Improved hole injection / extraction using PEDOT : PSS interlayer coated onto high temperature annealed ITO electrode for e � cient device performances

Gnyaneshwar Dasi National Institute of Technology Warangal Thyda Lavanya National Institute of Technology Warangal Govindasamy Sathiyan Indian Institute of Technology Kanpur Raju Kumar Gupta Indian Institute of Technology Kanpur Ashish Garg Indian Institute of Technology Kanpur P. Amaladass Loyola Academy Degree and Post Graduate College THANGARAJU KUPPUSAMY (  ktr@nitw.ac.in ) National Institute of Technology Warangal https://orcid.org/0000-0002-3956-3260


Introduction
The recent technological developments and commercial applications of organic optoelectronic devices such as organic light emitting diodes (OLEDs) and organic photovoltaic cells (OPVs) have sparked wide research interest in order to improve the device e ciency and device durability for their potential use in the next generation at panel displays, solid state lighting, and renewable solar energy harvesting owing to its ease of large area device fabrication by roll-to-roll printing, light weight, the possibility to fabricate exible device, and cost effectiveness [1][2][3][4][5][6][7][8][9][10][11]. One of the several factors affecting the higher device e ciency and causing the failure of organic optoelectronic devices after continuous operation is the surface quality and/or bulk properties of indium tin oxide (ITO) lm (anodic electrode) [12][13][14]. ITO is the n-type transparent conducting oxide (TCO) material most commonly used as thin anodic electrode in various electronic and/or optoelectronic devices such as transparent coatings for solar energy heat mirrors, liquid crystal at-panel displays, OLEDs, OPVs, photo-transistors, and lasers, due to its excellent surface adherence, hardness, chemical inertness, wide band gap, higher transparency in the visible region, good electrical conductivity, high infrared re ectance, and tunable high work function [8][9][10][11][12][13][14][15]. The as-deposited ITO lm is found to be less e cient for hole injection in OLEDs and hole collection or extraction process in the OPVs [16].
Since the ITO lm is in direct contact with the organic thin lm in the devices, the surface properties of ITO anodic electrode directly affect the hole injection/extraction process at ITO/organic interface and electrical properties of the devices [17,18]. Thus, the surface of ITO anode substrate has been modi ed by various methods such as chemical (Aquaregia, RCA), plasma (O 2 , Ar, H 2 ), and UV-ozone treatments in order to improve the device e ciencies [14,19,20].The post-deposition thermal annealing of ITO lms prepared by various techniques such as direct current (dc) and radio frequency (rf) magnetron sputtering, electron beam evaporation, pulse laser method, spray pyrolysis, and the sol-gel method, has exhibited improved structural, optical and electrical properties. Maurya et al. has studied the post-thermal treatment of rf reactive sputtered ITO lms and reported the improved crystalline structure with minimum resistivity of 8.3 x 10 − 4 Ωcm and higher transparency of 90% in the visible region for the lms annealed at 400 ºC [21]. Cho et al. have reported that the as-deposited amorphous ITO lm prepared by electron beam evaporation system exhibited the crystalline structure with lower sheet resistance and higher transmittance over 85 % in the visible region after post-annealing at 300 ºC in an oxygen environment [22]. In contrary, the post-annealing (under the normal ambient, air) of rf magnetron sputtered ITO thin lms caused slight increase of resistivity upto 250 ºC which is attributed to free electron scattering by inoized impurities, whereas the resistivity abruptly increased above 250 ºC because of the change in the free electron scattering mechanism by chemisorbed oxygen atoms adsorbed at grain boundaries [23].
In support of the above results, our earlier studies on ITO substrates annealed at different temperatures under the normal ambient revealed that as annealing temperature increases upto 300°C, the ITO lm quality improves with slight increase in the sheet resistance (92 Ω/€ for pristine and 105 Ω/€ for 300°C), enhancing the hole injection properties at the ITO/organic interface in hole-only devices, whereas the annealing of ITO at 400°C forms the wrinkle kind of surface morphology with higher sheet resistance of 185 Ω/€, limiting the hole current through the interface in the device [24]. These reports show that the lm quality (bulk), surface chemical and morphological properties of ITO thin lm are the important parameters to make it more e cient to be used as anodic electrode in the optoelectronic devices. In the recent years, a polymer poly (3,4-ethylenedioxythiophene):poly(styrene sulfonate)(PEDOT:PSS) has been most extensively studied for their wide use in conductive layers, capacitors, antistatic coatings, and organic optoelectronic devices due to its high conductivity, high transparency, easy lm deposition, and mechanical exibility [25][26][27]. When the solution processed PEDOT:PSS layer is introduced in light emitting diodes and organic solar cells as buffer layer, it facilitates the hole transport and hole injection/extraction process at the interface, thus enhancing the e ciency and lifetime of the devices [28,29].
In this report, we study the optical, structural, and morphological properties of the bilayer structure of PEDOT:PSS lm coated onto pristine and annealed (200, 300, 400°C) ITO lms using various techniques such as X-ray diffraction (XRD), UV-visible transmittance, scanning electron microscopy (SEM), and fourprobe method. We fabricate the hole-only devices (HODs) based on PEDOT:PSS coated pristine or annealed ITO lm (anodic electrode) to study the hole injection/extraction characteristics of the structure in the devices. This bilayer structure is also utilized in the OPVs for the improved device performances.

Experimental Details
ITO coated glass substrates (TECHINSTRO, ITO-TIX005) with the thickness of ~ 200 nm were cut into 2×2 cm 2 area and were annealed at 200, 300, and 400°C in the tubular furnace under the normal ambient for 1 hour. The pristine and annealed ITO lms were rinsed and ultrasonically cleaned in soap solution, deionized water, acetone, isopropyl alcohol (IPA) and nally in deionized water for 10 minutes, and dried for 10 minutes at 120°C in each step using vacuum oven to evaporate the solvents used, ngerprints and other oil contaminations on the ITO surface. These ITO substrates were cleaned using plasma cleaner (Harrick Plasma, PDC-002) under the ambient pressure of 800 micron for 10 min to remove the unwanted ions on the ITO surface and followed by UV light treatment for 10 minutes under the normal ambient using UV light curing system (Kaivo, UV-1318) to increase the work function and/or adherence of ITO lms. The PEDOT:PSS solution (Sigma-Aldrich, Catalog No.: 739316) was ltered using 0.45µm syringe lter and spin-coated onto the cleaned pristine and annealed ITO lms at 5500 rpm for 60 seconds using programmable spin-coating unit (Apex Instruments, spinNXG-P2 model) and dried at 120°C for 10 minutes. The thickness of spin-coated PEDOT:PSS lm was measured to be around 45 nm using stylus pro lometer (Bruker's DektakXT stylus). The optical transmittance spectra of the bilayer structure of PEDOT:PSS lm coated onto the pristine and annealed ITO lms were measured using ultraviolet -visible (UV-vis) spectroscopic technique (Agilent, Carry 5000). The measurements of sheet resistance of the bilayer structure were carried out using the current/voltage source/measure unit (Keithley SMU, 2450 model) by four-point collinear probe method at room temperature. The X-ray diffraction spectra were recorded using XRD (PANanalytical Netherlands, XPERT-PRO model) with Cu K-alpha (X-ray source) radiation (wavelength (λ) of 1.5406 Å). The surface morphological studies were carried out using SEM (TESCAN, VEGA3LMU model).

Fabrication of Hole-Only devices (HODs)
The pristine and annealed (200, 300, and 400°C) ITO lms were patterned for the device area of 4 mm 2 by chemical etching process using hydrochloric acid (HCl) and zinc dust, and thoroughly cleaned by the above substrate cleaning procedure. The Hole-only devices (HODs) were fabricated with the device structure

Fabrication of Organic Photovoltaic Cells (OPVs)
The 15 mg/mL of P3HT and PCBM each with the volume ratio of 1:1 were dissolved in chlorobenzene solvent and stirred at 600 rpm at 50°C in glove box under the N 2 ambient for 12 hours followed by ltration using 0.45 µm syringe lter (Solution A). The PEDOT:PSS solution ltered using 0.45 µm syringe lter (Solution B) was spin-coated onto the cleaned pristine or annealed ITO lms as a buffer layer at 5500 rpm for 60 sec and dried at 120°C for 10 minutes on hot plate. The P3HT:PCBM (active) layer was spin-coated using solution A onto the PEDOT:PSS layer at 800 rpm for 60 sec and dried at 150°C for 10 minutes on hot plate. Lithium uoride (LiF) as electron injection layer (EIL) and aluminum (Al) as cathodic electrode were deposited using metal mask for the device area of 4 mm 2 by thermal evaporation process at ~ 5×10 − 6 Torr. The structure of fabricated OPVs is of ITO (pristine or annealed at 200, 300, 400°C)/PEDOT:PSS (45 nm)/P3HT:PCBM (100 nm)/LiF (1 nm)/Al (120 nm). The fabricated OPVs were characterized using current/voltage source measurement unit (Keithley's SMU unit, 2450 model) and the photocurrent was measured under AM 1.5 solar simulator source of 100 mW/cm 2 .

Results And Discussion
The bilayer structure of PEDOT:PSS layer spin-coated onto the pristine and annealed (200, 300, and 400°C) ITO lms is characterized by using various studies. Figure 1 shows the optical transmittance spectra of PEDOT:PSS coated pristine and annealed (200, 300, and 400°C) ITO lms. It is observed that the PEDOT:PSS/pristine ITO bilayer structure is having good transparency (96 % at 550 nm) in the visible region without any signi cant change for increasing annealing temperature of ITO lm (95 % for 200°C and 96 % for 300°C at 550 nm). It has been reported that annealing the bare ITO lm under the normal ambient forms the light absorbing/scattering centers in the lm and slightly decreases the transparency in the visible region [24]. The transmittance of PEDOT:PSS/annealed (400°C) ITO bilayer structure is considerably decreased in the UV region (33 % at 325 nm) when compared to other lms (58 % for pristine, 61 % for 200°C and 57 % for 300°C at 325 nm) without any signi cant change in its transmittance in the visible region (97 % for 400°C at 550 nm), which may be attributed to the signi cant change in the ITO lm and/or in the ITO/PEDOT:PSS interface properties upon annealing of ITO at high temperature of 400°C [23,24].
The direct optical bandgap energy (E g ) of PEDOT:PSS/pristine or annealed ITO bilayer structure is determined from transmittance spectra using the following formula.
(αhυ) 2 = A(hυ − E g ) ……………………….. (1) Where, α (α = 2.303(1/T)/t, T -transmittance, t -total lm thickness) is the absorption coe cient, hυ is the photon energy, and A is the constant. The inset of Fig. 1 shows the plot of (αhυ) 2 versus hυ, and the optical band gap (E g ) energies were obtained by extrapolating the linear portion of (αhυ) 2 against to the photon energy (hυ) where the absorption coe cient (α) is equal to zero. The PEDOT:PSS coated pristine ITO lm exhibited the band gap energy (E g ) of 3.75 eV which is consistent with the reported values [24].
No signi cant change in the optical band gap (3.73 eV for 200°C and 3.67 eV for 300°C) is observed for annealing of ITO lm upto 300°C in the bilayer structure and the similar trend has been reported in the case of bare annealed ITO lm below 300°C [24]. The PEDOT:PSS/annealed (400°C) ITO bilayer structure showed a slight decrease in the optical band gap (3.43 eV), attributed to the quantum con nement effect due to increase in crystallite size upon annealing [24,30]. Where D is the crystallite size, λ is the X-ray wavelength (1.5046 Å), β is the full width half maxima and θ is the diffraction angle. It is observed that the average crystallite size is increasing as annealing temperature of ITO lm in the bilayer increases (Fig. 3), which is attributed to combining of native crystalline grains into large size upon annealing under the normal ambient and resulting in the improved ITO lm quality [24].
The dislocation density (δ) is de ned as the length of dislocation lines per unit volume of the crystal. It is calculated for PEDOT:PSS/pristine or annealed (200, 300, and 400°C) ITO bilayer using the following equation [31]. The estimated values are given in Table 1 and shown in Fig. 4. Where δ is the dislocation density, n (= 1 for minimum dislocation density) is the factor, and D is the crystallite size. The strain induced in the lattice sites due to the lattice mismatch in the crystalline material is called as lattice strain (ε) and it is estimated from the Williamson-Hall equation [32]. ε = β hkl /4tanθ ……………………….. (4) Where ε is the lattice strain, β is the FWHM, θ is the diffraction angle. From the Fig. 4, the trend of decreasing lattice strain with increase of annealing temperature of ITO lm in the bilayer structure is attributed to the observed decrease of dislocation density contributed by decreasing of grain boundaries due to growing crystallites [33,34].  Figure 8 shows the current density-voltage (J -V) characteristics of the HODs. The HOD -A based on pristine ITO exhibits the on-set of hole injection above 0.5 V below which the leakage current dominates and hence the hole injection current can barely be extracted [36]. It is also observed that the hole injection on-set is hindered at around 0.5 V for other HODs (B -D) due to the leakage current domination. The hole current density of HOD -C at lower bias voltage is observed to be slightly less (45 µA/cm 2 at 1 V, 0.28 mA/cm 2 at 3 V, and 3.67 mA/cm 2 at 6 V) when compared to that of HOD -A (83 µA/cm 2 at 1 V, 0.28 mA/cm 2 at 3 V, and 2.80 mA/cm 2 at 6 V) and HOD -B (97 µA/cm 2 at 1 V, 0.34 mA/cm 2 at 3 V, and 3.24 mA/cm 2 at 6 V) without any signi cant change in hole current density at higher deriving voltages as shown in Fig. 8.

Conclusion
The annealing of ITO lm at 400°C reveals higher transparency in the visible region and improves the lm quality upon aggregation of native grains into large crystallites, resulting in decreased dislocation density and lattice strain, and leaving the wrinkle kind of surface morphology. The spin coating of PEDOT:PSS lm onto the wrinkle kind of surface morphology smoothens the surface and absorbs the UV light signi cantly. The higher hole current density of the HODs based on ITO annealed (400°C)/PEDOT:PSS interlayer may be associated with decreased potential barrier at the ITO/PEDOT:PSS interface for hole injection process into the device due to improved interfacial (energy levels and/or surface area) properties. The OPV using the annealed (400°C) ITO/PEDOT:PSS buffer layer exhibits the e ciency three times higher (η = 1.69 %) than that (η = 0.48 %) of pristine ITO based device. These results show that the ITO lm (annealed at 400°C)/PEDOT:PSS interface favors the effective hole injection/extraction process in the devices for the improved performances. The spin coating of PEDOT:PSS layer onto the annealed (400°C) ITO anodic electrode could be an effective way to improve the device performances of OLEDs and OPVs for the commercial applications.