Development of Novel Conjugated Polyelectrolytes as Water

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Development of Novel Conjugated Polyelectrolytes as Water-processable Interlayer Materials for High Performance Organic Photodiodes Seongwon Yoon, Jea Woong Jo, Seong Hoon Yu, Jae Hoon Yun, Hae Jung Son, and Dae Sung Chung ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b00112 • Publication Date (Web): 23 Feb 2017 Downloaded from http://pubs.acs.org on February 27, 2017

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Development of Novel Conjugated Polyelectrolytes as Water-processable Interlayer Materials for High Performance Organic Photodiodes Seongwon Yoon,† Jea Woong Jo,‡ Seong Hoon Yu,† Jae Hoon Yun,‡ Hae Jung Son,*,‡ and Dae Sung Chung*,†

†

Department of Energy System Engineering, Daegu Gyeongbuk Institute of Science and

Technology (DGIST), Daegu 42988, Republic of Korea ‡

Photo-electronic Hybrids Research Center, Korea Institute of Science and Technology, Seoul

02792, Republic of Korea.

ACS Paragon Plus Environment

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Abstract A series of novel conjugated polyelectrolytes composed of two different building blocks with different composition ratios were designed and synthesized for application as a functional layer in high-performance organic photodiodes (OPDs). A homopolymer and two random copolymers were prepared using different molar ratios of dibromo 1,4-bis(4-sulfonatobutoxy)benzene (SPh) and dibromo 1,4-bis(4-tetraethyleneglycol)benzene (EGPh): EG20 with SPh:EGPh ratio of 0.8:0.2 and EG40 with the ratio of 0.6:0.4. Structural analyses by two dimensional grazingincidence X-ray diffraction and near-edge X-ray absorption fine structure spectroscopy studies proved that a higher EGPh content could induce more organized polymer chains with face-on orientation of EG20 and EG40. Such an orientation of EG20 and EG40 along with the ordered crystalline organization yielded effective molecular dipole moments in the thin films, when applied as interlayer between ZnO and an active layer of inverted OPDs. As confirmed by ultraviolet photoelectron spectroscopy, the increase in EG content gradually shifted the workfunction of the ZnO, facilitating the inverted OPD to simultaneously achieve a decrease in dark current and enhancement in photocurrent. The synergetic effects introduced by the newly designed EG20 and EG40 resulted in significantly improved OPD performances with high specific detectivity up to 2.1 ×1013 Jones, 3-dB bandwidth of 72 kHz, and linear dynamic range of 110 dB.

KEYWORDS water-soluble, conjugated polyelectrolytes, organic photodiode, work function, low dark current, high-detectivity


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In recent years, conjugated polyelectrolytes (CPEs) have been widely applied as promising interface layer materials for optoelectronic devices such as organic photovoltaic (OPV) devices.1-7 For example, CPEs are employed as interface layers between the active layer and electrodes, and are effective in controlling energy level alignment.8,9 The introduction of a CPE can suppress unnecessary charge recombination10 or enhance charge extraction, both of which are directly associated with enhanced quantum efficiency of the OPV devices. Liu et al. reported the use of zwitterionic CPEs for enhancing the OPV performance.11 They modified the work function of a Ag electrode from 4.52 to 3.56 eV, and the fabricated OPVs displayed a power conversion efficiency (PCE) increase of > 500% when compared to the reference devices. Xie and coworkers showed an improved PCE enhancement of 2.99% to 4.08% in fabricated P3HT:PCBM-based OPVs by using CPEs as interlayers.12 They used CPEs such as PTN-Br on a ZnO layer and achieved an efficient electron injection due to the shift in the work function of an ITO/ZnO electrode from 4.4 to 3.93 eV. Oh et al. have also reported the effects of two different water-soluble CPEs on solar cell efficiency by testing various electrode materials, such as Al, Ag, Au, and Cu.13 Interestingly, the introduction of the CPEs modified the work function of Cu from 4.68 to 4.11 eV, increasing the PCE by > 400 %. Chen et al. reported the performance enhancement of OPVs by means of rapid dipole orientation using an ionic liquid crystal along with the CPEs.14 The work function of an ITO electrode shifted from ~4.9 to ~4.0 eV and resulted in the enhancement of solar cell efficiency from 6.6% to 7.5%. Although most of the research on CPEs is focused on OPVs, it is believed that CPEs are more promising materials for organic photodiode (OPD) applications. An OPD is a unit device of an image sensor, and therefore maximization of the signal-to-noise ratio is the most important parameter to achieve. The most widely accepted figure-of-merit of an OPD is the specific


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detectivity (D*), which enables comparison of the signal-to-noise ratios of photodiode devices with different device geometries. D* can be obtained from the equation ∗ =

√∙

, where e

is the elementary charge, λ the wavelength of incident light, A the area of the active layer, EQE the external quantum efficiency, h the Planck constant, c the speed of light, and in the noise current.15 Here, the noise current is known to have several sources such as shot noise, thermal noise, and 1/f noise. In most cases, the noise current is largely dependent on shot noise and therefore, suppressing the dark current can be an effective method to suppress the noise current level. Thus, decreasing the dark current without sacrificing the photocurrent is actually crucial to realize high performance OPDs. In this context, CPEs are considered as the most suitable materials, which can improve the photocurrent and simultaneously decrease the noise level, by a proper alignment of the energy levels between the CPE and its neighboring layers. Herein, we synthesized a series of conjugated polyelectrolytes composed of two different building blocks with different composition ratios. As shown in Scheme 1, a homopolymer (1T) and two random copolymers were prepared using different molar ratios of dibromo 1,4-bis(4sulfonatobutoxy)benzene (SPh) and dibromo 1,4-bis(4-tetraethyleneglycol)benzene (EGPh). The change in SPh content can change the total amount of the ionic groups in the polymer, which may affect the physical properties of the CPE such as energy levels and molecular dipole moments. The ethylene glycol-based building blocks are electron-rich, and can thus tune the energy levels of the CPE layer coated on metal oxides. Moreover, the ethylene glycol groups (EG) can enhance the intermolecular interactions between the polymer chains so that more aligned crystalline structure can be achieved, and also increase solubility of the resulting copolymers in aqueous solutions.16-18


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The CPEs developed in this work were used as interlayers in an OPD composed of poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b’]dithiophene-2,6-diyl-alt-(4-(2ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)]

(PBDTT-FTTE)

and

[6,6]-phenyl C71 butyric acid methyl ester (PC71BM) as donor and acceptor materials, respectively. PBDTT-FTTE is a well-known low-bandgap donor material and forms an appropriate type-II junction with PC71BM, which is essential for efficient exciton dissociation. Our studies reveal that introduction of our CPEs at the ZnO/active layer interface can successfully decrease the dark current down to 2.4 nA/cm2 and increase specific detectivity up to 2.1 ×1013 Jones at -2 V of the OPDs, which corresponds to a noise-equivalent power of 14.3 fW/Hz0.5. The physical properties of the CPEs, and their working mechanism in OPDs was investigated in terms of energy levels and packing structures on the ZnO layer.


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Methods Materials: 1T was synthesized according to the method reported elsewhere.19 Synthetic details for EG20 and EG40 are in the Supporting Information. PBDTT-FTTE and PC71BM were purchased from Solarmer and Nano-C, respectively and used without further purification. Photodiode fabrication: ITO-patterned glass substrates were mechanically cleaned with aqueous hydrochloric acid solution, followed by sequential sonication in detergent solution, distilled water, acetone and 2-propanol, for 15 mins each. After blowing with N2-gas to remove excess 2propanol, zinc acetate solution was spin-coated onto the cleaned substrates for ZnO layer deposition. 1 g of zinc acetate dihydrate and 10 mL of 2-methoxyethanol were mixed with 280 mg of ethanolamine as stabilizer to make sol-gel based ZnO solution in advance. After deposition of ZnO layer, the substrates were annealed at 200 °C for 30 min to terminate the solgel reaction. For interlayer deposition, aqueous 1T, EG20 and EG40 solutions were prepared with a concentration of 3 mg/mL and then spin-coated onto the ZnO films with a thickness of ~10 nm. A solution consisted of 24 mg of PBDTT-FTTE and 36 mg of PC71BM in 1.5 mL of chlorobenzene was spin-coated onto the CPEs-deposited substrates to form an active layer thin film with a nominal thickness of ~380 nm. Then MoO3 and Au were deposited by sequential thermal evaporation to form thickness values of 30 nm and 100 nm, respectively. Characterization: The chemical structures of compounds were identified by 1H NMR (Avance DPX-300) using dimethyl sulfoxide-d6 (DMSO-d6) as a solvent and tetramethylsilane as an internal reference material. Molecular weights and distribution of polymers were estimated by GPC (Waters) with a refractive index detector (Waters 2414). DMF was used as an eluent, and polystyrene standards were used for calibrating the molecular weights of polymers. Electron spin


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resonance (ESR) spectra were measured with an ESR spectrometer (JES-TE200, JEOL). The optical absorption spectra were obtained using an UV–Vis spectrophotometer (Lambda 35, Perkin Elmer). Dark current and photocurrent measurements, including detectivity measurement were performed with a combination of Keithley 2400 sourcemeter and Oriel Cornerstone 130 1/8 m monochromator. An intensity-tunable laser (λ = 650 nm) and neutral density filters were used to measure linear dynamic range and 3-dB frequency. For 3-dB frequency measurement, TDS5052 digital phosphor oscilloscope was also used (Tektronix). Noise currents were directly measured from the Stanford Research SR830 Lock-in Amplifiers and the collected noise current data were normalized by the input bandwidth. For the collection of two dimensional grazing incidence x-ray diffraction (2D-GIXD) and near edge X-ray absorption fine structure (NEXAFS) data, PLS-II 3C and 4D beam line at the Pohang Accelerator Laboratory (PAL) in Korea were used, respectively. All the measurements were performed at room temperature.


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Results and Discussion

Scheme 1 Chemical structures and synthetic schemes of CPEs. As shown in Scheme 1, we synthesized a homopolymer of 1T and two random copolymers via Suzuki coupling reactions of a thiophene diborate compound with different molar feed ratios of SPh and EGPh for polymerization (SPh:EGPh was 0.8:0.2 for EG20; 0.6:0.4 for EG40). 1T was synthesized according to a reported procedure.20 The detailed synthesis and characterization of the monomers and polymers are described in the Supporting Information (SI). Molecular weights were measured using gel permeation chromatography (GPC) after ion exchanges of each polymer with tetrabutylammonium bromide in order to increase solubilities in an organic solvent, such as chlorobenzene. The polymers showed an Mn (number-average of molecular weights) of 9.4 kg mol−1, 14.3 kg mol-1, and 16.6 kg mol-1 for 1T, EG20, and EG40, respectively. 1T, EG20, and EG40 are more selectively soluble in aqueous media than organic solvents. This enables water-processed thin layers of the polymer to be well conserved even after the deposition of upper active layers using organic solutions.


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Figure 1 (a) The absorption spectra of CPEs in aqueous solutions. (b-d) GIXD patters of thin films of (b) 1T, (c) EG20 and (d) EG40, (e) out-of-plane and (f) in-plane line-cut data corresponding to b-d. (g-i) NEXAFS data measured under various incident light angle of thin films of (g) 1T, (h) EG20 and (i) EG40. (j) π* transition intensities versus incidence angle. The solid lines represent the fitted curves Figure 1(a) shows the UV-Vis absorption spectra of the CPEs in solution phase with different composition ratios. Regardless of the composition ratios, all the CPEs showed a characteristic absorption peak at 413 nm, which corresponds to the π-π transition. At the same time, a very weak absorption feature at 600–800 nm, which can be assigned to a radical cation (or polaron), tends to decrease as the EG content increases. Therefore, it is expected that the polaron generation decreases with the increase in EG content, as supported by electron spin resonance analysis (Figure S1).


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2D-GIXD measurements were performed in order to investigate the effect of chemical structural change of the CPEs on the polymer packing properties in a film, and the results are summarized in Figures 1(b)–(f). Notably, EG20 and EG40 show (l00) Bragg diffraction peaks with series up to (200), indicating the formation of lamellar crystalline domains, while 1T (without the EG moiety) shows nearly featureless scattering patterns. The measured interlamellar distance of EG40 based on the (100) diffraction peak is ~1.90 nm, which is typical of conjugated polymers considering the length of the ethylene glycol side chains.18 It seems that the charged end groups of the SPh moiety prohibits intermolecular lamellar stacking of 1T, while the increased proportions of ethylene glycol side chains in EG20 and EG40 enables partial lamellar stacking. The crystalline orientation of the CPEs were further investigated by means of near edge X-ray absorption fine structure (NEXAFS) spectroscopy as summarized in Figures 1(g)–(i). In these spectra, the features at ~285.3, 287–291, and 292–307 eV can be assigned to the π* (C=C), σ* (C–S), and the π* (C=O) orbitals mixed with Rydberg orbitals and several σ* orbitals, respectively.20 In particular, by analyzing the incident light angle dependencies of the π* (C=C) orbital peak intensities, we can calculate the dichroic ratio (R) as summarized in Figure 1(j). The R values of EG20 and EG40 are negative while that of 1T is positive, implying that EG20 and EG40 exist in preferential face-on orientations, while 1T preferentially exists in the edge-on orientation. In conjugated polymers, more developed lamellar stacking is very often related to a higher charge carrier mobility. In addition, in vertical device structures, the face-on orientation is more beneficial for charge transport when compared to the edge-on orientation. It is widely reported that polymers having preferential face-on orientation positively affect the power conversion efficiency when they are used as photoactive materials for OPVs.33−36 Therefore,


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EG20 and EG40 can potentially lead to a more efficient charge extraction when applied as an interface layer of OPDs.

Figure 2 (a) The secondary cut-off region of UPS to show work function shift as a result of the introduction of CPEs and (b) the schematic energy level diagram to highlight the effect of CPEs for efficient hole blocking at ZnO/PBDTT-FTTE:PC70BM interface of inverted OPDs operating under reverse bias mode. Figure 2(a) shows the secondary cutoff region of the spectra obtained by ultraviolet photoemission spectroscopy (UPS) for 1T, EG20, and EG40 layers (~10 nm) deposited onto ZnO films. It is evident that the workfunction (WF) of ZnO is shifted to a lower value of kinetic energy as the EG content increases in the copolymer. Such a decrease of WF of ZnO with the introduction of thin layers of CPE can be attributed to the formation of molecular and interface dipoles.1 The negative charges of the sulfonate groups of the synthesized CPEs make them preferentially adsorbed onto the positively charged terminal zinc ions of the ZnO surface. Therefore, an interfacial dipole is formed from the active layer toward the electrode, resulting in decreased WF of ZnO. Moreover, the high electron density of the ethylene glycol moiety can passivate the surface traps of ZnO,21 resulting in decreased WFs. It is possible that the highly organized polymer chains of EG20 and EG40 in the films, as demonstrated by GIXD studies, are favorable for maximizing the molecular dipoles, resulting in a higher degree of energy level


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shifts compared to 1T. This is very useful for reducing the dark current in inverse-structured OPDs, as schematically described in Figure 2(b). Considering that the photodiodes are operated under reverse bias, dark currents can be generated by hole injection from ITO/ZnO to the highest occupied molecular orbital (HOMO) of donor materials. As seen in the Figure 2(b), the energy gap (∆φ) for hole-injection increased from 0.81 to 1.09 eV as a result of CPEs insertion. According to the thermionic emission model, dark current under reverse bias mode can be considered as a function of energy barrier height: = exp (−

Δ )

, where A is the pre-exponential factor, T is the temperature, q is the elementary charge, k is the Boltzmann constant.39 Therefore, the increase of energy barrier height leads to the suppression of the dark current of the photodiode. In addition, after the changes in the energy levels, the CPE coated ZnO forms a near-ohmic contact with the LUMO levels of the acceptor material. Therefore, the introduction of EG20 and EG40 can potentially decrease the dark current and improve the photocurrent in the OPDs. To investigate the effect of the CPE interlayer on the OPD performance, we fabricated a bulk heterojunction-based OPD with PBDTT-FTTE as the electron donor and PC71BM as the electron acceptor. An inverted OPD structure was constructed by using ZnO as the electron transport/hole blocking layer and MoO3 as the hole transport layer. The aqueous 1T, EG20, and EG40 solutions were spin-coated onto the ZnO layer. The PBDTT-FTTE: PC71BM blend dissolved in chlorobenzene was spin-coated to form the active layer with a nominal thickness of 390 nm, which was the optimized thickness in our system. Because of the selective solubility of the CPEs in water, chlorobenzene used for the active layer deposition could be a near-ideal orthogonal solvent, without causing any significant damage to the pre-formed CPE layer.


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Figure 3. (a) The dark current and photocurrent curves with various interlayers (1T, EG20 and EG40). The used light intensity was 36.5 µW/cm2. (b) The calculated specific detectivity as a function of wavelength under -2 V bias. Figure 3(a) shows the current density-voltage (J-V) characteristics with various CPE layers. The dark current values reduced dramatically from 97.3 nA/cm2 for ZnO to 2.4 nA/cm2 for ZnO/EG40 at -2 V, accompanied simultaneously by a slight increase in photocurrent. As mentioned above, the decreased WF of the ITO/ZnO electrode can hamper the hole injection from ITO to HOMO of the donor material, resulting in the suppression of the dark current. At the same time, the near-ohmic contact for electrons enhances the photocurrent. Besides, we observed that the dark J-V curves in reverse bias region become more flattened after inserting the CPEs. The flattened dark J-V curves imply that the tunneling current between the interlayer and the active layer was dramatically suppressed37 and it is widely known that the main source of the tunneling current is the surface traps.38 Therefore, the insertion of CPEs resulted in not only decreasing the WF, but also suppressing the tunneling-induced dark current.


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Figure 4. (a) The measured noise current, (b) response plot as a function of switching frequency of light and (c) linear dynamic range of the fabricated OPDs with EG40 as an interlayer. In (b) and (c), The measured bias was -2 V. To measure the specific detectivity more precisely, we carried out the noise current measurements to analyze the actual noise in the fabricated devices. The results for EG40 are shown in Figure 4(a) and the shot noise limit is also displayed for comparison. The measured noise current was just above ~2–3 times of the shot noise limit calculated from the dark current, implying that the shot noise and thus dark current are the main source of noise in the fabricated OPDs. As summarized in Figure 3(b), the calculated D* values of OPDs with EG40 reached over 1013 Jones for the entire visible range (400–700 nm), which is comparable to Si-based inorganic photodiodes22 and these results are comparable to the results reported as highperformance OPDs (Table S1).30-32 Fast response time against incident light is one of the most important performance criteria for OPDs. We observed that the responsivity of the OPD decreases as a function of increasing switching frequency of the incident light by a combination of oscilloscope, laser diode, and function generator, as seen in Figure 4(b). The calculated 3-dB frequency was ~72 kHz, which is sufficiently high for image sensing applications and comparable with inorganic photodiodes such as GaN or GaAsP photodiode.23,24 The 3-dB frequency (f3dB) can be calculated by the equation:


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1 !"

=

1 !",

%$+

1 !",'

, where f3dB,RC and f3dB,t are the RC-limited and transit-time limited 3-dB frequency, respectively.15 From the dark J-V curve and geometric capacitance calculation,30 the series resistance and the capacitance of the optimized device were found as 453 Ω·cm2 and 4.3 nF/cm2, respectively, which correspond to 81.5 kHz of f3dB,RC. Because this value of f3dB,RC is nearly similar to the actually measured f3dB, it can be said that the temporal response of the photodiode is limited by RC time, which corresponds to the response time of ~ 7.74 µs. Finally, we measured the linear dynamic range (LDR), which is another figure-of-merit for PDs. LDR is the range of intensity of the incident light that responsivity maintains constant.25 The measurements were conducted with the abovementioned laser diode and the modulation frequency was 35 Hz. As seen in Figure 4(c), the measured LDR was 110 dB, corresponding to 5.5 orders of magnitude. Here LDR was obtained by LDR = 20 log10(jmax/jmin), where jmax and jmin are maximum and minimum detectable current density, respectively.15 The obtained LDR value is more-or-less smaller than the highest values reported recently,23,25-28 however, it can be attributed to the fact that our instruments and the neutral density filter limit the minimum irradiance to 18.5 nW/cm2. Theoretically, the lower limit of the LDR is determined by the noise current29 and according to this theory, the estimated LDR will be over 170 dB, corresponding to 8.5 orders of magnitude. Besides, the measured slope of LDR was close to unity (0.95) implying that the fabricated OPD was a nearly ideal without any unexpected charge recombination or charge traps. Conclusion


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Interface engineering is a crucial stage in the optimization of multi-layered optoelectronic devices. CPEs with mechanical robustness and flexibility are very promising materials for interface-modifications in modern organic electronics. In this work, we designed and synthesized novel conjugated polyelectrolytes using SPh and EGPh building blocks with various composition ratios, and applied them as interlayer materials in OPDs. It was seen that increasing the EGPh content can induce more effective intermolecular lamellar stacking and preferential face-on orientation, which is very beneficial for charge extraction and interface dipole generation. More importantly, the increase of EG content gradually shifted the WF of the electrode and therefore, the OPD simultaneously achieved a decrease in dark current and an enhancement in photocurrent, when the CPE is used as an electron transport/hole blocking layer in inverted OPDs. The dual effects by the newly designed CPEs, EG20 and EG40, resulted in significantly improved OPD performances with a high specific detectivity of up to 2.1 ×1013 Jones, bandwidth of 72 kHz at -3 dB, and LDR of 110 dB. Acknowledgement This research was supported by Space Core Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (Grant No. NRF-2014M1A3A3A02034707). Supporting Information. The supporting materials of polymer synthesis, ESR, CV, DSC, TGA spectra, responsivity and EQE spectra and the comparison of figure-of-merits for OPDs are available free of charge on the ACS Publication website http://pubs.acs.org. AUTHOR INFORMATION


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Corresponding Author D. S. Chung, *E-mail: [email protected] H. J. Son, *E-mail: [email protected] Author Contributions S. Yoon and J. W. Jo contributed equally to the work.


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Table of Contents

We synthesize novel water-soluble conjugated polyelectrolytes named as “1T”, “EG20” and “EG40” and apply them as interlayers for organic photodiode application. Since the ethyleneglycol-based contents in the synthesized polyelectrolytes facilitate charge extraction and interface dipole generation, the optimized photodiode shows a low dark current of 2.4 nA/cm2 and a high detectivity of 2.1 × 1013 Jones at -2 V bias.


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Scheme 1 Chemical structures and synthetic schemes of CPEs. 43x23mm (600 x 600 DPI)


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Figure 1 (a) The absorption spectra of CPEs in aqueous solutions. (b-d) GIXD patters of thin films of (b) 1T, (c) EG20 and (d) EG40, (e) out-of-plane and (f) in-plane line-cut data corresponding to b-d. (g-i) NEXAFS data measured under various incident light angle of thin films of (g) 1T, (h) EG20 and (i) EG40. (j) π* transition intensities versus incidence angle. The solid lines represent the fitted curves. 475x328mm (96 x 96 DPI)


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Figure 2 (a) The secondary cut-off region of UPS to show work function shift as a result of the introduction of CPEs and (b) the schematic energy level diagram to highlight the effect of CPEs for efficient hole blocking at ZnO/PBDTT-FTTE:PC71BM interface of inverted OPDs operating under reverse bias mode. 454x172mm (96 x 96 DPI)


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Figure 3. (a) The dark current and photocurrent curves with various interlayers (1T, EG20 and EG40). The used light intensity was 36.5 µW/cm2. (b) The calculated specific detectivity as a function of wavelength under -2 V bias. 287x137mm (96 x 96 DPI)


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ACS Photonics

Figure 4 (a) The measured noise current, (b) response plot as a function of switching frequency of light and (c) linear dynamic range of the fabricated OPDs with EG40 as an interlayer. In (b) and (c), The measured bias was -2 V 441x145mm (96 x 96 DPI)


Development of Novel Conjugated Polyelectrolytes as Water

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