Zinc oxide nanorod doped graphene for high efficiency organic photovoltaic devices

Abstract

We report the synthesis of zinc oxide nanorod (ZnR) doped graphene (G) for high performance organic photovoltaic (OPV) devices based on a low bandgap polymer, poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7) blended with a fullerene derivative (PC₇₁BM). The use of ZnR–G in a PTB7:PC₇₁BM-based inverted OPV device leads to substantial enhancement in device performance in contrast to the reference devices. The improved performance is attributed to the improved charge carrier separation and prolonged lifetime of the generated electron–hole pairs. In addition, we also found that extra pathways for the disappearance of the charge carriers exist due to the interactions between the excited ZnR and G, which shows that the ZnR–G have a lower recombination rate of electrons and holes under UV light illumination.

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Introduction

Today, one-dimensional (1D) nanostructures are considered to be a promising backbone for various nanoscale and optoelectronic devices due to their small dimension. Thus, they are efficient for the transport of electron–hole pairs in nanoscale electronic and photonic structures.¹ Back in 1991, Iijima et al.² discovered one-dimensional carbon nanotubes which generated great research attention due to their distinctive geometrical morphologies, the combination of quantum confinement in the nanoscale dimension, the bulk properties in another dimension, novel physical and chemical properties, and potential application in nanoscale optical and electric devices.^3–5 Interestingly, 1D nanostructures are frequently deployed in organic photovoltaics (OPVs) due to their unique properties, uniform morphology, and efficient light-induced charge separation and transport.^6–10 Amongst 1D nanostructure materials, zinc oxide nanorods (ZnR), transparent wide bandgap semiconductors with distinctive structure and properties have been integrated into many nanoscale electronic devices including light-emitting devices,¹¹ field-effect transistors,¹² ultraviolet lasers,¹³ chemical sensors,¹⁴ photovoltaics,¹⁵ cathodoluminescence devices,¹⁶ and photocatalysts.^9,17 Nonetheless, there are still major drawbacks that hinder the utilization of pristine ZnR in OPV; for example, uneven surface topography and a high recombination rate of generated electron–hole pairs.

Recently, extensive research has been completed in order to enhance the electrical and optical properties of ZnR through various approaches, such as synthesis of ZnR arrays,^9,17 and mixing with other elements including metals,¹⁸ semiconductors,¹⁹ and carbon-based materials.^15,20–22 In the later case, graphene, another class of carbon material arranged in a honeycomb structure, has become the center of attention due to its high optical transparency, high electron conductivity and mobility, and high surface area.^23–25 These unique properties make graphene an important backbone for synthesizing new materials.^23–25 Furthermore, graphene significantly impacted many nanoscale and optoelectronic devices such as lithium ion batteries,^26,27 supercapacitors,^28,29 photochemical water splitting,^30,31 photocatalysis,^31,32 hydrogen storage,^33,34 and energy generation devices such as fuel cell,^35,36 microbial bio-fuel cells,^37,38 enzymatic bio-fuel cells,^39,40 organic photovoltaic,^41–46 and dye sensitized photovoltaic.^47,48

Chang et al., demonstrated the growth of ZnR/graphene in various substrates via a seeded solution growth method and also fabricated thin-film photoconductors based on ZnR/graphene as high sensitivity visible-blind ultraviolet sensors, in their latest discovery.²⁰ In addition, Lee et al. successfully demonstrated the integration of ZnR/graphene in the next-generation electronic and optoelectronic systems.²¹ Additionally, Yang et al. also demonstrated a combination of graphene oxide (GO)/ZnR by introducing amino groups into ZnR, and later amidating carboxyl groups of GO.²² Since then, there have been reports on ZnR/graphene. However, most reported works started with a deposition of a carbon layer on a SiO₂/Si/polymer substrate and were followed by the growth ZnR. Others began with an introduction of surface amino groups onto the outer wall of ZnR then grafted with GO sheets. Previous work were also keen on optical, electronic, morphology, and utilization in dye sensitized solar cells,¹⁵ ultraviolet sensors,²⁰ and multifunctional conductors.²¹ To the best of our knowledge, work on the incorporation of ZnR/graphene in the OPV device has been made unavailable.

Therefore, herein we report a synthesis surfactant-free hydrothermal process to prepare ZnR/graphene hybrid film with a small-diameter (∼20 nm) sized distribution, and fully utilized their unique potential as an electron transport layer (ETL) in OPV. Moreover, the effect of different ETLs on the device performance has also been investigated, and the probable justification for the improvement of device performance is also discussed. In this work, we illustrate that the integration of graphene into the matrix of 1D ZnR could potentially improve the electron transport process, and promises plausible incorporation of 1D ZnR and two-dimensional (2D) graphene for potential OPV applications. To the best of our knowledge, this is the first time the facile, high-yield and surfactant-free method has been used for the fabrication of ZnR/graphene in OPV. It also enriches the chemistry of morphology-dependent functional properties of ZnR in the application of OPV. This could open new pathways to further utilize the 1D nanostructure, such as with nanorods, nanotubes, and nanowires as the ETL in OPV.

Fig. 1 shows the X-ray diffraction (XRD) patterns of ZnR and ZnR/graphene thin films. The diffraction peaks of ZnR can be assigned according to JCPDS no. 36-1451. The peaks at 2θ values of 32°, 34.6°, 36.4°, 47.6°, 56.9°, 63.1°, 68.2°, 69.1°, and 77.1° can be assigned to (100), (002), (101), (102), (110), (103), (112), (201), and (202) crystal planes, respectively. As seen from the ZnR/graphene XRD pattern, the (002) reflection peak at 2θ ∼34.6° is remarkably sharpened, indicating the formation of nanorods along the c-axis,⁴⁹ that the peaks belong to ZnR, and that graphene can easily be distinguished. The strong and sharp peaks of ZnR show that the ZnR attached onto graphene were still highly crystalline, like the pristine ones. The presence of graphene can still be observed at 2θ ∼24° (002).⁵⁰ It is worth noting that the FWHM of ZnR/graphene is 0.2794 compared to that of 0.3016 for ZnR.

Fig. 1 XRD patterns of pristine ZnR, and ZnR/graphene.

The presence of graphene on ZnR was further verified by Fourier Transform Infrared Spectroscopy (FTIR), as shown in Fig. 2. For ZnR/graphene, the bands at 3412, 1637, 1402, and 1131 cm⁻¹ can be assigned to the –OH stretching vibrations, skeletal vibrations of unoxidized graphitic domains, O–H deformations of the C–OH groups, and C–O stretching vibrations, respectively. For the ZnR plot, a broad band at 3400 cm⁻¹ can be attributed to the hydroxyl groups and bands at 1573 and 1402 cm⁻¹ are due to the bridging mode of acetate bonded to Zn at the ZnO surface.^51,52

Fig. 2 FTIR spectra of ZnR, and ZnR/graphene.

To monitor the structural changes during an attachment process between ZnR and graphene, we performed Raman spectroscopy and thermogravimetric analysis (TGA) as shown in Fig. 3. From the Raman spectrum of ZnR/graphene (Fig. 3a), the ratio of the D and G bands (I_D/I_G) improved compared to that of graphene film. This corresponds to a decrease in the average sp² domain sizes.⁵³ Fig. 3b depicts a total weight loss of 4% (ZnR) in the temperature range of 100–800 °C, which is most likely due to the desorption of bound water.⁵⁴ For ZnR/graphene, the remarkable weight loss around 200 °C implies to the weight loss of graphene, while the weight loss at higher temperatures corresponds to the hydrothermal process.

Fig. 3 (a) Raman spectra and (b) TGA plots of ZnR, and ZnR/graphene.

Transmission electron microscopy (TEM) images are scanned to analyze the in-depth topography of the ZnR and the effect of the introduction of the graphene on the microscopic structure of the ZnR. As shown in Fig. 4a, ZnR were highly crystalline, and preferentially grown along the (001) reflection. The length and diameter of the ZnR are 5–20 nm and 15–30 nm, respectively. During the attachment process, it is assumed that ZnR was attached tightly to graphene rather than random and loose contact. Thus, covalent coupling is employed to achieve anchoring.

Fig. 4 SEM images of (a) ZnR, and (b) ZnR/graphene.

To further verify the close interfacial contact between ZnR and the 2D graphene, the scanning electron microscopy (SEM) analysis of the ZnR/graphene was used. Based on Fig. 4b, it is obvious that the 2D structure of graphene with clear wrinkles was still preserved after the hydrothermal treatment of graphene and ZnR. In the ZnR/graphene, the ZnR was decorated by graphene, several of which were wrapped by graphene. Next, we demonstrate that the good interfacial contact of graphene and ZnR attained during the hydrothermal process provides efficient photogenerated charge carriers for the transfer process across the interface.^55–60

It is known that the unique properties of graphene make it possible to enhance the lifetime of photoexcited electron–hole pairs generated from ZnR as well as improve the efficient transfer of charge carriers. These phenomena can be observed from the photoluminescence (PL), and an electrochemical impedance (EIS) analysis. Generally, it is accepted that ZnO and carbon materials are good electron donors and electron acceptors, respectively. Therefore, the synergistic effect between these two components would effectively reduce recombination of photogenerated electron–hole pairs, and at the same time lead to an increased charge carrier separation.⁶¹ The improved charge carrier separation, and the prolonged lifetime of photogenerated electron–hole pairs can be confirmed by the PL spectra. As shown in Fig. 5 (under excitation wavelength of 365 nm), a yellow-green band can be seen from 450–800 nm with a peak of 580 nm, which can be attributed to the oxygen vacancies in the ZnO lattice.²²

Fig. 5 Photoluminescence spectra of ZnR, and ZnR/graphene.

On the other hand, one can obviously see a decrease in intensity that corresponds to an effective interfacial charge transfer process. This suggests that additional pathways for the disappearance of the charge carriers exist due to the interactions between the excited ZnR and graphene.^58,62 This demonstrates that the ZnR/graphene have a lower recombination rate of electrons and holes under UV light illumination. This is due to the fact that the electrons are excited by the valence of the conduction band of ZnR, and later transfer to graphene; thus, preventing a direct recombination of electrons and holes. Graphene is regarded to be a good electron acceptor material that can completely prevent electron–hole pairs recombination. Due to its unique 2D π-conjugation structure, the graphene in ZnR/graphene is part of the separation region of the photogenerated electrons and holes.^63,64

To further evaluate the benefit of ZnR/graphene over ZnR in terms of improving the charge carriers' transfer, the electrochemical impedance spectroscopy (EIS) measurement was also used. EIS is a very powerful tool that can characterize charge carrier migration. It can be observed from Fig. 6 that the ZnR/graphene shows a significantly suppressed semicircle at high frequencies compared to the pristine ZnR. This implies a decrease in the solid state interface layer resistance, as well as the charge transfer resistance across the solid–liquid junction on the surface.⁶⁵ In summary, the integration of ZnR with graphene improves the transfer of photogenerated charge carriers.

Fig. 6 Nyquist impedance plot of ZnR, and ZnR/graphene.

Based on the promising results above, we investigated the effect of electron transport layers on the performance of PTB7:PCBM-based single junction solar cells, with an inverted structure of indium tin oxide (ITO)-coated glass/ETL/PTB7:PCBM/C₆₀-bis/Ag. Three electron transport layers were studied herein: ZnO, ZnR, and ZnR/graphene. The current–voltage (J–V) characteristics of the devices are shown in Fig. 7a, and the corresponding photovoltaic parameters are tabulated in Table 1. The ZnO-based device demonstrated a short-circuit current density (J_sc) of 13.70 mA cm⁻², an open-circuit voltage (V_oc) of 0.75 V, and a fill factor of 57.88%, with a power conversion efficiency of 5.95%, while the ZnR-based device exhibited a relatively high PCE of 6.31%, with a J_sc of 14.44 mA cm⁻², an V_oc of 0.75 V, and a FF of 58.44%. Although both ZnO and ZnR are indeed good electron transport layers for the PTB7:PCBM-based OPV, its differences in mobility (6.72 cm V⁻¹ s for ZnO and 120.8 cm V⁻¹ s for ZnR) can largely influence the morphology and J–V of PTB7:PCBM devices. The distinctive feature is when ZnR/graphene are integrated into the electron transport layer, in which the PCE increased to 7.86%, along with a J_sc of 16.20 mA cm⁻², an V_oc of 0.75, and a FF of 64.69%. The J_sc values of all devices are in agreement with the values calculated from the integrated incident photon-to-current conversion efficiency (IPCE) spectra (Fig. 7b), which confirmed the accuracy of the reported PCE values. It is worth noting that 7.86% represents one of the highest PCE for the combination one and 2D based OPV devices.

Fig. 7 (a) J–V characteristics of organic solar cells based on PTB7:PCBM with different electron transport layers measured under AM 1.5 illumination, and (b) external quantum efficiency based on different electron transport layers.

Table 1 Photovoltaic parameters extracted from Fig. 7

ETL	Jsc (mA cm^−2)	FF (%)	Voc (V)	PCE
ZnO NR/graphene	16.20	64.69	0.75	7.86
ZnO	14.44	58.44	0.75	6.31
ZnO NR	13.70	57.88	0.75	5.95

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The chemical states of ZnR/graphene thin film are analyzed by X-ray photoelectron spectroscopy (XPS) as illustrated in Fig. 8. A wide survey spectrum (Fig. 8a) shows the peaks of C 1s, O 1s, and Zn 2p for the ZnR/Graphene. The C 1s XPS spectrum of the ZnR/graphene is shown in Fig. 8b.

Fig. 8 XPS of ZnR/graphene thin film. (a) Wide spectrum, (b) C 1s, (c) O 1s, and (d) Zn 2p.

The binding energy at ∼285.3 eV is assigned to C–C and C–C/C–H bonds.^66–68 Fig. 8c displays that the peak at ∼531 eV of O 1s is attributed to the surface oxygenated species, such as C–O or O–H. The appearance of the C–O bond again confirms the interaction between 2D graphene and 1D ZnR.

In Fig. 5d, two binding energies peaks observed in the Zn 2p XPS spectrum are located at ∼1021 and ∼1044 eV, corresponding to the Zn 2p3/2 and Zn 2p1/2, respectively. The binding energies are slightly lower than the binding energies of the Zn 2p core level of bulk ZnO (∼1022 eV and ∼1046 eV),⁶⁹ which suggest that there is an interaction between graphene and ZnR. Therefore, the grown ZnR are significantly attached onto the graphene layer via the bonding between graphene and ZnR.

Conclusions

In summary, through an easy solution process, we have created a novel material consisting of 1D ZnR and 2D graphene. This new electron transport layer favors rapid charge transport among nanorods, resulting in higher a performance at the macroscopic scale. When used as an electron transport layer in PTB7:PCBM-based OPV, the resulting OPV demonstrates a high power conversion efficiency of 7.86%. This work also provides a paradigmatic structural design for high performance optoelectronic and electronic devices.

Experimental

Synthesis of ZnR

Commercial zinc oxide (ZnO), powders (analytic grade), and hydrogen peroxide (H₂O₂, 30%) were purchased from Argos Organic Chemicals. All chemicals were used without any further purification. The synthesis of ZnR was based on the modified hydrothermal method,⁷⁰ in which different amounts of H₂O₂ and ZnO powder were utilized. Firstly, the ZnO powder, as a precursor, was added to 20 mL H₂O₂ under continuous magnetic stirring for 1 h. Upon completion, the solution was then transferred into a 50 mL Teflon-lined stainless steel autoclave and hydrothermally treated at 453 K for 24 h in an oven. The final products were cooled to room temperature, separated by filtration, washed with distilled water until the pH value was neutral, and subsequently the products were dried at 333 K overnight.

Thin film characterization

The X-ray diffraction (XRD) patterns were collected on an X'Pert PRO X-ray diffractometer with Cu Kα radiation. The photoluminescence (PL) spectra for solid samples were investigated with an excitation wavelength of 365 nm on an Edinburgh FL/FS900 spectrophotometer. The electrochemical impedance spectroscopy (EIS) was conducted on a Precision PARC workstation. Transmission electron microscopy (TEM) was performed on a Jeol JEM 2100 with an accelerating voltage of 200 kV. Scanning electron microscopy (SEM) was performed using a Hitachi FE-SEM S-4800 operated at 1 kV. Fourier transform infrared (FTIR) spectra were obtained from a JASCO FT-IR 4100 spectrophotometer using KBr discs with a scan range of 500–4000 cm⁻¹. Raman spectra were collected on a Jobin Yvon T64000 Triple Raman Spectrometer. Thermogravimetric analysis (TGA) was performed from 100 to 800 °C with a heating rate of 20 °C min⁻¹ using a TA Instruments Q5000 under nitrogen atmosphere. The film thickness was measured by Dektak AlphaStep Profiler. The current density–voltage (J–V) characteristics were recorded with a Keithley 2410 source unit. The EQE measurements were performed using an EQE system (Model 74000) obtained from Newport Oriel Instruments USA and HAMAMATSU calibrated silicon cell photodiode used as a reference diode. The wavelength was controlled with a monochromator 200–1600 nm.

Acknowledgements

This work was supported by the Human Resources Development program (No. 20134010200490) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Trade, Industry and Energy. M. A. M. T also would like to thank MOHE, National University of Malaysia for providing financial assistance with grant number ICONIC-2013-005. W. J. D. S. and F. K. S. would like to thank CAPES-PNPD Project No. 3076/2010 for their financial assistance.

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