Grain boundary engineering of organic semiconductor films in organic transistors

Organic field‐effect transistors (OFETs) show great application potential in organic electronic and optoelectronic fields due to their excellent mechanical flexibility, low cost, and solution processing. However, grain boundaries (GBs) disrupt the aggregation state of organic semiconductor (OSC) films and hinder electrical performance and stability, which limits the application of OFETs. Besides, the sensitive nature of GBs is widely used in sensing, but detailed descriptions of the GBs are scarce. This review aims to fill this knowledge gap. The role of GBs and their effect on the performance and stability of OFETs are analyzed, followed by a detailed summary of the characterization of GBs. Then, strategies for suppressing the negative effects of GBs and utilizing the sensitive nature of GBs for application are proposed. Finally, potential research directions for GBs in OFETs are discussed.


INTRODUCTION
20][21][22][23][24][25][26][27] The OSC active layers are mainly prepared by vacuum thermal evaporation and solution processing.The prepared OSC films are usually polycrystalline due to the weak van der Waals forces between OSC molecules and the limitations of processing techniques.[30][31] Therefore, the presence of GBs disrupts the aggregation state of OSC films.As a crystal structural defect, GBs can lead to the formation of localized electronic

THE ROLE OF GBS IN OSC FILMS
GBs are interfaces between grains with the same structure but different orientations (Figure 1A), which are widely present in OSCs, perovskites, inorganic non-metallic mate-rials etc. [41][42][43][44] In contrast to the internal molecules of the grain, the molecules at the GBs are irregularly arranged and deviate from the equilibrium position.Therefore, the properties of GBs determine the mechanical properties, stability, electrical performance, and other important characteristics of the materials.As a structural defect within OSC films, GBs can lead to the localization of carriers.However, it is still controversial whether the local properties caused by GBs act as traps or potential barriers during the transport of carriers.One view is that GBs act as traps for carrier transport.For p-type OSCs, it is reported that GBs generate donor-like states near the highest occupied molecular orbital (HOMO) in the band gap at a certain temperature.If these states are close to the HOMO edge, they are shallow traps (a few k B T).If these states are far from the HOMO edge, they are deep traps (several k B T), where k B is Boltzmann's constant and T is the temperature (Figure 1B).These traps can temporarily capture carriers until they are released by external stimuli such as electric fields, heat, or photons. [45]In the multiple-trap and release (MTR) model, the holes are captured at the shallow traps generated by GBs or other defects during transport, and then released into the valence band by external stimuli continue to participate in carrier transport.If holes are captured by deep traps, they require more energy to return to the valence band.In addition to GBs, there are many factors that can generate traps, such as water and oxygen in the environment. [45]If there are too many traps within the OSC films, it is difficult for the captured holes to return to the valence band.Holes can only participate in charge transport processes by hopping and tunnelling between traps, which leads to the hopping model (Figure 1C).The trap model is better to describe the mobility variation data independent of gate voltage (V G ). Street et al. used the trap model to qualitatively and quantitatively explain the data in pentacene-based OFETs. [46]In the trap model, shallow and deep traps generated by GBs often lead to different effects.[49] However, Zhang  et al. and Xie et al. suggested that deep traps also affect the mobility. [50,51]There is no clear and exact boundary between shallow and deep traps, and there is usually a transition region (with a width of a few k B T). [48] Another view is that GBs are potential barriers for carrier transport.It is reported that GBs not only capture carriers but also repel other carriers with the same sign, forming a spacecharge region (SCR) with a built-in potential at the GBs.For p-type OSCs, GBs can capture holes, and thus produce a downward local band bending, whose degree depends on the height of potential barrier (E B ) of SCR. [52,53]The carriers around the local bended band can be thermally activated to participate in the carrier transport through the potential barriers (Figure 1D).The typical feature of the barrier model is that the mobility increases with the increase of V G , as V G can reduce the E B , thereby promoting carrier transport.Horowitz and Hajlaoui observed that the E B depends not only on V G but also on the size of the grains.They argued that it depends on the relationship between grain size(L) and Debye length (L D ).If L > 2L D , the transport behaviour of carriers only depends on the local E B at the GBs, where carriers are thermally activated at high temperatures and tunnelling dominates at low temperatures.When L < L D , the defects are uniformly distributed on the OSC films.At this time, the E B at the GBs is significantly reduced. [54]Matsubara et al. used the diffusion theory of carriers to modify Horowitz and Hajlaoui's barrier model.The E B and mobility of pentacene-based OFETs were calculated to be 150 meV and 1.0 cm 2 /(V⋅s), respectively.The mobility was lower than the calculated result, indicating that GBs are not the only influencing factor, there are other factors that limit the transport of carriers. [55]Verlaak et al. considered the doping of OSCs to modify the barrier model.They concluded that a higher concentration of dopants could reduce the area of the SCR and effectively shield the charge of the GBs, thereby reducing the E B and improving the mobility of the devices.They also demonstrated that the high mobility reported for many pentacene-based OFETs is due to unintentional doping rather than the formation of a favourable film morphology. [56]Schon et al. also revised the barrier model by suggesting that E B is not only determined by the V G but also the trap density and the position of Fermi level. [57]Therefore, in the barrier model, many factors such as V G , grain size, carrier concentration, trap density, and dopant concentration together control the E B .

EFFECT OF GBS ON THE PERFORMANCE AND STABILITY OF OFETS
As a structural defect within OSC films, GBs have an impact on the performance and stability of OFETs.Firstly, GBs cause a shift from band-like transport in organic single crystals to thermally activated transport in polycrystalline OSC films.Secondly, the electrical performance is severely limited.37]58]

Transport mechanism
In inorganic semiconductors, atoms are bound by covalent bonds.The electron orbitals of the bonded atoms overlap and split into consecutive energy bands.The carriers move as plane waves in the energy band, which are mainly affected by the lattice scattering.An increased temperature could enhance lattice scattering and reduce the transfer capacity and mobility.The typical feature of the band transport is that the mobility gradually decreases with increasing temperature.For OSCs, the molecules are connected by weak van der Waals interactions.Highly conjugated and tightly arranged organic single crystals are capable of forming separated and narrow energy bands.Therefore, the band-like transport can be observed in OSC single crystals.However, the presence of a large number of GBs in polycrystallineOSC films disrupts the highly conjugated structure, leading to thermally activated transport.Whether GBs act as traps or potential barriers, carriers can acquire more energy to promote transport as the temperature increases.Consequently, the presence of traps (such as GBs) in polycrystallineOSC films leads to a shift of the transport mode.[3,2-b]thiophene (DNTT) films.They found that the intrinsic carrier transport within the grains in the polycrystalline OSC films is band-like transport.The decrease in Hall mobility (μ Hall ) of polycrystalline OSC films is due to the presence of traps at the GBs. [59]Xu et al. obtained similar conclusions with Yamagishi et al. through experimental results.They prepared n-type organic semiconductor 6,13-bis((triisopropylsilyl)ethynyl)-5,7,12,14-tetraazapentacene (TIPS-TAP) films using three methods of drop-cast, dip-coated, and vacuum-deposited.The drop-cast films contain few GBs between aligned domains, while the dip-coated films have no GBs between the singlecrystalline ribbons.In contrast, the vacuum-deposited films contain more GBs in random directions (Figure 2A-C).The results showed that the highest mobility of the devices prepared by the first two methods is more than 11 cm 2 /(V⋅s), but the highest mobility of the devices formed by vacuumdeposited is only 6.8 cm 2 /(V⋅s) (Figure 2D-F).They tested the temperature dependence of electron mobility of the three drop-cast OFETs.When the temperature decreases from 310 to 230 K, the mobility of the A1-based OFET increases from 9.3 to 12.3 cm 2 /(V⋅s), which is a sign of band-like transport (Figure 2G).Further decreasing temperature leads to a decrease of mobility, indicating that the transport mechanism becomes thermally activated transport caused by shallow traps.The same temperature dependence is observed from the other two drop-cast OFETs, which achieves the maximum mobility at 240 and 250 K, respectively.The band-like transport is also observed from the three dip-coated OFETs in the higher temperature range (Figure 2H).However, the vacuum-deposited OFETs exhibits a monotonically decreasing mobility upon cooling from 310 to 100 K, which F I G U R E 2 (A-C) Optical images for drop-cast, dip-coated, and vacuum-deposited films of 6,13-bis((triisopropylsilyl)ethynyl)-5,7,12,14tetraazapentacene (TIPS-TAP).(D-F) Transfer curves of the best-performing of drop-cast, dip-coated, and vacuum-deposited organic field-effect transistors (OFETs).(G-I) Temperature dependence of field effect mobility for drop-cast, dip-coated, and vacuum-deposited OFETs.(A-I) Reproduced with permission. [35]Copyright 2016, Wiley-VCH.

Yamagishi et al. tested the Hall coefficient
shows thermally activated transport (Figure 2I).They found that GB is a key factor in suppressing band-like transport by studying the temperature dependence of device mobility with different GB density. [35]Choi et al. tested the Hall effect on blended OSC films.They blended the small molecule semiconductor 2,7-dioctyl [1]benzothiano [3,2b][1]benzothiaophene (C 8 -BTBT) and the conjugate polymer indacenodothiophene-benzothiazole (C 16 IDT-BT) and utilized C 60 F 48 as a dopant (Figure 3A).Polarized optical microscopy shows a closely packed polycrystalline OSC films structure (Figure 3B).The measured field-effect mobility (μ FET ) exceeds 6 cm 2 /(V⋅s), but the Hall mobility (μ Hall ) is substantially smaller than the μ FET (Figure 3C,D), and the Hall carrier density (n Hall ) is greater than the capacitively defined carrier density (n FET ) (Figure 3E,F).By establishing a GB model with capacitive charges, it was found that GB is the reason for the underdeveloped Hall effect.Therefore, it is best to use single-crystal devices when studying the transport mechanism of carriers, as GBs can affect the transport mechanism. [34]

Electrical performance
The localized electronic states at the GBs also affect the electrical performance of OFETs.The main electrical parameters of OFET include mobility, V T , and on/off current ratio (I on /I off ).GBs can hinder the transport of carriers within OSC films, greatly reducing the mobility of devices.Moreover, GBs capture a large number of carriers, resulting in a higher V G for the device to form conductive channels.Therefore, the V T and I on /I off of the device also change.Overall, GBs lead to the degradation of the device's electrical performance.Zhou et al. directly probed the carrier transport in initial molecular layers of polycrystalline OSC films and found that the GBs in single-layer OSC films severely limit the transport of carriers, reducing the mobility of devices.In this case, the carriers can only transport in a plane.The effects of GBs are reduced as the increase of OSC filmthickness.Due to the vertical interlayer transfer of carriers, they can bypass the GB region with the highest resistance. [36]It is an effective means to study the influence of GBs on the electrical performance of devices by simulation.Meier et al. modelled the thickness, energy, and other parameters of the GBs, and found that the parameters of the GBs control the position of the Fermi level, which severely limits the electrical parameters of the device. [60]Whether the transfer curve of the device is consistent during the forward and backward scanning of the V G is also an important parameter of the electrical performance of the device.After conducting the forward and backward scanning of the pentacene device, it was found that the transfer curve of the device has obvious hysteresis, which is also caused by the influence of local trap states at the GBs during the operation of the device. [61] I G U R E 3 (A) Chemical structures of 2,7-dioctyl (A-F) Reproduced with permission.[34] Copyright 2019, Wiley-VCH.(G) Atomic force microscopy (AFM) topography and corresponding current maps of an unstressed and electrically stressed OFET.

Stability
The stability of OFETs determines whether they can be commercialized.][20][21][22][23] Most studies on GB-related stability issues focus on the operation stability and storage stability.The operation instability of OFETs manifested as the serious performance degradation when it is biased for a long time, which manifests as a shift in the V T , a reduction in mobility, an increase in subthreshold slope (SS), and increased hysteresis in the device transport characteristics.At present, the reason for the instability of device bias is still in debate.Sirringhaus et al. used SKPM to study the surface potential changes of pentacene films under a bias voltage.The results showed that the surface potential in the thin region of the OSC films varies with the variation of V G , effectively proving the evidence of preferential charge capture at the GBs.Charge capture during operation may be an important reason for device bias instability. [30]The corresponding relationship between GB density and operation stability can also be used to prove the influence of GBs on stability.The larger grain size at the first layer results in a smaller V T shift.By controlling the GB density in 5,11-bis(triethylsilylethynyl) anthridithiophene (TES-ADT) films, devices with high GB density exhibit poor electrical performance and stability.More GBs provide more charge capture sites, which is an important reason for bias instability. [37]Mueller et al. demonstrated that the GBs are the cause of poor operational stability through testing the morphology and current diagram before and after applying a bias voltage (Figure 3G).After applying a continuous bias voltage, the GB resistance increased by more than 150% (from 2 ± 0.2 to 5 ± 1.5 GΩ), while the resistance in the grains remained unchanged (Figure 3H). [62]he properties of GBs are very easily affected by molecules such as water and oxygen in the environment, thereby affecting the storage stability of the devices.The a-sexithiophene (α−6T)-based OFETs were exposed to the environment of air, oxygen, helium, and nitrogen for spectroscopy and electrical performance tests.There is no change in the environment of helium and nitrogen, but there is an obvious increase in the trap density in the environment of air and oxygen, which leads to the reduction of device performance. [63]Further research was conducted on the reasons for the unstable performance of devices in air, and it was found that the main reason for the unstable performance is the formation of oxygen-related traps at the GBs when exposed to air. [64]Water in the air also plays a principal role.By studying the electrical performance of devices at different humidity levels, the current and mobility of all devices decreased with an increase of relative humidity.The degradation of device performance caused by high humidity is mainly due to the charge capture of polar water molecules at the GBs, affecting the stability of devices during storage. [65] A-E) Reproduced with permission. [58]Copyright 2009, American Chemical Society.
(Figure 4C).They compared the mobility and V T changes of single-crystal and polycrystalline C 3 F 7 CH 2 -PTCDI-(CN) 2 devices during storage, and found that the mobility and V T of single-crystal devices remain unchanged after being stored in air for more than 5 weeks (Figure 4D,E).However, the degradation of polycrystalline devices exceeds an order of magnitude.GBs are the reason for this stability difference.They believed that the mechanism of degradation may be the reorientation of GBs microstructure, and there are two possible mechanisms.One is the diffusion of inorganic small molecules, such as N 2 , O 2 , or H 2 O from the ambient air into the OSC films.The embedding of these molecules in semiconductors will cause distortion of the surrounding lattice, leading to the introduction of scattering defects.The other mechanism is the redistribution or relaxation of conjugated molecules (especially those at the GBs) to a thermodynamically more stable configuration that may be characterized by larger intermolecular distances and smaller carrier mobility. [58]

THE CHARACTERIZATION OF GBS
Both trap and barrier models can capture carriers at the GBs, resulting in differences in potential and conductivity properties between GBs and grains.With this property, GBs can be effectively characterized.Kelley and Frisbie characterized the GB resistance of 10 9 -10 10 Ω for a 1 μm boundary length by using conducting probe atomic force microscopy (CP-AFM).
The GB resistance is one order of magnitude larger than that of a single sexithiophene (6T) grain, and an increase in V G can effectively reduce the GB resistance. [66]Puntambekar et al. measured the potential drop of 5-10 mV at the GBs using Kelvin probe force microscopy (KPFM), indicating the capture of holes at the GBs. [67]Sirringhaus and coworkers also utilized scanning Kelvin probe microscopy (SKPM) to investigate the potential variation of thin intergrain regions in pentacene OSC films with applied gate bias, obtaining clear evidence of GBs preferential charge capture. [30]Yogev et al. studied the GBs of pentacene films with different thicknesses using KPFM.They believe that the accumulation of holes at the GBs in OSC films (<30 nm) is caused by negative charge capture on the dielectric layer.In thick films, the change in surface potential is mainly caused by the capture of positive charges at the GBs themselves. [68]Huang et al. also used KPFM to study the surface morphology and potential distribution of p-type semiconductor copper phthalocyanine (CuPc) and n-type semiconductor copper hexafluorophthalocyanine (F 16 CuPc) (Figure 5A-D).Based on the spatial distribution of surface potential at the GBs, they confirmed the donor and acceptor trap states present at the GBs of p-CuPc and n-F 16 CuPc films, respectively. [69]Qian et al. grew  E-J) Reproduced with permission. [70]Copyright 2015, American Chemical Society.
CuPc films on p-sexiphenyl (p-6P) and conducted in situ KPFM electrical tests (Figure 5E,F).They tested the positions with different orientation of GBs by morphology and voltage drop (Figure 5G,H), and found that the difference in the inter-grain orientation at the GBs would significantly affect the properties such as voltage drop (Figure 5I,J).The larger the width and orientation differences of GBs, the higher the voltage drop at the GBs. [70]lectronic force microscopy (EFM) is used to study the capture of carriers by traps in OSC films.The results showed that most of the trap positions could be filled at a time scale of 30 s, and the current and voltage characteristics exhibited good saturation behaviour at this time scale. [71]Hirose et al. performed electrical transport measurements on α−6T grains by using a dual-probe atomic force microscopy (DP-AFM) system which has two independently controlled cantilever probes.By changing the distance between the grain and the probe, the test data found that GB is the main reason for limiting electrical performance. [72]Roesner et al. combined scanning transmission X-ray microscopy (STXM) with confocal Raman microscopy to detect charge capture during transistor operation.They used the high sensitivity and specificity of Raman microscopy to locate captured charges at the GBs. [73]

STRATEGIES FOR SUPPRESSING THE NEGATIVE IMPACT OF GBS
GBs can cause negative effects on the performance of OFETs, such as changes in the transport mechanisms, degradation of electrical performance, and reduced stability.The most direct way to eliminate the negative effects of GBs is to fabricate the single-crystal devices.[76] This section focuses on the use of interface engineering, process optimization of film preparation, post-processing of devices, and other regulatory strategies to control the density, orientation, and other parameters of GBs in OSC films to suppress negative effects.

Interface engineering
The interface is key to the performance of the device. [28,77]or bottom gate devices, the properties of dielectric layers significantly affect the aggregation state of OSC films during subsequent processing.The surface energy and roughness of the dielectric layer directly affect the stacking arrangement of OSC molecules, which affects the grain size and GB density of OSC films.Small molecule OSC films deposited on dielectric layers with low roughness and high surface energy will result in larger grain size and lower GB density.However, there are different conclusions on whether lower GB density can lead to higher mobility.Several studies have concluded that large grains and low GB density can effectively reduce the effect of GB defects on devices and obtain high mobility. [78]Other studies have shown that OSC films obtained on dielectric layers with lower surface energy may have smaller grain size and higher GB density, but tighter grain connections facilitate carrier transport. [79]Therefore, GB density is not the only factor to affect device performance.
There are other factors need to be considered, such as the tightness of grain connections, GBs' orientation, and surface properties of the dielectric layers.The following content will introduce the comprehensive control of the surface properties of dielectric layers through different interface engineering strategies to control the aggregation state of OSC films and thereby regulate the performance of devices.Since 1997, the strategy of using self-assembled monolayers (SAMs) to change the surface roughness, surface energy, and surface polarity of dielectric layers had been applied in the field of OFETs to reduce the number of GBs in OSC films.De Oteyza et al. modified the SiO 2 dielectric layer with octadecyltrimethoxysilane (OTMS), and found that the deposited F 16 CuPc films formed highly ordered microcrystals.The higher degree of order leads to a decrease of GBs in the F 16 CuPc films, which results in an order of magnitude increase in mobility. [80]Similarly, Yamada et al. modified the SiO 2 dielectric layer with hexamethyldisilazane (HDMS).They were able to controllably adjust the average grain size of dibenzotrithiofuvalene (DBTTF) films between 0.2 and 20 μm.The device with low GB density obtained through HDMS modification not only effectively reduces the SS but also achieves a high mobility of 0.55 cm 2 /(V⋅s). [81]The number of SAMs can also significantly change the number of GBs and the device performance of the obtained OSC films.Zhang et al. prepared 2,6-diphenyl anthracene (DPA)-based OFETs with different octadecyltrichlorosilane (OTS) modifications (Figure 6A,B).By controlling the reaction temperature and time, the contact angles on the dielectric layer are 44.1 • , 70.6 • , and 98.7 • , respectively.This effectively demonstrates the modification of low-density, middle-density, and highdensity OTS on the same SiO 2 substrate (Figure 6C).Compared with low-density OTS, DPA formed larger grain size, fewer GBs, and better molecular ordering on highdensity OTS surfaces (Figure 6D-F).The mobility obtained by high-density OTS modified devices is 3.26 cm 2 /(V⋅s), which is about two orders of magnitude higher than that on low-density OTS surface (Figure 6G-I). [78]n addition to SAMs, polymer dielectric layers are often used due to their excellent film-forming properties and simple processing techniques.Polymer dielectric layers with different surface properties are very suitable for regulating the growth of OSC films and the density of GBs.The surface potential of polymers significantly affects the grain size of pentacene films.Polyvinyl difluoride (PVDF), polystyrene (PS), polymethyl methacrylate (PMMA), and polyvinyl alcohol (PVA) were selected as dielectric layers to regulate surface potential.When the surface energy is below 47 mN/m, the size and mobility of semiconductor grains increase with the increase of surface energy.When the surface energy is above 47 mN/m, the grain size and mobility decrease with the increase in surface energy. [82]This phenomenon may be determined by the matching degree between the surface potential of the polymer dielectric layer and the surface energy of pentacene itself. [83]Polymer dielectric layers cannot only improve device performance by controlling the GB density, but also reduce interface defects and achieve smaller hysteresis and higher mobility due to their fewer surface defects, smoother roughness, and inherent flexibility.This strategy of utilizing polymer dielectric layers is more in line with future development trends. [84,85]n addition to the use of SAMs and polymer dielectric layers to reduce the GB density of OSC films, the following strategies also achieve effective GB modulation.Yoshida et al. used N,N'-dimethylperylene-3,4,9,10bis(dicarboximide) (MePTC) as the second layer of semiconductor coating on the first layer of pentacene semiconductor to form two-layer OSC structure.The top layer of MePTC is equivalent to a packaging layer, which reduces the electron capture effect at the GBs, prevents the adsorption of water molecules at the GBs, and effectively improves the mobility and current of pentacene-based OFETs. [86]The strategy of inserting a buffer layer at the interface to promote semiconductor growth can effectively suppress interface defects and effectively reduce the GB density within the OSC films to improve device mobility and reduce channel resistance. [87,88]V-ozone treatment is also used for interface modification.This treatment can not only change the surface energy of the surface but also clean the SiO 2 surface.After one minute of UV-ozone treatment, C 8 -BTBT-based OFETs have larger grain size, and the mobility reaches 6.5 cm 2 /(V⋅s). [89]For bottom-contact devices, the interfacial properties of the electrodes can significantly affect the growth of semiconductor molecules and the performance of the device.Ji et al. used nanosphere lithography technology to form electrodes with nanopore structures.The introduction of this nanopore structure electrode leads to the formation of a nanopore structure pentacene layer with small GBs at the electrode interface, which induces the growth of pentacene.Therefore, the mobility of the bottom gate bottom contact pentacene-based OFETs reaches 0.46 cm 2 /(V⋅s) by enhancing charge carrier injection, which is about 20 times higher than that of electrodes without nanopore structure. [90]

Process optimization of the film preparation
The external conditions during the preparation of OSC films can significantly affect the crystallization process of OSC films.Therefore, controlling the preparation conditions can effectively modulate the aggregation state of OSC films, thereby affecting the GBs.Vacuum thermal evaporation is one of the most commonly used methods for preparation of OSC films.The evaporation rate, substrate temperature, and film thickness during the vacuum evaporation process can be accurately controlled, which is conducive to modulate GB by controlling process conditions.Generally speaking, low deposition rate and moderate substrate temperature are beneficial for obtaining low GB density and better film continuity.The higher the deposition rate, the more nucleation sites the semiconductor has on the substrate, resulting in more grains and higher GB density.The polycrystalline OSC  A-I) Reproduced with permission. [78]Copyright 2016, Springer Nature.films obtained by perylene at different deposition rates can clearly be observed, and the slow evaporation rate leads to larger grain size, which is also applicable to other semiconductor materials such as pentacene. [91,92]Wen et al. changed the grain size and GB depth of n-type semiconductors N,N′-dioctyl perylene diimide (PDI-C8) and N,N′-ditridecyl perylene diimide (PDI-C13) by optimizing the films growth and deposition rate, improving the device performance and air stability.When the substrate temperature is high, OSC molecules can diffuse onto the substrate and obtain sufficient energy to migrate along the surface.OSCs have a higher probability of reaching the equilibrium position and forming ordered stacking.When the substrate temperature is too high, the molecules gain too much energy and leave the substrate, exacerbating secondary evaporation, making it difficult for the molecules to be deposited, generating more GBs and reducing the performance of the device.Therefore, the synergistic control of deposition rate and substrate temperature can effectively control the GB density of the OSC films. [93]Kumar et al. and Pandma et al.were able to grow slender rod-shaped crystals by carefully studying the substrate temperature and deposition rate of CuPc films growth, overcoming the problems of poor performance and stability caused by a large number of GBs, and effectively improving the performance of the device. [94,95]Yadav et al. controlled the GBs in OSC films by optimizing the conditions for film growth, effectively reducing the contact resistance of p-type CuPc and n-type F 16 CuPc caused by GBs and improving carrier mobility, establishing a correlation between the contact resistance and the aggregation state of the OSC films. [96]lthough the vacuum thermal evaporation has the advantages of high film crystallization quality, it also has the disadvantages of high cost and is not suitable for polymers.One of the biggest advantages of OSCs is the low-temperature, large-area, and rapid preparation.The OSC films prepared by the solution method meet these advantages.However, the OSC films prepared by the solution method also have a large number of GBs, which could also cause many negative effects.The blending strategy of OSCs promotes the solution processability of OSC films and facilitates the formation of better morphology. [97]he blending strategy has several important advantages.Firstly, it can effectively adjust the GB density and size through the amount of polymer.Secondly, it can form a self-encapsulated structure and interpenetrating nano network, which effectively suppresses the influence of water and oxygen in the air on GBs and improves the operational stability and storage stability of the devices.Shen et al. selected soluble PMMA as the dielectric layer and used insulating polymer PS and C8-BTBT for spin coating.On the one hand, the soluble dielectric layer PMMA and PS effectively control the crystal structure of the OSC films.On the other hand, PS can fill in GBs through phase separation, effectively improving the stability and mobility of  A-D) Reproduced with permission. [99]Copyright 2020, American Chemical Society.
devices.The mobility of the prepared transistor exceeds 7 cm 2 /(V⋅s) and the I on /I off is greater than 10 7 . [98]Under external mechanical forces, the crystallinity of small molecule polycrystalline OSC films will be disrupted and the distance between GBs will be stretched further, thereby seriously damaging their electrical performance.Zhao et al. designed and synthesized a polymer binder (PB), the new PB have a naphthalenediimide-dithiophene π-conjugated backbone end-functionalized with PDI units (Figure 7A).PB was blended with small molecule semiconductor N,N′−1H,1Hperfluorobutyl dicyanoperylene-carboxydiimide (PDIF-CN 2 ) and spin-coated to obtain OSC films, and then the top gate bottom contact OFETs were prepared (Figure 7B).By testing the EDS distribution of fluorine (F) and sulfur elements (S), it was found that F elements are evenly distributed throughout the entire films, while S elements (only present in PB) are concentrated at the GBs (Figure 7C).Unlike traditional phase separation, PB is mainly distributed in the GB to connect two grains, acting as a "spring" to resist external mechanical stress damage to the OSC films.The device still maintains excellent charge transfer performance even with a bending radius of 3 mm (Figure 7D).This strategy achieves excellent mechanical flexibility of small molecule polycrystalline semiconductors. [99]ontrolling the number of GBs through the morphology of the OSC films is effective in suppressing the negative impact of GBs on the devices.On the other hand, the GBs' orientation also plays an important role.It is difficult to control the GBs' orientation in OSC films prepared by vacuum thermal evaporation, while the solution can easily influence the GBs' orientation by external conditions.Rivnay et al. obtained anisotropic N,N′-bis(n-octyl)-(1,7&1,6)-dicyanoperylene-3,4:9,10-bis(dicarboximide) (PDI8-CN 2 ) films through the solution method and demonstrated a relatively ordered polycrystalline OSC films structure through polarized optical microscopy (Figure 8A).The stacking mode of the small molecule semiconductor polycrystalline OSC films discussed in this work can be divided into two types: Covariate slipstacked molecules such as (PDI8-CN 2 ) and herringbone-type molecular films.Both of these stacking methods have slightly poorly oriented low-angle GBs (blue area) and highly oriented high-angle GBs (red area) (Figure 8B).High-angle GBs may have inherent small transfer integrals, or they may exhibit large disordered regions between grains, aiding in activation energy barriers related to capture at the boundaries, thereby hindering inter-grain transport.On the contrary, low-angle GBs may pose lower barriers to transport.They controlled the parallel and vertical relationship between the transport channel and GBs' orientation, the modulation of carrier mobility could be achieved by about two orders of magnitude (Figure 8C,D).The temperature dependence of the mobility further highlights the anisotropy of electrical performance (Figure 8E).Controlling GBs' orientation in anisotropic OSC films can effectively limit the influence of GBs on the electrical performance of devices. [29]e et al. used inorganic silica nanoparticles to manipulate the morphology of TIPS-pentacene films.The drop-cast films improved morphological uniformity at ≈10% SiO 2 loading  A-E) Reproduced with permission. [29]Copyright 2009, Springer Nature.
(Figure 9A-D), which also leads to a 3-fold increase in average mobility.The SiO 2 nanoparticles mostly aggregate at TIPS-pentacene GBs, and 10% nanoparticle concentration effectively reduced the undesirable crystal misorientation without considerably compromising TIPS-pentacene crystallinity. [100]In the process of forming TIPS-pentacene by solution method, the inert gas Ar can be used to control the direction and morphology of GB growth.When the gas direction was parallel to the current direction, the OFETs showed a high mobility of 0.53 cm 2 /(V⋅s) with a small deviation. [101]Loo et al. studied the effect of the orientation of interspersulite boundaries (ISBs) on the resistivity in TES-ADT films.They prepared low-angle ISB samples with an angle of molecular orientation mismatch of 0 ± 20 • and highangle ISB samples with an angle of molecular orientation mismatch of 90 ± 20 • (Figure 9E), and tested the morphology of ISB and the current after applying voltage (Figure 9F,G).The results showed that the current of low-angle ISB samples decreases little, while the current of high-angle ISB samples decreases by an order of magnitude (Figure 9H).In subsequent work, the densities and energy levels of shallow traps within and across TES-ADT spherulites were quantified.The trap densities within a single grain are 7 × 10 10 cm −2 , while the trap densities in ISBs are as high as 3 × 10 11 cm −2 , and the activation energy of charge transfer increases from 34 meV within the grain to 50-66 meV in ISBs. [102,103]n addition to the strategies for controlling preparation conditions of OSC films mentioned above.Rubinger et al. used a binary solvent consisting of a host solvent and a high-boiling point solvent additive and studied the effect of additive content on the carrier transport.The mobility of 8% dichlorobenzene content increased by three times, indicating the reduced density of the GBs in the device channel. [104]occoli et al. via seeded supersonic beams to improve the morphological and electrical performance of the OSC films.Using the supersonic beams, they increased the grain size and limited the formation of GBs.As a consequence, the mobility was enhanced by one order of magnitude. [105] I G U R E 9 (A-D) Polarized optical microscopy images of drop-cast TIPS pentacene films with SiO 2 concentrations of 0%, 5%, 10%, and 15%.Reproduced with permission. [100]Copyright 2011, Wiley-VCH.(E) Optical micrographs of triethylsilylethynyl anthracylthiophene (TES-ADT) films.(F) AFM height images of TES-ADT films.(G) Corresponding current maps.(H) Corresponding histograms.(E-H) Reproduced with permission. [103]Copyright 2012, American Chemical Society.

Post-processing of OFETs
The annealing process is a common post-process technology to improve organic optoelectronic devices. [106]Heating OSC films at appropriate temperatures can effectively improve the uniformity and crystallinity of the films, alter the structure of GBs, and reduce the number of GBs, thereby improving the performance of the device.However, when the annealing temperature is too high (>150 • C), it will significantly destroy the morphology of OSC films, increase the roughness of the films, and damage the performance of the devices.Therefore, it is necessary to choose an appropriate temperature for the annealing process.Bao et al. found that the improvement of device performance after annealing is due to structural changes in crystal domains and GBs at the atomic scale, including larger crystal domains with enhanced crystallinity, reduced π-π stacking distances, and reduced GB disorder.For the first time, they directly observed and analyzed the microstructure changes of GBs and crystal domains at the atomic scale. [107]Mochizuki et al. found that the deposited films of 2,5-bis(4-biphenylyl)thiophene (BP1T) and 1,4-bis(5-phenylthiophen-2-yl)benzene (AC5) both showed microcrystalline phases, and the initial microcrystals grow 10-100 times under heat treatment.Heat treatment led to a dense crystalline layer, and the heat treatment of the deposited OSC films is an effective to grow microcrystals in the thiophene/phenylene co-oligomers (TPCOs) deposited films.It was inferred that one of the main mechanisms of thermal crystallization involves the storage of strain energy around GBs. [108] By using the annealing strategy to process poly(9,9-dioctylfluorene-cobithiophene) (F8T2) films, Koiwai et al. found that electron mobility grad-ually increases with the increasing annealing temperature.The improvement of the interface geometry of F8T2 films is related to the reduction of GBs, resulting in an increase in mobility and bipolar characteristics.The appearance of electron conduction is not only due to an increase in grain size but also due to improved connectivity between GBs. [109] Through heat treatment, Ma et al. increased the grain size and reduced the GB density, inhibiting the adsorption of water and oxygen, enabling n-type semiconductor PDI-C13 to exhibit good air stability, and reducing the contact resistance of its devices. [110]olvent vapor annealing (SVA) is a technique widely used to optimize the microstructure of OSC films.Using SVA can effectively control the grain size and GB density of the OSC films.113] Wu et al. used different steam treatments for pentacene-based OFETs and found that methyl ethyl ketone (MEK) steam significantly improved device performance.Analysis showed that MEK steam treatment can repair water-induced defects in GBs, reduce deep trap charge localization, and improve charge transport. [114]lasma technology also has important applications in passivating the GBs of OSC films.Oxygen plasma treatment can introduce oxygen into the GBs of pentacene films, and passivate defects at the GBs, resulting in an increase in hole densities, improving the conductivity of devices, and having no impact on the morphology of the films. [115,116]There is a direct way to reduce the negative impact of GB effect on OFETs.By reducing the length of the channel, the number of GBs in the channel can be reduced to a smaller  [31] Copyright 2020, the Partner Organisations.value.Ji et al. used photolithography technology to prepare PS-based shadow masks with interdigital patterns and successfully produced top contact devices with a channel length of 5 μm through subsequent processes (Figure 10A).The optical microscope demonstrated that the channel length is as low as 5 μm (Figure 10B,C).By effectively reducing the number of GBs in the channel, the maximum mobility of DPA-based OFETs with a channel length of 5 μm can reach 19.22 cm 2 /(V⋅s) (Figure 10D), and the strategy can also be extended to pentacene, CuPc, and F 16 CuPc. [31]

UTILIZING THE SENSITIVE NATURE OF GBS FOR APPLICATION
Strategies for suppressing the negative effects of GBs can effectively improve the electrical performance and stability of OFETs to facilitate commercialization.Besides, OFETs are widely used in sensing due to their unique charge transfer properties, which can act as both signal transducers and signal amplifiers.Over the past three decades, various sensors based on OFETs have been developed, including temperature sensors, chemical sensors, gas sensors etc. GBs play an irreplaceable role in OFET-based sensors.
GBs usually show high sensitivity and response to the external environment.Therefore, organic polycrystalline OSC films have wide applications in the field of sensing.In the barrier model, E B plays an important role in the transport of thermally activated charges.Huang et al. obtained DNTT films with average grain sizes of 200, 350, and 520 nm by carefully controlling the grain size (Figure 11A-C), and tested their temperature-dependent transfer curves (Figure 11D-F), which exhibits significant temperature-dependent differences.They adjusted the E B by changing grain size, thereby effectively controlling the temperature dependence of carriers (Figure 11G).When E B is high, carriers must rely on thermal activation to cross the barrier.Therefore, electrical performance is closely related to temperature, achieving high thermal sensitivity sensors (Figure 11H,I).When E B is low, carriers can also cross the barrier at lower temperatures, achieving thermally insensitive DNTT-based OFETs.As a result, they realized thermally insensitive and thermally sensitive OFETs by precisely tuning the E B of the GB. [52]andal et al. studied the integration of angiotensinconverting enzyme 2 (ACE2) molecules in pentacene GBs through statistical analysis of the transverse correlation length and interface width of rough surfaces.They confirmed the uniform coating of ACE2 molecules in the GBs to achieve a better conductive channel for the receptor to enter the semiconductor/dielectric interface of OFETs.They observed a detection time of less than 1 min and a sensitivity of 94%, which is the highest reported value. [117]FETs are excellent candidate for gas sensing.Analytes can interact with OSCs in reversible or irreversible interactions, including hydrogen bonding, charge transfer, hydrophobic interactions, dipole-dipole interactions etc.These interactions may occur within the bulk of the OSCs, at the GBs, or at device interfaces (metal/semiconductor, or semiconductor/dielectric). The interaction of analytes with OFETs causes changes in electrical signals, such as mobility and V T of OFETs, resulting in the detection of the analytes.Reproduced with permission. [52]Copyright 2020, Springer Nature.
GBs play an irreplaceable role in gas sensors in OFETs.On the one hand, GBs can interact with the analyte to achieve sensing.On the other hand, GBs as channels for analyte diffusion can facilitate their interactions with OFETs.This section mainly introduces the interaction between the analyte and the GB to achieve sensing application.[120] The sensing of chemical gases by GBs mainly stems from the dipole properties of analyte molecules, and electrical performance responses cannot be observed for non-polar analytes.The charge transport in OSCs is quite sensitive to the local polar environment.Due to increased disorder or other non-uniformity at the GBs, the distribution of localized band tails at the GBs is much wider than that within grain.Therefore, when presented in the environment of polar analytes, polar organic vapours cause an increase in the polarization of semiconductors at the GBs, leading to an increase in the capture of carriers at the GBs and a decrease in current to achieve sense. [118]Therefore, the mechanism of GB sensing is the charge capture induced by the dipole of the analyte.The dipole field around the analyte molecule may be sufficient to reduce the current passing through the conducting state of OFETs.These local fields may be concentrated at the GBs, and analytes may bind at the GBs.Therefore, the sensing of chemical vapor is closely related to the GB density of the films.The higher the GB density, the stronger the sensing ability of the films.At the same time, the greater the polarity of the analyte, the easier it is to detect.The controllable GB density achieved by the above strategy can effectively regulate the sensing performance of the device.The chemical vapor sensing realized by OFET is not a single GB effect but also includes doping and other effects.The analyte can act on multiple interfaces, which could be used as sensing applications. [119]

CONCLUSION AND OUTLOOK
The role of GBs as traps or potential barriers has been extensively studied.Regardless of whether they are traps or potential barriers, they are generally detrimental to the performance of devices, leading to reduced mobility and environmental stability.Related characterization techniques were used to characterize the GBs.Therefore, a large amount of work has been done to regulate the GB density of OSC films through interface engineering and preparation strategies of OSC films.Post-processing of the devices is also used to minimize the negative impact of GBs on the OFETs.Besides, the sensitive nature of GBs makes them important for applications in sensing.However, there are still some problems in the study of GBs at present.Firstly, the properties of GBs have a wide range of adjustability, and little work has focused on utilizing these properties to achieve more applications.For articles related to the application of GB, the main focus is still on utilizing the properties of GBs themselves to achieve chemical sensing.However, if more functional molecules responsive to light, force, and heat can be introduced into GBs, utilizing the response of functional molecules can achieve wider applications.Secondly, regarding the impact of GBs on stability, molecules at the GBs in polycrystalline OSC films deviate from equilibrium positions, exhibit large distortions, and have high kinetic energy and GB energy.At high temperatures or with longer storage times, it is beneficial for grain growth and GB flattening, resulting in discontinuous and increased roughness of the films.Therefore, the electrical performance of the OFETs is degraded or even lost.However, few works focused on passivating the GB to achieve higher thermal stability and storage stability.How to solve this high-energy state of the GB to improve the thermal stability and storage stability of devices is very important for the industrial application of OFETs.
[1]benzothieno[3,2-b][1]benzothiophene (C 8 -BTBT) small molecule, indacenodithiophenebenzothiadiazole (C 16 IDT-BT) conjugated polymer, and C 60 F 48 molecular dopant.(B) Polarized optical microscopy of a spin-cast blend film.(C, D) The effect carrier mobilities (μ FET ) extracted from the four-probe FET and the Hall carrier mobilities (μ Hall ) measured by Hall effect.(E, F) The field effect carrier density (n FET ) calculated from the gate-channel capacitance and the Hall carrier density (n Hall ) measured by Hall effect.

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I G U R E 5 (A) Topography and surface potential images of copper phthalocyanine (CuPc).(B) Topography and surface potential of the line segment in (A).(C) Topography and surface potential images of copper hexafluorophthalocyanine (F 16 CuPc).(D) Topography and surface potential of the line segment in (C).(A-D) Reproduced with permission. [69]Copyright 2009, Springer Nature.(E) Molecular structure of CuPc and p-sexiphenyl (p-6P).(F) Schematic diagram of Kelvin probe force microscopy (KPFM) measurement.(G, H) Topography and voltage drop images of CuPc films grown on p-6P layer.(I, J) Voltage drop at different positions and GBs' orientation.(

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I G U R E 6 (A) Schematic diagram of OFET and molecular structure of 2,6-diphenyl anthracene (DPA).(B) Schematic diagram and molecular structure of Octadecyltrichlorosilane (OTS) monolayer with different densities on Si/SiO 2 surface.(C) Contact angles of OTS surface with low, middle, and high-density OTS.(D-F) Atomic force microscopy (AFM) images of DPA films with low, middle, and high-density OTS.(G-I) Transfer curves of DPA-based OFETs on low, middle, and high-density OTS. (

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I G U R E 7 (A) Chemical structures of N,N′−1H,1H-perfluorobutyl dicyanoperylene-carboxydiimide (PDIF-CN 2 ) and polymeric binder (PB).(B) Schematic diagram of the device structure.(C) EDS element maps of fluorine and sulphur elements.(D) Electron mobility variations of OFETs under different bending tests.(

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I G U R E 1 0 (A) A schematic of the manufacture of the device arrays based on the polymer mask.(B) Optical microscope image of devices arrays with a 5 μm-channel length based on 2,6-diphenyl anthracene (DPA) through the polymer mask.(C) Optical microscope image of the details of the conducting channel.(D) Electrical performance of OFETs with different channel lengths.(A-D) Reproduced with permission.

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I G U R E 1 1 (A-C) Topography images of DNTT polycrystalline films with grain sizes of 200, 350, and 520 nm, respectively.(D-E) The temperature dependent transfer curves of OFETs with grain sizes of about 200, 350, and 520 nm.(G) Bar chart of the relationship between sensitivity and grain size.(H) Temperature resolution curve of the sensor measured at 37-38 • C with a precision of 0.2 • C. (I) Cyclic stability of temperature sensor from 25 • C to 45 • C. (A-I)