Binary solvent engineering for small-molecular organic semiconductor crystallization

Abstract

Solution processed, flexible electronics has garnered great research attention in the last decade, and has found promising applications in semiconductor device fabrication such as in thin film transistors and organic gas sensors. Binary solvents have been demonstrated to exert an important impact on the semiconductor dissolution, crystal growth, phase segregation, film morphology, crystal alignment, film crystallinity and charge transport of organic semiconductors. In this article, we conducted a comprehensive review on the effect of engineering binary solvents on the crystallization of organic semiconductors. By studying the organic semiconductor 6,13-bis(triisopropylsilylethynyl)pentacene as a representative example, we showcase that the optimization of solvent choices can play a vital role in modulating the solvent evaporation, intermolecular interaction, supramolecular aggregation, semiconductor nuclei, crystal orientation and charge carrier mobilities. Based on a detailed review of these important works, we wish to shed light on the great potential of fine-tuning the solvent choices in order to optimize the charge transport and electrical performance of flexible electronic devices.

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1. Background and challenges

In recent years, flexible electronics, which is mainly the application of solution processable organic semiconductors in electronic device fabrication, has attracted considerable research attention.^1–8 Significant progress has been achieved in improving the charge carrier mobilities and air stabilities of organic semiconductors. Various small-molecular solution processed organic semiconductors, such as 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS pentacene),^9–11 5,6,11,12-tetraphenyltetracene (rubrene),^12–14 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C₈-BTBT),^15–17 and 2,7-didodecyl[1]benzothieno [3,2-b][1]benzothiophene (C₁₂-BTBT)^18–21 based thin film transistors have been reported with mobilities close to or even higher than 10 cm² V⁻¹ s⁻¹ by various research groups. These organic semiconductors also demonstrated enhanced electrical stability when exposed to air,²² bias,²³ or illumination.²⁴ These high charge carrier mobilities and exceptional stabilities have opened up more pathways for implementing organic semiconductors in the fabrication of high performance semiconductor devices including organic thin film transistors,^25–28 organic gas sensors,^29–33 organic optoelectronic devices,^34–40 and complimentary circuits.^41–43

Nevertheless, a challenging issue in organic semiconductor growth is the control of crystallization and morphology. This is mainly caused by the intrinsic crystal misorientation when the organic semiconductor is grown in solution, as reported in many small molecular organic semiconductors. For example, TIPS pentacene, when deposited via the method of drop casting, can exhibit dendritic structures of morphology with each organic crystal pointing in different directions.^44–49 The organic semiconductor 5,6,11,12-tetrachlorotetracene was also reported to exhibit multiple layers of misoriented wires stacking upon one another.⁵⁰ The 2,5-di-(2-ethylhexyl)-3,6-bis(5′′-n-hexyl-2,2′,5′,2′′]terthiophen-5-yl)-pyrrolo[3,4-c]pyrrole-1,4-dione (SMDPPEH) semiconductor from drop casting in a single solvent showed star-shaped organic crystals while a majority of semiconductors formed aggregations on the substrate without full crystallization.⁵¹ Other issues besides the crystal misorientation include poor film coverage on the substrate⁵² and abundant grain boundaries.^53–55 These obstacles have unfortunately led to the failure of many endeavors to apply small molecular organic semiconductors in the fabrication of the aforementioned high performance semiconductor devices.

In order to more precisely manipulate the crystal orientation and enhance the film morphology, enormous efforts have been made to explore possible pathways to control the organic semiconductor growth in a more consistent manner. One successful pathway to obtain a desirable morphology is to mix organic semiconductors with polymer additives.^56,57 Amorphous polymers including poly(α-methylstyrene) (PαMS),^58–62 poly(methyl methacrylate) (PMMA),^63–68 polystyrene (PS),^69–74 and poly(triaryl)amine (PTAA)^75–79 can both improve the semiconductor film uniformity and induce a vertically phase-separated active layer structure. This further forms a semiconductor sublayer with an elevated semiconductor concentration at the dielectric layer interface,^80,81 and/or a polymer encapsulation layer at the air interface,^82,83 which favors the charge transport and air stability of the transistor device. Other types of polymers, such as conjugated polymers^84–89 and semicrystalline polymers,^90–93 have also been reported for their capability to tune the polymorphism, diffusivity, surface energies and nucleation of organic semiconductors. Another pathway is to apply external forces to align the crystal growth direction at the same time when the semiconductor undergoes crystallization.⁹⁴ This gives rise to miscellaneous alignment methods, such as air force navigation,^95,96 pinning confinement,^97,98 solution shearing,^99–104 blade coating,^105,106 zone casting,^107–109 and selective patterning.^110–112 Regardless of polymer mixing or external alignment, it is still imperative to control the various factors that critically influence semiconductor crystallization. One of these factors is the choice of solvents that are used to dissolve organic semiconductors. The solvent choices not only impact the solubility the semiconductor, but also influence the nucleation,^113,114 crystallization,^115,116 alignment,^117,118 phase segregation,^119,120 film morphology,¹²¹ and charge transport.^122,123

2. Binary solvent effect on crystallization

The solvent choices can be divided into single solvent and binary solvents. A single solvent should possess good solubility of the organic semiconductor. As compared to a single solvent, binary solvents are composed of a main solvent and an additive solvent, in which the organic semiconductor may find varying solubility. The engineering and optimization of binary solvent choices have various advantages and exert beneficial effects on enhancing the crystal alignment, tuning phase segregation, controlling nucleation sites, reducing grain boundaries, and eliminating charge trap centers of the organic semiconductors. It is important to note that the direction of crystal alignment is determined by the direction of the solution drying. In this section, we will discuss the advantages of optimizing solvent choices in greater detail.

Crystal alignment: as mentioned above, solution based growth of organic semiconductors can result in inferior crystal alignment. Poor crystal alignment can further cause considerable mobility variations as measured from a batch of transistor devices,^124–131 which make them infeasible for flexible electronics applications. By intentionally aligning the crystal orientation at different angles to the direction from the source to drain contacts, the mobility variations can reach one order of magnitude.^132–136 In this regard, a binary solvent system has been proved to be highly effective in aligning the crystal orientations when combined with other external alignment methods. It is important to note that without applying the binary solvent method, some alignment methods, such as the controlled evaporative self-assembly method (CESA),⁵¹ cannot solely eliminate the random orientations of organic semiconductor crystals.

Phase segregation: when an organic semiconductor is mixed with a polymeric additive to form a binary system, the system can likely go through vertical phase segregation,¹³⁷ lateral phase segregation, or a combination of both phase segregations.^138,139 The type and extent of phase segregation can strongly influence the crystallization process, semiconductor morphology and charge transport.^140–143 The boiling point of the solvent is an important factor, among various other factors, that impact the phase segregation between the semiconductor and polymeric additive.^144,145 In particular, binary solvents with a high boiling point can possess a low evaporation rate. As a result, the organic semiconductor deposited in binary solvents with a high boiling point has sufficient time to diffuse within the semiconductor/polymer mixture and forms more pronounced phase segregation with the polymer.

Nucleation sites: instead of dissolving the organic semiconductor in a single solvent, a binary solvent system can be used to better modulate the nucleation and crystallization process of the semiconductor.⁵⁰ The binary solvent system is primarily composed of a “good” solvent (in which the semiconductor has a good solubility) as the main solvent and a “bad” solvent (in which the semiconductor has limited or little solubility) as the additive solvent. When the organic semiconductor is first dissolved in the main solvent and then injected into a larger volume of the additive solvent, dramatically reduced solubility in the binary solvent can promote nucleation of the organic semiconductor. The subsequent oversaturation upon solvent deposition and evaporation further promotes the semiconductor nucleation, which exerts a greater effect on facilitating the crystallization and crystal orientation alignment.

Grain boundaries: the different solvent choices and binary solvents can effectively control the grain width of the organic semiconductor crystals. In particular, binary solvents with a higher boiling point can take longer to dry out, which allows more time for the organic semiconductor to crystallize and form crystals with enlarged grain width. Crystalline defects exist at the grain boundaries as trap centers of charge carriers.^146–150 Increased size of the crystal domains can reduce the amount of crystalline defects and thereby clear the pathways for charge transport.^151–155

A desirable morphology of drop-casted organic semiconductors would include good crystal alignment, absence of grain boundary, high-quality charge transport interface, as well as morphology uniformity. Specifically, crystal alignment favors charge carrier mobility, which can be 10 times higher from crystals perpendicular to the source and drain contacts than the counterpart based on crystals parallel with the contacts.¹⁵⁶ The absence of grain boundary and negligible crystalline defects benefit charge transport at the interface between the dielectric layer and organic semiconductor layer. Good film uniformity not only improves charge transport but also enhances device-to-device mobility consistency.

The effect of crystalline deformities from the grain boundaries on the organic semiconductor charge transport can be further understood based on the “grain width-dependent mobility model”. Assume L is the length of the channel, and n is the number of grain boundaries. Then L can be reasonably divided into the sum of crystal length L_G and the sum of grain boundary L_GB in the channel:

L = nL_G + (n − 1)L_GB (1)

Given the small dimension of L_GB (approximately 1–2 nm) in connection in series, L_GB is much smaller than L. The total effective mobility μ_E can be calculated based on both the mobility at the crystal grains (μ_G) and the mobility at grain boundaries (μ_GB):^157–159

Merging eqn (1) and (2):

Since n (n ≫ 1) is a large number, (n − 1) is approximated to n. The grain boundary and crystal length can be correlated by designating W_G as the grain width:

By designating and , then eqn (3) becomes after merging eqn (5) into (3):

Eqn (6) dictates a “grain width-dependent mobility model” and shows the effective total mobility μ_E is proportionally dependent on the grain width. Thereby, a larger grain width benefits effective mobility thanks to the fewer crystalline defects.

3. Binary solvent technique

A binary solvent system is mainly composed of a main solvent as well as an additive solvent. When the additive solvent is mixed with the main solvent, the molecular structure of the additive solvent can have a significant impact on tuning the evaporation rate of the binary solvent system as well as on modifying the dissolving of the solute. In a solution that is composed of both solute and solvent, three different types of intermolecular interactions can exist, between solute and solute, between solvent and solvent, as well as between solute and solvent. The solvent's affinity for solute material can determine the extent of these intermolecular interactions and regulate other semiconductor crystallization and film formation processes. The Hansen's solubility theory can be employed to estimate the affinity of the solvent for the solute.

Three different Hansen's solubility parameters, including ∂_p, ∂_d, and ∂_h, represent the polar, dispersive and hydrogen bonding component of the intermolecular interaction force.^160,161 The similarity of these three Hansen's solubility parameters can give rise to good solubility of the solute in the solvent. The mismatch parameter R can be quantitatively obtained based on the following equation:

Eqn (7) can be utilized to calculate the mismatch parameter and provides a useful baseline for quantitatively assessing the solubility of a solute in a given solvent, and essentially denotes that a larger mismatch parameter leads to lower solubility in the given solvent. The criteria of choosing appropriate binary solvents include the dielectric constant, boiling point as well as the Hansen's solubility parameters. The higher dielectric constant of a solvent means a higher polarity and thereby greater ability for the solvent to stabilize the charges. Similar boiling points of the binary solvents can ensure simultaneous solvent evaporation and greater control of the semiconductor evaporation. In terms of the Hansen's solubility parameters, both solvents in the binary solvent system will have similar solubility parameters. As similar solubility parameters result in a small mismatch parameter based on eqn (7), a preferred choice of solvent shall thereby possess a small value of mismatch parameter.

In the following section, we will mainly review the application of binary solvents in controlling the single crystal growth and the crystal alignment and of organic semiconductors. We will focus our discussion on the effect of binary solvents on improving the semiconductor crystallization, thin film morphology and charge carrier mobilities of thin film transistors. It is important to note that in addition to thin film transistor applications, the binary solvent method has also been reported in the application of gas sensors and optoelectronic devices. For instance, Ogbeide et al. reported a binary solvent of isopropanol (IPA) and 2-butanol as the ink solvent carrier in order to suppress the adverse coffee-ring effect, which improves the printing uniformity and enhances the detection limit of the gas sensor.¹⁶² Fo et al. reported binary solvents composed of 1-chloronaphthalene (CN) and diphenyl ether (DPE) as additives to improve the crystallinity and phase segregation domain of the acceptor material, which in turn inhibits charge recombination and enhances power conversion efficiency.¹⁶³ In addition to the organic semiconductors, the binary solvent system has also been studied on other types of semiconductors such as small molecular MoS₂, and C₆₀. For example, Majd et al. reported a binary solvent method composed of N,N-dimethylformamide (DMF) and IPA to obtain two-dimensional MoS₂ nanosheets with an enlarged surface area for application in detecting miRNA-155 in breast cancer.¹⁶⁴ Wang et al. reported a binary solvent of chitosan and tetrahydrofuran to homogeneously disperse MoS₂ nanosheets with ultrathin thickness to tune the smoke safety property of the epoxy (EP) nanocomposites matrix.¹⁶⁵ Ghasemi et al. reported a binary solvent method composed of DMF and N-methyl-2-pyrrolidone (NMP) to modulate the size and thickness of MoS₂ nanoflakes.¹⁶⁶ The resultant MoS₂ nanoflakes with few-layer thickness without lateral dimensional shrinking were applied in phototransistor applications and exhibited a decent response to laser excitation. Zhao et al. reported a binary solvent (m-xylene as the main solvent, and IPA, EtOH and MeOH as the secondary/antisolvent solvent) based vapor diffusion method to grow millimeter-length C₆₀ single crystals and to improve photodetector responsivity by 10 times.¹⁶⁷

3.1. Application in single crystal growth

Kim et al. reported a solvent exchange method in order to grow TIPS pentacene microribbon single crystals.¹⁶⁸ The triisopropylsilyl side groups of TIPS pentacene provide conformational flexibility, giving rise to good solubility in various hydrophobic solvents such as toluene, while enhanced density of the bulky groups allows tight packing of the backbones and maximizes the π–π interactions. Consequently, the TIPS pentacene organic semiconductor is insoluble in solvents with more polarity such as acetonitrile. Thereby, TIPS pentacene was first dissolved in toluene, before the solution was injected into a large volume of acetonitrile. The minimized interactions with the acetonitrile solvent promoted the intermolecular π–π interactions of TIPS pentacene, and gave rise to the formation of long-extended, single crystal microribbons after the solution was deposited via a simple drop casting method for crystallization, as shown by the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of Fig. 1(A) and (B), respectively. Fig. 1(C) shows the selected area electron diffraction pattern of a TIPS pentacene microribbon, which demonstrated high crystallinity of the microribbon. Thin film transistors were fabricated based on the microribbon crystal (Fig. 1(D)), producing an enhanced hole mobility of 1.42 cm² V⁻¹ s⁻¹. This work shows that the single crystalline TIPS pentacene 1D microribbons exhibit a preferred growth direction along the [010] direction which is also parallel with the π–π stacking direction of TIPS pentacene small molecules, resulting in an improved charge carrier mobility.

Fig. 1 (A) SEM image and (B) TEM image of a TIPS pentacene microribbon crystal grown by using the solvent exchange method. (C) Selected area electron diffraction pattern of a TIPS pentacene microribbon. (D) Polarized optical image of a transistor device based on a TIPS pentacene microribbon as the active layer. Reproduced from ref. 168, with permission from Wiley.

He et al. combined a binary solvent exchange method with a nucleation agent to control and disperse the self-assembly process of TIPS pentacene microribbon single crystals.¹⁶⁹ TIPS pentacene, when drop casted from a single solvent toluene, formed misoriented bulk crystals as shown by the polarized optical microscopic images in Fig. 2(a). A solvent-exchange method composed of both toluene and acetonitrile was employed to alter the crystal growth from bulk crystals to microribbon crystals. However, the resultant microribbon crystals were aggregated (Fig. 2(b)). Thereby, the nucleation agent 4-hexylbenzoic acid (HBA) was employed to disperse the aggregated microribbons. HBA was first dissolved in toluene, and was then mixed with acetonitrile at a volume ratio of 1 : 50. Subsequently, the TIPS pentacene solution was injected into the HBA/toluene/acetonitrile mixture. After deposition via drop casting, HBA self-assembled onto the substrate forming an interfacial layer, while its hydrophobic tails underwent interactions with the alkyl chains of TIPS pentacene molecules. Consequently, effective dispersion of TIPS pentacene microribbons with more uniform coverage and morphology were observed in the optical images of Fig. 2(c) and (d). Thin film transistors incorporating the dispersed microribbons as the active layer demonstrated a hole mobility of 0.36 cm² V⁻¹ s⁻¹ as well as a 9 times increase in average mobility when compared to the counterpart based on the pristine crystals. This work showcases that the self-assembled process from the HBA nucleation agent can effectively modulate the nucleation and crystallization of the organic single crystals while simultaneously improving the film uniformity by dispersing single crystal aggregations.

Fig. 2 Polarized optical images of (a) pristine TIPS pentacene bulk crystals, (b) TIPS pentacene microribbon crystals, and (c and d) TIPS pentacene microribbon crystals incorporating the HBA small molecule as a nucleation agent. Images (a–c) have the same scale bar. Reproduced from ref. 169, with permission from The Japan Society of Applied Physics.

Li et al. reported a binary solvent based approach to fabricate large TIPS pentacene single crystals with sizes extending to millimeters.¹⁷⁰ Two different binary solvent systems, including IPA/toluene and EtOH/toluene, exhibit a positive azeotropic point but opposing polarities. The TIPS pentacene solutions were drop casted in an ambient clean room environment or in a nitrogen environment. Different ratios of the binary solvents were tested, which gave rise to the distinct thin film morphology of the resultant TIPS pentacene crystals as shown in the polarized optical images of Fig. 3. The TIPS pentacene morphology abruptly transitioned to single crystals with a large domain size from polycrystalline films, as the solvent composition of the binary solvents reached the azeotropic point. In particular, the azeotropic point is at 50.1/49.9 for the IPA/toluene solvents and at 59.8/40.2 for the EtOH/toluene solvents. Accordingly, the charge carrier mobility of self-assembly TIPS pentacene large single crystals were enhanced by 4 times to 0.73 cm² V⁻¹ s⁻¹. Besides, the single crystal-based transistor devices showed a threshold voltage close to zero, implying a high-quality interface between the active layer and the gate dielectric layer. The improved interface quality was attributed to the high crystallinity, well-defined facets of the single crystals, the specific thermodynamics of the azeotrope mixture, as well as the mechanical flexibility of crystal ribbons. This work shows azeotropic binary solvent mixtures provide an effective method to fabricate organic single crystals via a self-assembled process, with crystals ranging from small needles to large parallelepipeds dependent on the binary solvent ratios with respect to the azeotropic point.

Fig. 3 Cross-polarized optical images showing the TIPS pentacene morphology abruptly changed from polycrystalline films to single crystals with a large domain size as the binary solvent ratios reached the azeotropic point: at 50.1/49.9 for the IPA/toluene solvents as shown in (A), and at 59.8/40.2 for the EtOH/toluene solvents as shown in (B). All images share the same scale bar of 200 mm. Reproduced from ref. 170, with permission from Wiley.

Minemawari et al. reported a binary solvent system, including chlorobenzene as the main solvent and DMF as the antisolvent, to deposit single-domain TIPS pentacene film by using a double-shot ink-jet printing (DS-IJP) technique.¹²² A piezoelectric IJP apparatus mounted with dual IJP heads was employed for the alternate deposition of semiconductor solution microdroplets. Various printing conditions were tested including droplet volume, travel velocity, repetition frequency and deposition sequence of the antisolvent ink and solution ink. Without adding the antisolvent, inkjet printing TIPS pentacene resulted in a nonuniform film with undesirable coffee rings. Thereby, a different deposition sequence involving the main solvent and antisolvent was tested on the morphology of the resultant thin film morphology. For the first sequence, TIPS pentacene/antisolvent ink was deposited first followed by overprinting of the TIPS pentacene/chlorobenzene ink. Optical microscopic images showed plate-like crystals with a large thickness. For the second sequence, TIPS pentacene/chlorobenzene ink was deposited before the TIPS pentacene/antisolvent ink instead. This gave rise to large single crystal domains of TIPS pentacene, yielding a hole mobility of 0.042 cm² V⁻¹ s⁻¹. The improved hole mobility was due to uniform thin films free of pinholes, enlarged crystal domains as well as uniform film thickness. This work indicates that the crystalline preference of a small molecular organic semiconductor should be a key consideration when optimizing the inkjet printing processes.

Balakrishnan et al. reported the growth of one-dimensional propoxyethyl-PTCDI single crystalline nanobelts using a solvent exchange method.¹⁷¹ The propoxyethyl side chain of propoxyethyl-PTCDI has conformational flexibility and provides sufficient solubility in various hydrophobic solvents such as chloroform. On the other hand, the perylene backbones exhibit tight packing as a result of the small side chain size and increased density. These properties resulted in insolubility of propoxyethyl-PTCDI in more polar solvents. Therefore, when the propoxyethyl-PTCDI/chloroform solvent was injected into a polar solvent such as methanol, it promoted the nanocrystal phase to self-assemble and to form nanobelts. The solution was drop casted onto the substrate for various characterizations. As shown in the SEM image of Fig. 4(A), the propoxyethyl-PTCDI nanobelts were in well-defined nanostructures. The TEM images presented in Fig. 4(B) and (C) showed uniform nanobelt structures. The diffraction pattern indicated sharp diffraction spots, as shown in the inset of Fig. 4(C). The AFM image of propoxyethyl-PTCDI nanobelts was presented in Fig. 4(D), showing an average thickness of 100 nm. This work showcases that a simple self-assembling process in a binary solvent system results in the formation of the uniaxial crystal structure of the nanobelt.

Fig. 4 (A) SEM photo of propoxyethyl-PTCDI nanobelts grown based on the solvent exchange method. (B) TEM image of propoxyethyl-PTCDI nanobelts, with the zoom-in TEM image shown in (C). (D) AFM image of propoxyethyl-PTCDI nanobelts with an average thickness of 100 nm. Reproduced from ref. 171, with permission from the American Chemical Society.

Chae et al. reported DPE and CN as additives to the main solvent in order to tune the crystallization and thin film morphology of TIPS pentacene single crystalline domains.¹⁷² TIPS pentacene was first dissolved in the anisole solvent and the solvent additives were mixed at different concentrations including 0.25%, 0.5% and 1% for DPE as well as 0.2%, 0.25% and 0.5% for CN. The TIPS pentacene solution was drop casted onto substrates preheated at various temperatures for crystallization. Polarized optical images indicated 0.5% DPE solvent gave rise to the formation of V-shaped microcrystals with an enlarged crystal width. In addition, 0.2% CN solvent additive caused the TIPS pentacene crystals to form highly packed crystal domains with improved uniformity in the grain size distribution. Accordingly, the highest mobility of 0.73 cm² V⁻¹ s⁻¹ and 0.71 cm² V⁻¹ s⁻¹ was obtained from the TIPS pentacene thin film transistors based on the 0.5% DPE and 0.2% CN solvent additive, respectively. The enhanced charge transport was mainly a result of the enlarged grain width and reduced grain boundaries. This work demonstrates a facile solvent additive approach to improve the charge carrier mobility by tuning the grain boundary density and crystalline defects.

The various binary solvent papers reviewed in this section, along with the author, semiconductor material, binary solvent choice, crystallization method, result and mobility, are summarized in Table 1. Although the mobility values reported in section 1 are slightly higher than those based on solvent exchange in section 3, it is important to point out that the higher mobilities can be partially attributed to the following factors. Organic semiconductors reported in more recent years, such as C₈-BTBT, tend to have higher mobilities, reaching approximately 50 cm² V⁻¹ s⁻¹. Also, external alignment methods such as solution shearing, blade coating and bar coating have been reported to enhance the mobility values. Besides, blending organic semiconductors with polymer additives such as polystyrene has been shown to improve the charge transport properties.¹⁷³ Therefore, the various advantages of the solvent exchange method as reported in this review article shed light on more universal application on other types of high mobility organic semiconductors as well as on combination with these external alignment and polymer blending methods. In general, controlling the binary solvent choices and compositions provide an effective method to modulate the important morphological factors such as crystal orientation, phase segregation, and grain size. Binary solvents composed of a “good” solvent and a “bad” solvent effectively modulate the semiconductor solubility, nucleation, crystallization and crystal alignment. Besides, solvents with a high boiling point possess a slow evaporation rate and provide sufficient time for solute diffusion, which not only promotes more pronounced phase segregation, but also results in an enlarged grain size with reduced grain boundaries and crystalline defects.

Table 1 Summary of the publications reviewed in this section, including the semiconductors, type, types of the binary solvent, and mobility

Author	Semiconductor	Binary solvent	Crystallization method	Result	Mobility (cm^2 V^−1 s^−1)
Kim et al.	TIPS pentacene	Toluene/acetonitrile	Drop casting	Solvent exchange method promoted the intermolecular π–π interactions and formed single crystalline microribbons	1.42
He et al.	TIPS pentacene	Toluene/acetonitrile	Drop casting	Solvent exchange method, combined with a nucleation agent, was used to disperse the aggregated microribbons	0.36
Li et al.	TIPS pentacene	IPA/toluene, EtOH/toluene	Drop casting	Binary solvents with a positive azeotropic point but opposing polarities gave rise to large single crystals	0.73
Minemawari et al.	TIPS pentacene	Chlorobenzene/DMF	Ink-jet printing	Adding DMF as an antisolvent ink eliminated the coffee ring and led to large single crystal domains	0.042
Balakrishnan et al.	Propoxyethyl-PTCDI	Chloroform/methanol	Drop casting	Binary solvent promoted the nanocrystal phase to self-assemble and form well-defined nanostructures	Not reported
Chae et al.	TIPS pentacene	Anisole/DPE or CN	Drop casting	Binary solvent with solvent additive led to V-shaped microcrystals and highly packed enlarged crystal domains	0.73

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3.2. Application in crystal alignment

He et al. combined the binary solvent method with a controlled solvent evaporation method to realize the aligned crystallization of a p-type organic semiconductor 5,6,11,12-tetrachlorotetracene.⁵⁰ The binary solvent system is composed of a chloroform solvent as the “good” solvent and a methanol solvent as the “bad” solvent. The organic semiconductor 5,6,11,12-tetrachlorotetracene was first dissolved in the chloroform solvent and the solution was then injected into methanol, which caused supramolecular aggregations to form as indicated by the blue shift in the UV-vis absorption spectra. These supramolecular aggregations serve as seeds for the 5,6,11,12-tetrachlorotetracene semiconductor to nucleate and crystallize. In the meantime, a controlled solvent evaporation method was applied to induce capillary force, which aligned the growth direction of the 5,6,11,12-tetrachlorotetracene semiconductor. In order to optimize the thin film morphology and crystal alignment, different ratios between chloroform and methanol including 1 : 0, 10 : 1, 5 : 1, and 3 : 1 were tested. A simple drop casting method was adopted to deposit the organic semiconductor solution. As shown in the polarized optical images of Fig. 5, without the methanol solvent, the 5,6,11,12-tetrachlorotetracene crystal growth based on the pure chloroform solvent showed random patterns, leaving a majority of the substrate uncovered with crystals. In contrast, both crystal alignment and substrate coverage were enhanced at 10 : 1 and 5 : 1 ratios of the chloroform/methanol binary solvent. When the ratio further changed to 3 : 1, the alignment started to wane. Electrical characterization results indicated that the 5,6,11,12-tetrachlorotetracene based thin film transistors with chloroform/methanol binary solvent at 10 : 1 exhibited an improved hole mobility of 1.1 cm² V⁻¹ s⁻¹, which was attributed to the improved crystal alignment and coverage. This work shows the solvent exchange method effectively modulates the supramolecular aggregations, nucleation formation and crystallization of the organic semiconductor 5,6,11,12-tetrachlorotetracene, and when in combination with the controlled solvent evaporation method, can powerfully lead to the directional alignment of crystal growth.

Fig. 5 Polarized optical images showing the thin film morphologies of 5,6,11,12-tetrachlorotetracene crystals grown by drop casting in binary solvents and applying the controlled solvent evaporation method. The images in (a–d) correspond to chloroform/methanol binary solvent at ratios of 1 : 0, 10 : 1, 5 : 1, and 3 : 1, respectively. The images in (a–d) have the same scale bar of 100 microns. Reproduced from ref. 50, with permission from Elsevier.

A similar method of combining controlled evaporation with a binary solvent system was also employed to realize the organized crystallization of SMDPPEH.⁵¹ In this work, Bi et al. used chloroform as the good solvent and ethanol as the bad solvent. The controlled evaporative self-assembly method, abbreviated as “CESA”, involved the placement of a Pinner on the substrate to induce capillary force. The SMDPPEH solution was deposited onto the substrate with a preset Pinner via a simple drop casting method. The resultant thin film morphology of SMDPPEH, in terms of crystal orientation alignment, film coverage as well as grain width, was observed to correlate to the different ratios between chloroform and ethanol. The misorientation angle was calculated to more accurately evaluate the change of crystal alignment with different binary solvent ratios. At ratios of 15 : 1, 10 : 1, 5 : 1, 1 : 1 and 1 : 5, average misorientation angles of 6 ± 5°, 4 ± 3°, 4 ± 2°, 11 ± 7° and 30 ± 20° were measured, respectively, which indicated the greatest advances in alignment at both 10 : 1 and 5 : 1 ratios. The aligned SMDPPEH crystal based OTFTs at the binary solvent ratio of 5 : 1 exhibited an enhanced mobility of 0.016 cm² V⁻¹ s⁻¹. The binary solvent method in combination with the CESA method as reported in this work effectively controls the contact line pinning, enhances the evaporation rate at the droplet edge, elevates the solution concentration, facilitates outward flow of solute, promotes nucleation formation, and improves crystallization.

Abdullah et al. reported the impact of anisole/decane binary solvents on the crystallization and thin film morphology of TIPS pentacene.¹¹³ When the composition of the anisole/decane binary solvent ranged between 96/4 wt% and 85/15 wt%, an azeotropic composition is maintained in the solution mixture, giving rise to a constant boiling point at 152 °C as well as constant binary solvent composition during the solvent evaporation. TIPS pentacene solution was drop casted at a temperature of 30 °C onto a slightly tilted substrate and allowed to dry for 3 hours. As the content of decane increased, the interaction between the solute and solvent became weakened, which promoted the nucleation of TIPS pentacene and consequently the formation of large crystals. The TIPS pentacene film deposited from pure anisole solvent showed grain like nanodots with a roughness of 6.3 nm. When the anisole was mixed with decane at 96/4 wt% and 90/10 wt%, the TIPS pentacene film changed to a terrace like morphology with a larger surface roughness. To fabricate the thin film transistors, a polyvinyl pyrrolidone (PVP) and poly(melamine-co-formaldehyde) hybrid layer was used as the gate dielectric before TIPS pentacene was drop casted in the binary solvents to form the active layer. A hole mobility of up to 0.16 cm² V⁻¹ s⁻¹ was obtained based on the 93/7 wt% binary solvent. This work shows that the binary solvents composed of anisole and decane can effectively modulate the mean size of nanodots of TIPS pentacene as well as its average surface roughness, promoting step-flow crystal growth, layer ordering and long-range alignment.

Bharti et al. studied the correlation between the dissimilarity of binary solvents, the molecular aggregation, and the charge transport of organic semiconductors.¹²³ In this work, TIPS pentacene was dissolved in different solvents including single solvent toluene, as well as binary solvents of toluene/benzene, toluene/cyclohexane, and toluene/hexane. A simple drop casting method was used to dispense the TIPS pentacene solution on a tilted substrate. The addition of hexane as a nonsolvent effectively resulted in a weaker repulsive force between the TIPS pentacene solute molecules and thereby stronger molecular aggregations due to enhanced molecular interactions. Terracing structures of TIPS pentacene were obtained based on the single solvent. The addition of cyclohexane and hexane, which exhibit more different structures from that of toluene, resulted in more irregular terracing structures of TIPS pentacene. In particular, hexane has the most dissimilar structure with toluene and hence promotes the strongest molecular aggregation of TIPS pentacene. As a result, TIPS pentacene crystals grown based on the toluene/hexane binary solvent showed improved crystallinity and the highest hole mobility of 0.15 cm² V⁻¹ s⁻¹. This work demonstrates that increased dissimilarity of the additive solvent in the binary solvent system makes it more challenging to overcome intermolecular forces and thereby favors molecular aggregation of TIPS pentacene, leading to enhanced crystallinity.

Lim et al. reported the effect of solvent mixture including chlorobenzene as the main solvent and hexane, o-dichlorobenzene or dodecane as the additive solvent to tune the crystal growth and alignment of TIPS pentacene.¹¹⁸ An ink-jet printer mounted with a single-nozzle piezoelectric head and a two-axis motorized positioning system was employed to deposit 40–50 picoliters of the TIPS pentacene solution onto the substrate which is 0.5 mm away from the head. TIPS pentacene deposited from chlorobenzene resulted in a homogeneous film but undesirable molecular misorientations, as shown in the optical image of Fig. 6(a). Thereby, various solvents including hexane, o-dichlorobenzene or dodecane, were mixed with chlorobenzene as a solvent additive. The resultant thin film morphology was presented in Fig. 6(b)–(d), respectively. The addition of hexane resulted in thicker aggregation of TIPS pentacene at the perimeter of the inkjet printed droplets. Mixing o-dichlorobenzene with the main solvent led to a more uniform morphology (Fig. 6(b)). Besides, hexane, which has a high boiling point, allows a recirculation flow of the solution and also induces a Marangoni flow that contradicts the convective flow (Fig. 6(c)). TIPS pentacene crystals deposited from the chlorobenzene/dodecane binary solvent showed orientation enhancement, as shown in the optical image of Fig. 6(d). As a result, the best average hole mobility of 0.12 cm² V⁻¹ s⁻¹ was obtained from the inkjet printed TIPS pentacene based on the chlorobenzene/dodecane binary solvent. This work reveals that in order to obtain highly aligned TIPS pentacene crystals with high crystallinity, it is important to ensure the minor solvent has a higher boiling point as well as lower surface tension than the main solvent, which induces a recirculation flow opposite to the convective flow and promotes crystal self alignment.

Fig. 6 Optical microscopic images of inkjet printed TIPS pentacene films based on different solvent choices including (a) chlorobenzene, (b) chlorobenzene/hexane, (c) chlorobenzene/o-dichlorobenzene, and (d) chlorobenzene/dodecane. The volume ratio of chlorobenzene is maintained at 25% for all cases. Reproduced from ref. 118, with permission from Wiley.

Zhao et al. reported a binary solvent method to grow large scaled TIPS pentacene crystals with orientation alignment.¹⁷⁴ A Marangoni effect-controlled oriented growth (MOG) method was used to enable large-area deposition of TIPS pentacene solution and allowed the substrate to move in an upward and downward manner, which promoted contact line pinning, liquid membrane spreading, film formation and crystal alignment. The binary solvent system is composed of toluene and carbon tetrachloride with different boiling points. Carbon tetrachloride has a low boiling point and high surface tension, whereas toluene has a high boiling point and low surface tension. Since the solvent evaporation had a faster rate at the contact line than inside the liquid interior, the composition of toluene in the binary solvent at the contact time increased over time due to its higher boiling point. As a result, the contact line had a lower surface tension, which created a Marangoni flow from the contact line towards the liquid interior. This Marangoni flow further counterbalanced the convective flow and promoted a recirculation flow, giving rise to well-aligned TIPS pentacene crystals with large scaled domain size. As a result, the TIPS pentacene thin film transistors based on the binary solvent showed a mobility of 0.7 ± 0.22 cm² V⁻¹ s⁻¹. The MOG method in combination with the binary solvent method is effective in promoting directed formation of semiconducting small molecules and self-organization of highly aligned crystalline structures.

Lada et al. reported a binary solvent system in order to tune the morphology and charge transport of TIPS pentacene.¹⁷⁵ The solvent system is composed of mesitylene as the main solvent, and a series of additive solvents including two types of alkyl aromatics (cumene and indane), two types of aromatic acetates (butyl and amyl acetate) and two types of aromatic ethers (anisole and 4-methylanisole), which give rise to a range of boiling points close to that of the main solvent. TIPS pentacene and the polymer additive PS finds good solubility in the main solvent and additive solvent, respectively. The TIPS pentacene solution with PS additive was deposited via spin coating. Optical images showed that TIPS pentacene film exhibited small crystallite size when deposited in the single solvent, generating a low mobility of 0.1 cm² V⁻¹ s⁻¹. In contrast, adding anisole as the additive solvent yielded a considerable increase in the crystal size of TIPS pentacene, and thereby generated a much higher saturation mobility of 1.16 cm² V⁻¹ s⁻¹ as well as a lower threshold voltage due to the reduced number of grain boundaries. This work demonstrates choosing the appropriate solubility differentials of the binary solvents promoting phase segregation, crystallite size as well as active layer flatness.

The various binary solvent papers reviewed In this section, along with the author, semiconductor material, binary solvent choice, crystallization method, result and mobility, are summarized in Table 2.

Table 2 Summary of the publications reviewed in this section, including the semiconductors, type, types of the binary solvent, and mobility

Author	Semiconductor	Binary solvent	Crystallization method	Result	Mobility (cm^2 V^−1 s^−1)
He et al.	5,6,11,12-Tetrachlorotetracene	Chloroform/methanol	Drop casting	Binary solvents promoted supramolecular aggregations to form as nucleation seeds	1.1
Bi et al.	SMDPPEH	Chloroform/ethanol	Drop casting	Binary solvents along with a “CESA” method reduced crystal misorientation and improved alignment	0.016
Abdullah et al.	TIPS pentacene	Anisole/decane	Drop casting	Binary solvents promoted semiconductor nucleation and formation of large crystals	0.16
Bharti et al.	TIPS pentacene	Toluene/benzene, cyclohexane, or hexane	Drop casting	Solvent additive with the most dissimilar structure promoted the strongest molecular aggregation and improved crystallinity	0.15
Lim et al.	TIPS pentacene	Chlorobenzene/hexane, o-dichlorobenzene or dodecane	Ink-jet printing	Binary solvent with dodecane additive led to enhanced orientation alignment	0.12
Zhao et al.	TIPS pentacene	Toluene/carbon tetrachloride	MOG method	Binary solvent created Marangoni flow that counterbalanced convective flow and promoted crystal alignment	0.7 ± 0.22
Lada et al.	TIPS pentacene	Mesitylene/various aromatics-based solvents	Spin coating	Binary solvent incorporating hexane increased the crystal size of TIPS pentacene and reduced grain boundaries	1.16

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4. Summary and outlook

In this work, we have reviewed a binary solvent technique for modulating the organic semiconductor nucleation, controlling the crystallization behavior, improving the film crystallinity and enhancing the charge transport of organic semiconductor based thin film transistors. Using TIPS pentacene as a primary example for the general solution processed, organic semiconductors, we discussed the effects of engineering different solvent choices on the organic semiconductor thin film morphologies, phase segregation, grain width and crystal orientation. We also explored the important correlation between the different solvent choice, thin film morphology and charge carrier mobility of the organic semiconductor based thin film transistors. These works demonstrated that binary solvents can provide an effective approach to promote supramolecular aggregation, counterbalance undesirable convective flow, and enhance crystal orientation alignment when combined with miscellaneous external alignment methods.

Organic semiconductor based thin film transistors can be implemented in a variety of device applications such as photodetectors, gas sensors and logic circuits. The exploration and optimization of the solvent choices for solution processed organic semiconductors becomes more important when future endeavors are made to expand the application of organic semiconductors in these related electrical device fabrications. Future research in the following aspects will shed light on the pathway to achieve high performance flexible electronics. First of all, binary solvent is an effective method to induce the crystallization of single crystals. A single crystal is free of grain boundaries and crystalline defects, which can yield ultra-high charge carrier mobilities and thereby have significant applications for high performance flexible electronic devices.^176–180 Besides, ternary solvents, as opposed to a single solvent or binary solvent, have been extensively demonstrated to control the domain of size of perovskite films for enhancing solar cell performances.^181,182 The ternary solvent system could be a potential pathway to control the crystallization of solution processed organic semiconductors since it has been reported to effectively improve the semiconductor morphology uniformity, enhance the crystal alignment in a long range order, control the domain size, and modulate the interface. These factors exert positive effects on organic semiconductor charge transport: improved crystal morphology uniformity and long-range alignment improves crystal bridging of contact electrodes and ensures efficient charge transport; enlarged domain sizes help diminish grain boundaries, crystalline defects and charge trap centers; improved quality of the interface between the semiconductor active layer and dielectric layer gives rise to better charge transport. Furthermore, due to the increasingly urgent call for a green environment, green solvents such as n-amyl acetate or 1,3-dioxolane provide an effective alternative to the binary solvent while simultaneously bringing little to no toxicity to the environment.^183,184 Finally, miscellaneous external alignment methods such as bar-casting,^185–188 dip-casting,^189–194 drop-pinned crystallization,^195–198 off-center coating^199–205 have been previously reported as highly effective in aligning the crystal orientation, patterning crystals on a large area, improving areal coverage and continuity, which is preferred to enhance charge transport capability. While previous binary solvent studies mainly employ drop casting as the crystallization method as shown in this review article, combining these methods with the binary solvent method, thereby, would further improve the charge carrier properties and expand the application in organic optoelectronics or organic circuits. We expect that future efforts in these aforementioned aspects will expedite the progress of flexible electronic devices.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

S. Bi acknowledges the support from the Science and Technology Project of Liaoning Province (20180540006).

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