Electron-accepting Unit Research Articles

Molecular engineering of “A-D-D-A” dual-emitting-cores emitters with thermally activated delayed fluorescence and aggregation-induced emission characteristics

Endowing thermally activated delayed fluorescence (TADF) emitter with aggregation-induced emission (AIE) peculiarity is of great significance for realizing more promising commercial applications. Herein, two new dual-emitting-cores emitters with a structure of acceptor-donor-donor-acceptor (A-D-D-A), namely 2DBT-BZ-2Cz and 2DFT-BZ-2Cz, were designed and synthesized to explore their luminescence trait. The emitters, adopting dual carbazole as donor segments and dual phenyl ketone in peripheral skeleton as electron acceptor units, were featured with small singlet (S 1 )–triplet (T 1 ) splitting energy (ΔE ST ) of 0.02 eV and 0.01 eV. The efficient thermally activated delayed fluorescence (TADF) characteristics and aggregation-induced emission property make them suitable for nondoped OLED devices. The solution-processed green OLEDs based on 2DBT-BZ-2Cz demonstrated greater device performance with current efficiency of 20.7 cd A −1 and maximum luminescence of 10,000 cd m −2 . This work thus provides the direction to explore luminogens of dual-emitting-cores with TADF and AIE features as promising candidates in solid state lighting. Two novel AIE-active TADF materials with dual-emitting cores, namely 2DBT-BZ-2Cz and 2DFT-BZ-2Cz, were designed and synthesized, in which efficient separation of frontier molecular orbital (FMO) and highly twisted molecular structure induced similarly small ΔE ST , obvious TADF characteristic and AIE feature. • Two TADF emitters with dual emitting cores are constructed with the similar molecular configuration of A-D-D-A. • Efficient separation of frontier molecular orbital induced similarly narrow ΔE ST and distinct TADF characteristic. • Highly twisted molecular structure of the dual emitting cores restricted the π-π packing and abduct AIE feature.

Side chain engineering of semiconducting polymers for improved NIR-II fluorescence imaging and photothermal therapy

Development of efficient agents for NIR-II fluorescence imaging (NIR-II FI) offers opportunities to facilitate NIR-II FI in biomedicine and life science. Although semiconducting polymers (SPs) are proved to be excellent candidates for NIR-II FI, their fluorescence quantum yields are generally low mainly because of the accelerated nonradiative decay by the vibrational overlap between the ground state and excited state due to the small energy gap of NIR-II SPs. We herein propose a side chain engineering of SPs for enhanced NIR-II FI and imaging-guided photothermal therapy (PTT). The fluorescence optimization is realized by tuning the conformation and bulk of side chains of SPs, which demonstrates that the bulky branched side chains remarkably contribute to the fluorescence enhancement relative to linear or short branched side chains. Density functional theory (DFT) calculation indicates that bulky branched side chain-attached SP (P3) has the largest dihedral angle between electron donor and acceptor units, which suggests the largest intramolecular steric hindrance that restricts the free rotation in the molecule, thus boosting the radiative decay to generate fluorescence. Water-dispersible nanoparticles with high tumor specificity fabricated from the optimal P3 are employed for in vivo applications, which show excellent NIR-II FI performance and PTT efficacy toward tumor. Our study therefore proposes a feasible molecular guideline to amplify the NIR-II brightness of SPs for improved phototheranostics.

Structure–property relationship of donor–acceptor type conjugated copolymers with thienoisoquinoline and benzothiadiazole units

We synthesized a new series of donor–acceptor (D-A) type conjugated copolymers containing two electron-accepting units, namely thienoisoquinoline (TIQO) and benzothiadiazole (BT) (PTIQO-HBT) or fluorinated BT analogs (PTIQO-FBT and PTIQO-FFBT) connected to thiophene-based electron-donating units. The optical and electrochemical properties, planarity of conjugated backbone, molecular crystalline structure, and charge transport characteristics were investigated to comprehensively study the structure–property relationship of these semiconducting copolymers. When the BT unit carries 0, 1, and 2 fluorine, both the highest occupied and the lowest unoccupied molecular orbital energy levels of the conjugated copolymers gradually decrease. DFT calculations showed no significant difference in the energy gap or the copolymer backbone planarity. However, the grazing incidence X-ray diffraction results confirm that the PTIQO-FBT copolymer film has a longer coherence length of π–π stacking between the molecular chains than PTIQO-HBT and PTIQO-FFBT, which may affect the intermolecular charge transport. Organic field-effect transistors (OFETs) employing the three TIQO-based D-A type semiconductor copolymer films as active layer all exhibited ambipolar characteristics that enable both hole and electron transport. Particularly, the PTIQO-FBT based OFETs (hole mobility: 0.030 cm2/V·s, electron mobility: 0.023 cm2/V·s) demonstrate higher performance with well-balanced hole and electron transport, as compared to the other two copolymers in terms of field-effect mobility. Therefore, introducing a single fluorine into the BT unit in the TIQO-based D-A type conjugated copolymers results in the most appropriately controlled ground-state charge transfer for efficient intramolecular charge transport along the polymer backbone, as well as higher molecular film crystallinity.

Impact of Five-membered Heterocyclic Rings on Photophysical Properties Including Two-photon Absorption Character

Abstract A series of dipolar-type terphenyl derivatives 1–3 featuring an electron donor (p-NMe2C6H4 group) and acceptor (p-NO2C6H4 group) unit was synthesized and their photophysical properties were investigated. By using aromatic heterocycles (thiophene, furan, pyrrole) as the π-linkers between the donor and acceptor units, significant effects on the bathochromic shift and enhancement of the two-photon absorption wavelength and the two-photon absorption cross-sections were observed, respectively.

Electron-Accepting π-Conjugated Compound Containing Cyano-Substituted Naphthobisthiadiazole as Nonfullerene Acceptor in Organic Solar Cells

AbstractThe incorporation of electron-accepting units into π-conjugated systems can allow the modulation of the physical properties and frontier orbital energy levels of the molecules. An electron-accepting π-conjugated compound (CNNTz-TR) containing cyano-substituted naphthobisthiadiazole (CNNTz) was synthesized via a nucleophilic substitution reaction as the key step. Owing to the presence of the cyano groups, CNNTz-TR affords a low-lying lowest unoccupied molecular orbital energy level. Organic solar cells based on the blend films of CNNTz-TR and a low-bandgap donor exhibited moderate power conversion efficiencies. The results showed that embedding the CNNTz unit into the π-conjugated backbone is an effective approach for designing electron-accepting semiconducting materials.

Tricyclic or Pentacyclic D Units: Design of D-π-A-Type Copolymers for High VOC Organic Photovoltaic Cells.

Although there are several electron-donating (D) units, only the classic benzo[1,2-b:4,5-b']dithiophenes (BDT) unit was utilized to develop D-π-A-type copolymers for high-voltage organic photovoltaic (OPV) cells. Hence, in this work, we chose two tricyclic D units, BDT and benzo[1,2-b:4,5-b']difurans (BDF), together with one pentacyclic ring, dithieno[2,3-d;2',3'-d']benzo[1,2-b;4,5-b']dithiophenes (DTBDT), to comprehensively study the effect of different D units on the optoelectronic properties and photovoltaic performance. By copolymerized with the benzo[1,2,3]triazole (BTA) electron-accepting unit, the final copolymers J52-Cl, F11, and PE52 were combined with a nonfullerene acceptor (NFA) F-BTA3 according to the "Same-A-Strategy." As we preconceived, all the three single-junction OPV cells can obtain high open-circuit voltage (VOC) over 1.10 V. Although the tricyclic D unit of BDF exhibits a slightly lower VOC of 1.12 V because of its mildly larger energy loss of 0.698 eV, its higher carrier mobilities and exciton dissociation efficiency strikingly boost the short-circuit current (JSC) and fill factor, which contribute to a comparable PCE of 10.04% with J52-Cl (10.10%). However, the DTBDT-based polymer PE52 shows the worst performance with a PCE of 6.78% and a VOC of 1.14 V, owing to the higher bimolecular recombination and disordered molecular stacking. Our results indicate that tricyclic D units should be a better choice for constructing D-π-A-type polymers for high-voltage photovoltaic materials than the pentacyclic analogues.

Non-Fullerene Acceptors with an Extended π-Conjugated Core: Third Components in Ternary Blends for High-Efficiency, Post-Treatment-Free Organic Solar Cells.

The synthesis of four non-fullerene acceptors (NFAs) with a "A-π-D-π-A" structure, in which the electron-donating core is extended, was achieved. The molecules differed by the nature of the solubilizing groups on the π-spacer and/or the presence of fluorine atoms on the peripheral electron-accepting units. The optoelectronic properties of the molecules were characterized in solution, in thin film, and in photovoltaic devices. The nature of the solubilizing groups had a minor influence on the optoelectronic properties but affected the organization in the solid state. On the other hand, the fluorine atoms influenced the optoelectronics properties and increased the photo-stability of the molecules in thin films. Compared to reference ITIC, the extended molecules showed a wider absorption across the visible range and higher lowest unoccupied molecular orbital energy levels. The photovoltaic performances of the four NFAs were assessed in binary blends using PM6 (PBDB-T-2F) as the donating polymer and in ternary blends with ITIC-4F. Solar cells (active area 0.27 cm2 ) showed power conversion efficiencies of up to 11.1 % when ternary blends were processed from non-halogenated solvents, without any thermal post-treatment or use of halogenated additives, making this process compatible with industrial requirements.

Synthesis, Properties, and Photovoltaic Characteristics of Arch- and S-shaped Naphthobisthiadiazole-based Acceptors

Fine-tuning physical properties by structural modification is important for developing organic semiconducting materials. In this work, we designed and synthesized new electron-accepting compounds containing naphtho[1,2-c:7,8-c']bis([1,2,5]thiadiazole (vNTz) or naphtho[1,2-c:5,6-c']bis[1,2,5]thiadiazole (NTz) groups as electron-accepting units; these units are structural isomers. The vNTz-based compounds have an arch-shaped molecular backbone with C2v symmetry, whereas the NTz-based compound forms an S-shaped molecular backbone with C2h symmetry. Property measurements showed unique behavior originating from the vNTz core. An organic solar cell comprising the vNTz-based compound and poly(3-hexylthiopehene) showed a power conversion efficiency of 2.06%. This result demonstrates the potential of vNTz as an electron-accepting unit in organic semiconducting materials.

Preparation and characterization of organic soluble polyimides with low dielectric constant containing trifluoromethyl for optoelectronic application

Polyimide (PI) films have excellent comprehensive properties and bright prospects in the next generation of optoelectronic and microelectronic applications. PIs were synthesized by aromatic diamines and alicyclic dianhydrides considered to be the most effective method for preparing polyimides with high light transmittance and low dielectric constant. In order to improve the overall performance of polyimide, the introduction of a large trifluoromethyl group into the main chain is also considered a powerful strategy. In this article a novel diamine containing 6 trifluoromethyl groups has been designed and synthesized by Suzuki reaction. Afterwards, PIs were synthesized with fluorinated aromatic dianhydride and alicyclic dianhydride through a two-step method. The two PIs have excellent solubility: PI-a and PI-b exhibited excellent solubility in common polar solvents. Such as DMF, DMAc, NMP and DMSO at room temperature, and PI-a was soluble in low-boiling solvents such as ethyl acetate, CH2Cl2 and CHCl3 at room temperature. PI-b was soluble in low-boiling solvents acetone and CHCl3 at room temperature, which was rarely seen in other aromatic polyimides. Soluble in common organic solvents, especially low boiling point solvents, made these polymers easy to process and suitable for processing technology in solution. This was an important modification for PIs. They also showed good high temperature resistance: the decomposition temperature (T5%) of two PIs were 520 °C and 475 °C. In addition, the residual rate at 790 °C was 50% and 49% respectively. Two PI films had a very low dielectric constant 2.26 and 2.24, respectively, which was attributed to the hyperbranched structure and strong electron-withdrawing fluorine-containing groups. PI-a and PI-b also had low water absorption of 0.66% and 0.72%, respectively. The transmittance of PI-a and PI-b high transparent films at 450 nm were 74% and 82%, respectively. The significant optical modification was mainly due to the weak electron-accepting alicyclic unit and the large trifluoromethyl electron withdrawing group. This work mainly provided an effective strategy for synthesizing comprehensive excellent performance PIs with high transmittance, excellent solubility, low water absorption, high heat resistance and low dielectric constant.

Benzothiadiazole-based non-fullerene acceptors

The power conversion efficiencies (PCEs) of organic solar cells (OSCs) have been greatly improved with the rapid development of non-fullerene acceptors (NFAs) in recent five years. Among a great deal of electron-accepting units (A units), benzothiadiazole (BT) is the most important one and has been widely used to construct high-efficiency NFAs due to its unique electron-withdrawing properties, plane rigidity and structural adjustability. The optoelectronic properties of BT-based small molecules can be efficiently modulated with the location of BT unit in the molecular backbone. Typically, BT unit is used as an A 1 unit to construct A 2 -D-A 1 -D-A 2 and A 2 -A 1 -D-A 1 -A 2 type acceptors. Furthermore, DA’ D -type fused BT-units can be utilized as a central segment to design A-DA’D-A type NFAs, which promote the rapid progress of OSCs from 2019. Judicious molecular engineering can finetune the energy levels of frontier molecular orbitals, absorption spectra, and the π-π staking properties of these materials. In turn, high-efficiency photovoltaic devices can be obtained by combining these BT-based NFAs with suitable p-type polymers. The highest PCE of OSC reached up to 18%, which could be mainly attributed to the emerge of the fused-BT-based molecule Y6. This review firstly made a precise classification of BT-based NFAs, and then summarized the relationship of chemical structures, molecular properties, and device performance. Finally, we looked forward to the challenges and design directions of this kind of molecules in the future. Benzothiadiazole (BT) is the most important electron-accepting (A) unit and has been widely used to construct high-efficiency non-fullerene acceptors (NFAs). This review firstly made a precise classification of BT-based NFAs: single-BT, double-BT, multiple-BT and ring-fused BT. Then, we summarized the relationship of chemical structures, molecular properties, energy loss and photovoltaic performance. Finally, the challenges and design directions of this kind of molecules are prospected. • A comprehensive review on the development of BT-based non-fullerene acceptors (NFAs) was made. • BT-based acceptor is an eximious material to obtain power conversion efficiency of more than 18%. • This kind of acceptor has been systematically classified and the structure-property relationships were deeply summarized. • An innovative prospect for the future development and the direction of industrialization of BT-based acceptors was made.

Modulating the middle and end-capped units of A2-A1-D-A1-A2 type non-fullerene acceptors for high VOC organic solar cells

The power conversion efficiencies (PCEs) of organic solar cells (OSCs) have been improved rapidly in the last few years, due to the development of excellent non-fullerene acceptor (NFA) materials. However, compared to the high PCE, the open-circuit voltage ( V OC ) of OSCs is still relatively low. Recently, we have demonstrated that the “Same-A-Strategy” (SAS) is feasible to achieve high V OC , where the same electron-accepting (A) unit is utilized to construct polymer donor and NFA. To investigate the structure-properties relationship, here we chose six benzotriazole-based NFAs (BTA1, BTA11, BTA3, BTA13, BTA7 and BTA17) with A 2 -A 1 -D-A 1 -A 2 type molecular backbone, where indacenodithiophene (IDT) and indacenodithieno[3,2- b ]thiophene (IDTT) as middle electron-donating unit, rhodanine (R), 2-(1,1-dicyanomethylene)rhodanine (RCN), and malononitrile (M) as the terminal A 2, respectively. When paired with a p-type polymer J52-Cl containing BTA unit, OSCs based on six material combinations can realize ultra-high V OC of 1.09–1.33 V with PCEs of 0.13–10.5%. The results indicate that the middle and end-capped units play a vital role to extend the application of SAS, and IDT and RCN are promising choices as the core and end-capped groups. Six benzotriazole-based non-fullerene acceptors are combined with a benzotriazole-based polymer J52-Cl for high-voltage organic solar cells. The results indicate IDT and RCN are better choices as the middle and end-capped groups for “Same-A-Strategy” material combinations. • By modulating the middle and end capped units, six benzotriazole (BTA)-based non-fullerene acceptors are developed to combine with a BTA-based polymer. • Organic solar cell based on all the six material combinations can achieve high V oc of 1.09–1.33V. • The PCEs of OSCs based on these six non-fullerene acceptors various from 0.13% to 10.5%.

Carbazole-based D-π-A molecules: Determining the photophysical properties and comparing ICT effects of π-spacer and acceptor groups

4-(9′-Hexylcarbazol-3′-yl)benzaldehyde (Cz-Ph-CHO: 4) and 4-(9′-hexylcarbazol-3′-yl)benzylidenemalononitrile (Cz-Ph-CN: 5) were synthesised with the structure of D-π-A, where carbazole, phenylene and formyl/dicyanovinyl groups act as electron donor (D), pi-spacer (π) and electron acceptor (A) units, respectively. The thermal, electrochemical, optical and intramolecular charge transfer (ICT) properties of compounds 4 and 5 were investigated. Compounds 4 and 5, in particular their ICT behaviour, were also compared with the closely related structure, 2-(9′-hexylcarbazol-3′-yl)-5-pyridinecarbaldehyde (Cz-Py-CHO: 7). For the purpose of tuning chemical structure to obtain targeted properties, electrochemical data and absorption and emission measurements suggest that the dicyanovinyl unit in compound 5 is a better acceptor than formyl in compound 4, and that pyridine in compound 7 is a better π-spacer than benzene in compound 4, in exerting ICT characteristics such as fluorosolvatochromism and Stokes shifts.

Symmetric and Asymmetric Push-Pull Conjugates: Significance of Pull Group Strength on Charge Transfer and Separation.

The effect of acceptor strength on excited-state charge transfer (CT) and charge separation (CS) in central phenothiazine (PTZ)-derived symmetric 1 (PTZ-(TCBD-TPA)2) and asymmetric 2 (PTZ-(TCBD/DCNQ-TPA)2) push-pull conjugates, in which triphenylamine (TPA) acts as end capping and 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) and cyclohexa-2,5-diene-1,4-ylidene-expanded TCBD (DCNQ) act as electron acceptor units, is reported. Due to strong push-pull effects, intramolecular CT was observed in the ground state, extending the absorption into the near-infrared region. Electrochemical, spectroelectrochemical, and computational studies coupled with energy-level calculations predicted both 1 and 2 to be efficient candidates for ultrafast CT. Subsequent femtosecond transient absorption studies along with global target analysis, performed in both polar and nonpolar solvents, confirmed such processes in which the CS was efficient in asymmetric 2, having both TCBD and DCNQ acceptors in polar benzonitrile, while in toluene, only CT was witnessed. This work highlights the significance of the number and strength of electron acceptor entities and the role of solvent polarity in multimodular push-pull systems to achieve ultrafast CS.

Influence of the Location of Electron-Donating 3,4-Ethylenedioxythiophene (EDOT) Moiety in the A–π–D–π–A Type Conjugated Molecules on the Optoelectronic Properties and Photovoltaic Performances

A–π–D–π–A type conjugated small molecules play an indispensable role in organic photovoltaics. Understanding the relationship between the molecular structure and performance is a fundamental question for the further rational design of high-performance organic materials. To red-shift the absorption spectrum of benzo[1,2-b:4,5-b']dithiophene (BDT) based A–π–D–π–A type compounds, an electron-donating 3,4-ethylenedioxythiophene (EDOT) moiety was introduced into the π-conjugation bridge unit. Two new compounds with EDOT next to the central BDT core (COOP-2HT-EDOT-BDT) or next to the terminal electron acceptor unit (COOP-EDOT-2HT-BDT) were synthesized and characterized. The compound COOP-2HT-EDOT-BDT showed higher molar extinction coefficient (εabs max = 1.06 × 105 L mol−1 cm−1), lower optical band gap (E g = 1.56 eV) and high HOMO energy level (E HOMO = −5.08 eV) than COOP-EDOT-2HT-BDT (εabs max = 0.96 × 105 L mol−1 cm−1, E g = 1.71 eV, E HOMO = −5.26 eV), which is attributed to the intensive interaction between the EDOT unit and the HOMO orbital, as confirmed by the theoretical calculation results. However, the higher power conversion efficiency of 3.58% was achieved for the COOP-EDOT-2HT-BDT:PC61BM-based solar cells, demonstrating that the electron-donating EDOT unit adjacent to the electron-withdrawing end-capped group (COOP) is a better way to achieve high-performance photovoltaic materials.

Difluorodithieno[3,2-a:2′,3′-c]phenazine as a strong acceptor for materials displaying thermally activated delayed fluorescence or room temperature phosphorescence

A novel strong electron-acceptor unit, 9,10-difluorodithieno[3,2-a:2′,3′-c]phenazine (DTPz), is synthesized and applied in the design of two donor-acceptor type emitters displaying long-lived delayed emission. Using either 9,9-dimethyl-9,10-dihydroacridine (DMAC) or triisopropyl-substituted benzo[1,2-b:4,5-b']dithiophene (BDT-TIPS) as the donor component, push-pull type chromophores exhibiting charge-transfer emission are obtained and found to afford either thermally activated delayed fluorescence (TADF) for DMAC or room temperature phosphorescence (RTP) for BDT-TIPS.

Donor-acceptor 2D covalent organic frameworks for efficient heterogeneous photocatalytic α-oxyamination

Covalent organic frameworks (COFs) have received widespread interest due to their high porosity, excellent crystallinity, tailorable structures, and broad application prospects. It has been demonstrated that proper combination and arrangement of electron donor and acceptor units in 2D conjugated COF lattice could promote efficient charge separation and electron transfer, and thus is beneficial for photocatalysis. In this article, three donor-acceptor (D-A) 2D COFs were prepared by Schiff base reaction of electron acceptor 4,4′,4″,4‴-(benzo[1,2-d:4,5-d′]bis(oxazole)-2,4,6,8-tetrayl)tetraaniline (BBO) with different electron donors: thieno[3,2-b]thiophene-2,5-dicarbaldehyde (TT), benzo[1,2-b:4,5-b′]dithiophene-2,6-dicarboxaldehyde (BDT) and terephthalaldehyde (Ph), respectively. These D-A 2D COFs exhibited prominent photocatalytic activity towards α-oxyamination of 1,3-dicarbonyl with 2,2,6,6-tetramethyl-1-piperdinyloxy (TEMPO) upon visible light irradiation. Among these D-ABBO-COFs, DTT-ABBO-COF exhibited the highest photocatalytic rates, which can be ascribed to the more negative highest occupied molecular orbital (HOMO) and narrower bandgap. The excellent stability, high activity and superior recyclability render DTT-ABBO-COF as a potential and environmentally friendly heterogeneous catalyst for α-oxyamination.

Naphthalenothiophene imide-based polymer exhibiting over 17% efficiency

Summary Developing novel electron-deficient monomers is crucially important for the future development of non-fullerene organic solar cells. Here, we report a new electron-deficient monomer naphthalenothiophene imide (NTI) and NTI-based copolymers PNTB and PNTB-2T. The strong electron-withdrawing ability of NTI could effectively lower the HOMO level of the polymer that facilitates the tuning of the polymer’s physical properties while simultaneously maintaining the low-lying HOMO value. Polymer chains modification by introducing extra thiophenes results in the closed π−π stacking and ordered polymer chain packing of PNTB-2T and is proven to be an effective strategy to increase the device performance. Thus, solar cell devices for PNTB-2T:Y6 exhibit an efficiency of 16.72%, which is further enhanced to 17.35% by the third component PC71BM, whereas that for PNTB:Y6 is only 3.81%. Importantly, PNTB-2T-based devices exhibit excellent batch-to-batch reproducibility. These features make this family of polymer donors very promising and the design strategies will enlighten future development of high-performance polymer donors for organic solar cells (OSC).

Twisted Intramolecular Charge Transfer—Aggregation-Induced Emission Fluorogen with Polymer Encapsulation-Enhanced Near-Infrared Emission for Bioimaging

Fluorescence probes with strong near-infrared (NIR) emission and water solubility are considered useful visualization tools for localization marking as well as investigating cell migration and tran...

Controlling Energy Gaps of π-Conjugated Polymers by Multi-Fluorinated Boron-Fused Azobenzene Acceptors for Highly Efficient Near-Infrared Emission.

We demonstrate that multi-fluorinated boron-fused azobenzene (BAz) complexes can work as a strong electron acceptor in electron donor-acceptor (D-A) type π-conjugated polymers. Position-dependent substitution effects were revealed, and the energy level of the lowest unoccupied molecular orbital (LUMO) was critically decreased by fluorination. As a result, the obtained polymers showed near-infrared (NIR) emission (λPL =758-847 nm) with high absolute photoluminescence quantum yield (ΦPL =7-23%) originating from low-lying LUMO energy levels of the BAz moieties (-3.94 to -4.25 eV). Owing to inherent solid-state emissive properties of the BAz units, deeper NIR emission (λPL =852980 nm) was detected in film state. Clear solvent effects prove that the NIR emission is from a charge transfer state originating from a strong D-A interaction. The effects of fluorination on the frontier orbitals are well understandable and predictable by theoretical calculation with density functional theory. This study demonstrates the effectiveness of fluorination to the BAz units for producing a strong electron-accepting unit through fine-tuning of energy gaps, which can be the promising strategy for designing NIR absorptive and emissive materials.

Novel D-A-D type small-molecular hole transport materials for stable inverted perovskite solar cells

Hole transport materials (HTMs), as a critical role in the hole extraction and transportation processes, highly influence the efficiency and stability of perovskite solar cells (PSCs). Despite that several efficient dopant-free HTMs have been reported, there is still no clear structure-property relationship that could give instructions for the rational molecular design of efficient HTMs. Thus, in this work, a series of donor–acceptor-donor (D–A–D) type carbazole-based small molecules, TM-1 to TM-4, have been carefully designed and synthesized. By varing the electron acceptor unit from benzene to pyridine, pyrazine and diazine, their packing structure in single crystals, optical and electronic properties have shown a great difference. While as dopant-free HTM in p-i-n type PSCs, TM-2 improved the device photovoltaic performance with a power conversion efficiency from 15.02% (based on PEDOT:PSS) to 16.13%. Moreover, the unencapsulated device based on TM-2 retains about 80% of its initial efficiency after 500 h storage in ambient environment, showing the superior stability. • A series of donor–acceptor-donor (D–A–D) type carbazole-based small molecules, TM-1 to TM-4, have been designed and synthesized, showing a great difference in their single crystal packing structure, optical and electronic properties. • As the dopant-free HTM in p-i-n type PSCs, TM-2 improved the device photovoltaic performance with a power conversion efficiency from 15.02% (based on PEDOT:PSS) to 16.13% along with a superior stability.