Linear Alkyl Side Chains Research Articles

Synergistically manipulating the shape of alkyl-chain and asymmetric side groups of non-fullerene acceptors enables organic solar cells to reach 18.5% efficiency

Side-chain modification and asymmetric design for non-fullerene acceptors (NFAs) have been proven to be effective methods for harvesting high-performance organic solar cells (OSCs). Combining the two molecular design strategies, we adopted phenyl chain and alkyl chains with different shapes to develop two novel asymmetric NFAs, named BTP-P2EHC11 and BTP-P2EHC2C4. Compared with BTP-P2EHC2C4 attached 2-ethylhexyl side chain, BTP-P2EHC11 with linear alkyl side chain have slightly red-shifted absorption and intensive absorption strength. Moreover, the PM6:BTP-P2EHC11 blend film presents higher and more balanced charge mobilities, reducing charge recombination, tighter intermolecular packing and more favorable fibrous network morphology with appropriate phase separation than PM6:BTP-P2EHC2C4, which lead to significantly enhanced short-circuit current density (JSC) of PM6:BTP-P2EHC11-based devices. Thus, the OSCs based on PM6:BTP-P2EHC11 achieve a superior power conversion efficiency (PCE) of 18.50% with a good trade-off among open-circuit voltage (VOC) of 0.876 V, JSC of 26.85 mA cm−2 and fill factor (FF) of 78.65%, while PM6:BTP-P2EHC2C4-based device exhibits a lower PCE of 17.49%. Our investigation elucidates that the combination of finely optimizing the shape of alkyl-chain and asymmetric side groups of NFAs could pave a promising avenue toward morphology optimization and performance promotion of OSCs.

Designing high‐performance nonfused ring electron acceptors via side‐chain engineering

AbstractThe side‐chain has a significant influence on the optical properties and aggregation behaviors of the organic small molecule acceptors, which becomes an important strategy to optimize the photovoltaic performance of organic solar cells. In this work, we designed and synthesized three brand‐new nonfused ring electron acceptors (NFREAs) OC4‐4Cl‐Ph, OC4‐4Cl‐Th, and OC4‐4Cl‐C8 with hexylbenzene, hexylthiophene, and octyl side chains on the π‐bridge units. Compared with OC4‐4Cl‐Ph and OC4‐4Cl‐Th, OC4‐4Cl‐C8 with linear alkyl side chain has more red‐shift absorption, which is conducive to obtaining higher short‐circuit current density. Additionally, the OC4‐4Cl‐C8 film exhibits a longer exciton diffusion distance, and the D18:OC4‐4Cl‐C8 blend film displays faster hole transfer, weaker bimolecular recombination, and more efficient exciton transport. Furthermore, The D18:OC4‐4Cl‐C8 blend films may effectively form interpenetrating networks that resemble nanofibrils, which can facilitate exciton dissociation and charge transport. Finally, OC4‐4Cl‐C8‐based devices can be created a marvellously power conversion efficiency (PCE) of 16.56%, which is much higher than OC4‐4Cl‐Ph (12.29%)‐ and OC4‐4Cl‐Th‐based (11.00%) ones, being the highest PCE among the NFREA based binary devices. All in all, we have validated that side‐chain engineering is an efficient way to achieve high‐performance NFREAs.

The effect of extended alkyl length on molecular orientation of nonfullerene acceptor: A case study in as-cast polymer solar cells

In this work, the effective electronic coupling on the indacenodithiophene (IDT)-based nonfullerene core was investigated by changing the length of the linear alkyl side chain. Extended linear alkyl side chain reduces the extinction coefficient in thin films and promotes the tendency of nonfullerene molecules to edge-on orientation. The theoretical calculation illustrates that the enhanced electronic coupling stem from short alkyl side chain could be beneficial for face-on orientation and charge transport. For the as-cast solar cells, the devices based on IDIC with the short alkyl side chain possessed the best efficiency. Our result manifests that the linear alkyl chain on the core of nonfullerene acceptor dominates molecular packing mode, light absorption, bulk heterojunction size, charge carrier transport and recombination dynamics.

Alkyl Chain Engineering of Low Bandgap Non-Fullerene Acceptors for High-Performance Organic Solar Cells: Branched vs. Linear Alkyl Side Chains.

In this work, we report the synthesis and photovoltaic properties of IEBICO-4F, IEHICO-4F, IOICO-4F, and IDICO-4F non-fullerene acceptors (NFAs) bearing different types of alkyl chains (2-ehtylhexyl (EH), 2-ethylbutyl (EB), n-octyl (O), and n-decyl (D), respectively). These NFAs are based on the central indacenodithiophene (IDT) donor core and the same terminal group of 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (IC-2F), albeit with different side chains appended to the thiophene bridge unit. Although the side chains induced negligible differences between the NFAs in terms of optical band gaps and molecular energy levels, they did lead to changes in their melting points and crystallinity. The NFAs with branched alkyl chains exhibited weaker intermolecular interactions and crystallinity than those with linear alkyl chains. Organic solar cells (OSCs) were fabricated by blending these NFAs with the p-type polymer PTB7-Th. The NFAs with appended branched alkyl chains (IEHICO-4F and IEBICO-4F) possessed superior photovoltaic properties than those with appended linear alkyl chains (IOICO-4F and IDICO-4F). This result can be ascribed mainly to the thin-film morphology. Furthermore, the NFA-based blend films with appended branched alkyl chains exhibited the optimal degree of aggregation and miscibility, whereas the NFA-based blend films with appended linear alkyl chains exhibited higher levels of self-aggregation and lower miscibility between the NFA molecule and the PTB7-Th polymer. We demonstrate that changing the alkyl chain on the π-bridging unit in fused-ring-based NFAs is an effective strategy for improving their photovoltaic performance in bulk heterojunction-type OSCs.

High performance polymerized small molecule acceptor by synergistic optimization on π-bridge linker and side chain

The polymerized small-molecule acceptors have attracted great attention for application as polymer acceptor in all-polymer solar cells recently. The modification of small molecule acceptor building block and the π-bridge linker is an effective strategy to improve the photovoltaic performance of the polymer acceptors. In this work, we synthesized a new polymer acceptor PG-IT2F which is a modification of the representative polymer acceptor PY-IT by replacing its upper linear alkyl side chains on the small molecule building block with branched alkyl chains and attaching difluorene substituents on its thiophene π-bridge linker. Through this synergistic optimization, PG-IT2F possesses more suitable phase separation, increased charge transportation, better exciton dissociation, lower bimolecular recombination, and longer charge transfer state lifetime than PY-IT in their polymer solar cells with PM6 as polymer donor. Therefore, the devices based on PM6:PG-IT2F demonstrated a high power conversion efficiency of 17.24%, which is one of the highest efficiency reported for the binary all polymer solar cells to date. This work indicates that the synergistic regulation of small molecule acceptor building block and π-bridge linker plays a key role in designing and developing highly efficient polymer acceptors.

Human risk assessment of 4-n-nonylphenol (4-n-NP) using physiologically based pharmacokinetic (PBPK) modeling: analysis of gender exposure differences and application to exposure analysis related to large exposure variability in population.

As a toxic substance, 4-n-nonylphenol (4-n-NP) or 4-nonylphenol (4-NP) is widely present in the environment. 4-n-NP is a single substance with a linear-alkyl side chain, but 4-NP usually refers to a random mixture containing various branched types. Unfortunately, human risk assessment and/or exposure level analysis for 4-n-NP (or 4-NP) were almost nonexistent, and related research was urgently needed. This study aimed to analyze the various exposures of 4-n-NP (or 4-NP) through development of a physiologically based-pharmacokinetic (PBPK) model considering gender difference in pharmacokinetics of 4-n-NP and its application to human risk assessment studies. A PBPK model was newly developed considering gender differences in 4-n-NP pharmacokinetics and applied to a human risk assessment for each gender. Exposure analysis was performed using a PBPK model that considered gender differences in 4-n-NP (or 4-NP) exposure and high variabilities in several countries. Furthermore, an extended application was attempted as a human risk assessment for random mixture 4-NP, which is difficult to accurately evaluate in reality. External-exposure and margin-of-safety estimated with the same internal exposure amount differed between genders, meaning the need for a differentiated risk assessment considering gender. Exposure analysis based on biomonitoring data confirmed large variability in exposure to 4-n-NP (or 4-NP) by country, group, and period. External-exposures estimated using PBPK model varied widely, ranging from 0.039 to 63.875mg/kg/day (for 4-n-NP or 4-NP). By country, 4-n-NP (or 4-NP) exposure was higher in females than in males and the margin-of-safety tended to be low. Overall, exposure to 4-n-NP (or 4-NP) in populations was largely not safe, suggesting need for ongoing management and monitoring. Considering low in vivo accumulation confirmed by PBPK model, risk reduction of 4-n-NP is possible by reducing its use.

Structure–Property–Processing Relations of Short‐Chain Branched Poly(butylene terephthalate) (PBT) with Biobased Comonomers

AbstractPoly(butylene terephthalate) (PBT) is difficult to foam due to its unfavorable rheological behavior (low melt strength, no strain hardening). In particular, a high expansion and a homogeneous cell morphology are difficult to achieve. This can be altered successfully by addition of multifunctional chain extenders. Chain extenders cause nondefined and rarely understood changes in the polymer architecture usually described as branching. In this contribution, the synthesis of two series of PBT copolyesters with defined short‐chain branched units is presented. Dilinoleic derivatives with linear C9and C7alkyl side chains are employed to reflect short‐chain branches and are incorporated into PBT in various molar ratios. Characterization by NMR spectroscopy and size exclusion chromatography demonstrates the random chain structure and high molar masses of the terpolyesters. Incorporation of dilinoleic derivatives results in the reduction of PBT crystallinity, decreased glass transition temperatures, and altered rheological behavior, in particular of extensional rheology characterized by strain hardening. The comparison to control copolyesters without branches proves that strain hardening is caused by the branches. A higher concentration of branches induces stronger strain hardening, resulting in successful foaming. It is demonstrated that the new terpolyesters have properties comparable with PBT treated with chain extenders.

Investigation of the Side Chain Effect on Gas and Water Vapor Transport Properties of Anthracene-Maleimide Based Polymers of Intrinsic Microporosity.

In the present work, a set of anthracene maleimide monomers with different aliphatic side groups obtained by Diels Alder reactions were used as precursors for a series of polymers of intrinsic microporosity (PIM) based homo- and copolymers that were successfully synthesized and characterized. Polymers with different sizes and shapes of aliphatic side groups were characterized by size-exclusion chromatography (SEC), (nuclear magnetic resonance) 1H-NMR, thermogravimetric (TG) analysis coupled with Fourier-Transform-Infrared (FTIR) spectroscopy (TG-FTIR) and density measurements. The TG-FTIR measurement of the monomer-containing methyl side group revealed that the maleimide group decomposes prior to the anthracene backbone. Thermal treatment of homopolymer methyl-100 thick film was conducted to establish retro-Diels Alder rearrangement of the homopolymer. Gas and water vapor transport properties of homopolymers and copolymers were investigated by time-lag measurements. Homopolymers with bulky side groups (i-propyl-100 and t-butyl-100) experienced a strong impact of these side groups in fractional free volume (FFV) and penetrant permeability, compared to the homopolymers with linear alkyl side chains. The effect of anthracene maleimide derivatives with a variety of aliphatic side groups on water vapor transport is discussed. The maleimide moiety increased the water affinity of the homopolymers. Phenyl-100 exhibited a high water solubility, which is related to a higher amount of aromatic rings in the polymer. Copolymers (methyl-50 and t-butyl-50) showed higher CO2 and CH4 permeability compared to PIM-1. In summary, the introduction of bulky substituents increased free volume and permeability whilst the maleimide moiety enhanced the water vapor affinity of the polymers.

Ordered hexagonal columnar liquid crystalline self-assembly of mesogen-free sulfonylated side-chain chiral polyethers and their high dielectric property

To achieve large spontaneous polarization for ferroelectric liquid crystalline polymers, we have designed a series of mesogen-free isotactic polyethers with sulfonylated side chains. Different from prior studies using linear alkyl side chains, in this work a methyl branch was implemented in the alkyl side chains of the chiral polyethers. As a result of the continuous twist from the methyl branches and strong dipole-dipole interactions among neighboring sulfonyl groups, a helical chain conformation with tilted side chains was achieved. When the branched side chains were short (e.g., C5) and double chirality existed in both the main chain and side chains, a crystalline structure was obtained. When the branched side chains were long (e.g., ≥ C8) and single chirality existed in the main chain, an ordered hexagonal columnar liquid crystalline phase was induced. Their high-field dielectric property was studied by electric displacement-electric field (D-E) loop tests; however, no ferroelectric behavior was observed up to 200 MV/m. It was considered that the dipole-dipole interaction among neighboring sulfonyl groups in the helical chains was too strong to allow easy ferroelectric flipping.

Recent Advances in Molecular Design of Organic Thermoelectric Materials

Recent Advances in Molecular Design of Organic Thermoelectric Materials

Drag reduction assessment of some new copolymers of 1-hexene and maleic anhydride in light crude oil

Poly(1-hexene-co-maleic anhydride) derivatives containing various lengths of linear alkyl side chains (C11–C22) were experimentally evaluated as drag reduction agents in the turbulent flow of Iraqi crude oil using a circulating flow loop system. Good drag reduction performances were obtained. The results showed that the drag reduction has been influenced by the flow rate, concentration, chemical structure, molecular weight, and temperature. Their mechanical stabilities were also investigated using a cylindrical rotator device, and a good relationship was obtained between copolymers' drag reduction efficiency and branch length, in which longer branches significantly enhance copolymer mechanical stability.

Molecular Origin of Carbon–Oxygen‐Bridge Isomerization Induced Reverse Aggregation Ability in Acceptor–Donor–Acceptor Electron Acceptors for Organic Solar Cells

For bulk heterojunction organic solar cells (OSCs), controlling molecular self‐aggregation during solution processing is crucial to obtain ideally phase‐separated morphology and high device performance. Recently, fused‐ring regiochemistry, for example, carbon–oxygen (CO)‐bridge isomerization, has been found to effectively modulate the aggregation structures and photovoltaic properties of acceptor–donor–acceptor (A–D–A) small‐molecule acceptors (SMAs). Strikingly, the relative aggregation ability for the CO‐bridge isomers turns out to be reverse after simultaneous replacement of the linear alkyl side chains with branched ones and fluorination of the end groups. Herein, to understand the molecular origin of such an observation, the aggregation behaviors of three pairs of CO‐bridge isomeric SMAs in solutions are systematically investigated by atomistic molecular dynamics simulations. Because of the large side‐chain steric hindrance around the fused‐ring core, the molecular self‐aggregation for all of these SMAs is dominated by end‐group π–π stacking. Moreover, the end‐group π–π interaction is controlled by the synergistic effect of CO‐bridge isomerization, side‐chain branching, and end‐group fluorination, which are responsible for the reversal of the aggregation ability of the isomeric SMAs. This work provides the rationalization of experimental observations and is helpful for modulating the blending morphologies for high‐efficiency OSCs based on CO‐bridge SMAs.

Pronounced Dependence of All-Polymer Solar Cells Photovoltaic Performance on the Alkyl Substituent Patterns in Large Bandgap Polymer Donors.

For all-polymer solar cells which are composed of polymer donors and polymer acceptors, the effect of alkyl side chains on photovoltaic performance is a matter of some debate, and this effect remains difficult to forecast. In this concise contribution, we demonstrate that three alkyls namely branched alkyl 2-butyloctyl (2BO), long linear alkyl n-dodecyl (C12), and double-short linear alkyl n-hexyls (DC6) incorporated into the side chains of large bandgap polymer donor PBDT-TTz can induce considerable, of significance, and different electronic, optical, and morphological parameters. Systematic studies shed light on the critical role of the double-short linear alkyl n-hexyls (DC6) in (i) producing large ionization potential value, (ii) increasing propensity of the polymer to order along the π-stacking direction, (iii) generating polymer crystallites with more preferential "face-on" orientation, consequently, (iv) improvement of carriers transportation, (v) suppression of charge recombination, (vi) reduction of energy loss in all-polymer devices. In parallel, we unearth that the PBDT-TTz with double-short linear alkyl n-hexyls (DC6) represents the highest efficiency of 8.3 %, whereas, the other two PBDT-TTz analogues (2BO, C12) yield efficiencies of less than 3 % in optimized all-polymer solar cells. Though branched or long linear alkyl side chains (2BO, C12) have been applied to provide the solution processability of conjugated polymers, motifs bearing multiple short linear alkyl substituents (DC6) are proved critical to the development of high performing polymers.

Measurement of Solvation Ability of Solvents by Porphyrin-Based Solvation/Desolvation Indicators.

A new solvent scale, solvation ability (SA), was developed to arrange solvents in the order of their SA for large π-conjugated compounds. The SA of a solvent was determined in a binary solvent system of an assessed solvent and a standard “good” solvent (GS) or “poor” solvent (PS), chloroform or methylcyclohexane, respectively, in the presence of two types of solvation/desolvation indicators, 1Zn2 and 2Zn2. The latter comprises bis(imidazolylporphyrinatozinc) linked via a 1,3-butadiynylene moiety having linear alkyl and hydrophilic side chains, respectively. GSs and PSs give extended (E-) and stacked (S-) supramolecular polymers of the indicators, respectively. SA values are defined as vol % of the standard solvent added to an assessed solvent to give the balance point where comparable amounts of E- and S-polymers of the indicators coexist. GSs and PSs have positive and negative signs, respectively. In this study, the SA of 25 solvents was determined. The SA values with indicator 1Zn2 were as follows: ethyl acetate (−81), hexane (−66), toluene (−50), cyclohexane (−47), CCl4 (−25), chloroform (50), and nitrobenzene (79).

Development of Prediction Models on Base-Catalyzed Hydrolysis Kinetics of Phthalate Esters with Density Functional Theory Calculation.

Many phthalate esters (PAEs) are chemicals of high production volume and of toxicological concern. The second-order rate constant for base-catalyzed hydrolysis ( kB) is a key parameter for assessing environmental persistence of PAEs. However, the kB values for most PAEs are lacking, and the experimental determination of kB encounters various difficulties. Herein, density functional theory (DFT) methods were selected by comparing empirical kB values of five PAEs and five carboxylic acid esters with the DFT-calculated ones. Results indicate that PAEs with cyclic side chains are more vulnerable to base-catalyzed hydrolysis than PAEs with linear alkyl side chains, followed by PAEs with branched alkyl side chains. By combining experimental and DFT-calculated second-order rate constants for base-catalyzed hydrolysis of one side chain in PAEs ( kB_sidechain), quantitative structure-activity relationship models were developed. The models can differentiate PAEs with the departure of the leaving group (or the nucleophilic attack of OH-) as the rate-determining step in the hydrolysis and estimate kB values, which provides a promising way to predict hydrolysis kinetics of PAEs. The half-lives of the investigated PAEs were calculated and vary from 0.001 h to 558 years (pH = 7∼9), further illustrating the necessity of prediction models for hydrolysis kinetics in assessing the environmental persistence of chemicals.

High‐Performance All‐Small‐Molecule Solar Cells Based on a New Type of Small Molecule Acceptors with Chlorinated End Groups

AbstractWhile a wide variety of nonfullerene acceptors are developed and perform well in combination with polymer donors, only a few nonfullerene acceptors can work well with small molecule donors. Here, all‐small‐molecule solar cells with high performance enabled by a new type of small molecule acceptors (F‐0Cl, F‐1Cl, and F‐2Cl), which contain linear alkyl side chains and end groups substituted with various number of chlorine atoms, are reported. End group chlorination leads to redshifted absorption, enhanced crystallinity, and high electron mobility. These properties make them competitive as electron acceptors for all‐small‐molecule solar devices. When combined with two popular small molecule donors DRTB‐T and DRCN5T, these nonfullerene acceptors offer power conversion efficiencies up to 10.76 and 9.89%, which are among the top efficiencies reported in all‐small‐molecule solar cells and indicate the great potential of all‐small‐molecule solar devices.

Benzodithiophene-Dithienylbenzothiadiazole Copolymers for Efficient Polymer Solar Cells: Side-Chain Effect on Photovoltaic Performance.

A new series of low band gap D-A alternating polymers based on 4,5-bis((2-ethylhexyl)oxy)benzo[2,1- b:3,4- b']dithiophene (BDT) and 5-fluoro-4,7-bis(4-alkylthien-2-yl)benzo[ c][1,2,5]thiadiazole bearing different size of lateral alkyl substituents, namely, PfBB- n, n = 8, 10, 12, 14, and 16, was designed and synthesized for high-performance bulk heterojunction (BHJ) polymer solar cells (PSCs). PfBB- n-bearing linear alkyl side chains exhibited strong and controllable aggregation in both solution and solid states, which gives rise to a significant bathochromic shift of the absorption cut-off down to ∼780 nm in thin film. In addition, the strong and wide absorption (350-800 nm) of PfBB- n polymers can compensate for the relatively weak absorption of PC71BM, particularly in the 300-400 range nm to enhance light harvesting of such an active blend. BHJ solar cells based on PfBB- n:PC71BM blends as an active layer showed power conversion efficiency (PCE) in the range 7.8-9.7%. Because of the strong stacking interchain interactions, PfBB-12-based PSC exhibited aggregation-induced spectral broadening, superior structural order, higher exciton dissociation, higher and more balanced charge carrier mobilities, as well as reduced recombination losses. As a result, PfBB-12-based device afforded the best PCE of 9.7%, with the highest short-circuit current density ( Jsc) of 16.6 mA cm-2 and open-circuit voltage ( Voc) of 0.92 V among devices fabricated. These results demonstrate that the alkyl side chain of the polymer significantly affects the absorption, morphology, and electronic properties of the active blend of PfBB- n/PC71BM, which would provide an alternative useful tool to fine-tune the device performance. Our results also highlight that the electron-rich benzo[2,1- b:3,4- b']dithiophene building block, BDT, is highly useful for the construction of low band gap D-A polymer for highly efficient PSCs.

Efficient blue-to-transmissive electrochromic transitions of alkylated quinoxaline-thiophene based donor-acceptor type conjugated polymers

Two electrochromic copolymers, P1 and P2, were synthesized from Stille coupling reactions of dibrominated phenazine having branched and linear alkyl side-chains, respectively, with distannyl thiophene. The prepared donor–acceptor type electrochromic polymers showed good solution processability, superior switching characteristics, and high redox stability. In electrochromic measurements, the polymers exhibited reversible blue-to-transmissive switching at an applied voltage of 0.9 V and 1.2 V, respectively, with transmittance changes of 18.1% at 597 nm and 25.3% at 653 nm for P1 and P2, respectively. They showed high coloration efficiencies of 232.5 and 132.6 cm2/C, respectively, in the visible region and long-term stability for more than a hundred cycles. Furthermore, the polymers were completely transparent in the bleached state, accompanied by high NIR absorption, upon oxidation. These results manifest that the alkyl side-chains on the quinoxaline acceptor unit could dictate the intermolecular interactions of polymer chains affecting the electrochromic performances. The findings of this study suggest the strategy of designing and preparing efficient electrochromic polymers with selective absorption in long visible and NIR regions for smart windows and displays.

Cyclopentadithiophene-Benzothiadiazole Donor-Acceptor Polymers as Prototypical Semiconductors for High-Performance Field-Effect Transistors.

Donor-acceptor (D-A) conjugated polymers are of great interest as organic semiconductors, because they offer a rational tailoring of the electronic properties by modification of the donor and acceptor units. Nowadays, D-A polymers exhibit field-effect mobilities on the order of 10-2-100 cm2 V-1 s-1, while several examples showed a mobility over 10 cm2 V-1 s-1. The development of cyclopentadithiophene-benzothiadiazole (CDT-BTZ) copolymers one decade ago represents an important step toward high-performance organic semiconductors for field-effect transistors. The significant rise in field-effect mobility of CDT-BTZ in comparison to the existing D-A polymers at that time opened the door to a new research field with a large number of novel D-A systems. From this point, the device performance of CDT-BTZ was gradually improved by a systematic optimization of the synthesis and polymer structure as well as by an efficient solution processing into long-range ordered thin films. The key aspect was a comprehensive understanding of the relation between polymer structure and solid-state organization. Due to their fundamental role for the field of D-A polymers in general, this Account will for the first time explicitly focus on prototypical CDT-BTZ polymers, while other reviews provide an excellent general overview on D-A polymers. The first part of this Account discusses strategies for improving the charge carrier transport, focusing on chemical aspects. Improved synthesis as an essential stage toward high purity, and high molecular weight is a prerequisite for molecular order. The modification of substituents is a further crucial feature to tune the CDT-BTZ packing and self-assembly. Linear alkyl side chains facilitate intermolecular π-stacking interactions, while branched ones increase solubility and alter the polymer packing. Additional control over the supramolecular organization of CDT-BTZ polymers is introduced by alkenyl substituents via their cis-trans isomerization. The last discussed chemical concept is based on heteroatom variation within the CDT unit. The relationships found experimentally for CDT-BTZ between polymer chemical structure, solid-state organization, and charge carrier transport are explained by means of theoretical simulations. Besides the effects of molecular design, the second part of this Account discusses the processing conditions from solution. The film microstructure, defined as a mesoscopic domain organization, is critically affected by solution processing. Suitable processing techniques allow the formation of a long-range order and a uniaxial orientation of the CDT-BTZ chains, thus lowering the trapping density of grain boundaries for charge carriers. For instance, alignment of the CDT-BTZ polymer by dip-coating yields films with a pronounced structural and electrical anisotropy and favors a fast migration of charge carriers along the conjugated backbones in the deposition direction. By using film compression with the assistance of an ionic liquid, one even obtains CDT-BTZ films with a band-like transport and a transistor hole mobility of 10 cm2 V-1 s-1. This device performance is attributed to large domains in the compressed films being formed by CDT-BTZ with longer alkyl chains, which establish a fine balance between polymer interactions and growth kinetics during solvent evaporation. On the basis of the prototypical semiconductor CDT-BTZ, this Account provides general guidelines for achieving high-performance polymer transistors by taking into account the subtle balance of synthetic protocol, molecular design, and processing.

Disubstituted perylene diimides in organic field-effect transistors: Effect of the alkyl side chains and thermal annealing on the device performance

Abstract Using a series of eight substituted perylene diimides (PDIs) we have explored the effects of the side chains and thermal annealing of the semiconductor films on their electrical characteristics in organic field-effect transistors (OFETs). Clear correlations between the thermally-induced improvement in the crystallinity of PDI films, their phase transition temperatures determined by differential scanning calorimery (DSC) measurements and the electrical characteristics of the OFETs have been revealed. It has been also demonstrated that the best charge carrier mobilities are delivered by PDIs showing the highest enthalpies of the phase transitions, manifesting particularly strong intermolecular interactions in these crystalline solids. On the other hand, the length of alkyl side chains represents a crucial parameter governing the device performance. PDIs with the linear C6-C8 alkyl side chains showed the highest charge-carrier mobilities when annealed in optimal regimes, which correlates well with the thermal and structural characteristics of these materials. Finally, we show that DSC can be considered as a very useful technique to speed up the discovery of new promising semiconductor materials via screening for the highest phase transition enthalpies and performing film annealing near the phase transition temperatures.