Letter www.acsami.org
p‑Type NiO Hybrid Visible Photodetector John Mallows,† Miquel Planells,† Vishal Thakare,‡ Reshma Bhosale,‡ Satishchandra Ogale,*,‡,§ and Neil Robertson*,† †
School of Chemistry, University of Edinburgh, Joseph Black Building, David Brewster Road, Edinburgh, Scotland EH9 3FJ, U.K. CSIR-National Chemical Laboratory, Pashan Road, Pashan, Pune, Maharashtra 411008, India § Department of Physics and Centre for Energy Science, Indian Institute of Science Education and Research, Dr. Homi Bhabha Road, Pune 411008, India ‡
S Supporting Information *
ABSTRACT: A novel hybrid visible-light photodetector was created using a planar p-type inorganic NiO layer in a junction with an organic electron acceptor layer. The effect of different oxygen pressures on formation of the NiO layer by pulsed laser deposition shows that higher pressure increases the charge carrier density of the film and lowers the dark current in the device. The addition of a monolayer of small molecules containing conjugated π systems and carboxyl groups at the device interface was also investigated and with correct alignment of the energy levels improves the device performance with respect to the quantum efficiency, responsivity, and photogeneration. The thickness of the organic layer was also optimized for the device, giving a responsivity of 1.54 × 10−2 A W−1 in 460 nm light. KEYWORDS: photodetector, nickel oxide, organic, surface modifier, oxygen vacancies, pulsed laser deposition
T
here are many examples of p−n junction inorganic photodetectors and hybrid inorganic−organic photodetectors using n-type transparent, conducting, metal oxide semiconductors such as ZnO.1 In contrast, p-type metal oxides such as NiO have been much less studied, although more recently, interest in devices such as p-type dye-sensitized solar cells has emerged. NiO is also prevalent in other applications such as resistive switching, catalysts, batteries, and electrochromics.2,3 NiO is a wide-band-gap, from 3.6 to 4.0 eV, electron-donating p-type semiconductor with cubic structure that is essentially transparent as a thin layer.4 In undoped NiO, electrical conduction is primarily due to the hopping of holes associated with Ni2+ vacancies, where each vacancy contributes two holes for conduction.5 In order to build on the growing interest in p-type metal oxide devices, we have studied NiO/ organic hybrid devices as photodetectors. We are currently unaware of visible-light photodetectors using a junction formed from a p-type inorganic metal oxide layer and an organic electron acceptor layer. The study of any similar devices has been limited to diodes, and specific reports of this combination as photodetectors are not reported; only a few hybrid photodetectors using other crystalline p-type materials have been produced.1,6−9 Here, in order to expand the material combinations used and build a broader range of device possibilities, we report a hybrid photodetector device using a junction formed between a p-type NiO layer and (E)-2-[3cyano-5,5-dimethyl-4-[2-(pyren-1-yl)vinyl]furan-2(5H)ylidene]malononitrile (Pyr_TCF;Figure 1).10 Pyr_TCF was © XXXX American Chemical Society
Figure 1. (left) Pyr_TCF. Surface modifiers: (center) Anth_COOH; (right) Napth_COOH.
chosen because of its light-absorption properties with a relatively high molar absorption coefficient of 27140 cm−1 M−110 and favorable alignment of the highest occupied molecular orbital (HOMO) energy level with the NiO valence-band level for efficient hole injection. Strong π−π interactions between molecules may also aid electron transfer through the crystalline layer. Pyr_TCF consists of a polycyclic aromatic unit, around which the HOMO is located, and an electron-withdrawing tricyanofuran unit, around which the lowest unoccupied molecular orbital (LUMO) is located, linked by a π bridge, giving a donor−π-acceptor molecule. The suggested charge-transfer mechanism for the device is light absorption in the organic layer, exciting an electron from the HOMO to the LUMO of Pyr_TCF. Charge separation Received: October 1, 2015 Accepted: December 10, 2015
A
DOI: 10.1021/acsami.5b09291 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Letter
ACS Applied Materials & Interfaces occurs at the NiO/Pyr_TCF interface because of the built-in electric field, and an electron is conducted through the organic layer to the aluminum (Al) electrode and the hole transported through the NiO to the fluorine-doped tin oxide (FTO) electrode. The HOMO is regenerated with an electron from the valence band of NiO, which is, in turn, regenerated from the FTO electrode, which is connected via an external circuit to the Al electrode (Figure 2). Increasing the applied bias increases
Figure 3. (left) XRD pattern of a NiO film pulsed-laser-deposited on a FTO-coated glass substrate. (right) Mott−Schottky plot of p-type NiO films grown under varying oxygen pressures.
To investigate the effect of oxygen vacancies in the NiO layer on charge conduction and the device efficiency, a planar NiO layer was grown on the FTO glass substrate by PLD at varying oxygen pressures, with lower oxygen pressures increasing the density of oxygen vacancies in NiO. The charge carrier density of the NiO films grown at different oxygen pressures was investigated by measuring the capacitance of the films over a range of applied potentials using the Mott−Schottky relationship:16
Figure 2. (left) Energy-level diagram for a FTO/p-NiO/Pyr_TCF/Al photodetector device. (right) Energy-level diagram for a FTO/p-NiO/ surface modifier/Pyr_TCF/Al photodetector device.
the electric field, widening the depletion region and allowing for increased photogeneration and collection. A potential difficulty in hybrid devices is charge transfer at the interface and how the materials forming the interface interact; therefore, as in previous work,11 we used surface modifiers (Figure 1) in an attempt to improve the performance of the device. Synthesis and characterization data of the surface modifiers along with further device studies are included in the Supporting Information, while the main points are covered in this paper. Transparent NiO films of 100 nm thickness were grown on FTO substrates by pulsed laser deposition (PLD) at oxygen pressures of 1 × 10−1, 1 × 10−3, and 5 × 10−5 mbar, an energy density of 2 J cm−2, an energy per pulse of 80 mJ, a pulse frequency of 10 Hz, and a substrate temperature of 400 °C. Devices that include a surface modifier were then placed in a 0.5 mM solution of (E)-3-(anthracen-9-yl)-2-cyanoacrylic acid (Anth_COOH) or (E)-2-cyano-3-(naphthalen-1-yl)acrylic acid (Napth_COOH) in ethanol for 24 h, rinsed with ethanol, and allowed to air-dry. The organic Pyr_TCF layer was spin-coated at 2000 rpm for 30 s from 20, 40, 60, 80, 100, or 150 mM solutions of Pyr_TCF in chlorobenzene.10 A total of 0.25 cm2 of Al of 100 nm thickness was deposited on the organic layer by thermal evaporation for charge collection. A UV, blue, green, or red LED light source was shone on the device from 3.5 cm above using a Keithley 2400 sourcemeter passing 0.3 A through the LED. The device was scanned from +2 to −2 V with a step potential of 0.05 V and the current response recorded to generate an I−V curve on a Keithley 4200 SCS semiconductor characterization system. X-ray diffraction (XRD) of a NiO film pulsed-laser-deposited on FTO (Figure 3) shows distinct peaks at 2θ of 37.4°, 43.2°, 62.8°, 75.1°, and 79.3°, which can be attributed to the cubic NiO diffracting planes, as confirmed by the peak positions from the ICSD,12 and also peaks at 26.4°, 37.2°, 51.4°, 61.3°, and 65.4°, which can be attributed to FTO. The intense peak at 37.4° indicates that the NiO crystals are exposed along the 111 plane because the 200 plane indicated by a peak at 43.2° is less prominent.13,14 The 111 crystal plane is most likely to be covered by hydroxyl groups, which potentially aid in the adsorption of surface modifiers through a dehydration reaction with the carboxyl groups.15
1/CSC 2 = (2/eε0εND)(E − E FB − kT /e)
where CSC = capacitance of the space charge region (F), ND = charge carrier density (hole concentration for a p-type semiconductor, cm−3), ε0 = permittivity of free space = 8.854 × 10−12 F m−1, ε = dielectric constant of the material (values vary between 12.717 and 11.918 at 25 °C for NiO) with a value of 11.9 used here, E = applied potential (V), EFB = flat-band potential (V), and e = electronic charge = −1.603 × 10−19 C. A Mott−Schottky plot was produced showing the inverse of the square of the capacitance of the space charge region of the film as a function of the potential (Figure 3). Using the Mott− Schottky relationship, the charge carrier density (ND) was calculated from the slope of the linear part of the graph and the flat-band potential from the x-axis intercept of the extrapolated linear line.16 ND = (2/eε0ε)/slope
The negative linear slope of all plots show that all of the films are p-type.16 As the oxygen pressure decreases, the slope of the Mott−Schottky plot becomes steeper, showing a decrease in the density of the charge carriers (Table 1). Also, as the oxygen Table 1. Mott−Schottky Data for NiO Films oxygen pressure (mbar) −1
1 × 10 1 × 10−3 5 × 10−5
ND (cm−3)
EFB (V)
6.74 × 1016 5.08 × 1015 2.82 × 1015
0.18 0.19 0.83
pressure decreases, the flat-band potential of the material shows a slight increase from 0.2 to 0.8 V versus the redox potential of the electrolyte (Table 1). Higher oxygen pressure increases the ratio of oxygen to nickel, effectively increasing the amount of nickel vacancies and therefore increasing the hole doping. Higher oxygen pressure provides a higher charge carrier density in the film. This effect is due to a change of the stoichiometry in NiO, altering the electron correlation of the oxygen-derived band, which splits the band across the Fermi level because of Coulomb repulsion between electrons, an effect common in transition-metal oxides.19 B
DOI: 10.1021/acsami.5b09291 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Letter
ACS Applied Materials & Interfaces
with the HOMO levels obtained by cyclic voltammetry measurements and the optical band gap (Eg opt) obtained by UV−vis measurements (Figure 5):20
Considering the effect of oxygen pressure on devices, the dark current in the lower-oxygen-pressure devices is significantly higher than that in comparable devices at higher oxygen pressures when measured at 2 V bias (Figure 4), showing an
E LUMO (eV) = Eg opt + E HOMO
The resulting energy-level diagram (Figure 2) suggests a possible charge-transfer mechanism for the Anth_COOH surface modifier device, with the HOMO level slightly below the NiO valence band aiding charge transfer. There is also possibly a secondary light-absorption and charge-transfer mechanism due to the surface modifier. The Napth_COOH device displays an inferior alignment of the energy levels, which may hinder the charge-transfer mechanism. This is reflected in the superior performance of the device using Anth_COOH (Table 2). The thickness of the organic layer was optimized for devices and compared both in self-powered mode and under applied bias. Comparing devices with the same NiO layer and Anth_COOH surface modifier allows optimization of the thickness of the organic layer, with the photoresponse ratio increasing with layer thickness and optimum thickness provided by spin coating from a 100 mM solution of Pyr_TCF. The FTO/NiO/Pyr_TCF/Al device, which has no surface modifier, generates photocurrent when under illumination and operates in photovoltaic mode and under forward and reverse bias conditions up to −200 mV (Figure 4). The external quantum efficiency (EQE), which is the ratio of incident photons converted to charge carriers, and responsivity, which is a measure of the current output per optical input, are two of the defining parameters of photodetectors. The device shows higher photocurrent density, EQE, and responsivity than other comparable p-type hybrid devices from the literature (Table 2). The first positive dark current in the device occurs at 250 mV, showing a light/dark current ratio of 43.3. The addition of a surface modifier to the NiO surface further increases the photoresponse and response rates compared with the device without a surface modifier (Tables 2 and S6 and S7) possibly because of removing trap states from the NiO surface and aiding charge transfer. The addition of the Anth_COOH surface modifier to the device increases all performance parameters, including the response, relaxation times, and stability, indicating that it successfully aids charge transfer and π-stacking in the system. The device shows higher EQE and responsivity than other p-type hybrid devices, despite those incorporating nonplanar interfaces, while also having higher responsivity than some inorganic devices (Table 2). The device is also comparable to n-type hybrid photodetectors, although further work is needed to fully match the performance. At 0 V bias, the optimized device with the Anth_COOH surface modifier shows a light/dark current ratio of 3 under UV light, 20 under blue LED, and 8 under green and red LED illumination. Because of the large band gap of the NiO layer, this confirms that light absorption and charge generation must be occurring in the organic layer or no photocurrent would be present under higher wavelength illumination. A higher response was expected under the green LED because of the λmax value of Pyr_TCF in CHCl3 occurring at 533 nm;10 however, because this was measured in solution, the absorption peak may shift in the film. The Napth_COOH modifier shows a slight increase in the photoresponse, EQE, and responsivity at 0 V; however, at 2 V bias, a decrease is seen in all parameters compared to the device with no surface modifier, potentially
Figure 4. (left) Comparison of the dark current measured at 2 V bias for Pyr_TCF/NiO devices of two different organic layer thicknesses, with the NiO layer at two different oxygen pressures. (right) J−V curve of a NiO (1 × 10−1 mbar film)/Pyr_TCF (100 mM solution) photodetector under a blue-light LED.
effect opposite to that expected from the Mott−Schottky results, potentially due to surface interactions of the organic layer with NiO. A lower dark current is beneficial to the operation of devices; therefore, higher oxygen pressures were used for the production of p-type NiO films by PLD for the reported devices. The addition of a monolayer of a surface modifier with a carboxyl anchoring group and conjugated π system to the NiO surface and its effect on aiding charge transfer between the NiO and Pyr_TCF layers by wetting the NiO surface and directing π-stacking of Pyr_TCF was investigated. A dehydration reaction occurs between the carboxyl group of the surface modifier and the hydroxyl groups at the NiO surface,15 chemisorbing the molecule to the surface with aromatic π systems pointing out, which through π−π interactions with the polycyclic aromatic part of Pyr_TCF is expected to hold the HOMO part of the molecule near the NiO surface. Surface modifiers (Figure 1) with two or three aromatic rings were used individually in devices in this paper and compared to the device with no surface modifier. To further examine the role and charge-transfer mechanism of the surface modifiers, the energy levels were calculated from the electrochemical data. The HOMO energy levels were calculated using the equation E HOMO (eV) = −[Eox onset − E1/2(Fc/Fc+)] − 4.80 eV
where Eoxonset and E1/2(Fc/Fc+) are the onset oxidation potentials of the irreversible first oxidation for the samples and the halfwave potential for ferrocene, respectively (Figure 5), while the value −4.8 eV is taken as the HOMO energy level of ferrocene against a vacuum. The LUMO energy levels were calculated
Figure 5. (left) Cyclic voltammogram of Napth_COOH and Anth_COOH compared with ferrocene in 0.1 M TBAPF6 in MeCN. (right) UV−vis spectra of 0.07 mM solutions of Napth_COOH and Anth_COOH in dichloromethane. C
DOI: 10.1021/acsami.5b09291 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Letter
ACS Applied Materials & Interfaces Table 2. Performance Characteristics of Hybrid Photodetectors Incorporating p-Type Inorganic Material device
applied bias (V)
light source
2 0 2 0 2 0 0 2 2 0 0
460 nm LED (Pin = 0.037 W) 460 nm LED (Pin = 0.037 W) 460 nm LED (Pin = 0.037 W) 460 nm LED (Pin = 0.037 W) 460 nm LED (Pin = 0.037 W) 460 nm LED (Pin = 0.037 W) 200 W halogen (Pin = 5.6 mW) 550 nm light (Pin = 3.15 mW) 520 nm light (Pin = 2.52 mW) UV xenon lamp 470 nm LED (Pin = 0.018 W)
NiO/Pyr_TCF (100 mM) NiO/Napth_COOH/Pyr_TCF NiO/Anth_COOH/Pyr_TCF p-Si/MEH-PPV:C606 GaP (NW)/PCBM7 p-Cd3P2/PCBM8 n-ZnO/p-NiO9 N:ZnO/SpiroMeOTAD1 a
photocurrent density (A cm−2) 4.63 1.38 4.36 4.78 5.69 1.33 2.45 3.74 6.55
× × × × × × × × ×
10−6 10−7 10−6 10−7 10−6 10−6 10−8 10−7 10−8
8 × 10−5
Iphoto/Idark
EQE (%) × × × × × ×
10−2 10−3 10−2 10−3 10−2 10−3
2.01 −3.87 1.08 57.98 11.22 20.37
3.37 1.01 3.18 3.49 4.15 9.70
169.91a 4.69a
2.66 × 10−2a 4.88 × 10−3a
1600
responsivity (A W−1) 1.25 × 10−2 3.72 × 10−4 1.18 × 10−2 1.29 × 10−3 1.54 × 10−2 3.60 × 10−3 1.63 × 10−2 1.17 × 10−2a 2.04 × 10−3a 4.93 × 10−4 6.5 × 10−3
Values calculated from the results reported in the papers.
*E-mail:
[email protected].
because of the misaligned energy levels. This is highlighted because the Napth_COOH device does not perform as well as the device with Anth_COOH, which has better aligned energy levels. The Napth_COOH device shows the highest light/dark current ratio ratio of all devices at 0 V applied bias; however, the light/dark current ratio at 0 V is an unreliable characteristic because the dark current under no applied bias should theoretically be 0 A cm−2; therefore, any dark current measured at 0 V is due to residual charge capacitance in the film. In conclusion, a novel hybrid heterojunction photodetector is reported using the FTO/p-NiO/PYR_TCF/Al configuration, which successfully generates photocurrent when illuminated with visible light, both under applied bias and in self-powered photovoltaic mode. The use of a higher oxygen pressure upon deposition of NiO by PLD is shown to provide a higher charge carrier density in the film but a lower dark current in the device. The addition of a monolayer of a small organic surface modifier to NiO (FTO/p-NiO/Anth_COOH/Pyr_TCF/Al) has also been shown to increase the photocurrent, Iphoto/Idark ratio, quantum efficiency, responsivity, response rates, and stability of devices, presumably by aiding charge transfer and directing πstacking of the organic electron acceptor layer. Tuning of the surface modifier can alter the device performance, highlighting the need for the correct alignment of the energy levels. Further tuning of the surface modifier could potentially increase the device efficiency. Optimization of the NiO layer could also improve the device performance because thinner layers will be more transparent and more conductive, while the use of a mesoporous NiO layer would greatly increase the area of the interface, which would increase charge separation of the photogenerated excitons and collection compared to the planar layer used. Overall, the promising initial work reported here opens up several avenues for further optimization within a new approach to hybrid photodetectors.
■
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS The authors thank the U.K.−India Education and Research Initiative (UKIERI) and EPSRC Apex project for financial support.
■
(1) Game, O.; Singh, U.; Kumari, T.; Banpurkar, A.; Ogale, S. ZnO (N)−Spiro-MeOTAD Hybrid Photodiode: an Efficient Self-Powered Fast-Response UV (Visible) Photosensor. Nanoscale 2014, 6, 503− 513. (2) Ye, J.; Li, Y.; Gao, J.; Peng, H.; Wu, S.; Wu, T. Nanoscale Resistive Switching and Filamentary Conduction in NiO Thin Films. Appl. Phys. Lett. 2010, 97, 132108. (3) Wei, Z. P.; Arredondo, M.; Peng, H. Y.; Zhang, Z.; Guo, D. L.; Xing, G. Z.; Li, Y. F.; Wong, L. M.; Wang, S. J.; Valanoor, N.; Wu, T. A Template and Catalyst-Free Metal-Etching-Oxidation Method to Synthesize Aligned Oxide Nanowire Arrays: NiO as an Example. ACS Nano 2010, 4, 4785−4791. (4) Irwin, M. D.; Buchholz, D. B.; Hains, A. W.; Chang, R. P.; Marks, T. J. p-Type Semiconducting Nickel Oxide as an Efficiency-Enhancing Anode Interfacial Layer in Polymer Bulk-Heterojunction Solar Cells. Proc. Natl. Acad. Sci. U. S. A. 2008, 105, 2783−2787. (5) Biju, V.; Khadar, M. A. Dielectric Properties of Nanostructured Nickel Oxide. J. Mater. Sci. 2003, 38, 4055−4063. (6) Yakuphanoglu, F. Photovoltaic Properties of the Organic− Inorganic Photodiode based on Polymer and Fullerene Blend for Optical Sensors. Sens. Actuators, A 2008, 141, 383−389. (7) Chen, G.; Xie, X.; Shen, G. Flexible Organic-Inorganic Hybrid Photodetectors with n-type Phenyl-C61-Butyric Acid Methyl Ester (PCBM) and p-type Pearl-like GaP Nanowires. Nano Res. 2014, 7, 1777−1787. (8) Chen, G.; Liang, B.; Liu, X.; Liu, Z.; Yu, G.; Xie, X.; Luo, T.; Chen, D.; Zhu, M.; Shen, G.; Fan, Z. High-Performance Hybrid Phenyl-C61-Butyric Acid Methyl Ester/Cd3P2 Nanowire Ultraviolet− Visible−Near Infrared Photodetectors. ACS Nano 2014, 8, 787−796. (9) Ni, P.-N.; Shan, C.-X.; Wang, S.-P.; Liu, X.-Y.; Shen, D.-Z. SelfPowered Spectrum-Selective Photodetectors Fabricated from n-ZnO/ p-NiO Core−Shell Nanowire Arrays. J. Mater. Chem. C 2013, 1, 4445−4449. (10) Planells, M.; Pizzotti, M.; Nichol, G. S.; Tessore, F.; Robertson, N. Effect of Torsional Twist on 2nd Order Non-Linear Optical Activity of Anthracene and Pyrene Tricyanofuran Derivatives. Phys. Chem. Chem. Phys. 2014, 16, 23404−23411. (11) Planells, M.; Abate, A.; Snaith, H. J.; Robertson, N. Oligothiophene Interlayer Effect on Photocurrent Generation for
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b09291. Surface modifier synthesis and characterization data, device fabrication and further studies, layer thickness optimization, and experimental information (PDF)
■
REFERENCES
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. D
DOI: 10.1021/acsami.5b09291 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Letter
ACS Applied Materials & Interfaces Hybrid TiO2/P3HT Solar Cells. ACS Appl. Mater. Interfaces 2014, 6, 17226−17235. (12) Sasaki, S.; Fujino, K.; Takeuchi, Y. X-ray Determination of Electron-Density Distributions in Oxides, MgO, MnO, CoO, and NiO, and Atomic Scattering Factors of their Constituent Atoms. Proc. Jpn. Acad., Ser. B 1979, 55, 43−48. (13) Venter, A.; Botha, J. R. Optical and Electrical Properties of NiO for Possible Dielectric Applications. S. Afr. J. Sci. 2011, 107, 1−6. (14) Mahmoud, S. A.; Shereen, A.; Tarawnh, M. A. Structural and Optical Dispersion Characterisation of Sprayed Nickel Oxide Thin Films. J. Mod. Phys. 2011, 2, 1178. (15) Cappus, D.; Xu, C.; Ehrlich, D.; Dillmann, B.; Ventrice, C.; Al Shamery, K.; Kuhlenbeck, H.; Freund, H.-J. Hydroxyl Groups on Oxide Surfaces: NiO (100), NiO (111) and Cr 2 O 3 (111). Chem. Phys. 1993, 177, 533−546. (16) Bott, A. W. Electrochemistry of Semiconductors. Curr. Sep. 1998, 17, 87−92. (17) Oliver, P.; Watson, G.; Parker, S. Molecular-Dynamics Simulations of Nickel Oxide Surfaces. Phys. Rev. B: Condens. Matter Mater. Phys. 1995, 52, 5323. (18) Rao, K.; Smakula, A. Dielectric Properties of Cobalt Oxide, Nickel Oxide, and their Mixed Crystals. J. Appl. Phys. 1965, 36, 2031− 2038. (19) Peng, H. Y.; Li, Y. F.; Lin, W. N.; Wang, Y. Z.; Gao, X. Y.; Wu, T. Deterministic Conversion Between Memory and Threshold Resistive Switching via Tuning the Strong Electron Correlation. Sci. Rep. 2012, 2, 442. (20) Hong, W.; Sun, B.; Aziz, H.; Park, W.-T.; Noh, Y.-Y.; Li, Y. A Conjugated Polyazine Containing Diketopyrrolopyrrole for Ambipolar Organic Thin Film Transistors. Chem. Commun. 2012, 48, 8413−8415.
E
DOI: 10.1021/acsami.5b09291 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
