N-type Small Molecules Research Articles

Modulating crystallinity and mixed ionic–electronic conduction properties via terminal side chain engineering of n-type small molecules

Via versatile terminal side chain modification in n-type small molecular mixed conductors, a superior figure of merit μC* of 14.1 F V−1 cm−1 s−1 in OECTs is achieved, along with robust synaptic tunability and excellent non-volatility in OENSs.

N-type small molecule electrolyte cathode interface layer with thickness insensitivity for organic solar cells

Interface engineering has a critical impact on the performances of organic solar cells (OSCs). And cathode interface layer (CIL) with thickness insensitivity is urgently pursued to improve the possibility of industrialization of OSCs. N-type self-doping has been proven effective in increasing electron mobility. Here, four novel n-type small molecule electrolytes (SMEs) with diverse counter anions (CAs), PDIN-BF4, PDIN-BPh4, PDIN-BPhF4, and PDIN-BIm4 were synthesized and employed as cathode interface layers (CILs). Among them, PDIN-BIm4-based OSCs with PM6:Y6 active layer achieved the most glorious electron mobility and thickness insensitivity with a power conversion efficiency (PCE) of 16.98 % due to outstanding self-doping effect and interfacial regulation ability. However, the multi-F atoms on PDIN-BPhF4 may prevent self-doping progress and impede electron transport, thus leading to a low PCE of 11.53 %. Meanwhile, the PDIN-BIm4-based device can maintain over 80 % of the optimal PCE with a thickness of 43 nm or storing in a glove box for 600 h. In addition, PM6: BTP-eC9-based device with PDIN-BIm4 CIL acquired a PCE of 17.82 %, highlighting the broad applicability of PDIN-BIm4. Our work demonstrates that the introduction of CAs into n-type organic materials helps promote the progress of efficient and stable OSCs.

N-Type Small Molecule Electron Transport Layers with Excellent Surface Energy and Moisture Resistance Siloxane for Non-Fullerene Organic Solar Cells.

Electron transport layers (ETLs) generally contain polar groups for enhancing performance and reducing the work function. Nevertheless, the polar group with high surface energy may cause inferior interfacial compatibility, which challenges the ETLsto balance stability and performance. Here, two conjugated small molecules of ETLs with low surface energy siloxane, namely PDI-Si and PDIN-Si, are synthesized. The siloxane with low surface energy not only enhances the interfacial compatibility between ETLs and active layers but also improves the moisture-proof stability of the device. Impressively, the amine-functionalized PDIN-Si can simultaneously exhibit conspicuous n-type self-doping properties and outstanding moisture-proof stability. The optimization of interfacial contact and morphology enables the PM6:Y6-based OSC with PDIN-Si to achieve a power conversion efficiency (PCE) of 15.87%, which is slightly superior to that of classical ETL PDINO devices (15.27%), and when the PDIN-Si film thickness reaches 28nm, the PCE remains at 13.19% (≈83%), which indicates that PDIN-Si has satisfactory thickness insensitivity to facilitate roll-to-roll processing. Excitingly, after 120 h of storage in an environment with humidity above 45%, the unencapsulated device with PDIN-Si as ETL remains at 75% of the initial PCE value, while the device with PDINO as ETL is only 50%.

Exploring the Influence of P3HT on PTCA Crystallization and Phase Behavior in Thin Films.

The thermal properties and alignment of crystallinity of materials in thin films play crucial roles in the performance and reliability of various devices, especially in the fields of electronics, materials science, and engineering. The slight variations in the molecular packing of the active layer can make considerable differences in the optical and thermal properties. Herein, we aim to investigate the tuning of the physical properties of a blended thin film of n-type small organic molecules of perylene-3,4,9,10-tetracarboxylic acid (PTCA-SMs) with the mixing of the p-type polymer poly(3-hexylthiophene) (P3HT). The resulting thin films exhibit an enhanced surface crystallinity compared to the pristine material, leading to the formation of long crystallites, and these crystallites are thermally stable in the solid state, as confirmed by X-ray diffraction (XRD), atomic force microscopy (AFM), and thermal analysis using variable-temperature spectroscopic ellipsometry (VTSE) and differential scanning calorimetry (DSC). We believe that the crystalline structure of the obtained P3HT/PTCA-SMs blends is a combination of edge-on and face-on orientations, which enable the potential use of this material as an active layer in organic electronics.

Synaptic transistor based on PVK mixed with oxadiazole and its logic gate application

Because of the simplicity of device integration, high energy efficiency, and the ability to achieve robust neuromorphic calculation, the explore of artificial synaptic devices has attracted extensive attention. In this work, an electronic synaptic organic transistor based on a mixture of p-type polymer and n-type small molecule semiconductor colloids by spin coating was designed to achieve synaptic plasticity. Typical synaptic behavior, including excitatory postsynaptic current and paired-pulse facilitation, is simulated using electrical impulse stimulation. In addition, the transition from short-term plasticity to long-term plasticity were realized by adjusting input pulse operation. More importantly, the logic function of “AND” and “NAND” gate is designed and realized based on the operability of organic transistor pulse modulation. Our work offers a new approach for the exploitation of organic multifunctional neuromorphic electronic devices.

Low-cost material combination based on PTQ10 and completely non-fused nonfullerene acceptor for high VOC organic photovoltaics

It is a great challenge to develop low-cost photovoltaic materials, including p-type polymers and n-type small molecules, to fabricate organic photovoltaic (OPV) cells for outdoor and indoor applications. Among large number of nonfullerene acceptors (NFAs), non-fused NFAs have attracted much attention because of the advantages of simple synthesis and easy chemical structure modification. Although the highest power conversion efficiencies (PCEs) of OPVs based on non-fused NFAs have reached 15%, the open-circuit voltage (VOC) is relatively low and hinders indoor application. Here, we designed a completely non-fused NFAs, Cl-BTA33, to combine with a classic low-cost polymer PTQ10. Compared with nonhalogenated BTA33, Cl-BTA33 possesses a higher molar absorption coefficient, stronger crystallinity, and tighter π-π stacking due to the stronger intermolecular interactions. Under AM 1.5 M illumination, PTQ10:Cl-BTA33 combination realizes an improved PCE of 12.16% compared to PTQ10: BTA33-based device (PCE = 8.68%). The enhancement comes from the higher and more balanced carrier mobility, more efficient exciton dissociation, weaker charge recombination, more ordered molecular stacking, and more suitable phase separation. In addition, the large area (1.0 cm2) indoor OPV based on PTQ10: Cl-BTA33 achieves a high PCE of 24.28% under a 2700 K LED illumination at 1000 lx. This study proves that chlorination on the π-bridge unit is a facile and effective strategy to modify the properties and improve the photovoltaic performance of low-cost, completely non-fused NFAs.

TFT-CN/P3HT blending active layer based two-component organic field-effect transistor for improved H2S gas detection

To prepare OFET-based sensors by low-cost solution method and improve the sensing ability of polymer OFETs to hydrogen sulfide (H2S), a n-type small molecule dicyanomethylene-terminated quinoid compound (TFT-CN) was blended with P3HT and used as the semiconductor active layer. The introducing of TFT-CN into two-component OFET based sensors greatly improves the sensitivity and selectivity for H2S gas and reduces the limit of detection to 10 ppb level due to the strong binding energy between TFT-CN and H2S. The blending ratio has a great influence on the sensing performance and the sensors show the highest sensitivity for various concentrations of H2S when the ratio of TFT-CN: P3HT is 1: 6 in weight, and the sensitivity can be increased five times than that of the pure P3HT-based sensors. This work proves that it is a simple, effective and low-cost solution method to improve the sensing performance of OFET-based sensors by introducing a functional molecule into conventional polymers which will inspire the performance improvement and application expansion of many traditional materials.

Naphthalene diimide-based n-type small molecule organic mixed conductors for accumulation mode organic electrochemical transistors.

Organic mixed ionic-electronic conductors (OMIECs), which transport both ionic and electronic charges, development are important for progressing bioelectronic and energy storage devices. The p-type OMIECs are extensively investigated and used in various applications, whereas the n-type ones lag far behind due to their moisture and air instability. Here, we report the synthesis of the novel n-type naphthalene diimide (NDI)-based small-molecule OMIECs for organic electrochemical transistors (OECTs). The electro-active NDI molecule with the linear ethylene glycol side chains is a promising candidate for n-type channel material to obtain accumulation mode OECTs. This NDI-based small-molecule OMIEC, gNDI-Br2, demonstrates ion permeability due to the attachment of the glycol side chains with optimized ionic-electronic conductions. OECT devices with gNDI-Br2 channel material displays excellent performance in water and ambient stability. OECTs fabricated with two different concentrations, 50 mg mL-1 and 100 mg mL-1 of gNDI-Br2 demonstrate a transconductance value of 344 ± 19.7 μS and 814 ± 124.2 μS with the mobility capacitance product (μC*) of 0.13 ± 0.03 F cm-1 V-1 s-1 and 0.23 ± 0.04 F cm-1 V-1 s-1, respectively. These results demonstrate the n-type OMIEC behaviour of the NDI-based small-molecule and its applicability as an OECT channel material.

N-type small molecule electron transport materials with D-A-D conjugated core for non-fullerene organic solar cells

Two D (donor)-A (acceptor)-D-type small-molecular conjugated electrolytes (SMCEs) electron transport materials (ETMs) with self-doping effects, namely DPPN2TPA and DPPM2TPA, were successfully synthesized by introducing diketopyrrolopyrrole (DPP) as the electron acceptor, triphenylamine (TPA) as the electron donor and quaternary ammonium or imidazolium salts as polar pendants. Impressively, the electron-deficient group DPP is double-doped by the amine-based groups and the electron-rich groups TPA, thus DPPN2TPA and DPPM2TPA exhibit prominent intramolecular charge transfer (ICT) and distinct n-type self-doping property. These characters can increase the electron mobility and ameliorate the work function (WF) of the stable metal Ag to create ohmic contact, making for significant elevated photovoltaic performance of the SMCEs. Especially, DPPM2TPA emerges more prominent optoelectronic property than DPPN2TPA due to the extra interfacial dipoles generated by the tertiary amine of imidazole moieties. The PM6:Y6-based non-fullerene organic solar cells (NOSCs) with DPPN2TPA and DPPM2TPA as ETMs achieve excellent power conversion efficiency (PCE) of 15.93 % and 16.40 %, respectively. It is noticeably superior to the NOSC with classical ETM PDINO (15.35 %). Delightfully, the PCE remained at 14.64 % (≈90 %) when the thickness of DPPM2TPA film reached 37 nm, which manifests that DPPM2TPA has satisfactory thickness insensitivity and can be used to manufacture large-scale devices. Furthermore, DPPM2TPA has conspicuous universality, when DPPM2TPA ETM was applied to the device with PM6: BTP-eC9, a remarkable PCE of 17.31 % was realized. These results demonstrate the potential of DPPM2TPA for industrialized large-scale manufacturing and afford an effective approach for exploring high-performance D-A-D SMCEs NOSCs.

Organic Photovoltaic Cells Based on Nonhalogenated Polymer Donors and Nonhalogenated A-DA'D-A-Type Nonfullerene Acceptors with High VOC and Low Nonradiative Voltage Loss.

Compared with other all-inorganic/organic-inorganic hybrid solar cells, the large voltage loss (Vloss) of organic photovoltaic (OPV) cells, especially the nonradiative voltage loss (ΔVnonrad), limited the further improvement of performance. Although A-DA'D-A-type Y-series nonfullerene acceptors (NFAs) largely improve the power conversion efficiencies (PCEs) to 18%, the open-circuit voltage (VOC) of this kind of material was still restricted to below 1.0 V. Herein, we designed and synthesized a narrow bandgap (Eg = 1.41 eV) acceptor BTA77 with an A-DA'D-A-type backbone containing a nonhalogenated terminal group to achieve high electroluminescence efficiency and high VOC. Combined with the nonhalogenated polymer PBDB-T with a conjugated thiophene side chain, BTA77 realized a VOC of 0.944 V, a Vloss of 0.552 V, and a PCE of 13.75%, which is one of the highest PCEs based on nonhalogenated A-DA'D-A-type acceptors with VOC > 0.9 V. After further blending with the nonhalogenated donor polymer PBT1-C with a conjugated phenyl side chain, the VOC increases to 1.021 V with a super low ΔVnonrad of 0.14 V owing to the greatly improved electroluminescence external quantum efficiency (EQEEL) of 4.42 × 10-3. Our results indicate that there is still a large room to decrease the ΔVnonrad and increase VOC by synergistic molecular engineering of p-type polymers and n-type small molecules.

Core-twisted tetrachloroperylenediimide additives improve the crystallinity of perovskites to provide efficient perovskite solar cells

Defects at the grain boundaries and surfaces of metal-halide perovskite films function as non-radiative recombination centers that decrease the performance and/or stability of perovskite solar cells (PSCs). The incorporation of additives into perovskite films can improve their quality and, thereby, provide high-performance PSCs. In this study, we introduced core-twisted tetrachloroperylene diimide (ClPDI) derivatives (n-type small molecules) into the lead iodide precursor solution and used a two-step deposition method to prepare methylammonium lead triiodide (MAPbI3) perovskite films. We examined the effects of the terminal functional groups of the ClPDI derivatives—n-butyl (ClPDI-C4), dimethylaminopropyl (ClPDI-C3DMeA), and aminopropyl (ClPDI-DPA) — on their ability to control the crystallization rate and modulate the film morphology. The power conversion efficiency of the PSC prepared without an additive (15.88%) improved to 16.88 and 18.77% after incorporation of ClPDI-C4 and ClPDI-DPA, respectively, because the strong interactions between these additives and the perovskite passivated defects, slowed the rate of perovskite crystal growth, and increased the grain size. Thus, the use of ClPDI additives in perovskite films can effectively enhance both the efficiency and stability of PSCs.

A high mobility air-stable n-type organic small molecule semiconductor with high UV–visible-to-NIR photoresponse

An organic semiconductor with high carrier mobility and efficient light absorption over a wide spectral range is of the most important yet challenging material for constructing a broadband responsive organic photodetector. However, the development of such organic semiconductors, especially for air-stable n-type organic small molecule semiconductors, is still at an early stage. Here we report the fabrication of high-performance n-type semiconducting crystalline nanosheets and the development of air-stable field-effect transistors, phototransistors, with high response over a broad spectrum. The n-type small molecule semiconductor is assembled into a crystalline nanosheet based on the solvent-phase interfacial self-assembly method. N-type field-effect transistors with high electron mobility are fabricated and their electrical performances exhibit excellent air stability. Impressively, the demonstrated phototransistors exhibit an ultrahigh responsivity over a wide spectral range from 365 to 940 nm, with a maximum photoresponsivity of 9.2 × 105 A W−1 and specific detectivity of 5.26 × 1013 Jones, which is the best performance among the reported n-type organic small molecule-based phototransistors.

High-performance floating-gate organic phototransistors based on n-type core-expanded naphthalene diimides

In the field of organic phototransistor, achieving both broad-spectral and high photosensitivity has always been a big challenge. The innovation of device structure has previously proven to be a possible solution to this problem. Here in this study, a novel organic phototransistor based on a high mobility n-type small molecule as the conducting layer and an isolated bulk heterojunction light-absorbing layer as the floating gate has been demonstrated in this study. With the special designed device structure, the phototransistor shows extremely high sensitivity to broad spectral and weak light irradiation, and the photoresponsivity and photocurrent/dark-current ratio of the device can reach up to 4840 mA/W and 1.8 × 105 respectively. For conclusion, this study suggests a potential way to obtain high-performance phototransistors at room temperature, which will further promote the commercial application of organic phototransistors.

A novel n-type organic semiconductor comprising a 1,5-naphthyridine-2,6-dione unit

The first examples of 1,5-naphthyridine-2,6-dione (NTD)-based n-type small molecules, NTDT-DCV and NTDP-DCV with an electron-withdrawing dicyanovinyl terminal unit and different aromatic bridging groups were synthesized and characterized.

Near-IR Absorbing Molecular Semiconductors Incorporating Cyanated Benzothiadiazole Acceptors for High-Performance Semitransparent n-Type Organic Field-Effect Transistors

Small band gap molecular semiconductors are of interest for the development of transparent electronics. Here we report two near-infrared (NIR), n-type small molecule semiconductors, based upon an acceptor–donor–acceptor (A-D-A) approach. We show that the inclusion of molecular spacers between the strong-electron-accepting end group, 2,1,3-benzothiadiazole-4,5,6-tricarbonitrile, and the donor core affords semiconductors with very low band gaps down to 1 eV. Both materials were synthesized by a one-pot, 6-fold nucleophilic displacement of a fluorinated precursor by cyanide. Significant differences in solid-state ordering and charge carrier mobility are observed depending on the nature of the spacer, with a thiophene spacer resulting in solution processed organic field-effect transistors (OFETs) exhibiting excellent electron mobility up to 1.1 cm2 V–1 s–1. The use of silver nanowires as the gate electrode enables the fabrication of a semitransparent OFET device with an average visible transmission of 71% in the optical spectrum.

Effects of the Polarity and Bulkiness of End-Functionalized Side Chains on the Charge Transport of Dicyanovinyl-End-Capped Diketopyrrolopyrrole-Based n-Type Small Molecules.

When designing organic semiconductors, side-chain engineering is as important as modifying the conjugated backbone, which has a significant impact on molecular ordering, morphology, and thus electronic device performance. We have developed three dicyanovinyl-end-capped donor-acceptor diketopyrrolopyrrole-based n-type small molecules (C2C9CN, SiC4CN, and EH4PCN) bearing an identical length of alkyl spacer yet different end-functionalized side chains (i.e., alkyl-, siloxane-, and phosphonate-end pendants). The effects of the end-functionalized side chains on the intrinsic molecular properties, microstructure, and charge transport of the small-molecule series were investigated. In comparison with the alkyl-end side chains, incorporating siloxane-end side chains into the backbone facilitates 2D edge-on oriented high intergrain connectivity/crystallinity and compatibility with the substrate surface, whereas the phosphonate-end analogues have an adverse effect on the film-forming quality due to high polarity. Thereby, an organic field-effect transistor fabricated by SiC4CN shows the best electron mobility up to 1.59 × 10-1 cm2 V-1 s-1 along with a high current on/off ratio >105. This study contributes to our understanding of the role of the end-functionalized side chains (e.g., the effects of polarity and bulkiness of the end groups) for the development of high-performance semiconductors.

Interfacial and Permeating Modification Effect of n-type Non-fullerene Acceptors toward High-Performance Perovskite Solar Cells

Interface passivation in the electron transport layer (ETL) has emerged as a very important and challenging topic for the improvement of stability and efficiency of perovskite solar cells (PSCs). Here, we introduce the n-type small organic molecule Y6 that acts as an effective ETL modifier through surface engineering. As a result, the simple PCBM + Y6 ETL led to significantly stronger light absorption, higher electron extraction ability and transportability, and reduced recombination loss in regular MAPbI3 PSCs. The power conversion efficiency can be significantly increased from 17.39 to 20.02% in inverted p-i-n MAPbI3 PSCs without additional alternation, with an increment of over 15%, along with higher open-circuit voltage and short-circuit current. What is more, the devices with the Y6-modified ETL also exhibited better long-term stability compared to the control devices. This indicates the feasibility of enhancing absorption over a wider light spectrum in a single-junction cell and gaining a comprehensive understanding of interface engineering between the ETL and perovskite layer.

Controlled recrystallization from the melt of the organic n-type small molecule semiconductor 2-decyl-7-phenyl-[1]benzothieno[3,2-b][1]benzothiophene S,S,S′,S′-tetraoxide

Recrystallization from the melt of the n-type BTBT derivative 2-dectyl-7-phenyl-[1]benzothieno[3,2-b][1]benzothiophene S,S,S',S'-tetraoxide (Ph-BTBTOx2-10) reveals a defined crystal structure of the molecule. It leads to the formation of alternating nano-segregated layers consisting of parallelly stacked aromatic units and alkyl units. This polymorph appears to be the thermodynamic stable phase. Charge carrier mobility measurements indicate an electron mobility of 4*10−6 cm2 V−1 s−1 for this phase.

Selenium-containing core-expanded naphthalene diimides for high performance n-type organic semiconductors

The incorporation of heavy atoms into molecular backbone is an extremely straightforward strategy for fine-tuning the optoelectronic properties of organic semiconductors. However, it is rarely studied in n-type small molecules. Herein, by selenium substitution of NDI3HU-DTYM2, two Se-decorated core-expanded naphthalene diimides (NDI) derivatives DTYM-NDI3HU-DSYM ( 1 ) and NDI3HU-DSYM2 ( 2 ) were synthesized. In comparison with the reference S-containing compound NDI3HU-DTYM2, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of 1 and 2 were fine-tuned with ∆HOMO of about 0.2 eV, ∆LUMO of 0.1 eV and the narrowed HOMO-LUMO gaps. More surprisingly, the as-spun organic thin film transistors (OTFTs) based on 1 and 2 both showed µ e,sat values as high as 1.0 cm2 V−1 s−1, which are 2-fold higher than that of NDI3HU-DTYM2 with the same device structure and measurement conditions. In addition, the single crystal OFET devices based on Se-containing compound NDI2BO-DSYM2 showed a high µ e,sat value of 1.30 cm2 V−1 s−1. The molecular packing of NDI2BO-DSYM2 in single crystals (two-dimensional supramolecular structure formed by intermolecular Se···Se interactions) is quite different from that of a S-containing compound NDI-DTYM2 (one dimensional supramolecular structure formed by intermolecular π-π stacking). Therefore, the Se substitution can cause dramatic change about molecular stacking model, giving rise to high n-type OTFT performance. Our results demonstrated an effective strategy of the heavy atom effect for designing novel organic semiconductors.

SWIR Photodetection and Visualization Realized by Incorporating an Organic SWIR Sensitive Bulk Heterojunction.

Short‐wavelength infrared (SWIR) photodetection and visualization has profound impacts on different applications. In this work, a large‐area organic SWIR photodetector (PD) that is sensitive to SWIR light over a wavelength range from 1000 to 1600 nm and a SWIR visualization device that is capable of upconverting SWIR to visible light are developed. The organic SWIR PD, comprising an organic SWIR sensitive blend of a near‐infrared polymer and a nonfullerene n‐type small molecule SWIR dye, demonstrates an excellent capability for real‐time heart rate monitoring, offering an attractive opportunity for portable and wearable healthcare gadgets. The SWIR‐to‐visible upconversion device is also demonstrated by monolithic integration of an organic SWIR PD and a perovskite light‐emitting diode, converting SWIR (1050 nm) to visible light (516 nm). The most important attribute of the SWIR visualizing device is its solution fabrication capability for large‐area SWIR detection and visualization at a low cost. The results are very encouraging, revealing the exciting large‐area SWIR photodetection and visualization for a plethora of applications in environmental pollution, surveillance, bioimaging, medical, automotive, food, and wellness monitoring.