Narrowband Organic Photodiodes Based on Green Light Sensitive

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Narrowband Organic Photodiodes Based on Green Light Sensitive Squarylium Wenhai Li, Hang Guo, Ziang Wang, and Guifang Dong J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b03412 • Publication Date (Web): 26 Jun 2017 Downloaded from http://pubs.acs.org on June 27, 2017

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The Journal of Physical Chemistry

Narrowband Organic Photodiodes Based on Green Light Sensitive Squarylium

Wenhai Li 1, Hang Guo 1, Ziang Wang, and Guifang Dong* ADDRESS: Key Laboratory of Organic Optoelectronics and Molecular Engineering of Ministry of Education, Department of Chemistry, Tsinghua University, Beijing, 100084, P. R. China

ABSTRACT: organic photodiodes (OPD) are becoming promising candidates for narrowband detection to meet new requirements on detectors, such as smaller size, higher resolution, lower power consumption, and the ability to interact with a variety of interfaces. In this paper, a new narrowband green-light sensitive squarylium material with donor-acceptor-donor structure and its application in photodiode are reported. Due to the material with intense and sharp absorption in the green color and simple single-layer structure, the device shows excellent color-selective. The green EQE of the device reaches 66% while the blue EQE is low to 10%. The device also shows significantly low dark current of ~5.4 nA·cm-2 at −2.5 V and high specific detectivity of 7.7 × 1012 Jones. The strategy for designing narrowband OPD can be extended to other wavelength region by replacing substituent group in the donor-acceptor-donor structure of the squarylium and using the single-layer structure.

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1. Introduction The image sensor market is considered to be in the process of rapid development 1

due to the expanded applications in digital imaging, education, environmental

monitoring, optical communications, and machine vision. As an important component in image sensor, photodetectors for spectrally selective light detection need a total upgrade to serve new applications. New requirements are being placed on narrowband detectors, such as smaller size, higher resolution, lower power consumption, and the ability to interact with a variety of interfaces.2 The traditional narrowband detectors that use inorganic broadband photodetector with additional color filters suffer from poor mechanical flexibility, complex structures, and light waste.3-5 And thus, organic photodiodes (OPD) are becoming promising candidates for narrowband detection,6 due to their simple structure and the potential to realize high color-separation, large-area and flexible optoelectronic devices.2, 7 However, at present, narrowband OPD meet two major challenges: 1. the lack of materials with narrowband and high absorption; 2. and the nonnegligible absorption in unwanted regions resulted from using two or more materials with different absorption in common donor-acceptor structure of organic photodiode.8-9 Though a method for fabricating narrowband organic photodiodes by tuning the internal quantum efficiency was reported in recent, 10

the devices based on this method show low external quantum efficiency (EQE) and

a significant mismatch between absorption and EQE, which limits its application in stacked-type sensors. New materials and structures are in urgent need for organic


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photodiodes. In this paper, we reported a new narrowband green-light organic photodiode using squarylium material with donor-acceptor-donor (D-A-D) structure. The device is based on a simple single-layer structure,11 which don’t need to introduce another material. And through the optical field optimization in the device, our device shows high EQE (66%) in green region at -2.5 V while the EQE at 450 nm is low to 10%. The full width at half maximum (FWHM) is about 110 nm. The device also shows significantly low dark current of ~5.4 nA·cm-2 at −2.5 V and high specific detectivity of 7.7 × 1012 Jones. The results revealed that the device has a promising potential for future narrowband applications.

2. Experiment and Results First, we need to develop the green-light sensitive materials. A squarylium molecule with red-light absorption, 1,3-bis[(3,3-dimethylindolin-2-ylidene) methyl] squaraine (ISQ, figure 1a), has been developed in our lab.12 The molecular is typical D-A-D structure. We plan to tune the absorption wavelength of ISQ to get green-light sensitive narrowband materials. Here, we maintain the backbone, and replace the substituent group 2, 3, 3-trimethyl -3H-indole with 2, 4- dimethyl-3H- pyrrole. And squarylium molecular 1,3-bis[(3,5-dimethylazole-2-ylidene) methyl] squaraine (PSQ, figure 1a) is synthesized. The structure of product was confirmed by 1H NMR spectra (figure S1). The absorption of the two molecular in CH3OH solvent and in thin film is shown


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in figure 1b. Compared with ISQ, the absorption of PSQ shows an obvious blue-shift, and the peak of absorption curve is located in green region (550 nm, in CH3OH). And for both molecular, the absorption bands become broader and red-shift in thin film due to strong charge transfer interactions and tendency to aggregate in squarylium.13 The energy gap of PSQ is higher than ISQ because of the weaker donor strength of PSQ as compared with that of ISQ.14-15 Figure 1c represents the optimized molecular configuration of the S1 excited state and ground state of PSQ at the B3LYP level of the DFT method with the 6-31g(d) basis set, calculated using Gaussian09.16 The total reorganization energy and the dipole moment difference (∆µ)between the ground state and the excited state are also calculated. The very low total reorganization energy (0.057 eV) is consistent with the small ∆µ value (0.0009 debye), indicating that the configuration of the S1 excited state is almost the same with that of ground state. We also calculated the resonance parameter c2 value, which can be used to characterize the charge transfer properties of molecules, through expression (1)17-18: 1 c = (1 − ∆ × (4 + ∆ (1 2

µtr is transition dipole moment, which is calculated to be 9.671 debye through quantum calculation. And the calculated c2 is 0.500, indicating that the PSQ is close to the cyanine limit and have therefore cyanine-like character. Therefore, the PSQ has a very sharp and strong absorption at 555 nm. The crystal packing of PSQs was shown in Figure 1d. Clear face to face packing is found in the cell. The closest interplanar distance of PSQ is only 3.448 Å, lower


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than the interplanar distance of ISQ (tested as 3.6 Å).

12

The shorter π−π stacking

distance may indicate stronger intermolecular interactions and thus the absorption spectra of PSQ (FWHM= 135 nm) was broader than that of ISQ (FWHM= 115 nm). In addition, it may benefit the π–π carrier transport between neighboring molecules and we can hope a good charge mobility performance for PSQ used in single crystal field effect transistor in the future.12, 19 The lowest unoccupied molecular orbital (LUMO) level and highest occupied molecular orbital (HOMO) level of PSQ are experimentally measured to be 3.27 and 5.33 eV respectively by cyclic voltammetry. We choose ITO/PEDOT: PSS electrode and Al electrode as anode and cathode respectively to fabricate a single-layer device with structure of ITO (140 nm)/ PEDOT: PSS (40 nm)/ PSQ/ Al (110 nm).11 The fermi level is 5.2 eV for ITO/ PEDOT: PSS and 4.3 eV for Al.(Figure 2a) For the large energy gap between HOMO of PSQ and fermi level of Al, high hole injection barrier was formed in Al/PSQ interface. Similarly, ITO/ PEDOT: PSS /PSQ interface has high electron injection barrier. Thus, a low darkcurrent for device can be expected. Then, we need to determine the optimum thickness of PSQ in the device. We obtained the refractive index n and extinction coefficient k of PSQ through ellipsometer (figure S2). With the transfer matrix simulation,

20

we calculate the

actual light absorption spectra of the active layer in the single-layer device with different thickness of PSQ. The result is shown in figure 2b. As we can see, when the


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thickness of PSQ film increase from 40 nm to 55 nm, the absorption in the whole spectra increases. However, when the thickness further increase to 70 nm, the increase of peak absorption of device is limited due to absorption saturation,21 while the absorption in blue region increases rapidly. And this will cause the absorption broader. Thus, to balance high EQE and narrow band response, a device with 55 nm PSQ is then fabricated. Figure 3a shows the dark current density (Jd) of the PSQ device under a bias ranging from −4 to 2 V. As we have predicted, the device shows quite low reverse bias dark current, approximately 0.9 × 10−9 A·cm−2 at −1 V and 5.4 × 10−9 A·cm−2 at −2.5 V, due to the high charge injection barrier in the two electrode/semiconductor interface. The EQE is then tested at -2.5 V. As shown in Figure 3b, the device has quite high EQE (66%) at 600 nm when the EQE at 450 nm is low to 10%. The relation of peak EQE and external bias is also given in figure 3b. when the external bias is set to 0, the EQE of device is almost close to 0. And it has a rapid growth with the increase of bias, indicating that charge separation in our single-layer device is external bias assisted.

In general, organic photodiode devices typically rely on

heterojunctions for high efficient exciton separation. To find out the reason for high external quantum efficiency in single-layer squarylium devices, the hole and electron mobility (Figure 3c) of PSQ material were tested by Time-of-Flight (TOF) measurements. According to the Figure 3c, the PSQ materials exhibit relatively high hole mobility of 2.69×10-3 cm2 V-1s-1 and electron mobility of 1.44×10-3 cm2 V-1s-1 at


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the field intensity of 8×107 V/m, respectively. These results confirm that PSQ is a good bipolarity material and could efficiently transport free carriers, which are separated from electron-hole pair created via photoexcitation under external electric field without assistance from the bulk-heterojunction. According to the Figure 3b, The FWHM is about 110 nm, indicating that the device has good color-selective property. Here, the FWHM of OPD’s EQE curves was lower than UV-Vis absorption spectra of PSQ film, and it’s probably due to the optical cavity effect. The different structures between the PSQ OPD and PSQ thin film (with or without the Al electrode) strongly influence the optical field distribution and then light absorption.11, 22 It can be further confirmed by calculated light absorption spectra through transfer matrix simulation of the active layer in OPD device and in the single film of PSQ (figure S3). The detectivity (D*) of a photodetector is also estimated through the following expression (2): 7 D∗ =

( × × /ℎ

(2

(2 where q is the electron charge, λ is the light wavelength, h and c are Planck constant, and the speed of light in vacuum, respectively. The inferred detectivity results are shown in Figure 4a. The maximum of 7.7 × 1012 Jones appeared at 600 nm under a bias of −2.5 V. However, Equation 2 assumes that the shot noise from dark current is the dominant factor of noise current of OPD, and probably overestimates the real D*.23-24 Thus the detectivity calculated by equation 2 is the upper limit of D*,


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and we will try to improve our test condition and provide a more accurate result in further study. The linear response is also critical performance for OPD. Thus, we test the relation of photocurrent density with light intensity. As Figure 4b show, the device shows linear response to light intensity in more than 6 orders, from approximately 0.5 µW·cm−2 to a value of 29.8 mW·cm−2. The linear dynamic range (LDR), parameter which represents the performance of OPD in linear response, is calculated to be 48 dB according to the tested data. However, the tested LDR is limited by our instrument, which has a limited light intensity range. So in order to further evaluate the linear response of our device, we calculated the upper limit of LDR according to expression (3): 25 LDR = 10log

$%& (3 '$

where noise equivalent power (NEP) is the minimum light power which can be detected, and the value of NEP is reciprocal of detectivity. Psat is the saturation light intensity at which the photocurrent began to deviate from the linearity. The upper limit of LDR of the PSQ device is then calculated to be 106 dB, and the result shows that our device have a potential to be used in some applications needing measure light density accurately.26

3. Experimental method Materials

preparation:

The

synthetic

raw

material

for

PSQ

3,

4-dihydroxy-3-cyclobutene-1, 2-dione and 2, 4- dimethyl-3H- pyrrole was purchased


8

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from Sigma–Aldrich.3, 4-dihydroxy-3-cyclobutene-1, 2-dione and 2, 4- dimethyl-3Hpyrrole were added in molar ratio 1:2 and heated in mixed solvent (toluene: n-butanol = 1:1) for 5 h at 110 °C and then heated azeotropically using a Dean–Stark trap for 2 h. Then we removed extra solvents by vacuum distillation. Column chromatography and recrystallization were utilized in turn to purify the precipitate and golden flaky crystals PSQ were obtained at last. 1H NMR (600 MHz, CDCl3) δ: 2.36 (s, 6H, C(CH3)), 2.59-2.62 (d, 6H, C(CH3)), 6.10 (s, 2H, = CH-C), 9.90-10.00 (d, 2H, N-H); Device Fabrication: The organic photodiode was fabricated on top of completely cleaned commercial indium tin oxide (ITO) substrate. Firstly, a layer of PEDOT: PSS (Clevios PVPCH 8000, purchased from Heraeus) was spin-coated on ITO substrate at 3000 rpm for 60 s. Then the PEDOT: PSS film were annealed at 200 °C for 15 min. After cooling, the organic layers PSQ was deposited through a shadow mask at a rate of 1 Å/s by thermal evaporation. Finally, Al cathode was deposited at 5 Å/s with the thickness of 110 nm. The whole process of thermal evaporation was under a vacuum degree of 5×10-4 Pa, And the deposition rates and thickness of the layers were monitored by oscillating quartz monitors. Characterization and testing: The thicknesses of films were tested through Atomic Force Microscope method. The I-V curves of OPD were measured by Keithley 4200 Semiconducting System. The EQE measurements were performed with a 300 W Xe lamp equipped with a manual monochromator and the electrical characteristics were collected by a Keithley 2400 Source Meter. And the absorption of


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materials was measured by UV-vis spectrophotometer (Agilent8453). The refractive index (n) and extinction coefficient (k) values of different layers were obtained by variable angle spectroscopic ellipsometry (SENTECH850).

4. Conclusion In summary, we have introduced a narrowband response organic photodiode based on squarylium and single-layer structure. The dark current low to 5 nA·cm-2 at −2.5 V because of the high injection barriers. Under the bias of -2.5 V, the EQE of the OPD reach to 66% at 600 nm, when the EQE at 450 nm is low to 10%. The device shows a narrow response spectrum with a FWHM of 110 nm, indicating its high color-selective performance and potential to satisfy the needs of applications in future digital imaging, optical communications, and machine vision. The strategy for obtaining green-light organic material by change substituent group in the D-A-D structure can be extended to material with absorption in other wavelength region, such as near-infrared.12-13 And with the combination of the single-layer structure, it can be an appealing alternative solution solve the material limits in a narrowband photodetector.

ASSOCIATED CONTENT

Supporting Information. 1H NMR spectra of PSQ in CDCl3 solvent; optical


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constants of each layer of our device; The simulated actual absorption of 55-nm PSQ in the device and single film on glass. AUTHOR INFORMATION

Corresponding Author *Guifang Dong: E-mail: [email protected]

Author Contributions ‡

W. H. Li and H. Guo contributed equally to this work.

ACKNOWLEDGMENT The authors would like to thank the National Key R&D Program of “Strategic Advanced Electronic Materials” (No 2016YFB0401100), the Natural Science Foundation of China (Grant No. 61474069) and Tsinghua University Initiative Scientific Research Program. REFERENCES 1. Lineback, R. A Market Analysis and Forecast for the Optoelectronics, Sensors/Actuators, and Discretes. IC Insights: Arizona, USA, 2014; pp 4-28. 2. Jansen-van Vuuren, R. D.; Armin, A.; Pandey, A. K.; Burn, P. L.; Meredith, P. Organic Photodiodes: The Future of Full Color Detection and Image Sensing. Adv. Mater. 2016, 28 (24), 4766-4802. 3. Ihama, M.; Hayashi, M.; Maehara, Y.; Mitsui, T.; Takada, S. CMOS Color Image Sensor with Overlaid Organic Photoelectric Conversion Layers Having Narrow Absorption Band: Depression of Dark Current. [C]//Proc. SPIE. 2007; 6656: 66560A. 4. Nau, S.; Wolf, C.; Sax, S.; List-Kratochvil, E. J., Organic Non‐volatile


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Figures

Figure 1. (a) molecule structure of ISQ and PSQ; (b) absorption of ISQ and PSQ in CH3OH solution and in thin film; (c) the calculated S1 excited state and S0 ground of PSQ; (d) crystal packing of PSQ.


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Figure 2. (a) energy diagram of the single-layer organic photodiode based on PSQ; (b) the calculated absorption in organic photodiode based on structure of ITO (140 nm)/ PEDOT: PSS (40 nm)/ PSQ/ Al (110 nm) with different thickness of PSQ layer.


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Figure 3. (a) darkcurrent curves and photocurrent curves (@600 nm, 7.4 µW/cm2, the incident light is provided by a Xe lamp equipped with a manual monochromator) of PSQ organic photodiode; (b) the EQE curves of PSQ organic photodiode (left) and the relation between external bias and peak EQE (right); (c) charge mobilities of PSQ materials under different electric fields.


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Figure 4. (a) the calculated detectivity of green-light photodiode at -2.5 V; (b) the measured linear dynamic range for the green-light photodiode. The incident light source used for the measurement was a 520 nm OLED.


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TOC Graphic.


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Figure 1. (a) molecule structure of ISQ and PSQ; (b) absorption of ISQ and PSQ in CH3OH solution and in thin film; (c) the calculated S1 excited state and S0 ground of PSQ; (d) crystal packing of PSQ. 160x139mm (300 x 300 DPI)


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Figure 2. (a) energy diagram of the single-layer organic photodiode based on PSQ; (b) the calculated absorption in organic photodiode based on structure of ITO (140 nm)/ PEDOT: PSS (40 nm)/ PSQ/ Al (110 nm) with different thickness of PSQ layer. 75x119mm (300 x 300 DPI)



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Figure 3. (a) darkcurrent curves and photocurrent curves (@600 nm, 7.4 µW/cm2, the incident light is provided by a Xe lamp equipped with a manual monochromator) of PSQ organic photodiode; (b) the EQE curves of PSQ organic photodiode (left) and the relation between external bias and peak EQE (right); (c) charge mobilities of PSQ materials under different electric fields. 127x225mm (300 x 300 DPI)


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Figure 4 (a) the calculated detectivity of green-light photodiode at -2.5 V; (b) the measured linear dynamic range for the green-light photodiode. The incident light source used for the measurement was a 520 nm OLED. 75x114mm (300 x 300 DPI)


Narrowband Organic Photodiodes Based on Green Light Sensitive

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