Fabrication of High Performance, Narrowband Blue-Selective Polymer

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Fabrication of High Performance, Narrowband Blue-selective Polymer Photodiodes with Dialkoxynaphthalene-based Conjugated Polymer Seongwon Yoon, Yeon Hee Ha, Soon-Ki Kwon, Yun-Hi Kim, and Dae Sung Chung ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b01248 • Publication Date (Web): 29 Nov 2017 Downloaded from http://pubs.acs.org on December 1, 2017

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Fabrication of High Performance, Narrowband Blue-selective

Polymer

Photodiodes

with

Dialkoxynaphthalene-based Conjugated Polymer

Seongwon Yoon1, Yeon-Hee Ha2, Soon-Ki Kwon,3* Yun-Hi Kim2* and Dae Sung Chung1* 1

Department of Energy Science & Engineering, Daegu Gyeongbuk Institute of Science & Technology (DGIST), Daegu 42988, Republic of Korea

2

Department of Chemistry and RIGET, Gyeongsang National University, Jinju 52828, Republic of Korea

3

Department of Materials Engineering and Convergence Technology and ETI, Gyeongsang National University, Jinju 52828, Republic of Korea

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Abstract Here, we synthesized a dihexyloxynaphthalene-based conjugated polymer (PNa6-Th) to realize narrowband blue-selective polymer photodiode. The optical, electrochemical and thermal properties of the synthesized polymer were investigated. It was found that PNa6-Th exhibited a blue-selective absorption with a narrow full width at half maximum of ~100 nm and a wide optical band gap of ~2.52 eV. We constructed a planar heterojunction structure with PNa6-Th and ZnO as a blue-selective electron donor and non-absorbing acceptor, respectively; To enhance the photodiode performance, a minor amount of [6,6]-phenyl-C61butyric acid methyl ester (PCBM) was introduced on the donor layer. By introducing the PCBM on PNa6-Th layer, external quantum efficiency was increased from 5.4 % for pristine device to 37.8 % for 15 wt% PCBM-doped device, while the dark current values maintained nearly constant. This external quantum efficiency boost leads to high detectivity of 2.31×1012 Jones at -1 V. The physics behind the improved performance were fully discussed based on percolation pathway theory and space-charge-limited current analyses.

KEYWORDS external quantum efficiency, color-selectivity, polymer photodiode, low dark current, highdetectivity, non-absorbing acceptor


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Introduction Image sensors are widely used in imaging devices such as cameras and camcorders and, recently, they have also been used as high-performance sensors for smart cars. Accordingly, the importance of photodiodes, which are used as unit pixels in image sensors, has also been highlighted. So far, the photoactive layer of the photodiode in commercial image sensors consists of a Si semiconductor. However, Si displays panchromatic absorption behavior over the entire visible range and therefore, using color filters is inevitable to obtain distinct colors for full color detection. However, it has been reported that using color filters for color selection impinges on device performance not only because of the expensive process costs for color filters, but also by limiting the quantum efficiency and spatial resolution of the pixel.1 Therefore, it is important to develop new materials and architecture for the photodiode to realize full color detection without using color filters. Organic semiconductors are desirable candidates for color-filter-free photodiodes because of not only their better processability2-10 than the inorganic counterparts, but also their absorption range tunability and high absorption coefficient of over 104 cm-1.11-13 In particular, the absorption region of organic semiconductors can be controlled by adjusting the molecular structure of the materials and, therefore, color-selective photoactive layers can be realized by using color-selective organic semiconductors. In addition, when applying a colorselective photoactive layer in a stacked image sensor, as partially demonstrated by Sakai and coworkers,14 the blue-selective layer is located at the uppermost position; therefore, narrowband absorption is more important for blue light than for other colors to prevent damage to other color signals. Many studies have reported related to organic semiconductorbased color-selective photodiodes. However, there were several drawbacks to the previously


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reported photodiodes, including the use of thermal evaporation,15 low external quantum efficiency (EQE) (1 µm).17 To realize blue-selective photodiodes, we synthesized a dialkoxynaphthalene-based donor-donor type conjugated polymer (PNa6-Th) with well-defined absorption18 and investigated its optical, thermal and electrochemical properties. Donor-donor type conjugated polymer indicates the conjugated polymer consisted of two different donor monomers, different to the cases of many other donor-acceptor conjugated polymers. PNa6-Th was strategically designed to have well-defined blue absorption as well as good charge transport; dialkoxynaphthalene donor was employed for its strong π-stacking, linear planar π-system and good chemical and thermal stability resulted from deep highest occupied molecular orbital (HOMO) level and thiophene donor was used to improve the backbone planarity of the polymer.19-22 The synthesized PNa6-Th showed a narrowband absorption with a full width at half maximum (FWHM) of ~100 nm for blue light (central wavelength ~420 nm). We constructed a planar heterojunction structure using ZnO, which is a non-absorbing acceptor, together with PNa6-Th to achieve a blue-selective polymer photodiode (PPD). It is known that a planar heterojunction structure is an optimized structure for suppressing dark current because it strictly limits the undesired dark current injection at both electrodes.16 However, the limited donor/acceptor interfacial area compared to the bulk heterojunction can be a weakness of the planar heterojunction structure for efficient exciton dissociation. To overcome this issue, we added [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) in the PNa6-Th polymer film. PCBM has high absorption coefficients at blue region (400 – 500 nm) and low absorption coefficients at the rest of the visible region (500 – 700 nm) and therefore, we anticipated that adding a low weight percent of PCBM would only affect charge generation without damaging the absorption spectrum. By adding an appropriate amount of PCBM, the PCBM-added PPD showed a higher EQE compared to pristine devices and it is ACS Paragon Plus Environment

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speculated that the boost in EQE is originated from a synergistic effect of the expansion in donor-acceptor interfacial area (PNa6-Th/PCBM) and the increase of photocurrent due to the formation of percolation pathway. As a result, we could observe a high specific detectivity of 2.31×1012 Jones at -1 V and wide linear dynamic range (LDR) of 142 dB. Interestingly, notable thermal stability was also observed presumably due to the morphologically stable nature of planar heterojunction architecture of the optimized devices.


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Methods Materials: Synthesis method of PNa6-Th is described in Supporting Information. Soxhlet extraction was performed for the purification of the synthesized PNa6-Th. Zinc acetate dihydrate, 2-methoxyethanol and ethanolamine were purchased from Sigma-Aldrich and used without further purifications. Device Fabrication: The work function values of ITO and MoO3/Ag electrodes were measured

and optimized with 4D beamline at the Pohang Accelerator Laboratory (PAL)

in Korea. ITO-patterned glasses were cleaned by sonication with aqueous detergent solution, deionized water, acetone and 2-propanol sequentially and the cleaned substrates were blown with N2. Prior to deposition of ZnO layer, the substrates were exposed to O2-plasma and the ZnO precursor solution was spin-coated on the substrates. ZnO solution was prepared by mixing 1 g of zinc acetate dihydrate with 10 mL of 2-methoxyethanol. 280 mg of ethanolamine was added as stabilizer. After deposition of ZnO, the substrates were annealed at 200 oC for 30 min. 20 mg of PNa6-Th solution in 1 mL of chloroform in addition. In case of PCBM-added solution of 15 wt%, 20 mg of PNa6-Th and 3 mg of PCBM were mixed in 1 mL of chloroform. Each solution was stirred at least 4 hours for homogeneous mixture. PNa6-Th or PNa6-Th/PCBM solutions were spin-coated onto the ITO/ZnO substrate and annealed at 80 oC for 10 min. After the active layer deposition, 30 nm of MoO3 and 100 nm of silver electrode were sequentially deposited with thermal evaporation through the shadow masks. For thermal stability experiment, silver paste was applied to the fabricated samples for improved contact. After silver paste application, the samples were stored in vacuum condition to dry residual solvents in silver paste for at least 1 hr. Device Characterization: Current density – voltage characteristics, wavelength – detectivity curve and EQE curve were measured with a combination of Keithley 2400 sourcemeter and LabView-controlled Oriel Cornerstone 130 1/8 m monochromator. Two different light ACS Paragon Plus Environment

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sources were used to measure the linear dynamic range: a monochromatic light from 150 W Xe arc lamp from the monochromator was used for the intensity below 1 µW/cm2 and a laser diode (473 nm) modulated by AFG310 arbitrary function generator (Tektronix) was used for the intensity over 1 µW/cm2. All the light intensity values were calibrated with Siphotodetector. -3 dB frequency was measured with TDS5052 digital phosphor oscilloscope (Tektronix) and the laser diode. Noise current was directly measured from SR 830 Lock-in Amplifier and measured noise current values were normalized with the modulation bandwidth.


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Results and Discussion

Figure 1. The synthetic route of PNa6-Th

The synthetic routes for PNa6-Th are depicted in Figure 1. The polymer was obtained by Suzuki coupling reaction using 2,5-dibromothiophene and dialkoxynaphthalene borate. The structure of the obtained polymer was characterized by 1H-NMR spectroscopy (Figure S1). The polymer had good solubility in common organic solvent. From the gel permeation chromatography analysis, the weight average molecular weight of the polymer was ~25 kDa with a polydispersity index of 3.07 (Figure S2). The 5 % weight loss of polymer was measured at 300.5 °C by thermogravimetry analysis (Figure S3 (a)) and the polymer did not have any thermal transition by heating up to 250 °C. (Figure S3 (b)) We investigated the optical properties of the synthesized polymer by measuring its UV-visible absorption spectra (Figure 2(a)), which displayed absorption peaks at 416 nm (solution) and 426 nm (film) and absorption shoulders at ~430 nm (solution) and ~445 nm (film). The peaks near 420 nm can be attributed to a π-π* transition originated from the naphthalene moiety23 and the absorption shoulders are attributed to intermolecular interaction by π-stacking.24 In addition, we observed a slightly bathochromic shift of ~10 nm in the film phase, which results from J-aggregation-like molecular packing.15,25


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Figure 2. (a) The UV-vis. absorption spectra in solution phase (open circle) and film phase (open star) and (b) the absorption coefficient spectra of PNa6-Th and ZnO films. In the inset of Figure 2(a), the positions of absorption shoulder are expressed as arrows of corresponding colors. The synthesized PNa6-Th exhibited a high absorption coefficient of 9.4 × 104 cm-1 (Figure 2(b)) and it absorbs sufficient light (> 90 %) with a thin photoactive layer (~250 nm). The FWHM values extracted from absorption spectra of the synthesized polymer in solution and the film phase were 96 nm and 100 nm, respectively, which are sufficient narrowband absorptions in the blue wavelength region. We could obtain an optical bandgap, highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) by measuring the absorption onset of the film spectrum and by conducting the cyclic voltammetry measurement (Figure S4), respectively. The calculated optical bandgap was ~2.52 eV and HOMO and LUMO levels were -5.4 eV and -2.88 eV, respectively. To measure the device performance, we fabricated a polymer photodiode (PPD) using PNa6Th and ZnO as the photoactive materials. An inverted device geometry was adopted and MoO3 was used as a hole-transport layer, as seen in Figure 3(a). 15 wt% of PCBM compared to the mass of PNa6-Th was added on the PNa6-Th solution. The measured EQE spectra can be found in Figure 3(b). As seen in Figure 3(b), when the additional PCBM was not added,


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very low EQE of 5.4 % was observed. This phenomenon is known to be due to the lack of pn interfacial area, which was a disadvantage in our previous work.12

Figure 3. (a) The energy levels of the used materials, (b) the external quantum efficiency (EQE) spectra and (c) dark current density – voltage (J-V) characteristics of various wt% ratios of PCBM

By adding 15 wt% of PCBM, which is the typical acceptor weight percentage for percolation pathway formation,26-28 the EQE and photocurrent density were sharply increased. Since the EQE is directly proportional to the generated photocurrent and the percolation pathway leads to the increase of photocurrent, it is speculated that percolation pathway made of PCBM was formed between PNa6-Th and ZnO when the PCBM ratio reaches 15 wt%. However, PCBM percolation pathway may also occur electron injection from cathode (MoO3/Ag) by provide more favorable LUMO level to electron at cathode for electron injection, we measured dark current density – voltage (J-V) characteristics to investigate this point. As can be seen in Figure 3(c), the variation in dark current density is nearly constant until the ratio reaches 15 wt%. Unlike organic photovoltaics, suppression of dark current is as essential as increase of photocurrent for organic photodiodes to enhance the signal-to-noise ratio. Therefore, we can address that the addition of PCBM resulted in only an increase of EQE without sacrificing low dark current.


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Figure 4. (a) J-V Characteristics and (b) specific detectivity spectrum of the optimized PPD. Specific detectivity spectrum was measured at -1 V.

The J-V characteristics of the optimized PPD are depicted in Figure 4(a). A low dark-current density of 4.1 nA/cm2 at -1 V was obtained, which is attributed to the introduction of the planar heterojunction structure.16 The specific detectivity (D*) is one of the figure-of-merits of PPDs and it can be calculated by the following equation: ∗

√∙

, , where q is the

elementary charge in C, λ is the wavelength of the incident light, A is the active area (0.09 cm2), h is the Planck constant, c is the speed of light, and in is the total noise current.1 We calculated the specific detectivity from EQE values at Figure 3(b) and noise current displayed at Figure S5 and the calculated specific detectivity is shown at Figure 4(b). It is clearly shown that a narrowband detection for blue-light occurs with a narrow FWHM of 103 nm. It can be confirmed that the absorption spectrum of donor material (PNa6-Th) is well reflected in the spectral response of the final device without the disturbance of absorption of acceptor materials (PCBM, ZnO). To examine the electrical properties of each active layer films (PNa6-Th:PCBM, ZnO), we conducted space charge-limited current (SCLC) mobility measurement.29 Since it can be assumed that certain charge carrier flows only certain layer for photocurrent generation, i.e. photogenerated holes flow only through PNa6-Th:PCBM layer and photogenerated electrons


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flow only through ZnO layer, we fabricated hole-/electron-only devices consist of each layer as

active

layer

(hole:

ITO/PEDOT:PSS/PNa6-Th:PCBM/MoO3/Ag,

electron:

ITO/ZnO/LiF/Al). The measured hole and electron mobilities were µh = 1.90×10-5 cm2/Vs and µe = 2.39×10-6 cm2/Vs, respectively. From the measured thickness information of each active layer components (PNa6-Th:PCBM = ~320 nm, ZnO: ~40 nm), we can calculate the transit time. The transit time is the time taken for holes or electrons to move from the donor/acceptor interface of photoactive layer to the corresponding electrode. The transit time for each carrier can be calculated using the equation:

/ , where ttr is the transit time for corresponding charge carrier (holes [h] or electrons [e]), d is the corresponding film thickness, µ is the corresponding carrier mobility, E is the effective electric field, which can be calculated as E = V/(ddonor + dacceptor), and V is the applied voltage.30,31 From the equation and parameters, the calculated transit time values for hole and electron are ttr,h = 6.03×10-5 s and ttr,e = 6.06×10-5 s, respectively. Therefore, it can be confirmed that the difference in transit time is small down to few hundreds of nanoseconds. The transient response and linear dynamic range (LDR) are the other important figure-ofmerits of photodiodes. The -3 dB frequency was measured to determine the transient response; the -3 dB frequency is the frequency corresponding to -3 dB (~70.8 %) of the original signal. We measured the photo-signal-magnitude change as a function of the incident light frequency and the results can be shown in Figure 5(a). The measured -3 dB frequency was 9.1 kHz, which is sufficiently high for imaging applications. Theoretically, the -3 dB frequency (f3dB) can be


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Figure 5 (a) The frequency response plot, (b) linear dynamic range (LDR) plot at -1 V and (c) normalized dark current as a function of operating temperature of devices. The arrows in Figure 5(b) indicate the deviation points from LDR and the inset in Figure 5(c) depicts corresponding change of specific detectivity (D*) according to dark current.

calculated by the equation:

1

1 ,

1 ,

where f3dB,t and f3dB,RC are the transit-time-limited and RC-limited -3 dB frequency, respectively.1 From the transit time values in both charge carrier cases, ~60 µs, the calculated f3dB,t value was 9.3 kHz. Since the measured -3 dB frequency is nearly same as f3dB,t, it can be said that the measured f3dB is mainly determined by transit-time-limited -3 dB frequency. The LDR is the range over which the responsivity stays constant and it can be calculated via the equation: LDR 20log *+,-. /+, / 0, where jmax and jmin are the maximum and minimum detectable photocurrent density values, respectively. The measured LDR of the optimized device can be shown in Figure 5(b) and it was 142 dB, which corresponds to ~7 orders of magnitude. This value is superior to many inorganic-based photodetectors such as GaN (100 dB; 5 orders), InGaAs (132 dB; 6.6 orders) and comparable to Si (240 dB; 12 orders).32 Moreover, the slope of dashed line in Figure 5(b) was 0.986 and it implies that unwanted bimolecular recombination was minimized.33 Together with the -3 dB frequency results, it is


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confirmed that the optimized PPD is theoretically near ideal photodiodes. Thermal stability at high operating temperature over ambient temperature (25 oC) is considered as one of the most important factors for commercialization and industrial application of organic semiconductor-based devices. To investigate this issue, we conducted dark current measurement at high temperature (>25 oC). Samples were placed on hot plate of various temperature from 50 to 115 oC during dark current measurement and a sample with which PCBM contents are 0 wt% was also examined for comparison. The results of this investigation are summarized at Figure 5(c). In both cases of PCBM contents of 0 wt% and 15 wt%, dark current was maintained nearly same as the value at ambient condition until the operating temperature reached to ~90 oC and slight difference in dark current was made at 115 oC. It is presumably due to the relatively large interfacial area between polymer and PCBM. Because the photocurrent was nearly unchanged by the increased measurement temperature up to 90 oC, the optimized OPD with the PCBM content of 15 wt% showed outstanding thermal stability even in terms of D*, by maintating 50 % of its initial value during at least 20 minutes of thermal exposure to 90 oC (inset of Figure 5(c)). We could conclude that the nature of planar heterojunction of the optimized OPD in this work can not only block the dark current injection under the reverse bias, but also enhance thermal stability driven by morphological simplicity. Conclusion We designed and synthesized PNa6-Th with well-defined blue absorption by Suzuki coupling reaction and applied it as a photoactive layer in a PPD for narrowband absorption of blue light. Owing to its naphthalene moiety, PNa6-Th exhibited a wide bandgap of ~2.52 eV with an appropriate absorption range for blue PPD. We constructed a planar heterojunction PPD using PNa6-Th with various PCBM ratios and ZnO, which functioned as a non-absorbing


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acceptor layer. By introducing optimal amount of PCBM (15 wt%), EQE was enhanced from 5.4 % for 0 wt% to 37.8 % for 15 wt% while dark current density values were negligibly increased. Together with the low noise current originated from planar heterojunction nature, a high detectivity of 2.31×1012 Jones was observed. Moreover, we also obtained a fast and -3 dB frequency of 9.1 kHz and a wide LDR of 142 dB (corresponding to ~7 orders of magnitude). Moreover, morphological robustness of the strategically introduced planar heterojunction enabled remarkable thermal stability. We believe that this technique can expand to the key of performance enhancement of various planar heterojunction based optoelectronic devices such as bilayer structured Perovskite solar cells, dye-sensitized solar cells or non-absorbing acceptor-based color-selective photodiodes.

Acknowledgement This research was financially supported by the National Research Foundation of Korea (NRF) funded by Korea government (MSIP) (NRF-2015R1A2A1A10055620) and Space Core Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2014M1A3A3A02034707). This research was also supported by the MOTIE (Ministry of Trade, Industry & Energy (project number: 10051463) and KDRC (Korea Display Research Corporation) support program for the development of future devices technology for display industry. Supporting Information The supporting materials of polymer synthesis, 1H-NMR spectra, GPC, TGA, DSC spectra, cyclic voltammogram, noise current and J-V characteristics for SCLC measurement are available free of charge on the ACS Publication website http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors


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D. S. Chung, *E-mail: [email protected] S. -K. Kwon, *E-mail: [email protected] Y. -H. Kim, *E-mail: [email protected]


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Table of Contents

We synthesize narrowband blue-selective polymer named as “PNa6-Th” and apply it as photoactive donor with ZnO as non-absorbing acceptor for polymer photodiode application. To improve low external quantum efficiency derived from the limited interface, we add a small amount of [6,6]-phenyl-C61-butyric acid methyl ester and we obtain high peak quantum efficiency and detectivity of 37.8 % and 2.31×1012 Jones, respectively.


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Fabrication of High Performance, Narrowband Blue-Selective Polymer

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