Lowest Unoccupied Molecular Orbital Energy Levels Research Articles

Solution-Processable Triindoles as Hole Selective Materials in Organic Solar Cells

We report the use of two solution-processable triindoles, triazatruxene (TAT), and N-trimethyltriindole (TMTI), as hole selective materials in organic solar cells. The unique optical and electronic properties of these molecules make them suitable as a hole extracting/electron blocking layer, i.e. transparency in the visible region due to a wide bandgap, high LUMO (lowest unoccupied molecular orbital) energy level, modest HOMO (highest occupied molecular orbital) level, and high hole carrier mobility. TAT is shown to have a LUMO at -1.68 eV, a HOMO at -5.03 eV, and a bandgap of 3.35 eV, whereas TMTI has a LUMO at -2.05 eV, a HOMO at -5.1 eV, and a bandgap of 3.05 eV, obtained from cyclic voltammetry measurements and absorption spectroscopy. Planar heterojunction photovoltaic devices, consisting of a solution processed transparent TAT (or TMTI) layer and a vapor-deposited C60 layer, exhibited efficiencies of up to 0.71 % (or 0.87 %). In these bilayer devices, the excitons are primarily generated in the C60 layer and undergo dissociation in the interfaces via hole transfer from the C60 layer to the TAT (or TMTI) layer. Additionally, spin-casting methanol solution of TAT on the top of P3HT:PCBM bulk heterojunction in an inverted device produced a hole selective interfacial layer between the photoactive layer and anode, leading to a 26% efficiency increase as compared to a control device without the TAT layer.

Highly Efficient and Thermally Stable Polymer Solar Cells with Dihydronaphthyl‐Based [70]Fullerene Bisadduct Derivative as the Acceptor

AbstractThe efficiency of polymer solar cells (PSCs) can be essentially enhanced by improving the performance of electron‐acceptor materials, including by increasing the lowest unoccupied molecular orbital (LUMO) level, improving the optical absorption, and tuning the material solubility. Here, a new soluble C70 derivative, dihydronaphthyl‐based C70 bisadduct (NC70BA), is synthesized and explored as acceptor in PSCs. The NC70BA has high LUMO energy level that is 0.2 eV higher than [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM), and displays broad light absorption in the visible region. Consequently, the PSC based on the blend of poly(3‐hexylthiophene) (P3HT) and NC70BA shows a high open‐circuit voltage (Voc = 0.83 V) and a high power conversion efficiency (PCE = 5.95%), which are much better than those of the P3HT:PCBM‐based device (Voc = 0.60 V; PCE = 3.74%). Moreover, the amorphous nature of NC70BA effectively suppresses the thermally driven crystallization, leading to high thermal stability of the P3HT:NC70BA‐based solar cell devices. It is observed that the P3HT:NC70BA‐based device retains 80% of its original PCE value against thermal heating at 150 °C over 20 h. The results unambiguously indicate that the NC70BA is a promising acceptor material for practical PSCs.

Dithienocarbazole‐Based Ladder‐Type Heptacyclic Arenes with Silicon, Carbon, and Nitrogen Bridges: Synthesis, Molecular Properties, Field‐Effect Transistors, and Photovoltaic Applications

AbstractA new class of ladder‐type dithienosilolo‐carbazole (DTSC), dithienopyrrolo‐carbazole (DTPC), and dithienocyclopenta‐carbazole (DTCC) units is developed in which two outer thiophene subunits are covalently fastened to the central 2,7‐carbazole cores by silicon, nitrogen, and carbon bridges, respectively. The heptacyclic multifused monomers are polymerized with the benzothiadiazole (BT) acceptor by palladium‐catalyzed cross‐coupling to afford three alternating donor‐acceptor copolymers poly(dithienosilolo‐carbazole‐alt‐benzothiadiazole) (PDTSCBT), poly(dithienocyclopenta‐carbazole‐alt‐benzothiadiazole) (PDTCCBT), and poly(dithienopyrrolo‐carbazole‐alt‐benzothiadiazole) (PDTPCBT). The silole units in DTSC possess electron‐accepting ability that lowers the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of PDTSCBT, whereas stronger electron‐donating ability of the pyrrole moiety in DTPC increases the HOMO and LUMO energy levels of PDTPCBT. The optical bandgaps (Egopt) deduced from the absorption edges of thin film spectra are in the following order: PDTSCBT (1.83 eV) > PDTCCBT (1.64 eV) > PDTPCBT (1.50 eV). This result indicated that the donor strength of the heptacyclic arenes is in the order: DTPC > DTCC > DTSC. The devices based on PDTSCBT and PDTCCBT exhibited high hole mobilities of 0.073 and 0.110 cm2 V−1 s−1, respectively, which are among the highest performance from the OFET devices based on the amorphous donor‐acceptor copolymers. The bulk heterojunction photovoltaic device using PDTSCBT as the p‐type material delivered a promising efficiency of 5.2% with an enhanced open circuit voltage, Voc, of 0.82 V.

Molecular Design of Photovoltaic Materials for Polymer Solar Cells: Toward Suitable Electronic Energy Levels and Broad Absorption

Bulk heterojunction (BHJ) polymer solar cells (PSCs) sandwich a blend layer of conjugated polymer donor and fullerene derivative acceptor between a transparent ITO positive electrode and a low work function metal negative electrode. In comparison with traditional inorganic semiconductor solar cells, PSCs offer a simpler device structure, easier fabrication, lower cost, and lighter weight, and these structures can be fabricated into flexible devices. But currently the power conversion efficiency (PCE) of the PSCs is not sufficient for future commercialization. The polymer donors and fullerene derivative acceptors are the key photovoltaic materials that will need to be optimized for high-performance PSCs. In this Account, I discuss the basic requirements and scientific issues in the molecular design of high efficiency photovoltaic molecules. I also summarize recent progress in electronic energy level engineering and absorption spectral broadening of the donor and acceptor photovoltaic materials by my research group and others. For high-efficiency conjugated polymer donors, key requirements are a narrower energy bandgap (E(g)) and broad absorption, relatively lower-lying HOMO (the highest occupied molecular orbital) level, and higher hole mobility. There are three strategies to meet these requirements: D-A copolymerization for narrower E(g) and lower-lying HOMO, substitution with electron-withdrawing groups for lower-lying HOMO, and two-dimensional conjugation for broad absorption and higher hole mobility. Moreover, better main chain planarity and less side chain steric hindrance could strengthen π-π stacking and increase hole mobility. Furthermore, the molecular weight of the polymers also influences their photovoltaic performance. To produce high efficiency photovoltaic polymers, researchers should attempt to increase molecular weight while maintaining solubility. High-efficiency D-A copolymers have been obtained by using benzodithiophene (BDT), dithienosilole (DTS), or indacenodithiophene (IDT) donor unit and benzothiadiazole (BT), thienopyrrole-dione (TPD), or thiazolothiazole (TTz) acceptor units. The BDT unit with two thienyl conjugated side chains is a highly promising unit in constructing high-efficiency copolymer donor materials. The electron-withdrawing groups of ester, ketone, fluorine, or sulfonyl can effectively tune the HOMO energy levels downward. To improve the performance of fullerene derivative acceptors, researchers will need to strengthen absorption in the visible spectrum, upshift the LUMO (the lowest unoccupied molecular orbital) energy level, and increase the electron mobility. [6,6]-Phenyl-C(71)-butyric acid methyl ester (PC(70)BM) is superior to [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM) because C(70) absorbs visible light more efficiently. Indene-C(60) bisadduct (ICBA) and Indene-C(70) bisadduct (IC(70)BA) show 0.17 and 0.19 eV higher LUMO energy levels, respectively, than PCBM, due to the electron-rich character of indene and the effect of bisadduct. ICBA and IC(70)BA are excellent acceptors for the P3HT-based PSCs.

Three-carbazole-armed host materials with various cores for RGB phosphorescent organic light-emitting diodes

A series of three-carbazole-armed host materials containing various arylene cores, like benzene (1,3,5-tris(3-(carbazol-9-yl)phenyl)-benzene, TCPB), pyridine (2,4,6-tris(3-(carbazol-9-yl)phenyl)-pyridine, TCPY), and pyrimidine (2,4,6-tris(3-(carbazol-9-yl)phenyl)-pyrimidine, TCPM), were developed for red, green, and blue phosphorescent organic light-emitting diodes (OLEDs). An intramolecular charge transfer was observed for TCPY and TCPM with heterocyclic cores of pyridine and pyrimidine, giving bathochromic shifts in the photoluminescent spectrum and reduced energy band gaps in comparison with TCPB with a benzene core. In addition, lower energy singlet and triplet excited states, reduced lowest unoccupied molecular orbital (LUMO) energy level, smaller singlet–triplet exchange energy (ΔEST), and improved bipolarity were also achieved with introducing heterocycles of pyridine and pyrimidine instead of benzene. In contrast to the slightly decreased triplet energy (ET), a significantly decreased ΔEST was achieved by introducing heterocycles of pyridine and pyrimidine as the core, and the more nitrogen atoms in the central heterocycle, the smaller ΔEST is achieved. Reduced driving voltages were achieved for the green and red phosphorescent OLEDs by utilizing TCPY and TCPM as the host due to their decreased ΔEST and lower-lying LUMO energy level, proving that more carriers must be injected into the emitting layer through the host molecules rather than direct carrier trapping by the dopant. Moreover, improved efficiency and suppressed efficiency roll-off were also achieved for the green and red phosphorescent OLEDs based on TCPY and TCPM due to their improved bipolarity and thus improved carrier balance.

Di(4-methylphenyl)methano-C60 Bis-Adduct for Efficient and Stable Organic Photovoltaics with Enhanced Open-Circuit Voltage

A new class of fullerene bis-adducts—di(4-methylphenyl)methano-C60 bis-adduct (DMPCBA), di(4-fluorophenyl)methano-C60 bis-adduct (DFPCBA), and diphenylmethano-C60 bis-adduct (DPCBA)—were rationally designed and easily synthesized. Compared to the lowest unoccupied molecular orbital (LUMO) energy level of PC61BM (−3.95 eV), the double functionalization effectively raises the LUMO energy levels of these fullerene materials to ca. −3.85 eV, regardless of the substituent groups (CH3–, F–, and H−) at the para-position of the phenyl rings. This phenomenon suggests that the plane of the phenyl groups is preferentially parallel to the fullerene surface, leading to poor orbital interactions with C60 and negligible electronic effect. Importantly, such geometry sterically protects and shields the core C60 structure from severe intermolecular aggregation, rendering it intrinsically soluble, morphologically amorphous, and thermally stable. The device based on the P3HT:DMPCBA blend exhibited an open-circuit voltage (Vo...

Electron-Transporting Oligothiophenes Containing Dicyanomethylene-Substituted Cyclopenta[b]thiophene: Chemical Tuning for Air Stability in OFETs

We have synthesized new electron-transporting oligothiophenes containing dicyanomethylene-substituted cyclopenta[b]thiophene as an active material for the fabrication of solution-processable n-type organic field-effect transistors (OFETs). The influence of the number of dicyanomethylene groups as well as the position of hexyl groups was investigated in detail by performing photophysical and electrochemical measurements. Results revealed that the optical energy gaps and the lowest unoccupied molecular orbital (LUMO) energy levels can be controlled by changing the number of dicyanomethylene groups. In contrast, the position of hexyl groups has little influence on molecular electronic properties. X-ray diffraction and atomic force microscopy measurements revealed that spin-coated thin films of the new compounds had a crystalline structure. OFETs based on these compounds were evaluated in vacuum and air-exposed conditions, and the electron mobility of up to 0.016 cm(2) V(-1) s(-1) was achieved. Furthermore, we demonstrated that the air stability of the OFETs depends on the LUMO energy level of the compounds.

Alternating Copolymers of Cyclopenta[2,1‐b;3,4‐b′]dithiophene and Thieno[3,4‐c]pyrrole‐4,6‐dione for High‐Performance Polymer Solar Cells

AbstractA series of alternating copolymers of cyclopenta[2,1‐b;3,4‐b′]dithiophene (CPDT) and thieno[3,4‐c]pyrrole‐4,6‐dione (TPD) have been prepared and characterized for polymer solar cell (PSC) applications. Different alkyl side chains, including butyl (Bu), hexyl (He), octyl (Oc), and 2‐ethylhexyl (EH), are introduced to the TPD unit in order to adjust the packing of the polymer chain in the solid state, while the hexyl side chain on the CPDT unit remains unchanged to simplify discussion. The polymers in this series have a simple main chain structure and can be synthesized easily, have a narrow band gap and a broad light absorption. The different alkyl chains on the TPD unit not only significantly influence the solubility and chain packing, but also fine tune the energy levels of the polymers. The polymers with Oc or EH group have lower HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) energy levels, resulting higher open circuit voltages (Voc) of the PSC devices. Power conversion efficiencies (PCEs) up to 5.5% and 6.4% are obtained from the devices of the Oc substituted polymer (PCPDTTPD‐Oc) with PC61BM and PC71BM, respectively. This side chain effect on the PSC performance is related to the formation of a fine bulk heterojunction structure of polymer and PCBM domains, as observed with atomic force microscopy.

BODIPY-Based Donor–Acceptor π-Conjugated Alternating Copolymers

Four novel π-conjugated copolymers incorporating 4,4-difluoro-4-borata-3a-azonia-4a-aza-s-indacene (BODIPY) core as the “donor” and quinoxaline (Qx), 2,1,3-benzothiadiazole (BzT), N,N′-di(2′-ethyl)hexyl-3,4,7,8-naphthalenetetracarboxylic diimide (NDI), and N,N′-di(2′-ethyl)hexyl-3,4,9,10-perylene tetracarboxylic diimide (PDI) as acceptors were designed and synthesized via Sonogashira polymerization. The polymers were characterized by 1H NMR spectroscopy, gel permeation chromatography (GPC), UV–vis absorption spectroscopy, and cyclic voltammetry. Density functional theory (DFT) calculations were performed on polymer repeat units, and the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels were estimated from the optimized geometry using B3LYP functional and 6-311g(d,p) basis set. Copolymers with Qx and BzT possessed HOMO and LUMO energy levels comparable to those of BODIPY homopolymer, while PDI stabilized both HOMO and LUMO levels. Semiconductor behav...

Synthesis and photovoltaic properties of low-bandgap 4,7-dithien-2-yl-2,1,3-benzothiadiazole-based poly(heteroarylenevinylene)s

Three novel low-bandgap copolymers containing alkylated 4,7-dithien-2-yl-2,1,3-benzothiadiazole (HBT) and different electron-rich functional groups (dialkylfluorene (PFV-HBT), dialkyloxyphenylene (PPV-HBT) and dialkylthiophene (PTV-HBT)) were prepared by Horner polycondensation reactions and characterized by 1H NMR, gel permeation chromatography, and elemental analysis. The alkyl side chain brings these polymeric materials good solubility in common organic solvents, which is critical for the manufacture of solar cells in a cost-effective manner. The copolymers exhibit low optical bandgap from 1.48 to 1.83 eV. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the copolymers were measured by cyclic voltammetry. Theoretical calculations revealed that the variation laws of HOMO and the LUMO energy levels are well consistent with cyclic voltammetry measurement. The bulk heterojunction photovoltaic devices with the structure of ITO/PEDOT-PSS/polymer:PCBM/LiF/Al were fabricated by using the three copolymers as the donor and (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) as the acceptor in the active layer. The device based on PTV-HBT:PCBM (1:4 w/w) achieved a power conversion efficiency of 1.05% under the illumination of AM 1.5, 100 mW/cm2. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011.

Bipolar Heteroleptic Green Iridium Dendrimers Containing Oligocarbazole and Oxadiazole Dendrons for Bright and Efficient Nondoped Electrophosphorescent Devices

Bipolar heteroleptic green light-emitting iridium (Ir) dendrimers G(OXD) and G(DOXD) have been designed and synthesized under mild conditions in high yields, in which the first C^N and second O^O ligands are functionalized with oligocarbazole- and oxadiazole-based dendrons, respectively. To avoid affecting the optical properties of the emissive iridium core, all the functional moieties are attached to the ligands through a flexible spacer. Compared with the unipolar dendrimer G(acac), dendrimers G(OXD) and G(DOXD) exhibit the close emission maxima of 511-512 nm and photoluminescence quantum yield of 0.39-0.40 in a solution of toluene. Moreover, on going from G(acac) to G(OXD) and G(DOXD), we have found that the introduction of oxadiazole fragments decreases the lowest unoccupied molecular orbital (LUMO) energy levels to facilitate the electron injection and electron transporting, while their highest occupied molecular orbital (HOMO) energy levels remain unchanged. This means that, we can individually tune the HOMO and LUMO energy levels based on the heteroleptic structure to ensure the relative independence between the hole and electron in the emitting layer (EML), which is a favorable feature for bipolar optoelectronic materials. As a result, a bilayer nondoped electrophosphorescent device with G(DOXD) as the EML gives a maximum luminous efficiency of 25.5 cd A(-1) (η(ext): 7.4%) and a brightness of 33,880 cd m(-2). In comparison to G(acac) (17.2 cd A(-1), 17,680 cd m(-2)), both the efficiency and brightness are improved by about 1.5 and 2 times, respectively. These state-of-the-art performances indicate the potential of these bipolar heteroleptic iridium dendrimers as solution-processible emitting materials for nondoped device applications.

Air‐Stable n‐Type Organic Field‐Effect Transistors Based on Solution‐Processable, Electronegative Oligomers Containing Dicyanomethylene‐Substituted Cyclopenta[b]thiophene

Solution-processable, electronegative, π-conjugated systems containing dicyanomethylene-substituted cyclopenta[b]thiophene were synthesized as potential active materials for air-stable n-type organic field-effect transistors (OFETs). Electrochemical measurements revealed that these compounds exhibited electrochemical stability and that the lowest unoccupied molecular orbital (LUMO) had an energy level less than -4.0 eV. Flash-photolysis time-resolved microwave conductivity (FP-TRMC) measurements were performed, and the value of intradomain electron mobility was determined to be as high as 0.1 cm(2) V(-1) s(-1) . The OFETs were fabricated by spin-coating thin films of the compounds as an active layer. The electron mobility of the OFETs was 3.5×10(-3) cm(2) V(-1) s(-1) in vacuum. Furthermore, electron mobility of the same order of magnitude and stable characteristics were obtained under air-exposed conditions. X-ray diffraction measurements of the spin-coated thin films revealed the difference of molecular arrangements depending on the inner conjugated units. Atomic force microscopy measurements of crystalline-structured films exhibited the formation of grains. The accomplishment of air-stability was attributed to the combined effect of the low-lying LUMO energy level and the molecular arrangements in the solid state, avoiding both the quenching of electron carriers and the intrusion of oxygen and/or moisture.

Cyclopentadithiophene-Based Organic Semiconductors: Effect of Fluorinated Substituents on Electrochemical and Charge Transport Properties

Thiophene-based semiconductors are often hole conductors that have been converted to electron-transporting materials by incorporation of electron-withdrawing groups at terminal positions, such as fluorinated substituents. This conversion of an otherwise p-type material to n-type material is often attributed to the lowering of the lowest unoccupied molecular orbital (LUMO) energy level due to the increased electron affinity in the molecule. Yet, it is not clear if lowering of LUMO energy level is a sufficient condition for yielding n-type material. Herein, we report small-molecule semiconductors based on cyclopentadithiophene (CPD), which can be orthogonally functionalized at two different positions, which allows us to tune the frontier orbital energy levels. We find that simply lowering the LUMO energy level, without inclusion of fluoro groups, does not result in conversion of the otherwise p-type material to n-type material, whereas incorporation of fluorinated substituents does. This indicates that char...

A Simple and Effective Modification of PCBM for Use as an Electron Acceptor in Efficient Bulk Heterojunction Solar Cells

Abstract[6,6]‐phenyl‐C‐61‐butyric acid methyl ester (PCBM) and poly(3‐hexylthiophene) (P3HT) are the most widely used acceptor and donor materials, respectively, in polymer solar cells (PSCs). However, the low LUMO (lowest unoccupied molecular orbital) energy level of PCBM limits the open circuit voltage (Voc) of the PSCs based on P3HT. Herein a simple, low‐cost and effective approach of modifying PCBM and improving its absorption is reported which can be extended to all fullerene derivatives with an ester structure. In particular, PCBM is hydrolyzed to carboxylic acid and then converted to the corresponding carbonyl chloride. The latter is condensed with 4‐nitro‐4’‐hydroxy‐α‐cyanostilbene to afford the modified fullerene F. It is more soluble than PCBM in common organic solvents due to the increase of the organic moiety. Both solutions and thin films of F show stronger absorption than PCBM in the range of 250–900 nm. The electrochemical properties and electronic energy levels of F and PCBM are measured by cyclic voltammetry. The LUMO energy level of F is 0.25 eV higher than that of PCBM. The PSCs based on P3HT with F as an acceptor shows a higher Voc of 0.86 V and a short circuit current (Jsc) of 8.5 mA cm−2, resulting in a power conversion efficiency (PCE) of 4.23%, while the PSC based on P3HT:PCBM shows a PCE of about 2.93% under the same conditions. The results indicate that the modified PCBM, i.e., F, is an excellent acceptor for PSC based on bulk heterojunction active layers. A maximum overall PCE of 5.25% is achieved with the PSC based on the P3HT:F blend deposited from a mixture of solvents (chloroform/acetone) and subsequent thermal annealing at 120 °C.

Influence of Sulfur Oxidation on the Absorption and Electronic Energy Levels of Poly(thienothiophene) Derivatives

Two poly(thienothiophene) derivatives containing thieno[3,2-b]thiophene-4,4-dioxide unit were synthesized by Pd-catalyzed Stille coupling method. They were poly(3,6-dihexyl-thieno[3,2-b]thiophene-4,4-dioxide vinylene) (P2) and poly(2,5-diyl-3,6-dihexyl-thieno[3,2-b]thiophene-4,4-dioxide)-co-(2,5-diyl-thiophene) (P4). Poly(3,6-dihexyl-thieno[3,2-b] thiophene vinylene) (P1) and poly(2,5-diyl-3,6-dihexyl-thieno[3,2-b] thiophene)-co-(2,5-diyl-thiophene) (P3) were synthesized for comparison with P2 and P4. After sulfur oxidation on the thienothiophene units, the absorption peaks of the polymer solutions were red-shifted from 540 nm of P1 to 625 nm of P2 and from 445 nm of P3 to 520 nm of P4. The absorption peaks of the polymer films were red-shifted more significantly from 542 nm of P1 to 630 nm of P2 and from 480 nm of P3 to 564 nm of P4. The lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy levels also decreased a lot after the sulfur oxidation. In comparison with P1, the LUMO and HOMO energy levels of P2 decreased by 0.59 and 0.35 eV, respectively. The levels were 0.87 and 0.39 eV lower in the LUMO and HOMO energy levels of P4 than in that of P3.

Programmable digital memory devices based on nanoscale thin films of a thermallydimensionally stable polyimide

We have fabricated electrically programmable memory devices with thermally and dimensionally stablepoly(N-(N′,N′-diphenyl-N′-1,4-phenyl)-N,N-4,4′-diphenylene hexafluoroisopropylidene-diphthalimide) (6F-2TPA PI) films and investigated theirswitching characteristics and reliability. 6F-2TPA PI films were found to reveal a conductivity of1.0 × 10−13–1.0 × 10−14 S cm−1. The 6F-2TPA PI films exhibit versatile memory characteristics that depend on the filmthickness. All the PI films are initially present in the OFF state. The PI films with a thickness of>15 to<100 nm exhibit excellent write-once-read-many-times (WORM) (i.e. fuse-type)memory characteristics with and without polarity depending on the thickness.The WORM memory devices are electrically stable, even in air ambient,for a very long time. The devices’ ON/OFF current ratio is high, up to1010. Therefore, these WORM memory devices can provide an efficient, low-costmeans of permanent data storage. On the other hand, the 100 nm thick PI filmsexhibit excellent dynamic random access memory (DRAM) characteristicswith polarity. The ON/OFF current ratio of the DRAM devices is as high as1011. The observed electrical switching behaviors were found to be governed by trap-limitedspace-charge-limited conduction and local filament formation and further dependent on thedifferences between the highest occupied molecular orbital and the lowest unoccupiedmolecular orbital energy levels of the PI film and the work functions of the top and bottomelectrodes as well as the PI film thickness. In summary, the excellent memory properties of6F-2TPA PI make it a promising candidate material for the low-cost mass production ofhigh density and very stable digital nonvolatile WORM and volatile DRAM memorydevices.

A Comparison of Indoline Dyes as Photosensitizers in Dye-Sensitized Solar Cells

Properties of four indoline dyes were studied by means of density functional theory (DFT) and time- dependent density functional theory (TD-DFT) with the goal of finding an excellent photosensitizer for use in dye-sensitized solar cells. Theoretical results showed that the frontier molecular orbital structures of indoline dyes are suitable for electron injection from the excited states of indoline dyes to a TiO2 electrode. Calculated UV-visible absorption spectra of indoline dyes in vacuum match well with solar radiation spectra. The calculated energy levels of these dye molecules demonstrate that indoline dyes can be used as photosensitizers for TiO2 nanocrystalline solar cells together with the I-/I3- electrolyte. The lowest unoccupied molecular orbital (LUMO) energy levels of indoline dyes are higher than the conduction band edge of the TiO2 crystal, which ensures a high efficiency of electron transfer from indoline dyes to TiO2 electrodes. As the highest occupied molecular orbital (HOMO) energy levels of indoline dyes are lower than those of I-/I3- , molecules that donated electrons can receive electrons from the electrolyte. By comparison with the experimental data, the transfer efficiency of dye-sensitized solar cells may be determined mainly by the LUMO energy levels. The working lifetime of a dye-sensitized solar cell depends mainly on the stability of the dye molecule. From an analysis of the bond length of chemical bonds, we find that the stability of the four indoline dye molecules is basically the same. We further show that indoline dye 1 (ID1) has the highest LUMO energy level and the highest molecular stability. Its absorption spectra match solar radiation spectra well in an ethanol solution, therefore, it is the best photosensitizer among the analyzed dyes for application in dye-sensitized solar cells.

Synthesis and characterization of poly(fluorene vinylene) copolymers containing thienylene–vinylene units

AbstractNarrow‐band‐gap 2,5‐thienylene‐divinylene (ThV) units were incorporated into the poly(fluorene vinylene) backbone via a Gilch reaction as an energy trap with various feed ratios; this yielded pronounced changes in the electrochemical and optical properties of the material. The energy levels of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the polymers {poly(9,9‐di‐iso‐octylfluorene vinylene) [poly(fluorene vinylene‐co‐thiophene vinylene (FV))], C1, and C2} were estimated to be −5.53 to −5.10 eV and −2.98 to −2.84 eV, respectively, by cyclic voltammetry measurements. In comparison with poly(FV), the HOMO energy levels of polymers poly(fluorene vinylene‐co‐thiophene vinylene (FV) (90 : 10) (C1) and poly(fluorene vinylene‐co‐thiophene vinylene (FV) (80 : 20) (C2) were significantly increased, but their LUMO energy levels were slightly decreased. The optical properties were investigated by absorption and emission spectra of the polymers. The good spectral overlap between the emission of poly(FV) and the absorption of polymers C1 and C2 revealed a sufficient energy transfer from the majority of 9,9‐di‐iso‐octylfluorene vinylene units to the minority of ThV units. The reduction of self‐absorption losses of polymers C1 and C2 due to spectral separation caused by the incorporation of ThV units could be indirectly confirmed by nonlinear optical (NLO) properties. The result of the NLO properties of the polymers showed that the third‐order NLO coefficients of poly(FV), C1, and C2 were 8.1 × 10−10, 1.35 × 10−9, and 1.51 × 10−9 esu, respectively. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Donor-π-Acceptor Conjugated Copolymers for Photovoltaic Applications: Tuning the Open-Circuit Voltage by Adjusting the Donor/Acceptor Ratio

A class of new conjugated copolymers containing a donor (thiophene)-acceptor (2-pyran-4-ylidene-malononitrile) was synthesized via Stille coupling polymerization. The resulting copolymers were characterized by 1H NMR, elemental analysis, GPC, TGA, and DSC. UV-vis spectra indicated that the increase in the content of the thiophene units increased the interaction between the polymer main chains to cause a red-shift in the optical absorbance. Cyclic voltammetry was used to estimate the energy levels of the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) and the band gap (Eg) of the copolymers. The basic electronic structures of the copolymers were also studied by DFT calculations with the GGA/B3LYP function. Both the experimental and the calculated results indicated an increase in the HOMO energy level with increasing the content of thiophene units, whereas the corresponding change in the LUMO energy level was much smaller. Polymer photovoltaic cells of a bulk heterojunction were fabricated with the structure of ITO/PEDOT/PSS (30 nm)/copolymer-PCBM blend (70 nm)/Ca (8 nm)/Al (140 nm). It was found that the open-circuit voltage (Voc) increased (up to 0.93 V) with a decrease in the content of thiophene units. Although the observed power convention efficiency is still relatively low (up to 0.9%), the corresponding low fill factor (0.29) indicates considerable room for further improvement in the device performance. These results provided a novel concept for developing high Voc photovoltaic cells based on donor-pi-acceptor conjugated copolymers by adjusting the donor/acceptor ratio.

Energy gaps in zero-dimensional graphene nanoribbons

The finite size effects on the electronic structure of graphene ribbons are studied using first principles density functional techniques. The energy gap [difference between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)] dependence for finite width and length is computed for both armchair and zigzag ribbons and compared to their one-dimensional (infinite length) cases. The results suggest, in addition to quantum confinement along the width of the ribbon, an additional finite size effect emerges along the length of ribbons only for metallic armchair ribbons. The origin of additional quantum confinement in these structures is analyzed based on the energy states near the Fermi energy: both HOMO and LUMO energy levels for metallic armchair ribbons are delocalized entirely on the ribbons while for nonmetallic ribbons, these states are localized at the edges only. The results are discussed in light of effect of passivation on the electronic properties of graphenes and their impact on nanoelectronic devices based on graphenes.