Design and Properties of Intermediate-Sized Narrow Band-Gap Conjugated Molecules Relevant to Solution-Processed Organic Solar Cells

Control of Multi-Fluorination Number and Position in D-π-A Type Polymers and Their Impact on High-Voltage Organic Photovoltaics

Exploring the structure-performance relationship of high-voltage organic solar cells (OSCs) is significant for pushing material design and promoting photovoltaic performance. Herein, we chose a D-π-A type polymer composed of 4,8-bis(thiophene-2-yl)-benzo[1,2-b:4,5-b']dithiophene (BDT-T) and benzotriazole (BTA) units as the benchmark to investigate the effect of the fluorination number and position of the polymers on the device performance of the high-voltage OSCs, with a benzotriazole-based small molecule (BTA3) as the acceptor. F00, F20, and F40 are the polymers with progressively increasing F atoms on the D units, while F02, F22, and F42 are the polymers with further attachment of F atoms to the BTA units based on the above three polymers. Fluorination positively affects the molecular planarity, dipole moment, and molecular aggregations. Our results show that VOC increases with the number of fluorine atoms, and fluorination on the D units has a greater effect on VOC than on the A unit. F42 with six fluorine atom substitutions achieves the highest VOC (1.23 V). When four F atoms are located on the D units, the short-circuit current (JSC) and fill factor (FF) plummet, and before that, they remain almost constant. The drop in JSC and FF in F40- and F42-based devices may be attributed to inefficient charge transfer and severe charge recombination. The F22:BTA3 system achieves the highest power conversion efficiency of 9.5% with a VOC of 1.20 V due to the excellent balance between the photovoltaic parameters. Our study provides insights for the future application of fluorination strategies in molecular design for high-voltage organic photovoltaics.

Energetically More Stable Singlet Cyclopentane-1,3-diyl Diradical with π-Single Bonding Character than the Corresponding σ-Single Bonded Compound

Carbon-carbon σ-single bonds are crucial for constructing molecules like ethane derivatives (R3C-CR3), which are composed of tetrahedral four-coordinate carbons. Molecular functions, such as light absorption or emission, originate from the π-bonds existing in ethylene derivatives (R2C═CR2). In this study, a relatively stable cyclopentane-1,3-diyl species with π-single bonding system (C-π-C) with planar four-coordinate carbons is constructed. This diradicaloid is energetically more stable than the corresponding σ-single bonding system. The π-electron single bonding system provides deeper insights into the chemical bonding and the physical properties derived from the small energy gaps between the bonding and antibonding molecular orbitals.

Intramolecular Palladium Catalyst Transfer on Benzoheterodiazoles as Acceptor Monomers and Discovery of Catalyst Transfer Inhibitors

Intramolecular catalyst transfer on benzoheterodiazoles was investigated in Suzuki-Miyaura coupling reactions and polymerization reactions with t Bu3 PPd precatalyst. In the coupling reactions of dibromobenzotriazole, dibromobenzoxazole, and dibromobenzothiadiazole with pinacol phenylboronate, the product ratios of monosubstituted product to disubstituted product were 0/100, 27/73, and 89/11, respectively, indicating that the Pd catalyst undergoes intramolecular catalyst transfer on dibromobenzotriazole, whereas intermolecular transfer occurs in part in the case of dibromobenzoxazole and is predominant for dibromobenzothiadiazole. The polycondensation of 1.3 equivalents of dibromobenzotriazole with 1.0 equivalent of para- and meta-phenylenediboronates afforded high-molecular-weight polymer and cyclic polymer, respectively. In the case of dibromobenzoxazole, however, para- and meta-phenylenediboronates afforded moderate-molecular-weight polymer with bromine at both ends and cyclic polymer, respectively. In the case of dibromobenzothiadiazole, they afforded low-molecular-weight polymers with bromine at both ends. Addition of benzothiadiazole derivatives interfered with catalyst transfer in the coupling reactions.

A Review on the Materials Science and Device Physics of Semitransparent Organic Photovoltaics

In this review, the current state of materials science and the device physics of semitransparent organic solar cells is summarized. Relevant synthetic strategies to narrow the band gap of organic semiconducting molecules are outlined, and recent developments in the polymer donor and near-infrared absorbing acceptor materials are discussed. Next, an overview of transparent electrodes is given, including oxides, multi-stacks, thin metal, and solution processed electrodes, as well as considerations that are unique to ST-OPVs. The remainder of this review focuses on the device engineering of ST-OPVs. The figures of merit and the theoretical limitations of ST-OPVs are covered, as well as strategies to improve the light utilization efficiency. Lastly, the importance of creating an in-depth understanding of the device physics of ST-OPVs is emphasized and the existing works that answer fundamental questions about the inherent changes in the optoelectronic processes in transparent devices are presented in a condensed way. This last part outlines the changes that are unique for devices with increased transparency and the resulting implications, serving as a point of reference for the systematic development of next-generation ST-OPVs.

Effects of Side-Chain Engineering with the S Atom in Thieno[3,2-b]thiophene-porphyrin to Obtain Small-Molecule Donor Materials for Organic Solar Cells

To explore the effect of the introduction of heteroatoms on the properties of porphyrin materials, a new porphyrin-based derivative small-molecule donor named as PorTT-T was designed and synthesized based on alkyl-thieno[3,2-b]thiophene(TT)-substituted porphyrins. The linker bridge and end groups of PorTT-T were the same as those of XLP-II small-molecule donor materials, while the side-chain attached to the core of thieno[3,2-b]thiophene(TT)-substituted porphyrin was different. Measurements of intrinsic properties showed that PorTT-T has wide absorption and appropriate energy levels in the UV-visible range. A comparison of the morphologies of the two materials using atomic force microscopy showed that PorTT-T has a better surface morphology with a smaller root-mean-square roughness, and can present closer intermolecular stacking as compared to XLP-II. The device characterization results showed that PorTT-T with the introduced S atom has a higher open circuit voltage of 0.886 eV, a higher short circuit current of 12.03 mAcm⁻², a fill factor of 0.499, a high photovoltaic conversion efficiency of 5.32%, better external quantum efficiency in the UV-visible range, and higher hole mobility.

Energy-Level Manipulation in Novel Indacenodithiophene-Based Donor-Acceptor Polymers for Near-Infrared Organic Photodetectors

Organic photodetectors (OPDs) are promising candidates for next-generation digital imaging and wearable sensors due to their low cost, tuneable optoelectrical properties combined with high-level performance, and solution-processed fabrication techniques. However, OPD detection is often limited to shorter wavelengths, whereas photodetection in the near-infrared (NIR) region is increasingly being required for wearable electronics and medical device applications. NIR sensing suffers from low responsivity and high dark currents. A common approach to enhance NIR photon detection is lowering the optical band gap via donor-acceptor (D-A) molecular engineering. Herein, we present the synthesis of two novel indacenodithiophene (IDT)-based D-A conjugated polymers, namely, PDPPy-IT and PSNT-IT via palladium-catalyzed Stille coupling reactions. These novel polymers exhibit optical band gaps of 1.81 and 1.27 eV for PDPPy-IT and PSNT-IT, respectively, with highly desirable visible and NIR light detection through energy-level manipulation. Moreover, excellent materials' solubility and thin-film processability allow easy incorporation of these polymers as an active layer into OPDs for light detection. In the case of PSNT-IT devices, a photodetection up to 1000 nm is demonstrated with a peak sensitivity centered at 875 nm, whereas PDPPy-IT devices are efficient in detecting the visible spectrum with the highest sensitivity at 660 nm. Overall, both OPDs exhibit spectral responsivities up to 0.11 A W^-1 and dark currents in the nA cm^-2 range. With linear dynamic ranges exceeding 140 dB and fast response times recorded below 100 μs, the use of novel IDT-based polymers in OPDs shows great potential for wearable optoelectronics.

Optoelectronic and charge transport properties of D-n-A type 1,3,5-triazine derivatives: A combined experimental and DFT study

The synthesis of five D-n-A type star-shaped octupolar molecules is presented in the current work. The exploration of the potential applicability of molecules under study in organic optoelectronics as electron or hole transporting materials is carried out by DFT methods. All the molecules have a 1,3,5-triazine core, which acts as an electron acceptor (A). Phenyl ring and pyridine ring act as electron donors (D) in AZ and PZ series of molecules respectively. The donor and acceptor core are connected by -NH bridge (n). The crystal structure of a molecule in the PZ series is elucidated. Thermogravimetric studies are carried out to confirm the thermal stability of molecules. The frontier molecular orbitals of molecules are characterized with the help of cyclic voltammetry. With the assistance of DFT methodologies, the whole research presented in this work focuses on the electronic excitations, reorganization energies, electron affinity, ionization potential and features of frontier molecular orbitals of molecules. The investigation of the variation of optoelectronic properties of molecules with changing patterns of nucleophilic substitution on 1,3,5-triazine core and presence of a hetero (nitrogen) atom in the donor part of the molecule is also accomplished.

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.

Gradual Fluorination on the Phenyl Side Chains for Benzodithiophene-Based Linear Polymers to Improve the Photovoltaic Performance

To study the impact of introducing fluorine atoms onto the conjugated phenyl side chains of benzo[1,2-b:4,5-b']dithiophene (BDT)-based copolymers, three novel donor-π-acceptor (D-π-A) alternative polymers PE40, PE42, and PE44 were designed and synthesized. The phenyl-substituted-BDT, thieno[3,2-b]thiophene, and benzo[d][1,2,3]triazole (BTA) served as the donor, π-bridge, and acceptor units, respectively, to enable linear polymer backbones. When introducing two or four fluorine atoms into the phenyl side units of PE40, the polymers PE42 and PE44 demonstrate a gradual decrease of energy levels and an increase of crystallinity in the pristine and blend films. It was noted that the increase in fluorine atoms gradually improved the performance parameters of polymer solar cells (PSCs) with Y6 as the acceptor. The PE40:Y6 device yielded a power conversion efficiency (PCE) of up to 7.07% with a short-circuit (J_SC) of 21.36 mA cm^-2, an open-circuV_OC) of 0.65 V, and a fill factor (FF) of 0.51, and PE42:Y6 exhibited a better PCE of 10.11% (J_SC = 23.25 mA cm^-2, V_OC = 0.74 V, and FF = 0.59), while PE44:Y6 exhibited the best PCE of 13.62% (J_SC = 25.29 mA cm^-2, V_OC = 0.82 V, and FF = 0.66). The suitable energy offsets between the donor and the acceptor, high and balanced charge-carrier mobility, and the optimal morphology of the blend film contributed to the high performance of PE44:Y6 combination. Our results demonstrate that introducing more fluorine atoms onto the phenyl side units of BDT is a prospective approach to break the trade-offs between V_OC, J_SC, and FF, and finally improve the performance of PSCs.

Quantum Interference and Substantial Property Tuning in Conjugated Z-ortho-Regio-Resistive Organic (ZORRO) Junctions

Coherence is a significant factor in nanoscale electronic insulator technology and necessitates an understanding of the structure-property relationship between constructive and destructive quantum interference. This is particularly important in organic dielectric circuitry, which is the subject of this work. It is known that molecular wires composed of (i) meta-substituted phenylene rings, (ii) cross-conjugated double bonds (orthogonal to the molecular long axis), and (iii) single bonds can dramatically reduce electrical transmission. Here we add to these tools the use of an unexplored molecular shape to create strong and counterintuitive interference: a fully conjugated molecular wire with a structure that is forced back on itself in a Z shape, thereby exhibiting remarkably low conductance (G = 0.43 × 10^-9 S) even though the phenylene arrangements are ortho- rather than meta-disposed. We call these Z-shaped molecules having ultralow conduction Z-ortho-regio-resistive organics (ZORROs). Here we analyze a series of ZORRO molecules and find them to have significant insulating properties in the coherent electron-transport regime due to interfering transmission pathways in the phenylene rings. Importantly, we find that both electron-withdrawing (fluorine) and electron-donating (methoxy) substituents enhance the transmission, which is not desirable. The former is due to the suppression of the destructive quantum interference at the F site, thereby enhancing the overall transmission, much like a Büttiker probe. The latter is due to a methoxy unit resonance additive effect, akin to oxygen doping, and positively contributes to the transmission. We then examine the effects of replacing the phenylene rings with 4,5- and 3,4-disubstituted thiophenes and how this ZORRO modification further reduces the transmission. An ultralow conductance of 0.13 × 10^-9 S and a relatively high dielectric constant (ε_r) of ∼5 are predicted for the 3,4-thiophene ZORRO derivative, which closely resembles two cross-conjugated units, making it an intriguing candidate for a gate dielectric material.

Highly Efficient Indoor Organic Solar Cells by Voltage Loss Minimization through Fine-Tuning of Polymer Structures

Herein, we report a detailed study on the optoelectronic properties, photovoltaic performance, structural conformation, morphology variation, charge carrier mobility, and recombination dynamics in bulk heterojunction solar cells comprising a series of donor-acceptor conjugated polymers as electron donors based on benzodithiophene (BDT) and 5,8-bis(5-bromothiophen-2-yl)-6,7-difluoro-2,3-bis(3-(octyloxy)phenyl)quinoxaline as a function of the BDT's thienyl substitution (alkyl (WF3), alkylthio (WF3S), and fluoro (WF3F)). The synergistic positive effects of the fluorine substituents on the minimization of the bimolecular recombination losses, the reduction of the series resistances (R_S), the increment of the shunt resistances (R_Sh), the suppression of the trap-assisted recombination losses, the balanced charge transport, the finer nanoscale morphology, and the deeper highest occupied molecular orbital (E_HOMO) are manifested versus the alkyl and alkylthio substituents. According to these findings, the WF3F:[6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM)-based organic photovoltaic device is a rare example that features a high power conversion efficiency (PCE) of 17.34% under 500 lx indoor light-emitting diode light source with a high open-circuit voltage (V_OC) of 0.69 V, due to the suppression of the voltage losses, and a PCE of 9.44% at 1 sun (100 mW/cm²) conditions, simultaneously.

The Effect of Acceptor Structure on Emission Color Tuning in Organic Semiconductors with D-π-A-π-D Structures

A series of novel donor-acceptor D-π-A-π-D compounds were synthesized and characterized in order to determine the influence of different acceptor units on their properties. The introduction of acceptor moieties had a direct impact on the HOMO and LUMO energy levels. Fluorescence spectra of compounds can be changed by the choice of an appropriate acceptor and were shifted from the green to the near-infrared part of spectra. Due to observed concentration induced emission quenching, the green exciplex type host was used to evaluate the potential of synthesized molecules as emitters in organic light emitting diodes (OLEDs).

Energy level gamut-a wide-angle lens to look at photoelectronic properties of diketopyrrolopyrrole-benzothiadiazole-based small molecules

Demands in the field of molecular design for optimized bandgap and proper energy levels to obtain high efficiencies are growing progressively in organic electronics. In the present work, we designed a series of molecules based on diketopyrrolopyrrole (DPP) and benzothiadiazoles (BT). We also studied the efeect of the presence and position of the nitrogen atom as an effective heteroatom. Finally, we optimized the energy levels of the designed structures to find the most favorable donor properties along with fullerene and non-fullerene (NF) acceptors in bulk heterojunction (BHJ) solar cell systems. To shed new light on the electronic characteristics of the designed structures, we developed a correction gamut of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels. The gamut is a span that predicts the occurrence of practical HOMO or LUMO with high probability from density functional theory computations in the gas phase. The model was validated using experimental energy level values of a similar structure as reference material. The results obtained by the new pathway of combining the idea of energy level gamuts with the modified Scharber model for NF BHJ suggested that the designed structures can afford power conversion efficiencies (PCE) for NF-BHJ of 8.5-10.5%. Graphical abstract Improved approach for predicting power conversion efficiencies (PCE) of designed molecules.

Solution-Processed Semitransparent Organic Photovoltaics: From Molecular Design to Device Performance

Recent research efforts on solution-processed semitransparent organic solar cells (OSCs) are presented. Essential properties of organic donor:acceptor bulk heterojunction blends and electrode materials, required for the combination of simultaneous high power conversion efficiency (PCE) and average visible transmittance of photovoltaic devices, are presented from the materials science and device engineering points of view. Aspects of optical perception, charge generation-recombination, and extraction processes relevant for semitransparent OSCs are also discussed in detail. Furthermore, the theoretical limits of PCE for fully transparent OSCs, compared to the performance of the best reported semitransparent OSCs, and options for further optimization are discussed.

Photoinduced electron transfer in conjugated molecules containing different donor in an external electric field

Photoinduced electron transfers in different conjugated donor polymer containing with PC₆₀BM as the acceptor (X3:PC₆₀BM and X6:PC₆₀BM) were theoretically investigated. A detailed theoretical analysis of Marcus electron transfer rate in an external electric field was also performed. By discussing the effect of electric field on ΔG, λ, V_DA and charge separation rate, we have come to some conclusions. The results reveal that the free energy change and electronic coupling matrix are significantly influence by the vector properties of external electric field. Thus, the electron transfer process appears critically influenced by external electric fields. Our results will provide meaningful information on enhancing the photoelectric conversion efficiencies of BHJ solar cells.

Significant Influence of a Single Atom Change in Auxiliary Acceptor on Photovoltaic Properties of Porphyrin-Based Dye-Sensitized Solar Cells

The rational design of porphyrin sensitizers is always crucial for dye-sensitized solar cells (DSSCs), since the change of only a single atom can have a significant influence on the photovoltaic performance. We incorporated the pyridothiadiazole group, as a stronger electron-withdrawing group, into the commonly well-established skeleton of D-porphyrin-triple bond-acceptor sensitizers by a single atom change for a well-known strong electron-withdrawing benzothiadiazole (BTD) unit as an auxiliary acceptor. The impact of the pyridothiadiazole group on the optical; electrochemical; and photovoltaic properties of D⁻π⁻A porphyrin sensitizers was investigated with comparison for a benzothiadiazole-substituted SGT-020 porphyrin. The pyridothiadiazole-substituted SGT-024 porphyrin dye was red-shifted so that the absorption range might be expected to achieve higher light harvest efficiency (LHE) than the SGT-020 porphyrin. However, all the devices were fabricated by utilizing SGT-020 and SGT-024, evaluated and found to achieve a cell efficiency of 10.3% for SGT-020-based DSSC but 4.2% for SGT-024-based DSSC under standard global AM 1.5G solar light conditions. The main reason is the lower charge collection efficiency of SGT-024-based DSSC than SGT-020-based DSSC, which can be attributed to the tilted dye adsorption mode on the TiO₂ photoanode. This may allow for faster charge recombination, which eventually leads to lower Jsc, Voc and power conversion efficiency (PCE).

Kinetic Versus Thermodynamic Orientational Preferences for a Series of Isomorphic Molecular Semiconductors

Due to the anisotropic nature of charge transport through most organic semiconductors, the orientation of the conjugated backbone is of great relevance because it may affect final device properties. Herein, we present a set of four nearly isostructural molecular organic semiconducting materials whose orientation changes drastically with a two-atom change in the conjugated framework. We investigate the X-ray diffraction patterns of these materials in the thin film, both as-deposited from solution and following melt-annealing. Following melt-annealing of the films, crystallites of all four materials orient edge-on with respect to the substrate, which indicates that this orientation is thermodynamically preferred. We can infer that the initial face-on orientation of some of the materials is due to kinetic trapping during the spin-coating process. Previous observations from the literature suggest that the edge-on orientation is the thermodynamically preferable state for many organic semiconducting materials. However, a cohesive explanation for this phenomenon remains elusive.

Post-polymerisation functionalisation of conjugated polymer backbones and its application in multi-functional emissive nanoparticles

Backbone functionalisation of conjugated polymers is crucial to their performance in many applications, from electronic displays to nanoparticle biosensors, yet there are limited approaches to introduce functionality. To address this challenge we have developed a method for the direct modification of the aromatic backbone of a conjugated polymer, post-polymerisation. This is achieved via a quantitative nucleophilic aromatic substitution (SNAr) reaction on a range of fluorinated electron-deficient comonomers. The method allows for facile tuning of the physical and optoelectronic properties within a batch of consistent molecular weight and dispersity. It also enables the introduction of multiple different functional groups onto the polymer backbone in a controlled manner. To demonstrate the versatility of this reaction, we designed and synthesised a range of emissive poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT)-based polymers for the creation of mono and multifunctional semiconducting polymer nanoparticles (SPNs) capable of two orthogonal bioconjugation reactions on the same surface.

π-Bridge modification of thiazole-bridged DPP polymers for high performance near-IR OSCs

Thiophene-bridged and thiazole-bridged diketopyrrolopyrrole (DPP) polymers for near-infrared (near-IR) photovoltaic applications have been investigated via density functional theory (DFT) and Marcus charge transfer theory. Compared with thiophene-bridged DPP polymers, thiazole-bridged polymers have higher ionization potentials (IPs) but poorer optical absorption and worse charge transport capability. Different beneficial substituents replaced the hydrogen atoms (H) on the thiazole rings for the sake of reversing the disadvantages of thiazole-bridged DPP polymers and making these compounds better near-infrared absorbing materials. In order to gain deep insight into the impact of π-bridge modification on the photoelectronic properties of DPP polymers, their electronic structures, absorption capabilities, intramolecular charge transfer properties and charge transport performances have been analyzed. The calculated results reveal that π-bridge modification is a feasible way to improve the light-absorbing capability, electron excitation properties and charge transport performance of thiazole-bridged DPP polymers. It is expected that π-bridge modification can also work for other polymers containing π-bridge units. We hope that our research efforts will be helpful in the designing of new near-IR absorbing materials and could motivate further improvement of organic solar cells.

Structure-Property Relationships of Semiconducting Polymers for Flexible and Durable Polymer Field-Effect Transistors

We report high-performance top-gate bottom-contact flexible polymer field-effect transistors (FETs) fabricated by flow-coating diketopyrrolopyrrole (DPP)-based and naphthalene diimide (NDI)-based polymers (P(DPP2DT-T2), P(DPP2DT-TT), P(DPP2DT-DTT), P(NDI2OD-T2), P(NDI2OD-F2T2), and P(NDI2OD-Se2)) as semiconducting channel materials. All of the polymers displayed good FET characteristics with on/off current ratios exceeding 10⁷. The highest hole mobility of 1.51 cm² V^-1 s^-1 and the highest electron mobility of 0.85 cm² V^-1 s^-1 were obtained from the P(DPP2DT-T2) and P(NDI2OD-Se2) polymer FETs, respectively. The impacts of the polymer structures on the FET performance are well-explained by the interplay between the crystallinity, the tendency of the polymer backbone to adopt an edge-on orientation, and the interconnectivity of polymer fibrils in the film state. Additionally, we demonstrated that all of the flexible polymer-based FETs were highly resistant to tensile stress, with negligible changes in their carrier mobilities and on/off ratios after a bending test. Conclusively, these high-performance, flexible, and durable FETs demonstrate the potential of semiconducting conjugated polymers for use in flexible electronic applications.

Small Molecules Derived from Thieno[3,4-c]pyrrole-4,6-dione (TPD) and Their Use in Solution Processed Organic Solar Cells

In this work, microwave synthesis, chemical, optical and electrochemical characterization of three small organic molecules, TPA-TPD, TPA-PT-TPD and TPA-TT-TPD with donor-acceptor structure and their use in organic photovoltaic cells are reported. For the synthesis, 5-(2-ethylhexyl)-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione was used as electron withdrawing fragment while the triphenylamine was used as electron donating fragment. Molecular electronic geometry and electronic distribution density were established by density functional theory (DFT) calculations and confirmed by optical and chemical characterization. These molecules were employed as electron-donors in the active layer for manufacturing bulk heterojunction organic solar cells, where [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) was used as electron-acceptor. As cathode, Field's metal (FM), an eutectic alloy (Bi/In/Sn: 32.5%, 51%, and 16.5%, respectively) with a melting point above 62 °C, was easily deposited by drop casting under vacuum-free process and at air atmosphere. Prepared devices based on TPA-TPD:PC71BM (1:4 w/w ratio) presented a large VOC = 0.97 V, with JSC = 7.9 mA/cm², a FF = 0.34, then, a power conversion efficiency (PCE) of 2.6%.

A Nonchlorinated Solvent-Processable Fluorinated Planar Conjugated Polymer for Flexible Field-Effect Transistors

High carrier mobilities have recently been achieved in polymer field effect transistors (FETs). However, many of these polymer FET devices require the use of chlorinated solvents such as chloroform (CF), chlorobenzene (CB), and o-dichlorobenzene (DCB) during fabrication. The use of these solvents is highly restricted in industry because of health and environmental issues. Here, we report the synthesis of a low band gap (1.43 eV, 870 nm) semiconducting polymer (PDPP2DT-F2T2) having a planar geometry, which can be readily processable with nonchlorinated solvents such as toluene (TOL), o-xylene (XY), and 1,2,4-trimethylbenzene (TMB). We performed structural characterization of PDPP2DT-F2T2 films prepared from different solvents, and the electrical properties of the films were measured in the context of FETs. The devices exhibited an ambipolar behavior with hole dominant transport. Hole mobilities increased with increasing boiling point (bp) of the nonchlorinated solvents: 0.03, 0.05, and 0.10 cm² V^-1 s^-1 for devices processed using TOL, XY, and TMB, respectively. Thermal annealing further improved the FET performance. TMB-based polymer FETs annealed at 200 °C yielded a maximum hole mobility of 1.28 cm² V^-1 s^-1, which is far higher than the 0.43 cm² V^-1 s^-1 obtained from the CF-based device. This enhancement was attributed to increased interchain interactions as well as improved long-range interconnection between fibrous domains. Moreover, all of the nonchlorinated solutions generated purely edge-on orientations of the polymer chains, which is highly beneficial for carrier transport in FET devices. Furthermore, we fabricated an array of flexible TMB-processed PDPP2DT-F2T2 FETs on the plastic PEN substrates. These devices demonstrated excellent carrier mobilities and negligible degradation after 300 bending cycles. Overall, we demonstrated that the organized assembly of polymer chains can be achieved by slow drying using high bp nonchlorinated solvents and a post thermal treatment. Furthermore, we showed that polymer FETs processed using high bp nonhalogenated solvents may outperform those processed using halogenated solvents.

Dithieno[3,2-b:2',3'-d]pyrrole-benzo[c][1,2,5]thiadiazole conjugate small molecule donors: effect of fluorine content on their photovoltaic properties

Two new small molecule donors, namely ICT4 and ICT6 with D₁-A-D₂-A-D₁ architecture having 2,4-bis(2-ethylhexyl)-4H-dithieno[3,2-b:2',3'-d]pyrrole (EHDTP, D₁) and 4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b']dithiophene (OBDT, D₂) as the terminal and central donor, and benzo[c][1,2,5]thiadiazole (BT for ICT4) and 5,6-difluorobenzo[c][1,2,5]thiadiazole (F2BT for ICT6) as the acceptor (A) moieties, are synthesized and their optical, electronic and photovoltaic properties are investigated. Both ICT4 and ICT6 have considerable solubility in various solvents and possess efficient light absorption ability [ε (×10⁵ mol^-1 cm^-1) is 0.99 and 1.06, respectively for ICT4 and ICT6] and appropriate frontier molecular orbital energy offsets with [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM). Bulk heterojunction solar cells (BHJSCs) are fabricated using ICT4/ICT6 and PC₇₁BM as donors and acceptors, respectively and BHJSCs with two-step annealed (thermal followed by solvent vapor annealing) active layers of ICT4 and ICT6 show overall power conversion efficiencies (PCEs) of 5.46% and 7.91%, respectively. The superior photovoltaic performance of the ICT6 based BHJSCs is due to the favourable morphology with a nanoscale interpenetrating network in the ICT6:PC₇₁BM active layer induced by the fluorine atoms on the BT acceptor, which significantly enhances the dissociation of excitons, charge transport and the charge collection efficiency, and suppresses bimolecular recombination in the BHJ. The observed higher PCE of 7.91% indicates that ICT6 is one of the best BT based donor material for small molecular BHJSCs.

Pyridalthiadiazole acceptor-functionalized triarylboranes with multi-responsive optoelectronic characteristics

A new class of Ar₂B-π-A dyads and A-π-B(Ar)-π-A triads that feature strong organic acceptor moieties (A = pyridalthiadiazole, PT) attached to a central triarylborane were synthesized via Stille cross-coupling of ArB(Th-SnMe₃)₂ (Th = thiophenediyl, Ar = 2,4,6-tri-tert-butylphenyl (Mes*) or 2,4,6-tris(trifluoromethylphenyl) (^FMes)) with one or two equivalents of dibromopyridalthiadiazole. Single-crystal X-ray crystallography data for the triad Mes*B(Th-PT-Br)₂ indicate a highly coplanar conformation, which is ideal for extended π-conjugation and favors intermolecular π-stacking. Despite the presence of Br substituents, these compounds exhibit strong photoluminescence in THF solution with quantum yields reaching up to 52%. Further extension of conjugation by coupling with 2-hexylthiophene leads to additional bathochromic shifts to give a highly soluble and strongly red-emissive material. All these compounds undergo facile reduction, first of the PT substituents and then at more negative potentials for the borane moiety. Upon chemical reduction with in THF, an intramolecular charge transfer (ICT) pathway from the reduced PT moieties to boron is enabled and this results in a change of the color to blue. Theoretical calculations reveal that, due to the electron-withdrawing effect of the PT moieties, not only the PT-centered LUMOs themselves but also the LUMO+1 or LUMO+2, which show contributions from the p orbital of boron, experience a significant decrease in energy; they are much lower in energy than those of typical conjugated triarylboranes. The relatively low energy of both the PT-centered LUMOs and boron-centered LUMO+1 or LUMO+2 opens up multiple pathways for reaction with highly nucleophilic fluoride anions. Evidence for very strong F^- binding to boron is obtained in the case of the more sterically accessible ^FMes derivatives. Fluoride anion binding leads to an electron-rich borate moiety and as such generates an ICT pathway to the electron-deficient PT moieties; the direction of this ICT is opposite to that observed upon chemical reduction. For the Mes* derivatives, F^- binding is hindered, resulting in competing reduction of the PT acceptors. Finally, the electron acceptor character of the hexylthiophene derivative is exploited in electron-only diodes that show an average electron mobility of 6.4 ± 1.6 × 10^-5 cm² V^-1 s^-1.

Regioregular narrow-bandgap-conjugated polymers for plastic electronics

Progress in the molecular design and processing protocols of semiconducting polymers has opened significant opportunities for the fabrication of low-cost plastic electronic devices. Recent studies indicate that field-effect transistors and organic solar cells fabricated using narrow-bandgap regioregular polymers with translational symmetries in the direction of the backbone vector often outperform those containing analogous regiorandom polymers. This review addresses the cutting edge of regioregularity chemistry, in particular how to control the spatial distribution in the molecular structures and how this order translates to more ordered bulk morphologies. The effect of regioregularity on charge transport and photovoltaic properties is also outlined.

The continued search of high-performance semiconductors for organic electronics calls for smart molecular designs. Here, Ying et al. review the recent development of synthesizing regioregular narrow bandgap conjugated polymers and the effect of regioregularity on transistor and photovoltaic performance.

Fluorine-Induced Highly Reproducible Resistive Switching Performance: Facile Morphology Control through the Transition between J- and H-Aggregation

Improving the reproducibility and air-endurance of organic resistance switching (RS) devices, in particular multilevel-cell RS devices, is critical for the confirmation of its competency to realize big data storage capability. However, such enhancement still remains challenging. In this report, we demonstrated that fluorine (F)-embedding should be an effective way to enhance the overall performance of RS devices. Four new azo-cored analogues (IDAZO, FIDAZO, F₂IDAZO, and F₄IDAZO) have been designed and synthesized. These four compounds have similar structures with different numbers of F substituents. Interestingly, UV-vis measurements reveal that upon F-embedding, an exceptional transition from molecular J-aggregation to H-aggregation is achieved. As a result, the morphology of RS films becomes more and more uniform, as determined by AFM and XRD. Meanwhile, the hydrophobicity of RS film is promoted, which further improves the device atmospheric stability. The total RS reproducibility increases to 96% (the uppermost value), and the tristage RS reproducibility rises to 64%, accompanied by a more stable OFF state and lower logic SET voltages. Our study suggests F-embedding would be a promising strategy to achieve highly reproducible and air-endurable organic multilevel-cell RS devices.

Electrical Performance of a Molecular Organic Semiconductor under Thermal Stress

The high temperature performance oforganic field-effect transistorsbased on a molecular organic semiconductor with intermediate dimensions, namely X2, is evaluated. Hole mobility is stable, even at 200-250 °C. Changes in device characteristics at high temperature are reversible across multiple cycles of high temperature operation. Measurements at high temperature exhibit larger hysteresis, while at low temperature one observes the emergence of ambipolar transport.

1,3-Bis(thieno[3,4-b]thiophen-6-yl)-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione-Based Small-Molecule Donor for Efficient Solution-Processed Solar Cells

A small molecule TBTT-1 with 5-(2-ethylhexyl)-1,3-bis(2-(2-ethylhexyl)thieno[3,4-b]thiophen-6-yl)-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione (TBTT) as the central moiety was designed and synthesized for solution-processed bulk-heterojunction solar cells. TBTT-1 exhibits a broad absorption with a low optical band gap of approximately 1.53 eV in the thin film. An optimized power conversion efficiency (PCE) of 7.47% with a high short-circuit current of 14.95 mA cm^-2 was achieved with diphenyl ether (DPE) as additive, which is the highest PCE for TPD-based small-molecule solar cells. According to the detailed morphology investigations, we found that DPE processing helped to enhance π-π stacking and reduce the scales of phase separation, which led to improved exciton splitting and charge transport in BHJ thin film, and thus enhanced device performance.

Effects of solvent additive on "s-shaped" curves in solution-processed small molecule solar cells

A novel molecular chromophore, p-SIDT(FBTThCA8)₂, is introduced as an electron-donor material for bulk heterojunction (BHJ) solar cells with broad absorption and near ideal energy levels for the use in combination with common acceptor materials. It is found that films cast from chlorobenzene yield devices with strongly s-shaped current-voltage curves, drastically limiting performance. We find that addition of the common solvent additive diiodooctane, in addition to facilitating crystallization, leads to improved vertical phase separation. This yields much better performing devices, with improved curve shape, demonstrating the importance of morphology control in BHJ devices and improving the understanding of the role of solvent additives.

Controlling Intramolecular Conformation through Nonbonding Interaction for Soft-Conjugated Materials: Molecular Design and Optoelectronic Properties

To address the intrinsic contradiction between high optoelectronic properties and good processability in organic π-conjugated molecules, we propose that soft-conjugated molecules (SCMs), conformationally locked by intramolecular nonbonding interactions, can benefit from both nonplanar molecular structures in solution for processing and rigid coplanar structures in the solid state for enhanced optoelectronic properties. Computational results reveal that nonbonding pairs of S···N, N···H, and F···S are strong enough to prevail over thermal fluctuations, steric effects, and other repulsive interactions to force the adjacent aromatic rings to be planar; thus, constructed SCMs display delocalized frontier molecular orbitals with frontier orbital energy levels, band gaps, reorganization energies, and photophyscial properties comparable to those of rigid-conjugated molecules because of their stable planar soft-conjugation at both ground and excited states. The understanding gained from the theoretical investigations of SCMs provides keen insights into construction and modification of soft-conjugations to harmonize the optoelectronic property and processability in conjugated molecules for advanced optoelectronic applications.

Enhancement of charge transport properties of small molecule semiconductors by controlling fluorine substitution and effects on photovoltaic properties of organic solar cells and perovskite solar cells

We prepared a series of small molecules based on 7,7'-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b']dithiophene-2,6-diyl)bis(4-(5'-hexyl-[2,2'-bithiophene]-5-yl)benzo[c][1,2,5]thiadiazole) with different fluorine substitution patterns (0F-4F). Depending on symmetricity and numbers of fluorine atoms incorporated in the benzo[c][1,2,5]thiadiazole unit, they show very different optical and morphological properties in a film. 2F and 4F, which featured symmetric and even-numbered fluorine substitution patterns, display improved molecular packing structures and higher crystalline properties in a film compared with 1F and 3F and thus, 2F achieved the highest OTFT mobility, which is followed by 4F. In the bulk heterojunction solar cell fabricated with PC₇₁BM, 2F achieves the highest photovoltaic performance with an 8.14% efficiency and 0F shows the lowest efficiency of 1.28%. Moreover, the planar-type perovskite solar cell (PSC) prepared with 2F as a dopant-free hole transport material shows a high power conversion efficiency of 14.5% due to its high charge transporting properties, which were significantly improved compared with the corresponding PSC device obtained from 0F (8.5%). From the studies, it is demonstrated that low variation in the local dipole moment and the narrow distribution of 2F conformers make intermolecular interactions favorable, which may effectively drive crystal formations in the solid state and thus, higher charge transport properties compared with 1F and 3F.

Role of Interfacial Layers in Organic Solar Cells: Energy Level Pinning versus Phase Segregation

Organic photovoltaics (OPVs) are assembled from a complex ensemble of layers of disparate materials, each playing a distinct role within the device. In this work, the role of the interface that bridges the transparent anode and the bulk heterojunction (BHJ) in an OPV device was investigated. The surface characteristics of the electrode interface affect the energy level alignment, phase segregation, and the local composition of the bulk heterojunction (BHJ), which is in close contact. The commonly used ITO/PEDOT:PSS electrode was tailored with a thin, low-band-gap polymer overlayer, called PBDTTPD-COOH, a variant of the established donor polymer, PBDTTPD. Three BHJs that were composed of a donor polymer and PC71BM, were examined, including the donor polymers PBDTTPD, PCDTBT, and PTB7, within the following OPV device stack: ITO/(interfacial layer or layers)/BHJ/LiF/Al/Mg. It was found that modification of the ITO/PEDOT:PSS electrode with PBDTTPD-COOH resulted in statistically significant increases of power conversion efficiency for the PBDTTPD- and PCDTBT-based donor polymer:PC71BM BHJs, but not for the PTB7:PC71BM BHJ. Ultraviolet photoelectron spectroscopy (UPS) enabled determination of the respective energy level diagrams for these three different polymers relative to the ITO/PEDOT:PSS/PBDTTPD-COOH electrode, and revealed no injection barrier in all three polymer/substrate pairs. The observed differences of efficiency were not, therefore, electronic in origin. ToF-SIMS depth profiling and detailed experiments to determine surface energies strongly suggested that the greatest factor influencing device performance was a significant change of the local composition of the BHJ at this interface. When favorable accumulation of the donor polymer at the

PEDOT

PSS/interfacial layer was observed, the result was higher OPV device efficiencies. These results suggest that for each BHJ, the surface energies of the electrodes need to be carefully considered, as they will influence the local composition of the BHJ and resulting device performance.

Synthesis and characterization of donor–acceptor type conducting polymers containing benzotriazole acceptor and benzodithiophene donor or s-indacenodithiophene donor

Two D–A type copolymers were synthesized and characterized by a series of methods. Both copolymers showed high optical contrasts, fast response time and high coloration efficiency. In summary, they could be candidates for electrochromic applications.