Self-Assembly of 1-Pyrenemethanol on ZnO Surface toward

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The Self-assembly of 1-Pyrenemethanol on ZnO Surface toward Combined Cathode Buffer Layers for the Inverted Polymer Solar Cells Xiang Cai, Tao Yuan, Xiangfu Liu, and Guoli Tu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10399 • Publication Date (Web): 02 Oct 2017 Downloaded from http://pubs.acs.org on October 4, 2017

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ACS Applied Materials & Interfaces

The Self-assembly of 1-Pyrenemethanol on ZnO Surface toward Combined Cathode Buffer Layers for the Inverted Polymer Solar Cells Xiang Cai1, Tao Yuan1, Xiangfu Liu and Guoli Tu*

Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, People’s Republic of China

*Corresponding author: [email protected] 1

Xiang Cai and Tao Yuan contributed equally to this work

Abstract Solid alcohols 1-Pyrenemethanol (PyM) was first introduced to modify the zinc oxide (ZnO) layer which is used in the inverted polymer solar cells (PSCs) as a cathode buffer layer (CBL). As a low cost industrial product, the PyM can modify the surface defects and improve the electron mobility of ZnO CBL, which can be attributed to the self-assembled of PyM on the ZnO surface due to the hydrogen bonds and the conjugated structure in PyM. With a blend of PTB7:PC71BM as active layer, the device with ZnO/PyM CBL exhibited a notable power conversion efficiency (PCE) of 8.26%, which is better than that control devices based on bare ZnO CBL (7.26%). With the addition of PyM, the device based on PTB7-Th:PC71BM showed a higher PCE of 9.10%, obvious improving from 7.79% in control devices. There was not obvious change on the device performance with the increasement of PyM solutions concentration, indicating that the device fabrications are thickness-insensitive. These results could be particularly useful in solution processing of buffer layer materials to high-efficiency organic solar cells. KEYWORDS: 1-Pyrenemethanol, zinc oxide modifier, inverted polymer solar cells, hydrogen bonding, thickness-insensitive. 1

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Introduction Polymer solar cells (PSCs) have attracted widespread attention during the past decade because of the unique merits such as lightweight, flexible and low-cost solution processing et.al1-4. Although the power conversion efficiency (PCE) have approached the value of 13%5, the stability of PSCs becomes a critical issue for practical applications6. The typical photovoltaic device structure of conventional PSCs, consisting of indium tin oxide (ITO) anode, hole-transporting (normally PEDOT:PSS), active layer, and active metal cathode (normally Ca/Al), are very sensitive to water and oxygen in the air7. Compared with the typical device, the inverted PSCs show much better air-stability8-9, due to using n-type metal oxides (ZnO10, TiOx11, SnO212, etc) as cathode buffer layers (CBL) and high-work-function metals as top anode. And ZnO has become the most popular CBL in inverted PSCs, due to the high electron mobility, appropriate energy levels, nice transparency, low cost and environmental stability

13-15

,. However, solution-processed ZnO CBL also

possess high densities of surface defects, for instance, the surface groups and dangling bonds acting as trapping centers for photo generated charge16-17, and as an inorganic material ZnO exist poor interfacial contact compatibility with the upper organic active layer18, which was harmful to electron extraction19. To overcome these disadvantages of ZnO CBL, some strategies like interface engineering have been applied, included doping ZnO with metal or polymer and surface modification of ZnO. Interface engineering played a significant role in the inverted PSCs. Firstly, the interface works can modify the work function of ZnO to form an ohmic contact with the acceptor of active layer, which can help collect electrons from the acceptor. Secondly, they can passivate the surface defects and suppress their appearance of the solution-processed ZnO. Doping ZnO with aluminum (Al)20-21or gallium (Ga)22 could increase the electrical conductivity of ZnO CBL and enhanced devices performance of inverted PSCs. Compared the bare ZnO, ZnO:polymer CBL possessed advantages of smooth surface, good ohmic contacts and less thin film defects. ZnO:poly(ethylene 2


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(PEO)23,

oxide)

ZnO:poly[(9,9-bis(3'-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfl uorene] (PFN)24, ZnO:polyethylenimine (PEI)25, and ZnO:polyvinylpyrrolidone (PVP)26 have been well applied in inverted PSCs. There exist lots of organic modified materials including conjugated polyelectrolytes (CPEs), Neutral polymers, and small molecules, etc. The CPEs were widespread reported, such as PFN27, PFNBr28, PFEOSO3Na29, PTMAHT30 and so on. The neutral polymers, for which the key examples reported to date was PEI31, Some typical representatives include: fullerene derivatives (C60-SAM)32-33, ionic liquids34, perylene diimide (PDI)35 derivatives, ethanedithiol (EDT)16 and some solvent treatment36, etc. However, all these interfaces are too expensive or critical ultrathin thickness demand, limits their future application in industry37-39. So it is worthwhile to find an environmentally friendly and inexpensively small-molecule cathode interlayer materials. In this study, we first introduced a solid alcohols PyM to modify ZnO, which was used as CBL in inverted PSCs. The PyM is a mature industrial products that could be self-assembled on the surface of ZnO due to hydrogen bonds. PyMs could modify ZnO surface defects and their conjugated structure π-π stack can help the electrons transferring from the active layer to electrode, consequently one order of magnitude incensement of electron mobility, which has been observed in ZnO/PyM modified devices compared to the control devices without PyM. By the introduction of ZnO/PyM CBL, a high PCE of 9.10% was achieved with poly[4,8-bis(5-(2-ethylhexyl) thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene-alt-3-fluorothieno[3,4-b]thiophene-2-ca rboxylate] (PTB7-Th) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as active layer, which was improved about 17% comparing to the ZnO control devices. Interestingly, the increasing concentration (3-15 mg/ml) of PyM solution spin-coating on the ZnO layer had negligible effect on the device performance, which indicating that

PyM

is

a

thickness-insensitive

material.

The

active

layer

of

thieno[3,4-b]thiophene/benzo-dithiophene (PTB7) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) was also applied and the enhanced PCE from 7.26% to 8.26%. 3



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The above results would benefit commerical fabrication of PSCs.

Experimental section Materials: PTB7, PTB7-Th and PC71BM were purchased from Scholon Organic Optoelectronics

Technology

(Beijing)

Co.

1,8-Diiodooctane

(DIO,

98%),

1-Pyrenemethanol (PyM, 98%), ethanolamine, methanol, Zinc acetate dihydrate (Zn(CH3COO)2·2H2O, ≥99.0%), molybdenum trioxide (MoO3) and silver are from Sigma-Aldrich Inc. Chlorobenzene and 2-methoxyethanol were purchased from Aladdin. ZnO film Preparation: Zinc acetate dihydrate and ethanolamine (1: 1 by concentration) were dissolved in 2-methoxyethanol under vigorous stirring for one night40. The prepared ZnO precursor solution was spin-coated on ITO and then annealed at 200 °C for 1 hour. PyM was dissolved in methanol, then diluted to different concentrations to control the thickness of PyM on ITO/ZnO. After spin-coating the PyM solution, ITO substrate was annealed at 110 °C for 10 min. Fabrication and Characterization of PSCs: ITO substrates were cleaned by cleaning agents, acetone, isopropyl alcohol ultrasonic cleaning 30min. Then the substrates were dried in the drying oven. The active layer PTB7:PC71BM or PTB7-Th:PC71BM, dissolved in chlorobenzene/1,8-diiodoctane (97:3 vol%)) with a weight ratio of 1:1.5 41-42

was spin coated on top of the PyM coated ZnO layer. MoO3 (6 nm) and Ag

electrode (100 nm) were deposited respectively onto active layers by thermal evaporation. The effective device area is 9 mm2. The PCEs of the resulting PSCs divices were measured by Keithley 2400 under the illumination of AM1.5G 100 mW/cm2. The absorption transmission spectra and PL spectrum was measured by SHIMADZU UV-3600 UV-VIS-NIR spectrophotometer and Edinburgh FLS920 fluorescence spectrometer, respectively. Time-resolved photoluminance decay were measured with a 478 nm light pulse from a HORIBA Scientific DeltaPro fluorimeter. AFM (Atomic Force Microscope) images were 4


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obtained through Veeco Dimension 3100 NanoScope in a tapping mode measured by Bruker RTESPA-300 probes. The XPS was measured on an ESCALAB 250Xi X-ray photoelectron spectrometer (XPS) (Thermo Fisher Scientific). The water contact angles of ZnO film and ZnO/PyM film were measured on a drop shape analyzer (DSA100, KRUSS), ESR spectra were measured on a Bruker A300 spectrometer.

Results and discussion To demonstrate the influences of PyM on the efficiency, the devices with the structure ITO/ZnO/PyM/ PTB7-Th:PC71BM/MoO3/Ag were first fabricated, as shown in Figure 1a. Different concentrations of PyM methanol solution were also investigated. The J-V curves characteristics of all the devices, including the control device without PyM interlayer and a comparative device with methanol treatment are shown in Figure 2a. The detailed parameters are summarized in Table 1. Compared to the control device with bare ZnO CBL, the efficiency of the device using ZnO/methanol shows slightly enhancement from 7.79% to 7.92% (~ 2%), which was similar to the literature36. The VOC of bare ZnO, ZnO/methanol, ZnO/PyM are 0.77, 0.77, and 0.78 V, respectively, showing negligible change. It can be noted that JSC and FF were significantly improved with ZnO/PyM CBL. When the concentration of PyM is 3 mg/mL, the JSC is kept above 17 mA/cm2 and the FF is above 64.5%, corresponding to about 8.8% improvement of the device efficiency. When the PyM concentration is 5 mg/mL, a higher efficiency of 9.10% was achieved, which obtained a 17% enhancement compared to the control device. With higher concentration of PyM, for example 9 or 15 mg/mL, the device also exhibited efficiency of 8.83% and 8.72% respectively, indicating that the PTB7-Th: PC71BM based device are not sensitive to thickness of PyM. Compared with device with methanol treatment on the ZnO, it can be concluded that the further improvement of devices performance is mainly ascribe to the existence of conjugated structure in PyM. The dark J-V curves 5



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of the devices with ZnO/PyM and ZnO/methanol was illustrated in Figure 2b. Both the methanol treatment and the PyM modified ZnO can reduce the dark current of the device, benefit to obtain higher JSC and FF43-44. Inspired by the above result, the performances of PyM in the PTB7: PC71BM system were also tested. The J-V curves of the ZnO/PyM modified PTB7: PC71BM devices were plotted in Figure S1. The control devices showed PCE of 7.26%, JSC of 15.2 mA/cm2, VOC of 0.73 V and FF of 65.6%. After methanol treatment of ZnO, a slight effect on the efficiency has also been observed from the PTB7: PC71BM based devices. Using the optimal concentration of PyM (5mg/ml), a higher JSC of 15.2 mA/cm2 and a higher FF of 69.5% was obtained, resulting 14% increasing of PCE to 8.26%. To find out the reasons for the improved performances in the devices, the transmission of ITO, ITO/ZnO, ITO/ZnO/PyM, and ITO/ZnO/PyM/CB were measured to test whether the PyM would influence the light transmittance, which is important for the inverted PSCs efficiency. As shown in Figure 2c, ITO and ITO/ZnO possess high transmittance in the visible region. The transmission of ITO/ZnO is almost the same between 400-800 nm and a little less between 300-400 nm after the PyM was coated. This reducing results from the absorption of PyM, which is also a proof of the presence of PyM on ZnO surface. Considering that PyM has some solubility in the chlorobenzene (CB) from the solutions to prepare the active layers, then CB was used to wash the surface of PyM layer to imitate the process of device fabrications. It can be found that the transmission of ITO/ZnO/PyM/CB did not change substantially between 400-800 nm. The absorption of PyM film was still present between 300 nm and 400 nm and the increasing of transmittance can be attributed to the rinsing by CB. The Figure 2d shows the absorption spectra of ITO/ZnO/PTB7:PC71BM, ITO/ZnO/PyM/PTB7: PC71BM and pure PyM at 300 nm to 800 nm. The addition of PyM did not have effect on the absorption of PTB7:PC71BM active layer, the enhanced absorption of ITO/ZnO/PyM/ PTB7:PC71BM near 350 nm can be attributed to the PyM, this also demonstrates that PyM is still present at the ZnO surface after the coating of the PTB7: PC71BM solution. So the increase in 6


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current density using ZnO/PyM devices is not due to the increased absorption of active layer. XPS was measured to analyses the surface chemistry properties of the PyM treated ZnO films, and the survey spectra were presented in Figure 3a. After PyM treatment, the Zn 2p peak slightly shifts toward lower binding energy by 0.06 eV (Figure 3b), indicating that there formed a hydrogen bond among ZnO with PyM and then the electron cloud density of Zn atoms has changed45. Furthermore, Figure 3c shows the O 1s core level spectra, which can be divided into two peaks. Respectively, 530.1 eV peak represents the oxygen atoms form O–Zn bonding and 531.4 eV for the oxygen-deficient defects46 The relative intensity of the 531.4 eV peak decreased significantly after the PyM was treated, which means that the oxygen-deficient defects were lowered in the ZnO film, this is the one of the reasons for the improvement of JSC and FF in the device. The surface morphology of the device had a great influence to device performance, especially for the film prepared by solution. The influences of PyM on the morphology were studied by AFM. The AFM images of ITO/ZnO, ITO/ZnO/PyM and ITO/ZnO/PyM/CB films were investigated and shown in Figure 4a-c. The root-mean-square (RMS) for ZnO on ITO is 3.43 nm and it can be found that the ZnO film was corrugated coated on ITO glass. After the introduction of PyM on ITO/ZnO, surface roughness of the ITO/ZnO/PyM was slightly increased (RMS 3.81 nm), which can be attribute to the PyM aggregates along the surface of the protrusion on the ZnO film. After treated with CB solution, it was observed that the surface of the films became smooth with significantly reduced roughness and the grooves on the surface in the film were partly filled. This means the CB solution would remove some of the redundant PyM and bringing a portion of the PyM into the groove of the ZnO surface. Then the ZnO/PyM surface will be smooth enough to get good contact with the active layers. Morphology change of PTB7:PC71BM active layer with the introduction of PyM were also studied (Figure 4d-e). Some difference in RMS (2.25 to 2.44 nm) was observed, which implying that the incorporation of the PyM film will influence the morphology of PTB7:PC71BM active layer. This may ascribe to the hydrophilicity 7



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change in ITO/ZnO surface after PyM coating. As we known, the hydrophilicity/hydrophobicity properties of the interface layer have great influences on the morphology of active layer. The water contact images for ITO/ZnO and ITO/ZnO/PyM was measured in Figure S2. ITO/ZnO had a relatively strong hydrophilicity with a small water contact angle of 43.9°. After the introducing of PyM, the contact angle of ITO/ZnO/PyM is 53° and the hydrophobicity was enhanced. This is originated from the hydrophobic conjugate structure in PyM. The hydrophobic enhancement of the interfacial layer realized a good compatible to the hydrophobic active layer. The surface elements on the active layer were analyzed by XPS, as shown in Figures S3. The XPS spectra from the top surfaces of PTB7:PC71BM blend films with substrates of ZnO and ZnO/PyM respectively were investigated. The PTB7:PC71BM blend films on both the types substrates exhibit similar XPS peaks at around of 290.48, 167.28, 537.38, and 692.28 eV binding energies respectively, which can be assigned to C 1s , S 2p, O 1s, and F 1s of the top surfaces of PTB7:PC71BM blend films. As shown in the atomic proportions from Tables S1, the weight ratios of PTB7:PC71BM at the top surfaces valuated by C/S atomic ratios from calculation of the with the XPS measurement are about 19.05:1 and 17.27:1, respectively. The F and O atomic ratio also increased at top surfaces of PTB7:PC71BM blend films on ZnO/PyM. This verified that the PTB7 was enriched at the top surface of PTB7:PC71BM blend films on substrates of ZnO/PyM47, which agree with the AFM results. The Kelvin probe microscopy was performed to investigate the work function changes in PyM covered ITO/ZnO layers. Energy level diagram of the inverted devices with ZnO or ZnO/PyM CBL as shown in the Figure1c. The corresponding work function graph was presented in the Figure 5c. The work function of the untreated ITO electrode surface is 4.75 eV. After ZnO and ZnO/PyM modification, the work functions of ITO/ZnO and ITO/ZnO/PyM are 4.45 eV and 4.28 eV respectively. The work function of ITO/ZnO/PyM/CB can be increased to 4.43 eV after the CB rinsing, which was only 0.02 eV lower than ITO/ZnO/PyM. The surface properties of 8


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ITO/ZnO/PyM/CB are closer to ITO/ZnO. and the slight decrease in work function may due to the interaction of hydrogen bonds between PyM and ZnO. So the device current and efficiency of the improvement in ZnO/PyM buffer layer was not come from the change of the work function. To investigate the electron transfer ability from the PC71BM to ZnO, photoluminescence (PL) spectra of ZnO/PC71BM and ZnO/PyM/PC71BM were measured for further study. If existing the energy transfer between PC71BM to ZnO, PL emission intensity will be weaken, indicating the less charge carrier recombination and more electrons transferred from PC71BM to ZnO.48-50 Compared with ZnO/PC71BM, the PL spectrum (excitation: 470 nm) of ZnO/PyM/PC71BM exhibited a weaker intensity at emission peak of 716 nm, indicating the decrease of charge carrier recombination (Figure 5a). To further demonstrate the accelerated electronic transfer process from PC71BM to ZnO, corresponding time-resolved transient photoluminescence (TRTPL) spectra were also measured by monitoring the emission peak at 716 nm. As seen from Figure 5b, the 716 nm emission decay time is slightly decreased for ZnO/PyM/PC71BM, suggesting that electron transportation from PC71BM to ZnO was more efficient, which contributed to the suppression of charge carrier recombination in the acceptor. To study the influence of PyM modified ZnO on the electron transport properties in the device, the charge carrier mobility were measured using the space charge limited current method51 (SCLC) and J-V curves were measured in the dark (Figure 5d). SCLC can be characterized by the Mott-Gurney square law: 9 V2 J = ε rε 0 µ0 3 8 L

Electron-only devices with the construcution of ITO/CBL/PTB7:PC71BM/Al (Figure 5d) was fabricated. The electron mobility of the devices using ZnO as CBL is 1.53 × 10-5 cm-2 V-1 s. After the introducing of PyM, the electron mobility was increased an order of magnitude to 1.63 × 10-4 cm-2 V-1 s. The enhanced device mobility would contribute to higher JSC and FF in devices. Although PyM has a little solubility in CB solution, the hydrogen bond between PyM and ZnO can keep some 9



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of the PyM on surface of ZnO, which has proven by the absorption and transmission spectra. The residual part of the PyM formed hydrogen bonds with ZnO and were adsorbed on the surface of the ZnO, which will modify the ZnO surface defects without obvious influence on the work function of the surface. To verify whether PyM will increase the conductivity of ZnO in the light due to its own absorption, the devices with the structure ITO/ZnO/PyM/Al and ITO/ZnO/Al (Figure 6a) were fabricated. As the result shows, the light indeed increase the conductivity of part of the ZnO, and after modification of PyM, the conductivity of ZnO is really enhanced. However, the increasement of the conductivity is negligible52-53. Moreover, the ESR spectroscopy study was performed as the reported work54-55. The doping sample (PyM@PC71BM) was prepared by dissolving PyM and PC71BM in chlorobenzene and then dried. All samples in solid state were studied under the light condition. As seen from Figure 6b, there was no signal for PyM sample and meanwhile very weak ESR signal for PC71BM was detected, however, an obviously enhanced ESR signals was observed for PyM@PC71BM, indicating PyM and PC71BM have a good electron transport channel that under light conditions. It is possible that PyM forms a good π-π stack bridge between ZnO and PC71BM and then enhanced the electron conductivity. Thus, the π-π stack in conjugated structure in PyM may provide a favorable electron transport channel between the active layer and ZnO, which can accelerate the electrons transfer from PC71BM to ZnO and enhances the JSC and FF in devices, as shown in Figure 6c.

Conclusions In conclusion, the low-cost 1-pyrenemethanol (PyM) was introduced as CBL in inverted PSCs. The PyM could self-assemble on the ZnO surface by hydrogen bonding and modified the ZnO surface defects. Compared with the control devices without PyM, the obtained ZnO/PyM modified devices realize efficient electrons transfer from active layer to electrode, resulting one order of magnitude incensement 10


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of electron mobility, which can be attributed to the conjugated structure in PyM. And device efficiency of 9.10% based on PTB7-Th:PC71BM active layer was obtained after introducing of PyM, about 17% enhancement was achieved comparing to the bare ZnO device. The PTB7: PC71BM active layer was also tested and a 14% increased efficiency was obtained. Due to the CB rinsing process from the active layer solution, the concentration of PyM had small influence on the device performance, which indicating that PyM works like a thickness-insensitive material. These results are particularly useful to develop high efficiency and easy fabrication of PSCs buffer layer materials.

Acknowledgements This work was supported by the National High Technology Research and Development Program of China (863 Program, No. 2015AA033404), and the National Natural Science Foundation of China. (21574049) for ﬁnancial support.

Supporting Information Light J–V curves for PTB7:PC71BM based solar cells; Water contact angle images of ZnO film and ZnO/PyM film; XPS spectra PTB7 to PC71BM at the top surfaces of PTB7:PC71BM blend films on the substrates of ZnO and ZnO/PyM are included.

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Table 1. Device performance of inverted PSCs using ZnO, ZnO/methanol and ZnO/PyM film. PyM

PTB7-Th:PC71BM

PTB7:PC71BM

VOC

JSC 2

FF

PCE

(mg/mL)

(V)

(mA/cm )

(%)

(%)

ZnO

-

0.77

16.6

60.9

7.79

ZnO/methanol

-

0.77

16.7

61.7

7.92

ZnO/PyM

1

0.78

16.7

62.1

8.06

ZnO/PyM

3

0.78

17.0

64.7

8.63

ZnO/PyM

5

0.78

18.1

64.5

9.10

ZnO/PyM

7

0.78

17.4

65.8

8.99

ZnO/PyM

9

0.78

17.6

64.4

8.83

ZnO/PyM

15

0.78

17.7

63.1

8.72

ZnO

0.73

15.2

65.6

7.26

ZnO/methanol

0.73

15.3

67.0

7.46

0.73

16.3

69.5

8.26

ZnO/PyM

5

17



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Figure 1. (a) Solar cell device structure, (b) Energy level diagram of the inverted devices with ZnO or ZnO/PyM CBL, and (c) Chemical structures of PTB7, PTB7-Th, PC71BM and PyM.

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Figure 2. (a) J–V curves for PTB7-Th:PC71BM solar cells fabricated using PyM solution concentrations in the range 1–15 mg/mL. J–V curves for PTB7-Th:PC71BM solar cells with ZnO CBL or ZnO/methanol CBL are also shown. (b) The J–V characteristics of the optimized PTB7-Th:PC71BM PSCs using ZnO or ZnO/ PyM film in the dark. (c) The transmittance spectra of pristine ITO, ITO/ZnO, ITO/ZnO/PyM, and ITO/ZnO/PyM/CB films, (d) The absorption spectra of ITO/ZnO/PTB7:PC71BM and ITO/ZnO/PyM/PTB7:PC71BM.

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Figure 3. (a) XPS survey spectra of ZnO film and ZnO/PyM film, (b) Zn 2p spectra of the ZnO film and ZnO/PyM film, and (c) O 1s spectra of the ZnO film and ZnO/PyM film.

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Figure 4. Atomic force microscopy (AFM) images (sizes: 5µm × 5µm) of the (a) ITO/ZnO, (b) ITO/ZnO/PyM, (c) ITO/ZnO/PyM/CB, (d) ITO/ZnO/PTB7: PC71BM, and (e) ITO/ZnO/PyM/ PTB7:PC71BM.

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Figure 5. (a) PL and (b) TRTPL spectra of ZnO/PC71BM and ZnO/PyM/PC71BM composite films, (c) Work function images from ITO/ZnO, ITO/ZnO/PyM and ITO/ZnO/PyM/CB films, and (d) J−V characteristics of electron injection devices without and with the PyM interlayer.

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Figure 6. (a) I−V curves of the devices for ITO/ZnO/PyM/Al and ITO/ZnO/Al in the dark and under 1000 W/m2 AM 1.5G illumination. (b) ESR spectra of PyM, PC71BM, and PyM @ PC71BM in solid-state (c) Schematic diagram of electronic transfer from PC71BM to ZnO.

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Table of content

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Self-Assembly of 1-Pyrenemethanol on ZnO Surface toward

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