Side-Chain Effect on Cyclopentadithiophene/Fluorobenzothiadiazole-Based Low Band Gap Polymers and Their Applications for Polymer Solar Cells

NIR-II organic small molecule probe for labeling lymph nodes and guiding tumor imaging

Organic small molecule fluorescent groups have injected new material support into the field of medical imaging due to their unique luminescence mechanism and easy tuning of structure. The great potential of NIR-II window imaging forces us to continuously optimize the structure of organic fluorophores to design better fluorescent molecules for fluorescence imaging-guided surgery. An ideal organic small molecule fluorescent group: it can penetrate into the inside of the organism, clearly present the internal structure and the edge contour of different tissues, so as to perfectly achieve internal imaging and accurately guide external surgery. In vivo, fluorescent groups do not damage normal tissues and organs. However, problems such as low quantum yield and poor biocompatibility greatly limit the clinical transformation of NIR-II fluorescent small molecules. To avoid the shortcomings of NIR-II fluorescent probes as much as possible and better realize image-guided surgery, in this experiment, the biplane donor unit was incorporated into the twisted D-π-A-π-D structure to expand the conjugated structure of the fluorescent group, which not only realized NIR-II emission, but also had high quantum yield and biosafety.

Simple Tricyclic-Based A-π-D-π-A-Type Nonfullerene Acceptors for High-Efficiency Organic Solar Cells

Nonfused-ring electron acceptors have attracted much attention in recent years due to their advantages of simple synthetic routes, high yields, low costs, reasonable power conversion efficiencies (PCEs), and so on. Herein, three simple A-π-D-π-A-type acceptors (DTC-BO-4F, DTS-BO-4F, and DTP-BO-4F) comprising a tricyclic fused-ring core, two 2,5-bis(alkyloxy)phenylene spacers, and two difluorinated terminal groups (DF-IC) were developed. Compared with DTS-BO-4F, DTC-BO-4F and DTP-BO-4F exhibit higher molar extinction coefficients, stronger crystallinity, and more orderly stacking. The PBDB-T:DTC-BO-4F-based blend film shows suitable phase separation and higher and more balanced charge mobilities. Finally, the photovoltaic devices based on DTC-BO-4F give an outstanding PCE of 13.26% with a small nonradiative voltage loss of 0.23 eV.

Dithieno[3,2-b:2',3'-d]silole-based conjugated polymers for bioimaging in the short-wave infrared region

The short-wave infrared window (SWIR, 900–1700 nm) fluorescence imaging has been demonstrated to have excellent imaging performance in signal/noise ratio and tissue penetration compared to the conventional NIR biological window (NIR-I, 700–900 nm). Conventional organic SWIR fluorescent materials still suffer from low fluorescence quantum efficiency. In this work, a donor unit with sp³ hybrid configuration and an acceptor unit with small hindered alkyl side chains are employed to construct donor–acceptor (D–A) type conjugated polymers P1 and P2, which were substituted with one or two fluorine atoms. These structural features can alleviate the aggregation-caused quenching (ACQ) and contribute to charge transfer, resulting in a significantly improved fluorescence quantum efficiency. The SWIR fluorescent quantum efficiencies of P1 and P2 nanoparticles are 3.4% and 4.4%, respectively, which are some of the highest for organic SWIR fluorophores reported so far. Excellent imaging quality has been demonstrated with P2 nanoparticles for SWIR imaging of the vascular system of nude mice. The results indicate that our design strategy of introducing sp³ hybrid configuration and small hindered alkyl side chains to fabricate conjugated polymers is efficient in improving the fluorescent quantum efficiency as SWIR fluorescent imaging agents for potential clinical practice.

A D–A type polymer with a SWIR fluorescence quantum efficiency of 4.4% was obtained after structural optimization.

Molecular Understanding of How the Interfacial Structure Impacts the Open-Circuit Voltage of Highly Crystalline Polymer Solar Cells

Herein, we study the origin of differences in open-circuit voltage (V_OC) for polymer:fullerene solar cells employing highly crystalline conjugated polymers (PTzBT) based on the same thiophene-thiazolothiazole backbone with different side chains. By analyzing the temperature dependence of V_OC and cyclic voltammogram, we find that the difference in V_OC originates in the different cascaded energy structures for the highest occupied molecular orbital (HOMO) levels in the interfacial mixed phase. Furthermore, we find that this is due to the stabilization of HOMO caused by the different branching of side chains on the basis of density functional theory calculation. Finally, we discuss the molecular design strategy based on side-chain engineering for ideal interfacial cascaded energy structures leading to higher V_OC and photocurrent simultaneously.

Relation between the Electronic Properties of Regioregular Donor-Acceptor Terpolymers and Their Binary Copolymers

By analyzing the optical band gap and energy levels of seven different regioregular terpolymers in which two different electron-rich donor moieties are alternating with a common electron-deficient acceptor unit along the backbone, we establish a direct correlation with the properties of the corresponding binary copolymers in which one donor and one acceptor are combined. For this study, we use diketopyrrolopyrrole as the common acceptor and different π-conjugated aromatic oligomers as donors. We find that the optical band gap and frontier orbital energies of the terpolymers are the arithmetic average of those of the parent copolymers with remarkable accuracy. The same relationship is also found for the open-circuit voltage of the bulk heterojunction solar cells made with the ter- and copolymers in combination with [6,6]-phenyl-C₇₁-butyric acid methyl ester. Comparison of these findings with data in the literature suggests that this is a universal rule that can be used as a tool when designing new π-conjugated polymers. The experimental results are supported by a semiempirical quantum chemical model that accurately describes the energy levels of the terpolymers after parametrization on the energy levels of the copolymers and also provides a theoretical explanation for the observed arithmetic relations.

A 3-Fluoro-4-hexylthiophene-Based Wide Bandgap Donor Polymer for 10.9% Efficiency Eco-Friendly Nonfullerene Organic Solar Cells

Nonfullerene organic solar cells (NFOSCs) are attracting increasing academic and industrial interest due to their potential uses for flexible and lightweight products using low-cost roll-to-roll technology. In this work, two wide bandgap (WBG) polymers, namely P(fTh-BDT)-C6 and P(fTh-2DBDT)-C6, are designed and synthesized using benzodithiophene (BDT) derivatives. Good oxidation stability and high solubility are achieved by simultaneously introducing fluorine and alkyl chains to a single thiophene (Th) unit. Solid P(fTh-2DBDT)-C6 films present WBG optical absorption, suitable frontier orbital levels, and strong π-π stacking effects. In addition, P(fTh-2DBDT)-C6 exhibits good solubility in both halogenated and nonhalogenated solvents, suggesting its suitability as donor polymer for NFOSCs. The P(fTh-2DBDT)-C6:3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(5-hexylthienyl)-dithieno[2,3-d:2',3'-d']-s-indaceno[1,2-b:5,6-b']dithiophene (ITIC-Th) based device processed using chlorobenzene/1,8-diiodooctane (CB/DIO) exhibits a remarkably high power conversion efficiency (PCE) of 11.1%. Moreover, P(fTh-2DBDT)-C6:ITIC-Th reaches a high PCE of 10.9% when processed using eco-friendly solvents, such as o-xylene/diphenyl ether (DPE). The cell processed using CB/DIO maintains 100% efficiency after 1272 h, while that processed using o-xylene/DPE presents a 101% increase in efficiency after 768 h and excellent long-term stability. The results of this study demonstrate that simultaneous fluorination and alkylation are effective methods for designing donor polymers appropriate for high-performance NFOSCs.

Functionalizing benzothiadiazole with non-conjugating ester groups as side chains in a donor–acceptor polymer improves solar cell performance

Side chain ester substitution on donor–acceptor based conjugated polymers used as solar harvesters in a bulk-heterojunction (BHJ) polymer solar cell (PSC) can improve harvesting properties, phase separation in the active layer and PSC performance.

New Fused Pyrrolopyridine-Based Copolymers for Organic Solar Cell

A fused pyrrolopyridine core having substituents on the nitrogen atom instead of the carbon atom of the indoloindole unit is developed as a new donor unit for organic electronics. The new donor-acceptor copolymers, PDHPHBT, PDHPFBT, and PDHP2FBT, are synthesized using the new donor unit, well-known benzothiadiazole derivatives containing fluorine atoms as the acceptor. The thermal, optical, and electrochemical properties of these novel copolymers are reported. A solar cell using PDHPFBT with diphenyl ether has an open-circuit voltage, short-circuit current, fill factor, and power conversion efficiency of 0.86 V, 11.32 mA cm^-2 , 0.59%, and 5.68%, respectively, under AM 1.5G illumination (100 mW cm^-2 ) in the absence of annealing.

Near-Infrared Ternary Tandem Solar Cells

The paucity of near-infrared (NIR) organic materials with high absorption at long wavelengths, combined with large diffusion lengths and charge mobilities, is an impediment to progress in achieving high-efficiency organic tandem solar cells. Here a subcell is employed within a series tandem stack that comprises a solution-processed ternary blend of two NIR-absorbing nonfullerene acceptors and a polymer donor combined with a small-molecular-weight, short-wavelength fullerene-based subcell grown by vacuum thermal evaporation. The ternary cell achieves a power conversion efficiency of 12.6 ± 0.3% with a short-circuit current of 25.5 ± 0.3 mA cm^-2 , an open-circuit voltage of 0.69 ± 0.01 V, and a fill factor of 0.71 ± 0.01 under 1 sun, AM 1.5G spectral illumination. The success of this device is a result of the nearly identical offset energies between the lowest unoccupied molecular orbitals (LUMOs) of the donors with the highest occupied molecular orbital (HOMO) of the acceptor, resulting in a high open-circuit voltage. A tandem structure with an antireflection coating combining these subcells demonstrates a power conversion efficiency of 15.4 ± 0.3%.

An Imide-Based Pentacyclic Building Block for n-Type Organic Semiconductors

A new electron-deficient unit with a fused 5-membered heterocyclic ring was developed by replacing a cyclopenta-1,3-diene from electron-rich donor indacenodithiophene (IDT) with a cyclohepta-4,6-diene-1,3-diimde unit. The imide bridge endows dithienylbenzenebisimide (BBI) with a fixed planar configuration and low energy levels for both the highest occupied molecular orbital (HOMO; -6.24 eV) and the lowest unoccupied molecular orbit (LUMO; -2.57 eV). Organic field-effect transistors (OFETs) based on BBI polymers exhibit electron mobility up to 0.34 cm²  V^-1  s^-1 , which indicates that the BBI is a promising n-type building block for optoelectronics.

Polymer side-chain substituents elucidate thermochromism of benzodithiophene–dithiophenylacrylonitrile copolymers – polymer solubility correlation of thermochromism and photovoltaic performance

pBCNx polymers with different side-chain substituents for thermochromism and photovoltaic studies.

A General Protocol for the Polycondensation of Thienyl N-Methyliminodiacetic Acid Boronate Esters To Form High Molecular Weight Copolymers

Thienyl di-N-methyliminodiacetic acid (MIDA) boronate esters are readily synthesized by electrophilic C–H borylation producing bench stable crystalline solids in good yield and excellent purity. Optimal conditions for the slow release of the boronic acid using KOH as the base in biphasic THF/water mixtures enables the thienyl MIDA boronate esters to be extremely effective homo-bifunctionalized (AA-type) monomers in Suzuki–Miyaura copolymerizations with dibromo-heteroarenes (BB-type monomers). A single polymerization protocol is applicable for the formation of five alternating thienyl copolymers that are (or are close analogues of) state of the art materials used in organic electronics. The five polymers were produced in excellent yields and with high molecular weights comparable to those produced using Stille copolymerization protocols. Therefore, thienyl di-MIDA boronate esters represent bench stable and low toxicity alternatives to highly toxic di-trimethylstannyl AA-type monomers that are currently ubiquitous in the synthesis of these important alternating copolymers.

Impact of Backbone Fluorination on π-Conjugated Polymers in Organic Photovoltaic Devices: A Review

Solution-processed bulk heterojunction solar cells have experienced a remarkable acceleration in performances in the last two decades, reaching power conversion efficiencies above 10%. This impressive progress is the outcome of a simultaneous development of more advanced device architectures and of optimized semiconducting polymers. Several chemical approaches have been developed to fine-tune the optoelectronics and structural polymer parameters required to reach high efficiencies. Fluorination of the conjugated polymer backbone has appeared recently to be an especially promising approach for the development of efficient semiconducting polymers. As a matter of fact, most currently best-performing semiconducting polymers are using fluorine atoms in their conjugated backbone. In this review, we attempt to give an up-to-date overview of the latest results achieved on fluorinated polymers for solar cells and to highlight general polymer properties’ evolution trends related to the fluorination of their conjugated backbone.

Design, synthesis and optical properties of small molecules based on dithieno[3,2-b:2′,3′-d]stannole and stannafluorene

A series of stable Sn-containing heteroaromatic conjugated oligomers, dialkyl dithieno[3,2-b:2′,3′-d]stannole (DTSn) and stannafluorene (SnF) derivatives, were designed and synthesized.

Novel donor–acceptor polymers based on 7-perfluorophenyl-6H-[1,2,5]thiadiazole[3,4-g]benzoimidazole for bulk heterojunction solar cells

Three polymers with 7-perfluorophenyl-6H-[1,2,5]thiadiazole[3,4-g]benzoimidazole as a building unit were synthesized for BHJ solar cells, and a PCE of 1.92% was obtained.

Nanoscale phase separation control in rationally designed conjugated polymer solar cells processed using co-additives

A conjugated polymer based on benzo[1,2-b:4,5-b′]dithiophene with a thiophene-conjugated side chain andN-alkylthieno[3,4-c]pyrrole-4,6-dione was synthesized.

Dithieno[3,2-b:2′,3′-d]silole-based low band gap polymers: the effect of fluorine and side chain substituents on photovoltaic performance

The effect of the F atom and the side-chain of dithieno[3,2-b:2′,3′-d]silole-based polymers on the optical, carrier mobility and photovoltaic properties was investigated.

Recent progress of high performance polymer OLED and OPV materials for organic printed electronics

The development of organic printed electronics has been expanding to a variety of applications and is expected to bring innovations to our future life. Along with this trend, high performance organic materials with cost-efficient fabrication processes and specific features such as thin, light weight, bendable, and low power consumption are required. A variety of organic materials have been investigated in the development of this field. The basic guidelines for material design and the recent progress of polymer-based organic light-emitting diodes (OLEDs) and organic photovoltaic cells (OPVs) are reported.

Evaluation of heterocycle-modified pentathiophene-based molecular donor materials for solar cells

Two novel solution-processable acceptor-donor-acceptor (A-D-A)-structured organic small molecules with diketopyrrolopyrrole (DPP) as terminal acceptor units and pentathiophene (PTA) or pyrrole-modified pentathiophene (NPTA) as the central donor unit, namely, DPP2(PTA) and DPP2(NPTA), were designed and synthesized. We examined the effects of changing the central bridging heteroatoms of the five-ring-fused thienoacene core identity from sulfur [DPP2(PTA)] to nitrogen [DPP2(NPTA)] in the small-molecule donor material. Replacement of the bridging atom with a different electronic structure has a visible effect on both the optical and electrical properties: DPP2(NPTA), which contains much more electron-rich pyrrole in the central thienoacene unit, possesses red-shifted absorption and a higher HOMO level relative to DPP2(PTA) with the less electron-rich thiophene in the same position. More importantly, substitution of the bridging atoms results in a change of the substituting alkyl chains due to the nature of the heteroatoms, which significantly tailored the crystallization behavior and the ability to form an interpenetrating network in thin-film blends with an electron acceptor. Compared to DPP2(PTA) with no alkyl chain substituting on the central sulfur atom of the PTA unit, DPP2(NPTA) exhibits improved crystallinity and better miscibility with PC71BM probably because of a dodecyl chain on the central nitrogen atom of the NPTA unit. These features endow the DPP2(NPTA)/PC71BM blend film higher hole mobility and better donor/acceptor interpenetrating network morphology. Optimized photovoltaic device fabrication based on DPP2(NPTA)/PC71BM (1.5:1, w/w) has resulted in an average power conversion efficiency (PCE) as high as 3.69% (the maximum PCE was 3.83%). This study demonstrates that subtle changes and tailoring of the molecular structure, such as simply changing the bridging heteroatom in the thienoacene unit in D/A-type small molecules, can strongly affect the physical properties that govern their photovoltaic performances.

Quantifying Charge Extraction in Organic Solar Cells: The Case of Fluorinated PCPDTBT

We introduce a new and simple method to quantify the effective extraction mobility in organic solar cells at low electric fields and charge carrier densities comparable to operation conditions under one sun illumination. By comparing steady-state carrier densities at constant illumination intensity and under open-circuit conditions, the gradient of the quasi-Fermi potential driving the current is estimated as a function of external bias and charge density. These properties are then related to the respective steady-state current to determine the effective extraction mobility. The new technique is applied to different derivatives of the well-known low-band-gap polymer PCPDTBT blended with PC70BM. We show that the slower average extraction due to lower mobility accounts for the moderate fill factor when solar cells are fabricated with mono- or difluorinated PCPDTBT. This lower extraction competes with improved generation and reduced nongeminate recombination, rendering the monofluorinated derivative the most efficient donor polymer.

Highly efficient and facile alkylation of 4H-cyclopenta-[2,1-b:3,4-b′]dithiophene in water

A facile and highly economical methodology for the dialkylation of 4H-cyclopenta-[2,1-b:3,4-b′]dithiophene is reported.