Suppression of Time‐Dependent Donor/Acceptor Interface Degradation by Redistributing Donor Charge Density

Poor operation stability is a major hurdle for the wide application of organic photovoltaic (OPV) devices. While most attention is given to environmental threats to device stability, we herein show evidence from X‐ray photoemission spectroscopy (XPS) of an intrinsic time‐dependent chemical reaction at a donor/acceptor interface. Albeit with impressive device performance from boron subphthalocyanine chloride (SubPc)/fullerene (C₆₀) interface, the forming boride bonds at its interface hinders the interfacial exciton dissociation and leads to device deterioration. Due to the high electron affinity of molybdenum oxide (MoO₃) film, the incorporation of MoO₃ layer under the SubPc film has strong electron‐drawing property and leads to charge‐transfer complex (CTC) formation at the MoO₃/SubPc interface. The resulting charge redistribution in SubPc molecules effectively suppresses the further interfacial reaction at SubPc/C₆₀ junction. Our results provide insight for new degradation mechanisms of OPV devices and corresponding stability control via charge redistribution in the donor film.     

Formation of organic crystalline nanopillar arrays and their application to organic photovoltaic cells

To enhance the performance of organic photovoltaic (OPV) cells, preparation of organic nanometer-sized pillar arrays is fascinating because a significantly large area of a donor/acceptor heterointerface having continuous conduction path to both anode and cathode electrodes can be realized. In this study, we grew cupper phthalocyanine (CuPc) crystalline nanopillar arrays by conventional thermal gradient sublimation technique using a few-nanometer-sized trigger seeds composed of a CuPc and 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) stacked layer. We optimized the pillar density by tuning crystal growth condition in order to apply it to OPV cells.     

Fabrication and photovoltaic properties of fullerene and copper phthalocyanine derivative mixed Langmuir-Schaefer films

Surface pressure-area isotherms of mixed floating film of fullerene (C₆₀) and tri-(2,4-di-t-amylphenoxy)-(8-quinolinolyl) copper phthalocyanine (CuPc) were investigated at different molar compositions at air-water interface. The shapes of these isotherms and estimated areas per molecule are shown dependent on the molar compositions. C₆₀/CuPc mixed multilayers were fabricated onto various solid substrates by using Langmuir-Schaefer (LS) technique, and characterized by UV-VIS spectroscopy, electrochemical and photoelectrochemical measurements respectively. Uniform LS films were formed when the molar composition was less than 1 : 1 (C₆₀: CuPc). UV-VIS spectroscopic measurements showed that there was no distinct interaction between phthalocyanine and C₆₀molecules at ground state either in solution or in solid films. However, electron transfer appears to have occurred between two kinds of molecules under light illumination. Photoelectrochemical study revealed that phthalocyanine and C₆₀mixed system could be a potential candidate for photovoltaic applications.     

Theoretical investigation of the C<Subscript>60</Subscript>/copper phthalocyanine organic photovoltaic heterojunction

Molecular heterojunctions, such as the one based on copper phthalocyanine (CuPc) and carbon fullerene (C₆₀) molecules, are commonly employed in organic photovoltaic cells as electron donor-acceptor pairs. We have investigated the different atomic structures and electronic and optical properties of the C₆₀/CuPc heterojunction through first-principles calculations based on density functional theory (DFT) and time-dependent DFT. In general, configurations with the CuPc molecule “lying down” on C₆₀ are energetically more favorable than configurations with the CuPc molecule “standing up”. The lying-down configurations also facilitate charge transfer between the two molecules, due to the stronger interaction and the larger overlap between electronic wavefunctions at the interface. The energetically preferred structure consists of CuPc placed so that the Cu atom is above a bridge site of C₆₀, with one N-Cu-N bond of CuPc being parallel to a C-C bond of C₆₀. We also considered the structure of a periodic CuPc monolayer deposited on the (001) surface of a face-centered cubic (fcc) crystal of C₆₀ molecules with the lying-down orientation and on the (111) surface with the standing-up configuration. We find that the first arrangement can lead to larger open circuit voltage due to an enhanced electronic interaction between CuPc and C₆₀ molecules.      

Controlling Electronic States and Transport Properties at the Level of Single Molecules

Since molecular electronics has been rapidly growing as a promising alternative to conventional electronics towards the ultimate miniaturization of electronic devices through the bottom‐up strategy, it has become a long‐term desire to understand and control the transport properties at the level of single molecules. In this Research News article it is shown that one may modify the electronic states of single molecules and thus control their transport properties through designing and fabrication of functional molecules or manipulating molecules with scanning tunneling microscopy. The rectifying effect of single molecules can be realized by designing a donor–barrier–acceptor architecture of Pyridine–σ–C₆₀ molecules to achieve the Aviram–Ratner rectifier and by modifying electronic states through azafullerene C₅₉N molecules. The effect of the negative differential resistances can be realized by appropriately matching the molecular orbital symmetries between a cobalt phthalocyanine (CoPc) molecule and a Ni electrode. The electronic states and transport properties of single molecules, such as CoPc and melamine molecules, can be altered through manipulation or modifying molecular structures, leading to functionalized molecular devices.     

Photoelectron processes in heterojunctions based on organic films

The photoconductivity and photo emf have been studied in a heterostructure based on thin films of electron donor and acceptor organic compounds. It is established that the main process responsible for the photogeneration and separation of charge carriers is the decay of photoexcited molecular states at the interface between the donor and acceptor materials. The determining influence on the photoelectron processes in the organic heterostructure is produced by the energy structure of electron states at the interface. The role of the photoinduced intermolecular charge transfer in the development of photoconductivity in the given heterostructure is considered.     

Surface potential measurement of organic photo-diode consisting of fullerene/copper phthalocyanine double layered device

We investigated the surface potential built across the electrode/fullerene (C₆₀) or copper phthalocyanine (CuPc) interface and C₆₀/CuPc interface as a function of the thickness of the semiconductor film in the dark condition and under illumination. The surface potential of C₆₀ on Au, Al and Mg changes negatively with the increment of film thickness and it saturates at − 0.25, − 1.0 and − 1.5 V within 20 nm. The Fermi level alignment at C₆₀/electrode interface is established within ∼ 20 nm from electrode, and very high electric field exists due to the displacement of negative electronic charges from electrode into C₆₀. On the other hand, the surface potential of CuPc on ITO changes to + 0.1 V, and the work functions of C₆₀ and CuPc were estimated as 5.0 eV and 4.7 eV. C₆₀ film also accepts electrons from CuPc at hetero-junction interface, and the Fermi-level alignment was again obtained at C₆₀/CuPc interface under illumination. The built-in potential of ca. 0.3 V formed at C₆₀/CuPc interface was considered as the origin of the reduction of open-circuit voltage in ITO/CuPc/C₆₀/Au device compared with the optimum value of 0.6 V. On the other hand, the very high electric field formed at C₆₀/Mg contact improved the photovoltaic properties.     

Determination of first-order molecular hyperpolarizability of EAADCy and EDMAADCy using steady-state spectroscopic data and quantum-chemical calculations

Ethyl 5-(4-aminophenyl)-3-amino-2,4-dicyanobenzoate (EAADCy) and ethyl 5-(4-dimethylaminophenyl)-3-amino-2,4-dicyanobenzoate (EDMAADCy) organic molecules containing separate electron donor and electron acceptor groups belong to biphenyl derivatives in which a large dipole moment change between ground (S₀) and the first intramolecular charge transfer excited (S₁) states, as well as a large transition moment have been noted. The existence of electronically excited states with a strong intermolecular charge transfer (ICT) character is an essential prerequisite for large non-linear optical properties. Therefore, in this paper, we present a scrupulous analysis of the first-order hyperpolarizabilities of the studied molecules using an equivalent internal field model of an organic molecule. The calculated (using semiempirical calculations, CAChe WS 5.04) additive part of the first-order hyperpolarizability, β_add, values are discussed in relationship to the experimental data of the charge transfer hyperpolarizability, β_CT, obtained from steady-state spectroscopic measurements.     

Intramolecular Electron Transfers in Donor/Acceptor Linked Molecules Based on Endohedral Lanthanide Metallofullerenes

Chemical syntheses and intramolecular electron transferring behaviors in the electron donor/acceptor conjugates based on endohedral metallofullerenes, La₂@C₈₀ and La@C₈₂, are overviewed. A study on the photo-induced excited states of a La₂@C₈₀ derivative connected with an electron donor revealed the formation of a distinct radical ion pair state. A La@C₈₂ derivative linked with an electron donor demonstrated an unprecedented ion/anion pair state, and La₂@C₈₀ tethered with an acceptor showed a fullerene donor system, in which the fullerene acts as an electron donor. Using endohedral lanthanide metallofullerenes for intramolecular electron transferring systems opens a new door for developing novel molecular materials.     

Characterization of the CdS / Cu(In,Ga)Se2 interface by electron beam induced currents

The method of electron beam induced currents in junction configuration (JEBIC) has been employed to investigate carrier collection in Cu(In,Ga)Se₂ solar cells. A detailed analysis of JEBIC line-scans reveals unexpected carrier collection properties, which cannot be explained with common models. We ascribe this anomalous behavior to an electrostatic barrier effect at the Cu(In,Ga)Se₂ / CdS interface. We suggest the existence of a thin defect-layer on the surface of the Cu(In,Ga)Se₂ with high acceptor concentration and valence band edge that is energetically lower than that of the bulk. Using this model, we achieve a good agreement between experimental and simulated JEBIC line-scans. The influence of the barrier effect is considerably reduced by a metastable change of the interface properties induced by intensive electron irradiation of the interface. This effect is explained by a metastable decrease of the negative charge density in the defect-layer.     

Triphenylene‐Derived Electron Acceptors and Donors on Ag(111): Formation of Intermolecular Charge‐Transfer Complexes with Common Unoccupied Molecular States

Over the past years, ultrathin films consisting of electron donating and accepting molecules have attracted increasing attention due to their potential usage in optoelectronic devices. Key parameters for understanding and tuning their performance are intermolecular and molecule–substrate interactions. Here, the formation of a monolayer thick blend of triphenylene‐based organic donor and acceptor molecules from 2,3,6,7,10,11‐hexamethoxytriphenylene (HAT) and 1,4,5,8,9,12‐hexaazatriphenylenehexacarbonitrile (HATCN), respectively, on a silver (111) surface is reported. Scanning tunneling microscopy and spectroscopy, valence and core level photoelectron spectroscopy, as well as low‐energy electron diffraction measurements are used, complemented by density functional theory calculations, to investigate both the electronic and structural properties of the homomolecular as well as the intermixed layers. The donor molecules are weakly interacting with the Ag(111) surface, while the acceptor molecules show a strong interaction with the substrate leading to charge transfer and substantial buckling of the top silver layer and of the adsorbates. Upon mixing acceptor and donor molecules, strong hybridization occurs between the two different molecules leading to the emergence of a common unoccupied molecular orbital located at both the donor and acceptor molecules. The donor acceptor blend studied here is, therefore, a compelling candidate for organic electronics based on self‐assembled charge‐transfer complexes.     

Investigation of low resistance transparent MoO3/Ag/MoO3 multilayer and application as anode in organic solar cells

Depending on the resistivity and transmittance, transparent conductive oxides (TCO) are widely used in thin film optoelectronic devices. Thus doped In₂O₃ (ITO), ZnO, SnO₂ are commercially developed. However, the deposition process of these films need sputtering and/or heating cycle, which has negative effect on the performances of the organic devices due to the sputtering and heat damages. Therefore a thermally evaporable, low resistance, transparent electrode, deposited onto substrates room temperature, has to be developed to overcome these difficulties. For these reasons combination of dielectric materials and metal multilayer has been proposed to achieve high transparent conductive oxides. In this work the different structures probed were: MoO₃ (45 nm)/Ag (x nm)/MoO₃ (37.5 nm), with x = 5-15 nm. The measure of the electrical conductivity of the structures shows that there is a threshold value of the silver thickness: below 10 nm the films are semiconductor, from 10 nm and above the films are conductor. However, the transmittance of the structures decreases with the silver thickness, therefore the optimum Ag thickness is 10 nm. A structure MoO₃ (45 nm)/Ag (10 nm)/MoO₃ (37.5 nm) resulted with a resistivity of 8 × 10^− 5 Ω cm and a transmittance, at around 600 nm, of 80%. Such multilayer structure can be used as anode in organic solar cells according to the device anode/CuPc/C₆₀/Alq₃/Al. We have already shown that when the anode of the cells is an ITO film the introduction of a thin (3 nm) MoO₃ layer at the interface anode (ITO)/organic electron donor (CuPc) allows reducing the energy barrier due to the difference between the work function of ITO and the highest occupied molecular orbital of CuPc [1]. This property has been used in the present work to achieve a high hole transfer efficiency between the CuPc and the anode. For comparison MoO₃/Ag/MoO₃/CuPc/C₆₀/Alq₃/Al and ITO/MoO₃/CuPc/C₆₀/Alq3/Al solar cells have been deposited in the same run. These devices exhibit efficiency of the same order of magnitude.     

Morphological control of CuPc and its application in organic solar cells

We have prepared organic photovoltaic (OPV) cells possessing an ideal bulk heterojunction (BHJ) structure using the self-assembly of copper phthalocyanine (CuPc) as the donor material and fullerene (C(60)) as the acceptor. The variable self-assembly behavior of CuPc on a diverse range of substrates (surface energies) allowed us to control the morphology of the interface and the degree of carrier transportation within the active layer. We observed rod-like CuPc structures on indium-tin oxide (ITO), poly(3,4-ethylenedioxythiophene)-poly(4-styrenesulfonate) (PEDOT:PSS) and Au substrates. Accordingly, the interfaces and continuing transport path between CuPc and fullerene domains could be greatly improved due to the ideal BHJ structure. In this paper, we discuss the mechanisms of producing CuPc rod-like films on ITO, PEDOT:PSS and Au. The OPV cell performance was greatly enhanced when a mixture of horizontal and vertical CuPc rods was present on the PEDOT:PSS surfaces, i.e.?the power conversion efficiency was 50 times greater than that of the corresponding device featuring a planar CuPc structure.     

Tuning the Surface Electron Accumulation Layer of In2O3 by Adsorption of Molecular Electron Donors and Acceptors

In₂O₃, an n-type semiconducting transparent transition metal oxide, possesses a surface electron accumulation layer (SEAL) resulting from downward surface band bending due to the presence of ubiquitous oxygen vacancies. Upon annealing In₂O₃ in ultrahigh vacuum or in the presence of oxygen, the SEAL can be enhanced or depleted, as governed by the resulting density of oxygen vacancies at the surface. In this work, an alternative route to tune the SEAL by adsorption of strong molecular electron donors (specifically here ruthenium pentamethylcyclopentadienyl mesitylene dimer, [RuCp*mes]₂) and acceptors (here 2,2′-(1,3,4,5,7,8-hexafluoro-2,6-naphthalene-diylidene)bis-propanedinitrile, F₆TCNNQ) is demonstrated. Starting from an electron-depleted In₂O₃ surface after annealing in oxygen, the deposition of [RuCp*mes]₂ restores the accumulation layer as a result of electron transfer from the donor molecules to In₂O₃, as evidenced by the observation of (partially) filled conduction sub-bands near the Fermi level via angle-resolved photoemission spectroscopy, indicating the formation of a 2D electron gas due to the SEAL. In contrast, when F₆TCNNQ is deposited on a surface annealed without oxygen, the electron accumulation layer vanishes and an upward band bending is generated at the In₂O₃ surface due to electron depletion by the acceptor molecules. Hence, further opportunities to expand the application of In₂O₃ in electronic devices are revealed.     

Analysis of the edge emission of highly conductive CuGaTe2

Low temperature photoluminescence of CuGaTe₂ was studied using number of different samples. Totally 11 photoluminescence bands were detected in the edge emission region. It is shown that at least 6 bands have peak positions at higher energy than the lowest optical bandgap of CuGaTe₂. These bands were explained by using a model of resonant acceptor states (Fano-type resonances) in the valence band of CuGaTe₂. Thus, the electron from the conduction band or from the donor level recombines with holes from acceptor levels related to the different valence bands. The energetic distance between these valence bands is found to be 84 meV.     

A New Design Strategy for Efficient Thermally Activated Delayed Fluorescence Organic Emitters: From Twisted to Planar Structures

In the traditional molecular design of thermally activated delayed fluorescence (TADF) emitters composed of electron‐donor and electron‐acceptor moieties, achieving a small singlet–triplet energy gap (ΔE_ST) in strongly twisted structures usually translates into a small fluorescence oscillator strength, which can significantly decrease the emission quantum yield and limit efficiency in organic light‐emitting diode devices. Here, based on the results of quantum‐chemical calculations on TADF emitters composed of carbazole donor and 2,4,6‐triphenyl‐1,3,5‐triazine acceptor moieties, a new strategy is proposed for the molecular design of efficient TADF emitters that combine a small ΔE_ST with a large fluorescence oscillator strength. Since this strategy goes beyond the traditional framework of structurally twisted, charge‐transfer type emitters, importantly, it opens the way for coplanar molecules to be efficient TADF emitters. Here, a new emitter, composed of azatriangulene and diphenyltriazine moieties, is theoretically designed, which is coplanar due to intramolecular H‐bonding interactions. The synthesis of this hexamethylazatriangulene‐triazine (HMAT‐TRZ) emitter and its preliminary photophysical characterizations point to HMAT‐TRZ as a potential efficient TADF emitter.     

Studies on the Photocatalytic Electron Pooling of Graphene Oxide Hybrids Decorated with Electron Donor and Electron Acceptor Molecules

Photocatalytic behavior of a recently synthesized, single layer graphene oxide (SLGO) decorated with an electron donor, zinc phthalocyanine (ZnPc) and an electron acceptor, fulleropyrrolidine (C₆₀) donor-acceptor hybrid is demonstrated. Electron accumulation in the form of one-electron reduced product of methyl viologen was obtained in high yields in an electron pooling experiment involving the ZnPc-SLGO-C₆₀ hybrid and a sacrificial electron donor compared with control hybrids involving either ZnPc-SLGO or SLGO-C₆₀ hybrids. This novel property of ZnPc-SLGO-C₆₀ hybrid has been ascribed to the proximity effect offered by GO with covalently linked donor and acceptor entities on its surface. The present studies reveal that the ZnPc-SLGO-C₆₀ hybrid is a suitable catalyst for solar fuel production.     

Tuning the morphology and molecular orientation of copper hexadecafluorophthalocyanine thin films by solvent annealing

We present a detailed investigation of the molecular orientation transition and resulting morphology of copper hexadecafluorophthalocyanine (F₁₆CuPc) thin films induced by solvent annealing. The F₁₆CuPc molecules reorganize from small spherical or fibre-like crystals to large-size ribbon crystals which then dominate the resulting film properties. This reorganization and the formation of the ribbon crystals are closely related to the evaporation of solvent molecules and Ostwald ripening. The resulting thin films demonstrate morphological and structural characteristics with significant potential for application in high-performance organic electronic devices.     

Crystal Structure of C60 and C70 Compounds

Abstract

Fullerene intercalated compounds are the most intensively examined molecular materials to exhibit superconducting, ferromagnetic, optical non-linear and other properties. the fullerene C₆₀ or C₇₀ serve usually as electron acceptors in these materials. Although the electron acceptor properties of the fullerene are similar to those of the weak organic acceptors, the fullerene forms various C₆₀-based materials, namely clathrates, charge-transfer complexes and weak, molecular complexes. the search for cation species for fullerene-based materials is one of the routes towards progress in the design of materials with interesting physical properties.

Physical properties of the fullerene-derived molecular compounds are determined mainly by their crystal structure packing. Relatively large cavities in the fullerene solid can easily accommodate small units like solvent molecules or electron-donor organic compounds. An intercalation with these species is usually accompanied either by a lowering in crystal symmetry or by a change in the stacking arrangement of the C₆₀ spheres. These factors influence the interactions between fullerene (host) and an organic molecule (guest). Charge transfer between the electron donor molecule and the fullerene is usually weak and is hindered by unfavourable steric factors; it does not correlate with the ionization potential of the donor.

In this paper we present characteristic structures of one-, two-, and three-component fullerene compounds or else the structures of fullerene clathrates, neutral (van der Waals) complexes and ionic charge-transfer complexes. One can conclude that the stability and properties of the fullerene-based derivatives are defined by the steric compatibility between the three-dimensional donor and the spherical or elongated fullerene.     

Fill factor enhancement of organic solar cells based on a wide bandgap phosphorescent material and C60

Planar heterojunction organic solar cells using wide bandgap phosphorescent material bis[2-(4-tertbutylphenyl)benzothiazolato-N,C^2'] iridium (acetylacetonate) [(t-bt)₂Ir(acac)] as electron donor and fullerene (C₆₀) as electron acceptor were fabricated. A large open circuit voltage of 0.94 V was achieved due to low highest occupied molecular orbital level of (t-bt)₂Ir(acac). The effect of different hole transport layers and substrate heating were investigated to improve fill factor. It is shown that the open circuit voltage is strongly influenced by the interface energy barrier, whereas the fill factor is mainly limited by the charge carrier transport properties in active materials. The fill factor was significantly improved by either using hole transport layer with high carrier mobility or increasing the hole mobility of (t-bt)₂Ir(acac). A power conversion efficiency of 2.10% under AM 1.5 solar illumination at an intensity of 100 mW/cm² was achieved by heating the substrate during the deposition of active materials.