Preparation of Reduced Graphene Oxide: ZnO Hybrid Cathode

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Preparation of Reduced Graphene Oxide:ZnO Hybrid Cathode Interlayer using In-situ Thermal Reduction/Annealing for Interconnecting Nanostructure and its Effect on Organic Solar Cell Ding Zheng, Wei Huang, Pu Fan, Yifan Zheng, Jiang Huang, and Junsheng Yu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15411 • Publication Date (Web): 20 Jan 2017 Downloaded from http://pubs.acs.org on January 21, 2017

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ACS Applied Materials & Interfaces

Preparation of Reduced Graphene Oxide:ZnO Hybrid Cathode Interlayer using In-situ Thermal Reduction/Annealing for Interconnecting Nanostructure and its Effect on Organic Solar Cell Ding Zheng, Wei Huang, Pu Fan, Yifan Zheng, Jiang Huang, and Junsheng Yu*

State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Optoelectronic Information, University of Electronic Science and Technology of China (UESTC), Chengdu 610054, P. R. China

KEYWORDS: Organic solar cell, spray coating, reduced graphene oxide, zinc oxide, hybrid cathode interlayer, in-situ thermal reduction, in-situ thermal annealing.

ABSTRACT: A novel hybrid cathode interlayer (CIL) consisting of reduced graphene oxide and zinc oxide (ZnO) is realized in the inverted organic solar cells (OSCs). Dual-nozzle spray coating system and facile one step in-situ thermal reduction/annealing (ITR/ITA) method are introduced to precisely control the components of the CIL, assemble ZnO with graphene oxide, and reduce graphene oxide into in-situ thermal reduced graphene oxide (IT-RGO), simultaneously. The ZnO:IT-RGO hybrid CIL shows high electric conductivity, interconnecting nanostructure and matched energy level, which leads to a significant enhancement in the power conversion efficiency from 6.16 %

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to 8.04 % for PTB7:PC71BM and 8.02 % to 9.49 % for PTB7-Th:PC71BM based OSCs, respectively. This newly developed spray-coated ZnO:IT-RGO hybrid CIL based on one step ITR/ITA treatment has the high potential to provide a facile pathway to fabricate the large-scale, fast fabrication and high performance OSCs.

INTRODUCTION Bulk heterojunction (BHJ) organic solar cells (OSCs) have drawn much research interests in the past decades due to their potential advantages in achieving efficient, flexible, light-weight, large-scale and low-cost device manufacture.1-4 The rapid development of OSCs has recently led to remarkable power conversion efficiencies (PCE) of 10~11% by exploiting interfacial engineering,5 device structure optimization,6 and rational material synthesis.7 Compared with OSC based on conventional architecture, the inverted device structure has attracted particular interest,8-9 as it meets the research highlights of high PCE, high stability and roll-to-roll (R2R) compatibility.10-11 It has been demonstrated that the interfacial engineering i.e., inserting functional cathode interfacial layers (CILs) between ITO electrode and BHJ layer, plays a significant role in improving charge transportation and collection efficiency for the inverted OSCs.12-13 Over the past few years, various materials have been utilized as the CILs, such as n-type metal oxides (zinc oxide (ZnO), titanium oxide (TiOx), aluminum oxide (Al2O3), etc.),14-17 polymers (polyethylenimine, poly[(9,9-bis(3’-(N,N- dimethylamino)-propyl)-2, 7-fluorene)-alt-2, 7-(9, 9-dioctylfluorene)] (PFN), etc.),18-21 and metallic compounds (ZnS, CdS, etc.).22-23 Among them, ZnO has been proved as one of the most effective CILs for the inverted OSCs, mainly due to its high electron mobility, well optical transparency, and good stability towards moisture.16,

24-26

However, there are several

limits on the utilization of ZnO in further industrial application of OSCs. First of all, ZnO synthesized via sol-gel method requires high annealing temperature, long annealing time, and exposure of acid solvent,27 resulting in the increased energy consumption and low production speed, which is incompatible with the R2R process and flexible substrates.28 2


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For low-temperature solution-processed ZnO CILs, high density of defects, such as dangling bonds and surface states, will be generated. Moreover, poor spatial distribution of ZnO nanoparticles over a large area will restrict their contribution in high efficient inverted OSCs.29 Furthermore, conductivity and electron mobility of low-temperature processed ZnO are still unsatisfied. In this case, it is emergent to find a novel method to maintain the advantages of both high-temperature and low-temperature solution processed ZnO CILs without bringing in defects. It is worth noting that ZnO based hybrid films have been demonstrated as excellent composite CILs for high performance devices to avoid such defects. Numerous ZnO based hybrid CILs have been investigated, such as hybrid CILs of ZnO:polymer i.e. poly(ethylene oxide) (PEO), poly(ethylene glycol)

(PEG)

and

poly[9,9-bis(3'-(N,

propyl-2,7-fluorene)-alt-2,7-(9,9-dioctyluorene)]

(PFN),30-31

and

N-dimethyl)hybrid

CILs

of

ZnO:metallic oxides i.e. ZnO:tantalic oxide (Ta2O5), ZnO:titanium dioxide (TiOx).32-33 Recently, carbon-based materials such as graphene34-35, graphene oxide (GO)36 reduced graphene oxide (R-GO)37 and carbon nanotubes (CNT),38 have been verified as the excellent additives with ZnO to form hybrid CILs. With great electric conductivity, high charge carrier mobility, and excellent stability in ambient circumstance, these hybrid CILs can remarkable improve device performance.39-40 Compared with GO,41-42 R-GO has lower work function of 4.3 eV and matches well with the lowest unoccupied molecular orbital (LUMO) of (6, 6)-phenyl-C71-butyric acid methyl ester (PC71BM) (-3.7 eV). 35 Also, R-GO has better conductivity and electron mobility and more facile to be blended with ZnO as hybrid CIL than that of GO.37, 43-44 Representative techniques, including high-temperature reduction process under ultra-high vacuum,45-46 and thermal chemical process using toxic, long-term solvent,47-49 are now frequently utilized to produce R-GO. However, these complicated reduction methods are incompatible with low-temperature fast fabrication of BHJ OSCs, which become the major limitation for the application of ZnO:R-GO hybrid CIL. In this work, by utilizing a novel dual-nozzle spray coating system (DSCS),50 we can precisely control the film thickness and components of ZnO:R-GO hybrid CIL.

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Moreover, sophisticated reduction of GO into R-GO by utilizing a facile one step in-situ thermal reduction/annealing (ITR/ITA) method is realized within 15 s. Furthermore, to optimize the conductivity and energy level alignment of hybrid CIL, we inserted a thin in-situ thermal reduced graphene oxide (IT-RGO) layer between the ITO and ZnO:IT-RGO hybrid layer to form an IT-RGO/ZnO:IT-RGO hybrid CIL. By using ZnO:IT-RGO hybrid CIL, we obtain a great enhancement in the PCE from 6.16 % to 8.04 % for PTB7:PC71BM and 8.02 % to 9.49 % for PTB7-Th:PC71BM based OSCs, respectively. EXPERIMETNAL DETAILS Solution preparation The GO powder was prepared from graphite powder (particle diameter of 45 nm, 99.99 %, Sigma-Aldrich) according to the modified Hummers method.51 Then, the obtained GO powder (20 mg) was dispersed in deionized water (10 ml). The suspension was ultrasonicated by utilizing a sonication system for 4 h in a water bath. The resultant suspension was centrifuged at 7500 rpm for 15 min to remove the aggregated GO particles. Finally, a dark yellow suspension of GO with a concentration of ~2 mg/ml was obtained for device fabrication. ZnO precursor was prepared by dissolving zinc acetate dihydrate (Zn (CH3COO)2, 2H2O, Aldrich, 99.9 %, 1 g) and ethanolamine (Aldrich, 99.5 %, 0.28 g) in 2-methoxyethanol (Aldrich, 99.8 %, 10 ml) under vigorous stirring for 12

h

for

the

hydrolysis

reaction

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

in

air.

The

solution

dithiophene-2,6-diyl]

of

[3-fluoro-

2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl] (PTB7):PC71BM ratio of 1:1.5 wt. % in chlorobenzene (CB) with the addition of a small amount of 1.8-diiodooctane (DIO, Sigma, 99.9 %, CB:DIO= 97:3, vol. /vol.) with a total concentration of 20 mg/ml was

used.

The

solution

of

poly[4,8-bis(5-

(2-ethylhexyl)

thiophen-2-yl)benzo[1,2-b;4,5-b']dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3fluorothieno[3,4-b]thiophene)-2- carboxylate-2-6-diyl)] (PTB7-Th):PC71BM at a ratio of 1:1.5 wt. % was dissolved in 1, 2-dichlorobenzene (DCB) solvent at a concentration of 10 mg/ml with an identical volume of DIO. 4


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Figure 1. (a) Chemical structures of PTB7, PTB7-Th, PC71BM and GO; (b) Device architecture of OSC; (c) Schematic of fabrication process.

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Device Fabrication The chemical structures of organic materials and device architecture of OSCs are shown in Figure 1a and Figure 1b. The inverted OSCs are constructed as ITO/hybrid CILs (40 nm)/PTB7:PC71BM (100 nm) or PTB7-Th:PC71BM (95 nm) /MoO3 (15 nm)/Ag (100 nm). PTB7 (Molecular Weight 80000-200000), PTB7-Th (Average Molecular Weight 114000) and PC71BM were purchased from Solarmer Materials Inc. ITO-coated glass substrates with a sheet resistance of 10 Ω/sq were consecutively cleaned in an ultrasonic bath containing detergent, acetone, deionized water and ethanol for 15 min each step, and then dried at 80 oC for 1 h prior to use. Then, the substrates were treated by UV/ozone (PSD-UV3, 40w, NovaScan) for 10 min. A thin layer of IT-RGO baseplate was spray-coated on the top of ITO glass substrate with 200 oC by an air brush. The air brush was powered by N2 gas with a stream of 0.4 MP, and the injected flow rate of GO suspension was 12 µl/s. The nozzle was 12 cm above the substrate. Then the hybrid CILs was spray-coated by the DSCS with ZnO and GO solution for different time, respectively.52 For in-situ annealing treatment, we placed the substrate on a hot plate under various temperatures during the spraying process. The schematic of fabrication process of hybrid CIL is shown in Figure 1c. After that, the BHJ layer was spin-coated in the nitrogen box. Then, a MoO3 (99.98 %, Aldrich) layer was deposited onto the substrates at a rate of 1-2 Å/s at a pressure of 3 × 10-3 Pa in vacuum, followed by the deposition of Ag anode at a rate of about 10 Å/s under a pressure of 3 × 10-3 Pa without breaking the vacuum. Characterization and measurement The morphologies of the GO, IT-RGO and hybrid CILs were obtained by using Atomic Force Microscopy (AFM, Agilent, AFM 5500). X-ray Photoelectron Spectroscopy (XPS) and Ultraviolet Photoelectron Spectroscopy (UPS) were performed on Thermo Scientific ESCALAB 250Xi (dual source). Spectra were obtained after the surface of the film was etched for about 2 nm to minimize surface contamination. Raman Spectroscopy was collected by Acton TriVista Confocal Raman Spectroscopy System. The current density-voltage (J-V) characteristics of BHJ OSCs were measured with a

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simulated light source (CHF-XM35, Beijing Trusttech) with an illumination power of 100 mW/cm2. Electrical data were recorded using a Keithley 4200 source-measure unit. External quantum efficiency (EQE, SolarCellScan 100, Zolix) spectra were measured under the lump light passing through a monochromator, which was calibrated by a standard Si solar cell. An Agilent 4294A Precision Impedance Analyzer was employed for impedance spectroscopy measurements. The range of measured frequency was 40 Hz to 1 MHz; 50 Mv of modulation voltage without DC bias was used to extract the DC bias-dependent AC signal.53 RESULT AND DISCUSSION Fundamental structures and properties of spray-coated GO and IT-RGO AFM is utilized to characterize the morphology of GO (Figure S1). The thickness of GO is about 1.04 nm, which matches well with the thickness of a single layer GO sheet.54-55 Raman Spectrum is also conducted to characterize the ordered/disordered crystal structures and natural characteristics of the spray-coated GO and IT-RGO films (Figure 2a). It is well known that the G band corresponds to the first-order scattering of the E2g mode related to the vibration of sp2 bonded carbon atoms, and the D band as a breathing mode of point phonons of A1g symmetry,56-57 which is assigned to defects and disorders especially at the edges of graphene.58 The spray-coated GO film displays D band at 1351 cm-1and G band at 1603 cm-1. The D to G band intensity ratio (ID/IG) is 0.97, and the spray-coated IT-RGO film shows D band at 1354 cm-1 and G band at 1603 cm-1with a ID/IG of 0.89. The ID/IG reveals the level of disorder, as expressed by the sp3/sp2 carbon ratio. In our case, IT-RGO has depressed D band peak intensity and a smaller ID/IG ratio value than the GO sheet. As a result, in-situ thermal reduction process can reduce the defect quantity of graphene. Besides, it has been reported that the shape and position of 2D band (at ~ 2700 cm-1) is related to the layer numbers of graphene sheets.59 In this work, the 2D band peaks of GO film and IT-RGO film show similar shape and position at 2708 cm-1, and both have a relatively weak intensity, which indicates that there exist several single layer of GO sheets and IT-RGO sheets overlapping when fabricated by spray coating method.58 7



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Figure 2. Raman spectra (a) and XPS spectra (b) of the GO and IT-RGO film.

The chemical structures and elementary composition of GO and IT-RGO are measured by XPS. Figure 2b illustrates the curve fittings of the C1s peak of XPS spectra of spray-coated GO and spray-coated IT-RGO films. The binding energy of 285.1 eV is attributed to C–C/C–H bonds. The peaks which center at the binding energy of 286.3, 287.3 and 289.0 eV are assigned to C–OH, C=O, and O=C–OH functional groups, respectively.60-62 For GO film, considerable oxygen-containing functional groups are observed in the carbon peak, indicating the presence of GO. For the IT-RGO film, the peak at 287.3 eV (C=O bonds) nearly disappears and the peak at 289.0 eV (O=C–OH bonds) shows 60 % reduction than the spray-coated GO film without ITR. Thus, the decreased concentration of oxygen-containing functional groups after in-situ thermal reduction shows that ITR treatment is an effective and facile reduction method for the spray-coated GO film.

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Fundamental properties and morphology of various CILs Absorption spectra of ZnO, GO, IT-RGO, ZnO:GO and ZnO:IT-RGO films are shown in Figure 3a. Compared with GO, the IT-RGO film presented similar absorption curve except enlarged absorbance intensity in the visible region from 300 nm to 600 nm. This kind of color change was previously assigned to partial restoration of the π network within the carbon structure which can be considered as an evidence of the reduction of GO.63,64 Moreover, the absorption intensities in visible region of ZnO:GO and ZnO:IT-RGO are slightly increased when mixing the hybrid component of GO and IT-RGO. After all, all the hybrid CIL films show weak absorption in the solar spectrum and will not impede light absorption of active layers.

Figure 3. (a) UV-visible spectra of various CILs; (b) XPS spectra of the ZnO and the ZnO:IT-RGO films; (c) Schematic diagram of ITR/ITA process.

In addition, the I-V features of CIL films (ITO/CIL/Au) are displayed in Figure S3a, and the conductivity values of CIL films are listed in Table 3. The conductivity values of GO, R-GO, and IT-RGO are calculated as 2.12 × 10−4, 7.33 × 10−4, and 7.85 × 10−4 S/m, respectively. Consequently, the conductivities of R-GO, and IT-RGO after thermal

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reduction were enhanced. The conductivity of reference device based on ZnO determined to be 1.63 × 10−4 S/m. It is obvious that hybrid ZnO exhibit a better conductivity than the bare ZnO, thanks to the incorporation of highly conductive IT-RGO. As expected, the device based on IT-RGO/ZnO:IT-RGO show higher conductivity of 7.17 × 10−4 S/m than reference ZnO. Noticeably, this highly conductive hybrid CIL is favorable for charge recombination and facilitating electron extraction for OSCs. In addition, to investigate the chemical composition, XPS survey peaks of spray-coated ZnO and ZnO:IT-RGO hybrid films are shown in Figure 3b. Obvious Zn and O peaks are observed in ZnO film. The peaks located at binding energy of 1022 eV and 1045 eV correspond to Zn (2p3/2) and Zn(2p1/2), while the peak located at binding energy of 533 eV is attributed to O(1s). For ZnO:IT-RGO hybrid film, additional C peak is observed. Peak positions of Zn (2p) and O (1s) are identical to those in ZnO film, and the peak located at binding energy of 285 eV is attributed to C (1s). The peak intensity of Zn (2p3/2) in ZnO:IT-RGO hybrid film is decreased and peak intensity of O (1s) is inversely enhanced compared with ZnO film, which likely represents oxygen in Zn–O–C.65 It is possible that, after ITA treatment, chemical bond is formed between the GO and ZnO, which contribute to homogeneous and smooth hybrid film. In view of XPS results, we propose that oxygen from C–OH groups of GO, bonds with zinc during the ITA treatment of ZnO and GO mixed composite.46,

66

Then, ITR treatment removes

redundant oxygen-containing functional groups of GO to form ZnO:IT-RGO hybrid film. ITA and ITR treatment occur simultaneously, but they have entirely different effects for ZnO:IT-RGO hybrid film. Schematic diagram of the ITA and ITR treatment is shown in Figure 3c.

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Figure 4. (a) UPS results of bare ZnO, GO, IT-RGO and hybrid ZnO:IT-RGO films; (b) Optical band gap energy of ZnO and hybrid ZnO:IT-RGO films; (c) Energy level of the component materials used in the OSCs.

Furthermore, the energy level of the various hybrid CILs were measured by ultraviolet photoelectron spectroscopy (UPS). Figure 4a shows UPS measurement for ZnO, GO, IT-RGO, and ZnO:IT-RGO films. High binding energy cutoff (Ecutoff) region is shown in the left panel, while valence band (EVB) region is shown in the right panel. (These two split panel was measured by different scan modes of UPS.) From the examination of the Ecutoff region, the work function (WF) of GO is estimated to be 4.74 eV, while IT-RGO film shows reduced WF of 4.28 eV with ITR treatment, making the IT-RGO more facile to extract electrons from PC71BM. The optical energy band gap (Eg) is determined from the following eq. (1)67 1

[hωα (ω )]2 = B( hω − Eg )

(1)

where h is the plank constant, ω is the angular frequency and B is a constant. From Eq. (1), we can extract the band gap of ZnO and ZnO:IT-RGO. As shown in Figure 4b, Eg of 11



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ZnO and ZnO:IT-RGO hybrid film is 3.34 eV and 3.31 eV, respectively. The conduction band (CB) energy levels of ZnO and hybrid ZnO:IT-RGO can be estimated from the valence band (VB) energy levels and Eg. The VB energy levels are calculated from the UPS data using the following eq. (2) VB = hv - (Ecutoff - EVB)

(2)

where hv (21.21 eV) is the incident photon energy. The VB energy levels for ZnO, and hybrid ZnO:IT-RGO are -7.68 and -7.25 eV, respectively. Therefore, the corresponding CB energy levels are -4.34, and -3.94 eV. The increased CB energy level of hybrid ZnO:IT-RGO is ascribed to bonding of ZnO and IT-RGO. The increased CB energy levels can be beneficial to the electron extraction and transportation from the acceptor materials to the CIL, as shown in Figure 4c.

Figure 5. AFM images of (a) spray-coated GO; (b) spray-coated IT-RGO; (c) spray-coated ZnO; (d) ZnO:GO mixed film; (e) ZnO:IT-RGO hybrid film with ITA; (f) ZnO:R-GO hybrid film with PTA.

Moreover, to observe the nanostructure, morphologies of various CILs are characterized by AFM. Figure 5a shows AFM image (2 µm × 2 µm) of spray-coated GO film (spray coating for 5 seconds) on ITO substrate. Root-mean-square (RMS) roughness of spray-coated GO film is 0.71 nm and the film thickness is 4.57 nm, indicating that spray-coated GO film is composed of multilayer GO sheets. Because of the solution

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spray-coating process, several overlapped sheets with a rough planar structure are observed. In fact, compared with GO film, the RMS roughness of IT-RGO film (0.58 nm) film is decreased (Figure 5b, 2µm × 2µm). In the course of the spray deposition process of the IT-RGO, the in situ thermal treatment could fast the removal of the residual solution in GO droplets, and reduce the size of them. When the GO droplets are deposited onto substrate, the small GO droplets may cause the limited overlap of the GO sheets, resulting in the decrease of RMS roughness of IT-RGO film. Figure 5c shows the spray-coated ZnO film with a RMS roughness of 1.32 nm and a thickness of 41 nm. For the morphology of ZnO:GO mixed film, which is fabricated by DSCS (Figure 5d), small particles of ZnO are observed nearby the sheets of GO, and a disordered stack of ZnO and GO is presented with poor binding between ZnO and GO. The ZnO:GO blend film has a RMS roughness of 3.34 nm, resulting in a rough and discontinuous film morphology for the CIL. For the ZnO:IT-RGO with ITA (Figure 5e), small ZnO nano-crystals are evenly distributed on IT-RGO sheets backbones, which are observed distinctly on the surface of ZnO:IT-RGO hybrid film. The morphology shows a uniform and continuous film with well-organized bonding between ZnO and IT-RGO. Meanwhile, the RMS roughness of ZnO:IT-RGO hybrid film with ITA process is decreased to 1.77 nm with a total thickness of 45 nm. In contrast to the ZnO:IT-RGO hybrid film with ITA process, ZnO:R-GO hybrid film with post thermal annealing (PTA) shown similar morphology (Figure 5f). Nevertheless, maybe due to the long term of thermal annealing (1 h), the RMS roughness of ZnO:R-GO film with PTA treatment increase to 2.65 nm with a total thickness of 51 nm.

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Table 1. Comparison of device characteristics of PTB7-Th:PC71BM and PTB7-Th:PC71BM OSCs with different CILs. CILs PTB7:PC71BM ZnO (spin) ZnO (spray) ZnO:GO ZnO:R-GO with 200 oC PTA for 1 h ZnO:IT-RGO with 200 oC ITA IT-RGO/ZnO:IT-RGO with 200 oC ITA PTB7-Th:PC71BM ZnO (spin) ZnO (spray) ZnO:GO ZnO:R-GO with 200 oC PTA for 1 h ZnO:IT-RGO with 200 oC ITA IT-RGO/ZnO:IT-RGO with 200 oC ITA

VOC (V)

JSC (mA/cm2)

FF (%)

PCE (%)

0.74 0.73 0.67 0.74 0.74 0.74

-13.69 -13.50 -11.91 -14.53 -15.46 -16.32

65.94 62.51 52.42 67.72 66.07 66.61

6.68 6.16 4.18 7.28 7.56 8.04

0.74 0.76 0.69 0.77 0.76 0.78

-17.47 -17.14 -15.72 -17.90 -18.01 -18.61

64.40 61.57 43.11 63.01 64.45 65.40

8.33 8.02 4.68 8.68 8.82 9.49

Devices performance of BHJ OSCs To further investigate the influence of various hybrid CILs on the device performance, the OPV devices with various hybrid CILs are fabricated and their typical J-V curves are presented in Figure 6. The detailed characteristics of devices with different CILs including the open-circuit voltage (VOC), current density (JSC), fill factor (FF), PCE, are listed in Table 1. For the PTB7:PC71BM system (Figure 6a), the devices with bare spray-coated ZnO as CIL yielding a PCE and VOC of 6.16 % and 0.73 V. While the devices using ZnO:GO mixed CIL show significantly reduced PCE and VOC of 4.18 % and 0.67 V. This is ascribed to the unmatched WF of GO with 4.74 eV and discontinuous film morphology. After PTA treatment for 1 h, ZnO:GO mixed CIL turn into ZnO:R-GO hybrid CIL, resulting in the improved PCE from 4.18 to 7.28 %. It indicates that the energy levels of ZnO and R-GO are well aligned, thus promoting an efficient electron extraction from the active layer to electrode. Compared with ZnO:R-GO hybrid CIL device based on PTA treatment, the device based on ZnO:IT-RGO hybrid CIL also shows an enhanced PCE of 7.56 %. By adding a thin IT-RGO baseplate film between ITO and ZnO:IT-RGO, the

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device shows the highest PCE of 8.04 % with enhanced JSC of 16.32 mA/cm2 and FF of 66.61 %. This high performance is supported by the high conductivity values of IT-RGO and homogeneous morphology from facile ITA treatment. This ITA treatment can effectively bind ZnO and IT-RGO to form a hybrid nanostructure. Consequently, this highly conductive hybrid CIL facilitates the charge collection and reduces charge recombination in OSCs. To verify the universality of spray-coated ZnO:IT-RGO hybrid CILs in OSCs, the devices with a BHJ layer of low-band gap polymer PTB7-Th with PC71BM active layers are prepared. The J-V curves of PTB7-Th:PC71BM active layer based OSCs are displayed in Figure 6b, and detail performances are listed in Table 1. These devices show a similar tendency of enhanced performance, compared to PTB7:PC71BM system. Among these, the one with ZnO:IT-RGO hybrid CIL and thin IT-RGO baseplate exhibits the highest PCE of 9.49 % (VOC of 0.78 V, JSC of 18.41 mA/cm2, and FF of 65.40 %), which is also higher than that using pristine ZnO CIL (PCE = 8.02 %).

Figure 6. (a) J-V characteristics of PTB7:PC71BM and (b) PTB7-Th:PC71BM OSCs with different CILs; (c) EQE curves of PTB7:PC71BM and (d) PTB7-Th:PC71BM devices.

The EQE curves of PTB7:PC71BM OSCs with different hybrid CILs are shown in Figure 6c. EQE values for all devices exceed 55 % in the region of 385-685 nm except 15



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for device with ZnO:GO CIL. The EQE peak of device with ZnO:IT-RGO hybrid CIL is 71.14 % at 565 nm. The high EQE originates from an increase in the electron transport efficiency for the high electron mobility of ZnO:IT-RGO hybrid CIL. The EQE curves of the IT-RGO/ZnO:IT-RGO device present similar shape in the entire range of wavelength between 350 nm to 700 nm. It can be observed an increase of EQE up to 73.85 % at 565 nm. The enhancement in the EQE is also in good agreement with the improved JSC. These results indicate that the device with ZnO:IT-RGO hybrid CIL (ITA) could bring improved electron transfer efficiency and provide a well-contact interface between cathode and active layer leading to a sensationally high JSC. The EQE curves of OSCs fabricate with PTB7-Th:PC71BM active layer are shown in Figure 6d. The EQE results show the same tendency as the OSCs fabricated with PTB7:PC71BM active layer. Table 2. Fitted parameter of each element in an equivalent model of PTB7:PC71BM OSCs with different CILs. CILs

R1 (Ω)

IT-RGO/ZnO:IT-RGO (ITA) ZnO:R-GO (PTA) ZnO (spin) ZnO (spray) ZnO:IT-RGO (ITA)

4073.3 6915.4 9472.7 10541.2 4642.4

C1 (F)

R2 (Ω) -9

8.169 × 10 4.814 × 10-9 3.514 × 10-9 2.852 × 10-9 7.175 × 10-9

75.0 127.9 175.3 188.2 86.1

C2 (F)

R3 (Ω) -8

1.9351 × 10 1.1384 × 10-8 8.3142 × 10-9 7.4027 × 10-9 1.7009 × 10-8

62.5 106.1 145.4 164.3 71.2

To examine the electrical contacts of the interfaces in our devices, a circuit model is defined according to the sandwiched device structure of OSCs (Figure S2a).68 The shunt pair with resistance 1 (R1) and capacitance 1 (C1) corresponds to the BHJ layer. The capacitance of the active layer, C1, is usually called “chemical capacitance” as the devices are under forward bias.69 The shunt pair with resistance 1 (R2) and capacitance 2 (C2) corresponds to the two electrical contacts of the interfaces between interlayer and active layer, or the interfaces between interlayer and electrode. The resistance 3 (R3) corresponds to the electrodes include the resistance of electrodes and electrode interlayers. Fitting of Cole-Cole plot (Figure S2b) into the equivalent model gives us the fitted value of each element listed in Table 2. Compared with the device based on ZnO, The R2 of the interface junction decreases from 175.3 to 75.0 Ω when using the IT-RGO/ZnO hybrid CIL. Moreover, the C2 of the interface junction increases from 8.31 16


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× 10-9 to 1.94 × 10-8 F, simultaneously. It indicates that the interface between IT-RGO/ZnO hybrid and active layer shows a good ohmic contact correlation to carrier transport and high electron mobility. In addition, the R3 value also decreases from 145.4 to 62.5 Ω when using IT-RGO/ZnO hybrid CIL. The decreased R3 value results in the presence of lower interfacial resistance between the electrode and ZnO:IT-RGO hybrid CIL due to the IT-RGO baseplate with high electron mobility and conductivity value. The effect of electron-only devices with PTB7:PC71BM as active layers on the charge carrier transport properties is investigated by using SCLC model. The J-V curve of electron only devices with a configuration of ITO/CILs (40 nm)/PTB7:PC71BM (100 nm)/Bphen (5 nm)/Ag (100 nm) is presented in Figure S3b. The electron mobility in the electron-only devices can be calculated using Motte-Gurney law as eq. (3)

9 V2 J = εε 0 µ 3 8 d

(3)

where ε is the relative permittivity of polymer assumed to be 3, and ε0 is the vacuum dielectric constant of 8.85 × 10-12 F/m. V is the voltage, and d is the thickness of active layer. The electron mobility of whole devices with different CILs are listed in Table 3. The electron mobility of device based on ZnO is 2.04 × 10-3 cm2V-1S-1. By using PTA treatment, electron mobility of ZnO:R-GO based OSC is enhanced to 2.98 × 10-3 cm2V-1S-1. The electron motility of device based on ZnO:IT-RGO hybrid CIL with ITA treatment is 3.39 × 10-3 cm2V-1S-1, which is much higher than that based on ZnO. These results are in accordance with the aforementioned observations. The device based on IT-RGO/ZnO:IT-RGO CIL shows the highest electron mobility of 4.20 × 10-3 cm2V-1S-1.The high electron mobility of the electron only device is attributed to the IT-RGO/ZnO:IT-RGO CIL, this high conductivity hybrid CIL with suitable energy level could effectively block the hole and provide a well ohmic contact between CIL and active layer to enhance the electron transport property of the whole device. Evidently, the improvements of JSC and PCE of device based on hybrid CIL which mentioned above are mainly attributed to the improved electron mobility and conductivity of CIL.70 Table 3. Comparison of electrical conductivity of different CILs and electron mobility of different

17



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CILs with PTB7:PC71BM. CILs

ZnO

GO

R-GO

IT-RGO

conductivity (S/m)

1.63

2.12

7.33

7.85

-4

× 10

CILs with active layer

ZnO

µe (cm V-1S-1)

2.04

× 10

2

-4

× 10-3

× 10

-4

× 10

ZnO: IT-RGO

-4

IT-RGO/ZnO: IT-RGO

5.41

× 10

-4

7.17

× 10-4

ZnO:R-GO

ZnO:IT-RGO

IT-RGO/ZnO: IT-RGO

2.98 × 10-3

3.39 × 10-3

4.20 × 10-3

CONCLUSION In summary, we successfully demonstrate the invert OSCs with a great enhancement in PCE by introducing a nanostructured hybrid IT-RGO/ZnO:IT-RGO CIL, which is fabricated by DSCS with facile, fast, and one step ITR/ITA treatment. The one step ITR/ITA treatment, which reduces GO into IT-RGO and assemble ZnO with IT-RGO simultaneously,

significantly

improves

the

electron

conductivity

of

the

IT-RGO/ZnO:IT-RGO hybrid films. Moreover, this hybrid CIL has other good properties, including well-organized and smooth nanostructure thin film and suitable energy level for charge transfer. This hybrid CIL for inverted OSCs can boost the electron transport and extraction between the cathode and the active layer and reduce carrier recombination. The OSCs based on the IT-RGO/ZnO:IT-RGO CIL obtains a PCE of 9.49 % for PTB7-Th:PC71BM system and 8.04 % for PTB7:PC71BM. The spray-coated one step in-situ thermal treatment is a universal method for the development of novel hybrid CILs. The ITR/ITA treatment of spray-coated hybrid CILs provides a facile pathway to the large-scale and fast fabrication of the OSC as well as high potential for low-cost manufacturing and application versatility.

ASSOCIATED CONTENT Supporting Information. AFM of GO sheet, impedance spectroscopy measurements, and I-V curves of electron only devices.

18


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AUTHOR INFORMATION Corresponding Author *e-mail: [email protected]

ORCID Junsheng Yu: 0000-0002-7484-8114

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This research was funded by the Foundation for Innovation Research Groups of the National Natural Science Foundation of China (NSFC) (Grant No. 61421002), the NSFC (Grant Nos. 61675041), the Project of Science and Technology of Sichuan Province (Grant No. 2016HH0027). Dr. Jiang Huang also thanks for the financial support of the Fundamental Research Funds for the Central Universities (Grant No. ZYGX2010Z004). Wei Huang and Yifan Zheng gratefully acknowledge China Scholarship Council for partial support of this work.

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Figure 1. (a) Chemical structures of PTB7, PTB7-Th, PC71BM and GO; (b) Device architecture of OSC; (c) Schematic of fabrication process. Figure 1 221x293mm (300 x 300 DPI)


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Figure 2. Raman spectra (a) and XPS spectra (b) of the GO and IT-RGO film. Figure 2 118x169mm (300 x 300 DPI)



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Figure 3. (a) UV-visible spectra of various CILs; (b) XPS spectra of the ZnO and the ZnO: IT-RGO films; (c) Schematic diagram of ITR/ITA process. Figure 3 115x78mm (300 x 300 DPI)


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Figure 4. (a) UPS results of bare ZnO, GO, IT-RGO and hybrid ZnO: IT-RGO films; (b) Optical band gap energy of ZnO and hybrid ZnO: IT-RGO films; (c) Energy level of the component materials used in the OSCs. Figure 4 133x104mm (300 x 300 DPI)



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Figure 5. AFM images of (a) spray-coated GO; (b) spray-coated IT-RGO; (c) spray-coated ZnO; (d) ZnO: GO mixed film; (e) ZnO: IT-RGO hybrid film with ITA; (f) ZnO: R-GO hybrid film with PTA. Figure 5 102x61mm (300 x 300 DPI)


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Figure 6. (a) J-V characteristics of PTB7:PC71BM and (b) PTB7-Th:PC71BM OSCs with different CILs; (c) EQE curves of PTB7:PC71BM and (d) PTB7-Th:PC71BM devices. Figure 6 123x89mm (300 x 300 DPI)



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Graphic Abstract 95x54mm (300 x 300 DPI)


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Preparation of Reduced Graphene Oxide: ZnO Hybrid Cathode

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