Ultrasonic-Assisted Spin-Coating: Improved Junction by Enhanced

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Applications of Polymer, Composite, and Coating Materials

Ultrasonic-Assisted Spin-Coating: Improved Junction by Enhanced Permeation of Coating Material within Nanostructures Jong-Won Yun, Farman Ullah, Se-Jeong Jang, Do Hui Kim, Tri Khoa Nguyen, Ki Yeon Ryu, Shinuk Cho, Joon I. Jang, Dooyong Lee, Sungkyun Park, and Yong Soo Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04516 • Publication Date (Web): 22 May 2018 Downloaded from http://pubs.acs.org on May 22, 2018

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ACS Applied Materials & Interfaces

Ultrasonic-Assisted Spin-Coating: Improved Junction by Enhanced Permeation of Coating Material within Nanostructures Jong-Won Yun†, Farman Ullah†, Se-Jeong Jang†, Do Hui Kim†, Tri Khoa Nguyen†, Ki Yeon Ryu†, Shinuk Cho†, Joon I. Jang‡,*, Dooyong Lee§, Sungkyun Park§, and Yong Soo Kim†,* †

Department of Physics and Energy harvest Storage Research Center, University of Ulsan, Ulsan

44610, South Korea. ‡

Department of Physics, Sogang University, Seoul 04107, South Korea

§

Department of Physics, Pusan National University, Busan 46241, South Korea

KEYWORDS: ultrasonic-assisted spin-coating, coating technique, void removal, interfacial resistance, organic solar cell.

ACS Paragon Plus Environment

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ABSTRACT

Over the last decades, the spin coating (SC) technique has been widely used to prepare thin films of various materials in liquid phase on arbitrary substrates. The technique simply relies on centrifugal force to spread a coating solution radially outwards over the substrate. This mechanism works fairly well for solutions with low surface tension to form thin films of reasonable junctions on smooth substrates. Here, we present a modified SC technique, namely ultrasonic-assisted spin-coating (UASC), to form thin films of coating solution having high surface tension on rough substrates with excellent junctions. The UASC technique couples SC with an external ultrasonic wave generator to provide external perturbation to locally break down big drops of the coating material into smaller droplets via Rayleigh instability. Due to their lower mass, these tiny droplets gain low momenta and move slowly both in radial and azimuthal directions, giving them an enough time to effectively permeate within pores, thereby yielding excellent junctions. Furthermore, we also investigate the effect of junction improvement on conventional and inverted bulk heterojunction organic solar cells. Intriguingly, the organic solar cells fabricated by the UASC method showed an improved efficiency compared to typical SC owing to efficient charge transfer across the junction. These results clearly imply that UASC is a simple and powerful technique which can significantly enhance the device performance by improving the junction. Moreover, we believe that UASC can be more effective for the preparation of devices comprised of multilayers of different materials having complicated nanostructures.


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1. INTRODUCTION A number of techniques so far have been established for the preparation of thin films, such as molecular beam epitaxy, pulsed laser deposition, atomic layer deposition, and spray coating, etc.1-7 Among them spin coating (SC) is a simple, low-cost and widely used method for the deposition of various materials in liquid phase on an arbitrary substrate.4,8-12 However, thin films prepared by SC usually show poor junctions.15,16 The junction issues become more apparent in substrates having complicated nanostructures especially when the coating materials have high surface tension.13-14 A solution having high surface tension usually forms big droplets during the coating process due to strong attractive forces among the molecules. Each droplet behaves as if its surface is covered with an elastic membrane. During the SC process, these big drops gain large momenta due to their high mass and quickly move radially outwards over the top surface of the substrate without interacting with small pores within the substrate. This phenomenon inevitably creates voids at the junction region. Typically, these voids significantly reduce the device performance by trapping as well as scattering charge carriers.13-14 Pertinently, PEDOT:PSS with high surface tension (71.2 dyne/cm) is normally used as a hole transport layer (HTL) or a blocking layer on a transparent conducting oxide (TCO) substrate in bulk heterojunction organic solar cells (BHJ OSCs).17-19 In such a device, a junction containing voids increases the interfacial resistance, limiting charge transfer across the HTL/TCO interface, and hence reduces the device performance.18-20 This effect can be even more severe in devices comprised of substrates having complex nanostructures (nanorods, nanotubes, etc.) or low surface free energy. Previously, an attempt to improve the junction between the HTL and TCO has been made by exposing the TCO substrate to ultra-violet (UV) light before coating the material.21,22 Although this technique somehow improves the junction, it is comparatively


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complicated and needs to be done in the special N2 environment. Also, the substrate should be protected from oxygen even after the UV treatment.21,22 Here, we introduce a newly developed ultrasonic-assisted spin-coating (UASC) technique for significantly improving the junction. An ultrasonic wave generator (see the METHOD) is coupled with a conventional SC method to produce ultrasound waves during the coating process. The ultrasonic waves provide external energy to locally break down big drops of the coating material into tiny droplets, helping them to infuse within small pores present on the substrate. This enhances the connectivity between the thin film and the underlying substrate, thereby creating a void-free junction. We verified this hypothesis by inspecting the cross-sectional field emission scanning electron microscopy (FE-SEM) images of PEDOT:PSS and TiO2 solution having high surface tension, coated on rough F-doped indium tin oxide (FTO) and Al2O3 nanotubes substrates. Furthermore, we also studied the effect of junction improvement on the OSC efficiency. We prepared conventional and inverted BHJ OSCs by depositing the HTL (PEDOT:PSS) and the electron transport layer (ETL, TiO2) on the FTO substrates by UASC and SC. We also prepared another set of conventional OSCs by depositing the HTL layer by SC on a UV-treated FTO substrate. It was found that the OSCs prepared by UASC show the highest efficiency. The enhancement in the efficiency can be attributed to the improved junction. We believe that our UASC method can be further utilized for preparing high-quality thin films that in turn would improve the various physical performances of the films in general.

2. EXPERIMENTAL SECTION 2.1 Materials: Poly(3.4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) with pH

1,

poly(3-hexylthiophene)

(P3HT),

Poly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-


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b']dithio-phene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenedi (PTB7), [6,6]-phenyl C61-buthyric acid methyl ester (PC61BM), [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) and 1,8-diiodooctane (DIO) were bought from OSM Co. Chlorobenzene (98 %), 1-(chloromethyl) naphthalene (97 %) and titanium(IV) isopropoxide (TTIP, 97 %) were bought from Sigma-Aldrich. Al2O3 nanotubes (Al2O3 NTs) with 40 nm of pore diameters were bought from InRedox. 2.2 Ultrasonic-assisted spin-coating (UASC): To make our UASC system, we connected five ultrasonic vibrators (Saehansonic Co, BLT-40, 40 kHz, 300 W) to a conventional SC system (Midas Co, Spin 1200D) by epoxy (CHEMTROS Co, YS-303). The ultrasonic vibrators were located symmetrically. The coupling between the spin coater and ultrasonic vibrator can simply be achieved by firmly attaching the ultrasonic vibrator within the spin coater chamber such that maximum ultrasonic waves hit the sample. Moreover, the ultrasonic vibrator is not attached with the spinor therefore; it does not rotate during the coating process. The schematic diagram of UASC is shown in Fig. S11. 2.3 TiO2 coating on Al2O3 NTs: The TiO2 solution with 140 mM concentration was prepared by adding 0.015 mL of HNO3 (60 %) and 1.425 mL of TTIP to ethanol. 0.345 mL of DI water was added drop by drop to the mixture and was stirred for 5 min. The molar ratio of Ti:H2O:HNO3 was 1:4:0.04. To investigate the effect of UASC, the TiO2 solution was coated on Al2O3 NTs at 7000 rpm for 30 sec by both UASC and SC. The Al2O3 NTs coated with TiO2 were annealed at 450 for 1 hour. 2.4 Fabrication of organic solar cells (OSCs): Prior to fabrication of the OSC device, the FTO substrate (F:SnO2, Tec 20, 7 Ω/□) was cleaned by immersing it in a mixture of DI water,


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acetone, and 2-propanol (1:1:1 of volume ratio) and ultrasonicated for 30 min. Two types of conventional OSCs were prepared by using P3HT:PC61BM and PTB7:PC71BM as active materials coated on the HTL (PEDOT:PSS), respectively. The HTL was coated at 5000 rpm for 40 sec on the FTO substrate by SC and UASC. The sample was then kept at 120 °C on a hot plate for 10 min. The P3HT:PC61BM (1 wt%) as an active layer was prepared by dissolving 13.1 mg of P3HT and PC61BM (1:1 weight ratio) in a mixture of 1 mL of chlorobenzene and 30 μL of 1-(chloromethyl)naphthalene. The active layer was then coated at 800 rpm for 60 sec by SC and was kept at 100 °C for 10 min [conventional (P3HT:PC61BM) OSC, Fig. 2(a)]. In other conventional (PTB7:PC71BM) OSCs in which PTB7:PC71BM was used as an active layer, the PTB7:PC71BM (1 wt%) solution was prepared by dissolving PTB7 and PC71BM (1:1.5 weight ratio) in a mixture of chlorobenzene with 3 vol% of DIO. The solution was coated at 1000 rpm for 60 sec by SC [Fig. S6 (a)]. The Al electrode (100 nm) was deposited by thermal evaporation. Finally, the OSCs were annealed at 110 °C for 10 min. On the other hand, the fabrication of inverted OSCs involves the ETL. TiO2 as the ETL was coated at 1000 rpm for 30 sec on the FTO substrate by UASC. The sample was then dried at 300 °C and annealed at 450 °C for 1 h. The P3HT:PC61BM active layer was then coated at the same condition. Finally, about 5 nm of MoO3 as the HTL and about 60 nm of the Ag electrode were deposited by thermal evaporation. All steps were done in the glove box filled with N2. For comparative analysis, we also prepared UVtreated OSCs. 2.5 Characterization: Field emission scanning electron microscopy (FE-SEM, JEOL, JSM 7600F) and atomic force microscopy (AFM) were used to study the junction characteristics and surface morphology. The AFM images (scan area: 5 µm × 5 µm) were obtained using a Seiko ESweep atomic force microscope in a tapping mode. The current density-voltage (J-V) curves


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were measured by Keithley 2401 source meter under AM 1.5 illumination calibrated with a standard Si photodiode detector of a KG-3 filter (Newport Co., Oriel). Impedance measurement was carried out at room temperature using impedance spectroscopy (IviumStat, Ivium Tech) in the frequency range from 1 MHz down to 1 Hz, and the results were fitted by the Z-view program. The EQEs of the OSCs were obtained using a solar cell spectral response QE/IPCE measurement system (Newport Co, Oriel IQE-200). Contact angle (Phoenix 300, SEO) was measured by using ethanol (99.9 %), DI water, TiO2 solution and PEDOT:PSS solution, recorded for 3-times at each 5-points on both Si and FTO substrate. Ultraviolet photoelectron spectroscopy (UPS) study was carried out with a He(I) beam source (21.21 eV). A home-built Physical Properties Measurement System (PPMS) using a four probe van der Pauw configuration was used to characterize the electrical resistance.

3. RESULTS AND DISCUSSION The working mechanism of SC and UASC are schematically illustrated in Figs. 1(a) and 1(b), respectively. SC only uses centrifugal force to spread out a coating solution over the substrate. This mechanism can produce a thin film with a reasonably good junction on flat substrates provided surface tension of the solution is low. On the other hand, if the coating solution has high surface tension or the substrate has a rough surface or low surface free energy, the resultant thin film shows a poor junction. This phenomenon can be explained as follows: the solution tends to form large drops due to strong attractive forces among the molecules (high surface tension), as schematically depicted in Fig. 1(a). These drops can be assumed as small membranes. As the spinning speed increases, these drops quickly gain large momenta due to their high mass and spread over the substrate. The magnitude of mass and respective momentum (‫݌‬Ԧ) of each


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drop are represented by the drop size and the length of the arrow in Figs. 1(a) and 1(b), respectively. The tangential component of the momentum also compels the drops to move radially outwards beside the circular motion. The bigger the drop size, the higher will be the radial component of the momentum (due to larger centrifugal force). Thus, the drop will quickly spread over the substrate with no enough time for the coating material to permeate within pores of the substrate. This effect is represented by the spacing between the circles in Fig. 1(a). Moreover, due to strong attractive forces, the big drops of high surface tension tend to remain over the top edges and thus create voids between the film and the substrate, as schematically represented in bottom of Fig. 1(a). We resolved this serious issue simply by UASC. The idea behind using the ultrasonic wave generator is to provide external energy to these big drops through ultrasonic waves which will eventually break them into tiny pieces. Fig. 1(b) schematically illustrates the mechanism of the UASC coating process, which can be roughly understood in terms of Rayleigh instability.23-25 Ultrasonic waves provide external vibration (periodic perturbation) to the solution flowing on the substrate during the coating process. This periodic perturbation generates the tiny droplets by overcoming its internal cohesive energy.23-25 The tiny droplets that receive lower centrifugal force during SC move slowly in both circular and radial directions over the substrate surface and easily penetrate into narrow regions on the substrate, thereby yielding a void-free junction, and homogeneously deposit the solution over the entire substrate, as schematically shown in Fig. 1(b). Clearly, this dynamic process is absent in conventional SC. Initially, the UASC performance was tested on a porous substrate. For comparison, TiO2 and PEDOT:PSS solutions were coated on rough FTO substrates by SC and UASC and their junction property was examined by FE-SEM, as shown in Figs. 1(c) and 1(d) (see also Figs. S1 and S2).


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The interface in Fig. 1(c) clearly have voids, while the interface in Fig. 1(d) reveals a void-free junction, confirming that UASC comparatively produces a thin film with a better junction. Moreover, the cross-sectional images at different locations, starting from the center of the sample and close to the edge, were taken to investigate the homogeneity of the junction. Surprisingly, the junction show identical behavior irrespective of location [Figs. S1 and S2]. Furthermore, the UASC performance was tested in hard coating conditions. We prepared substrates of Al2O3 nanotubes (NTs) having 2 µm of tube length and 40 nm of pore diameter. The TiO2 solution was coated on the substrates using SC and UASC, respectively. The resultant morphology of the samples was directly compared using FE-SEM. The SC technique slightly fills the NTs with the TiO2 solution, as shown in Figs. 1(e) and 1(f). Although the TiO2 solution has relatively low surface tension, it only formed a thin film over the top of the Al2O3 NTs because of the small diameter of the tube that limits the permeation of the solution into the NTs. On the other hand, UASC completely filled the NTs having a very small diameter (~40 nm), as shown in Figs. 1(g)– 1(j). The morphological examination provides a clear evidence that UASC is a much better approach to achieve high-quality thin films with full coverage. In this sense, our novel method can be even more useful to coat solutions of various materials on complicatedly nanostructured substrates. To demonstrate the effect of UASC on the overall device performance, we applied the UASC method to coat the ETL (TiO2) of inverted BHJ OSCs and the HTL (PEDOT:PSS) of conventional BHJ OSCs formed on FTO substrates with a rough surface and compared the results to SC and UV treatment as well. Figs. 2(a) and 2(b) schematically show the inverted and conventional (P3HT:PC61BM) BHJ OSCs with active material, P3HT:PC61BM, respectively. Figs. 2(c)–2(f) show the cross-sectional FE-SEM images of the inverted and conventional


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(P3HT:PC61BM) OSCs prepared by SC (left) and UASC (right), respectively. The thicknesses in Figs. 2(c)–2(f) of each layer in the inverted and conventional (P3HT:PC61BM) OSCs are same, indicating that UASC does not affect the film thickness. Moreover, because the FTO substrate usually has a rugged surface and low surface free energy [S3 in supporting information (SI)], the SC method could not fill the voids within such a rough substrate, as evident in Figs. 2(c) and 2(e) (red rectangular boxes). On the other hand, the voids were not present when prepared by UASC as shown in Figs. 2(d) and 2(f). As mentioned earlier UASC applies periodic perturbation to HTL or ETL solution during the coating process and locally breaks the fluid into smaller droplets. These tiny droplets can then effectively permeate into the narrow holes on the substrate, enhancing the connectivity between the two materials and improving the junction. A void present at the junction can scatter or trap photo-generated electrons or holes while moving from the active material to the bottom electrode, affecting the device performance as schematically represented in Fig. S4. The effect and reproducibility of the UASC technique in both P3HT:PC61BM based inverted and conventional OSCs were investigated by applying this technique to more than 50 samples. Furthermore, UASC performance was also checked on conventional OSCs by using PTB7:PC71BM as an active material [see Fig. S6(a) for the schematic of a conventional (PTB7:PC71BM) OSC]. The corresponding statistics of solar cell parameters, including current density (JSC), open circuit voltage (VOC), fill factor (FF) and power conversion efficiency (Eff) of the P3HT:PC61BM based conventional OSCs are shown in Figs. 3(a)–3(d), respectively. The rectangular boxes in Fig. 3 represent 2σ plots. The top and bottom of each vertical line crossing the rectangular box represents the maximum and minimum values, while the center of the small rectangular box represents the average values of respective parameters. Moreover, JSC, VOC, FF


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and Eff of P3HT:PC61BM based inverted OSCs and PTB7:PC71BM based conventional OSCs are shown in Fig. S5 and S6, respectively. The parameter values of the three cases of the OSCs are summarized in Table 1. The values of Eff for conventional (P3HT:PC61BM) and conventional (PTB7:PC71BM) OSCs were found to increase up to 14.8% and 7.8%, respectively, by UASC mainly due to increase in JSC, as shown in Table 1. However, the increase in Eff in the inverted OSCs was due to improvement in both JSC and VOC. The UASC method mainly improves the junction connectivity and does not change the film thickness or morphology. Figs. S7, S8 and S9 show the surface morphology and electric properties (work function and sheet resistance) of PEDOT:PSS and TiO2 coated by SC and UASC, respectively. The root mean square (rms) of surface roughness, work function and sheet resistance are given in Table 2. The data confirm that UASC does not affect chemical or structural properties of the material. Therefore, in both of the conventional OSCs by UASC, the VOC values remain the same while the JSC values were found to increase owing to better charge transfer. In other words, the improved Eff values in both of the conventional OSCs can be attributed to the improved junction between PEDOT:PSS and FTO substrates. In contrast, the averaged VOC value in the inverted OSCs was enhanced from 0.50 V to 0.56 V by UASC. This is indeed expected as the compaction of the blocking layer (TiO2) typically influences VOC.26-29 The surface morphology of TiO2 shows porosity [Figs. S8(a)–S8(b)] rather than a compact layer as compared with PEDOT:PSS in Figs. S7(a)–S7(b). The UASC method might have increased the compactness of TiO2 during the deposition that resulted in enhanced VOC. Furthermore, our UASC approach generated conventional (P3HT:PC61BM) OSCs showing slightly better performance as compared to the UV treatment. The UV technique works pretty well if the substrate is used immediately after the treatment without interacting with oxygen. We believe


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that UASC is more effective for solutions having high surface tension. Our results showed the UASC technique enhances JSC of the conventional (P3HT:PC61BM) OSCs when using PEDOT:PSS as the HTL compared with the case of the inverted OSCs due to higher surface tension of PEDOT:PSS solution than TiO2 solution. The values for surface tension of both solutions were calculated by using Owen-Wendt equation and Thomas Young’s equations30-35 and are given in S3 in SI. The statistical data of the three OSCs clearly show that UASC is a reliable approach for thin film preparation having excellent junction properties. Finally, we also investigated the interfacial resistance in the P3HT:PC61BM based conventional OSCs to further verify the junction improvement by UASC. Fig. 4(a) shows the Nyquist plots of impedance results of SC, UV and UASC. Impedance responses can be modeled by an equivalent circuit as shown in Fig. S10: Fig. 4(b) is the blow-up of the blue box in Fig. 4(a). Two semicircles were observed for all OSCs corresponding to impedances Z1 and Z2 [Figs. 4(a) and 4(b)]. The values of RS and impedance Z1 [Fig. 4(b)] depend upon the interfacial resistance between the HTL and the substrate.36-40 The total impedance [Fig. 4(a)] represents the total resistance of the OSCs.40-42 A void-free junction decreases the interfacial resistance and leads to a reduction of the total resistance in the device. Resistances of the OSC by UASC (RS = 68 Ω, R1 = 1332 Ω and R2 = 27.7 kΩ) were found to be lower than those of the OSC by UV (RS = 96 Ω, R1 = 1404 Ω and R2 = 38.6 kΩ) and the OSC by SC (RS = 125 Ω, R1 = 1575 Ω and R2 = 51.4 kΩ), as indicated by red, blue and black colors in Figs. 4(a) and 4(b). The lower resistance of UASC clearly indicates a better HTL/FTO junction.43-44 J-V curves and the external quantum efficiencies (EQEs) of the conventional (P3HT:PC61BM) OSCs prepared by SC, UV and UASC are depicted in Figs. 4(c) and 4(d), respectively. All three devices exhibit nearly identical VOC


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about 0.57 V. However, JSC of SC and UASC OSCs were found to be 9.59 mA/cm2 and 10.69 mA/cm2, respectively. Furthermore, JSC of the UASC OSC is also higher than that by UV (10.58 mA/cm2). The higher JSC of the OSC by UASC can be attributed to efficient charge transfer across the interface where the improved junction lowers the interfacial resistance RS and impedance Z1. Furthermore, the OSC prepared by UASC showed ~14.8% and ~1.6% higher efficiency compared to the OSCs by SC and UV, respectively. A similar trend can also be observed in EQE plots [Fig. 4(d)]. These results further confirm that our UASC technique produces thin films with excellent junctions. It is worth mentioning that UASC, which uses ultrasonic waves during the coating process, does not induce any chemical or structural changes within the material (Table 2, Figs. 2(c)–2(f) and Figs. S7–S9). It just locally breaks down solution drops into smaller droplets by overcoming the cohesive energy. Therefore, the improvement in the device performance arises directly from the improved junction.

4. CONCLUSION In summary, we modified the conventional SC system by coupling it with the external ultrasonic wave generator. The UASC system uses ultrasonic waves (periodic perturbation) during the SC process and locally breakdowns the fluid of the coating material into tiny droplets via Rayleigh instability. These smaller droplets feel low centrifugal force, move slowly both in radial and circular directions, and easily permeates into nanostructures, thereby improving the junction as directly verified by the cross-sectional FE-SEM images. The better junction significantly improves the device performance owing to efficient charge transfer across the interface. We believe that this technique can be more useful in devices such as solar cells and the


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photovoltaic cells, which are comprised of multilayers of different materials on substrates having complex nanostructures.

ASSOCIATED CONTENT The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Cross sectional FE-SEM images of the TiO2 and PEDOT:PSS layers on the FTO substrates; Surface free energy of the FTO substrate and surface tension of TiO2 and PEDOT:PSS solutions measured by contact angle results; Schematic description of hole transfer generated from the active layer in organic solar cells prepared by conventional SC and UASC; Statistical data of inverted and conventional (PTB7:PC71BM) organic solar cells; Surface morphology of TiO2, PEDOT:PSS and P3HT:PC61BM layers coated by SC and UASC; Electric properties of TiO2 and PEDOT:PSS coated by SC and UASC; Schematic description of the equivalent circuit for curve simulation of impedance; Schematic description of the UASC setup.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (Joon I. Jang) & [email protected] (Yong Soo Kim) Author Contributions Y.S. Kim and J.I. Jang designed and supervised the project. J.-W. Yun performed a major portion of UASC development, sample preparation, optimization, and basic measurement. J.-W. Yun and F. Ullah wrote the manuscript and revised by T.K. Nguyen, J.I. Jang and Y.S. Kim. S.-J. Jang and D.H. Kim fabricated organic solar cells under supervision of S. Cho. K.Y. Ryu supported the


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development of the UASC machine. D. Lee performed UPS measurement and analysis under supervision of S. Park. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This research was supported by the Priority Research Centers Program (2009-0093818), the Basic Science Research Program (2017R1E1A1A01075350, 2017R1D1A1B03035539), and the Basic Research Lab Program (2014R1A4A1071686) through the National Research Foundation of Korea (NRF), funded by the Korean government.

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Table Legends Table 1. The average values of organic solar cell parameters for more than 50 samples in three cases of organic solar cells.

Inverted Conventional (P3HT:PCBM) Conventional (PTB7:PCBM)

JSC (mA/cm2) SC UASC 5.83 6.19 (6.2 %) 9.59 10.69 (11.5 %) 12.79 13.65 (6.8 %)

VOC (V) SC UASC 0.50 0.56 (12 %) 0.58 0.57 (1.7 %) 0.70 0.71 (1.4 %)

SC 0.40 0.59 0.62

FF UASC 0.44 (10 %) 0.61 (3.4 %) 0.64 (3.2 %)

SC 1.19 3.25 5.75

Eff (%) UASC 1.51 (26.9 %) 3.73 (14.8 %) 6.20 (7.8 %)

Table 2. Surface roughness (rms), work function and sheet resistance of PEDOT:PSS and TiO2 on FTO substrates prepared by SC and UASC, respectively.

PEDOT:PSS TiO2

rms (nm) SC UASC 24.6 23.8 14.5 13.3

Work function (eV) SC UASC 4.91 4.91 4.01 4.03

Sheet resistance (Ω/sq.) SC UASC 1.2 1.0 1.6 1.4


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Figure Legends Figure 1. Schematic representation of the working mechanism of (a) conventional SC and (b) UASC. The cross sectional FE-SEM images of TiO2 (left) and PEDOT: PSS (right) on FTO substrates prepared by (c) SC and (d) UASC. (e) Schematic illustration and (f) cross section FESEM image of TiO2 coated on Al2O3 NTs by SC. (g) Schematic illustration and (h) cross section FE-SEM image of TiO2 coated on Al2O3 NTs by UASC. (i) and (j) Zoom-in images of the areas in (h) enclosed by the red box and the blue box, respectively. Smaller droplets locally broken by UASC feel lower centrifugal force and move slowly both in radial and circular directions, giving them an enough time to easily permeate into nanostructures, thereby improve the junction. Figure 2. Schematic representation of (a) inverted and (b) conventional (P3HT:PC61BM) OSCs. Cross-sectional FE-SEM images of (c-d) inverted and (e-f) conventional (P3HT:PC61BM) OSCs by SC (left) and UASC (right), respectively. The thicknesses in each case were found to be approximately same, but the voids between the HTL or ETL/FTO junctions were effectively removed by UASC. Figure 3. The statistical data of more than 50 conventional (P3HT:PC61BM) OSCs. The results of conventional spin-coating (SC), UV treatment (UV) and UASC are represented in black, blue and red colors, respectively. The solar cell parameters including (a) JSC, (b) VOC, (c) FF and (d) Eff conclusively show that UASC is a reliable technique that yields better OSCs. The notation used in the graphs is shown at the right side of (d). Figure 4. (a) Nyquist representation of the impedance result of P3HT:PC61BM based conventional OSCs. (b) Expanded impedance result of Z1 [blue square in (a)]. The UASC-based OSC exhibits the lowest impedance which indicates that the interfacial resistance between the


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HTL and the substrate is improved by the removal of voids. (c) J-V characteristics and (d) EQE spectra of OSCs. The current density and EQE of UASC are increased due to the reduced interfacial resistance.


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The table of contents


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Figure 1. Schematic representation of the working mechanism of (a) conventional SC and (b) UASC. The cross sectional FE-SEM images of TiO2 (left) and PEDOT: PSS (right) on FTO substrates prepared by (c) SC and (d) UASC. (e) Schematic illustration and (f) cross section FE-SEM image of TiO2 coated on Al2O3 NTs by SC. (g) Schematic illustration and (h) cross section FE-SEM image of TiO2 coated on Al2O3 NTs by UASC. (i) and (j) Zoom-in images of the areas in (h) enclosed by the red box and the blue box, respectively. Smaller droplets locally broken by UASC feel lower centrifugal force and move slowly both in radial and circular directions, giving them an enough time to easily permeate into nanostructures, thereby improve the junction. 180x160mm (300 x 300 DPI)



Figure 2. Schematic representation of (a) inverted and (b) conventional (P3HT:PC61BM) OSCs. Crosssectional FE-SEM images of (c-d) inverted and (e-f) conventional (P3HT:PC61BM) OSCs by SC (left) and UASC (right), respectively. The thicknesses in each case were found to be approximately same, but the voids between the HTL or ETL/FTO junctions were effectively removed by UASC. 206x101mm (300 x 300 DPI)


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Figure 3. The statistical data of more than 50 conventional (P3HT:PC61BM) OSCs. The results of conventional spin-coating (SC), UV treatment (UV) and UASC are represented in black, blue and red colors, respectively. The solar cell parameters including (a) JSC, (b) VOC, (c) FF and (d) Eff conclusively show that UASC is a reliable technique that yields better OSCs. The notation used in the graphs is shown at the right side of (d). 180x127mm (300 x 300 DPI)



Figure 4. (a) Nyquist representation of the impedance result of P3HT:PC61BM based conventional OSCs. (b) Expanded impedance result of Z1 [blue square in (a)]. The UASC-based OSC exhibits the lowest impedance which indicates that the interfacial resistance between the HTL and the substrate is improved by the removal of voids. (c) J-V characteristics and (d) EQE spectra of OSCs. The current density and EQE of UASC are increased due to the reduced interfacial resistance. 176x136mm (300 x 300 DPI)


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Ultrasonic-Assisted Spin-Coating: Improved Junction by Enhanced

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