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Synthetic Approach To Achieve a Thin-Film Red-Selective Polymer Photodiode: Difluorobenzothiadiazole-Based Donor−Acceptor Polymer with Enhanced Space Charge Carriers Soo-Kwan Kim,† Sungmin Park,‡ Hae Jung Son,*,‡ and Dae Sung Chung*,† †

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Department of Energy Science & Engineering, Daegu Gyeongbuk Institute of Science & Technology (DGIST), Daegu 42988, Republic of Korea ‡ Photo-electronic Hybrids Research Center, Korea Institute of Science and Technology, Seoul 02792, Republic of Korea S Supporting Information *

ABSTRACT: We suggest a synthetic strategy to accelerate charge collection narrowing (CCN) of a polymer photodiode so that a welldefined color selectivity can be realized even with a thin film thickness, which is the most important requirement for commercial application of polymer photodiodes. A new polymer semiconductor POFPhDT2FBT is synthesized by copolymerizing a difluorobenzothiadiazole acceptor with a difluorinated donor moiety. A polymer photodiode with a structure of indium tin oxide/ZnO/ POFPhDT2FBT:PC70BM bulk-heterojunction (BHJ) (550 nm)/MoO3/Ag exhibits a high peak detectivity of ∼6 × 1012 jones at 650 nm with a narrow full width at half-maximum 6 × 1012 jones at 650 nm under −0.1 V), narrow full width at half-maximum (fwhm 2 μm) photoactivelayer thicknesses are used so that incident photons with high absorption coefficients are mostly absorbed at the illuminated front side according to the Beer−Lambert law.11 Therefore, within a thick polymer:phenyl-C70-butyric acid methyl ester (PC70BM) bulk-heterojunction (BHJ) photoactive layer, excitons generated from photons with high absorption coefficients have a very limited chances of charge carrier extraction by each collecting electrode as the required travel length is too long. In a strong contrast, incident photons with low absorption coefficients can be delivered to deeper thicknesses of the photoactive layer by the cavity-type absorption behavior, leading to significantly more efficient charge carrier collections.12 Therefore, in such a CCN method, incident photons corresponding to the absorption tail, not the absorption maxima, of the photoactive layer are responsible for the resulting detectivity spectrum. Based on this thick-junction CCN approach, R/G/B-selective polymeric photodiodes with high detectivities have been demonstrated. However, as explained earlier and also according to the Fujifilm’s and B

DOI: 10.1021/acs.macromol.8b01751 Macromolecules XXXX, XXX, XXX−XXX

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Figure 2. (a) Schematic energy level alignment of the photodiode with POFPhDT2FBT:PC70BM BHJ as an active layer, (b) dark and illuminated J−V characteristics, (c) noise current density spectrum, (d) calculated normalized detectivity and EQE spectra measured at −0.1 V of the optimized photodiode, and (e, f) schematic cartoons which comparatively explain the differences of (e) surface charge generation and (f) volume charge generation. The molecular weight (Mn) of the polymer, estimated using gel permeation chromatography (GPC) at 60 °C, was 7.8 kDa with a polydispersity index (PDI) of 1.48. The thermal property of the polymer was investigated by performing thermogravimetric analyses (TGA) of the polymers under a nitrogen flow (Figure S1). The 5% weight loss temperature (Td) of the POFPhDT2FBT polymer was 386 °C. The electrochemical properties of the polymer were studied using cyclic voltammetry (Figure S2). The highest occupied molecular orbital (HOMO) energy level was estimated from the onset oxidation potential to be −5.49 eV. The lowest unoccupied molecular orbital (LUMO) energy level, calculated from the HOMO energy level and optical energy band gap, is −3.5 eV. The optical energy band gap was estimated from the absorption onset in the film absorption spectrum of POFPhDT2FBT (see Figure 1a). Device Preparation. Polymeric photodiodes were fabricated based on prepatterned indium tin oxide (ITO) glass as a substrate. Following the conventional ZnO sol−gel method, the ZnO precursor solution was spin-coated onto ITO glass. For the ZnO precursor solution, zinc acetate dehydrate (1 g) and ethanolamine (0.28 mL) were mixed with 2-methoxyethanol (10 mL). The solution was stirred at 60 °C for 3 h and then spun onto the substrate with 2000 rpm for 30 s, followed by thermal annealing at 200 °C for 20 min to complete

photodiode exhibits an outstanding device stability under repeated operation over 24 h in inert conditions as well as good shelf stability under humid-air exposure. This might be attributed to an improved molecular stability of the polymer owing to fluorination on the degradable dialkoxybenzene donor unit.

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EXPERIMENTAL SECTION

Synthesis. Scheme 1 shows the synthesis of POFPhDT2FBT. The polymer was synthesized by copolymerizing 1,4-dibromo-2,5-difluoro3,6-bis((2-hexyldecyl)oxy)benzene (3) and 5,6-difluoro-4,7-bis(5(trimethylstannyl)thiophen-2-yl)benzo[c][1,2,5]thiadiazole (4) using Stille coupling reactions in a toluene solvent with Pd(PPh3)4 as the catalyst. Compound 3 was prepared from 1,4-difluoro-2,5-dimethoxybenzene through three reaction steps, as shown in Scheme S1. After performing the polymerization, the polymer was collected by precipitation in acetone and purified by performing successive Soxhlet extractions with methanol, ethyl acetate, hexane, and dichloromethane to remove the byproducts and oligomers. Detailed descriptions of the monomer syntheses, polymerization, and characterizations of the monomer and polymer are provided in the Supporting Information. C

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Macromolecules the sol−gel reaction.17,18 Synthesized POFPhDT2FBT and PC70BM were mixed in a 1:1 weight ratio and dissolved in chloroform to make a solution with concentration of 40 mg mL−1. The blended solution was spin-coated onto the surface of ZnO. The spin-coating condition was 1000 rpm for 60 s, creating an active layer thickness ∼550 nm. Finally, MoO3 (30 nm) and Ag (100 nm) were deposited on the POFPhDT2FBT:PC70BM film by using a thermal evaporator. Characterization. Dark J−V characteristics were measured using a LabView-controlled Keithley 2400 SourceMeter, and illuminated J− V characteristics and the specific detectivity spectrum were measured from the above-mentioned instrument with an Oriel Conerstone 130 1/8 m monochromator combined with a 150 W xenon arc lamp. For measuring noise current of photodiode, we used the Stanford Research SR830 lock-in amplifier connected to a Keithley 2400 and Newport chopper controller (Model: 75160) for synchronization of frequency. The lock-in amplifier was controlled by the homemade LabView program. For noise current collection, first, the noise voltage was point-by-point collected by using the above-mentioned LabView program. Next, normalized noise current data were calculated by using collected noise voltage, sensitivity of the lock-in amplifier, and input frequency. To conduct LDR measurements, monochromated light (650 nm) from a 150 W xenon arc lamp for light intensities below 45 μW cm−2 and a laser (650 nm) for light intensities up to 31.7 mW cm−2 was used with various neutral density filters. These light sources were calibrated using commercial Si photodetectors, and all measurements using the light sources were performed at a light modulation frequency of 1 Hz. All the measurements, except the shelf stability measurement, were performed in a N2-filled glovebox, and the shelf stability measurement was performed under ambient conditions. For measuring operational stability, the photodiode was exposed to continuous light pulse of 1 Hz and 45 μW cm−2 of light intensity in a N2-filled glovebox. The thickness values of polymer films stated in this article were measured by a DektakXT stylus profiler. GIXD measurements were performed using the 3C and 9A beamline at the Pohang Accelerator Laboratory (PAL).

behavior with only slowly decreasing peak intensity throughout the whole thickness. A photodiode was fabricated with a structure of indium tinoxide (ITO)/ZnO/BHJ/MoO3/Ag, as shown in Figure 2a. The LUMO level offset between POFPhDT2FBT and PC70BM is ∼0.8 eV, sufficiently high to provide an efficient charge separation. As the photodiode is operated under reverse bias, ZnO and MoO3 work as not only work-function tuning buffer layers but also as hole- and electron-blocking layers, respectively. The resulting J−V characteristics are summarized in Figure 2b. A low dark-current density of ∼16 nA cm−2 was measured under a reverse bias of −5 V, typical for highperformance organic photodiodes.3 It is worth noting that the measured photocurrent at the wavelength of 594 nm (peak of band I) is significantly lower than that at 650 nm (absorption tail). Based on the measured photocurrent density Figure 2b and the noise current density Figure 2c, the specific detectivity and external quantum efficiency (EQE) spectrum Figure 2d were obtained. The specific detectivity was calculated using the equation D* =

(2)

where λ is the wavelength of the incident light, A is the active area (0.09 cm2), EQE is the external quantum efficiency, h is the Planck constant, c is the speed of light, and inoise is the noise current.3 A remarkable red selectivity was obtained with the detectivity spectral band centered at 650 nm and fwhm of ∼80 nm as well as high peak detectivity of 6 × 1012 jones, which is so far the highest detectivity value among all the previously reported red-selective photodiode.7,10 Simultaneously, all other visible ranges were significantly quenched with low EQEs, smaller than 1.3%. This implies that the CCN mechanism was successfully realized in the POFPhDT2FBT:PC70BM BHJ film. Illuminated photons with a wavelength of 594 nm corresponding to the absorption peak of band I were mostly absorbed at the front side; the LUMO level offset of ∼0.8 eV between POFPhDT2FBT and PC70BM enabled charge separation at the front side. Consequently, under the reverse bias, photogenerated holes should diffuse along the distance of ∼550 nm to be collected by the MoO3/Ag electrode. If the hole mobility of POFPhDT2FBT is not sufficiently high to enable the diffusion along the length of ∼550 nm, most of the holes would become space charges within the BHJ, creating a strong localized electric field, which in turn inhibits the extraction of electrons.20 Notably, the thickness of our BHJ film is only ∼550 nm, which is much thinner than those in the previous CCN demonstrations.10 For example, in the case of a poly(3hexylthiophene) (P3HT):PC70BM BHJ film, no CCN was observed from its detectivity and the EQE spectrum when similarly thin film thickness of 550 nm was introduced, as summarized in Figure S3. The resulting detectivity spectrum only resembles the absorption spectrum of P3HT:PC70BM, without any CCN. To understand the physics of such a thinfilm CCN mechanism, a hole-only diode was fabricated with a structure of ITO/MoO3/POFPhDT2FBT/Au and analyzed by the space-charge-limited-current (SCLC) method,21,22 as shown in Figure S4. The obtained hole mobility was as low as ∼9.00 × 10−8 cm2 −1 −1 V s , which implies that under −5 V holes are collected by the counter electrode after ∼6 ms.

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RESULTS AND DISCUSSION Figure 1a shows ultraviolet−visible (UV−vis) absorption spectra of the POFPhDT2FBT film and its BHJ film with PC70BM. A typical dual-band feature is observed for the pristine POFPhDT2FBT with high band I and band II peak extinction coefficients of 15.5 × 104 and 9.4 × 104 cm−1 for incident photon wavelengths of 594 and 386 nm, respectively. Owing to the non-negligible absorption of PC70BM at short wavelengths, the BHJ film exhibited a similar but more flattened absorption feature. Considering the Beer−Lambert law I = I0 exp( −αt )

qλ A EQE hcinoise

(1)

where α is the extinction coefficient and t is the thickness of the absorbing film, the penetration depth for each photon wavelength can be calculated as 1/α.10 Therefore, the calculated penetration depth for a photon wavelength of 594 nm (peak of band I) is 67 nm, while that at 650 nm (absorption tail) is as deep as 330 nm. This phenomenon can be further quantitatively analyzed by the generalized transfer matrix method (GTMM).19 For this purpose, the reflective index n and k values were experimentally obtained using an integrating-sphere spectrophotometer. Figure 1b shows the calculated normalized electrical intensity (|E|2) distributions within the BHJ film with the experimentally determined thickness of ∼550 nm. Most of the 400 and 550 nm photons are absorbed at the illuminated surface while a negligible number of photons reach the counter electrode, MoO3/Ag. On the other hand, the 650 nm photons corresponding to the absorption tail of the BHJ film exhibit an obvious cavity-type D

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Figure 3. 2-D-GIXD images measured from (a) POFPhDT2FBT and (b) POFPhDT2FBT:PC70BM BHJ films.

Figure 4. (a) Dynamic response of POFPhDT2FBT:PC70BM BHJ under reverse bias of 0.1 V and a 650 nm illumination with various light intensities. (b) Frequency response of BHJ film; horizontally dashed line indicates −3 dB cutoff frequency. (c) Operating stability under reverse bias of 0.1 V, 650 nm illumination pulsed at 1 Hz, 45 μW cm−2. (d) Shelf stability of the optimized photodiode in ambient condition with high humidity (rh > 70%).

t=

d2 μV

film thickness of ∼550 nm. The overall schematic operation principle for the thin-film CCN mechanism is summarized in Figures 2e and 2f. Figure 2e shows that charge carriers from surface-generated excitons cannot be efficiently extracted due to long transit time in the case where polymers with enhanced space charge carriers are used, while Figure 2f shows that all the generated excitons can be separated within the thin film

(3)

where t is the transit time and d is the transit distance.20,23 This is a very long transit time, more than 1000 times longer than those of all other organic photodiodes,7 supporting the possibility of CCN mechanism in the POFPhDT2FBT:PC70BM BHJ film despite the rather thin E

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active layer when enhanced space charge carriers are not introduced. To understand the rather unique property of POFPhDT2FBT with large space charges and thus with an ability of realizing CCN within thin film thickness, twodimensional grazing-incidence X-ray diffraction (GIXD) analyses were performed for POFPhDT2FBT and its BHJ film. As summarized in Figures 3a and 3b, both POFPhDT2FBT and POFPhDT2FBT:PC70BM film exhibit powder-scattering-like rings, suggesting a polycrystalline structure with no preferred molecular orientation with respect to the structure. The (100) scattering due to the lamellar layer structure (q ∼ 0.29 Å−1) and (010) scattering due to the π−π intermolecular stacking (q ∼ 1.73 Å−1) are observed, confirming the intermolecular self-assembly, but only at a short-range with a near-isotropic direction. This result is different from those of other difluorobenzothiadiazole-based polymers.14 This probably occurs as the weak donor unit of fluorinated 3,6-bis((2-hexyldecyl)oxy)-1,4-bis(thiophen-2-yl)benzene decreases the ICT on the polymer backbone, and thus the polymer tends to be less planar and more randomly organized with a near-isotropic molecular orientation and short-range ordering. Consequently, the photodiode has large space charges even in a 550 nm thick diode thin film, thus successfully demonstrating the CCN mechanism. Other photodiode characteristics such as linear dynamic range (LDR) and −3 dB frequency are summarized in Figures 4a and 4b, respectively. LDR represents the linear relationship of the photocurrent as a function of the incident light power, which is very important to acquire clear images regardless of the surrounding environment.24 From the lowest linearly measurable 4.83 × 10−8 W/cm2 to the highest linearly measurable 3.20 × 10−2 W/cm2, the POFPhDT2FBT diode exhibited well-defined linearity, yielding a large LDR of 117 dB P (LDR = 20 log Pmax )25 or 6 orders of magnitude. This LDR

CONCLUSION In modern photodiode applications, particularly for image sensors, thin films are very important as they can significantly enhance the integrity of each image pixel or enable very useful functions such as a global shutter. However, most of the previous studies failed to maintain thin-film (∼550 nm) characteristics in the development of color-filter-free full-color organic photodiodes. In this study, we demonstrated that “thin-film CCN” can be realized by a suitable design/synthesis of weak donor-strong acceptor polymers showing nearisotropic molecular orientations with a short-range ordering in the film, which effectively contribute to enhance space charges within a thin BHJ film so that excitons with high extinction coefficients can be neglected. We demonstrated a high-performance red-selective polymer photodiode with a peak spectrum detectivity at 650 nm of up to 6 × 1012 jones, fwhm 70%); the results are summarized in Figure 4d. The diode maintained 100% of its initial EQE after storage for 12 days under ambient conditions without encapsulation.3,29 The high air stability of the diode could be attributed to the improved stability of the polymer as the electron-withdrawing fluorine atom might increase the chemical stability of the polymer where the dialkoxybenzene moiety is known to be easily degradable by oxygen in air.30,31

Notes

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b01751. General, 1H NMR, 13C NMR, TGA, GPC, synthesis, electrochemical properties, cyclic voltammetry (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(D.S.C.) E-mail [email protected]. *(H.J.S.). E-mail [email protected]. ORCID

Hae Jung Son: 0000-0002-0912-3483 Dae Sung Chung: 0000-0003-1313-8298 Author Contributions

S.K.K. and S.P. contributed equally.

min

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Space Core Technology Development Program funded by the Ministry of Education (Grant NRF-2014M1A3A3A02034707), DGIST HRHR Program funded by the Ministry of Science (18-01-HRSS-10), the Global Frontier R&D Program on Center for Multiscale Energy System (2015R1A1A1A05001115) and the KIST institutional programs.

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ABBREVIATIONS CCN, charge collection narrowing; BHJ, bulk heterojunction; POFPhDT2FBT, poly[(2,5-difluoro-3,6-bis(2-hexyldecyloxy)phenylene)-alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c][1,2,5]-thiadiazole)]; PC70BM, phenyl-C70-butyric acid methyl ester; CMOS, complementary metal oxide semiconductor; ICT, intramolecular charge transfer; GPC, gel permeation chromatography; PDI, polydispersity index; TGA, thermogravimetric analyses; HOMO, highest occupied molecular orbital; LUMO, lowest unoccupied molecular orbital; P3HT, poly(3hexylthiophene); ITO, indium tin oxide; SCLC, space-chargelimited-current; EQE, external quantum efficiency; GIXD, grazing-incidence X-ray diffraction; PAL, Pohang Accelerator F

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(19) Kim, S.-Y.; Lee, J.-H.; Shim, H.-S.; Kim, J.-J. Optical analysis of organic photovoltaic cells incorporating graphene as a transparent electrode. Org. Electron. 2013, 14, 1496−1503. (20) Armin, A.; Yazmaciyan, A.; Hambsch, M.; Li, J.; Burn, P. L.; Meredith, P. Electro-Optics of Conventional and Inverted Thick Junction Organic Solar Cells. ACS Photonics 2015, 2, 1745−1754. (21) Zuo, G.; Li, Z.; Andersson, O.; Abdalla, H.; Wang, E.; Kemerink, M. Molecular Doping and Trap Filling in Organic Semiconductor Host−Guest Systems. J. Phys. Chem. C 2017, 121, 7767−7775. (22) Blom, P. W. M.; De Jong, M. J. M.; Vleggaar, J. J. M. Electron and hole transport in poly(p-phenylene vinylene) devices. Appl. Phys. Lett. 1996, 68, 3308−3310. (23) Ng, T. N.; Wong, W. S.; Chabinyc, M. L.; Sambandan, S.; Street, R. A. Flexible image sensor array with bulk heterojunction organic photodiode. Appl. Phys. Lett. 2008, 92, 213303−213305. (24) Konstantatos, G.; Clifford, J.; Levina, L.; Sargent, E. H. Sensitive solution-processed visible-wavelength photodetectors. Nat. Photonics 2007, 1, 531−534. (25) Yoon, S.; Ha, J.; Cho, J.; Chung, D. S. Nonabsorbing AcceptorBased Planar Heterojunction for Color-Selective and High-Detectivity Polymer Photodiodes. Adv. Opt. Mater. 2016, 4, 1933−1938. (26) Guo, F.; Xiao, Z.; Huang, J. Fullerene Photodetectors with a Linear Dynamic Range of 90 dB Enabled by a Cross-Linkable Buffer Layer. Adv. Opt. Mater. 2013, 1, 289−294. (27) Asif Khan, M.; Kuznia, J. N.; Olson, D. T.; Van Hove, J. M.; Blasingame, M.; Reitz, L. F. High-responsivity photoconductive ultraviolet sensors based on insulating single-crystal GaN epilayers. Appl. Phys. Lett. 1992, 60, 2917−2919. (28) Sim, K. M.; Yoon, S.; Cho, J.; Jang, M. S.; Chung, D. S. Facile Tuning the Detection Spectrum of Organic Thin Film Photodiode via Selective Exciton Activation. ACS Appl. Mater. Interfaces 2018, 10, 8405−8410. (29) Deckman, I.; Lechene, P. B.; Pierre, A.; Arias, A. C. All-printed full-color pixel organic photodiode array with a single active layer. Org. Electron. 2018, 56, 139−145. (30) Chambon, S.; Rivaton, A.; Gardette, J.-L.; Firon, M.; Lutsen, L. Aging of a donor conjugated polymer: Photochemical studies of the degradation of poly[2-methoxy-5-(35-(3. L.; Woo, H. Y.; Shin, W. S.; Kim, J. Y. J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 317−331. (31) Atreya, M.; Li, S.; Kang, E. T.; Neoh, K. G.; Ma, Z. H.; Tan, K. L.; Huang, W. Stability studies of poly(2-methoxy-5-(25-(2 of the degradation of poly[2-methoxy-5-(35-(3. LPolym. Polym. Degrad. Stab. 1999, 65, 287−296. (32) Cumpston, B. H.; Jensen, K. F. Photooxidative stability of substituted poly(phenylene vinylene) (PPV) and poly(phenylene acetylene) (PPA). J. Appl. Polym. Sci. 1998, 69, 2451−2458.

Laboratory; LDR, linear dynamic range; fwhm, full width at half-maximum.

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DOI: 10.1021/acs.macromol.8b01751 Macromolecules XXXX, XXX, XXX−XXX