Impact of ZnO Photoluminescence on Organic Photovoltaic Performance

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Impact of ZnO Photoluminescence on Organic Photovoltaic Performance ke yang, Jiehao Fu, lijun hu, Zhuang Xiong, Meng Li, Xingzhan Wei, Zeyun Xiao, Shirong Lu, and Kuan Sun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b14224 • Publication Date (Web): 26 Oct 2018 Downloaded from http://pubs.acs.org on October 26, 2018

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ACS Applied Materials & Interfaces

Impact of ZnO Photoluminescence on Organic Photovoltaic Performance Ke Yang,†, ‡ Jiehao Fu,†, §, ‡ Lijun Hu,† Zhuang Xiong,† Meng Li,† Xingzhan Wei,§ Zeyun Xiao,§ Shirong Lu,§ Kuan Sun†, * † MOE

Key Laboratory of Low-grade Energy Utilization Technologies and Systems,

CQU-NUS Renewable Energy Materials & Devices Joint Laboratory, School of Energy & Power Engineering, Chongqing University, Chongqing 400044, China §

Organic Semiconductor Research Center, Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences, Chongqing 400714, China

KEYWORDS: Zinc oxide; organic photovoltaics; photoluminescence; interface; intrinsic defects

ABSTRACT: ZnO is a widely used electron transport material in the 3rd generation solar cells. Intrinsic defects rising from different synthetic methods and conditions lead to different fluorescent colors. The defect mechanisms have been explored in literatures, but the impact of which on organic photovoltaic cell (OPV) performance is rarely reported. Herein, three different ZnO nanoparticles showing blue, green 1 ACS Paragon Plus Environment

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and yellow emission colors are synthesized and incorporated into OPV. The ascast ZnO films result in vastly different OPV performances. It is found the sodium acetate as the by-product of the synthesis can significantly influence the interfacial contact. After removing the impurity via rinsing with polar organic solvents, the different ZnO nanoparticles can deliver similar power conversion efficiencies (PCEs) in three representative OPV systems. The PCEs reached 4%, 8% and 10% in P3HT:PC61BM, PTB7-Th: PC71BM and PBDB-T-SF:IT-4F based OPV cells, respectively. A series of characterizations indicate the intrinsic defect types do not affect the optical and electrical properties of the ZnO film, including photon transmittance, electrical conductivity, charge extraction from the active layer as well as electron mobility. The results together suggest the intrinsic defect in ZnO nanoparticles has little impact on OPV performance. Thus it might be necessary to revisit the strategies for defect engineering or passivation in oxide based interfacial materials.

1. Introduction ZnO is a typical n-type semiconductor with a bandgap around 3.3 eV1, 2. It is widely used as an electron transport layer (ETL) in the third generation solar cells, especially in organic photovoltaic cells (OPVs)3, 4, 5, 6, 7, due to facile synthesis8, solution processability9, suitable energy levels7,

8, 10, 11, 12,

reasonable electrical

conductivity13, tunable semiconductor properties14, etc. Over the years, a few 2 ACS Paragon Plus Environment

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synthetic protocols for ZnO nanoparticles (NPs) have been developed7, 14, 15, 16. Different ZnO synthetic routes and conditions result in slightly different physical properties, such as dispersity, particle size and photoluminescence2,

14, 17, 18.

Variation in photoluminescent colors arises from different forms of defects2. For example, blue emission is controlled by zinc interstitial defect; green is emitted from the substitutions of Zn by O; yellow is caused by oxygen interstitial2,

19.

Methods to tune these defects or passivate them have been developed, e.g. doping20, 21, 22, 23, 24, polymer covering25, 26, 27, 28, UV irradiation29, solvent washing30, etc. Despite so many defect controlling methods being reported, the impact of those defects or photoluminescent colors on OPV performance is rarely discussed. Herein, we studied three typical ZnO NPs with completely different photoluminescence colors made from popular and robust synthetic routes. These ZnO NPs exhibited blue, green or yellow emissions both in solutions and in films. Then their impact on photovoltaic performance was assessed in three representative OPV systems, i.e. medium-bandgap polymer:fullerene, lowbandgap polymer:fullerene, and low-bandgap polymer:non-fullerene systems. It was found the by-product from the ZnO NP synthesis could significantly diminish the OPV performance. After removing the by-product via solvent rinsing, the OPV performance could be independent from the photoluminescence color of ZnO. In other words, different defects in ZnO played an insignificant role in OPV 3 ACS Paragon Plus Environment


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performance. Characterizations showed rinsed ZnO films exhibited comparable optical transmittance, electrical conductivity, electron extraction capability, etc. This study provides a new insight into ZnO defects, which might reshape the strategies of interface engineering in the third generation solar cells.

2. Experimental 2.1. Materials Zinc acetate dihydrate (99 +% purity), sodium hydroxide (NaOH, 98+% purity), ethanol (99.7% purity), acetone (99.5% purity) and iso-propyl alcohol (IPA, 99.7%) were purchased from KESHI. Poly(3-hexylthiophene) (P3HT, Mw > 45000) was acquired from Lumtec. [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM, 99.5%) and [6,6]-Phenyl C71 butyric acid methyl ester (PC71BM, 99.5%) were ordered from Hyper Chemical. Zinc acetate (anhydrous), 2-methoxyethanol (99 +% purity), ethanolamine (99+% purity) and MoO3 (99.99% purity) were obtained from Adams Reagent. 1,2-Dichlorobenzene (o-DCB, 98%) and chlorobenzene (CB, 99.5%) were from General Reagent. 1, 8-Diiodooctane (DIO, 98%) was received from Alfa Aesar. Poly[[2,6’-4, 8-di (5-ethylhexylthienyl)benzo[1,2-b;3,3-b]-dithiophene][3fluoro-2 [(2-ethylhexyl) carbonyl] thieno[3,4-b]- thiophenediyl]] (PTB7-Th, Mw > 40000), PBDB-T-SF and IT-4F11 were from Organtec solar materials Inc. All materials were used as received. 4 ACS Paragon Plus Environment

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2.2. Preparation of ZnO NPs with different photoluminescent colors Blue photoluminescent ZnO was prepared following a modified method reported in a literature7. 500 mg of zinc acetate dehydrate was dissolved into 7.5 mL of 2-methoxyethanol, 127 uL of ethanolamine was added in and kept stirring at 60 °C for 1 hour and then at room temperature for 12 hours. Green photoluminescent ZnO was prepared referring to an established method in literatures15, 31. 440 mg of zinc acetate dihydrate and 144 mg of NaOH were dissolved in 20 mL ethanol separately under constant stirring at 60 °C. Then the fully dissolved solutions were cooled down to -10 °C in 30 min. After that, these two precursor solutions were mixed and vortex stirred for 1 min, then stored at -15 °C for 24 hr. Finally, 500 mL of acetone was added into the mixed solution to precipitate the ZnO NPs, which was harvested by centrifuge at 3 krpm for 1 min. They were re-dispersed into 20 mL of ethanol to form a clear solution. The synthesis of yellow photoluminescent ZnO NPs was similar to its green counterpart17. The difference was that after vortex stirring, the mixed solution was kept stirring at 60 °C for 5 hr. Precipitation and re-dispersing processes were the same as the protocols for green emissive ZnO. 2.3. Device fabrication OPVs with an inverted structure with an architecture of glass/ITO/ZnO/active layer/MoO3/Ag were fabricated. Pattered ITO glasses were washed by detergent, 5 ACS Paragon Plus Environment


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deionized water, acetone and IPA sequentially. Each washing step was carried out in an ultra-sonication bath for 10 min. After drying in a nitrogen flow, UV-ozone treatment was performed for 30 min. A 30 nm-thick ZnO film was obtained by spin coating at 3000 rpm for 30 s and dried at 150 °C for 10 min in air. Solvent rinsing was performed by spin coating 2-methoxyethanol on the ZnO films right after the thermal annealing. Then the ZnO covered ITO glass was transferred into a nitrogen filled glove box to deposit the active layer. For P3HT:PC61BM based cells, P3HT and PC61BM was dissolved into oDCB at a total concentration of 34 mg/mL with a donor/acceptor (D/A) ratio of 1:1. The solution was spin coated at 650 rpm for 60 s and then 1500 rpm for 3 s. Immediately after the spin coating, the wet P3HT:PC61BM films were kept in a closed petri-dish until dry. Finally, the active layer was thermally annealed at 125 °C for 10 min. For PTB7-Th: PC71BM based cells, donor and acceptor were dissolved in CB at 10 mg/mL and 15 mg/mL, respectively. 3 vol% DIO was added as solvent additive. Mixed solution was deposited at 1800 rpm for 60 s, and was then kept in low vacuum (0.1 mbar) for 3 hr to remove residue solvent. For PBDB-T-SF:IT-4F based cells, the materials were dissolved in CB:DIO (99.5:0.5 volume ratio) mixed solvent to make a concentration of 20 mg/mL, with a D/A ratio of 1:1. The solution was spin coated at 2500 rpm for 60 s, then annealed at 135 °C for 10 min. 6 ACS Paragon Plus Environment

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The cathode was formed by depositing 8 nm of MoO3 and 100 nm of Ag in a thermal evaporation chamber with a base pressure of 2×10-4 Pa. 2.4. Characterizations Photocurrent density−voltage (J−V) characteristics and device stability were recorded under AM 1.5G illumination provided by an AAA class solar simulator (SS-100A, San-Ei Electric) calibrated by a silicon reference cell. Film thickness was acquired with a Bruker DektakXT step profiler. Contact angle test was performed with a contact angle goniometer (YIKE-360A, Chende Yike). Photoluminescence test was performed by a fluorescence spectrophotometer (LS45, Perkin Elmer) in both film and solution. Atomic force microscopic (AFM) images were acquired by a Bruker Dimension EDGE in tapping mode and a Micronano D5-A in tapping mode. The scanning electron microscopy (SEM) images and energy dispersive spectrometer (EDS) were recorded on a field-emission SEM (JSM-7800F, JEOL). Fourier-transform infrared spectroscopy (FTIR) test was achieved with a FTIR spectrometer (Spectrum Two, Perkin Elmer) in powder testing mode. X-ray diffraction (XRD) test was performed with ZnO-G powder from vacuum dried solution and using a PANalytical Empyrean diffractometer equipped with Cu Kα radiation (λ = 1.5406 Å).

3. Results and Discussions 7 ACS Paragon Plus Environment


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Figure 1 Images of three different ZnO NP solutions under (a) white light or (b) under UV light (365 nm). (c) The corresponding photoluminescence (PL) emission spectra of the three solutions excited at 360 nm. As Figure 1(a) shows, the ZnO NP solutions are transparent under white light illumination, implying the ZnO NPs can be dispersed well in the solvent, and their particle sizes are much smaller than the wavelength of visible light. Under exposure of an ultraviolet lamp, these solutions emit three completely different colors, i.e. blue, green and yellow (Fig. 1(b)). The differences in photoluminescent colors are confirmed by PL spectra (Fig. 1(c)), with corresponding peaks located at 450 nm, 520 nm, 535 nm respectively for the blue, green and yellow emissive ZnO samples (denoted as ZnO-B, ZnO-G and ZnO-Y). Since the band gap of ZnO is ~3.3 eV1, 2, the corresponding emission peak should be at 375 nm. The red shift of the ZnO PL peak is related to the defects in ZnO NPs19, 32, 33, 34. The PL peaks 8 ACS Paragon Plus Environment

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of the three emission colors represent an energy level difference of 2.76 eV, 2.38 eV and 2.3 eV, respectively. These new energy levels are formed by Zn interstitials, substitutions of O at Zn atomic position and O interstitials2, 19. In order to investigate the influence of different ZnO defects on OPV performance, ZnO NPs with different emission colors were incorporated into OPVs as electron transporting layer (ETL). At the very beginning, the as-cast ZnO films without solvent rinsing were used as the ETL. P3HT:PC61BM based OPVs showed quite different performances, as reflected by the current density-voltage (J-V) curves and summarized photovoltaic parameters (Fig. 2(a) and Table 1). ZnO-B delivers the best photovoltaic performance, with an average power conversion efficiency (PCE) close to 4%. In great contrast, the performances of ZnO-G and ZnO-Y based OPVs are extremely poor. The PCE values are below 0.5%.

Figure 2 J-V curves of P3HT:PC61BM based OPV cells with different as-cast ZnO NPs without solvent rinsing (a) under AM 1.5G illumination and (b) in dark.

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Table 1 Average photovoltaic performances of P3HT:PC61BM based OPV cells with different as-cast ZnO NPs without solvent rinsing. The statistical data are the average of 4 devices. ZnO

Voc

Jsc

FF

PCE

Rs

Rsh

(V)

(mA/cm2)

(%)

(%)

(Ω cm2)

(kΩ cm2)

1.6±0.8

523±3

3.5±1.8

0.5±0.1

6.9±4.0

0.2±0.01

As-cast ZnO- 0.62±0.0 B

1

3.9±0. 9.9±0.4

As-cast ZnO- 0.35±0.1 G

0

2.1±0.9

63.0±2.5

2

28.4±11.

0.2±0.

7

2

As-cast ZnO- 0.30±0.2 Y

0

0.3±0. 2.3±0.7

49.0±7.0

2

More information was unveiled by dark current measurements (Figure 2b). The dark current of ZnO-G or ZnO-Y based OPVs under reverse bias are two orders of magnitude higher than that of ZnO-B based OPVs, which implies a worse rectifying characteristics. Series resistance (Rs) and shunt resistance (Rsh) were extracted at 1.5 V and 0 V of the dark J-V curves, respectively. On one hand, the Rs of ZnO-B based OPV is 1.6 Ω cm2, which is much smaller than that of ZnO-G (3.5 Ω cm2) and ZnO-Y (6.9 Ω cm2). On the other hand, the Rsh of the ZnO-B based 10 ACS Paragon Plus Environment

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OPV is 0.5 MΩ cm2, which is three orders of magnitude higher than that of ZnO-G (500 Ω cm2) and ZnO-Y (300 Ω cm2). In other words, under as-cast condition, ZnOG/Y causes a worse rectifying behavior than ZnO-B, thus hindering the effective charge extraction and utilization35. Further characterization on the ZnO films reveals an interesting phenomenon. On the as-cast ZnO-G/Y surface right after thermal annealing, the emergence of small droplets was observed under an optical microscope. Figure 3(a) captures the process. Within 90 s, numerous droplets emerged on the ZnO surface; and the droplet size kept increasing with time. Presumably, hygroscopic substance may exist on the surface of the as-cast ZnO-G/Y films. To test such a hypothesis, solvent rinsing with a number of polar organic solvents, e.g. 2-methoxyethanol, methanol, ethanol, IPA and n-butanol, was performed. After rinsing, this droplet formation has been completely suppressed, as depicted in Figure 3(b).



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Figure 3 Optical microscopic images showing (a) droplet formation with time on annealed as-cast ZnO-G/Y films in air and (b) the ZnO surface after being rinsed with a polar organic solvent. The magnification is 80 X. In order to probe the changes of ZnO surface before and after rinsing, SEM and AFM images were recorded. As depicted in Figure 4, the as-cast ZnO-B film exhibits a smooth and homogeneous surface. But for the as-cast ZnO-G film, there are flower-like patterns in micron scale on the surface; and pinholes are observed on the as-cast ZnO-Y film. After rinsing, no obvious change is found in ZnO-B. In contrast, the surface morphology of ZnO-G and ZnO-Y films is totally modified. The patterns are disappeared, leaving a more uniform surface with less contrast. While SEM provides surface morphology in micro-scale, AFM can show surface morphology in nanoscale as well as roughness. Due to the formation of droplets on the surface of ZnO-G/Y, the acquisition of clear AFM images for these two samples is unsuccessful (Fig. S1). Only ZnO-B sample shows a smooth surface with a roughness (Rq) of 1.3 nm. Rinsing process seems can wash away the hygroscopic substances on ZnO-G/Y samples. Clear yet similar surface morphologies are recorded for these two samples. The Rq values are also comparable (6 nm for ZnO-G and 7 nm for ZnO-Y). The morphology of ZnO-B is also changed after rinsing, with particles appear on surface. But the Rq is still 1.1 nm, almost unchanged. 12 ACS Paragon Plus Environment

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Figure 4 SEM images of different ZnO films before and after solvent rinsing process. The surface hydrophobicity was investigated by contact angle measurement. As shown in Figure S2, no matter whether it is rinsed or not, the ZnO-B displays a small contact angle of 3° when o-dichlorobenzene (oDCB) is dropped onto the surface. However, before rinsing, ZnO-G and ZnO-Y show a large contact angle of 23°, implying the surface is much more hydrophilic than ZnO-B. After solvent rinsing, the contact angle is reduced to 2~3°, quite comparable to that of ZnO-B. Since oDCB is the solvent for the active layer, bad wettability of oDCB can cause a bad contact between ZnO layer and active layer, thus the device performance is poor for ZnO-G/Y based OPVs.



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The surface energy of each type of rinsed ZnO films was estimated with OWRK method36. For ZnO-B/G/Y, their corresponding surface energy was 65.5 mN/m, 70.9 mN/m and 71.3 mN/m, respectively. The similar surface energy implied that the same contact conditions between each type of rinsed ZnO films and the photoactive layers could be achieved. In order to identify the ingredient of hygroscopic substance, FT-IR transmittance spectra of ZnO-G, ZnO-Y was acquired, as shown in Figure 5(a). Characteristic peaks were marked out with dash lines. For both ZnO-G and ZnOY samples, a strong absorption peak at 450 cm-1 corresponding to Zn-O stretching mode is observed, suggesting the successful formation of ZnO20, 37. Absorption peaks at 1550 cm-1 and 1400 cm-1 are assigned to electron resonance of carbonyl group (C-O) and carboxyl group (C=O) in carboxylate ion COO-38, 39, 40. Besides, antisymmetric stretching and symmetric stretching of single C-O bonds are also observed at 1042 cm-1 and 921 cm-139. Other absorption peaks around 3400cm-1 are due to O-H stretching

41, 42, 43,

mainly coming from the hydroxyl group or H

bond. The peaks around 3000 cm-1 and 2930 cm-1 are referring to alky groups42, 44, 45.

Those characteristic signals together suggest the presence of acetate in

ZnO-G and ZnO-Y. Since zinc acetate is a reactant and overdosed in our synthesis protocol, it is possible that zinc acetate and/or sodium acetate (NaAc), which is the by-product after reacting with NaOH, still remain in the ZnO-G/Y solutions. 14 ACS Paragon Plus Environment

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To further confirm such a hypothesis, FTIR transmittance spectrum of NaAc was acquired. Except for the Zn-O absorption peak, other characteristic peaks are all matched, implying that NaAc does exist in ZnO-G and ZnO-Y. Furthermore, a large contact angle of 24° is observed when o-DCB is dropped on NaAc film. The contact angle is similar to that of the as-cast ZnO-G and ZnO-Y samples (Fig. S3(a)). In addition, a similar droplet growing process is also observed on the NaAc surface with a more rapid growing speed (Fig. S3(b)). Energy dispersive X-ray spectroscopy (EDS) was carried out for further investigation on the compositional change of the ZnO films deposited on Si wafer before and after rinsing. As depicted by the pie chart in Figure 5(b), sodium is detected in the as-cast ZnO-G film, with molar ratio of more than a half of Zn; while after rinsing, the sodium content decreases by two orders of magnitude, from 2.31% to 0.03%. Meanwhile, carbon and oxygen contents are also decreased due to the removal of the sodium salt. XRD was also carried out to further confirm the existence of NaAc. As shown in Figure S4, the broadened diffraction peaks of ZnO appeared at 31.8°, 34.2°, 36.3°, 56.6°, 62.9° and 68.0°, corresponding to (100), (002), (101), (110), (103), (200) crystal planes, and matched with the JCPDS PDF#36-145146. Besides the ZnO diffraction peaks, all the additional diffraction peaks could match well with the JCPDS PDF#29-1158 and PDF#28-1030 of NaAc



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and its trihydrate. The above analysis confirms the by-product of NaAc is presented in the ZnO-G/Y samples and can be washed away by solvent rinsing.

Figure 5 (a) FT-IR spectrums of sodium acetate, ZnO-G and ZnO-Y powder; (b) EDS elemental analysis of ZnO-G films on silicon wafers before and after rinsing. After the hygroscopic remnants in ZnO films are removed by rinsing, the influence of photoluminescence on photovoltaic performance is re-evaluated in OPV systems. As presented by the J-V curves and summarized photovoltaic parameters in Figure 6(a), all three ZnO NPs performed equally well in P3HT:PC61BM based cells, e.g. open-circuit voltage (Voc) all reached 0.62 V, shortcircuit current density (Jsc) values were between 9.5 and 9.8 mA/cm2, fill factor (FF) achieved 65% and above, so the overall PCE were all around 4%. Their temporal stability performance was measured with P3HT:PC61BM OPVs based on each 16 ACS Paragon Plus Environment

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type of ZnO films before and after rinsing (Fig. S5). After rinsing, all the OPV devices are quite stable in air, much more stable than the devices with as-cast ZnO ETLs. But it is worth noting that the ZnO-B delivers very similar device stability performances for both as-cast and rinsed conditions. To make a comparison of different types of ZnO defects on the active layer, external quantum efficiency (EQE) and UV-vis absorption spectra were recorded (Fig. S6 (a) and (b)). After rinsing, the EQE spectra, integrated current densities and absorption spectra are quite comparable, suggesting that different ZnO defects cause insignificant influence on light absorption, charge generation and transport of the active layer in OPVs. AFM topographies of the photoactive blends on each type of rinsed ZnO ETLs were also acquired, as shown in Figure S6 (c). Comparable surface morphology and roughness were observed with corresponding surface roughness of 3.0 nm/3.6 nm/3.8 nm for ZnO-B/G/Y. It should be noted that the effective removal of NaAc by rinsing is related to the solvent types. Polar organic solvents such as 2methoxyethanol or methanol delivered slightly better OPV performance than IPA or butanol, when the ZnO layer was rinsed by these solvent for one time (Figure S7 and Table S1). However, the performance can reach to the same level after rinsing with poor solvent such as IPA for multiple times, as reflected in Figure S8



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and Table S2. This experiment highlights the necessity for complete removal of NaAc. The rinsing strategy also works well in other representative OPV systems, i.e. low-bandgap polymer:fullerene system (e.g. PTB7-Th:PC71BM) and low-bandgap polymer:non-fullerene system (e.g. PBDB-T-SF:IT-4F). As exhibited in Figure 6(b) and (c), the J-V curves for different ZnO NPs after rinsing are almost overlapping. The PCEs reached 8% and 10% for PTB7-Th:PC71BM based OPVs and PBDB-TSF:IT-4F OPVs, respectively. These results indicate that the photovoltaic performance of different OPV systems is independent from the photoluminescence color of ZnO NPs. In other words, different intrinsic defects in ZnO play an insignificant role in OPV performance.

Figure 6 J-V curves of (a) P3HT:PC61BM, (b) PTB7-Th: PC71BM and (c) PBDB-TSF:IT-4F based OPV cells with different solvent rinsed ZnO NPs under AM 1.5G illumination. Corresponding photovoltaic parameters are listed in each plot.


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To rule out the possibility that the defect type will be changed by rinsing, PL measurement was performed on ZnO films before and after thermal annealing and rinsing. As depicted in Figure S9(a), the photoluminescent peaks of ZnO-G and ZnO-Y were almost at the same position, thus the defect types are unchanged. For ZnO-B, a slight red shift from 430 nm to 460 nm was observed. 460 nm is equivalent to an energy difference of 2.7 eV, which still belongs to the zinc interstitial defect. Therefore, we can conclude that film treatment such as thermal annealing or solvent rinsing will not change the intrinsic defect of ZnO NPs. To understand why ZnO NPs with different intrinsic defects have little impact on OPV performance, optical and electrical properties of the ZnO layers are characterized. After forming the film, different ZnO NPs showed nearly identical transmittance spectra in the visible light range (Fig. S9(b)), and comparable electrical resistances (0.54 Ω cm2, 0.56 Ω cm2 and 0.54 Ω cm2 for ZnO-B/G/Y), as extracted from the J-V curves of an ITO/ZnO/Ag sandwich structure (Fig. S9(c)). On one hand, high transmittance of the ETL implies a small optical loss on the path of light propagation from transparent electrode to active layer, guaranteed enough photons to be captured by the active layer. On the other hand, small resistance brings less recombination loss. Comparable optical and electrical properties of different ZnO NPs suggest that different defect forms do not affect photon transmittance and electron transport through the ZnO films. 19 ACS Paragon Plus Environment


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Charge extraction from active layer by the different ZnO interlayers was accessed by PL quenching experiment. P3HT:PC61BM was deposited on glass or different rinsed ZnO NPs. Figure S9(d) showed the active layer on different ZnO ETLs exhibited the same PL intensity at 650 nm, which is lower than that of deposited on glass. A lower PL intensity is attributed to better charge extraction capability. Space charge limited current (SCLC) test of electron-only devices is also carried out to extract electron mobility of different ZnO interlayers (Fig. S9(e)). The mobility is 3×10-5, 3.2×10-5 and 4×10-5 cm2 V−1 s−1 for ZnO-B, ZnO-G and ZnOY, respectively. The comparable mobility values proved again the defect type does not affect charge transport in ZnO films. 4. Conclusions In summary, ZnO is one of the most popular electron transport materials used in organic photovoltaic cells. Various synthetic routes and conditions can lead to different intrinsic defects in ZnO NPs, thus producing different photoluminescent colors. We studied the impact of these intrinsic defects on OPV performance by synthesizing blue, green and yellow emissive ZnO NPs. The as-cast ZnO interlayers delivered vastly different OPV performances. It was found sodium acetate as the by-product of ZnO synthesis played the major role in deteriorating the device. Upon successful removal of NaAc by rinsing the as-cast ZnO films with polar organic solvents, different ZnO NPs were able to produce almost the same 20 ACS Paragon Plus Environment

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high PCE values in three representative OPV systems. Further characterizations proved that the defect types did not alter the photon transmission and charge transport through the ZnO interlayer. This finding provides a new perspective on defect control of ZnO, and oxide based interlayer in larger extent.

Supporting Information. AFM height images of ZnO films before and after solvent rinsing; contact angles of oDCB on different ZnO films before and after rinsing; contact angle measurement of oDCB on sodium acetate surface; the process of liquid droplet formation on an annealed sodium acetate film in air; XRD measurement of as cast ZnO-B; EQE, UVvis absorption measurement of P3HT:PC61BM OPV based

on each type of ZnO; AFM

test of P3HT:PC61BM blend on each type of ZnO; J-V curves of P3HT:PC61BM OPV cells with ZnO-B, ZnO-G and ZnO-Y interlayers that are rinsed with different solvents; the evolution of J-V curves with IPA rinsing for various times; statistics of OPV performance with different solvents rinsing on ZnO; averaged OPV performance parameters ZnO-G based cells rinsed by IPA for 1-5 times; photoluminescence of various ZnO films before and after treated; optical transmittance ZnO films; vertical conductivity test of different ZnO films; PL quenching test; SCLC test. AUTHOR INFORMATION 21 ACS Paragon Plus Environment


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Corresponding Author * Corresponding Emails: [email protected] Author Contributions ‡These

authors contributed equally.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by research grants from the Natural Science Foundation of China (61504015, 51702032), the Natural Science Foundation of Chongqing (cstc2017jcyjA0752), the Key Laboratory of Low-grade Energy Utilization Technologies and Systems (LLEUTS-2017004), Venture & Innovation Support Program for Chongqing Overseas Returnees (cx2017034, cx2017056) and Fundamental Research Funds for the Central Universities (106112017CDJQJ148805, 2018CDQYDL0051). REFERENCES (1) Ozgur, U.; Alivov, Y. I.; Liu, C.; Teke, A.; Reshchikov, M. A.; Dogan, S.; Avrutin, V.; Cho, S. J.; Morkoc, H. A Comprehensive Review of ZnO Materials and Devices. J. Appl. Phys. 2005, 98, 103.


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TOC


Impact of ZnO Photoluminescence on Organic Photovoltaic Performance

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