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Interface Engineering of Metal Oxides using Ammonium Anthracene in Inverted Organic Solar Cells Il Jeon, Sasa Zeljkovic, Kei Kondo, Michito Yoshizawa, and Yutaka Matsuo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09684 • Publication Date (Web): 03 Oct 2016 Downloaded from http://pubs.acs.org on October 5, 2016

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ACS Applied Materials & Interfaces

Interface Engineering of Metal Oxides using Ammonium Anthracene in Inverted Organic Solar Cells Il Jeon,† Sasa Zeljkovic,§ Kei Kondo, ‡ Michito Yoshizawa, ‡ and Yutaka Matsuo†,#* †: Department of Mechanical Engineering, School of Engineering, The University of Tokyo, 73-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan §: Department of Chemistry, Faculty of Sciences, University of Banja Luka, Mladena Stojanovica 2, 78000 Banja Luka, Bosnia and Herzegovina ‡: Laboratory for Chemistry and Life Science, Institute of Innovative Research, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan #: Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026, China

KEYWORDS: Organic solar cells, Electron-Transporting Layers, Vertical separations, fullerene catcher, Antracene, Polyaromatic amphiphiles

ACS Paragon Plus Environment

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ABSTRACT In this work, by casting water-soluble ammonium anthracene on metal oxides, the organic surface modifier re-engineered the interface of the metal oxide to improve charge transport. The energy level of ammonium anthracene increased the work function of indium tin oxide (ITO), functioning as a hole-blocker (electron-transporter). Solar cells in which ITO was treated by the ammonium anthracene produced an average power conversion efficiency (PCE) of 5.8% without ZnO, the electron-transporting layer. When the ammonium anthracene was applied to ZnO, an average PCE of 8.1% was achieved, which is higher than the average PCE of 7.5% for non-treated ZnO-based devices.


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Extensive efforts have been put into developing organic solar cells (OSCs) for their lowcost, light-weight, and potential flexibility.1-2 Efforts to avoid the low-work-function metals have created inverted architecture of the device, where the oxide electron-transporting layer (ETL), such as ZnO, has been used on an indium tin oxide electrode to invert the polarity of the bulk heterojunction devices.3 This design yields high efficiencies and prolongs the lifetime of OSCs by avoiding reactive layers, thereby mitigating the encapsulation requirement.4 Among the available ETLs, sol-gel-prepared colloidal nanoparticles have been most widely used for their high electron mobility and easy synthesis.5 However, there are limitations in using sol-gel ZnO as the ETL. First, high temperature process of the sol-gel ZnO fabrication limits their flexible application.6 This is because an annealing temperature above glass transition temperatures of flexible plastic substrates will melt the substrate. Secondly, oxygen vacancies (V(O)s) and zinc interstitials (Zn(i)) of ZnO induces near-surface defects with adsorbed oxygen7 and poor spatial distribution of the nanoparticles.8,9 This results in changes to the degree of doping and the Fermi level position.10–12 Moreover, carrier density and chemical potential of the adjacent organic layers will be affected. Hence, to realize efficient inverted OSCs, it is imperative that we develop materials that can replace ZnO or assist ZnO films.13 So far, there have been numerous studies on improving the contact by surface treatment: Water-soluble polyfluorenes with alkylphosphonates and alkylamine salts on the side chains were found to be effective in elevating open-circuit voltage (VOC) of OSCs.14,15 Application of carboxylic acids as a self-assembled monolayer on top of metal oxidie to modify the interfacial property is another example.16 Not only the ZnO interface, but also the interface of the active layer can be improved using self-assembled crosslinked fullerene17,18 To date, poly [(9,9-bis(3-(N,N-dimethylamino)propyl)-2,7- fluorene)-alt-2,7(9,9–dioctylfluorene)], which was originally used for light-emitting devices to enhance electron injection from high-work-function metals, has demonstrated the best performance as the ETL, replacing ZnO.19 Here we present ammonium anthracene (AA) as a surface modifier that has positively charged trimethylammonium groups (-N+(CH3)3) on one end and a bent polyaromatic group on the other.20 As an organic compound, they possess suitable highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels which can function as the ETL. When deposited on ITO, they offered ohmic contact for collection and allowed optimum photogenerated charge-carrier harvest, replacing ZnO. Using this method, the low-band gap


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polymer, thieno [3,4-b] thiophene/benzodithiophene (PTB7)-based OSCs gave an average power conversion efficiency of 5.8%, even without ZnO and any thermal process. When AA overcoated ZnO, it alleviated poor contact between ZnO and the active layer interface as evidenced by recovery of VOC loss. In addition, the V-shaped polyaromatic group could effectively catch fullerenes through multiple aromatic-aromatic interactions. This induced a thin phenyl-C71butyric acid methyl ester (PC71BM) layer as a barrier for hole carrier injection. From this, an average PCE of 8.1% was obtained which is higher than the 7.5% obtained for the non-treated reference devices.


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Water is nature's primary solvent, capable of dissolving biomolecules with no harm to the environment. AA used in this work possesses high polarity and thus has ability to form micellar spherical structures with core diameters of ~1 nm in water.20,21 Therefore, our approach qualifies for an aqueous process that is easy and fully amenable to roll-to-roll fabrication. For the surface treatment, AA dissolved in water was dropped slowly on either ITO or ZnO. After 20 minutes waiting time, the solution on the films was blown with a nitrogen gun. For some AA on ZnO, water rinsing was necessary to eliminate any unreacted AA to induce a monolayer (Figure 1). 20 minutes of waiting time was crucial as longer waiting times resulted in thicker and less uniform AA films, which led to decrease in device performance for the devices without ZnO.

Figure 1. Methods of AA surface treatments on (a) ITO and (b) ZnO.

Inverted OSCs were fabricated using different concentrations of AAs on ITO and ZnO. A mixture of poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PC61BM)


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photoactive materials was used due to their established reliability. PCEs of the devices according to the concentration are shown in Figure S1. According to the data, the devices with non-rinsed AAs on ITO performed substantially better than the rinsed ones. This is because a sufficiently thick layer of AA was required to avoid any shunt pathways between ITO and the active layer in the absence of ZnO. Concentrations of between 4 and 6 mg mL-1 of the AA solutions showed optimal results. This indicates that an interlayer that is too thick leads to a high series resistance (RS), while a too-thin layer cannot provide enough ohmic contact. Alternatively, for AAs applied to ZnO, the rinsed devices performed the best regardless of the concentration. The devices with AAs applied to ZnO without rinsing did not perform well. We can conjecture that a thin AA layer enhanced the ZnO performance, while a thick AA layer undermined the performance by having excessive ETL. After the AA concentrations have been optimized, average photovoltaic parameters of each condition were recorded, and shown in Table S1, along with the reference devices. As it can be seen from the data, the ITO/non-rinsed AA-based devices produced an average PCE of 2.58%, showing that AAs can function as the ETL, replacing ZnO. High VOC (0.59) indicates that AAs covered most of the surface uniformly and with sufficient fill factor (FF). This also indicates that AAs can function as a decent ETL.14,15,22 It was the short-circuit current density (JSC), which was lower than those of the ITO/ZnO reference, that was responsible for the moderate PCEs. The ITO/ZnO/rinsed AAs-based devices showed an average PCE of 3.08%, which is slightly higher than that of the reference device (2.91%). Higher fill factor (FF) resulted in the higher PCE. Increase in both FF and shunt resistance (RSH) proved that AAs enhanced the hole-blocking ability of ZnO.23 On the other hand, the JSC was again a little lower than that of the ITO/ZnO reference devices. It is worth noting that the upward facing concave polyaromatic ends of AA may have caught the fullerene through multiple aromatic-aromatic interactions,21 leading to vertical separation.24 This involves increases in FF. A PTB7 system, a mixture of PTB7 and PC71BM, was employed as the active layer to demonstrate the effect of AA with more boosted PCEs (Table 1). The same phenomena as the P3HT system was observed. AA can replace ZnO and produced an average PCE of 5.8% also producing decent VOC and FF values. When AA was used to enhance the ability of ZnO, the PCE increased and positioned higher than that of the ITO/ZnO reference devices, owing to the significantly increased FF.


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Table 1. Photovoltaic parameters of AAs-applied devices and a reference device using PTB7:PC71BM as the active layer under one sun (AM1.5G illumination, 100 mW cm-2). Corresponding J−V curves can be found in Figure S2.

Device

VOC (V)

JSC -2 (mA cm )

FF

RS 2 (Ωcm )

RSH 2 (Ωcm )

ITO/ZnO/AA

0.76±0.01

15.1±0.20

0.70±0.03

30

2.6 × 10

6

8.1±0.21

2.0 × 10

6

5.8±0.11

4

7.5±0.15

ITO/AA

0.60±0.00

14.8±0.18

0.63±0.01

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[Reference] ITO/ZnO

0.73±0.03

16.0±0.46

0.65±0.12

16

6.4 × 10

[Reference] ITO

0.16

0.98

0.32

-

-

PCE (%)

0.55

AA’s ability to function as an ETL can be explained by theory and analysis. For an effective ETL, it is important that efficient electron injection and hole blocking are obtained by decreasing the work function of ITO effectively.25 Therefore, having a high LUMO level is crucial. Gaussian calculation and photoelectron yield spectroscopy (PYS) in air was used to investigate the energetics of AA. From the Gaussian calculation in Figure S3, we can see that the AAs possess high-lying LUMO, which can decrease the work function ITO. Using Kelvin probe and PYS, we found the work function of ITO, the valence band of ZnO, and HOMO of AAs (Figure S4). From Figure S4a, we can see that ITO possessed a work function of around 4.7 eV, but with AAs on top, the energy level was around 5.7 eV (Figure S4b). This did not change significantly after rinsing with water, indicating that chemical adsorption occurred between the AAs and the ITO surface (Figure S4c). This 5.7 eV value was similar to the valence level of ZnO (5.5 eV) measured by PYS in Figure S4d. This indicates that the deep HOMO level of AAs is comparable to that of ZnO and confirms the effective hole-blocking property of AAs. Using the band gap calculated from the Gaussian analysis used in Figure S3, we can estimate the empirical LUMO level to be ca. 3.0 eV (Figure 2). While this can effectively decrease the work function of ITO effectively, it lies higher than the LUMOs of the donor materials. We suspect such energy mismatch could be attributed to the low JSC, energetically hindering the charge extraction.26


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Figure 2. Energy diagram of the solar cells used in this work

In order to clarify the low JSC further, UV-vis spectroscopy measurement was carried out (Figure S5). AA-treated ZnO showed a slight increase in absorption. This may have affected lowering the JSC. In addition, external quantum efficiency (EQE) was measured (Figure S6). The data reveals that the device with AA-treated ZnO show slightly lower EQE at short wavelengths, which could be directly related the observed lower JSC. It was observed that the increase in the thickness of AA in the ITO/ZnO/non-rinsed AAs device decreased both FF and JSC gradually. Decrease in FF is understandable because AA is an organic compound and organic compounds have lower carrier mobility than metal oxides;27 Space charge limited current (SCLC) measurement was conducted to clarify this. A marginal decrease in mobility was observed from non-rinsed AA-treated ZnO (Figure S7). Therefore, the decrease in FF with the increase in the thickness of AA was assumed to be a combination of low mobility and the loss of vertical separation effect as the aromatic groups alternates in layers (Figure S8). In this regard, the decrease in JSC combined with the increase in the thickness of AA can also be understood by a combination of the energetic mismatch hypothesis and the decreased transmittance of light. The interaction between AAs and the metal oxides was studied by X-ray photoelectron spectroscopy (XPS). The whole range spectra are shown in Figure S9. Figure 3a and 3c show that chlorine 2p peaks did not change after their application on metal oxides. This indicates that Cl- stayed almost unchanged after the surface treatment and water rinsing. On the contrary, there


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were interesting changes in nitrogen N 1s peaks (Figure 3b and 3d). Solid AA powder possessed two peaks: one on the left corresponding to nitrogen in trimethylammonium group (404 eV) and one on the right corresponding to the same nitrogen affected by the organic matrix surrounding it (401 eV) (Figure 3b). When AA was applied to ZnO and watered down, the original peak (404 eV) decreased and a new peak appeared at a lower binding energy (408 eV) to signify more negative nitrogen adsorbed to oxygen in metal oxides (Figure 3d). For the spectrum in which AA was not watered down, there was a weak peak (408 eV) indicating the adsorbed nitrogen, and a 404 eV nitrogen peak to indicate an unadsorbed AA stack. This peak proves AA interaction with the metal oxides and the positively charged, downward facing trimethylammonium side.28

Figure 3. XPS spectra of solid AAs for a) Cl 2p core and b) N 1s core; and both of rinsed AAs on ZnO and nonrinsed AAs on ITO c) Cl 2p core, and d) N 1s core.

Atomic force microscopy (AFM) was used to investigate the morphology of AAs and their fullerene catching ability. Figure S10 show that the root mean squared (r.m.s.) roughness decreased slightly after the AA treatment. This denotes that AA surface treatment improves


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morphology by filling up pinholes in metal oxide surfaces. The decrease in roughness was greater for the samples, in which AA was not rinsed (Figure S10a–d). To our surprise, its influence even reached the roughness of the P3HT:PCBM active layer (Figure S10e–f). Overall, the decrease in roughness was not significant enough to affect the devices performance. We applied 1,8-octanedithiol (OT), which is known to selectively dissolve PCBM,29 on the active layers to detect vertical separation. When OT was applied to glass/ITO/ZnO/rinsed AA, the active layer was peeled off from the substrate (Figure 4a and b). This was because OT dissolved a fullerene layer at the bottom of the active layer, which denotes the fullerene derivatives catching ability of AAs (Figure S11). This also happened with glass/ITO/non-rinsed AA revealing that even non-rinsed ANTH manifests some degree of fullerene-catching ability. According to Figure 4c and d, OT treatment resulted in slightly different morphologies to AAtreated and non-treated P3HT:PCBM layers. However, the change was not clear enough to claim vertical phase separation from the roughness.30

Figure 4. Pictures after OT treatment for a) glass/ITO/ZnO/P3HT:PCBM and b) glass/ITO/ZnO/rinsed AAs/P3HT:PCBM, where P3HT:PCBM layer has been peeled off. AFM images and r.m.s. roughness values after OT treatment for c) glass/ITO/ZnO/P3HT:PCBM and d) glass/ITO/ZnO/rinsed AAs/P3HT:PCBM.

Finally, stability of the AA-applied devices was studied. AA is a water soluble material and its hygroscopicity could affect the stability of the cells. According to our stability test result in Figure S12, the devices, in which AA was applied to ZnO ,show the same stability as the reference. On the other hand, the devices in which AA was applied to ITO, show rather significant decrease over time. This is because only a thin layer of AA was applied to ZnO, which was already washed by water. Therefore, its solubility in water does not affect the stability.


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In conclusion, we have successfully demonstrated AA, small-molecule aromatic compounds, functioning as water-soluble surfactants in inverted OSCs. The surface modifier was adsorbed to the metal oxides, ITO and ZnO, as proven by XPS. AA possessed high lying LUMO levels as evidenced by PYS and the Gaussian calculations, and, therefore, can function as the ETL. AA was found to be able to replace ZnO, and AA-treated inverted OSCs could produce a high efficiency without ZnO. AA also enhanced the functionality of ZnO when applied to its surface, as demonstrated by the increase in VOC and FF. In addition, AAs’ fullerene catching ability was observed by OT washing. With clear chemical and physical descriptors, we were able to show the anthracene derivatives improving charge harvesting across the ITO and ZnO interfaces. An understanding of such interactions is of paramount importance for the ability to tailor carrier extraction in solar cell devices. This finding will open new avenues of research into interfaces between organic semiconductors and transparent oxides.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]

Notes The authors declare no competing financial interest.

Supporting Information Supporting information contains information regarding experimental procedures, PCE evaluations, energetics analyses, other properties of AA, and fullerene catching ability.

ACKNOWLEDGEMENT


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This work was supported by a Grant-in-Aid for Scientific Research (15H05760 and 16H04187) and the CREST project. I.J. thanks the Japan Society for the Promotion of Science for financial support.

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Anthopoulos, T. D.; Stavrinou, P. N.; Bradley, D. D. C.; Nelson, J. Morphology Evolution via Self-Organization and Lateral and Vertical Diffusion in Polymer:fullerene Solar Cell Blends. Nat. Mater. 2008, 7 (2), 158–164.

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