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Dark Current Reduction Strategy via a Layer-by-Layer Solution Process for a High-Performance All-Polymer Photodetector Zhiming Zhong, Laju Bu, Peng Zhu, Tong Xiao, Baobing Fan, Lei Ying, Guanghao Lu, Gang Yu, Fei Huang, and Yong Cao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20981 • Publication Date (Web): 30 Jan 2019 Downloaded from http://pubs.acs.org on February 2, 2019

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ACS Applied Materials & Interfaces

Dark Current Reduction Strategy via a Layer-by-Layer Solution Process for a High-Performance All-Polymer Photodetector

Zhiming Zhong,† Laju Bu,§ Peng Zhu,† Tong Xiao,§ Baobing Fan,† Lei Ying,†,* Guanghao Lu,§ Gang Yu,† Fei Huang,†,* Yong Cao†

†

Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of

Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China §

School of Science and Frontier Institute of Science and Technology, Xi’an Jiaotong

University, Xi’an 710049, China

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ABSTRACT:

The

ideal

bulk-heterojunction

for

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high-performance

organic

photodetectors requires a morphology with a vertically gradient component to suppress the leaking current. Here we demonstrate an all-polymer photodetector with a segregated bulk-heterojunction active layer. This all-polymer photodetector exhibits a dramatically reduced dark current density due to its built-in charge blocking layer, with a responsivity of 0.25 A W-1 at approximately 600 nm, and specific detectivity of 5.68 × 1012 cm Hz1/2 W-1 as calculated from the noise spectra at 1 kHz. To our knowledge, this is among the highest performance values reported for photodetectors based on both polymeric donor and acceptor in the photoactive layer. These findings present a facile approach to improving the specific detectivity of polymer photodetectors via a layerby-layer solution process.

KEYWORDS:

All-polymer

photodetector,

layer-by-layer

process,

functionalized benzotriazole, gradient bulk heterojunction, noise spectral density


imide

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INTRODUCTION Organic photodetectors (OPDs) have recently emerged as ideal candidates for nextgeneration light sensing, primarily due to their specific advantages over their inorganic counterparts, such as their tunable band gap and photo-responsibility, high extinction coefficients, and lower crystal defect density.1-3 The specific merits of OPDs also include their remarkable processability, such that they can be printed from solutions on various kinds and shapes of substrates to achieve low cost, lightweight, flexible, and pattern-less image arrays.4-8 This significantly reduces the fabrication cost of inorganic counterparts, which are typically fabricated by epitaxial techniques and then cut and bonded to read-out integrated circuits.9 Additionally, in contrast with organic photovoltaics, which typically require standard one sun illumination (~100 mW cm-2), OPDs are generally operated in extremely weak light intensity (100 pW cm-2 ~ 1 mW cm-2).1 This means that the noise current must be suppressed to an extremely low level to make the device sensitive enough to differentiate light signals from noise. Although the noise current of OPDs is not yet fully understood, it is believed to be closely related to their dark current.10 Therefore, various strategies have been devoted to reducing dark current under inverse bias.11 One of the most straightforward ways to reduce the dark current of OPDs is to increase the thickness of the photoactive layer12,13, not only due to the increased resistance of bulk-heterojunction (BHJ) films according to Ohm’s law, but also because increasing film thickness reduces the pinholes of the photoactive layer, leads to the planarization of indium tin oxide (ITO) substrates, and keeps the diffusion of 3 ACS Paragon Plus Environment


evaporated metal particles from penetrating inside. However, increasing the thickness of the photoactive layer may also negatively affect the photocurrent due to the limited charge carrier mobility of organic semiconducting materials.14 An alternative strategy is to incorporate hole- and electron-blocking layers between the photoactive layer and the electrodes, which can form a p-i-n-like structure that effectively prevents the injection of electrons/holes from the anode/cathode under inverse bias, respectively.5,15 However, it remains challenging to find an interfacial layer with an appropriate energy level and charge transport ability. The photovoltaic performance of polymer solar cells fabricated by the conventional one-step solution method can be improved by using a layer-by-layer (LBL) solution processing procedure.16-20 The improved fill factor and rectification ratio imply that less interface recombination occurs or fewer electrons/holes are injected from the anode/cathode by forming pure donor/anode and pure acceptor/cathode contact, respectively, where the donor-rich interface can keep electrons away from the anode and the acceptor-rich interface can keep holes away from the cathode. Here we demonstrated highly sensitive polymer photodetectors with a gradient BHJ layer processed via the LBL solution processing strategy, conducted by taking advantage of the solubility difference between a p-type of the conjugated polymer PTzBI-Ph that contains an imide functionalized benzotriazole unit,21 and an n-type poly{[N,N′bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt5,5′-(2,2′bithiophene)} (with the commercial name of N2200). Both PTzBI-Ph and N2200 can be easily dissolved in p-xylene or chlorobenzene (solubility > 20 mg mL-1) 4 ACS Paragon Plus Environment

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at

room

temperature.

However,

N2200

can

be

easily

dissolved

in

2-

methyltetrahydrofuran (MeTHF), whereas PTzBI-Ph is barely dissolvable in MeTHF.22-24 This combination is a good way to process the bilayer photoactive layer by processing N2200 from the MeTHF solution on the top of the pre-fabricated PTzBIPh layer, which can form the built-in blocking layers of the gradient BHJ structure. Interestingly, the bilayer device based on PTzBI-Ph/N2200 exhibited a dark current density (Jd) of 1.24 × 10-9 A cm-2 at -0.1 V and the impressively high detectivity of 5.68 × 1012 cm Hz1/2 W-1, as calculated from the noise spectra.

RESULTS AND DISCUSSION The molecular structure and energy level alignment of the related polymers are shown in Figure 1a. The wide-band-gap PTzBI-Ph was selected as the electron-donating polymer,21 as it has poor solubility in MeTHF, whereas N2200 (with an average molecular weight of 75.1 kDa and a polydispersity index of 1.8) can be dissolved in MeTHF.23 This specific discrepancy in solubility facilitated the construction of a photoactive layer via an LBL processing procedure (see Figure S1 in the supporting information, SI). Specifically, N2200 was processed from its MeTHF solution on top of the pre-fabricated PTzBI-Ph layer, ultimately yielding a favorable vertical phase separation of the bilayer-structured photoactive layer (referred as PTzBI-Ph/N2200, and the one-step solution-processed blend film was referred as PTzBI-Ph:N2200). According to the Beer-Lambert law, absorption intensity is proportional to film thickness.25,26 To confirm the semi-orthogonal solvent system for sequential 5 ACS Paragon Plus Environment


processing, we measured the UV-vis absorption of these films before and after solvent rinsing. The UV-vis absorbance intensity of the PTzBI-Ph thin film remained at 80% of its original value after rinsing with neat MeTHF (Figure 1b).

Figure 1. (a) The molecular structure of PTzBI-Ph and N2200. (b) The absorbance spectra of PTzBI-Ph film before and after rinsing with MeTHF. (c) Electrons (holes) can be injected under zero or inverse bias in traditional BHJ film. (d) Electron and hole injection are more difficult under zero or inverse bias in vertically gradient BHJ film.

The photoluminescence (PL) quenching of the PTzBI-Ph/N2200 films processed via the LBL procedure was measured under excitations of 550 nm and 700 nm (Figure 2). The excitations at 550 nm and 700 nm were favorable for PTzBI-Ph and N2200, respectively (as shown in Figure S2, SI). The PL from the PTzBI-Ph film showed nearly completed (~99%) quenching by the N2200 with 550 nm excitation, indicating efficient 6 ACS Paragon Plus Environment

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exciton separation. The absorption profile of the N2200 film was much weaker at 550 nm than that of PTzBI-Ph and overlapped with the PL spectrum of PTzBI-Ph (Figure S1, SI). Thus, the PL spectrum of N2200 (Figure 2a) can be attributed to Förster energy transfer from PTzBI-Ph to N2200 rather than direct excitation. During contact with PTzBI-Ph, the PL spectrum of N2200 only slightly decayed at 700 nm excitation, suggesting inefficient separation for the excitons.

Figure 2. (a) The PL quenching spectra of PTzBI-Ph and PTzBI-Ph/N2200 excited by 550 nm, with the inset zoomed in on the PTzBI-Ph/N2200 excited by 550 nm. (b) The PL quenching spectra of N2200 and PTzBI-Ph/N2200 excited by 700 nm. The measurement was conducted in films and their thicknesses were identical to those in the devices.

In order to reveal the difference morphology of films deposited by LBL and spincast from blended solution, we characterized the surface morphology by using atomic force microscopy (AFM), with relevant figures shown in Figure 3.

One can clearly

observe the pin-holes in the PTzBI-Ph:N2200 blend film (Figure 3a) and neat PTzBI7 ACS Paragon Plus Environment


Ph film (Figure 3b), while the surfaces of neat N2200 film (Figure 3c) and the PTzBIPh/N2200 film processed by LBL method were relatively smooth (Figure 3d).

Figure 3. The height images of (a) PTzBI-Ph:N2200 =2:1 wt blend film, (b) neat PTzBI-Ph film, (c) neat N2200 film and (d) PTzBI-Ph/N2200 bilayer film on ITO/PEDOT:PSS substrates.

To obtain insight into the morphology, we further used depth-dependent component analysis technology to analyze the vertical phase separation of the PTzBIPh/N2200 film processed using the LBL procedure. This method has been successfully applied to confirm the vertical phase segregation in thin film OPVs and organic fieldeffect transistor.27-29 By virtue of the complementary absorptions between PTzBI-Ph and N2200, the sublayer absorption spectra could be distinguished through UV-vis absorption spectroscopy measurement under low-pressure oxygen plasma etching. Consequently, from the evolution at different film-depth of the top surface, middle 8 ACS Paragon Plus Environment

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region, and bottom surface of the film were obtained (Figure 4a and 4b). The corresponding component ratio of sub-layers were also estimated from absorption spectra fitting by pure components according to Lambert-Beer law (Figure 4c and 4d). Although N2200 has a narrower energy gap than PTzBI-Ph, its absorption peaked at approximately 700 nm, demonstrating good overlap with the absorption profile of PTzBI-Ph. Thus, we alternatively focused on the characteristic absorbance peak at approximately 400 nm, which could be attributed to the π-π* transition of N2200, corresponding to a valley of the absorption profile of PTzBI-Ph. As illustrated in Figure 4a and 4c, the PTzBI-Ph/N2200 film exhibited a gradient morphology with a nearly pure donor on the bottom and acceptor enrichment on the top. Both are more favorable for photodiodes with the conventional device structure (bottom electrode as the anode and top electrode as the cathode) than for those with the inverted device structure. In contrast, the blend processed sample (PTzBI-Ph:N2200) demonstrated an obvious PTzBI-Ph absorption peak throughout the film (Figure 4b and 4d) as a result of relatively homogenous distribution of PTzBI-Ph along the vertical direction.



Figure 4. Film-depth dependent light absorption spectra of (a) LBL-processed film (PTzBI-Ph/N2200) and (b) blend-processed film (PTzBI-Ph:N2200). The spectra curves were vertically shifted for clarity. The corresponding component ratio of sublayers in (c) LBL-processed film (PTzBI-Ph/N2200) and (d) blend-processed film (PTzBI-Ph:N2200), estimated from absorption spectra fitting by pure components according to Lambert-Beer law.

With such a vertical phase separation, the all-polymer photodiode based on PTzBIPh and N2200 was fabricated via LBL solution processing, with the device structure of ITO/PEDOT:PSS (10 nm)/PTzBI-Ph (x nm)/N2200 (y nm)/Ca (10 nm)/Al (200 nm). The effect of thickness on device performance was investigated, with relevant data summarized in Table S1 (SI).

Although the devices with > 80 nm-thick PTzBI-Ph 10

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layer or > 60 nm-thick N2200 layer presented higher absorption than those of devices based on thinner films, both external quantum efficiency (EQE) and responsivity (R) values decreased, which can be attributable to the enhanced recombination loss.22 Moreover, compared to the device with a configuration of PTzBI-Ph (80 nm)/N2200 (60 nm), the device with thinner N2200 (40 nm) or PTzBI-Ph (60 nm) exhibited comparable EQE while much higher Jd. The optimized devices showed a broad EQE response from 300 nm to 850 nm. Of particular interest is that the EQE curve has a distinct peak over 50% at approximately 580 nm and decays steeply in the blue and red near-infrared region, providing a specific narrowband photodetecting application. It is worthy of noting that due to the pronounced overlap between the EQE spectrum of the fabricated bilayer device and the emission spectrum of the CsI (Tl)-based scintillator (Figure S3, SI), our device has the potential to be used in X-ray detectors.30-32 The R was calculated as 0.25 A W-1 at approximately 600 nm, which is among the highest values reported for all-polymer photodetectors.33-36 Control devices based on traditional BHJ film were also fabricated with optimized in thickness (Table S1, SI), with a similar device structure of ITO/PEDOT:PSS (10 nm)/PTzBI-Ph:N2200 (2:1, wt:wt)/Ca (10 nm)/Al (200 nm). However, under a bias lower than open-circuit voltage (VOC), the resulting devices presented much lower photocurrent densities (Jph) than those based on the PTzBI-Ph/N2200 bilayer processed using the LBL procedure (Figure 5a), probably due to the unfavorable morphology processed by chlorobenzene rather than MeTHF.37 It was reported that the gradient BHJ photodiodes with cascadelike energy levels would benefit the extraction of charge carriers, yielding higher 11 ACS Paragon Plus Environment


photocurrent.16 In contrast, the dark current density (Jd) at reversed bias of the PTzBIPh:N2200 (2:1, wt:wt) device was much higher than that obtained for the PTzBIPh/N2200 bilayer device, which can be attributable to the lower energy level barriers and the existence of pin-holes.38 Similar results were observed in the all polymer photovoltaics with inverted structure. Sequentially deposited OPVs shown lower dark current but higher photocurrent than the control BHJ device via blend process. The grazing incidence wide angle X-ray scattering analysis of the films indicated that the packing of the polymers was disrupted in the traditional BHJ films relative to that of the neat and sequentially deposited films.20 Moreover, Tsao and Huang et al. also reported that nanostructure and interfacial interaction have a great impact on performance of spray-coated OPDs, especially the Jd was highly related to the phaseseparated BHJ nanostructure of poly(3-hexylthiophene) and [6,6]-phenyl-C61-butyric acid methyl ester.39 The combination of these effects yielded an improvement of two orders of magnitude for the Jph/Jd of the device fabricated with PTzBI-Ph/N2200 compared with that of the PTzBI-Ph:N2200 counterpart device under the same illumination at -2 V (Figure 5a). In addition, the PTzBI-Ph/N2200 device exhibited excellent stability (Figure S4 and Table S2, SI). It is noted that the EQE slightly decreased after encapsulated by epoxy and cover glass then storing in ambient condition for 160 days.


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Figure 5. (a) The J-V characteristics of photodiodes based on PTzBI-Ph/N2200 or PTzBI-Ph:N2200 = 2:1 wt. in the darkness or sunlight simulator. (b) The spectral EQE and R of photodiodes based on PTzBI-Ph/N2200 or PTzBI-Ph:N2200 = 2:1 wt. under -0.1 V bias.

The dark current density (Jd) has a pronounced impact on D*, which is a figure of merit used to characterize the performance of photodetectors. The D* of OPDs is calculated as follows: 𝐷∗ =

𝑅 2𝑞𝐽𝑑

(1)

where R is the responsivity, q is the value of the electron charge, and Jd is the dark current density.40 Generally, Equation 1 assumes that the total noise of the photodetector is dominated by the shot noise in Jd. However, other noises, such as Johnson noise and flicker noise, cannot be ignored. Thus, D* requires an accurate assessment of dark current noise, in. To further investigate the performance of these photodetectors, the noise spectra were measured using a spectrum analyzer. The data are shown in Figure 6a. Corresponding to the tendency of Jd, the noise power density of the photodetector based on PTzBI13 ACS Paragon Plus Environment


Ph:N2200 was more than one order of magnitude higher than that of the one based on PTzBI-Ph/N2200. The noise spectrum followed a 1/f rule from 1 Hz to ~2 kHz, which became more independent of frequencies beyond 2 kHz. The 1/f noise was dominant at low frequencies, whereas frequency-independent type noises, such as shot noise and thermal noise, played a leading role at higher frequencies. According to the definition, D* can be derived as follows:41-43 𝐷 ∗ = 𝑅 × 𝐴 × ∆𝑓

𝑖𝑛2

(2)

where in is the integrated mean square noise spectral density in the device bandwidth, R is the spectral responsivity, A is the device area, and ∆f is the bandwidth. As illustrated in Figure 6b, the D* calculated according to the actual measured noise, including 1/f, shot, and thermal noise, was lower than that calculated from equation 1. Notably, Equation 1 must be used carefully to avoid overestimating D*. The D*max values under -0.1 V derived from Equation 2, assuming ∆f = 1/10/100/1,000 Hz, were 1.92 × 1011/3.95 × 1011/8.21 × 1011/5.68 × 1012 cm Hz1/2 W-1.

Figure 6. (a) The noise spectral density of photodiodes based on PTzBI-Ph/N2200 or 14 ACS Paragon Plus Environment

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PTzBI-Ph:N2200 = 2:1 wt. under -0.1 V bias. (b) The D* of photodiodes based on PTzBI-Ph/N2200 derived from different equations. The noise spectral density vs. frequency was obtained from the Fourier transform of the dark current vs time.

CONCLUSION In summary, we developed a high-performance all-polymer photodetector via an layerby-layer solution process based on the solubility difference between PTzBI-Ph and N2200. With advanced vertical separation and suitable interlayers, the dark current density of the PTzBI-Ph/N2200 photodiode in reverse bias was suppressed, leading to an advanced detectivity of 5 × 1012 cm Hz1/2 W-1. The R reached 0.25 A W-1 at a wavelength of approximately 600 nm, which is among the highest values reported for all-polymer photodetectors. Our photodetector demonstrated a pronounced overlap with the PL (~550 nm) of CsI (Tl) based scintillators, indicating great potential for its application in constructing X-ray detecting and imaging devices. These observations demonstrate that the gradient bulk-heterojunction blends prepared by layer-by-layer solution process can be used to develop high performance photodiodes. We surmise that this strategy may also effective for the construction of high-performance allpolymer photodetectors with inverted structure.

EXPERIMENTAL SECTION Materials: The PFN-Br, PTzBI-Ph, and N2200 were synthesized in our laboratory according to the procedures reported above. The PEDOT:PSS, Clevios™ P VP CH8000 15 ACS Paragon Plus Environment


was purchased from Heraeus Electronic Materials Division and diluted by adding 3 mL DI water to 1 mL PEDOT:PSS. All of the solvents used were purchased from Aldrich. All of the chemicals and materials were purchased and used as received unless otherwise noted. Device Fabrication: ITO-coated glass (Truly Semiconductors Ltd.) with a sheet resistance of 15 Ω to 20 Ω per square was used as the substrate. Before device fabrication, the substrates were thoroughly cleaned in sequence in an ultrasonic bath of acetone, isopropanol, detergent, de-ionized water, and isopropanol and subsequently dried in a baking oven over night. After oxygen plasma treatment for 1 min, a PEDOT:PSS layer (with thickness of about 10 nm) was first spin-coated on the ITO substrate and then baked in N2 at 100°C for 5 min. A 80-nm thick PTzBI-Ph layer was spin-coated onto the PEDOT:PSS from chlorobenzene solution with a concentration of 10 mg mL-1. A 60-nm thick N2200 was then spin-coated onto the PTzBI-Ph layer from the MeTHF solutions with a concentration of 4 mg mL-1. For the blend film, the PTzBIPh:N2200 = 2:1 wt. layer was cast from chlorobenzene solution at a total concentration of 12 mg mL-1. To complete the device, 10 nm Ca and 200 nm Al in sequence were evaporated through a shadow mask to form the top electrode in a vacuum with a pressure below 10-5 Pa. The device active area was 0.0516 mm2. Characterization: The thickness of the organic films was determined by a Dektak 150 surface profiler. An AFM (Bruker Multimode 8) was used to scan the height images in scanasys mode. The absorption spectra of the films were measured using a Shimadzu UV3600. The depth-dependent absorption spectra were acquired according to a 16 ACS Paragon Plus Environment

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previously reported method.27 The Jph-V characteristics of the devices under AM 1.5G (100 mW cm-2) were measured on a Keithley 2400 source-meter. The Jd-V characteristics of the devices were measured on a Keithley 236 source-meter in an electrically and optically shielded box. The EQE spectrum was carried out on a commercial EQE measurement system QE-R3011 (Enlitech Co., Ltd.) calibrated by a standard single crystal Si photovoltaic cell. The noise spectral density characteristics of the devices were recorded by a semiconductor parameter analyzer (Platform Design Automation, Inc. FS380 ProTM) in an electrically and optically shielded box.

ASSOCIATED CONTENT Supporting information The supporting information is available free of charge in the ACS Publication website. The relationship of semi-orthogonal solution processing and device structure. The normalized absorption and PL spectra of PTzBI-Ph, and the normalized absorption spectrum of N2200 in films. The detailed data of device performance. The device performance before and after aged. The overlap between device’s EQE spectrum and CsI (Tl)-based scintillator’s emission spectrum.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (L. Ying) *E-mail: [email protected] (F. Huang) 17 ACS Paragon Plus Environment


Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was financially supported by the National Natural Science Foundation of China (No. 21822505, 21634004, 51521002 and 21574103). Z. Zhong thanks the support from China Postdoctoral Science Foundation (No. 2018M643079).

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