Lowest Unoccupied Molecular Orbital Energy Levels Research Articles

A combined experimental and computational study of electrochemical and photophysical properties of new benzophenone derivatives functionalized with N-substituted-phenyl-1,3,4-thiadiazole-2-amine

New benzophenone derivatives with N-substituted-1,3,4-thiadiazole including different substituents were synthesized and characterized by using FTIR, NMR, UV/Visible, and fluorescence spectroscopies and cyclic voltammetry. The effect of –F, -Cl, –OCH3 and –NO2 substitutions at ortho-, meta-, para-positions on the photoluminescence properties of the molecules have been investigated. The results indicate that the electron-donating and electron-withdrawing moieties at the ortho-, meta-, and para-positions have an important effect on the photoluminescence properties of the molecules. The calculated the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels are in the range of (−5.55) to (−5.65) eV and (−2.83) to (−3.02), respectively. The lower LUMO energy levels were observed for the molecules with –OCH3 substituents due to the donor strength of the substituent. Excited states and electrochemical properties of the molecules were also comprehensively and quantitatively studied by Density Functional Theory and Time-Dependent Density Functional Theory calculations. Experimental trends were successfully revealed by calculated data.

Electric Field Control of Molecular Charge State in a Single-Component 2D Organic Nanoarray.

Quantum dots (QD) with electric-field-controlled charge state are promising for electronics applications, e.g., digital information storage, single-electron transistors, and quantum computing. Inorganic QDs consisting of semiconductor nanostructures or heterostructures often offer limited control on size and composition distribution as well as low potential for scalability and/or nanoscale miniaturization. Owing to their tunability and self-assembly capability, using organic molecules as building nanounits can allow for bottom-up synthesis of two-dimensional (2D) nanoarrays of QDs. However, 2D molecular self-assembly protocols are often applicable on metals surfaces, where electronic hybridization and Fermi level pinning can hinder electric-field control of the QD charge state. Here, we demonstrate the synthesis of a single-component self-assembled 2D array of molecules [9,10-dicyanoanthracene (DCA)] that exhibit electric-field-controlled spatially periodic charging on a noble metal surface, Ag(111). The charge state of DCA can be altered (between neutral and negative), depending on its adsorption site, by the local electric field induced by a scanning tunneling microscope tip. Limited metal-molecule interactions result in an effective tunneling barrier between DCA and Ag(111) that enables electric-field-induced electron population of the lowest unoccupied molecular orbital (LUMO) and, hence, charging of the molecule. Subtle site-dependent variation of the molecular adsorption height translates into a significant spatial modulation of the molecular polarizability, dielectric constant, and LUMO energy level alignment, giving rise to a spatially dependent effective molecule-surface tunneling barrier and likelihood of charging. This work offers potential for high-density 2D self-assembled nanoarrays of identical QDs whose charge states can be addressed individually with an electric field.

Enhancing Open-Circuit Voltage and Charge Transportation for Ternary Organic Solar Cells With Energy Cascade Double Nonfullerene Acceptors

An effective double nonfullerene acceptors ternary organic solar cell (OSC) was systemically investigated to improve the performance of OSCs by doping higher lowest unoccupied molecular orbital (LUMO) energy level nonfullerene acceptor ITIC-Th into the PBDB-T-SF: IT-4F active layer. Compared with binary OSCs, the power conversion efficiency of the optimal ternary OSCs with 20 wt% ITIC-Th content is improved from 9.84% to 11.0%, which was attributed to simultaneous improvements in the open-circuit voltage ( V oc) and fill factor with a slightly reduction in short-current density ( J sc). The optimal devices show appropriate cascaded LUMO energy level, small aggregation phases of film morphology and higher hole–electron mobility, which are favorable to charge dissociation and significantly beneficial for efficient charge generation and collection. Moreover, under 20 wt% ITIC-Th, the impedance spectra analysis reveals that the smoother surface of the active layer presents a good Ohmic contact for better charge transportation by establishing more potential pathways.

Machine Learning for Understanding the Relationship between the Charge Transport Mobility and Electronic Energy Levels for n‐Type Organic Field‐Effect Transistors

AbstractOrganic field‐effect transistors (OFETs) have received considerably more attention than inorganic‐based field‐effect transistors for use in next generation of organic circuits. There are a number of variables, for example, the ordering of the OFETs, their energy levels, and the material used for source/drain electrodes, that influence the magnitude of charge transport mobility. Importantly, a suitable energy level match between highest occupied molecular orbital (HOMO) or lowest unoccupied molecular orbital (LUMO) energy level and work function of the electrodes may have a large influence on the measured mobility. An informatics approach, specifically use of machine learning, is proposed for charge transport mobility prediction. Gradient Boosting and Random Forest regression algorithms are used to model previous experimental datasets and HOMO and LUMO energy levels of n‐type materials are optimized using expected machine‐learning methods. The results reveal that Random Forest model benefits the functional analysis of n‐type OFETs in three ways: 1) it provides better understanding of current n‐type organic materials, 2) it may guide the choice of n‐type organic materials and conducting electrodes, and 3) it measures the tradeoffs between the charge transport mobility and electronic energy levels for n‐type OFETs.

Fluorescence Detection of Prostate Cancer by an Activatable Fluorescence Probe for PSMA Carboxypeptidase Activity.

Prostate cancer (PCa) is a common malignant tumor among adult males, and convenient intraoperative detection of PCa would reduce the risk of leaving positive surgical margins, especially during nerve-sparing procedures. To achieve rapid, fluorescence-based visualization of PCa, we focused on the glutamate carboxypeptidase (CP) activity of prostate-specific membrane antigen (PSMA), a type II transmembrane glycoprotein that is attracting attention as a PCa biomarker. Based on our finding that aryl glutamate conjugates with an azoformyl linker are recognized by PSMA and have a sufficiently low LUMO (lowest unoccupied molecular orbital) energy level to quench the fluorophore through photoinduced electron transfer, we designed and synthesized a first-in-class activatable fluorescence probe for CP activity of PSMA. The developed probe allowed us to visualize the CP activity of PSMA in living cells and in clinical specimens from PCa patients and is expected to be useful for rapid intraoperative detection and diagnosis of PCa.

Dual Imide-Functionalized Unit-Based Regioregular D-A1-D-A2 Polymers for Efficient Unipolar n-Channel Organic Transistors and All-Polymer Solar Cells.

The demand for the development of more promising n-type semiconducting polymers with excellent electron mobilities and air stabilities is growing fast. In this study, we designed and synthesized a series of new dual imide-functionalized derivative-based regioregular D-A1-D-A2 copolymers with different side chains (namely, PNT-R, R = 2-decyltetradecyl (DT), 2-octadecyldodecyl (OD), and 2-hexyldecyl (HD)). These new polymers PNT-R showed strong electron affinities with deep lowest unoccupied molecular orbital (LUMO) levels down to -4.01 eV, indicating that they are promising electron-transporting materials. To optimize the electron mobility, side-chain engineering was adopted. Thus, the effects of the side-chain length on their optoelectronic and charge-transport properties as well as the performances of all-polymer solar cells (all-PSCs) were systematically investigated. Shortening the side-chain length significantly expanded the absorption range, deepened the LUMO energy level, strengthened the molecular packing properties, and developed more crystalline microstructures in the solid state, as evidenced by the ultraviolet-visible absorption spectra, cyclic voltammetry, synchrotron two-dimensional grazing-incidence wide-angle X-ray scattering, and atomic force microscopy measurements. Consequently, the highest electron mobility of 1.05 cm2 V-1 s-1 was achieved in PNT-HD-based organic thin-film transistors (OTFTs). Also, PNT-R polymers were successfully applied as electron acceptors in all-PSCs. In good agreement with the OTFT results, the highest power conversion efficiency of 6.62% was obtained for the PNT-HD-blend film due to its excellent short-circuit current ( Jsc) value (12.07 mA cm-2), which was much higher than that of the PNT-DT- and PNT-OD-based all-PSCs (7.67 and 10.19 mA cm-2, respectively). By further investigating the dependence of the Jsc and open-circuit voltage ( Voc) on the illuminated light intensity ( P), the high Jsc value of the PNT-HD-based device was found to originate from its highly suppressed mono- and bimolecular recombination as well as efficient exciton dissociation and charge transfer at the donor-acceptor interfaces. Overall, this study provides insights into the naphthalenediimide-based regioregular D-A1-D-A2 copolymers used in all-PSCs and offers important design guidelines for future development of n-type semiconducting polymers.

Unconjugated Side‐Chain Engineering Enables Small Molecular Acceptors for Highly Efficient Non‐Fullerene Organic Solar Cells: Insights into the Fine‐Tuning of Acceptor Properties and Micromorphology

Abstract2D conjugated side‐chain engineering is an effective strategy that is widely utilized to construct benzodithiophene‐based polymers. Herein, an unconjugated side‐chain strategy to design fused‐benzodithiophene‐based non‐fullerene small molecule acceptors (SMAs) via vertical aromatic side‐chain engineering on the ladder‐type core is employed. Three SMAs named BTTIC‐Th, BTTIC‐TT, and BTTIC‐Ph with thiophene, thieno[3,2‐b]thiophene, and benzene, respectively, as side chains, are designed and synthesized. Three SMAs exhibit similar absorption ranges but different lowest unoccupied molecular orbital (LUMO) energy levels due to the different strength of the δ‐inductive effect between vertical aromatic side chains and their electron‐rich core. Organic solar cells based on PBDB‐T:BTTIC‐TT achieve a power conversion efficiency (PCE) of 13.44%, which is higher than the PCE of devices based on PBDB‐T:BTTIC‐Th (12.91%) and PBDB‐T:BTTIC‐Ph (9.14%). The difference in device performance is investigated by electrical and morphological characterizations. A large domain size and different types of π–π stacking are found in the bulk heterojunction layer of PBDB‐T:BTTIC‐Ph blend film, which are detrimental to exciton dissociation and charge transport. Overall, it is demonstrated that when designing unconjugated side chains, thieno[3,2‐b]thiophene is superior to thiophene and benzene through its dual roles of promoting the LUMO energy level and optimizing the morphology. These results shed light on the side‐chain engineering of high‐performance non‐fullerene SMAs.

A Simple and Effective Method of Determining Lewis Acidity by Using Fluorescence

Summary The strength of a Lewis acid is an important parameter in establishing its utility for practical applications. Here, we report a universal fluorescence-based method for accurately determining the strength of a range of Lewis acids. Utilizing a fluorescent dithienophosphole oxide architecture, we are able to emulate the structural motif of the widely employed Gutmann-Beckett method. This allows us to precisely correlate the optical response to the strength of a Lewis acid. In comparison to commonly used methods that employ 31P NMR chemical shifts or quantum-chemical calculations, our method strongly suggests that using fluorescent Lewis acid-base adducts (FLAs) is more reliable. Moreover, our method provides a direct measure of the Lewis acidity of a given solution, independent of the Lewis base, with direct implications to the overall utility of a Lewis acid in practical applications.

Cationic IrIII Emitters with Near-Infrared Emission Beyond 800 nm and Their Use in Light-Emitting Electrochemical Cells.

Solid-state near-infrared (NIR) light-emitting devices have recently received considerable attention as NIR light sources that can penetrate deep into human tissue and are suitable for bioimaging and labeling. In addition, solid-state NIR light-emitting electrochemical cells (LECs) have shown several promising advantages over NIR organic light-emitting devices (OLEDs). However, among the reported NIR LECs based on ionic transition-metal complexes (iTMCs), there is currently no iridium-based LEC that displays NIR electroluminescence (EL) peaks near to or above 800 nm. In this report we demonstrate a simple method for adjusting the energy gap between the highest-occupied molecular orbital (HOMO) and the lowest-unoccupied molecular orbital (LUMO) of iridium-based iTMCs to generate NIR emission. We describe a series of novel ionic iridium complexes with very small energy gaps, namely NIR1-NIR6, in which 2,3-diphenylbenzo[g]quinoxaline moieties mainly take charge of the HOMO energy levels and 2,2'-biquinoline, 2-(quinolin-2-yl)quinazoline, and 2,2'-bibenzo[d]thiazole moieties mainly control the LUMO energy levels. All the complexes exhibited NIR phosphorescence, with emission maxima up to 850 nm, and have been applied as components in LECs, showing a maximum external quantum efficiency (EQE) of 0.05 % in the EL devices. By using a host-guest emissive system, with the iridium complex RED as the host and the complex NIR3 or NIR6 as guest, the highest EQE of the LECs can be further enhanced to above 0.1 %.

Conjugated Microporous Polymers with Tunable Electronic Structure for High-Performance Potassium-Ion Batteries.

Conjugated microporous polymers (CMPs) with π-conjugated skeletons show great potential as energy storage materials due to their porous structure and tunable redox nature. However, CMPs and the structure-performance relationships have not been well explored for potassium-ion batteries (KIBs). Here, we report the structure-engineered CMP anodes with tunable electronic structures for high-performance KIBs. The results demonstrate that the electronic structure of the CMPs plays an important role in enhancing potassium storage capability, including the lowest unoccupied molecular orbital (LUMO) distribution, LUMO energy level, and band gap, which can be finely tuned by synthetic control. It was revealed that the poly(pyrene- co-benzothiadiazole) (PyBT) with optimized structure delivers a high reversible capacity of 428 mAh g-1 and shows an excellent cycling stability over 500 cycles. Our findings provide a fundamental understanding in the design of CMP anode materials for high-performance potassium-organic energy storage devices.

Spectrum and dissociation properties of fluoro trichloro methane molecule in radiational field

The ozone layer in the stratosphere of the earth’s atmosphere, which can be destroyed by CFC-11 molecule, plays a crucial role in human survival because it can absorb most of the harmful radiation from the sun and effectively protect the earth’s biology. Therefore, it is of evident significance to investigate the properties of CFC-11 molecule. By Motivated by this and the adoption of B3LYP complex function at a level of 6-311++g(3df, 3pd) basis set, we carry out a series of theoretical studies of the Freon material CFC-11 (CFCl3) molecules, including the studies of the equilibrium structure, electric dipole moment, total energy, the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) level, energy gap, infrared and Raman spectrum, C—F dissociation characteristics of CFC-11 molecule, and the effect of the applied electric field on CFC-11 molecule as well. The results show that the maximum error between the theoretical calculation value and the experimental value is less than 2% for an optimized ground state structure; the C—F bond length and C—Cl bond length extend with the increase of electric field intensity, but the degree of change of C—F bond length is much stronger than that of C—Cl; the HOMO energy level and total energy go up and then come down as the external field rises, while the LUMO energy level goes up as the field increases. The energy gap <i>E</i><sub>g</sub> first increases and then decreases with the variation of <i>E</i><sub>H</sub> and <i>E</i><sub>L</sub>. The dipole moment without electric field is a minimum value, and the external electric field leads the molecular polarity to increase and the molecular activity to strengthen. The electric field influences the absorption intensity of infrared and Raman spectrum. The infrared and Raman spectrum move toward the long wave under the action of positive electric field, while they move toward the short wave under the action of negative electric field. The red- or blue-shift of infrared and Raman spectrum occur with the change of electric field. The electric field can be adopted as an auxiliary means to separate the overlapping or quasi-overlapping spectral lines. The potential well depth decreases with the increase of the reverse electric field until it vanishes, which causes the bound state ability of C—F bond of CFC-11 molecule to gradually degrade. This paper is expected to provide a feasible and effective tunable means for the final dissociation and degradation of CFC-11 molecules.

Elucidating the impact of N-arylanilino substituents of squaraines on their photovoltaic performances

Abstract Squaraines have attracted increasing research attention for organic photovoltaic applications because of their facile and low-cost synthesis, and intense and broad absorption in the visible and near-infrared (Vis-NIR) regions. Herein, three new squaraines, N-phenylanilino substituted SQ-EP, N-2-naphthylanilino substituted SQ-EN, and N-fluorenylanilino substituted SQ-EF, were developed for improving the hole mobility of aniline-based squaraines. The substitution effect of N-arylanilino groups on the optoelectronic properties of squaraines was investigated. These squaraines exhibited similar absorbance spectra, HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) energy levels, and hole mobilities in their neat films. However, compared to their neat films, the hole mobility of the SQ-EP-based BHJ blend film showed the smallest decrease when blending with PC71BM ([6,6]-phenyl C71 butyric acid methyl ester). Besides, morphological studies have shown that a phase separation of 20–30 nm can only be observed in the SQ-EP/PC71BM blend film. Consequently, a solution-processed bulk-heterojunction (BHJ) organic photovoltaic (OPV) device fabricated with the as-cast SQ-EP/PC71BM blend film exhibited obviously higher power conversion efficiency (PCE: 5.4%) than those of the SQ-EN/PC71BM (PCE: 4.3%) or SQ-EF/PC71BM systems (PCE: 2.8%). This is one of the highest recorded PCE values in aniline-based squaraines single-junction BHJ-OPV devices. This much-enhanced PCE can be attributed to the significant increases in short-circuit current density (Jsc) and fill factor (FF) of the SQ-EP/PC71BM-based OPV device.

Theoretical Investigation on Flavones and Isoflavones-Added Triphenylamine-Based Sensitizers for DSSC Application

Twelve novel dye molecules developed from D-π-A-based triphenylamine (TPA) dyes were studied to evaluate their suitability for dye-sensitized solar cells (DSSC), using density functional theory (DFT) and time-dependent density functional theory (TDDFT). The effects of flavone and isoflavone on triphenylamine-based dyes were studied. B3LYP and CAM-B3LYP density functionals combined with 6-311G(d,p) basis set were chosen for optimization and TDDFT respectively. Optimization geometry, highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy level, and absorption spectrum were calculated. The electron injection force and light-harvesting efficiency were calculated. We have found HOMO, LUMO energy levels, and electronic absorption spectrum suitable for DSSC application. Based on theoretical calculations, the usefulness of flavones and isoflavones-added triphenylamine dye molecules for DSSC application is examined.

Donor–Acceptor Conjugated Polymers Based on Bisisoindigo: Energy Level Modulation toward Unipolar n-Type Semiconductors

Four bisisoindigo (bis-IID) derivatives containing different numbers of electron withdrawing fluorine (F) or nitrogen (N) atoms and eight alternating copolymers of them with 2,2′-bithophene (BT) or 3,3′,4,4′-tetrafluoro-2,2′-bithiophene (4FBT) were synthesized. Depending on the numbers of F and N atoms incorporated, the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of the polymers were feasibly tuned in the ranges of −5.51 to −6.06 eV and −3.64 to −4.01 eV, respectively. Top gate and bottom contact (TGBC) organic thin-film transistors (OTFTs) were fabricated to study the semiconducting properties of the polymers. Five polymers, i.e., P0F-BT, P2F-BT, P4F-BT, P2N2F-BT, and P0F-4FBT that contain 0–4 F and N atoms in the repeating unit, with HOMO and LUMO energy levels above −5.90 and −3.80 eV, respectively, all showed ambipolar OTFT characteristics. Three other polymers P2F-4FBT, P4F-4FBT, and P2N2F-4FBT that contain 6 or 8 F and N atoms with HOMO ...

SYNTHESIS OF NEW NITROBENZYLIDENE DERIVATIVES AND PYRROLE-BASED COPOLYMERS FOR DYE-SENSITIZED SOLAR CELLS: EFFECT OF SUBSTITUENT ON OPTO-ELECTRICAL PROPERTIES OF DIP-COATED THIN FILMS

The mastery of the optoelectronic properties of conjugated copolymers by substituting their radicals is a promising way for increasing the light absorption and charge career transport in the organic devices active layer. In this paper, we present the chemical synthesis of four different conjugated benzaldehyde derivatives and pyrrole-based copolymers (P–P:B) followed by their conception in thin films on glass substrates by dip coating root from a solution in dichloromethane. UV–Vis measurements showed absorption in good part of the visible region, with an optical gap around 2 eV. Morphological properties observed by scanning electron microscope of the four P–P:B based thin films illustrated homogenous and continuous surfaces with roughness and surface shape that can be modulated according to Nitrobenzylidene derivative that contains the copolymer. First oxidation ([Formula: see text]) and reduction ([Formula: see text]) potentials of synthetized copolymers have been estimated by cyclic voltammetry which led us to estimate the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), respectively. The HOMO and LUMO energy level diagram of P–P:B compared with the most commonly used organic materials as donor/acceptor couples showed a cascade shape, which allowed us to opt for organic solar cells based on multiple active layers in our aim to improve its performance.

Unconventional anode interlayer universally improving solar cell efficiency

Organic photovoltaic cells typically employ an interlayer sandwiched between the active donor or acceptor materials and the respective electrodes. Conventionally, the employed anode interlayer (AIL) adjacent to the anode has a wide optical band gap and a significantly higher lowest unoccupied molecular orbital (LUMO) energy level, compared to the adjunct electron donor material, such as boron subphthalocyanine chloride (SubPc) studied here. In this report, we synthesized three novel AIL materials, NP-β-PCN, NPAPMLI and NPAPMLI, having narrow optical band gaps but matching the LUMO energy levels of SubPc in a planar heterojunction solar cell of ITO/SubPc/C60/BCP/Al. Upon insertion of an ultrathin (2 nm) AIL, the device power conversion efficiency is increased from 3.98% to as high as 4.92%, mainly due to the significant increase of short-circuit current density (JSC) from 5.97 to 7.11–7.65 mA cm−2. From the detailed morphological and photophysical studies, we have demonstrated that the employed unconventional materials of AIL are effective in exciton (of SubPc) blocking, thereby enhancing exciton diffusion towards the charge-separating interface of SubPc/C60 and hence JSC. Since these AIL materials all have a LUMO energy level very close to that of SubPc, the study reported here clarifies that the electron blocking is not a necessary property of an AIL material.

Nonfullerene Acceptor with "Donor-Acceptor Combined π-Bridge" for Organic Photovoltaics with Large Open-Circuit Voltage.

In this work, a kind of "donor-acceptor (D-A) combined π-bridge" based on the regioselective reactivity of monofluoro-substituted benzothiadiazole (FBT) to link a thiophene ring has been designed to construct a new A-π-D-π-A-type small molecular acceptor (IDT-FBTR) with indacenodithiophene (IDT) as a central core (D) and 3-octyl-2-(1,1-dicyanomethylene)rhodanine as an electron-withdrawing terminal group (A). Because of the strong intramolecular push-pull electron effect, the IDT-FBTR shows a strong and broad intramolecular charge-transfer absorption band in the range of 500-750 nm. Especially, as an electron-deficient FBT unit (A') and an electron-rich thiophene ring (D') in "D-A combined π-bridge" exert an "offset effect" to regulate the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) energy levels of the molecule, a relatively high LUMO energy level can be maintained for IDT-FBTR that is helpful to enhance the open-circuit voltage ( Voc) for highly efficient organic solar cells (OSCs). Therefore, the optimized OSC device based on IDT-FBTR as the acceptor and PTB7-Th as the donor shows a much high Voc of 1.02 V with a relatively low Eloss of 0.56 eV and a best power conversion efficiency of 9.14%.

Insights into the mechanism of metal-polymer contact electrification for triboelectric nanogenerator via first-principles investigations

Surface micro/nano structures and materials modification are the major methods to enhance the output performance of triboelectric nanogenerator (TENG). However, the microcosmic mechanism on how they take effect has not been clear yet. Herein, the typical material pair Al-PTFE of TENG is taken as the example to study the mechanism of metal-polymer contact electrification via first-principles investigations and provide theoretical basis for the optimized design of TENG.We demonstrate that the interface barrier of the contact materials is the more fundamental parameter related to contact electrification, rather than the effective work function difference which has been thought to be proportional to the amount of charge transfer. The relationship between charge transfer and interface distance is also investigated. We confirm that charge transfer is significantly affected by the stress on the contact region, and it can even occur without contact. Based on the results, we propose that the surface micro/nano structures design on TENG should aim at making the contact regions in an appropriate stress state instead of only increasing the contact area, and improving the specific surface areas in the effective range of contact electrification via nanostructures for generating extra charge transfer freely.In order to provide theoretical basis for surface modification on contact materials which has been little studied, the direction, driving force and the intrinsic cause of charge transfer are investigated. It is demonstrated that the electrons acceptor is the LUMO (lowest unoccupied molecular orbital) on PTFE surface, and the driving force of charge transfer is the electrostatic attraction generated by the electrons acceptor. In addition, the key role and intrinsic mechanism of the interface chemical bond in PTFE is studied. We propose that the fundamental goal of chemical modification on contact materials should be to lower the LUMO energy level on the surface of electron acceptor materials.

Synthesis and Characterization of a 2‐(1,1‐Dicyanomethylene) rhodanine‐based Nonfullerene Acceptor for OPVs

We synthesized an A–π–D–π–A‐type nonfullerene small molecule acceptor (Flu‐CNRH) based on fluorene and 2‐(1,1‐dicyanomethylene)rhodanine (CNRH) by a Knoevenagel condensation of the diformyl compound (Flu‐T‐CHO) and CNRH for the use as acceptors in organic photovoltaic cells (OPVs). Introduction of CNRH end groups effectively lowered the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of the acceptor compared to those of the corresponding rhodanine‐based acceptor (Flu‐RH). The HOMO and LUMO levels of Flu‐CNRH are −5.71 and −3.60 eV, respectively. For comparison, Flu‐RH has HOMO and LUMO energy levels of −5.58 and −3.53 eV, respectively. This lowered LUMO energy level enables Flu‐CNRH to be combined with low bandgap polymer donors PTB7 and PTB7‐Th by providing proper LUMO offsets between donor and acceptor. OPV cells were fabricated with the device configuration of ITO/PEDOT:PSS/active layer/LiF/Al. The best power conversion efficiency of 1.47% was obtained in the P3HT:Flu‐CNRH devices at a D:A ratio of 1:1.5 and with annealing at 140°C. Moreover, Flu‐CNRH showed compatibility with low bandgap polymer donor PTB7 (1.22%) with external quantum efficiency response at a longer wavelength region up to 750 nm, while Flu‐RH had no photovoltaic behavior with PTB7 and PTB7‐Th.

Advances in the Synthesis of Small Molecules as Hole Transport Materials for Lead Halide Perovskite Solar Cells.

Over hundreds of new organic semiconductor molecules have been synthesized as hole transport materials (HTMs) for perovskite solar cells. However, to date, the well-known N2, N2, N2', N2', N7, N7, N7', octakis-(4-methoxyphenyl)-9,9-spirobi-[9,9'-spirobi[9 H-fluorene]-2,2',7,7'-tetramine (spiro-OMeTAD) is still the best choice for the best perovskite device performance. Nevertheless, there is a consensus that spiro-OMeTAD by itself is not stable enough for long-term stable devices, and its market price makes its use in large-scale production costly. Novel synthetic routes for new HTMs have to be sought that can be carried out in fewer synthetic steps and can be easily scaled up for commercial purposes. On the one hand, synthetic chemists have taken, as a first approach, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the spiro-OMeTAD molecule as a reference to synthesize molecules with similar energy levels, although these HOMO and LUMO energy levels often have been measured indirectly in solution using cyclic voltammetry. On the other hand, the "spiro" chemical core has also been studied as a structural motif for novel HTMs. However, only a few molecules incorporated as HTMs in complete functional perovskite solar cells have been capable of matching the performance of the best-performing perovskite solar cells made using spiro-OMeTAD. In this Account, we describe the advances in the synthesis of HTMs that have been tested in perovskite solar cells. The comparison of solar cell efficiencies is of course very challenging because the solar cell preparation conditions may differ from laboratory to laboratory. To extract valuable information about the HTM molecular structure-device function relationship, we describe those examples that always have used spiro-OMeTAD as a control device and have always used identical experimental conditions (e.g., the use of the same chemical dopant for the HTM or the lack of it). The pioneering work was focused on well-understood organic semiconductor moieties such as arylamine, carbazole, and thiophene. Those chemical structures have been largely employed and studied as HTMs, for instance, in organic light-emitting devices. Interestingly, most research groups have reported the hole mobility values for their novel HTMs. However, only a few examples have been found that have measured the HOMO and LUMO energy levels using advanced spectroscopic techniques to determine these reference energy values directly. Moreover, it has been shown that those molecules, upon interacting with the perovskite layer, often have different HOMO and LUMO energies than the values estimated indirectly using solution-based electrochemical methods. Last but not least, porphyrins and phthalocyanines have also been synthesized as potential HTMs for perovskite solar cells. Their optical and physical properties, such as high absorption and good energy transfer capabilities, open new possibilities for HTMs in perovskite solar cells.