Control over the aggregated structure of donor–acceptor conjugated polymer films for high‐mobility organic field‐effect transistors

Donor–acceptor (D‐A) conjugated polymers have demonstrated great potential in organic field‐effect transistors application, and their aggregated structure is a crucial factor for high charge mobility. However, the aggregated structure of D‐A conjugated polymer films is complex and the structure–property relationship is difficult to understand. This review provides an overview of recent progress in controlling the aggregated structure of D‐A conjugated polymer films for higher mobility, including the mechanisms, methods, and properties. We first discuss the multilevel microstructures of D‐A conjugated polymer films, and then summarize the current understanding of the relationship between film microstructures and charge transport properties. Subsequently, we review the theory of D‐A conjugated polymer crystallization. After that, we summarize the common methods to control the aggregated structure of semi‐crystalline and near‐amorphous D‐A conjugated polymer films, such as crystallites and aggregates, tie chains, film alignment, and attempt to understand them from the basic theory of polymer crystallization. Finally, we provide the current challenges in controlling the aggregated structure of D‐A conjugated polymer films and in understanding the structure–property relationship.

polymers exhibits strong intermolecular charge transfer interactions, allowing their energy levels to be reasonably tuned. [12,13]Besides, D-A conjugated polymers with highly planar backbone and strong interchain π-π interaction are beneficial to charge transport. [14,15]As a result, D-A conjugated polymers have achieved excellent performance in OFETs and OSCs.
OFET is an organic semiconductor device that uses a variable electric field to control the current between the source and drain electrodes.A crucial parameter that determines the performance of OFETs is the charge mobility, which describes the drift velocity of charge carriers per unit applied electric field.Based on the species of charge carrier, D-A conjugated polymers can be divided into three types, p-type (hole), n-type (electron), and ambipolar (hole and electron).In the last dozen years, several species of D-A conjugated polymers have been developed for OFETs, such as diketopyrrolopyrrole (DPP)-based polymers, [16,17] isoindigo (IID)-based polymers, [18,19] indacenodithiophene The main content of this review.
(IDT)-based polymers, [20] naphthalenediimide (NDI)-based polymers, [21] etc.Their charge mobilities have exceeded 10 cm 2 V −1 s −1 , [22] which is higher than that of amorphous silicon (0.5-1 cm 2 V −1 s −1 ). [1]Moreover, the highest charge mobility of D-A conjugated polymer single crystal has reached 92.64 cm 2 V −1 s −1 , [23] which is approaching that of polycrystalline silicon (∼100 cm 2 V −1 s −1 ). [24,25][28] For semi-crystalline P3ATs, it is proposed that high crystallinity, low π-π stacking distance (d π-π ), well connected crystalline domains, alignment of films along the channel direction, etc., are critical for high charge mobility. [29,30]These principles have also been used to guide the control of the aggregated structure of D-A conjugated polymers.In addition, recent researches have demonstrated that D-A conjugated polymers with near-amorphous film structure can also show good charge mobilities (∼1 cm 2 V −1 s −1 ), and the study of near-amorphous conjugated polymers has become an interesting research topic. [31,32]35][36] Therefore, the control of the aggregated structure of D-A conjugated polymer films can be divided into two species, semi-crystalline and near-amorphous.
Before discussing how to control the aggregated structure of D-A conjugated polymers, it is necessary to understand their crystallization mechanism.Therefore, the main content of this review concerns crystallization theories, control methods, aggregated structures, and charge mobilities of D-A conjugated polymers, as shown in Figure 1.We first discuss the multilevel microstructures of D-A conjugated polymer films and the current understanding of charge transport in conjugated polymer films.Subsequently, we discuss the theory of D-A conjugated polymer crystallization, which is crucial for controlling the aggregated structure.After that, we review recent advances in controlling the aggregated structure of semi-crystalline and near-amorphous D-A conjugated polymer films for enhanced charge mobility.We attempt to understand them from the basic theory of polymer crystallization.Finally, we present a concise summary, and provide the current challenges in controlling the aggregated structure of D-A conjugated polymer films and in understanding the structure-property relationship.

MULTISCALE CHARGE TRANSPORT IN D-A CONJUGATED POLYMER FILMS
Polymers are long-chain structures with many degrees of conformational freedom, polydisperse chain lengths, and complex interactions.As a result, the structure of conjugated polymer films is multilevel, including molecular structures, packing structures, and phase behavior. [1]The multilevel microstructures in conjugated polymer films in turn lead to a multiscale charge transport process. [37,38]Therefore, it is necessary to understand the multilevel microstructures of D-A conjugated polymer films and how they affect charge transport.

Multilevel microstructures of D-A conjugated polymer films
A general schematic diagram describing the microstructures of D-A conjugated polymer films is shown in Figure 2. The molecular structure of D-A conjugated polymers includes the chemical structure of the repeating unit (the acceptor unit, the donor unit, and the alkyl side chain), degree of polymerization, chain conformation, etc. [1] The packing structures (multiple aggregates formed through interchain interactions) and phase behavior (size, orientation, and distribution of domains) proposed by Pei and coworkers [1] could also be regarded as the aggregated structure.Because of the semicrystalline nature of polymer materials, their aggregated structure consists of crystalline and amorphous domains.The crystalline and amorphous domains can be distinguished by F I G U R E 2 Schematic illustration of multilevel microstructures for donor-acceptor (D-A) conjugated polymers.The film is made up of crystallites (orange areas), aggregates (azury areas), and completely amorphous domains (light blue areas).In the amorphous domains, the possible shapes of polymer chains include loops, tie chains, cilia, and hooking networks.Illustrations (A) and (B) describe the representative structures of the molecular chain and the crystals, respectively.
technologies such as polarized optical microscopy, atomic force microscopy, transmission electron microscopy, etc.In general, a crystalline domain may not be monocrystalline, but rather a polycrystalline structure consisting of several crystallites separated by grain boundaries. [39,40]The structure of crystallites could be characterized by technologies such as X-ray diffraction (XRD), selected area electron diffraction (SAED), and scanning tunneling microscopy technologies, and the crystallites may adopt a face-on (with the π-π stacking direction perpendicular to the substrate) or edge-on (with the side chain stacking direction perpendicular to the substrate) orientation.However, the microstructure of amorphous domains is much harder to characterize.In early times, it was proposed that the amorphous phase is made up of random coils.[43] For traditional flexible polymers, the sizes of these short-range ordered structures are generally less than 5 nm, which is smaller than that of crystals (larger than 10 nm).To distinguish them, crystals are regarded as long-range ordered structures, and the short-range ordered structures are named as aggregates. [31]For D-A conjugated polymers, the existence of folded-chain is negligible because of their rigid backbones. [44]Therefore, the cluster model is suitable to describe the amorphous structure of D-A conjugated polymers, and the microstructures include macromolecular hooking networks and clusters. [42]Besides, the amorphous chains between two crystalline domains may have different sharps, including cilia, loops, and tie chains. [45,46]

Charge transport in conjugated polymer films
The transport of charge can be characterized by the charge mobility (μ), which describes the average velocity of charge carriers in a given electric field.At the molecular level, the charge mobility is a combined effect of electron tunneling and hopping motion, which is dominant at low temperature and higher temperature, respectively. [47]According to the Einstein relation and the semiclassical Marcus theory, the charge mobility of organic semiconductors in the high-temperature regime can be expressed as [47][48][49][50] where e, a, t, k B , T, ћ, and λ correspond to the electronic charge, transport distance, the transfer integral, the Boltzmann constant, temperature, the reduced Planck constant, and the reorganization energy, respectively.Therefore, increasing the transfer integral or decreasing the reorganization energy is beneficial for high charge mobility.The transfer integral describes the strength of the interaction between the two segments and is related to the energetic splitting of the electronic level. [30,50]It is governed by the relative position of the interacting segments and the shape of their frontier molecular orbitals. [49]In general, improving the conjugated length and decreasing the d π-π result in higher transfer integral, and thus lead to higher charge mobility. [30,51]The reorganization energy describes the change of the vibrational structure caused by the electron gain/loss process of the segment, and its value is smaller for longer conjugated segments. [30]he transportation of charge occurs through ordered and disordered domains in conjugated polymer films.According to the ordering of conjugated polymer chains, the microstructure of films can be classified into three types, as shown in Figure 3A-C. [31]The first type of films consists of three-dimensional (3D) long-range ordered domains (crystallites) and amorphous "spaghetti-like" regions, for example, high-molecular-weight regioregular poly(3-hexylthiophene) (P3HT) film. [31]In the crystalline domains, charge transport along the backbone direction is the fastest, the π-π stacking direction which is critical for interchain charge transfer is posterior, and the side chain direction is very slow.Therefore, an edge-on orientation is beneficial to charge transport, [52] because the channel direction in OFET is along the backbone or π-π stacking direction in this case.In the amorphous domains, the charge mobility is much slower or even F I G U R E 3 Schematic illustrations of microstructure for conjugated polymer films: (A) a long-range ordered film, (B) a short-range ordered film, and (C) a completely amorphous film.Reproduced with permission. [31]Copyright 2013, Macmillan Publishers.(D) Calculated charge mobility of typical poly(3hexylthiophene) (P3HT) films with various molecular conformation and chain structure.Reproduced with permission. [30]Copyright 2009, American Chemical Society.(E) Schematic illustrations of a high-crystalline P3HT-type film with poor interconnectivity in amorphous regions (top) and a low-crystalline RP33-type film with localized aggregates in amorphous regions (bottom).Reproduced with permission. [62]Copyright 2016, American Chemical Society.
negligible in comparison with that of crystalline domains. [53,54]The average overall mobility (μ ave ) of the whole film is the combination of mobility in the ordered and disordered domains (μ ord and μ dis , respectively), and can be expressed as [30] 1 where φ ord is the percentage of the ordered domains.According to Equation (2), the overall mobility is dominated by the mobility of the amorphous domains, and therefore it is necessary to decrease the percentage of the amorphous domains or increase the mobility of the amorphous domains.Besides, at the interface between order and disorder, the charges must overcome an energy barrier when moving from the ordered to the amorphous domain due to the larger bandgap in the amorphous domain compared to the ordered one and the absence of energetic overlap of electronic states. [31,55]As a result, the disordered domain acts as a significant barrier to charge transport.In reality, it has been proposed that the transport of charge carriers between crystallites is by means of tie chains for polymer semiconductors.In sum, the critical elements for designing high-mobility crystalline films include: high crystallinity, edge-on orientation, low d π-π , more tie chains, alignment of molecular chains, and crystalline domains along the channel direction.Taking P3HT film as an example, a general relationship between the microstructure and the charge mobility is shown in Figure 3D. [30]he second type of films consists of disordered aggregates (short-range ordering with a few molecular units) and amorphous domains.[58][59][60] The disorder in crystals can be quantitatively measured by the paracrystallinity parameter (g), which is defined as the standard deviation of lattice spacing fluctuations normalized by the average lattice spacing. [61]Salleo and coworkers calculated the g values in the π-π stacking direction of several D-A conjugated polymers, and found that their g values are in the range of 10%-15%, which is closer to that of amorphous structure (10%-20%). [31]However, these D-A conjugated polymers show good charge mobilities (∼1 cm 2 V −1 s −1 ), which are comparable to or even higher than those of traditional semi-crystalline conjugated polymers such as P3HT, poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2,-b]thiophene), and poly[5;50-bis(3-alkyl-2thienyl)−2,20-bithiophene].The activation energies for transport in these high-mobility D-A conjugated polymers are 76 ± 33 meV, which are similar to those of traditional semi-crystalline conjugated polymers (72 ± 33 meV) and much smaller than those of completely amorphous materials (230 ± 100 meV).Therefore, the presence of small and disordered aggregates is sufficient for high charge mobility. [31]Further evidence for this is proved by Son and coworkers. [62,63]They proposed a 3-hexylthiophene and thiophene copolymerization strategy to increase the connectivity between polymer chains by forming localized aggregates, as shown in Figure 3E.The planar thiophene units in random copolymer RP33 (33% thiophene) is harmful to crystallization, but they can form localized aggregates in amorphous regions.As a result, the field-effect mobility of RP33 is 1.37 cm 2 V −1 s −1 , which is much higher than that of highly crystalline P3HT (0.17 cm 2 V −1 s −1 ). [62]The third type of films are completely amorphous, such as regiorandom P3HT and phenylenevinylene-based polymers.These films have the poorest charge mobility.
Although the charge mobilities of D-A conjugated polymers with disordered aggregates can reach ∼1 cm 2 V −1 s −1 , which is higher than that of semi-crystalline P3HT. [31]n contrast to semi-crystalline P3HT, semi-crystalline D-A conjugated polymers have higher charge mobilities.On the one hand, D-A conjugated polymers contain more planar backbones and stronger π-π interactions, which are beneficial to the intrachain and interchain charge transport, respectively. [14,15,17]On the other hand, because of their large persistence length (>5 nm), D-A conjugated polymers generally form extended-chain crystals with the length direction along the backbone, [44] which favors charge transport.However, P3HT generally form folded-chain crystals with the length direction along the π-π stacking when its molecular weight is large enough, [64] which is relatively poorer for charge transport.Therefore, there is still a lot of room for improvement in the charge mobilities of D-A conjugated polymer films.][67][68] Furthermore, the charge mobility of D-A conjugated polymer single crystals can even reach 92 cm 2 V −1 s −1 . [23]The rules for designing high-mobility crystalline P3HT films could be used in semi-crystalline D-A conjugated polymer films.[35][36] Therefore, the control of the aggregated structure of D-A conjugated polymer films can be divided into two directions, semi-crystalline and near-amorphous.
In fact, for D-A conjugated polymers, the concept of semi-crystalline and near-amorphous is not very clear.In general, crystals are 3D long-range ordered, liquids are 3D disordered, and liquid crystals are two-dimensional (2D) or one-dimensional (1D) long-range ordered.For crystalline polymer materials, they are typically semi-crystalline due to the presence of amorphous domains.However, the g value of semi-crystalline D-A conjugated polymers (commonly considered) are much larger than that of crystals (<2%).In consideration of the fact that several D-A conjugated polymers have a large g value calculated from the grazingincidence wide-angle X-ray scattering (GIWAXS) data and an obvious melt peak measured by the fast scanning calorimetry, Martin and coworkers proposed a semi-paracrystalline model (a dense array of small paracrystalline domains embedded in a more disordered matrix) to describe their aggregated structure. [69]A paracrystalline can be regarded as an ordered structure with the g value between 2% and 12%. [70]Ye et al. further studied the structure of 11 spin-coated or drop-casted conjugated polymer films by GIWAXS and differential scanning calorimetry (DSC).According to the glass transition temperature (T g ) signal, melting temperature (T m ) signal, g values for (100) and (010) peaks, they divided these conjugated polymers into five species, 3D amorphous, oriented amorphous, highly disordered, 2D semiparacrystalline, and 3D semi-paracrystalline. Their work is very important for further understanding the microstructure of conjugated polymers in the future. [70]In this review, we simply discussed two types of D-A conjugated polymers, semi-crystalline (long-range ordered) and near-amorphous (short-range ordered), as proposed by Dong et al. [32] The reasons for this include: (1) the g value may be overestimated because the existence of amorphous domains, [69] (2) it is hard to further estimate the types of D-A conjugated polymers in the literatures, (3) the aggregated structure of D-A conjugated polymers is largely depend on the crystallization conditions or blends, [71][72][73] and (4) the essential differences between semi-crystalline, paracrystalline, and amorphous structure are the size and density of ordered structure, which are more accurate to describe the microstructure.Also, the size of short-range ordered structure for D-A conjugated polymers is not very clear.As mentioned in Section 2.1, the concept of short-range ordered is used to describe the ordered structure in amorphous materials, and the size of short-range ordered in traditional flexible polymers is <5 nm.Here, the concept of near-amorphous can be regarded as a medial structure between semi-crystalline and completely amorphous.We can approximately consider that the size of short-range ordered structure for D-A conjugated polymers is around the minimum size of a stable nucleus (r 0 ), which will be further discussed in Section 3.

THEORY OF D-A CONJUGATED POLYMER CRYSTALLIZATION
Before discussing the regulation of semi-crystalline and nearamorphous D-A conjugated polymer films, it is necessary to understand the crystallization mechanism of D-A conjugated polymers.The crystallization process involves two steps, nucleation and growth.The nucleation process is generally described by the classical nucleation theory (CNT), as shown in Figure 4A.When considering the formation of an embryo from the melt or in the solution, the change of surface energy (ΔG s ) is proportional to the square of radius (r), and the change of volume energy (ΔG v ) is proportional to −r 3 .The total free energy change (ΔG) is the sum of ΔG s and ΔG v .When the embryo size increases, ΔG increases first and then decreases. [74,75]The critical ΔG above whom the embryo could exist and further grow into a stable nucleus (ΔG ≤ 0, r ≥ r 0 ) can be expressed as ΔG*, and the accordingly critical radius is r*.According to the CNT, the steady-state rate of nucleation (J) which describes the number of nuclei formed per unit time per unit volume can be expressed as [74][75][76][77] where A is the pre-exponential factor.When the size of nucleus is larger than r 0 , the nuclei can exist stably and may be characterized by the DSC (ΔG < 0), otherwise, the nucleus Reproduced with permission. [71]Copyright 2022, Elsevier Ltd. (E) Schematic illustration of the anisotropic aggregation model.Reproduced with permission. [87]Copyright 2007, American Institute of Physics.
is unstable and may redissolve.Therefore, r 0 can be regarded as a size dividing line between crystalline and amorphous structures.
According to nucleation from amorphous state, on the surface of an existing crystal and at the corner of two intersecting crystal surfaces, the types of nucleation can be divided into primary, secondary, and tertiary nucleation, respectively. [78]mong them, primary nucleation is the slowest due to the generation of more interfaces.Primary nucleation can also be divided into homogeneous and heterogeneous nucleation, corresponding to nucleation from a homogeneous melt or solution and nucleation on the surface of impurities, respectively.For flexible polymers, there are two ideal types of homogeneous nucleation, intramolecular nucleation (chainfolding) and intermolecular nucleation (chain-extending). [79]owever, intramolecular nucleation is negligible for D-A conjugated polymers due to their rigid backbone.In consideration of the fact that the conformation of conjugated polymer chains generally changes from coil to rod before the formation of interchain π-π stacking, [80,81] a possible route for the nucleation process of D-A conjugated polymers is shown in Figure 4B.The polymer chains first form locally planarized conformation, then two locally planarized chains form a double-segment-stacked structure, and at last the double-segment-stacked structure grow larger and form a nucleus or aggregate.Intermolecular nucleation is very hard to occur in a homogeneous system, [79,82] even though the difficulty is reduced for D-A conjugated polymers because of their rigid backbone and strong interchain π-π stacking.In solution processing, controlling the solution-state aggregation is a general way to facilitate the nucleation process, which can also be regarded as a special heterogeneous nucleation, the self-seeding nucleation. [27,83,84]or the growth process, the most widely used Lauritzen and Hoffman theory is based on folded-chain model, [85] which is not suitable for D-A conjugated polymers.Establishing a quantitative growth theory for D-A conjugated polymers is a big challenge.On the one hand, the crystallization conditions of D-A conjugated polymers are unsteady, which is difficult to describe in theory.On the other hand, in situ observation of the crystallization process of D-A conjugated polymers is experimentally difficult due to the small size of the crystals and the presence of solvent.Moreover, simulation methods are also rarely used for D-A conjugated polymers due to their complex molecular structure.Recently, our group proposed the diffusion-conformational transition (D-CT) model to describe the growth process of D-A conjugated polymer crystals quantificationally, as shown in Figure 4C,D. [71]ased on the size of crystals along the backbone direction, we divided D-A conjugated polymer crystals into four types: local ordered structure (less than the extended-chain length), fibrils (between dozens of nanometers and a few microns), nanowires (larger than a few microns), and rhizoid crystals (branched fibrils and nanowires).We assume that the growth process involves two steps, where the chain segment first diffuses to the growth front and then the rest of the polymer chain transforms into the extended-chain conformation.We defined three rates, the average aggregate rate (v A ), the average diffusion rate (v D ), the average conformational transition rate (v T ), and established the relationship between the crystal morphology and these three speeds.We found that the v D /v A ratio determines how large the crystals can grow, and the v T /v A ratio determines whether the crystals could grow into the expected size.After discussing how common factors (concentration, temperature, spin-coating speed, chain rigidity, viscosity, molecular weight, etc.) affect the above three rates, we used the D-CT model to explain many crystallization behaviors of D-A conjugated polymers, for example, the length direction of D-A conjugated polymer crystals is along the backbone, the relationship between the solvent evaporation rate and crystal morphology, the effect of molecular weight on the crystallization process, the formation mechanism of rhizoid crystals, etc.We will discuss part of them in the following sections.For more details, please refer to our original work. [71]part from the growth of nuclei, the aggregation of crystallites or aggregates is also a possible factor affecting the aggregated structure of D-A conjugated polymers.In dilute solution, the formation of flexible polymer single crystals involves two steps, the formation of nanoclusters and the aggregation of nanoclusters, which can be described by the anisotropic aggregation model, as shown in Figure 4E. [86,87]or solution processed D-A conjugated polymers, this model is inapplicable because the concentration is much higher, [87] but crystallites or aggregates in the solution may also form aggregation when the solvent evaporates.The resulting associates are unlikely to have a 3D ordered structure like crystals, but some ordered stacks may form because of the strong π-π stacking.This could be a possible reason for high paracrystalline disorder in D-A conjugated polymer films.
It should be clarified that the concept of aggregation is broad.The aggregation of crystallites, aggregates and polymer chains may result in crystalline, semi-crystalline, paracrystalline, semi-paracrystalline, and amorphous structure.The crystallization process is a special aggregation process with the formation of long-range ordered structures, which can be characterized by XRD, DSC, SAED, etc.During the crystallization process of polymers, in most cases, due to their semi-crystalline nature, amorphous structures are also formed simultaneously.

REGULATION OF THE AGGREGATED STRUCTURE OF SEMI-CRYSTALLINE D-A CONJUGATED POLYMERS
According to the discussion in Section 2, the possible ways to increase the mobility of semi-crystalline D-A conjugated polymer films include controlling the crystalline structure, increasing the amount of tie chains, and promoting the alignment of films.Moreover, the ideal structures for charge transport are single crystals, which are also important for studying the intrinsic properties of charge transport and determining the performance limits of conjugated polymers. [88]able 1 summarizes the chemical structure, improvement of aggregated structure, the average mobility and OFET type of semi-crystalline D-A conjugated polymers via different strategies, which we will discuss in turn.

Controlling the crystalline structure
During solution processing, D-A conjugated polymers are firstly dissolved in the solvent (may contain cosolvent or additive) at an appropriate temperature, and then the solutions are cast into films using various methods.Thus, the possible ways to control the crystalline structure include regulating the aggregation of polymer chains in the initial solution, controlling the film formation process and post-annealing.

4.1.1
Regulating the solution-state aggregation D-A conjugated polymer chains generally adopt a singlechain conformation in solution when the solvent is good and the temperature is high.To form solution-state aggregation, the dissolution of polymer chains should be partly prevented.The formation of solution-state aggregation is a consequence of complex intramolecular and/or intermolecular interactions, which have recently been reviewed by Pei and coworkers, and the readers can refer to the correlating perspective paper. [89]ossible ways to achieve solution-state aggregation include lowering the temperature, using a poor solvent, adding solvent additives, etc. [84] Based on their behavior during film formation, the solution-state aggregation can be classified into two types.The first type of aggregation is "alive" and could act as nucleus, while the second type of aggregation is "dead" and will remain its structure to the film.In terms of crystallization, both single chains and nuclei are critical.On the one hand, single chains can participate in the crystallization process and form ordered structures, while "dead" aggregation cannot disentangle and transform into single chains. [90,91]On the other hand, the nucleation process of D-A conjugated polymers is hard, as we discussed above, and thus the heterogeneous nucleation ("alive" aggregation) in solution can facilitate crystallization.
Pei and coworkers studied the effect of temperature on the solution-state aggregation of four-fluorinated benzodifurandione-based oligo(p-phenylenevinylene) (PF 4 BDOPV) in 1-chloronaphthalene (CN) solution and the resulting aggregated structure of the spin-coated films, as shown in Figure 5A.At low temperatures, polymer chains form large aggregates with poorly packed structure in solution, and their motions are too weak to overcome the high crystallization energy barrier, resulting in disordered film structures.When the temperature is increased, the motion of the chains is enhanced, leading to smaller aggregates in the solution and a more ordered film structure.The best hole mobility is 2.63 ± 0.42 cm 2 V −1 s −1 when the temperature is 150 • C (with controlled chain motion), which is much higher TA B L E 1 Summary of chemical structure, improvement of aggregated structure, average mobility, and organic field-effect transistor type of semicrystalline donor-acceptor conjugated polymers via different strategies.F I G U R E 5 (A) Schematic illustration of the proposed crystallization process of four-fluorinated benzodifurandione-based oligo(p-phenylenevinylene) (PF 4 BDOPV) from the solution-state aggregates at different temperatures.Reproduced with permission. [92]Copyright 2020, Wiley-VCH GmbH.(B) Schematic illustration of the proposed structure of benzodifurandione-based oligo(p-phenylene vinylene) and bithiophene copolymer (PBDOPV-2T) aggregates in different solvents and the resulting film structure.Reproduced with permission. [95]Copyright 2017, WILEY-VCH Verlag GmbH & Co. KGaA.(C) Schematic illustration of polymorphism transitions of diketopyrrolopyrrole and quaterthiophene copolymer (D-PDPP4T-HD) with the decrease of chloroform:1,2,4-trichlorobenzene (CF:TCB) volume ratio.Reproduced with permission. [97]Copyright 2019, Nature Publishing Group.(D) Cryogenic transmission electron microscopy (Cryo-TEM) images of PNDI in CF solution with different content of ethylene glycol (EG).Reproduced with permission. [106]opyright 2022, American Chemical Society.

Polymers
than that of the film processed at 30 • C (0.012 ± 0.003 cm 2 V −1 s −1 ). [92]Janssen and coworkers also demonstrated that lower dissolution temperature of difluorobenzothiadiazole and quaterthiophene copolymer (PffBT4T-2DT) in chloroform (CF) can promote the formation of larger aggregations in solution.However, they found that larger aggregations result in wider fibrils and better π-π stacking in the monolayer film, and thus the charge mobility is higher. [93]To understand how the temperature affects the film structure, it is necessary to clarify the structure of the aggregations in the solution (ordered or disordered) and the role of them during the film formation process (promote the crystallization or remain the same).Except of the crystallinity and crystal sizes, the solution temperature can also affect the molecular orientation of D-A conjugated polymers, as demonstrated by Li and coworkers. [94]They found that the solutions of PffBT4T-2DT in 1,2,4-trichlorobenzene (TCB) contain 2D lamellar supramolecular nanostructure at low temperatures and the resulting films adopt a face-on orientation, while the solutions contain 1D worm-like nanostructure at high temperatures and the resulting films adopt an edge-on orientation.It is worth noting that the hole mobility is similar for PffBT4T-2DT films with different molecular orientation. [94]he possible reason is that long-range charge transport in D-A conjugated polymer films is mainly along the backbone, rather than the π-π stacking direction, and the role of ordered structure is to promote the interchain transport.The use of poor or mixed solvents is also a common approach to tune the solution-state aggregation of D-A conjugated polymers.Pei and coworkers studied the solution-state aggregation of benzodifurandione-based oligo(pphenylene vinylene) and bithiophene copolymer (PBDOPV-2T) in 1,2-dichlorobenzene (ODCB), methylbenzene (MB), and ODCB:MB (4:1) solutions, as shown in Figure 5B. [95]hey found that the polymer chains form 1D rod-like, 2D plate-like, and larger 1D rod-like (an intermediate state) aggregates in the above three solutions, respectively.The 1D rod-like aggregates can grow larger when the solvent evaporates, and the resulting films contain both highly crystalline domains and tie chains.However, the 2D plate-like aggregates can remain to the films when the solvent evaporates, but tie chains are lacking.The electron mobility of PBDOPV-2T films is highest (3.2 cm 2 V −1 s −1 ) when the mixed solvent was used, which is owing to the more ordered and well interconnected structure. [95]Diao and coworkers studied the effect of three types of solvent on the solutionstate aggregation of an isoindigo-bithiophene-based polymer (PII-2T), backbone solvent CN, side-chain solvent decane, and mutual solvent of the backbone and the side-chain ODCB. [96]They found that polymer chains form sidechain-associated amorphous aggregates, backbone-stacked semi-crystalline aggregates and chiral liquid crystal aggregates in the solution when CN, decane, and ODCB were used as the solvent, respectively.During the blade-coating process, amorphous aggregates can further form highly aligned crystalline domains and result in the best hole mobility (1.56 cm 2 V −1 s −1 ), while backbone-stacked semi-crystalline aggregates can only form disordered domains and the hole mobility is lower (0.50 cm 2 V −1 s −1 ).The chiral liquid crystal aggregates form twinned crystalline domains which are highly paracrystalline disorder and isotropic, and the resulting hole mobility is worst (0.11 cm 2 V −1 s −1 ). [96]Janssen and coworkers found that the solution-state aggregation of a diketopyrrolopyrrole and quaterthiophene copolymer (D-PDPP4T-HD) can change from β 1 phase to β 2 phase when the content of TCB in CF:TCB mixed solvent increases, as shown in Figure 5C. [97]The hole mobility of the films with β 1 and β 2 phase is 0.058 and 0.26 cm 2 V −1 s −1 , respectively, which is due to the low d π-π in β 2 phase. [97]nlike lowering the temperature and using a poor solvent, adding a small amount of solvent additives into the good solvent is a relatively "moderate" method to tune the solution-state aggregation of D-A conjugated polymers.100][101][102][103] Russell and coworkers reported that using ODCB and 1,8-diioctane (DIO) as additives can enhance the charge mobility of DPP-based polymer films, which is due to the enhanced edge-on orientation and the formation of more slender polymer fibrils in the films. [104]Park and coworkers also reported that adding 5% CN enhances the crystallinity of DPP-based polymers and thus the charge mobility of the films is increased. [105]Our group used ethylene glycol (EG) to improve the backbone planarization of poly{[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8bis(dicarboximide)−2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} (PNDI) in CF solution, and thus promoting the formation of aggregations, as shown in Figure 5D. [106]The aggregations can further promote the formation of films with more oriented molecular chain alignments and lower d π-π during the blade-coating process.As a result, the electron mobility of the PNDI film processed with 3% EG is 1.21 cm 2 V −1 s −1 , which is much higher than that of the film processed without EG (0.25 cm 2 V −1 s −1 ). [106]s we discussed above, regulating the solution-state aggregation is an effective approach to improve the aggregated structure of D-A conjugated polymers.However, it is not very clear how these aggregations form films.Although researchers have attempted to answer this question by characterizing solution-state aggregation in more concentrated solutions. [107]The actual solution-state aggregation of the D-A conjugated polymer during solvent evaporation may be different from that of a concentrated solution with the same solvent content, because the crystallization process is governed by the kinetics, which is related to the evaporation rate. [82]Therefore, further understanding of the crystallization kinetics of D-A conjugated polymers is in demand.

4.1.2
Controlling the film formation process Controlling the crystallization of D-A conjugated polymers during film formation is a direct way to regulate the crystalline structure.Kim et al. used an off-center spin-coating (OCSC) technology to obtain wet and aligned PNDI films, and then adjusted the evaporation temperature of the solvent to control the crystallization process, as shown in Figure 6A-C. [108]The solvent CN is a good solvent for PNDI, with very weak aggregation of polymer chains in the pristine solution.When the solvent evaporates, the PNDI molecule transforms into a solid state through a nucleation and growth process.At high temperatures, the high diffusion energy increases both the nucleation and growth rates, resulting in too many nucleation sites with limited growth space, and thus the polymer nanofibrils are randomly oriented in the film.On the contrary, at low temperatures, the nucleation and growth rates are low, and the resulting films exhibit no obvious crystalline morphologies.At moderate temperatures, where the nucleation sites are relatively small, they can grow to larger sizes due to the relatively fast growth rate, and the resulting films exhibit well-oriented fibrillar structures.The highest electron mobility is 3.43 cm 2 V −1 s −1 when the temperature is 100 • C, which is due to the longrange ordered fibrillar structure with preferable molecular orientation. [108]Our group studied the crystallization kinetics of PNDI using two selective solvents, the backbone solvent bromonaphthalene (BN) and the side chain solvent MB, as shown in Figure 6D. [109]When the solvent is BN, the newly dissolved solution contains only unimer coil conformations, which can first form side-chain ordered aggregates, and then induce the formation of backbone planarization during aging.These nuclei can further grow into nanowires with a width of 20 nm and lengths up to tens of micrometers during film drying.However, when the solvent is MB, the initial solution contains many rod-like aggregates, and the resulting film contains many small fibrils.The electron mobility of the nanowire film is twice that of the fibril film, indicating that long-range ordered nanowire is a potential structure for efficient charge transport. [109]1.3

Post-annealing
Post-annealing is the most widely used method to increase the crystallinity of conjugated polymers, and the typically strategies include thermal annealing (TA) and solvent vapor annealing (SVA).For TA, the films are generally annealed at a temperature above the glass transition temperature (T g ) and below the melt temperature (T m ).In this case, the motion of the chain segments is active, and thus the amorphous polymer chain can recrystallize.As a result, the crystallinity of the D-A conjugated polymer is increased.If the annealing temperature is higher than T m , the recrystallization process should be very slow to overcome the nucleation barrier.
In most cases, TA can substantially improve the charge mobility of D-A conjugated polymer films due to increased crystallinity.[111][112][113][114] There is one exception, Li et al. found that the improvement of TA on the hole mobility of DPP and β-unsubstituted The average electron mobilities of the PNDI films processed from different temperatures.Reproduced with permission. [108]Copyright 2019, WILEY-VCH Verlag GmbH & Co. KGaA.(D) Schematic illustrations of the proposed nucleation mechanism for PNDI in BN and MB solutions.Reproduced with permission. [109]Copyright 2021, American Chemical Society.
quaterthiophene copolymer (PDQT) films is limited, even though the increase of crystallinity upon TA is distinct. [115]In addition, Salleo and coworkers found that melt annealing of PNDT films resulted in a shift from 77.5% face-on to 94.6% edge-on orientation. [116]However, the electron mobility of the two different films is similar, [116] which agrees with the conclusion of Li and coworkers, [94] indicating that the molecular orientation of D-A conjugated polymers is not important for charge transport.
In the case of SVA, conjugated polymer films are firstly partly dissolved at a high vapor pressure (P diss or P seed ) and then recrystallization at a lower vapor pressure (P cryst ). [117]By adjusting the P seed and P cryst values, Ludwigs and coworkers demonstrated that the density and size of P3HT spherulites can be well controlled. [118]hey also used this method to improve the crystalline structure of a D-A conjugated polymer, poly{[4,4-bis-alkylcyclopenta-(2,1-b;3,4-b′)dithiophen]−2,6-diyl-alt-(2,1,3benzo-thiadiazole)−4,7-diyl} (PCPDTBT), but the effect of crystalline structure on charge mobility was not studied. [119]verall, SVA is widely used to improve the charge mobility of conjugated small molecules, but their application Electron mobility of PNDI films with different molecular weights as a function of strain.Reproduced with permission. [127]Copyright 2021, American Chemical Society.(D) Schematics of the purification processes, possible solution-state aggregations and the corresponding film structures of IID-based oligomers (IIDDT) with different molecular weights.(E) Grazing-incidence wide-angle X-ray scattering (GIWAXS) analysis on coherence length and paracrystalline disorder of (100) diffractions of IIDDT films with different molecular weights.(F) The dependence of charge mobility of IIDDT films on the molecular weight.Reproduced with permission. [128]Copyright 2023, Nature Publishing Group.in D-A conjugated polymers is limited. [120]The possible reason is that other methods, such as TA and solution-state aggregation, are more efficient and convenient.

Increasing the amount of tie chains
Although the amount of tie chains may be improved through several approaches, for example, the solution-state aggregation should be 1D rod-like rather than 2D plate-like and the crystallization condition should be v T ≤ v A << v D according to the D-CT model, the most effective method is increasing the molecular weight of D-A conjugated polymers.In general, the charge mobility of low-molecular-weight D-A conjugated polymer films is poor due to the lack of tie chains.[125] However, quantitative calculations of the tie chains are challenging and further understanding of the formation of tie chains is necessary.Our group used high-resolution transmission electron microscopy to characterize the refined structure of PNDI films with different molecular weights, and estimated the fraction of tie chains by the Huang-Brown model, as shown in Figure 7A-C. [127]e found that the fraction of the tie chains improves by seven orders of magnitude when the molecular weight is increased from 34.0 to 170 kDa.The higher fraction of the tie chains favors the charge mobility and stretchability of PNDI films. [127]Also, according to the D-CT model, a higher molecular weight causes smaller v T and v D values, the thermodynamically stable growth fronts are harder to reach for chain segments and the conformation transition process is harder to accomplish, resulting in smaller sized crystallites. [71]Pei and coworkers studied the solutionstate aggregations and the corresponding film structures of IID-based oligomers (IIDDT) with different molecular weights, as shown in Figure 7D-F. [128]They found that lowmolecular-weight IIDDT forms isolated fiber-like aggregates in the solution, and that these fibers can further grow and form highly ordered films with a low density of tie chains.In the case of the medium-molecular-weight IIDDT, the polymer chains form a sparse network of aggregates in the solution, which can further aggregate as the solvent evaporates and form a moderately ordered film with a moderate density of tie chains.For high-molecular-weight IIDDT, the polymer chains form dense fibrillar networks and severe entanglement in solution, which can be inherited directly into the film, resulting in a structure with a high density of tie chains and poor crystallinity.The charge mobility of high-molecularweight IIDDT films is the best, despite their low crystallinity and large paracrystalline disorder.

Promoting the alignment of films
[131] Chain alignment can also lead to rigid backbones and impeded torsional angles, which favor the intrachain orbitals overlap and the formation of interchain π-π stacking, resulting in more effective intrachain and interchain charge transport. [131]n the direction parallel to the alignment, charges mainly transport along the backbone, which is beneficial for high mobility.However, in the orthogonal direction, π-π stacking becomes the dominant path for charge transport, and the resulting mobility is poorer in general.There are many methods to improve the alignment of D-A conjugated polymer films, such as meniscus-guided coating (MGC), OCSC, selfassembling in a nanogrooved substrate (SAINS), etc. [129][130][131] The fundamental behind them is providing a force field (shearing field, centrifugal force field, capillary force field, electric field, magnetic field, etc.) to the polymer chains during the film formation process.Many large-scale solution processing methods, such as blade coating, bar coating, dip coating, zone casting, slot-die coating, and solution shearing, belong to MGC techniques. [132]The name "meniscus-guided" is based on the fact that a meniscus-like gas/liquid interface is translated across the substrate during the coating process, which determines the thickness and microstructure of films.The possible flows, gradients, and fluid mechanical phenomena that can influence the structure of D-A conjugated polymer films are shown in Figure 8A, and the detailed discussion can be found in the review article of Bao and coworkers. [133]he actual film formation process is very complex and can be influenced by many factors, such as the concentration, viscosity, shearing speed, temperature, surface energy, etc.Among them, the shearing speed is a key factor affecting the alignment of D-A conjugated polymer films.On the one hand, increasing the shearing speed promotes the alignment of polymer chains in the solution, and thus is crucial for achieving aligned films.On the other hand, too fast shearing speeds will result in wet films, and the aligned polymer chains can relax during drying, which is detrimental to the alignment of the films.Therefore, an optimal shearing speed is necessary to balance the above two effects to obtain highly aligned films, but it is difficult to priori predict the ideal coating conditions because the effect of shear forces is difficult to measure directly. [133]The aggregates and crystals in the solution can also form aligned structure during the MGC process. [134]For example, Geng and coworkers studied the mobility of bar-coated thiophene-flanked diketopyrrolopyrrole and selenophene copolymer (TDPP-Se) films with different molecular weights. [125]They found that all the polymers adopt 1D rod-like aggregation structures in the solution and their length increases monotonically with the molecular weight.Their results show that the alignment of TDPP-Se films with moderate molecular weight is best. [125]n many cases, the crystallinity of D-A conjugated polymers can be enhanced during the MGC process, [125,135] but how the aligned polymer chains and aggregates in the solution affect the crystallization process is not very clear.In order to answer the above questions, perhaps simpler research systems should be established to eliminate the interference of other factors.Besides, the MGC process may also affect the crystal form of conjugated polymers.For example, Peng and coworkers reported for the first time a meniscus-assisted solution-shearing (MASS) strategy to efficiently tune the polymorph II-to-I transformation of poly(3-butylthiophene) (P3BT) upon increased shearing speed. [136,137]As modulating the polymorph of conjugated polymers does not change their chemical compositions, such MASS-induced polymorph transformation is important to unravel the structureproperty relationship and may be readily extended to D-A copolymers.
[143][144] There is one exception, Bao and coworkers found that the mobilities of aligned diketopyrrolopyrrole and trithiophene copolymer (PDPP3T) films along the parallel and perpendicular directions are similar, and they proposed that the charge transport is primarily intermolecular along the π-π stacking because of the relatively low molecular weight. [145]he OCSC method was first proposed by Bao and coworkers to obtain highly aligned films of conjugated small molecules.The coating process is similar to spin coating, but a controllable centrifugal force field is introduced by placing the samples at a specified distance from the spin center, as shown in Figure 8C. [146][149] For example, Noh and coworkers used the OCSC method to prepare aligned films of four conjugated polymers from pre-aggregated solutions.They found that the aligned conjugated polymer films exhibit a larger anisotropy on the top surface in comparison with the bulk film, resulting in a high mobility improvement in top-gate/bottom-contact OFET devices. [147]Kim and coworkers also found that OCSC is beneficial to improve the charge mobility for an IIDbased conjugated polymer [148] and two quinoidal conjugated polymers. [149]Besides, Noh and coworkers reported a similar F I G U R E 8 (A) Summary of possible flows, gradients and fluid mechanical phenomena that can influence the aggregated structure of donor-acceptor (D-A) conjugated polymer films processed by meniscus-guided coating (MGC).Reproduced with permission. [133]Copyright 2015, WILEY-VCH Verlag GmbH & Co. KGaA.(C) Schematic illustration of the off-center spin-coating (OCSC) process.Reproduced with permission. [146]Copyright 2014, Nature Publishing Group.(D) Schematic illustration of the rotation coating process.Reproduced with permission. [150]Copyright 2017, American Institute of Physics.(E) Schematic illustration of the sandwich tunnel system.(F) Schematic illustration of the flow direction of polymer solutions exerted by the glass spacers with two opposite surface treatments, oleophilic 6-phenylhexyltrichlorosilane (PTS) (top) and oleophobic perfluorodecyltrichlorosilane (FDTS) (bottom), respectively.(G) The average saturation mobility of two D-A conjugated polymer films formed in the sandwich tunnel system with various surface treatments over the glass spacers.Reproduced with permission. [153]Copyright 2014, American Chemical Society.method to OCSC, the rotation coating technique, as shown in Figure 8D. [150]he strategy of TA induced SAINS was first proposed by Sirringhaus et al. to obtain aligned conjugated polymer films. [151]Heeger and coworkers used this strategy to prepare aligned D-A conjugated polymer films.They designed a sandwich tunnel system composed of two Si/SiO 2 substrates (8 mm × 8 mm) and two side spacers.The surface of the substrates was scratched by diamond lapping films with different nanoparticle diameters (100, 250, and 500 nm).A solution of poly [1,2,5]thiadiazolo- [3,4-c]pyridine] (PCDTPT) in ODCB was injected into the tunnel, and then slowly dried under the influence of capillary force in a nitrogen environment.The aligned films showed a highest average hole mobility of 6.7 cm 2 V −1 s −1 when the annealing temperature was 200 • C and the diameter of the nanoparticles was 100 nm. [152]Later, Heeger and coworkers improved the sandwich tunnel system by using two surface treated glass spacers (7.7 × 2.0 × 1.0 mm) to separate the two Si/SiO 2 substrates (12.2 × 7.7 × 0.5 mm), as shown in Figure 8E-G.
When the spacers are treated with superoleophilic 6phenylhexyltrichlorosilane, the polymer solution is drawn and flows toward the spacer (parallel to the nanogrooves).On the contrary, such flow toward the spacer is suppressed when the spacers are treated with oleophobic perfluorodecyltrichlorosilane.As a result, the highest hole mobility of highly aligned PCDTPT films reached 47 cm 2 V −1 s −1 . [153]eeger and coworkers further studied the effect of molecular weight on the mobility of aligned films obtained by slow drying in the nanogrooved substrates.They found that a medium-molecular-weight (50 kDa) results in the best hole mobility for aligned films, indicating that very high molecular weight is not necessary for achieving high mobility. [154]he strategy of self-assembly in nanogrooved substrates was also confirmed by Cho and coworkers. [155]In addition, Teng and coworkers found that highly aligned PCDTPT films can also be obtained by direct spin coating on the nanogrooved substrates when 5% CN is added to the solution. [156]here are many other methods to promote the alignment of D-A conjugated polymer films, such as epitaxial crystallization on oriented polyethylene substrates, .Reproduced with permission. [161]Copyright 2013, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.(D) Scanning electron microscopy (SEM) image (left), transmission electron microscopy (TEM) image (middle), and selected area electron diffraction (SAED) patterns (right) of dithieno[3,2-b:2′,3′-d]thiophene and diketopyrrolopyrrole copolymer (PDTTDPP) nanowires, the inset in the middle image shows the molecular packing of PDTTDPP nanowires.Reproduced with permission. [162]Copyright 2021, WILEY-VCH Verlag GmbH & Co. KGaA.(E) Optical microscope (OM) images and molecular packing of PDPP2TBDT and PDPP2TzBDT nanowires, the insets show the chemical structure of the two D-A conjugated polymers.Reproduced with permission. [167]Copyright 2015, WILEY-VCH Verlag GmbH & Co. KGaA.electrical blade coating, and magnetic field-guided selfassembly. [140,157,158]The readers can refer to the correlative reviews. [130,159]

Preparing single crystals
The conditions for preparing polymer single crystals are quite demanding.On the one hand, the nucleation process should be very slow to avoid the formation of excess nuclei or tangled aggregations.On the other hand, the pristine solution should contain a large number of free single chains to support the growth process.In general, single crystals of flexible polymers are grown from the dilute solutions with a small crystallization rate (a small degree of supersaturation).For D-A conjugated polymers, according to the D-CT model, small crystallization rates will result in crystals with high aspect ratios (∼100). [71]Therefore, D-A conjugated polymer single crystals are typically 1D structures with the length direction along the backbone, which are also known as nanowires.Perhaps one possible way to prepare 2D or 3D D-A conjugated polymer single crystals is to prevent the growth of crystals along the backbone direction, for example, in confined spaces.So far, the possible way to prepare D-A conjugated polymer single crystals or nanowires includes the whisker method, solvent vapor enhanced drop casting (SVED) method, template-assisted self-assembling method, etc.The whisker method was proposed by Ihn et al. to prepare P3HT nanowires. [160]In general, conjugated polymers are first dissolved in the solvent at a high temperature, and then the temperature of the solutions is slowly decreased to a lower temperature (the room temperature in most cases) and remain for a period of time, as shown in Figure 9A,B. [161]hoi and coworkers first prepared D-A conjugated polymer nanowires in dilute solution (∼0.01 mg mL −1 ) using a dithieno[3,2-b:2′,3′-d]thiophene (DTT) and DPP copolymer (PDTTDPP).The length of PDTTDPP nanowires was larger than 10 μm, and the width of nanowires was close to 1 μm, as shown in Figure 9D.They found that the SAED diffraction pattern did not change as the electron beam moving along the nanowire, indicating that the nanowire is a single crystal structure.The maximal hole mobility of single PDTTDPP nanowire is 7.0 cm 2 V −1 s −1 , which is almost one order of magnitude higher than that of polycrystalline films.They proposed that a more rigidity and planarity backbone is beneficial for preparing nanowires. [162]Later, Choi and coworkers further prepared nanowires of a 1,2-bis(5-(thiophen-2-yl)-selenophen-2-yl)ethene and DPP copolymer (DPPBTSPE) with different molecular weights.They found that the nanowires made of high-molecular-weight (68 kDa) polymer were wider and shorter than those made of lowmolecular-weight (8 kDa) polymer. [163]According to the D-CT model, low-molecular-weight polymers have higher v D values and their chains are more likely to reach the growth front along the backbone direction, leading to narrower nanowires. [71]The hole mobility of single DPPBTSPE nanowire is as high as 24 cm 2 V −1 s −1 . [163]Our group prepared D-A conjugated polymer nanowires in more concentrated solutions (∼1 mg mL −1 ) using a modified whisker method, the slow evaporation of the main solvent method.The solvent is composed of a low boiling point good solvent and a high boiling point marginal solvent.When the good solvent evaporates, the crystallization process begins and the solution is concentrated simultaneously.As a result, D-A conjugated polymer nanowires are prepared in concentrated solutions.We proposed that the slow crystallization rate and unimer coil chain conformation in pristine solutions are critical for the preparation of D-A conjugated polymer nanowires. [82,164]chematic diagram of the SVED method is shown in Figure 9C, [161] D-A conjugated polymer solution is dropcasted on the substrate in an airtight container that is saturated with solvent evaporation, allowing the polymers to crystallize slowly.Mullen and coworkers used the SVED method to prepare cyclopentadithiophene-benzothiadiazole copolymer (CDT-BTZ) nanowires.The length and width of CDT-BTZ nanowires were 5-20 μm and 0.3-0.6 μm, respectively.The hole mobility of single CDT-BTZ nanowire is 5.5 cm 2 V −1 s −1 , which is much higher than that of spin-coated films (0.67 cm 2 V −1 s −1 ) and aligned films through dipcasting (1.4 cm 2 V −1 s −1 ). [165]Zhan and coworkers used the same method to prepare bithiazole-thiazolothiazole copolymer (PTz) nanowires.The mobility of single PTz nanowire is 0.46 cm 2 V −1 s −1 , which is more than two orders of magnitude higher than that of thin films. [166]Wang and coworkers prepared nanowires of two DPP-based polymers, PDPP2TBDT and PDPP2TzBDT, as shown in Figure 9E.They found that PDPP2TBDT adopted an edge-on orientation, while PDPP2TzBDT adopted a face-on orientation.However, the mobility of the single nanowire device is similar for the two polymers, indicating that the orientation of the D-A conjugated polymers is not important for charge transport. [167]Pei and coworkers prepared D-A conjugated polymer microwires by well-controlled crystallization conditions.The mobility of microwires is also much higher than that of films. [161]he TASA method is performed using nano-templates to assist the self-assembly of D-A conjugated polymers.Sung and coworkers prepared single-crystal PCDTPT nanowires using a liquid-bridge-mediated nanotransfer molding method, as shown in Figure 10A. [23]PCDTPT solution was applied to the polyurethane acrylate molds containing nanoscale channels, in which PCDTPT molecules self-assembled and formed nanowires.When the mold was removed, highquality single-crystal PCDTPT nanowires with a width of 90 nm and a height of 180 nm were gotten, as shown in Figure 10B.The hole mobility of PCDTPT is excellent, with an average value of 72.94 cm 2 V −1 s −1 and a maximum value of 92.64 cm 2 V −1 s −1 , which is the highest mobility for polymer semiconductor up to now. [23]Zhan and coworkers used a template-dipping method to prepare 4,7-bis(5-bromo-4-dodecyl-thien-2-yl)-2,1,3-benzothiadiazole and 2,7-bis(4,4,5,5-tetramethyl-1,3,2dioxaborolan-2-yl)-9,9-dihexylfluorene copolymer (PFTBT) nanowires.The nanowires have a length of 15 μm and a diameter of 200-300 nm. [168]Recently, Zhao and coworkers used the same method to prepare N-alkyldiketopyrrolopyrrole and dithienylthieno [3,2-b]thiophene copolymer (DPP-DTT) nanowires, as shown in Figure 10C. [169]A drop of DPP-DTT solution (5 mg mL −1 ) was deposited onto an anodic aluminum oxide template, and the excess solution was removed using a scalpel.The DPP-DTT molecules are able to selfassemble and form ordered structures in the template as the solvent evaporates.When the template was removed using a NaOH solution, DPP-DTT nanowires were cleaned and dispersed in n-hexane.The DPP-DTT nanowires exhibited single-crystalline structure with a diameter of about 200 nm and a length of 3-15 μm.The highest hole mobility of single DPP-DTT nanowire is 14.2 cm 2 V −1 s −1 , which is much higher than that of films. [169]

REGULATION OF THE AGGREGATED STRUCTURE OF NEAR-AMORPHOUS D-A CONJUGATED POLYMERS
The most famous near-amorphous D-A conjugated polymer is IDT and benzothiadiazole (BT) copolymer (IDT-BT), which was firstly reported by Zhang et al. with a hole mobility of 1.0 ± 0.25 cm 2 V −1 s −1 . [170]Later, Zhang et al. reported a hole mobility up to 3.6 cm 2 V −1 s −1 for IDT-BT. [171]They proposed that charge transport in IDT-BT is quasi 1D (along the backbone direction), and only occasional intermolecular hopping through short-range π-π stacking is needed. [171]Venkateshvaran et al. further investigated charge transport in IDT-BT compared to other conjugated polymers by field-effect modulated Seebeck, transistor, and sub-bandgap optical absorption measurements.They found that the near-amorphous IDT-BT exhibits a lower degree of energetic disorder than crystalline or semi-crystalline conjugated polymers.Also, they proposed three guidelines for designing "disorder-free" conjugated polymers: (1) the repeat unit contains collinear conjugated units with only a single or a minimal number of torsion-susceptible linkages; (2) the gas-phase torsion potential is relatively steep with a minima ideally value around 180 • , 0 • , or both; and (3) one of the conjugated units contains long side-chain substitution on both sides. [172]At present, the study of near-amorphous conjugated polymers has become an interesting research topic.Common approaches to the design of high mobility near-amorphous conjugated polymer films include chemical and physical methods.][175][176][177] Here, we mainly focus on physical methods to optimize the aggregated structure of near-amorphous D-A conjugated polymer films, such as improving the size and connectivity of aggregates, promoting the alignment of polymer chains and decreasing the density of traps.Table 2 summarizes the chemical structure, improvement of aggregated structure, the average mobility, and OFET type of near-amorphous D-A conjugated polymers via different strategies, which we will discuss in turn.Reproduced with permission. [23]Copyright 2019, American Chemical Society.(C) Schematic illustration of the preparation of N-alkyldiketopyrrolopyrrole and dithienylthieno [3,2-b]thiophene copolymer (DPP-DTT) nanowires using the anodic aluminum oxide (AAO) template-assisted method.Reproduced with permission. [169]Copyright 2023, Springer Nature.TA B L E 2 Summary of chemical structure, improvement of aggregated structure, the average mobility, and organic field-effect transistor type of nearamorphous donor-acceptor conjugated polymers via different strategies.Reproduced with permission.Copyright 2023. [178]American Chemical Society.(D) Intermodulation atomic force microscopy (ImAFM) images of the pristine (left) and antisolvent treated (right) IDT-BT films.(E) Radial (rad), in-plane (IP), and out-of-plane (OOP) 1D GIWAXS profiles of IDT-BT films (left), and plot of the normalized peak area ratios of the crystalline with respect to the amorphous (right).Reproduced with permission. [179]Copyright 2023, Wiley-VCH GmbH.(F) Charge mobility of IDT-BT with different molecular weights.(G) Schematic illustrations of the film microstructure of IDT-BT with different molecular weights.Reproduced with permission. [180]Copyright 2021, American Chemical Society.

Improving the size and connectivity of aggregates
Similar to the semi-crystalline films, increasing the size and connectivity of aggregates in near-amorphous D-A conjugated polymer films also favors charge transport.Our group used a solvent additive chlorohexadecane (CHD) to regulate the aggregation of IDT-BT in films, as shown in Figure 11A-C.CHD has a much higher boiling point than the host solvent chlorobenzene (CB), and can act as a spacer molecule during the spin-coating process because of its strong interaction with both the backbone and the side chain of IDT-BT.During the TA process, polymer chains have a better motility in the existence of CHD, leading to more compact and continuous aggregates in the solid film, as confirmed by the coherence length data of the (001) peak and the simulation results.As a result, the hole mobility of IDT-BT film processed with 1% CHD reached 3.61 cm 2 V −1 s −1 , which is much better than that of CB processed film. [178]irringhaus and coworkers found that the OFF-state bias-stress stability, environmental stability, and device performance of IDT-BT OFETs can be improved simultaneously by a simple antisolvent (AN) treatment.The AN treatment was carried out by exposing the wet spin-coated IDT-BT films into an orthogonal solvent for 2 min and then annealing at 90 • C for 60 min to remove the solvent.In comparison with the pristine film, the AN treated film has a higher relative degree of crystallinity and longer coherence length (from 6.86 to 9.39 nm), indicating that AN treatment promotes crystallization and aggregation of the film, as shown in Figure 11D,E.They proposed that the formation of larger crystalline aggregates is desirable for achieving sufficient bias-stress stability in near-amorphous polymers. [179]ncreasing the molecular weight is an efficient way to increase the connectivity of aggregates for near-amorphous D-A conjugated polymers.Geng and coworkers studied the effect of molecular weight on the charge mobility of IDT-BT.They found that the hole mobility of IDT-BT increases with increasing molecular weight due to a larger charge transport range along the backbone.The highest hole mobility is 2.63 cm 2 V −1 s −1 when the molecular weight is 1049.6 kDa, as shown in Figure 11F.Besides, high molecular weight is also beneficial to enhanced elasticity and strength.They proposed that a larger number of entanglements in high-molecularweight IDT-BT films can prevent irreversible deformation and thus lead to enhanced mechanical properties, as shown in Figure 11G. [180]

Promoting the alignment of polymer chains
Similar to the semi-crystalline films, promoting the alignment of polymer chains in near-amorphous D-A conjugated  [181] Copyright 2017, American Chemical Society.(D) Linear (dashed lines) and saturation (solid lines) transfer characteristics of indacenodithiophene and benzothiadiazole copolymer (IDT-BT) OFETs processed without (left) and with (right) 2 wt% tetracyanoquinodimethane (TCNQ).(E) Chemical structures of (IDT-BT) 2 -H 2 O complexes with H 2 O acting as a H-donor or H-acceptor.Reproduced with permission. [183]Copyright 2016, Springer Nature Limited.(F) Schematic illustration of the charge transfer between IDT-BT and N,N-dimethylanilinium tetrakis(pentafluorophenyl)borate (DTB).(G) Variation of the mobility with an increasing amount of DTB.(H) Variation of the N ST with an increasing amount of DTB.Reproduced with permission. [184]Copyright 2022, Wiley-VCH GmbH.
polymer films also favors charge transport.Park et al. used an electrohydrodynamic-jet (EHD-jet) printing method to improve the chain alignment of an amorphous polymer poly[(1,2-bis-(2′-thienyl)vinyl-5′,5″-diyl)-alt-(9,9-dioctyldecylfluorene-2,7-diyl)] (PFTVT), as shown in Figure 12A-C.They characterized the alignment of polymer chains by the near-edge X-ray absorption fine structure spectroscopy and found that EHD-jet printed PFTVT on hexamethyldisilazane (HMDS, has a similar surface energy with PFTVT) modified substrate shows good chain alignment.As a result, the charge mobility of chain-aligned PFTVT is five times higher than that of the spin-coated PFTVT, indicating that chain alignment is also important for high mobility in near-amorphous D-A conjugated polymers. [181]

Decreasing the density of traps
Decreasing the density of traps is an effective way to improve the charge mobility and stability of OFETs.The charge carrier traps in conjugated polymers include intrinsic sources (dynamic disorder, structural defects, and chemical impurities) and extrinsic sources (dopants, interfacial effects, environmental effects, and bias-stress effects), which are discussed in detail in the review paper of Jurchescu and coworkers. [182]Here, we only discuss some factors related to the aggregated structure of conjugated polymers, such as structural defects, dopants, and impurities.Sirringhaus and coworkers found that the addition of solvent additives or solid additives can improve the operational and environmental stability of IDT-BT OFETs, as shown in Figure 12D.They proposed that the key factor in charge trapping and device degradation is the water incorporated in nanometersized voids within the polymer microstructure.The water molecules can form hydrogen-bonding configurations with the polymer backbones, as shown in Figure 12E.The addition of solvent additives or solid additives can remove water from the film and thus benefit the stability of OFETs.The main difference between solid additives and solvent additives is that the former do not impart long-term stability, as they can evaporate slowly from the film. [183]Li and coworkers used the organic salt p-dopant N,N-dimethylanilinium tetrakis(pentafluorophenyl)borate (DTB) to improve the hole mobility of IDT-BT OFETs, as shown in Figure 12F-H. [184]hey found that the p-doping cannot occur via electron transfer because the energy levels of IDT-BT and DTB are not favorable for the p-doping, and hypothesized that the ammonium cations in DTB may receive electrons from IDT-BT to induce ammonium radicals.The hole mobility of IDT-BT OFETs increases from 0.32 to 1.15 cm 2 V −1 s −1 when 1% DTB is added.Further addition of DTB is detrimental to the hole mobility.They measured the surface defect concentration (N ST ) by the low-frequency noise, and found that the trap density achieves a minimum value when 1% DTB is added, explaining the improvement of hole mobility. [184]Han and coworkers used four Lewis acid dopants to dope IDT-BT, and found that (C 6 H 15 O) + (BF 4 ) − -doped IDT-BT OFET exhibits enhanced device performance and air stability.They proposed that the formation of Lewis acid-base adducts can yield radical cations on the backbone, reducing the traps and promoting charge transport. [185]f course, the strategies of removing water molecules and doping are also applicable for semi-crystalline D-A conjugated polymers, as demonstrated by Sirringhaus and coworkers and Pei and coworkers. [183,186]Besides, the density of traps can also be reduced by improving the crystalline structure and crystalline domain boundary.For example, Peng and coworkers proposed an effective cocrystallization strategy involving two or more conjugated components of polythiophene-based rod-rod block copolymers into cocrystals, exhibiting higher charge mobilities than the corresponding homopolymers.89]

CONCLUSION AND OUTLOOK
The film microstructure of D-A conjugated polymers is a critical factor affecting their performance in OFET devices.However, the nature of polymer materials determiners that the microstructure of D-A conjugated polymer films is complex and multilevel, including molecular structure (chemical structure and chain conformation) and aggregated structure (packing structures and phase behavior).In this review, we have summarized recent progress in controlling the aggregated structure of D-A conjugated polymer films for high-mobility OFETs.Due to the strong rigidity of D-A conjugated polymer backbone, charge transport in D-A conjugated polymer films is mainly along the backbone, together with indispensable interchain transport along the π-π stacking direction through ordered structure (crystallites and aggregates).For semi-crystalline D-A conjugated polymers, the strategies to enhance charge mobility include improving the crystalline structure, increasing tie chains, promoting film alignment, and preparing single crystals, which have been learned from the experience of P3ATs.As a result, the charge mobility of semi-crystalline D-A conjugated polymer films can reach ∼10 cm 2 V −1 s −1 , and the charge mobility of D-A conjugated polymer single nanowire can be several times higher, approaching that of polycrystalline silicon.In contrast to P3ATs, for D-A conjugated polymers, the adoption of edge-on orientation does not seem to be necessary.The possible reason is that long-range charge transport in D-A conjugated polymer films is mainly along the backbone, rather than the π-π stacking direction.For near-amorphous D-A conjugated polymers, rigid and planar backbones, chain alignment, as well as larger sized and well-connected aggregates, are beneficial for high charger mobility.The charge mobility of near-amorphous D-A conjugated polymer films can exceed 1 cm 2 V −1 s −1 , which is better than that of amorphous silicon, bring a bright application prospect in flexible electronics.Despite the achievements of the past dozen years, many challenges remain to be addressed.
1.The crystallization mechanism of D-A conjugated polymers should be further understood.It seems that the formation of solution-state aggregation, aggregates and nuclei are related to the nucleation process, but how to describe them quantitatively is not clear.These initial aggregated structures may grow larger to form crystals or cluster together to form associates during subsequent solvent evaporation.A qualitative growth model has been proposed for D-A conjugated polymers, but a quantitative description remains a challenge due to the unsteady crystallization conditions.Therefore, theoretical and experimental breakthroughs are urgently needed to understand the crystallization mechanism of D-A conjugated polymers.2. The film structure should be better characterized.The ordered structure in D-A conjugated polymers can be characterized using several techniques, but the characterization of amorphous domains is very difficult.For example, the density of tie chains is very important for charge transport, but quantitative measurements of it have rarely been reported.In addition, it seems that the paracrystalline disorder of D-A conjugated polymers is very large, approaching that of amorphous structures.Then, are D-A conjugated crystals poorly ordered crystals with many defects or the associates of crystallites and aggregates?Also, in near-amorphous D-A conjugated polymer films, the coherence length of the (001) peak is in the range of 5-10 nm, which is larger than the size of short-range ordered structure (<5 nm) and smaller than the size of crystals (>10 nm).Therefore, a better understanding of the film structure of D-A conjugated polymers is needed.3. The aggregated structure of D-A conjugated polymer films should be better controlled.Although many methods have been proposed to improve the film structure, quantitative control of the density and size of nuclei or aggregates remains a challenge.For example, the possible reason for high paracrystalline disorder is the presence of too many primary nuclei or secondary nuclei.It is also unclear how to control the size, density and connectivity of aggregates for near-amorphous D-A conjugated polymers.In addition, the preparation of 2D and 3D single crystals is also a challenge, which is important for studying the intrinsic properties of charge transport and determining the performance limits of conjugated polymers.4. The relationship between film microstructure and charge mobility should be better understood.Charge mobility can be affected by the molecular structure and the aggregated structure.However, the molecular structure of D-A conjugated polymers can also affect their aggregated structure, making it difficult to establish a relationship between film microstructure and charge mobility.Besides, Equation (2) seems unsuitable or hard to apply directly for D-A conjugated polymers because charge transport between ordered domains is mainly through tie chains.So, how to establish a quantitative relationship between the aggregated structure and charge mobility?

C O N F L I C T O F I N T E R E S T S TAT E M E N T
The authors declare they have no conflicts of interest.

F I G U R E 4
(A) Diagram of the free energy change with increasing nucleus in classical nucleation theory.Reproduced with permission.[44]Copyright 2019, Wiley Periodicals, Inc. (B) Schematic illustration of the possible nucleation process of donor-acceptor (D-A) conjugated polymers.(C) Schematic diagram of the growth process of D-A conjugated polymers in diffusion-conformational transition (D-CT) model.(D) Diagram of how v A , v D , and v T values affect the morphology of D-A conjugated polymer crystals.

F
I G U R E 9 (A) Self-seeding process of donor-acceptor (D-A) conjugated polymers in solution.(B) Schematic diagram of D-A conjugated polymer crystals grown in solution (the whisker method).(C) Schematic diagram of D-A conjugated polymer crystals grown on substrates (the solvent vapor enhanced drop casting [SVED] method)
This work was supported by the National Natural Science Foundation of China (grant nos.51933010 and 22203028).

Chemical structure Method Improvement of aggregated structure Mobility (cm 2
V −1 s −1 ) (Continues) TA B L E 1 (Continued)