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Fabrication of Combustion-Reacted High performance ZnO Electron Transport Layer with Silver Nanowire Electrodes for Organic Solar Cells Minkyu Park, Sang-Hoon Lee, Donghyuk Kim, Juhoon Kang, Jung-Yong Lee, and Seung Min Jane Han ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17148 • Publication Date (Web): 05 Feb 2018 Downloaded from http://pubs.acs.org on February 7, 2018

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ACS Applied Materials & Interfaces

Fabrication of Combustion-Reacted High performance ZnO Electron Transport Layer with Silver Nanowire Electrodes for Organic Solar Cells

Minkyu Park,†, § Sang-Hoon Lee, †, § Donghyuk Kim,‡Juhoon Kang,† Jung-Yong Lee,*,† and Seung Min Han*,† §

†

equal contribution

Graduate School of Energy, Environment, Water, and Sustainability (EEWS), Korea

Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yousung-gu, Daejeon 34141, Republic of Korea ‡

Department of Material Science and Engineering, Korea Advanced Institute of Science and

Technology (KAIST), 291 Daehak-ro, Yousung-gu, Daejeon 34141, Republic of Korea

*

Corresponding authors

E-mail: [email protected] (Jung-Yong Lee), [email protected] (Seung Min Han)

Keywords: combustion, solution-processed, zinc oxide, calorific value, organic solar cell, light scattering.

Abstract Herein, a new methodology for solution-processed ZnO fabrication on Ag nanowire network electrode via combustion reaction is reported, where the amount of heat emitted during combustion was minimized by controlling the reaction temperature to avoid damaging the underlying Ag nanowires. The degree of participation of acetylacetones which are volatile fuels in the combustion reaction was found to vary with reaction temperature as revealed by 1 ACS Paragon Plus Environment

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thermogravimetric and compositional analyses. Optimized processing temperature of 180°C was chosen to successfully fabricate a combustion-reacted ZnO and Ag nanowire hybrid electrode with sheet resistance of 30 ohm/sq and transmittance of 87%. Combustion-reacted ZnO on Ag nanowires hybrid structure was demonstrated as an efficient transparent electrode and electron transport layer for the PTB7-Th based polymer solar cells. Superior electrical conductivity of combustion-reacted ZnO, compared to conventional sol-gel ZnO, increased the external quantum efficiency over all absorption range while a unique light scattering effect due to the presence of nanopores in the combustion derived ZnO further enhanced the external quantum efficiency in 450-550 nm wavelength range. Power conversion efficiency of 8.48% was demonstrated for the PTB7-Th based polymer solar cell with the use of combustion-reacted ZnO/AgNW hybrid transparent electrode.

1. Introduction Organic solar cells (OSCs) have attracted much attention due to not only its high performance, but its low cost, light-weight, and flexibility 1-4, making it a promising candidate for replacing conventional inorganic solar cells. Significant advances in power conversion efficiency (PCE) have been made with efficiencies of >10% being reported 5-7, and thus the OSC is advancing towards meeting the industrial power output requirement for flexible device applications. However, conventional transparent conducting oxides paired with OSCs, such as indium tin oxide (ITO), may be unsuitable for flexible solar cells due to its intrinsically brittle nature. Various 1D or 2D materials such as carbon nanotube, graphene, and metal nanowire have been introduced as potential replacements for ITO and among them

8-11

, silver nanowire

(AgNW) network has been extensively investigated for flexible transparent electrode application, owing to its excellent optical and electrical properties, low cost, solution based synthesis, as well as mechanical flexibility. 2 ACS Paragon Plus Environment

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The drawback of AgNW network application in solar cells, however, is in insufficient surface coverage; the AgNW network covers less than 40% of the entire electrode area, which leaves much of the solar cell without contact with a conductor that can lead to a decrease in the cell efficiency

12

. There have been numerous attempts to overcome this issue, and the use of an

auxiliary transparent conductor was found to be suitable. For example, PEDOT:PSS coating on AgNW can solve the coverage issue to improve the overall conversion efficiency of the device

13-15

, but the corrosive nature of PEDOT:PSS can harm the reliability of the AgNWs.

Sputter deposition of ZnO on AgNW network has also been reported to solve the coverage issue as well as enhancing the thermal and chemical stability of the AgNW electrode 16-18, but the cost associated with the high vacuum processes remains as a challenge. Application of ZnO nanoparticles to AgNW networks was suggested to solve the coverage issue with low cost, but use of fine particles typically results in low conductivity as well as being less effective as a passivation due to the presence of many large-sized pores and grain boundaries19-20. Thus, the development of solution-processed metal oxides with sufficient optical, electrical properties is of critical interest. In an inverted structure of OSCs, ITO is coupled with an electron transport layer (ETL) to serve as a hole blocking layer for enhanced efficiency. ZnO is a widely adopted choice for an ETL due to its high electron mobility as well as its potential use of a low cost, solution-based coating process such as the sol-gel method 21. When replacing the ITO with AgNW network, the ZnO overlayer should, therefore, serve as both the ETL and the auxiliary transparent electrode. However, the challenge is to synthesize high quality ZnO that can function as an auxiliary conductor without elevating the processing temperature beyond 200°C, above which AgNWs are known to be thermally unstable

22

. Therefore, there exists a need to develop an

alternative method for obtaining high quality ZnO layer via solution processing at a lower temperature while ensuring sufficient conductivity for the charge carriers to travel through the 3 ACS Paragon Plus Environment


ZnO layer before reaching the AgNW network. Among numerous studies in the development of producing high quality oxide thin film at low temperature, the combustion reaction method was recently highlighted to be effective in achieving high electrical and optical performance

23-24

. During the combustion-reacted ZnO

synthesis of thin film, high quality ZnO can be fabricated via combustion reaction at 200°C, and the reaction completes within a short time in comparison to sol-gel methods. Although the reaction temperature is at 200°C, the localized temperature due to release of the thermal energy is expected to be higher. Thus, if one can lower the thermal energy released during the combustion reaction, the combustion-reacted method could be a useful technique for achieving high quality ZnO ETL without damaging the underlying AgNW network. In this study, the fabrication of high quality ZnO was demonstrated by controlling the amount of heat emitted during combustion reaction that allowed for a high quality ZnO layer and also prevented thermal damage on the underlying AgNWs. Using the developed combustion reaction method, the ZnO/AgNW hybrid electrode was successfully fabricated and integrated into PTB7-Th:PC70BM based polymer solar cell, which demonstrated an improved efficiency in comparison to those with conventional sol-gel ZnO/ITO or sol-gel ZnO/AgNW electrodes. Furthermore, we report a unique optical behavior in which the combustion derived ZnO electrode with presence of nanopores improved light scattering to result in enhancement in the quantum efficiency.

2. Experimental section

2.1. Fabrication of silver nanowire transparent electrode and ZnO ETL

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Silver nanowire solution (Nanopyxis, Korea) is air-sprayed on pre-cleaned glass substrate to obtain a sheet resistance value of 25 ohm/sq. After spraying, AgNW network is annealed at 200°C in air for 10 min, followed by acetone wash for 1 min. For sol-gel ZnO coating, zinc acetate (sigma Aldrich) is dissolved in 2-methoxyethanol (sigma Aldrich) at concentration of 0.4M. Monoethanolamine (sigma Aldrich) is also dissolved in zinc acetate solution with 1:1 ratio for zinc precursor. After aging 24 hours in room temperature, solution is spin-coated on ITO substrate or on top of AgNW network at 6000 rpm followed by annealing at 200°C for 1 hour. For combustion ZnO, 1:1 ratio of zinc nitrate (sigma Aldrich) and zinc acetyleacetonate (sigma Aldrich) is dissolved in 2-methoxyethanol at Zn concentration of 0.2M. Undissolved large particle residues are removed by 0.45 µm syringe filter and the solution is diluted to 0.05M. The solution for combustion ZnO is spin-coated on top of AgNW at 2500 rpm for 30 sec, followed by annealing at 180 °C for 30 sec for each coating and annealing step. 8 times of coating with 0.05M solution are conducted for 50 nm thickness of combustion ZnO.

2.2. Device fabrication

PTB7-Th:PC70BM (1-materials : Nano-C) at a weight ratio of 1:1.5 was dissolved in chlorobenzene: 1,8-diiodoctane (97:3); It was spin coated on the prepared electrode (ZnO on ITO or AgNW) by 2000 rpm speed for 30 sec and methanol was immediately spin casted on the PTB7-Th:PC70BM layer. After that, substrates were loaded into a vacuum chamber (pressure < 1.0 x 10-7 Torr). Finally, a 10 nm layer of MoO3 and 150 nm layer of silver electrode were sequentially deposited by thermal evaporation. Others devices were fabricated with the same method on AgNW/ZnO hybrid electrodes instead of ITO. For flexible devices, exactly the same materials and method are used except for replacing willow glass (Corning).



2.3. Characterization

Field emission scanning electron microscopy (FE-SEM, Magellan400, FEI company) was used for characterization of the fabricated hybrid electrode morphology. TGA (TG209 F1 Libra, Netzsch) is used to measure the quantity of emitted heat during the combustion reaction. Sheet resistance values are obtained by 4-point measurement system (FPP-2000A, Dasoleng, Korea) and diffusive transmittance and haze were measured using UV-vis spectrometer (UV3600 plus, Shimadzu). XPS analysis is performed using Sigma Probe system (Thermo VG Scientific) with a base pressure of 1 x 10-9 Torr using AlKα sources. TOF-SIMS5 (ION-TOF GmbH) is used with Cs+ ion beam at 0.5 KeV energy. D/MAX-2500 x-ray diffractometer (Rigaku) is utilized to determine the phases and crystallinity of the ZnO thin film. TEM analysis for detailed microstructural analysis is performed using Titan cubed G2 60-300 (FEI company) with acceleration voltage of 80 kV. (AFM (Atomic Force Microscopy) measurement is used to evaluate the surface roughness of ZnO thin film by XE-100 (Park systems) with tapping mode.) For measuring electrical conductivity of doped/undoped ZnO, Kiethley 4200SCS (Tektronix) is used with 4-point van der peuw method and 2-point I-V measurement. The current density-voltage (J-V) characteristics are measured by a solar simulator (K3000 LAB55 System, McScience Inc.) under 100 mW•cm-2 with the shadow mask which has size of 0.0625 cm2. The quantum efficiency (QE) and absorption for calculating internal quantum efficiency are operated by a spectral measurement system (K3100 IQX, McScience Inc.).

3. Results and Discussion


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3.1 Effect of varied annealing temperature

ZnO solution for combustion reaction was spin-coated on top of the AgNW network and then annealed on a hot plate at a target temperature set in the range of 160°C-200°C. The coating and annealing processes were repeated eight times to obtain a ZnO thin film with thickness of 50 nm that is sufficiently thick to function as an ETL in OSCs. It is possible to obtain the desired thickness with fewer repetitions by using a higher concentration solution, however, this may lead to excessive heat release that can result in damaging the underlying AgNWs. The amount of solution per coating and the processing temperature was therefore optimized to control the calorific value. It should be noted that the rapid speed of combustion makes this methodology efficient in that the multiple coating, annealing and entire steps take only 10 min to accomplish phase conversion whereas the conventional sol-gel method typically requires 1h for full annealing at target temperature18. In order to examine the damage on the AgNWs during ZnO combustion reaction, the resistance of the electrode was monitored as a function of each ZnO layer deposition. It should be noted that the prepared AgNWs are thermally stable up to 10 min of annealing at 200°C in air without an abrupt increase in resistance. Spray-coated AgNWs were annealed at 200°C to form a network with fused in junctions that results in optimized electrical optical properties (T=95% R=20ohm/sq). ZnO solution for combustion reaction was then coated repeatedly at varying temperatures of 200°C, 180°C, and 160°C on top of the AgNWs. The overall sheet resistance evolution after each ZnO coating is shown in Figure 1a. Combustion reaction carried out at 180°C and 160°C showed a slower increase in sheet resistance as the number of cycles and eventually saturated at 25 ohm/sq and 22 ohm/sq, respectively. For the combustion reaction temperature of 200°C, the sheet resistance of the electrode increased more rapidly with each coating due to the excessive heat that caused the underlying AgNW to 7 ACS Paragon Plus Environment


ball-up to result in disconnection, which is a commonly observed thermal instability behavior in AgNWs (Figure 1c and d)

22

. More damage on the AgNW network at higher processing

temperatures indicated that larger heat emission occurred during combustion reaction. Therefore, the amount of heat emission is dependent on annealing temperature and the mechanism that caused such dependence was further investigated. In a combustion reaction, the amount of heat released is in theory determined by the enthalpy difference between the reactants vs. the products and thus should not vary with the reaction temperature. To clarify this mismatch between theory and experimental results, solution for the combustion-reacted ZnO was thermogravimetrically analyzed to compare the calorific value with respect to the reaction temperature. It should be noted that the TGA analysis condition differs from the actual coating and annealing process; the combustion reaction of ZnO is carried out by direct exposure to the pre-heated hot plate while the TGA analysis involves heating the solution from room temperature to the set point at a relatively slow heating rate. TGA analysis was conducted at heating rates of 2°C/min, 5°C/min, and 10°C/min. Interestingly, the amount of emitted thermal energy and temperature at which the reaction is initiated during the combustion reaction are greater at higher heating rates as shown in Figure 1b. It can be deduced that the release of thermal energy can be suppressed when the combustion reaction is carried out at a low temperature that would thus limit any damage on the underlying AgNW network after combustion ZnO coating, as shown on Figure 1a and b.

3.2. Mechanism of heat emission loss

The difference in calorific value in relation to the heating rate can be attributed to the different degree of reactant participation during the combustion reaction at different reaction 8 ACS Paragon Plus Environment

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temperatures. In the combustion reaction solution, nitrate serves as an oxidant and acetylacetonate acts as a fuel to drive combustion. Acetylacetonate is highly volatile in the form of acetylacetone 24, and can evaporate into air before and during the combustion reaction, and this loss of combustible fuel results in a decrease in the actual released heat and causes larger amount of unreacted nitrate to remain after the reaction terminates. The amount of nitrate that serves as an oxidant in the combustion reaction was analyzed as a function of different processing temperatures using time of flight secondary ion mass spectroscopy (TOF-SIMS). Higher nitrate intensities were detected in samples annealed at lower temperatures and these results are consistent with XPS results as shown on Figure 2 and Table S1. Considering that all N species originated from the nitrate, it can be explained that varied calorific values at each temperature result from the different degree of participations of fuel(acetylacetonate) and oxidant(nitrate). Another interesting point confirmed by TOF-SIMS analysis is that the proportion of these N species was much higher near the surface that constantly decreased with an increase in depth. Such gradient in concentration can be explained considering that each repeated annealing step may lead to the newly deposited solution to provide the necessary fuel to react with remaining nitrate reactant in the underlying layer. The fact that the trace content of C does not change with respect to annealing temperature also indicates that the varied degree of participation of fuel is represented in form of evaporation, not remaining as unreacted C species in ZnO.

3.3 The influence of varied calorific values on the quality of ZnO

Microstructure and composition of the fabricated combustion-reacted ZnO thin film are heavily dependent on the calorific value, which in turn is dependent on the annealing 9 ACS Paragon Plus Environment


temperature. As shown in the cross-section SEM images of combustion ZnO thin films at various annealing temperatures in Figure 3a, a clear relationship between annealing temperature and microstructure of the thin film can be observed. At low annealing temperatures, the ZnO thin film is composed of irregular shaped nanoparticles with some porosity rather than being a dense thin film. At higher annealing temperatures, denser arrangement of spherical grains with a low porosity that resembles a dense thin film is formed. Besides, there is small difference between 190°C and 200°C annealed sample and it means that the effect of heat emission must be similar such that similar compact microstructure with low density of pores was found in both cases of 190°C and 200°C annealed samples. Further magnified TEM image in Figure 3b showed presence of nano-sized pores inside the combustion-reacted ZnO thin film annealed at 200°C and these nanopores are resultant of trapped gas during the rapid combustion reaction 23. Mostly, one of the critical factors determining the quality of solution-processed metal oxide is the amount of phase conversion from hydroxide (M-OH) to oxide (M-O). Variation in the quantity of emitted heat was also shown to cause different degree of oxide formation as revealed by XPS and TOF-SIMS composition analysis. Figure 4 shows the XPS spectra of O 1s of combustion-reacted ZnO thin films at various temperatures. The percentage of metal oxide bonding (Eb=528.2 eV) in comparison to the metal hydroxide bonding (Eb=530 eV) showed that combustion-reacted ZnO annealed at 170°C has 78.6% Zn-O bonds while the ZnO annealed at 200°C has almost 80% for Zn-O bonds

21, 25

. TOF-SIMS analysis provided

similar results indicating intensity ratio between 68ZnO and 68ZnOH to be 8.33:1 and 2.56:1 at annealing temperatures of 200°C and 170°C, respectively, indicating that greater fraction of reactants form ZnO at a higher annealing temperature (Figure S1). Thus, both XPS and TOFSIMS compositional analyses confirmed that the difference in calorific value resulting from different annealing temperatures influenced the degree of dehydration reaction that is critical 10 ACS Paragon Plus Environment

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for the hydroxide to oxide phase transformation. Though combustion-reacted ZnO annealed at a lower temperature showed less Zn-O bond formation with more impurities than those annealed at high temperatures, its quality or performance such as the conductivity were still significantly improved over the conventional sol-gel ZnO as shown in Figure 5. 8 times coated and annealed ZnO thin films fabricated by both the combustion reaction and sol-gel methods were evaluated by XPS, XRD. In XPS analysis as shown on Figure 5a, Sol-gel ZnO thin film annealed at 200°C showed only 60% Zn-O bonding that is consistent with previous reports

18

, while the combustion-reacted ZnO

thin film showed almost 80% Zn-O bonding. The differences between combustion ZnO and conventional sol-gel ZnO extends to the crystallinity of the fabricated thin film. Glazed incidence angle X-ray diffraction (GIAXRD) analysis was conducted that showed higher degree of crystallinity for the combustion-reacted ZnO annealed at 200°C than that of the solgel ZnO (Figure 5b). It can be easily expected that the quality difference between the combustion-reacted ZnO and sol-gel ZnO cause changes in conductivity of each ZnO, and both Al-doped and undoped ZnO thin film fabricated by two distinct methods were compared. The change in the conductivity of Al-doped ZnO thin films produced by combustion reaction with respect of annealing temperature is plotted in Figure 5c. Al-doped combustion-reacted ZnO showed an enhanced conductivity at higher annealing temperatures due to the low porosity, reduction in the residual N content and increased oxide / hydroxide ratio. Especially, it is well-known that presence of impurities is quite critical factor to determine the electrical properties of semiconductors. Residual N elements which are major impurities in combustion-reacted ZnO can act as an acceptor in semiconductor oxide by substituting oxygen sites and function as a dopant compensator in the intrinsic or n-type Al-doped ZnO semiconductor, thereby reducing the doping effect

26-28

. Furthermore, NOx can be chemisorbed on the surface, which causes a 11 ACS Paragon Plus Environment


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decrease in conductivity by creating defect sites 29-31. A lateral conductivity of 0.217 S/cm at an annealing temperature of 200°C was obtained for the combustion-reacted Al-doped ZnO thin film, which is higher than that of 1.5 x 10-3 S/cm for sol-gel Al-doped ZnO obtained at 250°C marked as the dotted line in Figure 5c. For undoped ZnO case, combustion-reacted ZnO showed resistivity of 2.8 x 103 ohm˖cm in dark condition while sol-gel ZnO showed 1.9 x 105 ohm˖cm as shown in Figure 5d. Therefore, it can be said that both the combustionreacted Al-doped or undoped ZnO has relatively high conductivity compared with that of doped or undoped sol-gel ZnO.

3.4 Unique properties of combustion-reacted ZnO and application to organic solar cell

A PTB7-Th:PC70BM based organic solar cell (OSC) has a high power conversion efficiency that can be made flexible with appropriate choice of substrate and transparent electrode. Therefore, the proposed combustion-reacted ZnO/AgNW electrode was used as a replacement for ITO in the OSC. Combustion-reacted ZnO/AgNW electrode was fabricated using a 0.05M ZnO combustion solution and annealed at 180°C to avoid damage on the Ag NW network that can cause an abrupt increase in sheet resistance. Fabricated combustion-reacted ZnO/AgNW electrode shows a sheet resistance of 30 ohm/sq at 87% diffusive transmittance, as shown in Figure S3. AgNW network acted as a transparent electrode and ZnO thin film acted as an ETL of the OSC (transparent electrode / ZnO (50 nm) / PTB7-Th:PC70BM / MoO3 / Ag). Device with as-annealed combustion-reacted ZnO/AgNW hybrid electrode showed “S” shaped J-V curve as shown in Figure S4, which can be attributed to recombination traps on the surface of ZnO thin film saturated with N or NOx. This undesired phenomenon can be reduced after H2 reduction at 200°C, which removes the remaining oxidants


32-33

. H2 reduction also can

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enhance the conductivity of combustion-reacted ZnO while sol-gel ZnO reduced by H2 shows even deteriorated electrical property (Figure S4) due to the presence of undecomposed organic species. The photovoltaic characteristics of the devices fabricated with sol-gel ZnO/ITO, sol-gel ZnO/AgNW, and combustion-reacted ZnO/AgNW transparent electrode/ETL are shown in Figure 6a and the solar cell parameters for each device are summarized in Table 1. The best PCE of combustion-reacted ZnO/AgNW was 8.48%, higher than that of sol-gel ZnO/AgNW (7.91%) devices and even higher than that of the conventional sol-gel ZnO/ITO (7.97%). We compared devices with sol-gel ZnO/AgNW and combustion-reacted ZnO/AgNW to emphasize the role of combustion-reacted ZnO ETL in devices. The improved PCE was attributed to the increase in Jsc and FF because the Voc values were constant for all devices. In a ZnO/AgNW hybrid electrode, the ZnO plays an important role in providing a lateral conduction pathway in areas that are not covered by the AgNWs. Because the conventional sol-gel ZnO has high resistance especially at low annealing temperatures, the device exhibits high ohmic loss and charge carrier recombination. In contrast, the combustion-reacted ZnO exhibits high purity, crystallinity, and conductivity and can ensure efficient charge transfer from ZnO to the AgNW network in the lateral direction. Both combustion-reacted and sol-gel fabricated ZnO/AgNW hybrid structures had similar sheet resistance that is governed primarily by the AgNW network. However, because charge capturing ability is influenced by the conductivity or quality of ZnO 8, a more effective and efficient charge capturing capability can be expected from a combustion-reacted ZnO compared to the sol-gel ZnO. Figure 6b compares the external quantum efficiency (EQE) of combustion-reacted ZnO and sol-gel ZnO solar cells. The short circuit current integrated from EQE for combustion-reacted ZnO/AgNW increased more than 10% compared to sol-gel ZnO/AgNW. The improvement was observed in entire wavelength range, which indicated that the charge recombination was 13 ACS Paragon Plus Environment


reduced by replacing the sol-gel ZnO with combustion-reacted ZnO, leading to an enhancement in the internal quantum efficiency (IQE) as shown in Figure S5. High conductivity of the combustion-reacted ZnO could improve lateral charge transfer toward the AgNW electrode and also increase FF of the device 34. An additional increase in EQE in the range of 450-550nm is attributed to the increased absorption from light scattering effect of combustion-reacted ZnO, which has nanopores as explained below. Combustion reaction occurs in a shorter timeframe with more release of combusted gases compared to sol-gel reaction. As a result, nano-sized pores are formed within the ZnO layer as confirmed by SEM and TEM images shown in Figure 1c,d and Figure 3. Higher degree of porosity equates to greater density of air and ZnO interfaces within the ETL, where light is highly scattered as shown in Figure 6c,d and Figure S3,5. In the case of the 50nm thick ZnO thin film, the absolute haze value was not significant, however, this became pronounced as the hybrid with AgNW network. As shown on Figure 6c, the combustionreacted ZnO/AgNW electrode shows a higher haze value compared to the sol-gel ZnO/AgNW, indicating that combustion-reacted ZnO amplifies the light scattering effect of the AgNW network. This light scattering effect can be explained by Rayleigh scattering as the pore size in combustion ZnO thin film is about a few tens of nanometer as shown on Figure 3, which is less than 1/10 of the wavelength of visible light 35-37. Due to the difference in refractive index between ZnO and air, strong scattering takes place and this effect becomes more pronounced in the short wavelength region. Haze tends to become higher in the short wavelength region and it results in an increase of EQE enhancement values from 550nm to shorter wavelength for solar cell devices. However, below the 450nm, the absorption of light by ZnO is becoming dominant even though much higher scattering effects are exhibited. Therefore, enhanced light absorption of active layer by combustion ZnO’s scattering effect becomes negligible below 450nm. Through those light scattering and absorption by combustion ZnO, EQE 14 ACS Paragon Plus Environment

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enhancements are pronounced in range of 450-550nm. To demonstrate the improved light scattering ability, a red laser beam was directed through the side of the glass substrate with ZnO and is shown in Figure 6d, e and f. The incident laser beam passing through the ZnO/glass structure can be clearly observed due to the intense light scattering effect of the combustion-reacted ZnO thin film while it was not observed in the solgel ZnO thin film. Higher intensity of light was observed in combustion-reacted ZnO/AgNW hybrid electrode due to the amplified light scattering effect. Having developed a combustion-reaction ZnO thin film of high quality, conductivity, and crystallinity with optimized process conditions for OSC applications, we fabricated a flexible solar cell using the combustion-reacted ZnO/AgNW hybrid electrode. The demonstration takes full advantage of the flexibility of the AgNW network. A PTB7-Th:PC70BM based organic solar cell with combustion-reacted ZnO/AgNW hybrid electrode structure was successfully fabricated on a flexible willow glass with a PCE of 7.30% as shown in Figure S6a and b.

4. Conclusion

In this study, a new methodology in fabrication of solution-processed ZnO thin film via combustion reaction with minimal damage to the underlying AgNWs was developed. Heat emission during combustion reaction was shown to be controlled by varying annealing temperature; the mechanism was determined to be due to more loss of volatile fuel in combustion reaction at lower temperature that hence results in unreacted nitrate and thus less heat emission. The optimized combustion-reacted ZnO thin film on AgNW electrode was applied to PTB7-Th based organic solar cell to confirm a much improved power conversion efficiency value of 8.48% compared to the conventional ITO based device. The improvement 15 ACS Paragon Plus Environment


is attributed to the high conductivity and high light scattering ability of combustion-reacted ZnO. The advantages of combustion-reacted ZnO fabrication method includes cost effectiveness, scalability while allowing for all solution-processed, synthesis that is expected to have a significant impact in various flexible optoelectronic devices, especially in the field of organic solar cells.


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Fig. 1. Evidences of varied calorific value by different annealing temperature. (a) Sheet resistance variation as a function of number coating (0.05M, 2500 rpm). (b) Varied exothermic peaks under different heating rate condition for combustion ZnO solution. (c) SEM image (SE mode) of top surface of combustion ZnO coated silver nanowire network. Thickness of ZnO layer is less than 30nm to clearly observe both silver nanowires and ZnO. (d) SEM image (BSE mode) of exactly same position for image (c). Welding and ball-up behavior (red arrow) are observed by higher contrast of BSE mode.



Fig. 2. TOF-SIMS results for fabricated combustion ZnO under different annealing temperature. Data for exactly top surface is neglected to clearly compare to each case.


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Fig. 3. (a) Cross-sectional SEM images of combustion ZnO thin film under different annealing temperatures. (b) TEM image for combustion ZnO annealed at 200°C. Nano-sized pores are observed inside the thin film.



Fig. 4. O 1s XPS spectra of combustion ZnO thin film under different annealing temperatures. Top surfaces are etched to avoid surface contaminations.


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Fig. 5. Comparison of sol-gel ZnO and combustion ZnO in terms of phase conversion by XPS analysis (a) and crystallinity by XRD analysis (b). (c) Conductivity values of Al doped ZnO via combustion under different annealing temperature. Red dot line indicates the conductivity of sol-gel based Al doped ZnO annealed at 250°C and reduced at 200°C in hydrogen atmosphere. (d) I-V measurement of combustion ZnO and sol-gel ZnO under illumination of solar simulator and dark state. Thickness of ZnO thin film for both cases is 150 nm. Dimension of measured area of ZnO thin film is 25 mm (W) x 1 mm (L) x 150 nm.



Fig. 6. (a) J-V characteristics of PTB7-Th:PC70BM organic solar cell with ITO/ZnO(sol-gel), AgNW/ZnO(combustion) and AgNW/ZnO(sol-gel) structure. (b) External quantum efficiency (EQE) spectra of AgNW/ZnO(sol-gel) and AgNW/ZnO(combustion). Enhancement of EQE value as a function of wavelength is shown as yellow line. (c) Haze amplification effect of combustion ZnO comparing with sol-gel ZnO. (d) Optical images for representing light scattering ability of combustion ZnO thin film. Laser beam propagated through inside the glass substrate is clearly shown for combustion ZnO and hybrid with silver nanowires. (e,f) Schematics for laser incident experiment.


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Table 1. Device performance parameters of fabricated ptb7-th organic solar cell with ITO/ZnO(sol-gel), AgNW/ZnO(combustion), and AgNW/ZnO(sol-gel).

Solar cell parameters Cathode

Voc [V]

Jsc

FF

PCEavea [%]

PCEmax [%]

0.62±0.01

7.93±0.10

7.98

0.63±0.02

7.70±0.19

7.91

0.64±0.01

8.21±0.22

8.48

(EQE)[mA/cm2] Reference ITO

0.79±0.00

16.26±0.06 (16.86)

Ag NWs +

0.80±0.00

15.62±0.18 (15.41)

sol-gel ZnO Ag NWs +

0.80±0.00

16.48±0.17

Combustion

(17.00)

ZnO a) The average PCEs of ten cells.



AUTHOR INFORMATION Corresponding Authors *

E-mail: [email protected]

*

E-mail: [email protected]

Supporting information The Supporting Information is available free of charge on the … Additional TOF-SIMS data for combustion reacted ZnO thin film, optical and electrical properties for fabricated electrodes and flat and curved solar cells, experimental setup for laser incident experiment and surface roughness measurement for ZnO films. Author Contributions §

These authors contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT Prof. S. M. Han would like to acknowledge the financial support from the Technology Innovation Program and Industrial Strategic Technology Development Program (RCMS 10052790) National Research Foundation of Korea (NRF) grant funded by the Korea government (No. NRF- 2014R1A4A1003712). Prof. J. Y. Lee would like to acknowled ge the financial support by the KETEP of the Republic of Korea (No. 2016303001362 0 and 20163010012200). 24 ACS Paragon Plus Environment

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ToC figure

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