All-Polymer Photovoltaic Devices of Poly(3-(4-n-octyl)-phenylthiophene) from Grignard Metathesis (GRIM) Polymerization

Quantitative comparison of the copolymerisation kinetics in catalyst-transfer copolymerisation to synthesise polythiophenes

Polythiophenes are one of the most widely studied conjugated polymers. With the discovery of the chain mechanism of Kumada catalyst-transfer polymerisation (KCTP), various polythiophene copolymer structures, such as random, block, and gradient copolymers, have been synthesized via batch or semi-batch (sequential addition) methods. However, the lack of quantitative kinetic data for thiophene monomers brings challenges to experimental design and structure prediction when synthesizing the copolymers. In this study, the reactivity ratios and the polymerisation rate constants of 3-hexylthiophene with 4 thiophene comonomers in KCTP are measured by adapting the Mayo-Lewis equation and the first-order kinetic behaviour of chain polymerisation. The obtained kinetic information highlights the impact of the monomer structure on the reactivity in the copolymerisations. The kinetic data are used to predict the copolymer structure of equimolar batch copolymerisations of the 4 thiophene derivatives with 3-hexylthiophene, with the experimental data agreeing well with the predictions. 3-Dodecylthiophene and 3-(6-bromo)hexylthiophene, which have higher structural similarity to 3-hexylthiophene, show nearly equivalent reactivity to 3-hexylthiophene and give random copolymers in the batch copolymerisation. 3-(2-Ethylhexyl)thiophene with a branched side chain is less reactive compared to 3-hexylthiophene and failed to homopolymerize at room temperature, but produced gradient copolymers with 3-hexylthiophene. Finally, the bulkiest 3-(4-octylphenyl)thiophene, despite its ability to homopolymerize, failed to maintain chain polymerisation in the copolymerisation with 3-hexylthiophene, possibly due to the large steric hindrance caused by the phenyl ring directly attached to the thiophene center. This study highlights the importance of monomer structures in copolymerisations and the need for accurate kinetic data.

Group 16 conjugated polymers based on furan, thiophene, selenophene, and tellurophene

Five-membered aromatic rings containing Group 16 elements (O, S, Se, and Te), also referred as chalcogenophenes, are ubiquitous building blocks for π-conjugated polymers (CPs). Among these, polythiophenes have been established as a model system to study the interplay between molecular structure, solid-state organization, and electronic performance. The judicious substitution of alternative heteroatoms into polythiophenes is a promising strategy for tuning their properties and improving the performance of derived organic electronic devices, thus leading to the recent abundance of CPs containing furan, selenophene, and tellurophene. In this review, we first discuss the current status of Kumada, Negishi, Murahashi, Suzuki-Miyaura, and direct arylation polymerizations, representing the best routes to access well-defined chalcogenophene-containing homopolymers and copolymers. The self-assembly, optical, solid-state, and electronic properties of these polymers and their influence on device performance are then summarized. In addition, we highlight post-polymerization modifications as effective methods to transform polychalcogenophene backbones or side chains in ways that are unobtainable by direct polymerization. Finally, the major challenges and future outlook in this field are presented.

The Effect of Alkyl Chain Length on Well-Defined Fluoro-Arylated Polythiophenes for Temperature-Dependent Morphological Transitions

Understanding the relationship between the molecular structure and morphological behaviors of well-defined semiconducting polymers is essential for developing novel conjugated building blocks and determining the origin of the functional characteristics of semiconducting polymers. Here, we provide insights into the significant temperature-dependent morphological transitions of novel well-defined polythiophene derivatives with m-alkoxy-substituted fluoro-aryl side units: poly(3-(4-fluoro-3-(hexyloxy)phenyl)thiophene) (PHFPT) and poly(3-(4-fluoro-3-(dodecyloxy)phenyl)thiophene) (PDFPT). We found that these unique morphological transitions depend on the alkyl chain length of the substituted fluoro-aryl side units. In PHFPT with short alkyl chains, the thermal treatment promotes a crowded interdigitated packing structure, resulting in narrow lamellar spacings in its crystalline structure. In contrast, the long alkyl chain of PDFPT acts as a physical spacer and disturbs the crowded interdigitation. In addition, the thermal treatment induces the backbone planarization and an ordered packing morphology in PDFPT. These demonstrations provide a critical milestone for the phase transitions of semiconducting polymers with conjugated side units.

Increasing N2200 Charge Transport Mobility to Improve Performance of All Polymer Solar Cells by Forming a Percolation Network Structure

The poor electron transport ability of the polymer acceptor is one of the factors restricting the performance of all-polymer solar cells. The percolation network of conjugated polymers can promote its charge transfer. Hence, we aim to find out the critical molecular weight (MW) of N2200 on the forming of the percolation network and to improve its charge mobility and thus photovoltaic performance of J51:N2200 blend. Detailed measurements demonstrate that when the MW of N2200 is larger than 96k, a percolation network structure is formed due to the chain tangled and multi-chain aggregations. Analysis of kinetic experiments reveals that it is the memory of the N2200 long chain conformation and the extent of aggregation in solution are carried into cast films for the formation of the percolation network. Thus, the electron mobility increases from 5.58 × 10-6 cm2V-1s-1 (N220017k) to 9.03 × 10-5 cm2V-1s-1 when the MW of N2200 is >96k. It led to a balance between hole and electron mobility. The μh/μe decrease from 16.9 to 1.53, causing a significant enhancement in the PCEs, from 5.87 to 8.28% without additives.

Low-Bandgap n-Type Polymer Based on a Fused-DAD-Type Heptacyclic Ring for All-Polymer Solar Cell Application with a Power Conversion Efficiency of 10.7

An n-type polymer (A701) is designed and synthesized with an alternative A'-DAD-A'-D' backbone, where 1,1-dicyanomethylene-3-indanone (IC), dithienothiophen[3,2-b]-pyrrolobenzothiadiazole (TPBT), and benzodithiophene (BDT) are used as A', DAD, and D' units, respectively. A701 shows enhanced light absorption with a narrow bandgap of 1.42 eV and a high absorption coefficient of 6.85 × 10⁴ cm^-1 at 780 nm. It displays an uplifted LUMO (the lowest unoccupied molecular orbital) level of -3.80 eV. By introducing a high point solvent additive of 1,8-diiodooctane (DIO), all-polymer solar cells (all-PSCs) based on the PBDB-T:A701 blend exhibit efficient exciton dissociation, enhanced charge transport, and decreased bimolecular recombination. Thus, a high open-circuit voltage (V_OC) of 0.92 V, a short-circuit current (J_SC) of 18.27 mA cm^-2, and a fill factor (FF) of 0.64 are attained, affording an impressive power conversion efficiency (PCE) of 10.70%. The low voltage loss of 0.50 V and high efficiency of 10.7% are among the top values for all-PSCs. Our results indicate that the fused DAD-type heptacyclic ring can be utilized to construct not only nonfullerene small molecular acceptors but also promising polymer acceptors.

Isoindigo-3,4-Difluorothiophene Polymer Acceptors Yield "All-Polymer" Bulk-Heterojunction Solar Cells with over 7 % Efficiency

Poly(isoindigo-alt-3,4-difluorothiophene) (PIID[2F]T) analogues used as "polymer acceptors" in bulk-heterojunction (BHJ) solar cells achieve >7 % efficiency when used in conjunction with the polymer donor PBFTAZ (model system; copolymer of benzo[1,2-b:4,5-b']dithiophene and 5,6-difluorobenzotriazole). Considering that most efficient polymer-acceptor alternatives to fullerenes (e.g. PC₆₁ BM or its C₇₁ derivative) are based on perylenediimide or naphthalenediimide motifs thus far, branched alkyl-substituted PIID[2F]T polymers are particularly promising non-fullerene candidates for "all-polymer" BHJ solar cells.

Matchmaking in Catalyst-Transfer Polycondensation: Optimizing Catalysts based on Mechanistic Insight

Catalyst-transfer polycondensation (CTP) has emerged as a useful living, chain-growth polymerization method for synthesizing conjugated (hetero)arene-based polymers with targetable molecular weights, narrow dispersities, and controllable copolymer sequences-all properties that significantly influence their performance in devices. Over the past decade, several phosphine- and carbene-ligated Ni- and Pd-based precatalysts have been shown to be effective in CTP. One current limitation is that these traditional CTP catalysts lead to nonliving, non-chain-growth behavior when complex monomer scaffolds are utilized. Because these monomers are often found in the highest-performing materials, there is a significant need to identify alternative CTP catalysts. Recent mechanistic insight into CTP has laid the foundation for designing new catalysts to expand the CTP monomer scope. Building off this insight, we have designed and implemented model systems to identify effective catalysts by understanding their underlying mechanistic behaviors and systematically modifying catalyst structures to improve their chain-growth behavior. In this Account, we describe how each catalyst parameter-the ancillary ligand(s), reactive ligand(s), and transition metal-influences CTP. As an example, ancillary ligands often dictate the turnover-limiting step of the catalytic cycle, and perhaps more importantly, they can be used to promote the formation of the key intermediate (a metal-arene associative complex) and its subsequent reactivity. The fidelity of this intermediate is central to the mechanism for the living, chain-growth polymerization. Reactive ligands, on the other hand, can be used to improve catalyst solubility and accelerate initiation. Additional advantages of the reactive ligand include providing access points for postpolymerization modification and synthesizing polymers directly off surfaces. While the most frequently used CTP catalysts contain nickel, palladium-based catalysts exhibit a higher functional group tolerance and broader substrate scope (e.g., monomers with boron, magnesium, tin, and gold transmetalating agents). Overall, we anticipate that applying the tools and lessons detailed in this Account to other monomers should facilitate a better "matchmaking" process that will lead to new catalyst-transfer polycondensations.

From Fullerene-Polymer to All-Polymer Solar Cells: The Importance of Molecular Packing, Orientation, and Morphology Control

All-polymer solar cells (all-PSCs), consisting of conjugated polymers as both electron donor (P_D) and acceptor (P_A), have recently attracted great attention. Remarkable progress has been achieved during the past few years, with power conversion efficiencies (PCEs) now approaching 8%. In this Account, we first discuss the major advantages of all-PSCs over fullerene-polymer solar cells (fullerene-PSCs): (i) high light absorption and chemical tunability of P_A, which affords simultaneous enhancement of both the short-circuit current density (J_SC) and the open-circuit voltage (V_OC), and (ii) superior long-term stability (in particular, thermal and mechanical stability) of all-PSCs due to entangled long P_A chains. In the second part of this Account, we discuss the device operation mechanism of all-PSCs and recognize the major challenges that need to be addressed in optimizing the performance of all-PSCs. The major difference between all-PSCs and fullerene-PSCs originates from the molecular structures and interactions, i.e., the electron transport ability in all-PSCs is significantly affected by the packing geometry of two-dimensional P_A chains relative to the electrodes (e.g., face-on or edge-on orientation), whereas spherically shaped fullerene acceptors can facilitate isotropic electron transport properties in fullerene-PSCs. Moreover, the crystalline packing structures of P_D and P_A at the P_D-P_A interface greatly affect their free charge carrier generation efficiencies. The design of P_A polymers (e.g., main backbone, side chain, and molecular weight) should therefore take account of optimizing three major aspects in all-PSCs: (1) the electron transport ability of P_A, (2) the molecular packing structure and orientation of P_A, and (3) the blend morphology. First, control of the backbone and side-chain structures, as well as the molecular weight, is critical for generating strong intermolecular assembly of P_A and its network, thus enabling high electron transport ability of P_A comparable to that of fullerenes. Second, the molecular orientation of anisotropically structured P_A should be favorably controlled in order to achieve efficient charge transport as well as charge transfer at the P_D-P_A interface. For instance, face-to-face stacking between P_D and P_A at the interface is desired for efficient free charge carrier generation because misoriented chains often cause an additional energy barrier for overcoming the binding energy of the charge transfer state. Third, large-scale phase separation often occurs in all-PSCs because of the significantly reduced entropic contribution by two macromolecular chains of P_D and P_A that energetically disfavors mixing. In this Account, we review the recent progress toward overcoming each recognized challenge and intend to provide guidelines for the future design of P_A. We believe that by optimization of the parameters discussed above the PCE values of all-PSCs will surpass the 10% level in the near future and that all-PSCs are promising candidates for the successful realization of flexible and portable power generators.

A step towards the processability of insoluble or partially soluble functional and structural variants of polymers based on 3,4-alkylenedioxythiophene

This work describes the processability of insoluble or partially soluble polymers based on 3,4-alkylenedioxythiophene.

Fully conjugated block copolymers for single-component solar cells: synthesis, purification, and characterization

All-polymer solar cells using the preparative GPC separated block copolymer P3HT-b-PBIT2 as a simple active layer show a power conversion efficiency of 1.0%.

Pronounced Cosolvent Effects in Polymer:Polymer Bulk Heterojunction Solar Cells with Sulfur-Rich Electron-Donating and Imide-Containing Electron-Accepting Polymers

The performance of solar cells with a polymer:polymer bulk heterojunction (BHJ) structure, consisting of poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene-alt-3-fluorothieno[3,4-b]thiophene-2-carboxylate] (PTB7-Th) donor and poly[[N,N'-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)] (P(NDI2OD-T2)) acceptor polymers, was investigated as a function of cosolvent (p-xylene:chlorobenzene (pXL:CB)) composition ratio. A remarkable efficiency improvement (∼38%) was achieved by spin-coating the photoactive blend layer from pXL:CB = 80:20 (volume) rather than pXL alone, but the efficiency then decreased when the CB content increased further to pXL:CB = 60:40. The improved efficiency was correlated with a particular PTB7-Th:P(NDI2OD-T2) donor-acceptor blend nanostructure, evidenced by a fiber-like surface morphology, a red-shifted optical absorption, and enhanced PL quenching. Further device optimization for pXL:CB = 80:20 films yielded a power conversion efficiency of ∼5.4%. However, these devices showed very poor stability (∼15 min for a 50% reduction in initial efficiency), owing specifically to degradation of the PTB7-Th donor-component. Replacing PTB7-Th with a more stable donor polymer will be essential for any application potential to be realized.

High-performance all-polymer solar cells via side-chain engineering of the polymer acceptor: the importance of the polymer packing structure and the nanoscale blend morphology

The effectiveness of side-chain engineering is demonstrated to produce highly efficient all-polymer solar cells (efficiency of 5.96%) using a series of naphthalene diimide-based polymer acceptors with controlled side chains. The dramatic changes in the polymer packing, blend morphology, and electron mobility of all-polymer solar cells elucidate clear trends in the photovoltaic performances.

A divergent route to core- and peripherally functionalized diazacoronenes that act as colorimetric and fluorescence proton sensors

One-stop center for functional polycyclic aromatic hydrocarbons — a dichlorodiazaperylene intermediate has been synthesized and employed for the synthesis of highly functionalized coronene derivatives.

Combining core annulation and peripheral group modification, we have demonstrated a divergent synthesis of a family of highly functionalized coronene derivatives from a readily accessible dichlorodiazaperylene intermediate. Various reactions, such as aromatic nucleophilic substitution, Kumada coupling and Suzuki coupling proceed effectively on α-positions of the pyridine sites, giving rise to alkoxy, thioalkyl, alkyl or aryl substituted polycyclic aromatic hydrocarbons. In addition to peripheral group modulation, the aromatic core structures can be altered by annulation with thiophene or benzene ring systems. Corresponding single crystal X-ray diffraction and optical studies indicate that the heteroatom linkages not only impact the solid state packing, but also significantly influence the optoelectronic properties. Moreover, these azacoronene derivatives display significant acid-induced spectroscopic changes, suggesting their great potential as colorimetric and fluorescence proton sensors.

Functionalized Poly(3-hexylthiophene)s via Lithium-Bromine Exchange

Poly(3-hexylthiophene) (P3HT) is one of the most extensively investigated conjugated polymers and has been employed as the active material in many devices including field-effect transistors, organic photovoltaics and sensors. As a result, methods to further tune the properties of P3HT are desirable for specific applications. Herein, we report a facile postpolymerization modification strategy to functionalize the 4-position of commercially available P3HT in two simple steps–bromination of the 4-position of P3HT (Br–P3HT) followed by lithium−bromine exchange and quenching with an electrophile. We achieved near quantitative lithium–bromine exchange with Br–P3HT, which requires over 100 thienyl lithiates to be present on a single polymer chain. The lithiated-P3HT is readily combined with functional electrophiles, resulting in P3HT derivatives with ketones, secondary alcohols, trimethylsilyl (TMS) group, fluorine, or an azide at the 4-position. We demonstrated that the azide-modified P3HT could undergo Cu-catalyzed or Cu-free click chemistry, significantly expanding the complexity of the structures that can be appended to P3HT using this method.

Star-shaped isoindigo-based small molecules as potential non-fullerene acceptors in bulk heterojunction solar cells

Two novel star-shaped isoindigo-based small molecules with different cores of triphenylamine and benzene were designed and synthesized as non-fullerene acceptor materials in organic solar cells.

Furan-bridged thiazolo [5,4-d]thiazole based D–π–A–π–D type linear chromophore for solution-processed bulk-heterojunction organic solar cells

Novel furan-bridged thiazolo[5,4-d]thiazole based π-conjugated organic chromophore (RFTzR) was formulated and utilized for the fabrication of solution-processed small molecule organic solar cells (SMOSCs).

High-Performance All-Polymer Solar Cells Based on Face-On Stacked Polymer Blends with Low Interfacial Tension

We report highly efficient all-polymer solar cells with power conversion efficiencies of over 4.5% by highly intermixed blends of PTB7-Th donor and P(NDI2OD-T2) acceptor polymers. The low interfacial tension and the face-on π-π stackings of the all-polymer blends afforded desired nanophase morphology, which facilitates efficient charge transport from the active layer to each electrode. In addition, the incorporation of 1,8-diiodooctane additives was able to tune the degree of crystallinity and orientation of P(NDI2OD-T2) acceptors, resulting in remarkable enhancement of electron mobility, external quantum efficiency, and J_SC values.

All-polymer bulk heterojuction solar cells with 4.8% efficiency achieved by solution processing from a co-solvent

All-polymer solar cells with 4.8% power conversion efficiency are achieved via solution processing from a co-solvent. The observed short-circuit current density of 10.5 mA cm(-2) and external quantum efficiency of 61.3% are also the best reported in all-polymer solar cells so far. The results demonstrate that processing the active layer from a co-solvent is an important strategy in achieving highly efficient all-polymer solar cells.

All-Polymer Solar Cell with High Near-Infrared Response Based on a Naphthodithiophene Diimide (NDTI) Copolymer

Polymer-blend solar cells (all-PSCs) based on a copolymer of naphthodithiophene diimide and bithiophene (PNDTI-BT-DT) as a near-infrared absorber as well as an electron acceptor were fabricated in combination with PTB7 as an electron donor. Notably, the external quantum efficiency spectra of the all-PSCs demonstrated photoresponse up to 900 nm with the efficiency of 25% at 800 nm, which is much higher than that for the previously reported all-PSCs. Power conversion efficiency as high as 2.59% was achieved under the irradiation of simulated solar light (AM1.5, 100 mW/cm²). Both PNDTI-BT-DT and PTB7 formed a crystalline structure in the blend films similar to in the pristine films, leading to the efficient charge generation contributed from both polymers.

Control of nanomorphology in all-polymer solar cells via assembling nanoaggregation in a mixed solution

The formation of interconnected phase-separated domains on sub-20 nm length scale is a key requirement for all-polymer solar cells (all-PSCs) with high efficiency. Herein, we report the application of crystalline poly(3-hexylthiophene) (P3HT) nanowires via an O-dichlorobenzene/hexane mixed solution blended with poly{(9,9-dioctylfluorenyl-2,7-diyl)-alt-[4,7-bis(3-hexylthiophen-5-yl)-2,1,3-benzothiadiazole]-2',2″-diyl} (F8TBT) for the first time. The nanomorphology of P3HT:F8TBT all-PSCs can be controlled by P3HT nanowires. The improved film morphology leads to enhanced light absorption, exciton dissociation, and charge transport in all-PSCs, as confirmed by ultraviolet-visible absorption spectra, X-ray diffraction, transmission electron microscopy, atomic force microscopy, and time-resolved photoluminescence spectra. The P3HT nanowire:F8TBT all-PSCs could achieve a power conversion efficiency of 1.87% and a Voc of 1.35 V, both of which are the highest values for P3HT:F8TBT all-PSCs. This work demonstrates that the semiconductor nanowires fabricated by the mixed solvents method is an efficient solution process approach to controlling the nanomorphology of all-PSCs.

Control of miscibility and aggregation via the material design and coating process for high-performance polymer blend solar cells

A power conversion efficiency of 3.6% for an all-polymer solar cell, which is the highest ever reported, is achieved by introducing a conjugated side chain into a p-type polymer to improve the miscibility of the polymer blend and by adding small amounts of 1,8-diiodooctane to increase the aggregation of n-type polymer.