Tailoring the molecular weight of polymer additives for organic semiconductors

Abstract

A binary system comprising both an organic semiconductor and a polymer additive has attracted extensive research interests due to its great potential for use in high-performance, solution-processable electronic devices on flexible substrates. The molecular weight of polymer additives plays a critical role in modulating the crystal growth, enabling phase segregation, optimizing thin film morphology, and improving the charge transport of organic semiconductors. Here, we provide an in-depth review of the recent progress in studying amorphous and semicrystalline polymeric additives, including polystyrene, poly(α-methylstyrene), polymethyl methacrylate, and polyethylene oxide, and fully discuss the effect of the different polymer molecular weights on semiconductor crystallization, active layer composition, and the electrical performance of miscellaneous organic semiconductors. Using the representative examples of 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS pentacene), 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C₈-BTBT), and 2,8-difluoro-5,11-bis(triethylsilylethynyl)anthradithiophene (diF-TES-ADT), this work sheds light on utilizing these universal polymers with varying molecular weights to powerfully manipulate the charge transport of other high-mobility, solution-processable organic semiconductors.

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Zhengran He

Dr Zhengran He received his BS degree from the School of Electronic Engineering at Xi’an Jiaotong University, China, and PhD degree in Electronic Engineering at the University of Alabama, USA. His current research interests are high performance of optoelectronic transistors and the development of physics in organic–inorganic electronic devices.

Ziyang Zhang

Mr Ziyang Zhang received his BS degree from the School of Electronic Engineering at Xi’an Jiaotong University, China, and MS degree in Electronic Engineering at Columbia University, USA. His current research interests are organic electronics, machine learning and composites.

Sheng Bi

Dr Sheng Bi is an associate professor in the School of Mechanical Engineering, Dalian University of Technology (DUT), China. He received his BS degree from the School of Physics at Dalian University of Technology in 2011 and PhD degree in electronic engineering at University of Alabama, USA, in 2016. His current research is focused on the behaviors of excited states in the light-emitting diodes based on organic materials.

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1. Introduction and background

1.1 Recent progress in flexible electronics

In recent years, research on flexible electronics has witnessed rapid progress.^1–11 Great efforts have been made to study solvent choices, controllable crystallization, charge carrier mobilities, and organic electronic device applications of solution-processed small-molecular organic semiconductors, such as 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS pentacene),^12–17 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C₈-BTBT),^18–21 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)^22–24 and 2,8-difluoro-5,11-bis(triethylsilylethynyl)anthradithiophene (diF-TES-ADT).^25–28 For instance, based on the structural engineering of the organic semiconductor pentacene, the herringbone packing patterns in TIPS pentacene are interrupted by the attachment of the bulky side chains, which improves the solubility of TIPS pentacene in organic solvents.^29,30 The enhanced π–π stacking in TIPS pentacene is attributed to higher charge transport.³¹ These research endeavors have opened up vast opportunities for organic semiconductors to be applied in high performance organic electronic devices such as organic thin film transistors,^32–35 organic gas sensors,^36–38 photovoltaic devices^39–41 and logic circuits.^42–44 In this section, we will review the important studies that investigated the solution processability and charge carrier mobility of organic semiconductor based thin film transistors, as well as their application in fabricating organic gas sensors.

First, research studies have been devoted to exploring the solvent possibilities of organic semiconductors, allowing the modulation of semiconductor morphology and charge transport.^45–47 For instance, Kim et al. investigated the effect of different solvents on the thin film morphology and crystallinity of TIPS pentacene.⁴⁸ Solvents with high boiling points such as chlorobenzene and xylene resulted in dendritic morphology with higher crystallinity, whereas solvents with low boiling points such as chloroform led to an amorphous film with low crystallinity. Choi et al. studied the correlation between the solvent boiling point, grain size and charge transport.²⁹ Spin coating TIPS pentacene from chlorobenzene with a high boiling point produced crystals with a large grain size and high crystallinity, yielding a mobility that is 5 orders of magnitude higher than that from a low boiling point solvent such as chloroform. Hwang et al. reported the effect of different solvents including chlorobenzene and tetralin on the vertical phase segregation and compositional structure of TIPS pentacene/polymer blends.⁴⁹ Distinct phase segregation and enhanced crystallization were observed based on the solvent tetralin, attributed to higher mobility values. Ozorio et al. discovered how the different solvent choices impacted the vertical phase segregation and charge transport in the TIPS pentacene/poly(3-hexylthiophene) (P3HT) blends.⁵⁰ Solvent trichlorobenzene caused a moderate vertical phase segregation between TIPS pentacene and P3HT and gave rise to optimized TIPS pentacene film morphology and enhanced P3HT ordering, which yielded an output current that is 39 times larger as compared to the counterpart based on other solvents such as chloroform and toluene.

Besides, highly effective alignment methods have been shown to enhance the charge carrier mobilities of the organic semiconductors close to or over 10 cm² V⁻¹ s⁻¹. While miscellaneous alignment methods have been reported, they can be mainly summarized into the categories of blading coating, substrate patterning, slot-die coating and controlled evaporation based methods. First, blade coating based methods involve either a running blade over a stationary substrate or a stationary blade over a moving substrate.^51–54 The resultant film thickness is determined by the gap between the blade and substrate, the coating speed and the viscoelastic properties of the solution. Solution shearing, as an enhanced version of the blade coating method, involves the use of a tilted blade to deposit the organic semiconductor solutions.^55–59 Solution shearing has been reported to effectively tune the molecular packing, manipulate the crystal alignment and control the thin film morphology of organic semiconductors.^60,61 Peng et al. demonstrated a “dual solution-shearing” method and reported a mobility of 10 cm² V⁻¹ s⁻¹ using the organic crystals of 2,7-didecyl[1]benzothieno[3,2-b][1]benzothiophene (C₁₀-BTBT).⁶² Bilgaiyan et al. reported the solution shearing of a 6,6 bis(trans-4-butylcyclohexyl)-dinaphtho[2,1-b:2,1-f]thieno[3,2-b]thiophene (4H-21DNTT) organic semiconductor based thin film transistor and obtained a mobility of 10.5 cm² V⁻¹ s⁻¹.⁶³ The 4H-21DNTT based CMOS inverter circuit showed sharp switching behaviors with a 31.5 signal gain. Second, substrate patterning based methods involve the tuning of substrate wettability by using surfactant treatment and/or photolithography patterning, which can effectively confine the deposition of organic semiconductor solutions as well as the subsequent crystal growth and alignment. Zhao et al. demonstrated an “asymmetric-wettability topographical template” to control the ordered crystallization of 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C₈-BTBT), giving rise to a mobility of 8.7 cm² V⁻¹ s⁻¹.⁶⁴ Shen et al. reported the controlled crystallization and morphologies of the C₈-BTBT organic semiconductor via tuning substrate wettability with soluble polymer films as a modification layer.⁶⁵ The resultant C₈-BTBT film with optimized grain boundary and domain size exhibited a mobility higher than 7 cm² V⁻¹ s⁻¹. Deng et al. reported a “channel-restricted meniscus self-assembly (CRMS)” method, which combined photolithography patterning and dip coating, to grow uniform 2,6-diphenylanthracene (DPA) single crystal arrays.⁶⁶ The photoresist patterning confined the lateral growth of DPA single crystals, leading to a mobility of 30.3 cm² V⁻¹ s⁻¹. Third, slot die coating based methods deposit the organic semiconductor solution onto a moving substrate directly through a coating head at a tunable rate.^67–70 Lin et al. demonstrated highly ordered single crystalline TIPS pentacene films by using a roll-to-roll slot die coating method, and reported a mobility of 4.2 cm² V⁻¹ s⁻¹ for pristine TIPS pentacene.⁷¹ Xie et al. reported a roll-to-roll slot die coating method to fabricate thin film transistors.⁷² With Ag nanowires deposited by using the R2R coating method as the gate electrode, the C₈-BTBT/polystyrene (PS) based transistor showed a mobility of 18.3 cm² V⁻¹ s⁻¹. Fourth, controlled evaporation based methods involve the tuning and control of the solution flow behavior and evaporation rate in order to modulate the deposition and growth of organic semiconductors.^73–75 Wang et al. reported a “Marangoni effect-controlled growth” method to grow a highly oriented C₈-BTBT film with a herringbone stacking structure.⁷⁶ A high mobility of 25 cm² V⁻¹ s⁻¹ was obtained from the C₈-BTBT based transistors incorporating a long channel length. Soeda et al. reported the growth of an aligned organic semiconductor 3,11-didecyldinaphtho[2,3-d:2′,3′-d′]benzo[1,2-b:4,5-b′]dithiophene (C10-DNBDT) with controlled solvent evaporation.⁷⁷ By holding the C10-DNBDT solution at a moving blade edge, continuous solvent evaporation and supply at the same rate gave rise to aligned crystals extending in inches and a mobility of 10 cm² V⁻¹ s⁻¹.

The exceptional high mobilities present great opportunities for the organic semiconductors to be implemented in high performance organic electronic devices such as gas sensors. Manoli et al. demonstrated a P3HT thin film transistor based gas sensor for detecting acetone, ethanol and n-butanol vapors.⁷⁸ When exposed to the vapors, the P3HT thin film transistors showed a decreased drain current as a result of the enlarged potential barrier at the P3HT grain boundaries as charge trapping. Seo et al. reported a 5,11-bis(triethylsilylethynyl)anthradithiophene (TES-ADT) thin film transistor based gas sensor for detecting NO₂ gas.⁷⁹ Exposure to 0 ppm to 100 ppm NO₂ gas caused accumulation of charge carriers and thereby led to an increase in the on-current. Hou et al. studied the effect of grain boundaries on the sensing performance of TIPS pentacene transistor based gas sensors.⁸⁰ Solvent vapor annealing with toluene gave rise to a large density of grain boundaries and thereby higher sensitivity when NO₂ gas diffused into the grain boundaries and caused a more pronounced current increase. Shao et al. compared the effect of different solvents on the performance of TIPS pentacene transistor based gas sensors.⁸¹ Spin coating TIPS pentacene in solvent o-xylene was observed to yield abundant grain boundaries and thereby enhanced gas sensing performance of NO₂.

1.2 Challenges in controlling organic semiconductor crystallization

Despite the major advantages of organic semiconductors such as high mobility and solution processability, the crystallization of organic semiconductors based on solution processing can still be difficult to maneuver.^82–86 In this section, we employ TIPS pentacene as a benchmark semiconductor to discuss the main challenges that can limit the implementation of organic semiconductors in organic electronic applications. These challenges include crystal misorientation, thermal cracks and grain boundaries, which will be discussed in greater detail below.

First, when drop cast in solution, TIPS pentacene forms dendritic patterns of crystal ribbons, which can be characterized by considerable crystal misorientation and film gaps.^87–98 In a pivotal study conducted by Chen and coworkers,⁹⁹ the misorientation angle θ was measured between the TIPS pentacene crystal long axis and the direction from source to drain contact electrodes in the transistor device. This angle was used as the baseline to evaluate the crystal misorientation induced mobility variations. When the smallest angle θ corresponded to the crystal orientation being parallel with the source/drain electrode direction, the largest hole mobility of 0.04 cm² V⁻¹ s⁻¹ was measured from the TIPS pentacene based transistors. When the crystal orientation bridged the source/drain electrode direction at the largest angle θ, the smallest mobility of 0.004 cm² V⁻¹ s⁻¹ was obtained, which showed that the mobility dropped by 10 fold. Similarly, such crystal misorientation has been reported in two other p-type organic semiconductors, i.e. 5,6,11,12-tetrachlorotetracene¹⁰⁰ and 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),¹⁰¹ when grown from a single chloroform solvent via the drop casting method.

Besides, the charge transport in the organic semiconductors may be impeded by large-scale thermal cracks, which occur after the semiconductors are exposed to elevated temperature. Chen et al. reported that when TIPS pentacene was heated at 150 °C, the organic semiconductors started to show thermal cracks,¹⁰² which was linked to the lower surface energy in various planes of the semiconductor. While pristine TIPS pentacene based transistors exhibited a hole mobility of up to 1 cm² V⁻¹ s⁻¹ in this study, the counterpart with the occurrence of thermal cracks showed a much lower hole mobility of up to 0.2 cm² V⁻¹ s⁻¹, which counts for a significant reduction in mobility by 80%. The generation of thermal cracks in TIPS pentacene was also reported in a later study by Asaree-Yeboah et al. who applied a temperature gradient technique in order to align the crystal growth direction of TIPS pentacene.¹⁰³ These thermal cracks extended to a few hundred micrometers, posing a barrier for efficient charge transport in TIPS pentacene. Despite exerting a blocking effect on charge transport, thermal cracks at the domain boundaries have been reported to generate deep electron traps and enhance the stability of TIPS pentacene based thin film transistor photo responsivity. Cho et al. reported that thermal annealing induced cracks at the domain boundary of TIPS pentacene films and related both the stable photoresponsive characteristics and lower charge carrier mobility to the corresponding deep electron traps.¹⁰⁴

Moreover, the crystal growth of organic semiconductors can result in small grain widths.^105–109 Because crystalline defects and charge trap sites are prevalent at the grain boundaries, small grain widths can be generally associated with a greater amount of defects and traps, which charge carriers must circumvent along the charge transport pathway.^110–120 If we assume that the channel length L is equal to the length of crystals, or L_G, plus the length of crystal boundary, or L_GB, then we have:

L = (n − 1)L_GB + nL_G (1)

where n is the total number of crystals in the channel. If we denote the effective total hole mobility as μ_g and the mobility at the grain boundaries as μ_GB, μ_g can be correlated to μ_GB based on the following equation:^121–123

The length of crystals can be correlated with the width of grains, or W_G, via the following equation:

By assigning and , we have:

Eqn (6) shows a clear correlation between the width of grains and the effective mobility, which states that a small grain width from the solution-processed organic semiconductors can result in lower effective mobility.

2. Benefits of polymer-modulated crystallization

Miscellaneous polymer additives have been extensively studied for mixing with solution processable, small molecular organic semiconductors.^124–129 When the polymer additives are mixed with semiconducting small molecules, the resultant binary system can benefit from improved semiconductor morphology uniformity,^130–134 phase segregation-enhanced charge transport^135–139 and operational device stability.^140–147 In this section, we will briefly discuss the benefits of mixing organic semiconductors with different categories of polymer additives, including amorphous polymers, conjugated polymers and semicrystalline polymers.

2.1. Amorphous polymers

Mixing organic semiconductors with amorphous polymers can help improve the thin film morphology uniformity and, more importantly, can promote the formation of orientated crystals and reduce crystal misorientations.^148,149 For example, pristine TIPS pentacene has been reported to form random crystal ribbons or dendritic patterns of crystals by multiple groups.^150–152 The mixing of TIPS pentacene with amorphous polymer, such as poly(α-methylstyrene) (PαMS), can significantly reduce crystal misorientation.¹⁵³ He et al. reported that when drop cast in the solvent of toluene, the TIPS pentacene pristine film exhibited a large misorientation angle of 43.9° ± 27.8°, whereas the addition of PαMS with TIPS pentacene strongly prevented such random crystal morphology and reduced the misorientation angle to 2.2° ± 1°.³⁰

Another major benefit that rises from blending organic semiconductors with amorphous polymers is the occurrence of phase segregation between the organic semiconductor and polymer additive.^154–160 This phase segregation can include lateral phase segregation, vertical phase segregation, or a mixture of both. In particular, vertical phase segregation between the organic semiconductor and amorphous polymer can facilitate the formation of a more concentrated semiconductor layer at the interface between the active layer and the dielectric layer, expediting charge transport of the organic semiconductor. Also, a polymeric encapsulation layer that forms as a result of the vertical phase segregation can protect the semiconductor active layer from being exposed to the ambient environment, enhancing the operational stability of the thin film transistors.^161,162

2.2 Conjugated polymers

When organic semiconductors are blended with conjugated polymers, the binary system can give rise to intermolecular interactions between these two components.^163–168 For example, Chen et al. reported that when two different types of conjugated polymers, including P3HT and regiorandom pentacene–bithiophene polymer (PnBT–RRa), were mixed with TIPS pentacene, the resultant binary system exhibited tunable intermolecular interactions that arise from both π–π interactions and hydrophobic interactions. The different extent of molecular structural similarity between TIPS pentacene and the conjugated polymer additives determines the intensity of intermolecular interactions. A small structural similarity between TIPS pentacene and P3HT yielded less intimate mixing and intermolecular interactions, leading to a polymorph with greater changes in the cell parameters. A higher similarity between TIPS pentacene and PnBT–RRa resulted in more intimate mixing, stronger interactions, and a polymorph with smaller changes in the cell parameters. Such differences further modulate the thin film morphology and confine charge transport in the organic semiconductor.⁸⁹ Depending on the different types of conjugated polymers and extent of intermolecular interactions, the TIPS pentacene crystalline morphology can range from straight crystal needles to grass-like ribbons. Simultaneously, the conjugated polymer may exhibit cocrystallization along with the organic semiconductor. Thereby, each conjugated polymer yielded distinct modes of crystallization behavior and charge transport.

2.3 Semicrystalline polymers

When compared to the amorphous and conjugated counterparts, semicrystalline polymers^169–175 can preserve their own nucleation and crystallization events, which are independent of and also competing with the nuclei formation and crystal growth of the organic semiconductors. In the meanwhile, semicrystalline polymers are still partially amorphous, allowing the modification of diffusivity and surface energies of the organic semiconductor facets similar to amorphous polymers. Unlike conjugated polymers, semicrystalline polymers eliminate the polymer-caused π–π interactions as described above, which may be a complicated process unwanted in some organic semiconductor applications. Thereby, semicrystalline polymers provide a novel pathway to guide organic semiconductor crystallization and tune its morphology from an unexplored perspective.

3. Overview of polymers with varying molecular weights

In this section, we will provide an overview of the important studies that report mixing organic semiconductors with polymeric additives with different molecular weights, in order to tune the crystal growth, thin film morphology, phase segregation, and charge transport. These polymer additives will be discussed in the following category: PαMS, PS, polymethyl methacrylate (PMMA) and polyethylene oxide (PEO); their molecular structures are shown in Fig. 1(a–d). Fig. 1(e–h) show the molecular structures of the various organic semiconductors mixed with the polymer additives in this section.

Fig. 1 Molecular structures of the various polymer additives and small-molecular organic semiconductors discussed in this work, including (a) polystyrene (PS), (b) poly(α-methylstyrene) PαMS, (c) polymethyl methacrylate (PMMA), (d) polyethylene oxide (PEO), (e) poly(3-hexylthiophene) (P3HT), (f) 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C₈-BTBT), (g) 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS pentacene), and (h) 2,8-difluoro-5,11-bis(triethylsilylethynyl)anthradithiophene (diF-TES-ADT).

3.1 Polystyrene with varying molecular weights

PS is an amorphous and thermoplastic polymer that finds extensive applications in miscellaneous semi-finished products such as plastic films and foams. PS has excellent electrical insulation properties and outstanding optical clarity as a result of its lack of crystallinity. Besides, PS is chemically resistant to diluted bases and acids, but can be attacked by hydrocarbon solvents and UV exposure. The stiffness of the PS polymer backbone results in brittle nature and low impact strength. Furthermore, PS has a low limit of upper temperature for continuous processing. When mixed with organic semiconductors, PS typically causes pronounced vertical phase segregation,^129,176 enhances semiconductor wettability,¹⁷⁷ modulates active layer structure and polymorph,¹⁷⁸ reduces charge interfacial traps,¹⁷⁹ and improves charge transport reproducibility.¹⁸⁰

Leonardi et al. reported the effect of PS additive with different molecular weights, including 3 K and 100 K, on the vertical phase segregation, thin film morphology and stability of a donor–acceptor random copolymer, i.e. PDPP-TT(1)-SVS(9).¹³⁷ AFM images in Fig. 2 show a vertical phase segregation with a bottom PDPP-TT(1)-SVS(9) layer beneath a top PS layer, which could serve as an encapsulation layer and improve the stability of the transistor device. In particular, the PDPP-TT(1)-SVS(9)/PS blend film with the 3 K molecular weight exhibited a non-homogeneous surface, whereas the counterpart with the 100 K molecular weight showed a thicker PS layer with the existence of holes, which resulted from the increased viscosity of the blend solution. PDPP-TT(1)-SVS(9) based transistors with the addition of PS with 3K and 100K molecular weights demonstrated average mobilities of 0.22 cm² V⁻¹ s⁻¹ and 0.10 cm² V⁻¹ s⁻¹, respectively. The enhanced charge transport was a direct benefit from the passivation of surface hydroxyl groups by the vertically-segregated PS polymer additive.

Fig. 2 AFM images showing the topography of the PDPP-TT(1)-SVS(9)/PS mixture film, with different molecular weights of PS including (a) 3 K and (b) 100 K. Reproduced from ref. 181, with permission from Elsevier.

Niazi et al. studied the impact of PS additives with different molecular weights on the phase separation, composition and domain size of the organic semiconductor diF-TES-ADT.¹⁸² PS with different molecular weights including 2.2 K and 900 K was mixed with diF-TES-ADT. The mixture was deposited by using the blade coating method with a direction parallel with the source-to-drain electrodes. The thin film morphology of neat diF-TES-ADT and the diF-TES-ADT/PS mixture film is shown in the polarized optical images and AFM images in Fig. 3. It can be inferred that the film morphology and domain size depend on the molecular weight of PS. Besides, both phase segregation and active layer compositions were found to correlate to the molecular weight of PS. In particular, PS with a high molecular weight of 900 K caused the formation of a vertically-segregated bilayer structure: a top diF-TES-ADT layer and a bottom PS layer. Conversely, PS with a low molecular weight of 2.2K led to a mixture of diF-TES-ADT and PS in the top section. A thin film transistor incorporating the diF-TES-ADT/PS mixture as the active layer showed a mobility of up to 6.7 cm² V⁻¹ s⁻¹ based on the high molecular weight PS, which is 50-times higher than that based on the low molecular weight counterpart. The high performance of the transistor devices was attributed to the single crystalline diF-TES-ADT layer with large domains as a result of vertical phase segregation.

Fig. 3 Polarized optical images and corresponding AFM images of (a) neat diF-TES-ADT, (b) diF-TES-ADT blended with PS with a low molecular weight of 2.2 K, and (c) diF-TES-ADT blended with PS with a high molecular weight of 900 K. The direction of blade coating was parallel with the direction from the source to drain contact electrodes. Reproduced from ref. 182, with permission from Springer Nature.

Haase et al. reported the addition of PS with different molecular weights in order to reduce the device-to-device mobility variation of the organic semiconductor C₈-BTBT.¹⁸³ Different molecular weights of the PS polymer including 2 K, 20 K, 200 K and 2000 K were tested on the thin film morphology of the organic semiconductor. PS with a low molecular weight resulted in a spherulitic growth of solution-sheared C₈-BTBT films, whereas PS with a larger molecular weight yielded C₈-BTBT ribbons with enhanced film coverage, as shown in the microscopic optical images in Fig. 4. Accordingly, the highest mobility of 14.4 cm² V⁻¹ s⁻¹ was observed with PS with a molecular weight of 200 K, at an optimized blending ratio of 1 : 1. Finally, distribution of saturation mobility was studied for thin film transistors based on neat C₈-BTBT and the C₈-BTBT/PS mixture in a 1 : 1 ratio. The addition of the PS polymer resulted in a much narrower distribution of mobility as well as reduced standard deviation.

Fig. 4 Microscopic images of solution-sheared C₈-BTBT films from blends based on different PS molecular weights, including 2 K, 20 K, 200 K and 2000 K. Reproduced from ref. 183, with permission from Wiley.

Singsumphan et al. studied the effect of PS molecular weight on the polymer viscosity and drop-cast thin film morphology of organic semiconductor 9,10-bis-2-[tris(1-methylethyl)silyl]ethynyl anthracene (TIPS-anthracene).¹⁸⁴Fig. 5 shows the optical images and fluorescence images of drop cast TIPS-anthracene based on mixing with PS of different molecular weights including 13 K, 35 K and 280 K, respectively. For PS with a low molecular weight of 13 K, the mixed TIPS–anthracene exhibited a crystalline morphology of elongated needles. For PS with an intermediate molecular weight of 35 K, the needle shaped TIPS–anthracene crystals adopted alignment in different directions, indicating greater randomness of the thin film morphology. For PS with a high molecular weight of 280 K, the needle-like TIPS–anthracene crystals further suffered from reduced substrate coverage, which was caused by the high viscosity of PS and poor processability. This study indicated that tuning the polymer viscosity by employing polymer additives with different molecular weights can be a pathway to regulate the thin film morphology of organic semiconductors.

Fig. 5 (a–c) Optical images and (d–f) fluorescence images of TIPS–anthracene/PS blend films based on different PS molecular weights including 13 K, 35 K and 280 K. The scale bar is 200 μm. Reproduced from ref. 184, with permission from Elsevier.

Rocha et al. studied the mixing of TIPS pentacene with PS with different molecular weights, including 2 K, 20 K, 200 K and 2000 K, to improve the binary system's miscibility and charge transport.¹⁸⁵ At a mixing ratio of 1 : 1, the TIPS pentacene/PS mixture was deposited by using a solution shearing method. The resultant crystalline film exhibited different morphologies, dependent on different molecular weights of the PS polymer additive and different solution shearing speed. In particular, highly anisotropic crystalline ribbons and more isotropic spherulitic crystals were obtained based on low and high solution shearing speed, respectively. Electrical characterization results indicated that PS with a higher molecular weight yielded a higher charge carrier mobility as compared to the counterpart with 2 K molecular weight. The low mobility from low molecular weight PS can be attributed to the higher miscibility between PS and TIPS pentacene, preventing the phase segregation and the growth of highly crystalline TIPS pentacene ribbons. On the other hand, PS with a molecular weight larger than 20 K was observed to induce a phase segregation between TIPS pentacene and PS. The highest mobility of 12.3 cm² V⁻¹ s⁻¹ was demonstrated based on the TIPS pentacene/PS mixture with a 20K molecular weight.

Han et al. reported the effect of different PS molecular weights on the response of P3HT based gas sensors.¹⁸⁶ Two types of PS, including isotactic semicrystalline PS with molecular weights of 1.3 K, 250 K, 280 K and 290 K and amorphous PS with a molecular weight between 130 K and 290 K, were mixed with P3HT at a 4 : 1 ratio to form an active layer of the thin film transistors. When the P3HT based transistor devices were exposed to 10 ppm NH₃ gas, the gas sensing response increased as the molecular weight of isotactic semicrystalline PS increased. In contrast, a much lower sensing response was observed for the counterpart based on the amorphous PS polymer additive. The enhanced sensing performance by the isotactic semicrystalline PS with a larger molecular weight was attributed to the formation of vertically-segregated, more continuous P3HT microstructures, which provide an undisrupted pathway for the transport of charge carriers in the transistor device. A mobility of 0.03 cm² V⁻¹ s⁻¹ was reported from the P3HT/PS based transistors.

3.2 Poly(α-methylstyrene) with varying molecular weights

PαMS is an amorphous polymer with outstanding hardness, heat resistance and solvent resistance properties.¹⁸⁷ While PαMS in general finds good solubility in both aromatic and halogenated solvents, only PαMS with a low molecular weight can be soluble in methyl ethyl ketone. The physical properties of PαMS such as solvent resistance depend on the amount of monomer and low molecular weight polymer in the compositions. Due to its relative brittleness and lower thermal stability (as compared to PS), the molecular weight of PαMS should be kept at a minimum value to enable any molding and extrusion processes to occur. As a result, it is desirable to set the molecular weight of PαMS at a lower limit, in order to render the polymer more useful physical properties. In the field of organic electronics, PαMS has been extensively studied for mixing with various organic semiconductors in order to improve the uniformity of thin film morphology,¹⁸⁸ modulate semiconductor wettability,¹⁸⁹ reduce crystal misorientation,^30,190 control the formation of thermal cracks,¹⁹¹ enhance charge carrier mobilities,^192,193 recover electrical performance,¹⁹⁴ and improve the thin film transistor device stability.¹⁹⁵

Ohe et al. reported the impact of different molecular weights of the polymeric additive PαMS on phase segregation, thin film morphology and charge carrier mobility of the organic semiconductor TIPS pentacene.¹⁹⁶ The different molecular weights of the polymeric additive PαMS included 2 K, 20 K, 60 K, 100 K and 800 K. Source and drain contact electrodes were prepatterned on the substrate covered with a cross-linked poly(vinyl phenol) gate dielectric layer. Then PαMS was mixed with TIPS pentacene at a 1 : 1 weight ratio in solution before being spin coated onto the source and drain electrodes. Electrical characterization results showed while the addition of PαMS with a low molecular weight of 2 K resulted in a low mobility, PαMS with a molecular weight above 20 K yielded a much higher mobility of 0.1 cm² V⁻¹ s⁻¹. To study the composition in the vertical profile of the organic semiconductor film, time-of-flight secondary ion mass spectrometry was conducted on the TIPS pentacene/PαMS blends. The secondary Si⁻ ions and SiH⁻ ions were used to mark the existence of TIPS pentacene. The addition of a PαMS additive with a molecular weight of 2 K led to a homogeneous distribution of TIPS pentacene, whereas PαMS with a high molecular weight of 100 K promoted a vertical phase segregation between the polymer additive and TIPS pentacene. Furthermore, the polarized optical microscope images showed that the PαMS additive with a high molecular weight of 100 K enlarged the crystalline region on the surface. Flory–Huggins theory, which is indicative of the Gibbs free energy change upon mixing, was applied to understand phase segregation between TIPS pentacene and PαMS. Negative Gibbs free energy of mixing implies homogeneous mixing, whereas positive Gibbs free energy of mixing indicates that phase segregation can more easily occur. The low molecular weight PαMS resulted in a negative value of ΔG_m, indicating it is more likely for the blend film to undergo a homogeneous mixing. In contrast, as the high molecular weight PαMS yielded a positive value of ΔG_m, the vertical phase segregation between the polymer additive and organic semiconductor was promoted.

Kang et al. reported the impact of PαMS with different molecular weights on the TIPS pentacene phase segregation behavior and compositional structure by employing the neutron reflectivity method.¹⁹⁷ Two types of PαMS with different molecular weights at 1.3 K and 575 K were blended with TIPS pentacene at a 1 : 1 weight ratio before being spin coated on the substrate to form an active layer. After this, thermal annealing at 90 °C was conducted on the active layer. Neutron reflectivity profiles of TIPS pentacene/PαMS blend film are presented in Fig. 6(a and b) based on the different PαMS molecular weights. The addition of PαMS with a lower molecular weight of 1.3 K was observed to induce a vertically segregated film structure, which is composed of an almost pure TIPS pentacene top layer at the air surface, a PαMS rich layer in the middle section, and a TIPS pentacene rich bottom layer at the substrate interface. The thicknesses from top to bottom layer corresponding to the lower molecular weight PαMS were 177 Å, 326 Å and 70 Å, respectively. In contrast, the mixing of PαMS with a higher molecular weight of 575 K induced a similar vertically segregated film but with different compositions. The thicknesses of the top and bottom TIPS pentacene rich layer based on the higher molecular weight PαMS changed to 134 Å and 117 Å, respectively. To understand the effect of thermal annealing on the crystallinity of the film, grazing-incidence X-ray diffraction (GIXD) was conducted based on PαMS with a molecular weight of 575 K. The GIXD spectra presented in Fig. 6(c and d) showed that thermal annealing resulted in a highly crystalline film with desired crystal orientations, giving rise to a saturation mobility of 0.54 cm² V⁻¹ s⁻¹.

Fig. 6 Neutron reflectivity profiles of the TIPS pentacene/PαMS blend film with different PαMS molecular weights of (a) 1.3 K and (b) 575 K, before and after thermal annealing at 100 °C. The insets show the fitted concentration profiles. GIXD patterns of the TIPS pentacene/PαMS blend film with a molecular weight of 575 K, measured (c) after spin coating and (d) after thermal annealing at 90 °C. Reproduced from ref. 197, with permission from American Chemical Society.

3.3 Poly(methyl methacrylate) with varying molecular weights

PMMA is a thermoplastic polymer with optical transparency. PMMA shows outstanding properties such as high impact strength, scratch resistance, and lightweight. The incorporation of an adjacent methyl group in the PMMA structure inhibits close packing and free rotation, which results in an amorphous nature of PMMA.¹⁹⁸ PMMA has extensive applications such as in optics, polymer viscosity and biomedical engineering, and also finds important implementation in nanotechnology because of its easy processing and good compatibility with inorganic materials such as carbon nanotubes.¹⁹⁹ As one of the most studied polymers for blending with organic semiconductors, PMMA demonstrates exceptional capability for enhancing semiconductor growth,²⁰⁰ film crystallinity,²⁰¹ grain size,²⁰² phase segregation,²⁰³ device air stability¹⁹⁵ and charge transport.²⁰⁴ Besides, PMMA also shows excellent gate dielectric properties when employed as the polymer gate insulator material for thin film transistors^205,206 and gas sensors.²⁰⁷

Kim et al. studied how different molecular weights of PMMA impacted the polymorphism and charge transport of a C₈-BTBT organic semiconductor.²⁰⁸ PMMA with different molecular weights including 15 K, 120 K, 350 K, and 996 K were mixed with C₈-BTBT before spin coating the blends for crystal growth using a solvent vapor annealing method. As shown in the polarized optical microscopic images in Fig. 7, the crystallization and resultant dimensions of the C₈-BTBT rods, in terms of rod height, width and length, were observed to be dependent on the molecular weight of PMMA. In particular, the rod dimension becomes larger as the PMMA molecular weight increases. This was attributed to the reduced polarity of higher molecular weight PMMA, which allowed the free migration of C₈-BTBT on the PMMA surface to form enlarged dimensions. Electrical characterization results of C₈-BTBT/PMMA (molecular weight 120K) based thin film transistors indicated that the source-drain currents exhibited a tremendous increase by 3–4 orders of magnitude as the device was applied to thermal cycling. This was due to the increased resistivity of C₈-BTBT induced by the varying temperatures, at which point the C₈-BTBT molecules become oriented downward the substrate. A mobility of 15.6 cm² V⁻¹ s⁻¹ was obtained when heating was applied to the C₈-BTBT/PMMA blend based thin film transistors.

Fig. 7 Polarized optical microscopic images of the crystallization of C₈-BTBT rods on PMMA with various molecular weights including (a) 15 K, (b) 120 K, (c) 350 K, and (d) 996 K. Reproduced from ref. 208, with permission from Wiley.

3.4 Polyethylene oxide with varying molecular weights

Polyethylene oxide (PEO) finds important applications in pharmaceutics and is compatible with pharmaceutical dosage due to its thermoplastic behavior. When combined with other polymers, PEO can be utilized to realize controllable drug release.²⁰⁹ In particular, PEO with a high molecular weight is suitable for designing melt-extruded matrices with extended release. On the other hand, PEO is a semicrystalline polymer exhibiting unique crystallization behavior in stark contrast to the conjugated and amorphous counterparts. PEO can eliminate the potential impact of conjugated polymer induced π–π interactions and cocrystallization on the target organic semiconductor,³⁸ which is not a feasible process to optimize for simplified device fabrication procedures. With that said, PEO retains the amorphous nature which helps improve the semiconductor diffusivity and surface energies. The semicrystalline PEO is known for its own crystallization, which is independent of but competing with the crystal growth of the organic semiconductor. These traits of PEO uniquely offer a pathway to modulate solution-based organic semiconductor crystallization from a whole new perspective.

He et al. studied the effect of semicrystalline polymer additive PEO with different molecular weights including 8 K and 100 K on the crystallization and charge transport of organic semiconductor TIPS pentacene.²¹⁰ With the selection of 8K and 100K molecular weights, the PEO polymer with both light and heavy chain entanglements and its impact on modulating semiconductor crystallization can be compared. PEO was mixed with TIPS pentacene at a 1 : 1 weight ratio before the mixture was deposited to form the active layer via drop casting. Fig. 8(a–d) show the morphology of the TIPS pentacene/PEO (molecular weight of 8 K) crystals based on different sections of the same substrate. The charge transport direction in the organic crystals is highlighted by red arrows. The dark regions observed in Fig. 8 are the formation of PEO crystals, as a result of its own nucleation and crystallization process. The crystal dimensions of PEO crystals are typically under 20 microns, which is consistent with the observation in Fig. 8. In comparison, TIPS pentacene crystals exhibit clear longitudinal dimension extending to a few hundred of microns and also crystal orientation-induced color variations in the crystalline domains under a polarized light microscope. It can be referred that the TIPS pentacene crystals were aligned by the addition of the PEO polymer with a lower molecular weight of 8 K. The effects on modulating the TIPS pentacene crystallization can be attributed to the partially amorphous nature of PEO, which tunes the semiconductor diffusivity and surface energy of faceted evolution.

Fig. 8 (a–d) Four polarized microscope images of TIPS pentacene crystals with the PEO polymer additive (molecular weight of 8 K). The charge transport direction in the TIPS pentacene/PEO crystals is highlighted by the red arrows. The substrate uncovered by TIPS pentacene/PEO crystals is marked by the yellow triangles. The scale bar shown in (a) applies to all images. Reproduced from ref. 210, with permission from Springer Nature.

In contrast, blending TIPS pentacene with a PEO polymer with a higher molecular weight of 100 K resulted in a distinctively different thin film morphology as shown in the optical images in Fig. 9(a–d). A PEO polymer with a 100 K molecular weight did not benefit TIPS pentacene from reducing random crystal orientations, as indicated by the misoriented red arrows in various directions. Additionally, the film was composed of a top TIPS pentacene/PEO crystal layer as well as a bottom base layer, suggesting strong vertical phase segregation. The blending of PEO with a higher molecular weight likely enhances the interaction between the polymer hydroxyl groups and the silicon dioxide layer on the substrate, further attributing to the phase segregation as observed in the optical images in Fig. 9. Although no obvious crystal alignment was observed, the PEO additive with a molecular weight of 100K showed a more pronounced effect on promoting film formation and increasing coverage. TIPS pentacene/PEO mixture film was further used as the active layer for fabricating bottom-gate, top-contact thin film transistors. The addition of PEO with a lower molecular weight of 8 K resulted in a mobility of 0.017 cm² V⁻¹ s⁻¹, which is 5-fold larger as compared to that based on PEO with a molecular weight of 100 K.

Fig. 9 (a-d) Four polarized optical microscope images of TIPS pentacene crystals with the PEO polymer additive (molecular weight of 100 K). The charge transport direction in the TIPS pentacene/PEO crystals is highlighted by the red arrows. The uncovered substrate is marked by the yellow triangle. The scale bar shown in (a) applies to all images. Reproduced from ref. 210, with permission from Springer Nature.

The different studies reviewed in this section are summarized in Table 1, which lists the authors, polymer (with its molecular weight), semiconductor, result and mobility.

Table 1 Summary of the reviewed studies, including the authors, polymer (with its molecular weight), semiconductor, result and mobility

Authors	Polymer	Semiconductor	Result	Mobility
Leonardia et al.	PS (3 K, 100 K)	PDPP-TT(1)-SVS(9)	PS resulted in vertical phase segregation and formed a top encapsulation layer; thickness of the top layer depended on PS molecular weight	0.22 cm^2 V^−1 s^−1
Niazi et al.	PS (2.2 K, 900 K)	diF-TES-ADT	PS molecular weight impacted film morphology, phase segregation and compositions	6.7 cm^2 V^−1 s^−1
Haase et al.	PS (2 K, 20 K, 200 K, 2000 K)	C8-BTBT	Increase in molecular weight of PS resulted in spherulitic to ribbon-like morphology	14.4 cm^2 V^−1 s^−1
Singsumphan et al.	PS (13 K, 35 K, 280 K)	TIPS-anthracene	PS of different molecular weights induced different thin film morphologies due to difference in viscosity and processability	Not reported
Rocha et al.	PS (2 K, 20 K, 200 K, 2000 K)	TIPS pentacene	Different PS molecular weights impacted intermixing, phase segregation, morphology and charge transport	12.3 cm^2 V^−1 s^−1
Han et al.	PS (1.3 K, 250 K, 280 K and 290 K)	P3HT	PS with a high molecular weight resulted in a more continuous P3HT microstructure, enhancing gas sensor response	0.03 cm^2 V^−1 s^−1
Ohe et al.	PαMS (2 K, 20 K, 60 K, 100 K, 800 K)	TIPS pentacene	PαMS with a high molecular weight resulted in a positive Gibbs free energy upon mixing and vertical phase segregation	0.1 cm^2 V^−1 s^−1
Kang et al.	PαMS (1.3 K and 575 K)	TIPS pentacene	PαMS with a high molecular weight enhanced vertical phase segregation and induced a thicker TIPS pentacene at the charge transport interface	0.54 cm^2 V^−1 s^−1
Kim et al.	PMMA (15 K, 120 K, 350 K, and 996 K)	C8-BTBT	Crystal dimensions of C8-BTBT increased with molecular weight of PMMA; thermal cycling tunes resistivity and thereby charge transport of C8-BTBT	15.6 cm^2 V^−1 s^−1
He et al.	PEO (8 K, 100 K)	TIPS pentacene	PEO with 8 K molecular weight improved crystal alignment, whereas PEO with 100 K increased film coverage	0.017 cm^2 V^−1 s^−1

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4. Conclusions and outlook

In this article, we have reviewed the influence of polymer additives with different molecular weights on the crystallization, vertical phase separation, compositional structure, thin film morphology and charge carrier mobility of solution-processed organic semiconductors. We first discussed the general benefits of mixing organic semiconductors with amorphous polymers, conjugated polymers, and semicrystalline polymers, respectively. Then, we reviewed several extensively studied amorphous polymers PS, PαMS and PMMA, as well as semicrystalline polymer PEO, in the context of small-molecular organic semiconductors such as TIPS pentacene, C₈-BTBT and diF-TES-ADT. In particular, PS with different molecular weights impacted solution viscosity, processability, solute intermixing, phase segregation, crystal morphology and active layer compositions. PαMS with different molecular weights influenced Gibbs free energy upon intermixing, phase segregation, semiconductor layer thickness and charge transport. PMMA with different molecular weights led to different film interconnection, grain boundary, crystal dimension and charge transport. PEO with different molecular weights affected crystal alignment, film coverage, and charge carrier mobility. Using these representative organic semiconductor and polymer additive examples, we demonstrate in this work an easy and useful pathway to manipulate the charge transport of solution-processable organic semiconductors more effectively by tuning the molecular weight of the polymer additive component.

While current research is underway in many aspects of organic electronics, we believe future endeavors that focus on these following topics will likely yield exciting discoveries. First, chemical engineering of the polymeric structures is a useful way to tune the molecular weight but at the same time add new properties to the polymers. For instance, side chain engineering can bestow solubility in organic solvents,^211,212 regulate molecular packing,^213,214 and modulate intermolecular interactions.^215,216 Second, polymers with both a similar molecular weight and a similar molecular structure, but with different side chain lengths, present another way to tune molecular stacking,^217,218 thin film morphology,^219,220 and charge transport.^221,222 Third, polymers can be deposited as an additional dielectric layer which helps tune the dielectric properties,^223,224 eliminate substrate hydroxyl groups,²²⁵ reduce surface roughness,²²⁶ enable low-voltage operation,²²⁷ enhance film topography,²²⁸ control polarizability,²²⁹ and improve charge carrier mobility.^230,231 Fourth, while current polymer mixing studies are predominately carried out on p-type organic semiconductors, the n-type counterparts are equally important to realize the fabrication of electronic devices such as complementary inverts and logic circuits.^232,233 Mixing these polymer additives with n-type organic semiconductors can provide a more effective approach to control semiconductor crystallization, improve electron charge transport, and modulate surface passivation.^234,235 Future research in these aforementioned areas may shed light on further improving the overall electronic device performance.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

S. Bi would like to thank Science and Technology Project of Liaoning Province (20180540006).

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