OxideâPolymer Heterojunction Diodes with a Nanoscopic Phase

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Oxide-Polymer Heterojunction Diodes with a Nanoscopic Phase-Separated Insulating Layer xinan zhang, Binghao Wang, Wei Huang, Gang Wang, Weigang Zhu, Zhi Wang, Weifeng Zhang, Antonio Facchetti, and Tobin J. Marks Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b04284 • Publication Date (Web): 05 Dec 2018 Downloaded from http://pubs.acs.org on December 6, 2018

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Oxide-Polymer

Heterojunction

Diodes

with

a

Nanoscopic Phase-Separated Insulating Layer Xinan Zhang, †,‡,⊥ Binghao Wang, †,⊥ Wei Huang, † Gang Wang, † Weigang Zhu, † Zhi Wang, † Weifeng Zhang,* ,‡ Antonio Facchetti, *,† Tobin J. Marks, *,† †Department

of Chemistry and the Materials Research Center, Northwestern University, 2145

Sheridan Road, Evanston, IL, 60208 USA ‡School

of Physics and Electronics, Key Laboratory of Photovoltaic Materials, Henan University,

Kaifeng, 475004, China

ABSTRACT: Organic semiconductorinsulator blend films are widely explored for high-performance electronic devices enabled by unique phase separation and self-assembly phenomena at key device interfaces. Here we report the first demonstration

of

high-performance

hybrid diodes based on p-n junctions formed by a p-type poly(3-hexylthiophene) (P3HT)poly(methylmethacrylate) (PMMA) blend and n-type indium-gallium-zinc oxide (IGZO). The thin film morphology, microstructure, and vertical phase separation behavior of the P3HT films with varying contents of PMMA are systematically analyzed. Microstructural and charge transport evaluation indicates that the polymer insulator component positively impacts the morphology, molecular orientation, and effective conjugation length of the P3HT films, thereby enhancing the heterojunction performance. Furthermore, the data suggest that PMMA phase segregation creates 1 Environment ACS Paragon Plus

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a continuous nanoscopic interlayer between the P3HT and IGZO layers, playing an important role in enhancing diode performance. Thus, the diode based on an optimal P3HT-PMMA blend exhibits a remarkable 10-fold increase in forward current versus that of a neat P3HT diode, yielding an ideality factor value as low as 2.5, and a moderate effective barrier height with an excellent rectification ratio. These results offer a new approach to simplified manufacturing of low-cost, large-area organic electronics technologies. KEYWORDS: Organic semiconductor/insulator blend, hybrid diode, P3HT, IGZO, effective barrier height, ideality factor

Organic semiconductor-based electronic devices have received remarkable attention over the past decades due to their light weight, mechanical flexibility, and the potential to be fabricated at lower costs by solution processing.1-4 Furthermore, organic semiconductors can be blended with various materials which can tune key properties such as the optical cross-section, mechanical flexibility, environmental stability, and charge transport.5-6 In particular, solution-processed organic semiconductor-electrically insulating polymer blends have been widely investigated for the fabrication of organic thin film transistors (OTFTs) since the insulating polymer phaseseparates vertically and self-assembles at the semiconductor-gate dielectric interface.7-8 This approach enables the passivation of charge traps on the dielectric surface without the necessity of self-assembled monolayers, such as hexamethyldisilazane (HMDS) and octyltriethoxyl silane (OTS),9-10 or crosslinked polymer films, as well as manipulating the channel film morphology and microstructure for optimal charge transport.7, 11 The result are OTFTs with superior field-effect mobilities. More recently, blends comprising a semiconductor component and an insulating polymer have also been used in metal oxide TFTs,12-13 as well as in other devices, to enhance charge transport.14-15

2 Environment ACS Paragon Plus

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In addition to TFTs, diodes are also essential elements for realizing electronic circuitry in displays, radio-frequency identification tags, and other emerging electronic applications.16-18 The basic diode requirement is to deliver a high current rectification ratio and sufficient high current density for high speed operations. However, the diode electrical performance relies critically on the junction interfacial properties, such as effective barrier height, work function, and injection efficiency when operating the device in forward/reverse bias. Therefore, interface engineering is important for manipulating the above properties to enhance the device electrical metrics. For example, Tomasz et al. reported Ag/ZnO Schottky diodes where the rectification ratio increases significantly from 102 to about 105 by insertion of an ALD-deposited 2.5 nm thick HfO2 layer in between the Ag electrode and the ZnO layer.19 Organic-inorganic hybrid heterojunction diodes have also been investigated to merge the remarkable mechanical flexibility of organic materials with the well-established properties of inorganic compounds.20-21 Thus, Rajiv et al. fabricated P3HT/ZnO heterojunction diodes by incorporating an OTS monolayer at the heterojunction interface to tailor the ZnO surface work function and thereby enhance the injection efficiency and the rectification ratio from 102 to 103.22 In contrast, to our knowledge there are no reports of hybrid organic-inorganic heterojunction diodes using organic semiconductor-insulator blends, despite the attractions that such devices might offer. In this letter, we demonstrate high-performance hybrid heterojunction diodes using solutionprocessed poly(3-hexylthiophene) (P3HT)-poly(methyl methacrylate) (PMMA) blends as the ptype semiconductor and indium-gallium-zinc oxide (IGZO) as the n-type layer. Our results indicate that PMMA positively enhances the film morphology, semiconductor backbone molecular orientation, and effective conjugation length of the P3HT films, resulting in diodes with enhanced rectification and forward current vs. those based on a pristine P3HT layer.


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The present organic-inorganic heterojunction diodes were fabricated on indium-tin oxide (ITO) coated glass substrates, cleaned sequentially with acetone, water, and isopropanol. The IGZO films (60 nm) were first deposited on the ITO layer by radio frequency magnetron sputtering (Sputter-AJA, IGZO target = In:Ga:Zn= 1:1:1 at%, 99.999%) at room temperature. The sputtering process (pressure = 2 Pa, Power = 150 W) was carried out in a mixed atmosphere of Ar and O2 with the flow rate ratio of 20:1. IGZO film microstructure and morphology were assessed by Grazing Incidence X-ray diffraction (GIXRD, Figure S1a) and atomic force microscopy (AFM, Figure S1b) which reveal that all films are amorphous and very smooth, with an RMS roughness of 0.89 nm. Both P3HT (Mn = 85,000) and PMMA (Mn = 120,000) were purchased from SigmaAldrich and used without further purification. Semiconductor solutions with a total polymer concentration of 10 mg mL-1 were prepared by dissolving P3HT and PMMA with weight ratios equal of 10:0, 10:1, 10:2 and 10:3 in chloroform and stirring at 60 °C for 2 h for complete dissolution. Next, the solutions were spin-coated on the IGZO films at 2000 rpm for 30 s and the resulting films (~60 nm thick) were thermally annealed at 80 °C in vacuum for 3 h. Finally, Au top electrodes (40 nm, area = 1×10-4 cm2) were deposited by thermal evaporation through a shadow mask.

Figure 1. Current-voltage (I-V) characteristics of P3HT-PMMA/IGZO heterojunction diodes with increasing amounts of added PMMA (left to right).


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Table 1. Electrical parameters obtained from I-V characteristics of different P3HT-PMMA/IGZO heterojunction diodesa Device Forward current at (P3HT-PMMA) 2V (A) 10:0 (2.2±0.2)×10-4 10:1 (2.3±0.1)×10-3 10:2 (1.1±0.1)×10-3 10:3 (5.7±0.2)×10-4 aAverage

n

φb

3.1±0.07 2.5±0.05 2.7±0.08 2.9±0.09

0.93±0.08 0.89±0.07 0.88±0.05 0.87±0.05

Rectification ratio at ±2V ~103 ~104 ~104 ~103

of a minimum of 10 devices

All diodes were evaluated in ambient conditions using an Agilent B1500A semiconductor parameter analyzer, and Figure 1 shows the device current-voltage (I-V) characteristics as a function of differing PMMA contents in the P3HT. These curves demonstrate the rectifying behavior of the present devices, confirming the formation of a P3HT/ IGZO p-n junction. From the semi-log forward bias curves, all I-V characteristics consist of two linear regions with different slopes, one at a low bias ≤0.9 V and another at middle bias region (0.9V≤V≤1.5V) showing the exponential relationship between the current and voltage. Here, the middle bias region is the most interesting because it is dominated by the diffusion component of the current which enables extraction of the most important diode parameters. The I-V characteristics can be described using the standard thermionic emission diffusion theory and expressed as:22-23

[ ( ) ― 1]

𝐼 = 𝐼0 exp

𝑞𝑉 𝑛𝑘𝑇

I0 = 𝐴𝐴 ∗ 𝑇2exp ( ―

𝑞𝜑𝑏

𝑘𝑇 )

(1)

(2)

where q is the electron charge, T is the temperature in Kelvin, k is the Boltzmann’s constant, A is the effective diode area, and I0 is the saturation current derived from the straight intercept of Ln(I) at V = 0. A* is the effective Richardson’s constant which equals 41 A/cm2K2 for n-type IGZO.16 Here n is the ideality factor and φb is the effective barrier height:


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𝑞

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𝑑𝑉

(3)

𝑛 = 𝑘𝑇𝑑𝑙𝑛(𝐼) ∗

𝜑𝑏 =

2

(𝑘𝑇𝑞)ln (𝐴𝐴𝐼 𝑇 ) 0

(4)

Based on the above equations, n, φb as well as forward current and rectification ratio can be extracted, and the calculated electrical parameters are summarized in Table 1. Compared with the neat P3HT/IGZO diode, n and φb decrease, while the forward current and rectification ratio increase after P3HT blending with PMMA, indicating improved charge injection efficiency. From Table 1, the P3HT-PMMA(10:1)/IGZO diode exhibits remarkable features, such as 10 higher forward current than that of neat P3HT diode, a n value as low as 2.5, a moderate φb (0.89 eV) with an excellent rectification ratio (104). To understand how PMMA incorporation affects blend physical properties, P3HT-PMMA films with 0:0, 10:1, 10:2 and 10:3 ratios were investigated by AFM, Raman spectroscopy, and optical absorption measurements. Figure S2 shows surface morphology images of the P3HTPMMA blend films with different P3HT-PMMA weight ratios. Although all the films form similar crystalline domains, RMS roughness and the connectivity of the domains are significantly different. For the neat P3HT film, the RMS roughness is 9.2 nm with very low domain connectivity. However, the RMS roughness decrease to 6.3 nm, 5.5 nm and 6.0 nm as the PMMA content in the blend increases in the order, 10:1, 10:2, 10:3 P3HT-PMMA, respectively. The better domain connectivity likely originates from the enhanced pre-aggregation of P3HT by the presence of PMMA.25 The domain structures are due to the strong intermolecular interactions between adjacent conjugated P3HT backbones, resulting in enhanced charge hopping rates compared with that in amorphous structures where poor connectivity between polymer crystalline domains can


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reduce charge transport efficiency.11 Therefore, the lower surface roughness and morphological connectivity of the blended films should enhance diode performance.

Figure 2. (a) Raman spectra, (b) GIXRD patterns, (c) Normalized optical absorption spectra, (d) Crystallite coherence length along the a-axis (La) and effective conjugation length (Leff) of P3HT for the indicated blends. The molecular features of the P3HT-PMMA blend films with different component weight ratio were also examined by Raman spectroscopy using a 532 nm solid-state laser (Figure 2a). The Raman spectra are dominated by an intense band with a maximum at 1449 cm-1 characteristic of the C=C stretching vibration of the thiophene ring. We also observe other Raman bands at 726 (Cα-S-Cα՛ deformation), 1001 (Cβ-calkyl stretching), 1091 (Cβ-H bending), 1210 (Cα-Cα՛ + Cβ-H bending), and 1381 cm-1 (Cβ-Cβ′ stretching), which are all consistent with previous reports.26 As shown in the inset of Figure 2(a), the 1449 cm-1 peak downshifts 1.5, 2.5, and 1.8 cm-1 for the 10:1, 10:2 and 10:3 blends, respectively. The downshifted peak indicates that the local density of states


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of the C=C bond is reduced, implying an increase in conjugation length.11 Therefore, the P3HT films exhibit improved electronic quality after blending with PMMA. In particular, the P3HTPMMA (10:2) film exhibits the largest P3HT peak shift as well as greatest peak intensity, indicating a lower content of “isolated” C=C bonds than that in the other samples. The microstructures of the P3HT-PMMA blend films were next examined by GIXRD (Figure 2b). A strong primary diffraction peak at 2θ = 5.4° is assigned to the (100) lattice plane of the edge-on P3HT chain stacking, forming lamellar supramolecular structures stacked along the aaxis, as well as two weaker peaks corresponding to the planes (200) at 10.7° and the (300) at 15.9° which are observed in all films.27 The intensity of the (100) peak declines as the PMMA content increases, indicating reduced P3HT composition in blend films. The half-width of the (100) peaks is related to the Debye-Scherrer dimension of coherently scattering crystal domains. So, the P3HT crystallite size along the a-axis (La) can be estimated using the Scherrer equation:28 𝐾𝜆

𝐿𝑎 = 𝐵 × 𝑐𝑜𝑠𝜃

(5)

where K is the Scherrer constant, θ is the diffraction angle of the peak, and λ is the wavelength of the X-ray. Here, K was set to be 0.9. The derived La value are therefore 9.9 nm, 10.7 nm, 11.7 nm, and 11.1 nm for 10:0, 10:1, 10:2 and 10:3 blends, respectively. The La as a function of PMMA content is shown in Figure 2(d). The results indicate that the P3HT-PMMA (10:2) film has the most extensive crystallization, consistent with above Raman results. Figure 2c presents the normalized optical absorption spectra of the semiconductor blends. The maximum absorption peak assigned to the A1 band is found at ~2.2 eV. To date, experimental results and theoretical analyses for P3HT films suggest that the optical absorption for energies < 2.3 eV is due to the chains in the crystalline regions which have longer effective conjugation lengths (Leff). Here, Leff in terms of the number of P3HT repeat units in the various films can be


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estimated further according to the theoretical calculations.11 It has been demonstrated that the intensity of absorption peak (assigned as the A0 band) located at ~2.1 eV increases with increasing degree of crystallinity. Another absorption peak at ~2.4 eV can be attributed to the distorted portion or amorphous region. According to the H-aggregate model,29 the magnitude of the exciton bandwidth (W) can be estimated from the intensity ratios of the A1 and A0 bands, as expressed in Eq. (6): 𝐴0 𝐴1

1―

0.24𝑊 𝐸𝑝

1+

0.073𝑊 𝐸𝑝

≈(

)

(6)

where Ep is the energy of the main intramolecular vibration coupled with the electronic transition. If Ep is set as 0.18 eV (C=C stretching vibration in the thiophene ring), then the relevant W of all samples can be calculated. The W values are 89.3, 81.1, 73.2 and 76.4 meV for the 10:0, 10:1, 10:2 and 10:3 blends, respectively. With the W values, Leff for the different P3HT-PMMA films can be estimated by extrapolating the results of Gierschner et al. for an infinite one-dimensional (1D) stack under screened and unscreened intermolecular potentials,30-31 and the average Leff values were obtained and are plotted in Figure 2(d). The observed Leff values can be assumed to be the approximate crystallite size along the long axis direction or the c-axis. Therefore, the microcrystalline domain size within the films can be derived from the XRD and optical absorption spectroscopy measurements. From Figure 2d it can be seen that the PMMA blend enhances the ordering and crystallinity of the P3HT phase, which is consistent with previous report that diluting a semiconducting polymer in a high-bandgap insulating polymer effectively eliminates electron trapping in the semiconductor resulting in predominantly trap-free transport.24 Specifically, the P3HT-PMMA (10:2) film possesses the smallest W and longer La and Leff, which correspond to a larger crystalline domain size.


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Based on the above results, the P3HT-PMMA (10:2) film is the most crystalline, which typically translates into enhanced charge transport performance, possibly also in a diode architecture.22 However, the data of Table 1 clearly indicate that the P3HT-PMMA (10:1)/IGZO diode exhibits the highest performance. Because the current flows vertically from the P3HT to the IGZO layer at forward bias, not only the crystalline semiconductor portion but also the amorphous P3HT-PMMA regions play an important role in the diode characteristics. Thus, to determine whether vertical phase separation occurs in our blends on the IGZO surface, cross-sectional transmission electron microscopy (TEM) images and Energy-dispersive X-ray spectroscopy (EDX) line-scan profiles were acquired (Figure 3). For these measurements the P3HT-PMMA (10:3) blend was used to enhance contrast. From the TEM image of Figure 3a, it is clear that there is a light layer (~6 nm) in between the P3HT layer and the IGZO layer, which we attribute to the PMMA layer. The vertical phase separation between P3HT and PMMA is explained by their different surface energies, 35.4 VS. 44.0 mJ m-1, respectively.32 Thus, the PMMA segregation to the more hydrophilic IGZO surface is energetically more favorable than the segregation of P3HT to the IGZO surface. The P3HT-PMMA blend film morphology is further confirmed by the compositional line profiles by EDX, as shown in Figure 3b. The mapping scans of the C, S, O, In, Ga and Sn distributions shown in Figure S3 indicate continuous layer-by-layer deposition with negligible interpenetration. Because only P3HT contains sulfur atoms, the sulfur content was used to trace the P3HT within the blend films, which locates it at about 40-80 nm in the depth profile. Since only PMMA contains oxygen atoms, PMMA mainly localizes at the bottom of the blend film, at about 75 – 80 nm, a thickness in agreement with the TEM image. Moreover, the oxygen signal originates earlier than the indium one, further corroborating that an oxygen-containing species in the organic blend, thus PMMA, segregates in the region near the IGZO surface.


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Figure 3. (a) Cross-sectional TEM and (b) EDX chemical profiles of P3HT-PMMA (10:3) blend film.

Figure 4. UPS spectra of (a) representative cutoff and (b) valence band regions of IGZO and PMMA coated IGZO films. To investigate the influence of a thin PMMA layer on the IGZO film electronic structure, and thus explain the transport characteristics, ultraviolet photoemission spectroscopy (UPS) was performed to measure the film work function (WF). The UPS spectra of the bare IGZO film and that having an ultra-thin PMMA top layer (~2 nm) are shown in Figure 4 and the WF assessed from the secondary electron cutoff. The WF of IGZO increases from 3.96 eV to 4.14 eV after PMMA coating. It is well-established that metal oxide surfaces are hydrophilic in nature due to presence of surface OH groups, so deposition of PMMA on IGZO reduces the hydrophilicity which


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is known to enhance IGZO film WF.33-34 Note that the WF of P3HT measured here by UPS (4.37 eV, Figure 4) is similar to that in previous reports.35-36

Figure 5. Band diagrams of P3HT/IGZO heterojunction diodes. (a) non-equilibrium condition without PMMA, (b) non-equilibrium condition with PMMA, (c) equilibrium condition without PMMA, (d) equilibrium condition with low content PMMA and (e) equilibrium condition with high PMMA content. The curving lines in the band structure represent the relevant depletion region. Figure 5 summarizes the underlying physical mechanism for the P3HT/IGZO heterojunction diode behavior. Figures 5 a-b and c-e show the formation of a staggered heterojunction in nonequilibrium and in equilibrium conditions. Under the equilibrium condition, a barrier at the interface will form due to the WF difference between P3HT and IGZO. In this type of heterojunction, due to the electronic structures of the two materials, the electron diffusion barrier (qφb) is significantly lower relative to that for holes, and therefore electrons are the dominant charge carriers in this device. Thus, in forward bias conditions (Figure 5c) electrons from IGZO can be injected into the P3HT layer while in reverse bias both electrons and holes encounter a significant potential barrier. For the P3HT-PMMA blends having a low amount of insulating polymer, a thin PMMA interlayer forms at the P3HT/IGZO interface due to vertical phase separation, which increases the IGZO WF, decreasing φb and thus enhancing the forward current


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(Figure 5d)22, 37 However, when the PMMA content in the blend increases further, the insulator film thickness increases to a point where electrons can no longer tunnel through the barrier, resulting in suppressed electron flow (Figure 5e). Therefore, the electronic structure of the materials corroborates the P3HT-PMMA(10:1)/IGZO diode high electrical performance. Finally, to demonstrate all solution-processed junctions, IGZO films were also deposited by spin-coating of a solution combustion formulation. Details of film/device fabrication processes are reported in the Supporting Information. The solution-processed P3HT-PMMA(10:1)/IGZO diodes also exhibit excellent rectifying behavior (Figure S4) with a forward current of 7.610-4 A at +2V, a rectification ratio of 5103, as well as n = 2.8 and φb = 0.87 eV, demonstrating broad generality of our bilayer semiconductor structure for diode applications. In conclusion, high-performance hybrid diodes comprising spin-coated p-type P3HT-PMMA blend films and vapor/solution-processed n-type IGZO films were fabricated. The results clearly show that the insulating polymer greatly affects the morphological and microstructural properties of the organic blend deposited on top of the inorganic oxide film. More importantly, the PMMA phase separates vertically at the interface between the IGZO and the P3HT layers and reduces the work function of the underlying IGZO film. By varying the PMMA content in the organic blend the PMMA layer thickness varies, which affects electron transfer from the IGZO to the P3HT layers. The optimized diodes, fabricated with P3HT-PMMA(10:1)/IGZO, exhibit 10-fold larger forward currents than those based on neat P3HT film opening a new route to inexpensive hybrid electronic devices. ASSOCIATED CONTENT Supporting Information.


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The Supporting Information is available free of charge on the ACS Publications website. Additional figures including GIXRD, AFM, I-V curves and colored elemental mapping images (PDF) AUTHOR INFORMATION Corresponding Authors Tobin J. Marks E-mail: [email protected] Antonio Facchetti E-mail: [email protected] Author Contributions X. Z and B. W contributed equally. X. Z and B. W carried out the main part of experiments and drafted the manuscript. W. H. designed the experiment and polished the manuscript; G. W. participated in the measurements and performed the analysis. W. Z. and Z. W. participated in the design of the study. W. Z. gave suggestions on the experimental design. T. M. and A. F. supervised the experiments and led the work. All authors have approved the final version of the manuscript. Funding Sources We thank US-Israel Binational Science Foundation (BSF) (AGMT-2012250///02), AFOSR grant FA9550-18-1-0320, the Northwestern U. MRSEC (NSF DMR-1720139), and Flexterra Corp. for support

of

this

research.

A.

F.

thanks

the

Shenzhen

Peacock

Plan

project

KQTD20140630110339343 for support. X.Z. thanks the National Natural Science Foundation of China (Grant No. U1504625) and the youth backbone teacher training pro-gram in Henan province (Grant No. 2017GGJS021). This work made use of the J. B. Cohen X-Ray Diffraction Facility,


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EPIC facility, Keck-II facility, and SPID facility of the NUANCE Center at Northwestern U., which received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF NNCI-1542205); the MRSEC program (NSF DMR-1720139) at the Materials Research Center; the International Institute for Nanotechnology (IIN); the Keck Foundation; and the State of Illinois, through the IIN. Notes The authors declare no competing financial interest. Abbreviations P3HT, poly(3-hexylthiophene); PMMA, poly(methyl methacrylate); IGZO, indium-gallium-zinc oxide; OTFTs, organic thin-film transistors; AFM, atomic force microscope; GIXRD, grazing incidence X-ray diffraction; RMS, root mean square; TEM, transmission electron microscopy; UPS, ultraviolet photoemission spectroscopy; WF, work function

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