Abstract: The open-circuit voltage (VOC) of bulk-heterojunction solar cells based on [6,6]-phenyl C61-butyric acid methyl ester (PCBM) as electron acceptor and poly[2-methoxy-5(3′,7′-dimethyloctyloxy)-p-phenylene vinylene] (OC1C10-PPV) as an electron donor has been investigated. In contrast to the present understanding, it is now demonstrated that for non-ohmic contacts the experimental VOC is determined by the work function difference of the electrodes. A total variation of more than 0.5 V of the VOC was observed by variation of the negative electrode (cathode) work function. For ohmic contacts the VOC is governed by the LUMO and HOMO levels of the acceptor and donor, respectively, which pin the Fermi levels of the cathode and anode. The band bending created by accumulated charges at an ohmic contact produce a considerable loss in VOC of 0.2 V at room temperature. The experimentally observed voltage loss in VOC of 0.38 V due to the presence of ohmic contacts at both interfaces strongly limits the maximum open-circ...

Abstract: High-quality ZnO rods were formed directly on sapphire (0001) substrates by metalorganic chemical vapor deposition. The rods exhibited free exciton and very sharp bound exciton emissions at low temperatures. By increasing the excitation intensity, biexciton emission was observed. Temperature dependence of the emission spectra suggested that the emission peak at ∼3.315 eV, which had been attributed to neutral acceptor-bound exciton emission, is due to donor-acceptor pairs. The acceptor binding energy was determined to be about 107 meV, which agrees well with that estimated from a hydrogen-atom-like acceptor model.

Abstract: Although doping of a conjugated polymer by electron acceptors strongly facilitates exciton dissociation into geminate pairs of carriers, the yield of free carrier photogeneration can be high only at high doping levels, that is, in polymer/acceptor blends. We suggest a model that explains how excitons can efficiently dissociate into free carriers at an intrinsic polymer/acceptor interface despite the Coulomb interaction between the charges within precursor geminate pairs.

Abstract: Mg-doped AlN epilayers were grown by metalorganic chemical vapor deposition on sapphire substrates. Deep UV picosecond time-resolved photoluminescence (PL) spectroscopy has been employed to study the optical transitions in Mg-doped AlN epilayers. From PL emission spectra and the temperature dependence of the PL emission intensity, a binding energy of 0.51 eV for Mg acceptor in AlN was determined. Together with previous experimental results, the Mg acceptor activation energy in AlxGa1−xN as a function of the Al content (x) was extrapolated for the entire AlN composition range. The average hole effective mass in AlN was also deduced to be about 2.7 m0 from the experimental value of the Mg binding energy together with the use of the effective mass theory.

Abstract: The photophysical properties of a new dyad molecule composed of a covalently linked Zn-phthalocyanine (antenna/donor) and a C60 derivative (acceptor) have been investigated. We report experimental evidence of long-lived charge separation in the solid state with a lifetime several orders of magnitude higher than in solution. Such a long lifetime, unusual for phthalocyanine–fullerene dyads, is the basis for possible photovoltaic applications. A first demonstration of a working solar cell using phthalocyanine–fullerene dyads as the active material is presented. Though the power conversion efficiency under simulated solar illumination of 80 mW cm−2 is found to be moderate (0.02%), it is an encouraging result for application of C60 dyad molecules to photovoltaics.

Abstract: A Suzuki polycondensation reaction has been used to synthesize two copolymers consisting of alternating oligo(p-phenylene vinylene) (OPV) donor and perylene bisimide (PERY) acceptor chromophores. The copolymers differ by the length of the saturated spacer that connects the OPV and PERY units. Photoinduced singlet energy transfer and photoinduced charge separation in these polychromophores have been studied in solution and in the solid state via photoluminescence and femtosecond pump-probe spectroscopy. In both polymers a photoinduced electron transfer occurs within a few picoseconds after excitation of the OPV or the PERY chromophore. The electron transfer from OPV excited state competes with a singlet energy transfer state to the PERY chromophore. The differences in rate constants for the electron- and energy-transfer processes are discussed on the basis of correlated quantum-chemical calculations and in terms of conformational preferences and folding of the two polymers. In solution, the lifetime of the charge-separated state is longer than in the films where geminate recombination is much faster. However, in the films some charges are able to escape from geminate recombination and diffuse away and can be collected at the electrodes when the polymers are incorporated in a photovoltaic device.

Abstract: The synthesis, structure, and fluorescence properties of a series of new donor–π–acceptor (D–π–A) type compounds, with a trivalent boron, protected by two mesityl groups, as acceptor, and with various typical donors and different π-conjugated bridges, are reported All these stable organoboron compounds show intense single-photon excited fluorescence (SPEF) and two-photon excited fluorescence (TPEF) in a wide spectral range from blue to green, with the spectral peak position of the SPEF being basically the same as that of the TPEF The remarkably strong CB(mesityl)2 bonding, and the well-conjugated π-system, shown in X-ray crystal structures of two compounds, indicate some charge transfer features of the ground state Meanwhile, spectral data indicate that the charge transfer from donor to acceptor is greatly enhanced in the excited states Based on typical structural data and comprehensive spectral data, the following structure–property relationships can be drawn: 1) the moderate arylamino donor can more effectively enhance the SPEF and TPEF intensities than can the strong alkylamino donor; 2) stilbene is a better π-bridge than styrylthiophene for its capability of enhancing and blue-shifting the SPEF and TPEF of the corresponding D–π–A compounds; and 3) when compared to its boron-free precursors and other analogues, -B(mesityl)2 invariably and consistently acts as an effective SPEF and TPEF fluorophore in all this series of organoboron compounds, which may result from its strong π-electron-withdrawing and charge transfer-inducing nature in the ground-state and, more dominantly, in the excited-state Combining all the above positive structure factors, trans-4′-N,N-diphenylamino-4-dimesitylborylstilbene (compound 3) stands out as the optimized green SPEF and TPEF emitter This compound exhibits an SPEF quantum yield Φ of 091 at 522 nm in THF, a TPEF cross-section σ′ that is an order of magnitude larger than that of its boron-free precursor upon excitation by 800 nm femto-second laser pulses, and a two-photon absorption section σ of 30×10−48 cm4 s In the blue light region, trans-4′-N-carbazolyl-4-dimesitylboryl-stilbene (compound 4) shows significant SPEF and TPEF properties, with Φ=079 at 464 nm in THF and a large σ′ value, which is five times that of fluorescein upon excitation by 740 nm femto-second laser pulses

Abstract: Ultrafast electron-transfer dynamics of a catechol−TiO2 nanoparticle charge transfer (CT) complex and an intramolecular CT complex, [Ti(cat)3]2- were compared. Both complexes show similar CT bands, suggesting similar donor (catechol) and acceptor(Ti) orbitals are involved in CT transition. However, the electron is localized on one Ti center in the [Ti(cat)3] complex, but can delocalize into other Ti centers in TiO2 nanoparticles. The effect of charge delocalization on charge recombination dynamics was examined by comparing these complexes. In both cases, 400 nm excitation of the CT bands led to instantaneous promotion of an electron from a catechol ligand to a Ti(IV) center(s). In the molecular complex, the back electron-transfer dynamics from the Ti center to catechol ligand was observed on a 200 fs time scale. In the adsorbate-to-nanoparticle complex, back ET was much more complex, consisting of a 0.4 ps fast component and multiple slower components. The fast component was attributed to electrons trappe...

Abstract: We study spectrally resolved dynamics of Forster energy transfer in single monolayers and bilayers of semiconductor nanocrystal quantum dots assembled using Langmuir-Blodgett (LB) techniques. For a single monolayer, we observe a distribution of transfer times from ~50 ps to ~10 ns, which can be quantitatively modeled assuming that the energy transfer is dominated by interactions of a donor nanocrystal with acceptor nanocrystals from the first three shells surrounding the donor. We also detect an effective enhancement of the absorption cross section (up to a factor of 4) for larger nanocrystals on the red side of the size distribution, which results from strong, inter-dot electrostatic coupling in the LB film (the light-harvesting antenna effect). By assembling bilayers of nanocrystals of two different sizes, we are able to improve the donor-acceptor spectral overlap for engineered transfer in a specific (vertical) direction. These bilayers show a fast, unidirectional energy flow with a time constant of ~120 ps.

Abstract: Singlet excitation energy calculations for a series of acceptor para-substituted N,N-dimethyl-anilines that are dual (4-(N,N-dimethylamino)benzonitrile, 4DMAB-CN, 4-(N,N-dimethylamino)benzaldhyde, 4DMAB-CHO, 1-methyl-7-cyano-2,3,4,5-tetrahydro-1H-1-benzazepine, NMC7) and nondual (4-aminobenzonitrile, 4AB-CN, 3-(N,N-dimethylamino)benzonitrile, 3DMAB-CN, and 4-nitro(N,N-dimethyl) aniline, 4DMAB-NO2) fluorescent have been performed using time-dependent density functional theory (TDDFT). The B3LYP and MPW1PW91 functionals with a 6-311+G(2d,p) (Bg) basis set have been used to compute excitation energies. Ground-state geometries were optimized using density functional theory (DFT) with both B3LYP and MPW1PW91 functionals combined with a 6-31G(d) basis set. For most of the molecules presented in this study, potential energy surfaces have been computed according to the coordinates related to the three following mechanisms proposed in the literature: twisting, wagging, and planar intramolecular charge transfer (I...

Abstract: A novel donor−acceptor bisphthalocyanine (bis-Pc, 1) in which two different Pc units (Zn(II)-Pc and Ni(II)-Pc) are linked via vinylene spacers to the pseudopara positions of a central [2.2]paracyclophane moiety is described. The synthesis of 1 is achieved by two successive Heck reactions of pseudopara-divinyl[2.2]paracyclophane 9 with, sequentially, a zinc(II)- and a nickel(II)-iodophthalocyanine (4 and 5, respectively). The self-assembly ability of 1, which is the result of the complementary donor−acceptor character of its phthalocyanine units, has been assessed by a variety of techniques. It is revealed that 1 forms one-dimensional aggregates of nanometer-sized dimension, whereas equimolar mixtures of the donor and acceptor Pc subunits 2 and 3, although strongly interacting, do not give large arrays. The aggregates of 1 represent a novel type of supramolecular polymers based mainly upon donor−acceptor interactions.

Abstract: Density Functional Theory (DFT) calculations are performed to determine the accurate first static hyperpolarizability ( β ) of nitrogen bound low valent (M 0 ) group six metal carbonyls representing the class of chromophores displaying weak coupling between donor and acceptor. The nonlinear optical (NLO) response of this class of second-order NLO metal complexes is dominated by metal-to-ligand charge transfer excitations involving low lying, filled metal-carbonyl based orbitals and empty π ∗ orbitals of nitrogen bound ligand. We report novel organometallic systems with high β values. The full geometry optimizations of chromium and tungsten carbonyls were performed using DFT method at B3LYP/LanL2DZ level of theory using gaussian 98W. The calculations of the first hyperpolarizability ( β ) of these complexes were performed at the same level of theory. The calculated values of β were compared with available data in the literature. To understand the variation of β in these complexes, we examined the molecular HOMO and molecular LUMO generated via gaussian 98W. The present study concludes that these organometallic systems may contribute to the development of NLO materials.

Abstract: The surface structures formed upon deposition of O2 and Ga2O onto the technologically important arsenic-rich GaAs(001)-c(2×8)/(2×4) surface have been studied using scanning tunneling microscopy and spectroscopy, and the results are compared to density functional theory calculations. O2 chemisorbs by displacing first layer arsenic atoms bonded to second layer gallium atoms. Oxygen chemisorption pins the Fermi level at less than 5% monolayer coverage by creating a donor and acceptor site within the band gap originating from the gallium atom bonded between the two O atoms. In contrast, Ga2O chemisorbs by inserting into arsenic dimer pairs at elevated surface temperatures. A monolayer of Ga2O forms a (2×2) surface structure with a crystalline interface that is electronically unpinned: there are no states within the band gap. The unpinned interface results from Ga2O restoring the surface arsenic and gallium atoms to near-bulk charge.

Abstract: The Mg-acceptor activation mechanism and transport characteristics in a Mg-doped InGaN layer grown by metalorganic vapor phase epitaxy are systematically investigated through their structural, optical, and electrical properties. The In mole fraction was from 0 to 0.13, and the Mg concentration varied from 1×1019 to 1×1020 cm−3. X-ray rocking curves for Mg-doped InGaN layers indicate that the structural quality is comparable to that of undoped and Si-doped InGaN layers. Their photoluminescence spectra show emissions related to deep donors emerged at lower energy when Mg doping concentrations are above 2−3×1019 cm−3. The electrical properties also support the existence of these deep donors in the same Mg concentration range because the hole concentration starts to decrease at around the Mg concentration of 2−3×1019 cm−3. These results indicate that self-compensation occurs in Mg-doped InGaN at higher-doping levels. The temperature dependence of the hole concentration in Mg-doped InGaN indicates that the acceptor activation energy decreases with increasing In mole fraction. This is the reason the hole concentration in Mg-doped InGaN is higher than that in Mg-doped GaN at room temperature. In addition, the compensation ratio increases with doping concentration, which is consistent with the deep donor observed in PL spectra. For Mg-doped InGaN, impurity band conduction is dominant in carrier transport up to a relatively higher temperature than that for Mg-doped GaN, since the acceptor concentration for Mg-doped InGaN is higher than that of Mg-doped GaN.

Abstract: The association of acceptor cations (Fe′Ti and Mn″Ti) with oxygen vacancies in Fe- and Mn-doped SrTiO3 single crystals is investigated using in-situ EPR spectroscopy. Effective association enthalpies ΔassH0eff and entropies ΔassS0eff are determined, and ΔassH0eff is found to depend strongly on the dopant concentration. This dependence can be roughly described by a correction term in the chemical potential that is proportional to the cube root of the defect concentration. Extrapolation to infinite dilution yields an association enthalpy of −26 kJ mol−1. The association of oxygen vacancies can significantly reduce the ionic conductivity of these materials at temperatures up to ca. 200 °C (Fe′Ti) or even higher (Mn″Ti), and must therefore be taken into account in the respective defect chemical models at moderate or low temperatures.

Abstract: Photoluminescence (PL) properties of heavily P- and B-doped Si nanocrystals (nc-Si) are studied. By simultaneously doping two types of impurities, nc-Si exhibit strong PL at around 0.9 eV at room temperature. The temperature quenching of the PL is very small. Although the PL peak energy is very close to that of dangling-bond related PL previously observed, all of the observed properties, i.e., decay dynamics, degree of temperature quenching, etc., are apparently different. The transition between donor and acceptor states in nc-Si is the possible origin of the low-energy PL.

Abstract: The two-photon absorption (2PA) properties of a series of dyes containing donor and acceptor groups connected via a π-electronic bridge were studied by using a femtosecond Z-scan technique. It was found that the nature of the π-electronic bridge does not substantially affect the measured 2PA intensities. In the case of the substituted azobenzenes the decrease of the strength of the donor and/or acceptor groups leads to decrease of the 2PA intensity, the effect of the acceptor strength being greater.

Abstract: Photoinduced electron injection from the sensitizer Ru(dcbpy)2(NCS)2 (RuN3) into SnO2 and TiO2 nanocrystalline films occurs by two distinct channels on the femto- and picosecond time scales. The faster electron injection into the conduction band of the different semiconductors originates from the initially excited singlet state of RuN3, and occurs in competition with intersystem crossing. The rate of singlet electron injection is faster to TiO2 (1/55 fs-1) than to SnO2 (1/145 fs-1), in agreement with higher density of conduction band acceptor states in the former semiconductor. As a result of competition between the ultrafast processes, for TiO2 singlet, whereas for SnO2 triplet electron injection is dominant. Electron injection from the triplet state is nonexponential and can be fitted with time constants ranging from ~1 ps (2.5 ps for SnO2) to ~50 ps for both semiconductors. The major part of triplet injection is independent of the semiconductor and is most likely controlled by intramolecular dynamics in RuN3. The overall time scale and the yield of electron injection to the two semiconductors are very similar, suggesting that processes other than electron injection are responsible for the difference in efficiencies of solar cells made of these materials. (Less)

Abstract: Electronically coupled porphyrin arrays are suitable for artificial light harvesting antenna in light of a large absorption cross-section and fast excitation energy transfer (EET). Along this line, an artificial energy transfer model system has been synthesized, comprising of an energy donating meso-meso linked Zn(II) porphyrin array and an energy accepting 5,15-bisphenylethynylated Zn(II) porphyrin linked via a 1,4-phenylene spacer. This includes an increasing number of porphyrins in the meso-meso linked Zn(II) porphyrin array, 1, 2, 3, 6, 12, and 24 (Z1A, Z2A, Z3A, Z6A, Z12A, and Z24A). The intramolecular singlet-singlet EET processes have been examined by means of the steady-state and time-resolved spectroscopic techniques. The steady-state fluorescence comes only from the acceptor moiety in Z1A-Z12A, indicating nearly the quantitative EET. In Z24A that has a molecular length of ca. 217 A, the fluorescence comes largely from the acceptor moiety but partly from the long donor array, indicating that the intramolecular EET is not quantitative. The transient absorption spectroscopy has provided the EET rates in real time scale: (2.5 ps)(-1) for Z1A, (3.3 ps)(-1) for Z2A, (5.5 ps)(-1) for Z3A, (21 ps)(-1) for Z6A, (63 ps)(-1) for Z12A, and (108 ps)(-1) for Z24A. These results have been well explained by a revised Forster equation (Sumi formula), which takes into account an exciton extending coherently over several porphyrin pigments in the donor array, whose length is not much shorter than the average donor-acceptor distance. Advantages of such strongly coupled porphyrin arrays in light harvesting and transmission are emphasized in terms of fast EET and a large absorption cross-section for incident light.

Abstract: Unusual strength and directionality for the charge-transfer motif (established in solution) are shown to carry over into the solid state by the facile synthesis of a series of robust crystals of th...

Abstract: We report on the directional Forster resonance energy transfer (FRET) process taking place in single molecules of a first (T1P4) and a second (T2P8) generation of a perylenemonoimide (P)−terrylenediimide (T)-based dendrimer in which the chromophores are separated by rigid polyphenylene arms. At low excitation powers, single-molecule detection and spectroscopy of T1P4 and T2P8 dendrimers point to a highly efficient directional FRET from P donors to the central T acceptor, optical excitation at 488 nm resulting in exclusively acceptor emission in the beginning of the detected fluorescence intensity. Donor emission is seen only upon the bleaching of the acceptor. High-resolution time-resolved single-molecule fluorescence data measured with a microchannel plate photomultiplier reveal, for T2P8, a broad range of FRET rates as a result of a broad range of distances and orientations experienced by the donor−acceptor dendrimers when immobilized in a polymer matrix. Single-molecule data from T2P8 on 488 nm excitat...

Abstract: The first investigation on the distance dependence of a dissociative electron transfer process across peptide bridges is reported. This study was carried out by using a series of donor−peptide−acceptor systems in which the donor is a phthalimido moiety, the peptide bridges are provided by α-aminoisobutyric acid (Aib) homooligomers, and the acceptor is a peroxide functional group. The intramolecular electron transfer from the electrogenerated phthalimido radical anion to the peroxide was studied in comparison with the thermodynamic and kinetic information obtained with models of the acceptor and the donor. The intramolecular rate constants were determined in N,N-dimethylformamide by taking into account the corresponding intermolecular values. The experimental results point to an unusual non-exponential dependence of the intramolecular electron transfer rate on the number of bridge units. The same trend could be verified also by taking into account the actual donor−acceptor edge-to-edge distance. The peculi...

Abstract: Donor-sigma-acceptor-lipid molecules were prepared by using perylenetetracarboxylic diimide as the acceptor, starting from perylenetetracarboxylic dianhydride. One imide nitrogen was attached to a "swallowtail" lipid (a long alkyl tail connected at midchain), which imparts enough solubility to make the system tractable and provides a lipophilic region suitable for promoting Langmuir-Blodgett monolayer formation. The other imide link was to a donor group (pyrene, ferrocene, tetramethylphenylenediamine, phenyl) through a short alkyl sigma bridge. Features of the 1H and 13C NMR spectra of swallowtailed perylenediimides are interpreted as resulting from restricted rotation about the imide C-N bond; the 13C NMR spectra and stereochemistry of these molecules are contrasted with the case of the related bis-(2,5-di-tert-butylphenyl)perylenetetracarboxylic diimide.

Abstract: We describe the morphological, electrochemical, spectroelectrochemical characterization, and electrochromic device properties of films of poly(2,2‘-[10-methyl-3,7-phenothiazylene]-6,6‘-bis[4-phenylquinoline]) (PPTZPQ). Cyclic voltammograms of PPTZPQ polymer films showed reversible electrochemical oxidation accompanied with a color change that was switchable as a function of the potential. However, the reduction behavior of PPTZPQ was unstable and caused a loss of the electrochromic effect. The spectroelectrochemical behavior of PPTZPQ was interpreted as the combination of the properties of the constituent donor and acceptor moieties. Two PPTZPQ solid-state electrochromic devices were constructed using indium tin oxide (ITO) deposited on polyester and poly(ethylene oxide) as a polymeric electrolyte. The electrochromic active materials were PPTZPQ as the primary electrode and ladder poly(benzobisimidazobenzophenanthroline) (BBL) or vanadium pentoxide (V2O5) as the secondary electrode. The PPTZPQ−BBL and PPT...