Reduction Of Energy Gap Research Articles

Designing of near-infrared sensitive asymmetric small molecular donors for high-efficiency organic solar cells

Herein, we have designed four small molecular donors (SMDs) with Donor–Acceptor–Acceptor (D–Á–A) backbone having different acceptor units for highly efficient organic solar cells (OSCs). The specific molecular modeling has been made by replacing the additional acceptor unit (A) of recently synthesized TPA-DAA-MDN molecule (R) by employing different highly efficient acceptor units in order to improve the photovoltaic performances of the molecules. A theoretical approach (DFT and TD-DFT) has been applied to investigate the photophysical, opto-electronic and photovoltaic parameters of the designed molecules (DAA1–DAA4) and compared with the reference molecule (R). The red-shifting absorption of SMDs is the most important factor for highly efficient OSCs. Our all formulated molecules showed a red shifted absorption spectrum and also exhibit near IR sensitivity. Acceptor unit modification of R molecule causes reduction in HOMO-LUMO energy gap; therefore, all designed molecules offer better opto-electronic properties as compared to R molecule. A variety of certain critical factors essential for efficient SMDs like frontier molecular orbitals (FMOs), absorption maxima, dipole moment, exciton binding energy along with transition density matrix, excitation energy, open circuit voltages and charge mobilities of (DAA1–DAA4) and R have also been investigated. Generally, low values of reorganizational energy (hole and electron) offer high charge mobility and our all designed molecules are enriched in this aspect. High open circuit voltage values, low excitation energies, large dipole moment values indicate that our designed SMDs are suitable candidates for high-efficiency OSCs. Furthermore, conceptualized molecules are superior and thus are suggested to experimentalist for out-looking future progresses of highly efficient OSCs devices.

Designing Star-Shaped Subphthalocyanine-Based Acceptor Materials with Promising Photovoltaic Parameters for Non-fullerene Solar Cells.

Star-shaped three-dimensional (3D) twisted configured acceptors are a type of nonfullerene acceptors (NFAs) which are getting considerable attention of chemists and physicists on account of their promising photovoltaic properties and manifestly promoted the rapid progress of organic solar cells (OSCs). This report describes the peripheral substitution of the recently reported highly efficient 3D star-shaped acceptor compound, STIC, containing a 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile (IC) end-capped group and a subphthalocyanine (SubPc) core unit. The 3D star-shaped SubPc-based NFA compound STIC is peripherally substituted with well-known end-capped groups, and six new molecules (S1–S6) are quantum chemically designed and explored using density functional theory (DFT) and time-dependent DFT (TDDFT). Density of states (DOS) analysis, frontier molecular orbital (FMO) analysis, reorganization energies of electrons and holes, open-circuit voltage, transition density matrix (TDM) surface, photophysical characteristics, and charge-transfer analysis of selected molecules (S1–S6) are evaluated with the synthesized reference STIC. The designed molecules are found in the ambience of 2.52–2.27 eV with a reduction in energy gap of up to 0.19 eV compared to R values. The designed molecules S3–S6 showed a red shift in the absorption spectrum in the visible region and broader shift in the range of 605.21–669.38 nm (gas) and 624.34–698.77 (chloroform) than the R phase values of 596.73 nm (gas) and 616.92 nm (chloroform). The open-circuit voltages are found with the values larger than R values in S3–S6 (1.71–1.90 V) and comparable to R in the S1 and S2 molecules. Among all investigated molecules, S5 due to the combination of extended conjugation and electron-withdrawing capability of end-capped acceptor moiety A5 is proven as the best candidate owing to promising photovoltaic properties including the lowest band gap (2.27 eV), smallest λe = 0.00232 eV and λh = 0.00483 eV, highest λmax values of 669.38 nm (in gas) and 698.77 nm (in chloroform), and highest Voc = 1.90 V with respect to HOMOPTB7-Th–LUMOacceptor. Our results suggest that the selected molecules are fine acceptor materials and can be used as electron and/or hole transport materials with excellent photovoltaic properties for OSCs.

Bioactive Natural Small Molecule-Tuned Coassembly of Photosensitive Drugs for Highly Efficient Synergistic and Enhanced Type I Photochemotherapy.

Self-assembling natural small molecules (NSMs) with favorable anticancer activity are of increasing interest as novel drug delivery platforms without structural modification for biomedical applications. However, a lack of knowledge and practicability of NSMs as drug carriers limited their current biomedical application. Here, via a green and facile supramolecular coassembly strategy, we report and develop a series of carrier-free terpenoid natural small molecule-mediated coassembled photosensitive drugs for enhanced and synergistic chemo/photodynamic therapy. After screening 17 terpenoid NSMs, we identified 11 compounds that could form coassembled NSMs-Ce6 NPs with regulatable drug sizes. Analysis of the representative betulonic acid (BC)-mediated nano-coassemblies (BC-Ce6 NPs) reveals the high efficiency of the coassembly strategy and highlights the tremendous potential of NSMs as novel drug delivery platforms. Through molecular dynamics simulation and theoretical calculations, we elucidate the mystery of the coassembly process, indicating that the linear coplanar arrangement of BC dimeric units is primarily responsible for the formation of rod-like or spherical morphology. Meanwhile, we demonstrated that the reduced energy gap between the singlet and triplet excited states (ΔEST) facilitates efficient reactive oxygen species generation by promoting ·OH generation via a type I photoreaction mechanism. The assembled nanodrugs exhibit multiple favorable therapeutic features, ensuring a remarkably enhanced, synergistic, and secure combinatorial anticancer efficacy of 93.6% with highly efficient tumor ablation. This work not only expands the possibility of natural biodegradable materials for wide biological applications but also provides a new perspective for the construction of NSM-mediated nano-coassemblies for precision therapy.

Biological perspective of a triazine derivative with isatin/chalcone/acridone: DFT and docking investigations

The article highlights insights into biological activity using computational methods in products of 1,3,5-triazine-2,4-diamine with 1H-indole-2,3-dione (isatin)/(2E)-13-diphenylprop-2-en-1-one (chalcone)/10H-acridin-9-one (acridone). Biological activity is carried out using the method of electron density, 6-311++G(d,p) for molecular and electronic characteristics. Frontier molecular orbitals provide quantum mechanical descriptors to determine electronic properties. Studies show reduced energy gap with high kinetic stability and the index of electrophilicity is a consequence of delocalized sites of 1,3,5-triazine-2,4-diamine and nucleophilic sites are responsible for biological activity. Enhancement in electrophilicity index of products confirms electron acceptor between the reactants. The structural and molecular docking with different proteins shows that the product molecules are good drugs for the corresponding activity.

Impact of Se doping on optical and third-order nonlinear optical properties of spray pyrolysis fabricated CdS thin films for optoelectronics

Cadmium sulphide is known to have tremendous applications towards optoelectronic and nonlinear devices. Hence, here we have facilely casted the CdS films with diverse Se contents through low-cost spray pyrolysis technique. XRD study defends mono-phase formation of CdS having hexagonal system at all Se doping contents. Scherrer equation was employed to evaluate crystallite size in range of 15–25 nm. Vibrational study reveals the presence of fundamental modes of vibration of hexagonal CdS. EDX and SEM mapping studies approve the existence of Se and its homogeneous distribution all over the film. SEM micrographs shows the nanoscale grains formation on film surface and the size is increasing with Se doping. Optical study revealed that the grown films are of optimal quality with transparency in range of 60–75% with low absorbance and reflectance values. The refractive index values are noted to varied from 1 to 2.7 with wavelength and noticed to be reduced on Se content in UV–Vis region. The reduction in direct and indirect energy gap was found from 2.46 to 2.34 eV and 2.21 to 1.96 eV, correspondingly due to Se. PL emission profile contains an emission band at 528, 529, 529, 530 and 546 nm for 0.0, 0.5, 1.0, 2.5, 5.0 wt% Se:CdS films. Dielectric constant and loss were estimated. The nonlinear refraction (n2) and absorption coefficient (β) and third-order nonlinear susceptibility $$(\chi^{3} )$$ values were determined using Z-scan and observed in order of 10–8 cm2/W, 10–4 cm/W and 10–3 esu, correspondingly. The high values of $$\chi^{3}$$ propose the films for nonlinear applications.

Adsorption of SO2 molecule on Ni-doped and Pd-doped graphene based on first-principle study

In this study SO2 adsorption on intrinsic graphene (IG), Ni-doped graphene (NiG) and Pd-doped graphene (PdG) has been analyzed and studied using density functional theory (DFT) based on first principles. By analyzing the adsorption energy, bond length, bond angle, charge transfer, DOS, PDOS and frontier molecular orbitals, it can be proved that NiG and PdG have excellent SO2 adsorption performance than IG. PdG has better SO2 adsorption performance than NiG thanks to the strong overlapping of peaks in PDOS and the obvious reduction of energy gap. In particular, the stable adsorption of double SO2 on NiG and PdG means that Ni or Pd doping has practical value. Therefore, the study in this paper provides a feasible direction to the detection of SO2.

Computational studies of Distinct Anilines for Electrooptical properties

Significant studies of anilines reported biological activities like oxidation, polymerization, arylation, alkylation, protonation, conformation for photophysical and electrochemical properties using experimental and computational studies like CNDO, MINDO, PCILO that arise due to charge transfer interaction. These interactions still provide enough scope with analysis using electron density method to determine physical, chemical and electrooptical properties. The present work attributes family of anilines-aniline(AN), p-chloroaniline(CAN) and p-nitroaniline(NAN) with experimental studies like FTIR and FT Raman and Computational studies with Gauss View4.1 package using B3LYP–basis set 6-311++(d, p) for electrooptical properties. Interpretation of experimental spectra of FTIR and FT Raman confirm the wave numbers are in specified range corresponding to functional group and fingerprint regions. It is observed that intensity of the wave numbers in Raman spectra are reduced in comparison with infrared spectra. Molecular structures are visualized with molecular editor Avogadro for molecular properties. Computational studies are performed for optimized structures wherein the computed infrared spectra and Raman spectra are in agreement with experimental spectra. Studies of charge transfer interactions determine properties like dipole moment, EHomo, ELumo, energy gap, electrophilicity index, polarization and first order hyperpolarizability. A feature of work is NAN possess greater tendency in charge transfer interaction with reduced energy gap, high polarizability and first order Hyperpolarizability that enable to form complex for optical materials and devices.

Regulation of corundum band gap width by p elements and vacancy co-doping

The coherent potential approximation is applied to study the evolution of the electronic structure mainly in the vicinity of the energy gap of pure corundum (α-Al2O3) in dependence on p cation and anion impurities (Si, Ge, Sn, C, and N) and aluminum/oxygen deficiency. The study shows an energy gap reduction due to appearance of new impurity/vacancy peaks at the edges of the energy gap of undoped α-Al2O3, which agrees with experiments for some single impurities. Simultaneously doping with p elements and vacancies of α-Al2O3-based compounds Al2{O2.88C0.06}, {Al1.94Si0.03Ge0.03}{O2.88C0.03N0.06}, and {Al1.90Si0.03Ge0.03Sn0.03}{O2.88C0.03N0.06} with the 1.9 eV energy gap are proposed as new effective materials for applications in photocatalysis and photovoltaic and optical devices.

Ni-doped BaTi5O11: New brilliant yellow pigment with high NIR reflectance as solar reflective fillers

New brilliant yellow pigments of Ni-doped BaTi5O11 were synthesized via solid-state method. Pure monoclinic BaTi5O11 powders with high crystallinity were obtained when the calcination was performed at 1000 °C for 2 h. As-prepared pigments had particle size distribution that ranged from 0.5 to 2 μm Ni2+ and Ni3+ ions coexisted in the lattice of BaTi5O11 host. Ni-doping resulted in reduced energy gap of 2.44 eV for Ni-doped BaTi5O11 from 3.08 eV for undoped BaTi5O11 because of valence band shift. BaTi5O11 pigments with 5% Ni exhibited excellent color performance (L = 95.1, a* = −9.12, b* = 65.12, C* = 65.75, and ho = 97.97) and impressive NIR reflectance of 76.19%. Approximately, 10.3 °C decrease in temperature was observed for inner surface of ceramic tiles coated with cool pigments relative to the temperature of ceramic tiles with blank PU coating.

Fine-tuning of energy gap, FTIR, photoluminescence and photocatalytic behavior of Centella asiatica extract mediated Mn/Mg doped ZnO nanostructure

Mn, Mg two element doped ZnO nanostructure derived from Centella asiatica leaf extract with Mn = 2% and Mg = 0, 2 and 4% were prepared by bio-synthesis method. Hexagonal structure of ZnO was maintained throughout the Mn/Mg doping without any additional phase. The derived XRD intensity ratio ((I100/I002) was almost equal to one for Mg = 2% than other samples which signifies the tuning of size and shape towards the spherical size which was confirmed by electron microscopic images. The higher absorbance in UV and visible wavelengths at Mg = 4% is used to improve the light harvesting capacity and should increase the photocatalytic activity of composite samples. The tuning of energy gap by Mg doping is useful for the fabrication of opto-electronic devices. The presence of Zn–O–Mn/Mg local bond is confirmed by Fourier transform infra-red studies. The existing strong ultra-violet emission band and the visible blue and green emission bands were discussed based on electronic transition and intensity of defect centers. The elevated photocatalytic activity at Mg = 4% is due to the reduced energy gap (3.89 eV) than other samples and hence the higher absorption in the visible region and the creation of defect states.

Promising PVA/TiO2, CuO filled nanocomposites for electrical and third order nonlinear optical applications

In this paper, we report enhancing the structural, morphological, mechanical, linear and nonlinear optical properties of polyvinyl alcohol (PVA) encapsulated with titanium dioxide (TiO2) and copper oxide (CuO) nanoparticles. PVA/(x)TiO2(15-x)CuO nanocomposites for x = 0 wt%, 1 wt%, 5 wt%, 7.5 wt%, 10 wt%, 14 wt% and 15 wt% filling concentration are prepared using ex-situ and solvent casting technique. The XRD spectra of the prepared nanocomposites endorse the semi-crystalline nature of PVA nanocomposites. The atomic force microscope (AFM) image displayed the uniform grain structure for pure PVA and change in surface morphology for prepared nanocomposites. Universal testing machine (UTM) explored high tensile strength and Young's modulus of 1685.70 MPa for x = 10 wt% filling concentration. PVA/(x)TiO2(15-x)CuO nanocomposites shows an enhanced electrical conductivity of 3.21 × 10−8Scm−1 for x = 10 wt% filling concentration. UV–Vis spectroscopy exposed the reduction in optical energy gap with the increase in filling concentration. Photoluminescence (PL) studies spectacles maximum enhancement in PL intensity for x = 10 wt% filling concentration. The Z-scan technique shows third order nonlinear absorption coefficient of 8.17 × 10−4 cm/W, the nonlinear refractive index of 2.56 × 10−8 cm2/W and nonlinear optical susceptibility of 1.48 × 10−6 esu for optimum nanocomposites.

Role of defects in optical, photoluminescence and magnetic properties of Zn0.96−xNi0.04CrxO nanoparticles

Cr, Ni doubly doped ZnO nanoparticles for Cr = 0%–5% have been synthesized successfully by sol-gel route. X-ray diffraction spectra revealed the hexagonal wurtzite structure of the samples without any detectable secondary phases up to Cr = 3% and a secondary spine ZnCr2O4 phase was noticed at higher Cr doping (Cr = 5%). The rapid decrease in crystallite size derived from Debye formula by incorporation of Cr was understood in terms of the dissimilarity in ionic radii among Cr3+ (0.063 nm) and Zn2+ (0.074 nm). The shape of the synthesized nanoparticles and the generation of defect concentrations by Cr-doping in ithe host Zn-Ni-O lattice plays major role in the reduction of optical energy gap from Cr = 1% (Eg ≈ 4 eV) to Cr = 5% (Eg ≈ 3.87 eV). The broad and dominant infrared absorption peaks between 483 cm−1 and 490 cm−1 revealed the occurrence of Zn-O stretching mode with octahedral co-ordinates. The steady enhancement of green emission by Cr addition from photoluminescence spectra supports total increase of defect states like oxygen vacancies. Room temperature ferromagnetism was noted in whole Cr range where Cr = 3% scored maximum magnetization and dropped off for Cr = 5% due to the secondary ZnCr2O4 phase.

Electron Donor and Acceptor Influence on the Nonlinear Optical Response of Diacetylene-Functionalized Organic Materials (DFOMs): Density Functional Theory Calculations.

Herein, we report the quantum chemical results based on density functional theory for the polarizability (α) and first hyperpolarizability (β) values of diacetylene-functionalized organic molecules (DFOM) containing an electron acceptor (A) unit in the form of nitro group and electron donor (D) unit in the form of amino group. Six DFOM 1–6 have been designed by structural tailoring of the synthesized chromophore 4,4′-(buta-1,3-diyne-1,4-diyl) dianiline (R) and the influence of the D and A moieties on α and β was explored. Ground state geometries, HOMO-LUMO energies, and natural bond orbital (NBO) analysis of all DFOM (R and 1–6) were explored through B3LYP level of DFT and 6-31G(d,p) basis set. The polarizability (α), first hyperpolarizability (β) values were computed using B3LYP (gas phase), CAM-B3LYP (gas phase), CAM-B3LYP (solvent DMSO) methods and 6-31G(d,p) basis set combination. UV-Visible analysis was performed at CAM-B3LYP/6-31G(d,p) level of theory. Results illustrated that much reduced energy gap in the range of 2.212–2.809 eV was observed in designed DFOM 1–6 as compared to parent molecule R (4.405 eV). Designed DFOM (except for 2 and 4) were found red shifted compared to parent molecule R. An absorption at longer wavelength was observed for 6 with 371.46 nm. NBO analysis confirmed the involvement of extended conjugation and as well as charge transfer character towards the promising NLO response and red shift of molecules under study. Overall, compound 6 displayed large <α> and βtot, computed to be 333.40 (a.u.) (B3LYP gas), 302.38 (a.u.) (CAM-B3LYP gas), 380.46 (a.u.) (CAM-B3LYP solvent) and 24708.79 (a.u.), 11841.93 (a.u.), 25053.32 (a.u.) measured from B3LYP (gas), CAM-B3LYP (gas) and CAM-B3LYP (DMSO) methods respectively. This investigation provides a theoretical framework for conversion of centrosymmetric molecules into non-centrosymmetric architectures to discover NLO candidates for modern hi-tech applications.

The Study of Optical Energy Gap, Refractive Index, and Dielectric Constant of Pure and Doped Polyaniline with HCl and H2SO4 Acids

Polyaniline (PANI) salt in its pure and doped forms find extensive applications in making devices such as polymer light emitting diodes, photovoltaic, sensors, batteries, and super capacitors. PANI salt has been synthesized successfully through chemically oxidative polymerization of aniline in the presence of hydrochloric acid (HCl) and sulfuric acid (H2 SO4 ) using ammonium peroxydisulfate as an oxidizing agent. The absorption spectra of pure PANI salt and its doped state, in HCl and H2 SO4 media, have been studied in the wavelength range from 200 to 1100 nm using ultraviolet and visible near infrared spectrophotometer. Tauc’s formula, Lambert-Beer’s relation, and Fresnel’s formula were employed in the MATLAB program to calculate the optical energy gap, refractive index, and dielectric constant. Results showed that doping with HCl and H2 SO4 acidic mediums caused a reduction in the direct energy gap of the pure PANI from 2.69eV to 2.42 eV and 2.54 eV, respectively. The reduction in optical energy gap is associated with the increase in refractive index. The refractive index (2.92) has a higher value of PANI doped with HCl. Higher refractive index values are for better-structured films.

Doping Effects on the Reactivity of the MVO5– (M = V–Zn) Clusters in CO Oxidation Reaction

Doped metal oxide catalysts have important applications in heterogeneous catalysis, whereas the fundamental effects of doping on the reactivity of catalysts are elusive. Herein, benefiting from the study of a series of atomic clusters MVO5– (M = V–Zn) in catalytic CO oxidation by molecular O2 under thermal collision conditions, the crucial effects of doping on the electronic structure and then the catalytic reactivity of MVO5– were successfully rationalized by using mass spectrometry and theoretical calculations. We defined the energy gap of active orbitals that are closely related to electron transfer in MVO5– during CO oxidation. It was discovered that such an energy gap can modulate reaction thermodynamics, whereas the orbital overlap of MVO5– with CO controls the kinetics. The doped clusters MVO5– (M = Cr–Zn) have reduced energy gaps than that in host V2O5–, and doping can modify the orbital overlap of MVO5– (M = Cr–Zn) with CO. Only the MnVO5– and ZnVO5– clusters that have rational energy gaps and pr...

A theoretical investigation of the optoelectronic performance of some new carbazole dyes

Density functional theory and time-dependent approaches are applied for theoretical investigation of a new class of novel carbazole-based d–D–π–A-type dyes, where the carbazole moiety is the main electron donor, bithiophene behaves as a π-bridge, and cyanoacetic acid as an electron acceptor for all the studied dyes, whereas the terminal electron donor unit is varied to thiophene, thienothiophene, carbazole, dimethoxyphenyl, and indole. The influence of the terminal electron donor on the optoelectronics properties is investigated for the dyes in isolated state and in chloroform solvent. Their absorption spectra and electronic and structural properties are evaluated and discussed. The theoretical results show that all the dyes exhibit excellent optoelectronic properties. In particular, D5 with indole as the terminal electron donor moiety has potential for use as a sensitizer for nanocrystalline TiO2 solar cells based on its red-shifted absorption spectrum, reduced energy gap, lowest λtotal value, and higher $$\Delta G^{\text{Inject}}$$ and $$\Delta G^{\text{Reg}}$$ values.

Heterochiral Doped Supramolecular Coordination Networks for High-Performance Optoelectronics.

Chiral self-sorting has great potential for constructing new complex structures and determining chirality-dependent properties in multicomponent mixtures. However, it is still of great challenge to achieve high fidelity chiral self-discrimination. Besides, the researches on the coordination polymers or metal-organic frameworks for micro/nanooptoelectronics are still rare due to their low conductivity and difficulty in developing a rapid and simple scale-up synthetic method. Here, heterochiral supramolecular coordination networks (SCNs) were synthesized by the solvothermal reaction of naphthalene diimide enantiomers and cadmium iodide, using the chirality as a synthetic tuning parameter to control the morphologies. Intriguingly, heterochiral micro/nanocrystals exhibited photochromic and photodetecting properties. Furthermore, we also developed a simple and efficient doping method to enhance the conductivity and photoresponsivity of micro/nanocrystals using hydrazine. From experimental and theoretical studies, the mechanism was suggested as follows: the radicals in the singly occupied molecular orbital level of the ligands provide charge carriers that can undergo "through-space" transport between π-π stacked ligands and the electron transfer from adsorbed hydrazine to the SCNs results in reduction of energy gap, leading to increased conductivity. Our findings demonstrate a simple and powerful strategy for implementing coordination networks with redox ligands for micro/nanooptoelectronic applications.

Theoretical Study Toward Understanding the Electronic Properties of Armchair Hetero Nanotubes

Using the density functional theory ‘DFT’ code ‘SIESTA’ combined with a Green’s function scattering approach ‘GFSA’ code ‘GOLLUM’, we investigate the electronic properties of hetero nanotubes, density of states ‘DOS’, electronic band structure BS, transmission coefficient T(E) and current-voltage curve. We simulated four different hetero structures of nanotubes which are: 1- Hetero nanotube with one graphene ring and one boron nitride ring. 2- Hetero nanotube with two graphene ring and one boron nitride rings. 3- Hetero nanotube with three graphene ring and one boron nitride rings. 4- Hetero nanotube with four graphene ring and one boron nitride rings nanotubes. The results show that there is a clear reduction in energy gap ‘Eg’ of hetero nanotubes comparing with the insulator boron nitride (Eg 4.6 eV) and also this kind of hetero nanotubes have zero states at Fermi energy comparing with conductor carbon nanotube which has finite states at Fermi energy. This is could be a good method to design a semiconductor with desirable band gap.

Structural, electronic and energetic effects in heterocyclic fluorene derivatives fused with a fulvene unit

A set of 36 heterocyclic (B, N and O) fluorene (C) derivatives fused in nine ways with fulvene ring have been analyzed by means of different local aromaticity criteria. Molecular geometry of analyzed compounds were optimized at B3LYP/6-311++G(2d,2p) level of theory. The evaluation of the local aromaticity has been carried out through the use of the geometry-based harmonic oscillator model of aromaticity (HOMA) and the magnetism-based zz‐component of the nucleus independent chemical shifts calculated 1 Å above the ring center (NICS1zz) indices as well as one aromaticity index derived from the Quantum Theory Atoms in Molecules (QTAIM), i.e. the para-delocalization index (PDI). Additionally, the sum (H) of the kinetic (G) and potential (V) electron density energy in the ring critical points (RCPs) have been taken into consideration. These studies indicated that in 12 studied compounds the benzene ring fused with the fulvene loses its aromatic character. The fusion of heterocyclic fluorine derivatives with a fulvene ring significantly increases the energy of HOMOs and decreases the energy of LUMOs, which results in the energy gap reduction between the orbitals.

Energy level alignment at organic/inorganic semiconductor heterojunctions: Fermi level pinning at the molecular interlayer with a reduced energy gap.

The electronic properties of the organic/inorganic semiconductor heterojunction formed by para-sexiphenyl (6P) and three different faces of ZnO are investigated using photoelectron spectroscopy and X-ray absorption. While multilayer molecules stand almost upright with respect to the surface plane, we evidence the presence of a lying 6P interlayer, which exhibits a higher electron affinity. This is due to an energy gap narrowing because of the close vicinity of that interlayer to the higher dielectric constant ZnO and a more planar molecular conformation compared to molecules in the bulk. Both effects have a significant impact on the level alignment mechanisms at the three interfaces, i.e., surface electron push-back and Fermi level pinning. We disentangle the contribution of each effect to the level alignment for both standing and lying 6P layers and show that on ZnO(0001[combining macron]) only the push-back contributes, while on ZnO(101[combining macron]0) and ZnO(0001) Femi level pinning occurs in addition. In all three cases the lying 6P interlayer establishes the same work function to which the levels of the 6P multilayer align. Only the identification of the complex interplay of level alignment mechanisms and molecular degrees of freedom allows deriving a reliable picture of the energy levels at this heterojunction. This is important as the presence of an interlayer and its modified electronic states might go unnoticed, and conclusions on the correlation between purported interfacial energy levels and functionality of such semiconductor heterojunctions could be misleading.