Donor Acceptor Materials Research Articles

Synthesis, Molecular and Photovoltaic Properties of an Indolo[3,2‐b]indole‐Based Acceptor–Donor–Acceptor Small Molecule

AbstractIndolo[3,2‐b]indole, containing two fused indole units, is an unexplored but promising electron‐rich molecule for constructing donor–acceptor materials due to its planar, symmetric, and extended conjugated structure. We have successfully developed a new synthetic pathway to prepare 2,7‐diboronic ester‐indolo[3,2‐b]indole, which was then reacted with dithienodiketopyrrolo‐pyrrole acceptor to afford a new acceptor–donor–acceptor (A–D–A) conjugated molecule, 2,7‐bis(dithienodiketopyrrolo‐pyrrole)indolo[3,2‐b]indole (2,7‐DPPIIDPP). II is used to stand for indolo[3,2‐b]indole in order to emphasize that this compound is constructed from two indole units. The A–D–A linkage through the 2,7‐positions of II not only preserves the phenylene units in the para‐conjugation but also renders stronger electron‐donating strength. This material exhibited good thermal stability, high crystallinity, and broad UV/Vis absorption. The solution‐processed bulk heterojunction device using the configuration of ITO/PEDOT:PSS/2,7‐DPPIIDPP:PC71BM/Ca/Al exhibited a Voc of 0.72 V, a Jsc of 6.88 mA/cm2, and an FF of 49.6 %, leading to a power conversion efficiency (PCE) of 2.45 %.

Intramolecular Excimer Formation between 3,6-Di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione Chromophoric Groups Linked by a Flexible Alkyl Spacer

Bichromophoric molecules containing two 3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (DPP) moieties linked via aliphatic spacers of different length are synthesized. Optical absorption spectroscopy indicates that the molecules adopt an extended conformation in solution. Fluorescence spectroscopy shows that photons are emitted from the locally excited singlet state in an extended conformation. In sufficiently polar solvents, quenching of fluorescence occurs and fluorescence quantum yield (ΦF) and fluorescence lifetime (τF) measurements indicate formation of an intramolecular excimer as the quenching mechanism. The redox potentials of the molecules and the solvent polarity dependence of the quenching are consistent with significant charge-transfer character of the excimer state. Photoinduced absorption measurements show enhanced intersystem crossing to the triplet state in polar solvents. Results indicate that in donor-acceptor π-conjugated materials involving the DPP moiety, excimer-like interchain polaron pair excited states could play an important role in the photophysics because of their close proximity in energy to the lowest singlet excited state.

Charge Photogeneration in Donor–Acceptor Conjugated Materials: Influence of Excess Excitation Energy and Chain Length

We investigate the role of excess excitation energy on the nature of photoexcitations in donor-acceptor π-conjugated materials. We compare the polymer poly(2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[1,2-b;3,4-b']dithiophene)-4,7-benzo[2,1,3]thiadiazole) (PCPDTBT) and a short oligomer with identical constituents at different excitation wavelengths, from the near-infrared up to the ultraviolet spectral region. Ultrafast spectroscopic measurements clearly show an increased polaron pair yield for higher excess energies directly after photoexcitation when compared to the exciton population. This effect, already observable in the polymer, is even more pronounced for the shorter oligomer. Supported by quantum chemical simulations, we show that excitation in high-energy states generates electron and hole wave functions with reduced overlap, which likely act as precursors for the polaron pairs. Interestingly, in the oligomer we observe a lifetime of polaron pairs which is one order of magnitude longer. We suggest that this behavior results from the intermolecular nature of polaron pairs in oligomers. The study excludes the presence of carrier multiplication in these materials and highlights new aspects in the photophysics of donor-acceptor small molecules when compared to polymers. The former are identified as promising materials for efficient organic photovoltaics.

Theoretical investigations for organic solar cells

Organic solar cells (OSCs) exhibit tremendous potential in the world’s energy strategy. The achievement of high power conversion efficiencies (PCEs) can accelerate their practical applications. To maximise the PCEs of OSCs, it requires a full understanding of the elementary processes for the conversion of solar energy into electricity and finding optimal combinations of donor–acceptor materials in terms of optical, electronic and morphological properties, which can be obtained through the judicious selection of existing and new materials. Computational modelling plays an essential role in understanding and predicating the behaviour of OSC active materials at the molecular level. In this article, we review the successes and opportunities in using computational calculations in this field. We first provide a critical review of the current computational techniques used to assess the properties of isolated active component material in OSCs and the morphologies of device. We then briefly discuss the modelling of elementary processes in OSCs. Finally, special attention is paid to the theoretical design of materials for photovoltaic applications; in particular, a design strategy that involves spectral tailoring is presented. This microscopic modelling may allow experimentalists to choose materials based on the predicated properties rather than by trial and error.

Investigation on bistable resistance switching characteristics of an organic donor–acceptor material for high density data storage

In this paper, organic thin films of a donor-acceptor material N,N′-bis[4-(1,1-dicyanovinyl)phenyl]N,N′-bis(4-fluorophenyl)benzidine (TPD-FCN) were prepared by a vacuum vapor deposition method. The N,N,N′,N′-tetraphenylbenzidine moiety can be served as an excellent nonplanar electron donor and the 1,1-dicyanovinyl group is an excellent electron acceptor. The typical macroscopic I–V curves of the device based on TPD-FCN thin film exhibit a bistable resistive switching characteristics for memory application. Furthermore, stable nanoscale electrical information storage was achieved on the TPD-FCN thin film by scanning tunneling microscopy. The average size of the recorded marks is about 10 nm. Local I–V characteristics suggest that the formation of the recording dots is due to the local change of electrical property of the thin film, and the intermolecular charge transfer induced by an electric field is proposed as the reason for the information dot formation. The results indicate that TPD-FCN is a promising candidate for nanoscale recording materials.

Efficient hybrid solar cells using PbSxSe1−x quantum dots and nanorods for broad-range photon absorption and well-assembled charge transfer networks

We present hybrid solar cells with high efficiency utilizing a novel donor-acceptor combination of poly[2,6-(4,4'-bis-(2-ethylhexyl)dithieno[3,2-b:2',3'-d]silole)-alt-4,7(2,1,3-benzothiadiazole)] (PSBTBT) and a PbSxSe1-x inorganic semiconductor. Several nanocomposite parameters are evaluated in order to improve the hybrid device efficiency, including donor-acceptor materials, surface modification of the inorganic semiconductor, and mixture of quantum dots (QDs) and nanorods (NRs) in the polymer matrix. A high power conversion efficiency (PCE) of ~3.4% is attained from the optimal device with a PbS0.7Se0.3 QD : NR blending ratio of 0.3 : 0.7 (wt/wt) under air mass (AM) 1.5 solar illumination, which is attributed to the broad-range absorption of the solar energies and the efficient charge separation and transport dynamics. The optoelectronic and nanomorphological properties of these novel hybrid solar cells are described.

Triphenylene Substituted Pyrene Derivative: Synthesis and Single Molecule Investigation

A novel donor–acceptor material based on pyrene derivative with two substituted triphenylenes (Py-TP2) is synthesized via the Sonogashira coupling reaction. The structure and physical chemistry properties of the target molecule have been discussed, ranging from the traditional 1H NMR and high-resolution mass spectroscopy (HRMS), over UV and PL spectra, and to the surface science research. The results revealed that the Py-TP2 molecule shows a narrowed energy gap between LUMO–HOMO and a bathochromic shift of 27 nm in the solid state as compared to that in solution, which is important for its practical applications in optoeletronic devices. Moreover, combined with DFT calculations, our STM results clearly show that the Py-TP2 molecule assembled into a stable long-ranged zigzag structure on HOPG surface. The interesting results in this contribution will boost the physical chemistry study of other functional materials under such methods.

Time-Resolved Studies of Charge Recombination in the Pyrene/TCNQ Charge-Transfer Crystal: Evidence for Tunneling

Previous studies of solid-state tetracyanobenzene-based donor-acceptor complexes showed that these materials were highly susceptible to both laser and mechanical damage that complicated the analysis of their electron-transfer kinetics. In this paper, we characterize the optical properties of a pyrene/tetracyanoquinodimethane charge-transfer crystal that is much more robust than the tetracyanobenzene compounds. This donor-acceptor complex has a charge-transfer absorption that extends into the near-infrared, rendering the crystal black. We use time-resolved fluorescence and diffuse reflectance transient absorption to study its dynamics after photoexcitation. We show that the initially excited charge-transfer state undergoes a rapid, monoexponential decay with a lifetime of 290 ps at room temperature. There is no evidence for any long-lived intermediate or dark states; therefore, this decay is attributed to charge recombination back to the ground state. Fluorescence lifetime measurements demonstrate that this process becomes temperature-independent below 60 K, indicative of a thermally activated tunneling mechanism. The subnanosecond charge recombination makes this low-band-gap donor-acceptor material a poor candidate for generating long-lived electron-hole pairs.

Prediction of Remarkable Ambipolar Charge-Transport Characteristics in Organic Mixed-Stack Charge-Transfer Crystals

We have used density functional theory calculations and mixed quantum/classical dynamics simulations to study the electronic structure and charge-transport properties of three representative mixed-stack charge-transfer crystals, DBTTF-TCNQ, DMQtT-F(4)TCNQ, and STB-F(4)TCNQ. The compounds are characterized by very small effective masses and modest electron-phonon couplings for both holes and electrons. The hole and electron transport characteristics are found to be very similar along the stacking directions; for example, in the DMQtT-F(4)TCNQ crystal, the hole and electron effective masses are as small as 0.20 and 0.26 m(0), respectively. This similarity arises from the fact that the electronic couplings of both hole and electron are controlled by the same superexchange mechanism. Remarkable ambipolar charge-transport properties are predicted for all three crystals. Our calculations thus provide strong indications that mixed-stack donor-acceptor materials represent a class of systems with high potential in organic electronics.

Synthesis and unexpected halochromism of carbazole-functionalized dithienophospholes

A series of π-conjugated donor–acceptor (D/A) chromophores using dithieno[3,2-b:2′,3′-d]phosphole oxide as an acceptor and 3,6-carbazole as a donor component have been synthesized and characterized. The studies involved several molecular species with D–A, A–D–A, D–A–D architecture, as well as a polymeric species (D–A)n. The different photophysical properties of the systems can be attributed to the carbazole unit de facto acting as aniline species for the π-conjugated system. Treatment of the donor–acceptor materials with acids resulted in the significant red shift of the absorption and emission wavelengths and the process was found to be reversible. Investigations via Density-Functional Theory (DFT) calculations revealed the nature of the unexpected red shift upon protonation.

Synthesis and Characterization of Poly(5,8-quinoxaline ethynylene)s

A majority of conjugated organic polymers are electron-rich materials, with far fewer electron-poor (i.e., electron accepting) analogues. Here we report the synthesis and preliminary characterization of new class of electron-poor poly(arylene ethynylene)s (PAEs) that contain 5,8-quinoxaline ethynylene repeat units. While various PAE copolymers consisting of alternating electron-rich and electron-poor units display lower bandgaps than poly(phenylene ethynylene)s, the poly(5,8-quinoxaline ethynylene) (PQE) reported in this study has a higher electron affinity and lower bandgap (2.25 eV) than many of these donor–acceptor materials. In comparison to poly(5,8-quinoxaline)s (PQs), which do not have an ethynylene linkage between the quinoxalines, the PQE has a red-shifted absorption spectrum that is consistent with a more highly conjugated and planar backbone. In addition, the PQE has a lower electrochemical reduction potential than both a corresponding PQ and a donor–acceptor alternating PAE copolymer that contains the quinoxaline unit as the electron-poor component.

Hybrid Semiconductor Nanoparticles: π-Conjugated Ligands and Nanostructured Films

Hybrid inorganic–organic colloidal nanoparticles can be designed to achieve specific and complementary optoelectronic properties different from their sole organic and inorganic counterparts. The efficient coupling between organic and inorganic moieties facilitates optimization of these optoelectronic properties as single particles. Simply dispersing inorganic nanoparticles in an organic (polymer) matrix permits nanocomposite formation, but are usually prone to phase segregation. To achieve more-efficient energy transfer, charge carrier transport, and correspondence between energy levels of the inorganic and organic moieties, direct coupling is necessary. Ligand exchange with highly π-conjugated organic ligands or polymerization of optoelectronically active organic polymer on the surface of the inorganic nanoparticles, results in core–shell-like structures. This increases the surface-area-to-volume ratio contact between organic and inorganic moieties. Because of this advantage, energy transfer mechanism in hybrids can be tuned more efficiently for radiative or nonradiative decay. Recombination of excitons (bound electron–hole pairs) or the isolation of electrons (modulating charge transport) by controlling the conduction band-valence band (highest occupied molecular orbital–lowest unoccupied molecular orbital (HOMO–LUMO)) level becomes more tunable in donor–acceptor materials systems. Such optoelectronic property fine-tuning in a hybrid colloidal system can also be applied toward ultrathin film preparation and two-dimensional patterning (e.g., photovoltaic systems, light-emitting diode materials, sensors, and patterned arrays). A compatible organic shell facilitates greater solubility and dispersion of the inorganic core in a host polymer matrix. The use of a dendronic ligand provides an interesting method for facilitating surface functionalization, nanoparticle solubility, and electrochemical reactivity. By focusing on chalcogenide and semiconductor nanocrystals (NCs) or quantum dots (QDs), it is possible to limit these properties directly to charge carrier transport and energy transfer mechanisms. Each hybrid nanoparticle in essence carries the same properties replicated or dispersed within a film or a pattern. This review focuses on the design, preparation, and properties of such nanomaterial systems.

Poly(3-alkylthiophene) Nanofibers for Photovoltaic Energy Conversion

The use of nanostructured non-conventional semiconductors such as conjugated polymers and metal oxides (e.g. TiO2), opens promising perspectives towards a new generation of solar cells based on the concept of donor:acceptor bulk heterojunctions. In this concept donor material and acceptor material form interpenetrating networks allowing light absorption, charge transfer and charge transport throughout the entire bulk of the thin film. Since nanomorphology is of crucial importance for this type of solar cells, in this contribution the use of nanofibers in bulk heterojunction solar cells is explored in order to obtain highways for charge transport. We investigate in particular the use of P3AT (poly(3-alkylthiophene)) nanofibers and show that the polymer fraction aggregated into fibers can be easily controlled by temperature. We find an optimal efficiency at intermediate fiber fraction and show that it can be linked to the morphology of the active layer.

Synthesis and Characterization of Boron Azadipyrromethene Single-Wall Carbon Nanotube Electron Donor−Acceptor Conjugates

The preparation of a novel donor-acceptor material, consisting of a red/near-infrared (NIR) absorbing boron azadipyrromethene donor covalently attached to a highly functionalized single-wall carbon nanotube (SWNT) acceptor, which bears great potential in the field of organic photovoltaics, has been demonstrated. Both purification and covalent functionalization of SWNTs have been demonstrated using a number of complementary characterization techniques, including atomic force microscopy, Raman, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared, and NIR-photoluminescence spectroscopy, and a functionalization density of approximately 1 donor molecule per 100 SWNT atoms has been estimated by XPS. The redox behavior of the fluorophore has been investigated by electrochemistry and spectroelectrochemistry as well as by pulse radiolysis. The donor-acceptor properties of the material have been characterized by means of various spectroscopic techniques, such as UV-vis NIR absorption spectroscopy, steady-state and time-resolved fluorescence spectroscopy, and time-resolved transient absorption spectroscopy. Charge transfer from the photoexcited donor to the SWNT acceptor has been confirmed with a radical ion pair state lifetime of about 1.2 ns.

Charge and energy transfer processes in ruthenium(II) phthalocyanine based electron donor–acceptor materials—implications for solar cell performance

Six supramolecular electron donor–acceptor hybrids, based on a ruthenium(II) phthalocyanine [RuPc] coordinating different dendritic oligothiophene (DOT) ligands [Py-nT] (n = 3, 9, 21) in either one [RuPcCO(Py-nT)] or two [RuPc(Py-nT)2] axial positions, have been characterized by standard spectroscopic methods and their photophysical behavior has been established by using ultrafast and fast time-resolved techniques. Based on the spectrochemical and radiolytically generated [Py-nT] (i.e., one-electron reduction of [Py-nT]) and [RuPcCO(Py) or RuPc(Py)2] (i.e., one-electron oxidation of [RuPcCO(Py) or RuPc(Py)2]) features, the deactivation processes were assigned to a solvent independent energy transfer in RuPcCO(Py-3T) and RuPc(Py-3T)2 and a strongly solvent dependent charge transfer mechanism, which competes with the energy transfer and the intersystem crossing for RuPcCO(Py-9T), RuPc(Py-9T)2, RuPcCO(Py-21T) and RuPc(Py-21T)2.

One-Pot (Synthesis) Feeds Many Chemists

The strong electron-accepting properties of the benzo[1,2-c;4,5-c′]bis[1,2,5]thiadiazole (BBT) unit (4) have been widely exploited to make low bandgap donor-acceptor materials. This property is reflected in even the small oligomers 5 and 6 being reported to possess bandgaps of only 1.5 eV. Here, the authors present a one-pot synthesis of 4,8-dibromobenzo[1,2-c;4,5-c′]bis[1,2,5]thiadiazole, a convenient precursor for including the BBT unit in larger compounds/polymers, starting from the salt of the ­respective tetraaminobenzene.

Post annealing effect of flexible polymer solar cells to improve their electrical properties

Flexible polymer solar cells with an ITO/PEDOT/P3HT:PCBM/Al structure were fabricated using regioregular poly(3-hexylthiophene) (P3TH) polymer:(6,6)-phenyl C61-butyric acid methyl ester (PCBM) fullerene polymer as the photovoltaic (PV) bulk hetero-junction layer. The P3HT and PCBM used as the electron donor and electron acceptor materials were spin cast on indium tin oxide (ITO) coated polyethylene naphthalate (PEN) substrates. The optimum mixing concentration ratio of the P3HT:PCBM PV layer was found to be 4:4 wt.%, at which the maximum short circuit current density (J SC), open circuit voltage (V OC), fill factor (FF) and power conversion efficiency (PCE) values were about 3.8 mA/cm 2, 427 mV, 36.6% and 0.66%, respectively. To investigate the effects of the post annealing treatment, the as-prepared flexible polymer solar cells were post annealed at temperatures ranging from 150 °C to 180 °C for 5 min. The J SC and V OC values increased with increasing post annealing temperature from 150 °C to 170 °C, which may be due to the improvement of the light absorption coefficient of P3HT and improved ohmic contact between the PV layer and Al electrode film. The maximum J SC, V OC, FF and PCE values of the flexible polymer solar cell, which was post annealed at 170 °C for 5 min, were found to be about 4.3 mA/cm 2, 616 mV, 32.6% and 0.86%, respectively.

Nanostructured interfaces in polymer solar cells

The morphology in organic photovoltaics plays a key role in determining the device efficiency. We propose a method to fabricate bilayer devices with controlled nanostructured interfaces by combining nanoimprinting and lamination techniques. This technique allows us to achieve a network structure of donor-acceptor material with a ∼80 nm periodicity and ∼40 nm width. These structures have an abrupt interface between the donor and acceptor materials and show an increased effective interfacial area and photovoltaic performance compared to bilayer solar cells. In contrast to blend films, they will allow an in depth analysis of the influence of morphology on interfacial physical processes.

Photoinduced hole-transfer in semiconducting polymer/low-bandgap cyanine dye blends: evidence for unit charge separation quantum yield

Power-conversion efficiencies of organic heterojunction solar cells can be increased by using semiconducting donor-acceptor materials with complementary absorption spectra extending to the near-infrared region. Here, we used continuous wave fluorescence and absorption, as well as nanosecond transient absorption spectroscopy to study the initial charge transfer step for blends of a donor poly(p-phenylenevinylene) derivative and low-band gap cyanine dyes serving as electron acceptors. Electron transfer is the dominant relaxation process after photoexcitation of the donor. Hole transfer after cyanine photoexcitation occurs with an efficiency close to unity up to dye concentrations of approximately 30 wt%. Cyanines present an efficient self-quenching mechanism of their fluorescence, and for higher dye loadings in the blend, or pure cyanine films, this process effectively reduces the hole transfer. Comparison between dye emission in an inert polystyrene matrix and the donor matrix allowed us to separate the influence of self-quenching and charge transfer mechanisms. Favorable photovoltaic bilayer performance, including high open-circuit voltages of approximately 1 V confirmed the results from optical experiments. The characteristics of solar cells using different dyes also highlighted the need for balanced adjustment of the energy levels and their offsets at the heterojunction when using low-bandgap materials, and accentuated important effects of interface interactions and solid-state packing on charge generation and transport.

Integration of amyloid nanowires in organic solar cells

Amyloid nanowires were incorporated in organic photovoltaic devices in order to enhance the transport properties. Amyloid fibrils act as a template for donor-acceptor materials. The current-voltage characteristics under illumination and in the dark display a maximum for the fill factor and the space charge limit current, respectively, at an amyloid nanowire-donor-acceptor mass ratio of 0.014:1:1, associated to a better charge transport in the donor-acceptor domains. The absorption experiments display a redshift associated to a more planar polymer backbone with increasing concentration of amyloid fibrils. Amyloid nanowires present a significant effect on the donor-acceptor materials organization.