Nanoparticles for organic electronics applications

Recently, the research in solution-based, small-molecule organic semiconductors has achieved great progress, although their application in organic electronics devices is still restricted by a variety of issues, including crystal misorientation, morphological nonuniformity and low charge-carrier mobility. In order to overcome these issues, hybrid material systems that incorporate both organic semiconductors and additives have been successfully demonstrated to control crystal growth and charge transport of the organic semiconductors. In this work, we first review the recent advances in the charge-carrier mobility of the organic semiconductors, followed by a comparison of the different additives that have been reportedly blended with the semiconductors, including polymeric additives, small-molecule additives and nanoparticle based additives. Then we will review the important nanoparticles employed as additives to blend with solution-based, organic semiconductors, which effectively improved the semiconductor crystallization, enhanced film uniformity and increased charge transport. By discussing specific examples of various well-known organic semiconductors such as 6, 13-bis (triisopropylsilylethynyl) pentacene (TIPS pentacene), we demonstrate the essential relationship among the crystal growth, semiconductor morphology, dielectric properties, and charge-carrier mobilities. This work sheds light on the implementation of nanoparticle additives in high-performance organic electronics device application.


Introductions
The research of solution-processed organic semiconductors has achieved unprecedented progress in recent years [1][2][3][4][5][6][7]. Specially, they have been demonstrated with properties that would greatly benefit application in organic thin-film transistors (OTFTs), which include improved charge-carrier mobilities and expanded solubility in organic solvents [8][9][10][11]. Despite these advantages, their implementation in organic electronics devices still has many restrictions, which can be attributed to problems including crystal misorientation, morphological nonuniformity and low charge-carrier mobility [12,13]. These issues would further cause problems such as substrate poor coverage and inferior device performance consistency [14][15][16][17]. As a result, various hybrid material systems that incorporate both organic semiconductors and additive materials have been successfully demonstrated, which can take advantages of the merits from both components and contribute to the enhanced performance of organic electronics devices [18][19][20].
In the first section of this article, we will review the recent efforts made to advance the charge-carrier mobilities of the solution-processed, small-molecule, organic semiconductors. Then we will discuss the various issues of random crystal orientation and charge transport, which challenge the implementation of the organic semiconductors in organic electronics device fabrication. In the second section, we will compare the advantages of each type of additives, i.e. polymeric additive, small-molecule additive, and nanoparticle based additive. We will continue on to provide an in-depth review of the benefits of using nanoparticles as additives to effectively control the crystal growth, film morphology, substrate wettability, and charge carrier mobilities, which facilitate the application of the organic semiconductor in thin-film transistor and other electronics device fabrication. Throughout the discussion of these specific examples that mainly involve small-molecule organic semiconductors, we showcase that this work can be used to control the crystallization and electrical performance of other newly-discovered, high-performance, semiconducting materials.

Advances in organic electronics
In this section, we will review the various advances in charge-carrier mobilities and device application that have been recently achieved in the field of organic electronics. The following discussion will be mainly focused on various solution-processed, small-molecule, organic semiconductors.
The mobilities in solution-processed, small-molecule, organic semiconductors have been reported to be compared to or even far surpass the mobility of amorphous silicon. For example, Asare-Yeboah et al developed a temperature-based method which exposed the substrate to a gradient temperature and induced a solubility difference of a p-type small-molecule semiconductor 6, 13-bis (triisopropylsilylethynyl) pentacene (TIPS pentacene) [18]. TIPS pentacene crystals grew from the lower temperature side of the substrate towards the higher temperature side, forming well-aligned ribbons across the whole substrate. A mobility of up to 0.5 cm 2 V −1 s −1 has been reported from the TIPS pentacene OTFTs based on an ITO/PET flexible substrate by using the temperature-based alignment technique. Wade et al demonstrated a zone casting method to control the crystal alignment of both TIPS pentacene and 6,13-bis(triethylsilylethynyl) pentacene (TES pentacene) semiconductors [21]. Single crystalline crystals were obtained based on 50 μm s −1 zone-casting speed, yielding a mobility of 0.67 cm 2 V −1 s −1 and 0.037 cm 2 V −1 s −1 from TIPS pentacene and TES pentacene crystal based OTFTs, respectively. Lee et al demonstrated a tilting method to align the crystal growth of TIPS pentacene and obtained excellent alignment of the crystals with an average mobility of 0.3 ± 0.08 cm 2 V −1 s −1 [22]. Zhang et al reported the crystal alignment of TCNQ crystals with a brush writing method, resulting in both polycrystalline and single crystalline microstructural arrays with a mobility of 1.83 × 10 −3 cm 2 V −1 s −1 [23]. Becerril et al reported the growth of db-P2TP, dbo-P2TP, dho-P2TP, dho-P3TP, TMS-P2TP and TMS-4T semiconductors and obtained highly aligned crystalline ribbons, with a mobility of 0.129 cm 2 V −1 s −1 from elongated TMS-4T crystal based OTFTs [24]. Panidi et al reported a simple method to greatly enhance the charge-carrier mobilities in organic semiconductors via mixing a molecular Lewis acid, namely B(C 6 F 5 ) 3 , which results in effective pdoping in the organic semiconductors [25]. A mobility of 11 cm 2 V −1 s −1 and 8 cm 2 V −1 s −1 were demonstrated from the 2,7-dioctyl[1]-benzothieno[3,2-b] [1]benzothiophene:poly (indacenodithiophene-cobenzothiadiazole) (C 8 -BTBT:C 16 -IDTBT) and 2,8-difluoro-5,11-bis(triethylsilylethynyl) anthradithiophene: poly(triarylamine) (diF-TESADT:PTAA) based OTFTs, respectively.
More recently, researchers have devoted to the development of solution shearing based methods to control the crystal growth of solution-based, small-molecule, organic semiconductors. The application of the solution shearing technique has reportedly yielded even higher mobilities in the organic semiconductors including TIPS pentacene [26,27], diF-TES-ADT [28], and 2,9-di-decyl-dinaphtho-[2,3-b: 2′,3′-f]-thieno[3,2-b]-thiophene (C 10 -DNTT) [29], which can far supersede the electrical performance of amorphous silicon. Specially, Peng et al reported a mobility of 10.4 cm 2 V −1 s −1 from aligned single-crystalline, millimeter-scale C 10 -DNTT crystals grown with a solution shearing method [29]. Rocha et al combined the solution shearing method with the addition of a polystyrene polymer additive in order to align the crystal growth of TIPS pentacene, resulting in the formation of highly crystalline ribbons and spherulitic film [30]. An average mobility of 8.3 cm 2 V −1 s −1 and a highest mobility of 12.3 cm 2 V −1 s −1 were demonstrated from TIPS pentacene with confined crystal growth and continuous crystalline ribbons.
In addition to the implementation of solution-processed, small-molecule, organic semiconductors in the device fabrication of OTFTs, the semiconductor materials have also been used in application of other electronics devices, such as gas sensors. For example, Lee et al reported that TIPS pentacene organic semiconductor was used for the fabrication of gas sensor [31]. By allowing an optimum residual solvent amount, the fine-tuned spin coating time in this work led to two-dimensional (2D) growth of TIPS pentacene and the formation of 2D spherulite structure of TIPS pentacene. Its high surface coverage and porous structure further results in improved charge-carrier mobilities and an easy penetration of gas molecules into the charge transport channel, which enhances its gas sensing capability.

Challenges in organic electronics
As mentioned in the previous section, the application of solution-processed, small-molecule, organic semiconductors in the field of organic electronics has encountered various challenges. In this section, we will briefly discuss these restrictions, which include the misoriented semiconductor crystals, morphological nonuniformity and low charge-carrier mobility.
The crystal growth of many solution-processed, small-molecule, organic semiconductors is intrinsically anisotropic via typical solution deposition methods such as drop casting [32,33]. This essentially leads to considerable variations in the measured mobilities of the organic semiconductor based OTFTs, which has been previous reported in many semiconductor material systems including TIPS pentacene [34][35][36][37] and other semiconductors. In particular, Bi et al reported that p-type semiconductor 2,5-di-(2-ethylhexyl)-3,6-bis(5″-nhexyl-2,2′,5′,2″]terthiophen-5-yl)-pyrrolo [3,4-c]pyrrole-1,4-dione (SMDPPEH) (molecular structure shown in figure 1(a)) based OTFTs exhibited crystals with random directions and large gaps, as shown in the microscopic picture of figures 1(b), (c), after SMDPPEH was drop casted in a single solvent of pure chloroform without applying any external alignment method [38]. The measured mobilities in SMDPPEH ranged from 5.7×10 −5 cm 2 V −1 s −1 to 5.4×10 −4 cm 2 V −1 s −1 , which indicated variations of one order of magnitude. He et al reported that when a p-type semiconductor 5,6,11,12-tetrachlorotetracene (molecular structure shown in figure 1(d)) was drop casted in a single solvent of chloroform, it formed misoriented needles without neither long-range alignment nor continuous coverage on substrate (figure 1(e)) [15]. The 5,6,11,12tetrachlorotetracene based OTFTs exhibited mobilities that largely ranged from 0.006 cm 2 V −1 s −1 to 0.39 cm 2 V −1 s −1 , indicating mobility variations of almost three orders of magnitude. Chen et al reported the TIPS pentacene based OTFTs can exhibit mobilities that vary by one order of magnitude, depending on the angle between the crystal orientation and the source-to-drain direction [12]. A mobility of 0.004 cm 2 V −1 s −1 was measured from OTFTs which had TIPS pentacene crystals bridging the source-to-drain contact electrodes perpendicularly. In comparison, a 10-fold higher mobility was obtained when the TIPS pentacene crystals connected the electrodes in a parallel direction. The molecular structure of TIPS pentacene is shown in figure 1(f), and its typical randomly-located crystals via drop casting is shown in figures 1(g), (h), according to previous report [39].

Nanoparticles as additives
In order to address these aforementioned problems such as crystal misorientation, morphological nonuniformity and low charge-carrier mobility of the solution-processed, small-molecule organic semiconductor based OTFTs, the semiconductors were blended with various additives which can be categorized into polymeric additives, small-molecule additives and nanoparticle based additives. In this section, we will firstly compare the benefits of each type of additive materials based on discussion of their effects on the semiconductor morphology and charge transport. Then we will review some of the important nanoparticle based additives that have been reported to successfully control crystallization, morphology uniformity and enhance charge transport.

Small-molecule additives
Small-molecule additives have been employed to effectively tune the crystal growth of solution-processed, small-molecule, organic semiconductors. In particular, the loading of small-molecule additives can have the following benefits on the charge-carrier mobilities and device performance of OTFTs. First, the small-molecule additives have been reported to increase the crystallinity of the organic semiconductors and improve the orderliness of crystal structures, which is advantageous for charge transport [67][68][69][70]. Second, small-molecule additives can be utilized to modify the wettability of the organic semiconductor on the substrate, which leads to the formation of continuous crystalline film and is significant for the application in organic electronics device fabrication on large-scale substrates [71]. Third, previous study showed that small-molecule additives that exhibit similar molecular structure but different length of side group can modify the crystal growth of organic semiconductor. For example, a series of small-molecule additives, including 4-butylbenzoic acid (BBA), 4-hexylbenzoic acid (HBA), and 4-octylbenzoic acid (OBA), were blended with TIPS pentacene [72]. The smallmolecule additives formed an interfacial layer on the gate dielectric, which further facilitated the uniform deposition of seeds and allowed TIPS pentacene to become well oriented. Fourth, some small-molecule additives have also been reported to enhance the device stability of OTFTs [73]. This was reported to be related to their ability to repel the water molecules from being absorbed into the semiconductor active layer, which consequently enhanced the stability of the semiconducting OTFTs.

Nanoparticle based additives
The addition of nanoparticle based additives impacted the semiconductor growth, crystal orientation, substrate wettability and dielectric properties of the transistor device, resulting in enhanced charge-carrier mobilities [74][75][76]. First, nanoparticles were reported to be blended with organic semiconductors such as TIPS pentacene, which could effectively modify the crystallization of the organic semiconductor [77]. As a result, well-oriented TIPS pentacene crystals were formed across the substrate. One benefit from such improved crystal orientation is the reduction of mobility variations, since crystals are aligned in an orientation that more closely overlaps with the direction from the source to drain contact electrodes. Second, various nanoparticles were added to the dielectric layer, which led to enhanced dielectric constant and higher measured mobility of the small-molecule organic semiconductor based OTFTs. Third, nanoparticles were reported to be capable of modifying the surface energy of the substrate. Enhanced wettability of the semiconductor can lead to more uniform coverage on the substrate, which is specially significant for application for large-scale, organic electronics device on flexible substrate. Fourth, the addition of nanoparticle based additives can find application in other organic electronic devices such as biosensors [78,79].

Overview of nanoparticle based additives
In this section, we review the various nanoparticle-based additives that were reported to blend with both smallmolecule and polymeric organic semiconductors. These nanoparticles were added to tune the crystal growth, the semiconductor wettability, charge-carrier mobility, the dielectric constant of dielectric layer and other properties of organic electronics devices such as OTFTs and gas sensors.
He et al utilized silicon dioxide nanoparticles (SiO 2 NPs) to manipulate TIPS pentacene crystallization in an effort to improve device performance consistency [77]. By blending TIPS pentacene with SiO 2 NPs at different weight ratios, the crystal misorientation and poor film coverage of the pristine TIPS pentacene film can be largely mitigated ( figure 2(e)). Specifically, an average hole mobility of 0.13±0.04 cm 2 V −1 s −1 and 0.04±0.04 cm 2 V −1 s −1 was obtained with 10% SiO 2 NPs and without SiO 2 NPs loading, respectively, indicating a great increase in both average mobilities and performance consistency of TIPS pentacene OTFTs. The distribution of SiO 2 NPs in the vertical profile of the TIPS pentacene active layer was investigated, and a very small portion of SiO 2 NPs were found to be located at the interface between the active layer and insulator layer. Since most SiO 2 NPs are away from the interface, it would not adversely impact the charge transport of TIPS pentacene at the interface. Bright-field Transmission Electron Microscopy (TEM) was utilized to analyze the microstructural arrangement in the SiO 2 NPs/TIPS pentacene films, and results were presented in figures 2(a)-(c). The darker regions (i.e., smaller gray value in figure 2(d)) represent TIPS pentacene films or SiO 2 NPs, whereas the lighter ones (i.e., higher gray value in figure 2(d)) refer to amorphous-carbon supporting films. TIPS pentacene films with 10% SiO 2 NPs (figure 2(a)) are distinctively darker and have broader edge when compared to that of the neat TIPS pentacene films ( figure 2(b)). The TEM image of SiO 2 NPs is presented in figure 2(c), with an average diameter of 18 ± 4 nm based on 12 different measurements. The SiO 2 NPs/TIPS pentacene film has uniform electron contrast but shows gray value gradient only at the film edge, which can be attributed to the nanoparticle aggregations at the grain boundaries. Finally, a grayness plot is presented in figure 2(d) in order to quantitatively compare the width of the film edges. The two triangles in figure 2(d) correspond to the triangle in figures 2(a) and (b), respectively, allowing the calculation of the film edge width: 1.5 μm with 10% SiO 2 NPs and only 0.2 μm for the pure TIPS pentacene film. The aggregations of SiO 2 NPs at the grain boundaries of TIPS pentacene crystals attributed to the oriented crystal growth with improved alignment.
Afsharimani et al reported the addition of SiO 2 NPs into the polymer PVA in order to modify its dielectric properties, for application in α,ω-dihexylquaterthiophene (DH4T) based transistor fabrication [80]. Two types of dielectric layers were used in this study including the pure PVA film and PVA/SiO 2 NPs hybrid film. The PVA/SiO 2 hybrid film was found to exhibit a higher a root-mean-square (RMS) roughness of 3.8 nm as well as an elevated water contact angle of 52°, leading to a reduced surface energy of 12.8 mJ m −2 . The reduced surface energy is considered to be beneficial for the device stability of the DH4T based OTFTs since it can repel the adsorption of water molecules. When measured at 800 kHz, PVA film and the PVA/SiO 2 NPs hybrid film exhibited a capacitance value of 0.2 and 4.9 nF/cm 2 , which caused more charge carriers to accumulate in the channel of the hybrid devices and resulted in higher drain currents. Electrical characterization of the DH4T based OTFTs demonstrated ambipolar behavior, which was attributed to the reduced trap density from the hydroxyl group 'neutralization' of PVA. Slightly higher electron and hole mobilities were obtained in the range of 10 −4 cm 2 V −1 s −1 for devices with the hybrid film. Despite these improvements, however, the addition of the SiO 2 NPs led to more structural defects and larger leakage current.
Yamazaki et al reported the addition of silica nanoparticles (SNPs) into TIPS pentacene in order to improve the semiconductor coverage in the charge transport channel by tuning the wettability on the hydrophobic surface of poly(methylsilsesquioxane) (PMSQ) polymer gate dielectric [75]. The sol-gel synthesized colloidal SNPs were blended with TIPS pentacene at varied weight ratios including 0%, 0.01%, 0.1%, and 1%. The blend solution was then spin-coated onto a hydrophobic surface of PMSQ dielectrics, and the resultant morphology indicated that the wettability was greatly improved with NPs weight ratio at 0.1% and 1%. The enhanced wettability was due to the anchoring effect of TIPS pentacene onto the phenyl-modified SNPs. TIPS pentacene/ SNPs based OTFTs were fabricated by both spin coating and inkjet printing of the blends on the PMSQ polymer gate dielectric. Electrical characterization results indicated an improved mobility of up to 1.2×10 −3 cm 2 V −1 s −1 (spin coating) and 2.6×10 −2 cm 2 V −1 s −1 (inkjet printing) with 0.1% of NPs additive, which was accredited to the enhanced molecular ordering of the self-organized TIPS pentacene.
Similarly, Nagase et al reported the addition of SNPs into P3HT in order to improve the charge-carrier mobility of OTFTs by modifying the semiconductor wettability [81]. The surface of the SNPs was modified with phenyl surfactants (Ph-SNPs) and with methyl surfactants (Me-SNPs). Different average diameters of the SNPs were studied. The photographs of figure 3 show different wetting conditions of the P3HT/SNPs blends on ODTS-treated SiO 2 dielectric surface, which was dependent on the average diameter and weight ratio of the NPs. As figures 3(a) and (b) shows, the wettability generally improved at all weight ratios of Ph-SNPs, leading to the formation of continuous P3HT film, regardless of the average diameter size of the Ph-SNPs. In contrast, good wettability was only noticed at a large weight ratio of Me-SNPs (5:10), as shown in figure 3(c). Electrical characterization of the P3HT/SNPs based OTFTs on ODTS-treated substrates revealed higher mobility with the addition of larger-diameter SNPs, and correspondingly higher crystallinity was noted from X-ray diffraction spectra. In particular, a hole mobility of over 0.1 cm 2 V −1 s −1 was demonstrated from P3HT/SNPs based OTFTs with a 5:3 weight ratio and an average 38 nm diameter of the SNPs. The device configuration of P3HT/ SNPs based OTFTs was shown in the graphical representation figure 3(f).
Wang et al reported the hybrid layer of polymer/silicon nanoparticles (Si NPs) in application of organic transistor fabrication [82]. Specially, the Si NPs were modified with 2-hydroxyethyl methacrylate (HEMA) in order to facilitate their dispersion within the organic polymer layer of P-HEMA-&-GMA. Four different dielectric films were studied in this work: Si-5, Si-10, Si-15, and Si-20, which had a molar ratio of 5:95, 10:90, 15:85 and 20:80 between Si-HEMA and polymer. As shown in figure 4(a), para-exaphenyl (p-6P)/vanadylphthalocyanie (VOPc) semiconductor based OTFTs were fabricated with a dielectric bilayer incorporating both SiNx and polymer/Si NPs blend film. The morphology of the Si-HEMA was presented in the TEM image of figure 4(b), which showed that these modified NPs exhibited good dispersion and narrow dimension distribution. The dielectric thin film was found to exhibit increased dielectric constant, which ranged between 3.5 and 5.2. The enhanced dielectric constant with adding Si NPs was much higher than that of the organic dielectric films. As the AFM images of figure 4(c) shows, the addition of the Si NPs can facilitate the continuous growth of p-6P material. Also, the crystal grain size was found to increase with the increased molar ratio of the additive, which was considered to be beneficial for the device performance of OTFTs. The p-6P/VOPc based transistors demonstrated a mobility of up to 0.7 cm 2 V −1 s −1 based on the Si-10 film.
Park et al reported the utilization of gold nanoparticle (Au NPs) for application in TIPS pentacene based OTFTs and organic memory devices [83]. The device diagram was presented in figure 5(a). Negatively charged Au NPs were adsorbed onto a 3 nm multilayer-structure layer of polyelectrolytes (PEs) via a dipping process. The Au NPs array was presented in the Scanning Electron Microscopic (SEM) images of figure 5(b), which shows a uniform deposition and an average diameter of 16±3.2 nm. Then, 15 nm layer of HfO 2 was deposited as a tunneling oxide layer, followed by the deposition of TIPS pentacene as active layer and silver as source and drain contact electrodes via inkjet printing ( figure 5(c)). Characterization of the memory devices indicated both large  memory windows and decent data retention. In addition, the organic memory devices incorporated with Au NPs were found to exhibit good reliability with repeated cycles of programming/erasing. This work sheds lights on application in nanofloating gate memory devices printed on flexible substrates.
Hou et al reported the blending of BaTiO 3 nanoparticles (BaTiO 3 NPs) into the polymer matrix of P(VDF-TrFE)/PMMA in order to enhance its electrical property [76]. While the pristine polymer matrix of P(VDF-TrFE)/PMMA exhibited a rod-like structure, the loading of BaTiO 3 NPs at the weight ratios of 9%, 16% and 23% led to the formation of more compact crystal structure and reduced rod diameter, as shown in the Atomic Force Microscopy (AFM) images of figures 6(a)-(d) and SEM image of figures 6(e), (f). The dielectric constant of P(VDF-TrFE)/PMMA was found to increase with increased weigh ratio of the BaTiO 3 NPs. In particular, a dielectric constant of 9.3 was obtained with 23% weight ratio of BaTiO 3 NPs. TIPS pentacene OTFTs were fabricated with the P(VDF-TrFE)/PMMA/23% BaTiO 3 film as the dielectric layer. Electrical characterization demonstrated a mobility of 0.01 cm 2 V −1 s −1 and reduced leakage current under high bias condition, which was attributed to the good miscibility property of the nanoparticle with the P(VDF-TrFE)/PMMA polymer matrix.
Jang et al reported the addition of barium strontium titanate (BST) NPs into the dielectric layer of polyvinylphenol (PVP) polymer in order to improve the mobility of pentacene based TFTs [84]. The BST NPs have a diameter of less than 50 nm. The digital images of PVP and BTS NPs solutions were presented in figure 7(a). The existence of abundant OH groups from the PVP polymer facilitated the dispersion of the BST NPs within the dielectric layer. A higher weight ratio of NPs loaded to the dielectric layer led to a higher dielectric constant, which resulted in an increase of charge density located at the charge transport channel and higher mobilities. However, as the AFM images of figure 7 show, the increased weight ratios of BTS NPs at the same time led to increased surface roughness. In order to reduce the increased surface roughness, a thin polystyrene (PS) layer was formed on top of the PVP/BTS NPs blend dielectric layer. The highest mobility of 1.2 cm 2 V −1 s −1 was demonstrated from pentacene OTFTs based on the PVP/8% BST NPs/PS blend layer, which was attributed to the combined outcome of reduced surface roughness and increased dielectric constant.
Wang et al studied the effect of different thickness of silver (Ag) NPs on the charge-carrier mobility and memory property of pentacene based TFTs [85]. Ag NPs with a different thickness, including 1 nm, 5 nm and 10 nm, were embedded between two layers of organic semiconductor pentacene, as shown in the device configuration of figure 8(e). Electrical characterization results (figures 8(a)-(c)) demonstrate that the bottomgate, top-contact pentacene TFTs only exhibited saturation behavior in the output currents with a 1 nm and 5 nm thickness of Ag NPs, whereas no saturation behavior was observed for TFTs with 10 nm of Ag NPs. A hole mobility of 0.34 cm 2 V −1 s −1 and 0.1 cm 2 V −1 s −1 was obtained with 1 nm and 5 nm thick layer of Ag NPs. By applying a cyclic sweeping gate voltage, the devices showed a memory window that increased with an increase in the thickness of the Ag NPs film, as shown in figure 8(d), which was attributed to the larger trap density from Ag NPs. Also, the on/off current ratio was found to be 10 5 , 10 3 , and 10 with a thickness of 1 nm, 5 nm and 10 nm of the Ag NPs film. The significant drop in on/off current ratio was related to the connected Ag NPs as their size increased, as observed in the SEM images of figures 8(g), (h). This created a low-resistance path for current flow and led to increased off current.
Kim et al reported the employment of alumina nanoparticles (Al 2 O 3 NPs) to modify the dielectric property and to passivate defects on the gate layer for application in P3HT based OTFTs [86]. The silane-terminated polystyrene (PS) was grafted onto the core-shell Al 2 O 3 NPs via an Al-O-Si bond, leading to the formation of Al 2 O 3 -PS NPs. The Al 2 O 3 -PS NPs were further mixed with a polymer polystyrene (PS) or poly (methyl methacrylate) (PMMA) to serve as a hybrid dielectric layer for transistor application. Depending on composition of the nanoparticle component, the hybrid layer exhibited a dielectric constant that ranged from 2.59 to 7.79. When P3HT was used as a benchmark semiconductor, bottom-gate, top-contact OTFTs were fabricated with a   Si/SiO 2 /Al 2 O 3 -PS-NPs configuration, yielding a 6.3 nF/cm 2 capacitance at 1 kHz. The Al 2 O 3 -PS-NPs provided an effective passivation of the silanol groups on the SiO 2 substrate, which would otherwise serve as trap centers of charge carriers. The P3HT OTFTs with the Al 2 O 3 -PS-NPs showed a hole mobility of 1.4×10 −3 cm 2 V −1 s −1 , which was 10-fold higher than the value from devices without the additive. This work sheds lights on a new surface treatment for the charge trap centers on the oxide gate dielectrics. The various semiconductor materials, nanoparticle based additives, experimental process and result, and charge-carrier mobility were summarized in table 1.

Conclusions and outlook
In this article, we have reviewed the various nanoparticle based additives that have been reported to successfully blend with solution-processed organic semiconductors to control crystal orientation, increase substrate wettability, enhance dielectric properties and improve electrical performance of the semiconducting devices. Overall, these nanoparticle based additives can have the following benefits on the film morphology, charge transport and device performance. First, the nanoparticle based additives can control the crystallization of the small-molecule semiconductors, which further leads to enhanced crystallinity, improved crystal orientation and reduced mobility variation of the organic semiconductor based OTFTs. Second, the nanoparticle based additives can be employed to modify the wettability of the semiconductor on the substrate, which has significant application in large-area, high-performance organic electronics on flexible substrate. Third, the nanoparticle additives, when added into the dielectric layer, can enhance the dielectric constant, increase values of the measured mobility and improve the performance of memory devices. Besides the works that we have covered in this review article, there are still issues that need to be addressed in order to fully unlock the potentials of solution-processed, small-molecule organic semiconductors, such as degradation in air. Research efforts are still needed to enhance the stability of the organic semiconductor based devices when exposed to the ambient environment. We hope that the nanoparticle based additives as we reviewed in this work may shed lights on future endeavors in this filed.