high Voc than those of ITIC due to the introduction of weak electrondonating hexyl group on thiophene[23].

Recent advances and prospects of asymmetric non-fullerene small molecule acceptors for polymer solar cells

Recently,polymer solar cells developed very fast due to the application of non-fullerence acceptors.Substituting asymmetric small molecules for symmetric small molecule acceptors in the photoactive layer is a strategy to improve the performance of polymer solar cells.The asymmetric design of the molecule is very beneficial for exciton dissociation and charge transport and will also fine-tune the molecular energy level to adjust the open-circuit voltage(Voc)further.The influence on the absorption range and absorption intensity will cause the short-circuit current density(Jsc)to change,resulting in higher device performance.The effect on molecular aggregation and molecular stacking of asymmetric structures can directly change the microscopic morphology,phase separation size,and the active layer's crystallinity.Very recently,thanks to the ingenious design of active layer materials and the optimization of devices,asymmetric non-fullerene polymer solar cells(A-NF-PSCs)have achieved remarkable development.In this review,we have summarized the latest developments in asymmetric small molecule acceptors(A-NF-SMAs)with the acceptor-donor-acceptor(A-D-A)and/or acceptor-donor-acceptor-donor-acceptor(A-D-A-D-A)structures,and the advantages of asymmetric small molecules are explored from the aspects of charge transport,molecular energy level and active layer accumulation morphology.

Heteroatom substitution-induced asymmetric A–D–A type non-fullerene acceptor for efficient organic solar cells

Research on asymmetric A–D–A structured non-fullerene acceptors has lagged far behind the development of symmetric counterpart.In this contribution,by simply replacing one sulfur atom in indacenodithiophene unit with a selenium atom,an asymmetric building block Se PT and a corresponding asymmetric non-fullerene acceptor Se PT-IN have been developed.Asymmetric Se PT-IN achieved a high efficiency of 10.20% in organic solar cells when blended with PBT1-C,much higher than that of symmetric TPT-IN counterpart(8.91%).Our results demonstrated an effective heteroatom substitution strategy to develop asymmetric A–D–A structured non-fullerene acceptors.

A new chlorinated non-fullerene acceptor based organic photovoltaic cells over 12%efficiency

The method to fluorinate the terminal group has achieved remarkable success and been widely used to fine-tune the intrinsic properties of organic acceptor materials.Referring to chlorination,however,it gets less attention and remains ambiguous effect on organic photovoltaic(OPV)cells.Herein,a new non-fullerene acceptor named Y19 was reported with benzotriazole as the electron-deficient core and 2Cl-ICs as the strong electron-withdrawing end groups.Y19 exhibits a wide film absorption band from 600 nm to 948 nm and low LUMO(the lowest unoccupied molecular orbital)energy level of−3.95 eV.Photovoltaic devices based on PM6:Y19 show high-power conversion efficiency(PCE)of 12.76%with high open-circuit voltage(Voc)of 0.84 V,short-circuit current density(Jsc)of 22.38 mA/cm2 and fill factor(FF)of 68.18%.Broad external quantum efficiency(EQE)response of over 60%in the range of 480−860 nm can be obtained.This study demonstrates that chlorination,as a low-cost molecular design strategy,has its own superiorities to improve device performance and promote the potential application in OPV.

Spatial configuration engineering of perylenediimide-based non-fullerene electron transport materials for efficient inverted perovskite solar cells

Due to their excellent photoelectron chemical properties and suitable energy level alignment with perovskite,perylene diimide(PDI)derivatives are competitive non-fullerene electron transport material(ETM)candidates for perovskite solar cells(PSCs).However,the conjugated rigid plane structure of PDI units result in PDI-based ETMs tending to form large aggregates,limiting their application and photovoltaic performance.In this study,to restrict aggregation and further enhance the photovoltaic performance of PDI-type ETMs,two PDI-based ETMs,termed PDO-PDI2(dimer)and PDO-PDI3(trimer),were constructed by introducing a phenothiazine 5,5-dioxide(PDO)core building block.The research manifests that the optoelectronic properties and film formation property of PDO-PDI2 and PDO-PDI3 were deeply affected by the molecular spatial configuration.Applied in PSCs,PDO-PDI3 with threedimensional spiral molecular structure,exhibits superior electron extraction and transport properties,further achieving the best PCE of 18.72%and maintaining 93%of its initial efficiency after a 720-h aging test under ambient conditions.

Design and Synthesis of Acceptor-Donor-Acceptor Type Non-Fullerene Acceptors Using Oxindole-Based Bridge for Polymer Solar Cells Applications

Two acceptor-donor-acceptor(A-D-A)type non-fullerene acceptors(namely WH1 and WH7)containing the oxindole-based bridge are designed and synthesized for polymer solar cells(PSCs)applications.The bridge unit is introduced through a precursor(6-bromo-1-octylindoline-2,3-dione)that contains both bromine and carbonyl and provides the feasibility of the Pd-catalyzed cross-coupling reaction and the Knoevenagel condensation,respectively.This facile synthetic approach exhibits the potential to gain high performance non-fullerene acceptors through extendingπ-conjugated backbone with strong light-absorbing building blocks.The synthesis and properties of WH1 and WH7 are demonstrated with different endcap units,then PSCs are fabricated using PBDB-T:WH1 and PBDB-T:WH7 as the active layers,and attain an average power conversion efficiency(PCE)of 2.58%and 6.24%,respectively.Further device physics studies afford the deep insight of structure variation influence on the device performance.This work provides a facile non-fullerene acceptor design strategy and shows how structure variations impact the PSC performance.

Rational molecular engineering towards efficient heterojunction solar cells based on organic molecular acceptors

The selection of photoactive layer materials for organic solar cells(OSCs) is essential for the photoelectric conversion process.It is well known that chlorophyll is an abundant pigment in nature and is extremely valuable for photosynthesis.However,there is little research on how to improve the efficiency of chlorophyll-based OSCs by matching chlorophyll derivatives with excellent non-fullerene acceptors to form heterojunctions.Therefore in this study we utilize a chlorophyll derivative,Ce_(6)Me_(3),as a donor material and investigate the performance of its heterojunction with acceptor materials.Through density functional theory,the photoelectric performances of acceptors,i ncluding the fullerene derivative PC_(71)BM and the terminal halogenated non-fullerene DTBCIC series,are compared in detail.It is found that DTBCIC-C1 has better planarity,light absorption,electron affinity,charge reorganization energy and charge mobility than others.Ce_(6)Me_(3) has good energy level matching and absorption spectral complementarity with the investigated acceptor molecules and also shows good electron donor properties.Furthermore,the designed Ce_(6)Me_(3)/DTBCIC interfaces have improved charge separation and reorganization rates(K_(CS)/K_(CR)) compared with the Ce_(6)Me_(3)/PC_(71)BM interface.This research provides a theoretical basis for the design of photoactive layer materials for chlorophyll-based OSCs.

5H-Fluoreno [3,2-b:6,7-b’] Dithiophene Based Non-fullerene Small Molecular Acceptors for Polymer Solar Cell Application

Two novel non-fullerene small molecule acceptors were prepared with the conjugated backbone of 5 H-fluoreno[3, 2-b:6, 7-b’] dithiophene carrying the electron deficient unit of dicyanomethylene indanone(DICTFDT) and rhodanine(TFDTBR), respectively. The two acceptors exhibited excellent thermal stability and strong absorption in the visible region. The LUMO level is estimated to be at-3.89 eV for DICTFDT and-3.77 eV for TFDTBR. When utilized as the acceptor in bulk heterojunction polymer solar cells with the polymer donor of PBT7-Th, the optimized maximum power conversion efficiency of 5.12% and 3.95% was obtained for the device with DICTFDT and TFDTBR, respectively. The research demonstrates that 5 H-fluoreno[3, 2-b:6, 7-b’] dithiophene can be an appealing candidate for constructing small molecular electron acceptor towards efficient polymer:non-fullerene bulk heterojunction solar cells.

Small bandgap non-fullerene acceptor enables efficient PTB7-Th solar cell with near 0 eV HOMO offset

Three small bandgap non-fullerene(SBG NFAs) acceptors,BDTI,BDTI-2 F and BDTI-4 F,based on a carbon-oxygen bridged central core and thieno[3,4-b]thiophene linker,end-capped with varied electronwithdrawing terminal groups,were designed and synthesized.The acceptors exhibit strong absorption from 600 nm to 1000 nm.The optimal device incorporating designed NFA and PTB7-Th polymer donor achieves a power conversion efficiency of 9.11% with near 0 eV HOMO offset.The work presents a case study of efficient non-fullerene solar cells with small HOMO offsets,which is achieved by blending PTB7-Th with fine-tuned SBG acceptor.

Utilizing 3,4-ethylenedioxythiophene(EDOT)-bridged non-fullerene acceptors for efficient organic solar cells

A rational design of efficient low-band-gap non-fullerene acceptors(NFAs)for high-performance organic solar cells(OSCs)remains challenging;the main constraint being the decrease in the energy level of the lowest unoccupied molecular orbitals(LUMOs)as the bandgap of A-D-A-type NFAs decrease.Therefore,the short current density(J_(sc))and open-circuit voltage(V_(oc))result in a trade-off relationship,making it difficult to obtain efficient OSCs.Herein,three NFAs(IFL-ED-4 F,IDT-ED-4 F,and IDTT-ED-2 F)were synthesized to address the above-mentioned issue by introducing 3,4-ethylenedioxythiophene(EDOT)as aπ-bridge.These NFAs exhibit relatively low bandgaps(1.67,1.42,and 1.49 eV,respectively)and upshifted LUMO levels(-3.88,-3.84,and-3.81 eV,respectively)compared with most reported low-band-gap NFAs.Consequently,the photovoltaic devices based on IDT-ED-4 F blended with a PBDB-T donor polymer showed the best power conversion efficiency(PCE)of 10.4%with a high J_(sc) of 22.1 mA cm^(-2) and Voc of 0.884 V among the examined NFAs.In contrast,IDTT-ED-4 F,which was designed with an asymmetric structure of the D-p-A type,showed the lowest efficiency of 1.5%owing to the poor morphology and charge transport properties of the binary blend.However,when this was introduced as the third component of the PM6:BTP-BO-4 Cl,complementary absorption and cascade energy-level alignment between the two substances could be achieved.Surprisingly,the IDTT-ED-4 F-based ternary blend device not only improved the Jscand Voc,but also achieved a PCE of 15.2%,which is approximately 5.3%higher than that of the reference device with a minimized energy loss of 0.488 eV.In addition,the universality of IDTT-ED-2 F as a third component was effectively demonstrated in other photoactive systems,specifically,PM6:BTPe C9 and PTB7-Th:IEICO-4 F.This work facilitates a better understanding of the structure–property relationship for utilizing efficient EDOT-bridged NFAs in high-performance OSC applications.

Recent progress in side chain engineering of Y-series non-fullerene molecule and polymer acceptors

Organic solar cells(OSCs) have drawn considerable attention in the last decade due to the great potential of light weight,flexibility, and low-cost solution processing. Particularly, Y-series non-fullerene acceptors(NFAs) including small molecular acceptors(SMAs) and polymerized small molecular acceptors(PSMAs) have become research hot spots due to their excellent power conversion efficiency. Side chain engineering is crucial to adjust the solubility and crystallinity of NFAs, which will significantly affect the morphology of active layers and the efficiency of OSCs. However, the understanding of side chain engineering on NFAs is still limited and lacks a systematic review. This review aims to provide a brief summary of the recent developments in side chain engineering of NFAs, with a special focus on the design and application of Y-series SMAs and PSMAs for high-efficiency non-fullerene organic solar cells(NF-OSCs). In addition, the review also points out challenges and provides useful guidance regarding side chain regulation for Y-series NFAs.

Optimizing theπ-Bridge of Non-fullerene Acceptors to Suppress Dark Current in NIR Organic Photodetectors

Recently,the rapid development of non-fullerene acceptors(NFAs)has laid the foundation for performance improvements in near-infrared(NIR)organic photodetectors(OPDs).However,reducing the bandgap of NFAs to achieve strong absorption in the shorter-wave region usually leads to increased dark current density(J_(d))and decreased responsivity(R),severely limiting the detectivity(D*)of NIR-OPDs.To date,it remains challenging to manipulate the J_(d) of NIR-OPDs through rational structure engineering of NFAs.Herein,three NIR-NFAs,namely bis(2-decyltetradecyl)4,4′-(2′,7′-di-tert-butylspiro[cyclopenta[2,1-b:3,4-b′]dithiophene-4,9′-fluorene]-2,6-diyl)bis(6-(((Z)-1-(dicyanomethylene)-5,6-difluoro-3-oxo-1,3-dihydro-2H-inden-2-ylidene)methyl)thieno[3,4-b]thiophene-2-carboxylate)(TSIC-4F),bis(2-decyltetradecyl)6,6′-(2′,7′-di-tert-butylspiro[cyclopenta[2,1-b:3,4-b′]dithiophene-4,9′-fluorene]-2,6-diyl)bis(4-(((Z)-1-(dicyanomethylene)-5,6-difluoro-3-oxo-1,3-dihydro-2H-inden-2-ylidene)methyl)thieno[3,4-b]thiophene-2-carboxylate)(STIC-4F),and 2,2′-((2Z,2′Z)-(((2′,7′-di-tert-butylspiro[cyclopenta[2,1-b:3,4-b′]dithiophene-4,9′-fluorene]-2,6-diyl)bis(2,3-bis(5-(2-butyloctyl)thiophen-2-yl)thieno[3,4-b]pyrazine-7,5-diyl))bis(metha-neylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile(TPIC-4F),were designed using the thieno[3,4-b]thiophene(TT)and thieno[3,4-b]pyrazine(TPy)derivatives as theπ-bridge.Owing to the intramolecular S-S and S-N interactions,STIC-4F and TPIC-4F exhibited smaller backbone distortions than TSIC-4F.A significantly red-shifted absorption with a peak at 1015 nm was observed in TPIC-4F film,larger than that(ca.960 nm)for TSIC-4F and STIC-4F films.Moreover,OPDs operating in a photovoltaic mode were successfully fabricated,and TPIC-4F-based OPDs achieved the lowest J_(d) of 3.18×10^(-8) A/cm^(2) at-0.1 V.Impressively,although TPIC-4F-based OPDs exhibited the lowest R,higher shot-noise-limited specific detectivity(D_(sh)*)in 1000-1200 nm could be achieved due to its lowest J_(d).This study underscored the effectiveness of optimizing theπ-bridge structure of NFAs to suppress J_(d),ultimately attaining higher D_(sh)*in the NIR region.