Three-Phase Morphology Evolution in Sequentially ... - ACS Publications

Jan 5, 2018 - Department of Chemical and Biomolecular Engineering, Tandon School of Engineering, New York University, New York, New York. 11201 ...
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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Three-Phase Morphology Evolution in Sequentially SolutionProcessed Polymer Photodetector: Toward Low Dark Current and High Photodetectivity Hanyu Wang,†,‡ Shen Xing,† Yifan Zheng,†,‡ Jaemin Kong,‡,§ Junsheng Yu,*,† and André D. Taylor*,‡,§ †

State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Optoelectronic Information, University of Electronic Science and Technology of China (UESTC), Chengdu 610054, P. R. China ‡ Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06511, United States § Department of Chemical and Biomolecular Engineering, Tandon School of Engineering, New York University, New York, New York 11201, United States S Supporting Information *

ABSTRACT: Sequentially solution-processed polymer photodetectors (SSP PPDs) based on poly(3-hexylthiophene-2,5-diyl) (P3HT)/[6,6]-phenyl C71butyric acid methyl ester (PC71BM) are fabricated by depositing the top layers of PC71BM from an appropriate cosolvent of 2-chlorophenol (2-CP)/ o-dichlorobenzene (ODCB) onto the predeposited bottom layers of P3HT. By adjusting the ratio of 2-CP/ODCB in the top PC71BM layers, the resulting SSP PPD shows a decreased dark current and an increased photocurrent, leading to a maximum detectivity of 1.23 × 1012 Jones at a wavelength of 550 nm. This value is 5.3-fold higher than that of the conventional bulk heterojunction PPD. Morphology studies reveal that the PC71BM partially penetrates the predeposited P3HT layer during the spincoating process, resulting in an optimal three-phase morphology with one well-mixed interdiffusion P3HT/PC71BM phase in the middle of the bulk and two pure phases of P3HT and PC71BM at the two electrode sides. We show that the pure phases form high Schottky barriers (>2.0 eV) at the active layer/electrodes interface and efficiently block unfavorable reverse charge carrier injection by significantly decreasing the dark current. The interdiffussion phase enlarges the donor−acceptor interfacial area leading to a large photocurrent. We also reveal that the improved performance of SSP PPDs is also due to the enhanced optical absorption, improved P3HT crystallinity, increased charge carrier mobilities, and suppressed bimolecular recombination. KEYWORDS: polymer photodetectors (PPDs), P3HT/PC71BM, sequentially solution-processed, three-phase morphology, cosolvent

1. INTRODUCTION

(ITO)/poly(3,4-ethylen edioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/poly[5,7-bis(4-decanyl-2thienyl) thieno[3,4-b]diathiazole-thiophene-2,5] PDDTT: [6,6]-phenyl C61 butyric acid methyl ester (PC61BM)/Ba/Al, which demonstrated a high Jd of about 1 × 10−5 A/cm2 under a reverse bias of −2 V.14 Because both donor and acceptor phases are in contact with the same electrode in a normal BHJ PPD, the undesirable charge carrier injection, such as electron injection from the ITO anode to the acceptor or hole injection from the Al cathode to the donor, would decrease the D* of the PPDs by improving Jd.15 Previous efforts to suppress Jd are generally confined to inserting charge-blocking layers at the active layer/electrodes

Polymer photodetectors (PPDs) have attracted increased attention owing to the following substantial advantages over their inorganic counterparts: lower fabrication costs, large-area fabrication, tunable response spectra, lightweight, and flexibility.1−5 Research in recent years has primarily focused on solution-processed PPDs with high external quantum efficiency (EQE),6,7 low dark current density (Jd),8,9 high photodetectivity (D*),10 and wide and narrow response spectra.11,12 Similar to polymer solar cells, the bulk heterojunction (BHJ) structure, consisting of a conjugated polymer and a fullerene derivative as the electron donor and acceptor respectively,13 is often utilized to construct PPDs. The BHJ structure favors exciton diffusion and charge carrier separation, contributing to high EQE and photoresponsivity. However, the BHJ PPDs usually suffer a comparatively high Jd. For example, Cao et al. fabricated a BHJ PPD with a structure of indium tin oxide © XXXX American Chemical Society

Received: October 16, 2017 Accepted: January 5, 2018

A

DOI: 10.1021/acsami.7b15730 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) Schematic representation of the fabrication of SSP PPD with three-phase morphology induced by solvent swelling and interdiffusion, along with molecular structures of (b) P3HT, (c) PC71BM, (d) ODCB, and (e) 2-CP. SSP-0, SSP-5, SSP-10, and SSP-20% present the four SSP active layers that are prepared using different ODCB ratios in 2-CP (from 0 to 20%).

interfaces,16 molecular side-chain engineering,17 thickening the BHJ active layers,18 and fabricating bilayer or multilayer devices.19 For example, Zhou et al. improved the performance of BHJ PPDs by employing poly[N,N′-bis(4-butylphenyl)N,N′-bis(phenyl)-benzidine] (poly-TPD) as the electronblocking buffer layer, which can effectively reduce the Jd to 6.0 × 10−10 A/cm2 under a reverse bias of −0.5 V. Thus, the PPD exhibited a photodetection higher than 1.0 × 1013 Jones.20 However, these approaches often require complex processing and could be incompatible in other systems. Moreover, the reported approaches to suppress Jd usually cause a low charge injection efficiency under illumination, which would reduce the photocurrent density (Jph) at the same time.16,21 In addition to these above methods, sequential solutionprocessing to prepare the active layer by subsequent spincoating the donor and acceptor materials were developed.22−25 Because only one kind of layer (donor or acceptor layer) in the sequentially solution-processed (SSP) active layer contacts the electrode (anode or cathode), the SSP active layer effectively suppressed undesirable charge carrier injection when compared with the conventional BHJ counterpart. As is well known, the ratio of Jph/Jd determines the photodetectivity of PPD, in which a high Jph and a low Jd could make contribution to the ideal PPD. Previously, Shafidah et al. successfully reduced the Jd of PPD from 2.93 × 10−6 to 1.26 × 10−7 A/cm2 at −1 V by using a SSP structure. However, Jph of PPD in their case decreased from 8.32 × 10−3 to 3.00 × 10−3 A/cm2 due to the low exciton dissociation efficiency caused by the limited donor−acceptor (D−A) interfacial area in the common SSP active layer. This in turn limited the PPD photodetectivity.26 We suggest a new PPD architecture in which a SSP structure with a three-phase morphology of donor phase/D−A interdiffusion phase/acceptor phase carefully optimizes the D−A interfaces. Such a system is thought to be beneficial toward realizing a high-performance PPD with a low Jd and a high Jph. In this work, we design the SSP PPDs based on poly(3hexylthiophene-2,5-diyl) (P3HT) as the donor and [6,6]phenyl C71-butyric acid methyl ester (PC71BM) as the accepter fabricated by spin-casting a PC71BM layer that is dissolved in a cosolvent with different proportions of o-dichlorobenzene (ODCB) in 2-chlorophenol (2-CP) on the top of predeposited P3HT bottom layer. The parameters of PPD including Jd, Jph, EQE, and D* are investigated. The D* of the SSP PPD is significantly improved when compared with that of the BHJ PPD owing to the suppressed Jd and improved Jph. The

nanostructure morphology of P3HT/PC71BM interface in the SSP active layer is elucidated by atomic force microscopy (AFM) and X-ray diffraction (XRD). The charge carrier transport abilities are carried out by using a theoretical simulation of space-charge-limited current. Furthermore, the impedance spectroscopy is investigated to provide the resistance components of PPDs under dark condition.

2. EXPERIMENTAL SECTION The device structure is ITO/PEDOT:PSS (30 nm)/P3HT:PC71BM (120 nm)/Mg:Ag (100 nm) as shown in Figure 1. Before starting the PPD fabrication, the ITO substrates were cleaned by following our previous reported work.27 Briefly, after drying, the PEDOT:PSS was spin-coated on the UV-light-treated ITO substrates. After thermal annealing at 130 °C for 30 min, the substrates were moved into a highpurity glovebox (O2, H2O < 1 ppm). The photoactive layers were prepared by the BHJ and SSP processes. For the BHJ films, a solution mixture of P3HT (15 mg)/PC71BM (15 mg) in 1 mL of ODCB was spin-coated above the PEDOT:PSS layer, and then the photoactive layers were annealed at 140 °C for 10 min. For the SSP films, the P3HT solution (7 mg/mL in ODCB) was spin-coated on the top of PEDOT:PSS layer. Because of the high PC71BM solubility and low P3HT solubility of 2-CP,28 it was chosen as the main solvent of PC71BM without the dissolution of the P3HT underlayer. The PC71BM solution of 28 mg/mL with 0−20% cosolvent of ODCB in 2CP was directly dropped onto the P3HT layer. After swelling and interdiffusion for 0.5 min to develop a nanostructure in the film, PC71BM was spin-coated and then the SSP active layers were annealed at 140 °C for 10 min. Finally, Mg/Ag (9:1, w/w) as cathode via a metal shadow mask was deposited onto the substrates at a deposition speed of about 10 Å/s under high vacuum (3.0 × 10−5 Torr). The current density−voltage (J−V) curves were characterized by a Keithley 4200A-SCS Parameter Analyzer. The EQE spectra under the short-circuit and different reverse bias conditions were obtained under a xenon lamp light passing through a monochromator, which was calibrated by a standard silicon solar cell.27 The ultraviolet−visible (UV−vis) absorption spectra were recorded by a Shimazu UV-1700 Spectrophotometer. The active layers prepared for AFM (Agilent, AFM 5500) and XRD (RIGAKU, D/MAX-RC) measurements were the same with the active layers used for device fabrication. The impedance spectra were obtained by an Agilent Impedance Analyzer. Detailed characterizations of PPDs are described in the Supporting Information.

3. RESULTS AND DISCUSSION 3.1. Photodetective Performance. We illustrate the schematic representation of the SSP PPD fabrication and molecular chemical structures used in this work (Figure 1). B

DOI: 10.1021/acsami.7b15730 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 2. (a) Semi-log J−V characteristics of PPDs utilizing BHJ and SSP (with 0−20% ODCB in 2-CP) active layers in dark and under AM 1.5 light (100 mW/cm2) illumination, (b) Jd and on/off current ratio at −0.5 V of PPDs with BHJ and SSP active layers, (c) UV−vis absorption spectra of BHJ and SSP active layers, (d) measured EQE spectra of BHJ and SSP PPDs at bias of 0 V and −0.5 V, and (e) calculated R and (f) D* of BHJ and SSP PPDs at −0.5 V.

We can see from Table 1 that SSP-0% PPD shows the lowest Jph and on/off current ratio among the SSP PPDs, which should be ascribed to two reasons. (1) The thick P3HT layer (80 nm) in SSP-0% PPD, which is significantly larger than the exciton diffusion length of 20 nm.29 Therefore, the excitons generated in the SSP-0% active layer prefer to recombine, rather than separating into free charge carriers, causing insufficient charge transport and collection in the SSP-0% active layer. (2) The insufficient interdiffusion at the P3HT/PC71BM interface leads to a decrease in the exciton dissociation efficiency. We note that the on/off current ratio of SSP-10% PPD is 18-fold higher than that of the BHJ PPD. This is mainly attributed to the effective suppression of Jd and enhancement of Jph in SSP-10% PPD. To investigate the effect of swelling time of the PC71BM solution on the performance of the SSP PPDs, apart from the ODCB ratio in 2-CP, we compare the performance of SSP-10% PPDs with different swelling time from 0.5, 1, 2, to 3 min, as shown in Figure S2, and the relative values including Jph, Jd, and on/off current ratio at −0.5 V are listed in Table S1. The on/off current ratio of all of the PPDs are between 3.86 × 104 and 3.93 × 104. This result implies that the wide window time of this method could be adequate for mass production. We show the UV−vis absorption spectra (Figure 2c). Both the BHJ and the SSP active layers exhibit a PC71BM absorption peak at 380 nm and three absorption peaks for P3HT at 510, 550, and 600 nm, which correspond to the π−π* stacking of the P3HT main chain.30 The absorptions of SSP-0 and SSP10% active layers show enhanced vibronic peaks at 550 and 600 nm when compared with that of the BHJ active layer. This suggests that P3HT in the SSP active layer has a highly ordered structure.31 Because the aggregated PC71BM in the BHJ active layer prevents the continuously ordered packing of P3HT, the highly ordered P3HT is not usually observed in a common BHJ structure.

Four types of SSP active layers, which are denoted as SSP-0, SSP-5, SSP-10, and SSP-20%, are prepared using different ODCB ratios in 2-CP (from 0 to 20%). The semi-log J−V characteristics of the BHJ PPD and the SSP PPDs under light and dark conditions are presented in Figure 2a, and the values including Jph, Jd, on/off current ratio, and shunt resistance (Rsh) are listed in Table 1. The SSP-0% Table 1. Photocurrent Density, Dark Current Density, On/ Off Current Ratio, and Shunt Resistance of BHJ PPD and SSP PPDs at −0.5 V PPDs BHJ SSP-0% SSP-5% SSP-10% SSP-20%

Jph (A/cm2) 8.94 4.73 7.71 9.65 5.83

× × × × ×

10−3 10−3 10−3 10−3 10−3

Jd (A/cm2) 4.12 1.39 2.15 2.48 9.35

× × × × ×

10−6 10−7 10−7 10−7 10−6

on/off current ratio 2.17 3.40 3.59 3.89 6.24

× × × × ×

103 104 104 104 102

Rsh (Ω) 2.14 2.31 1.56 1.12 1.09

× × × × ×

106 107 107 107 106

PPD shows the lowest Jd of 1.39 × 10−7 A/cm2, which is decreased by more than 1 order of magnitude when compared with that of the BHJ PPD. The reduction in Jd is in correspondence with the change in Rsh, which increases from 2.14 × 106 to 2.31 × 107 Ω. As the ODCB ratio is increased to 5 and 10%, the Jd values keep the same magnitude with that of SSP-0% PPD. When the ODCB ratio is 20%, the Jd increases sharply to 9.35 × 10−6 A/cm2, which may be caused by excessive interdiffusion of the PC71BM, leading to the unfavorable charge carrier injection at the electrode/active layer interfaces. The J−V characteristics of PPDs under light are shown in Figure S1. All of the devices exhibit a slightly lower Jph at zero-biased condition. The Jph notably increases along with the reverse bias because of the increased charge extraction under an external electric field. C

DOI: 10.1021/acsami.7b15730 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 3. AFM height images (1 μm × 1 μm) of (a) as-cast P3HT film, (b) an P3HT film onto which a PC71BM layer had been spun from 2-CP and then subsequently removed by spin-coating the bilayer with 2-CP, (c) an P3HT film onto which a PC71BM layer had been spun from 2-CP/ODCB (9:1) and then subsequently removed by spin-coating the bilayer with 2-CP. The corresponding three-dimensional (3D) images are (d)−(f). The corresponding film configurations are (g)−(i).

interdiffusion between P3HT and PC71BM after the incorporation of 10% ODCB in 2-CP. The R is used to determine the conversion ratio from photons to charge carriers, as shown in Figure 2e and Table S3. The tendency of R corresponds to that of the EQE. The R values of SSP-10% PPD are 0.147, 0.302, and 0.246 A/W at 380, 550, and 620 nm, respectively, which are all larger than those of SSP-0% PPD. Compared with BHJ PPD, SSP-10% PPD exhibits a higher R in the range of 480−650 nm. As the most important figure of merit, D* is employed to determine the sensitivity of PPD. The calculated results are shown in Figure 2f and Table S4. SSP-10% PPD exhibits the highest D* in the wavelength range between 350 and 650 nm. The D* of SSP-10% PPD at 550 nm is 1.23 × 1012 Jones, which is 5.3-fold higher than the D* of the BHJ counterpart. The significant enhancement in D* is caused by significant suppression of Jd and improvement of Jph. As shown in Figure 2f, D* is higher than 5.00 × 1011 Jones in the spectral region of 360−640 nm, which demonstrates a high detection potential of the SSP PPD in the UV−vis range. 3.2. Morphology of SSP Active Layers. AFM is employed to investigate the nanostructure morphology at the P3HT/PC71BM interfaces of the SSP active layers. We present the AFM height images (tapping-mode) of the as-cast P3HT film and the P3HT layer on which the initially deposited PC71BM layer was removed in Figure 3. The PC71BM overlayer is removed by spin-coating with 2-CP solvent for three times. As shown in Figure S3, we find no spectroscopic evidence for any remaining PC71BM following such a treatment. It also can be seen from the X-ray photoelectron spectroscopy (XPS) spectra of P3HT S 2p peak (Figure S4) that the PC71BM overlayer is fully removed by 2-CP. The surface morphology of

As an important parameter in PPDs, EQE is defined as the rate between the numbers of charge carriers and incident photons. We show the EQE spectra of BHJ, SSP-0%, and SSP10% PPDs at the bias voltages of 0 and −0.5 V (Figure 2d) and list the EQE values of PPDs for different colors of lights at −0.5 V (Table S2). The EQE values of all the PPDs improved with the reverse bias. Considering the unbalance charge carrier transport of P3HT and PC71BM,32 a significant recombination of photogenerated charge carrier is expected for all of the PPDs at 0 V. Thus, applying a reverse bias of −0.5 V could reduce the charge recombination, resulting in the enhancement of charge carrier collection of PPDs. We observe that the EQE spectra shape keeps almost the same with the UV−vis absorption spectra. The EQE values for the BHJ PPD exceed 50% in the region between 400 and 600 nm, and the maximum EQE value is 65.5% at 540 nm. The EQE values of SSP-0% PPD are much lower than those of the BHJ PPD at 0 and −0.5 V. Because of insufficient interdiffusion at the P3HT/PC71BM interface and a thick P3HT layer as discussed above, the photogenerated charge carriers would be transported and collected inefficiently, leading to the low EQE values of SSP-0% PPD. In contrast, although the shape of the EQE spectrum for SSP-10% PPD is almost the same as that for the BHJ PPD in the wavelength region of 300−500 nm, the SSP-10% PPD shows enhanced EQE values in the wavelength region of 480− 650 nm, which is in accordance with the changes in UV−vis absorption spectra. The maximum EQE value of 68.6% is obtained at a wavelength of 545 nm for SSP-10% PPD, which is 1.96-fold larger than that obtained for SSP-0% PPD. The reason for the enhancement may be the appropriate D

DOI: 10.1021/acsami.7b15730 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces the as-cast P3HT film is presented in Figure 3a, and Figure 3d is the corresponding three-dimensional (3D) image. The ascast P3HT film has a smooth surface with a maximum peak-tovalley height of 12.5 nm and a root-mean-square (RMS) roughness of 1.17 nm. As for the interface of SSP-0% active layer, no great change is observed in the surface topography of P3HT, as shown in Figure 3b,e. This result further indicates that the interdiffusion of PC71BM into the P3HT layer does not occur during the formation of the SSP active layer. We show the nanostructure morphology of the P3HT/ PC71BM interface of the SSP-10% active layer (Figure 3c). After adding 10% ODCB into the PC71BM solution, the maximum peak-to-valley height and the RMS roughness are increased by 4.2- and 4.8-folds to 52 and 5.61 nm, respectively. The rougher texture is due to the interdiffusion of PC71BM into the P3HT underlayer. After the removal of PC71BM, a large number of craters and valleys left in the P3HT matrix can be observed in the 3D AFM image of Figure 3f. Compared with the SSP-0% active layer, the SSP-10% active layer has smaller P3HT grains, which are about 18 ± 3 nm. The decrease in the P3HT grain size can not only increase the P3HT/PC71BM interfacial area but also decrease the exciton recombination ratio. We illustrate the schematic device configurations of the SSP-0 and SSP-10% PPDs, respectively (Figure 3g−i). To further investigate the effect of swelling time on the morphology of the SSP active layers, the AFM height images of SSP-10% P3HT underlayers with different swelling times are measured, and the relative two-dimensional and 3D images are shown in Figure S5. The values of RMS roughness for P3HT underlayers with swelling times for 0.5, 1, 2, and 3 min are 5.61, 5.64, 5.62, and 5.64 nm, respectively. The little change in the RMS roughness with the extension of swelling time shows that the swelling time has negligible influence on the morphology of the SSP-active layers. As depicted in the corresponding 3D images in Figure S5, the maximum peak-to-valley heights of all of the films only fluctuate in a small range of 52−54 nm. This result also confirms that the swelling time will not cause deviations in the morphology evolution of the SSP active layers. To investigate the changes in P3HT packing in the SSP active layers, we utilize XRD to characterize P3HT crystallinity. In Figure 4, we observe an α-axis orientation at a peak of 2θ near 5.34°, corresponding to the (100) crystal plane.33,34 The π-conjugated P3HT molecule in P3HT/PC71BM film exhibits a reflection peak at 5.34°, with an intensity of 19 000. However, PC71BM removed P3HT film shares the same intensity of ∼31 000 with the pristine P3HT film, at the peak of 5.34°, indicating that the SSP structure could promote the ordered packing of P3HT. In addition, the same intensities of pristine P3HT and PC71BM removed P3HT films indicate that the interpenetration of PC71BM does not hinder the packing of the P3HT underlayer. This result corresponds well with the formation of a continuous hole transport passway, resulting in the improvement of Jph. 3.3. Single-Carrier Devices. To better understand the device characteristics, charge carrier transport behaviors of single-carrier devices are characterized under forward (hole injected by bottom Ag electrode, electron injected by top Mg/ Ag electrode) or reverse (hole injected by top Ag electrode, electron injected by bottom Mg/Ag electrode) bias, as presented in the schematic energy level diagrams in Figure S6. The relative double-logarithmic J−V curves are depicted in Figure 5a,b, and the calculated mobilities are listed in Table 2. Under forward bias, the hole and electron mobilities of the SSP

Figure 4. XRD images of (a) P3HT/PC71BM BHJ film, (b) pristine P3HT film, and (c) an P3HT film onto which a PC71BM layer had been spun from 2-CP/ODCB (9:1) and then subsequently removed by spin-coating the bilayer with 2-CP, which is marked as “PC71BM removed P3HT” in the inset.

devices are higher than those of the BHJ devices, which implies the formation of continuous hole and electron transport phases near the anode and cathode, respectively, in the devices with the SSP-10% active layer. We also highlight that the charge carrier transport in the SSP-10% device is more balanced than that in the BHJ device, leading to a comparatively high Jph. In the condition of a forward bias, holes and electrons are injected from the bottom Ag and the top Mg/Ag, respectively. On the contrary, the holes injection from the top Ag and electrons injection from the bottom Mg/Ag would happen in the reverse bias condition. According to the energy-level diagrams shown in Figure S6b, the holes could be injected comparatively easily from the bottom Ag to the P3HT highest occupied molecular orbital (HOMO) if more P3HT polymers are gathered near the bottom Ag, and accordingly, the electrons injection from the top Mg/Ag to the PC71BM lowest unoccupied molecular orbital (LUMO) should be much easier when more PC71BM enriched at the top Mg/Ag. The single-logarithmic J−V curves of the hole-only and electron-only devices are shown in the inset of Figure 5a,b. We can see that the J−V characteristics of the BHJ devices demonstrate similar features under both forward and reverse conditions. This result indicates that P3HT and PC71BM should be uniformly distributed in the BHJ active layer. However, the J−V characteristics of the SSP-10% devices under forward and reverse biases present obvious different features, implying that P3HT and PC71BM are enriched near the bottom and top surfaces, respectively. Thus, P3HT in SSP-10% PPD can effectively block the unfavorable reverse injection of electrons from Ag to PC71BM LUMO, resulting in the reduction of Jd. E

DOI: 10.1021/acsami.7b15730 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 5. Double-logarithmic J−V curves of (a) hole-only and (b) electron-only devices with the BHJ and the SSP-10% active layers under bottom and top injection. The inserted images are the single-logarithmic J−V curves of the corresponding devices. (c) Impedance spectra of BHJ and SSP PPDs. The inset shows the equivalent circuit used to model the impedance. (d) The linear dynamic range of BHJ and SSP devices at −0.5 V.

Table 2. Hole Mobility and Electron Mobility of the HoleOnly and Electron-Only Devices under Forward and Reverse Biases active layers BHJ forward bias BHJ reverse bias SSP-10% forward bias SSP-10% reverse bias

μh (cm2 V−1 s−1) 1.13 1.06 3.44 2.61

× × × ×

−4

10 10−4 10−4 10−5

Table 3. Electrical Parameters from the Impedance Value of PPDsa

μe (cm2 V−1 s−1) 2.87 2.65 3.02 1.43

× × × ×

−4

10 10−4 10−4 10−5 a

3.4. Impedance Analysis and Linear Dynamic Range Properties. To analyze the electrical behaviors of PPDs, the impedance spectroscopy is measured under dark condition, as shown in Figure 5c. We use the equivalent circuit in the inset of Figure 5c as a model. R1 is a series resistance coming from the active layer, the electrode, and the organic semiconductor/ metal electrode interface. C1 models the device’s dielectric effect. R2 originates from the parallel of the diode dynamical resistance and shunt resistance.16 Therefore, it contains characteristics of materials including charge carrier mobility and leakage current.35 The equivalent circuit is presented as eq 1 Z = R1 + 1/(1/R 2 + iωC1)

PPDs

R1 (Ω)

BHJ SSP-0% SSP-5% SSP-10% SSP-20%

25.1 25.4 25.1 25.2 25.0

R2 (Ω) 6.67 2.34 2.01 1.88 3.21

× × × × ×

105 106 106 106 105

C1 (F) 7.54 7.23 6.98 7.47 6.99

× × × × ×

10−10 10−10 10−10 10−10 10−10

The inset shows the equivalent circuit used to model the impedance.

that of the BHJ device. As R2 is related to the leakage current, these results imply that the SSP structure is in favor of the suppression of the leakage current. In addition, Jd is mostly decreased by the blocking effect of P3HT and PC71BM phases at the electrode/active layer interfaces and attributed to the suppression of unfavorable charge carrier injection, as shown in the dark J−V curves (see Figure 2a). Regarding the capacitance of C1, the values of C1 remains unchanged for all of the PPDs. This result means that the holes and electrons can be effectively collected by the corresponding electrodes without any accumulation. We show the dependence of Jph on Llight of PPDs, in which the slope is 0.966, 0.973, and 0.986 for BHJ, SSP-0%, and SSP10% PPDs, respectively (Figure 5d). The results indicate that the SSP structure is attributed to the reduction of bimolecular recombination. Thus, the photogenerated charge carriers can be effectively collected by the electrodes before recombination in

(1)

We list the electrical parameters of R1, R2, and C1 in Table 3 through fitting the curves according to the equivalent model. The values of R2 for SSP-0, SSP-5, and SSP-10% devices are all greater than 1.00 × 106 Ω, which are significantly larger than F

DOI: 10.1021/acsami.7b15730 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 6. Schematic band diagrams of (a) BHJ PPD and (b) SSP PPD in a reverse charge collection mode.

effectively block the unfavorable reverse charge carrier injections. Thus, when operated under reverse bias, the SSP PPD can simultaneously achieve a high photo response because of the Ohmic contacts for charge extractions and collections, and simultaneously, obtain a low Jd due to the high Schottky barriers for reducing unfavorable reverse charge carrier injection.

the SSP devices. The high slope of 0.986 for SSP-10% PPD implies effective charge carrier transport properties with a low charge carrier trap density and an appreciable charge carrier generation in the pure P3HT and PC71BM phases.36,37 3.5. Detail Analysis of the Device Dark Current. To understand the enhancement mechanism of SSP PPDs, we investigate the schematic band diagrams of BHJ and SSP PPDs in the reverse charge injection mode (Figure 6). Because PPDs are generally operated under a reverse bias, the Jd is mainly determined by the charge carrier injection from the electrodes and the charge carrier transport in the active layers.13,38 The reverse charge injection is determined by the Schottky barriers at the active layer/electrode interfaces, and the J−V characteristics of Schottky diode in dark condition can be expressed by the Shockley equation as eq 238 ⎧ ⎡ q(V + IR ) ⎤ ⎫ s Jd = Js ⎨exp⎢ ⎥ − 1⎬ ⎦ nkT ⎩ ⎣ ⎭

4. CONCLUSIONS In summary, we demonstrate highly efficient SSP PPDs with the P3HT and PC71BM fabricated by sequentially spin-casting a PC71BM top layer from the cosolvent with the optimized concentration onto the predeposited P3HT bottom layer. In this process, the PC71BM top layer can effectively penetrate the P3HT bottom layer and form an optimal three-phase morphology with one well-mixed interdiffusion P3HT/ PC71BM phase for efficient charge separation and two pure phases of P3HT and PC71BM for blocking unfavorable reverse charge carrier injection. As a result, SSP-10% PPD shows a suppressed dark current and increased photocurrent, leading to a maximum D* of 1.23 × 1012 Jones with 5.3 times improvement compared with its BHJ counterpart. In addition to inducing an optimal three-phase morphology, we reveal that the SSP structure could also (i) enhance the overall optical absorption of the film, (ii) induce an effective charge carrier transport pathway by improving the crystallinity and charge carrier mobility of P3HT, and (iii) suppress bimolecular recombination. These results suggest a new method for optimizing nanostructure morphologies toward the fabrication of high-performance PPDs with a low Jd and a high D*.

(2)

where I is the current in the device, Js is the reverse saturation dark current density, Rs is the series resistance, and n is the ideality factor. The Js is a function of eq 3 ⎛ ϕ′ ⎞ Js = A*T 2 exp⎜ − B1 ⎟ ⎝ kBT ⎠

(3)

where A* is the Richardson constant and ϕB is the injection barrier height, which is defined as the difference between the work function of electrode and the energy level of active layer.38−40 We illustrate that Ohmic contacts are formed at the active layer/metal electrodes interfaces for BHJ and SSP PPDs; therefore, the PPDs should be Schottky diodes (Figure 6a,b). In the BHJ PPD, the electrons are injected from the ITO/ PEDOT:PSS to the LUMO of PC71BM and the hole injection takes place from Mg/Ag to the HOMO of P3HT under reverse bias. The electrons and holes injection barriers of ϕB1 and ϕB2 in BHJ PPD are both 1.3 eV, as shown in Figure 6a. As for the SSP PPD, because the P3HT and PC71BM phases are formed in the corresponding electrodes, the increased Schottky barriers (ϕB1′ = 2.0 eV, and ϕB2′ = 2.4 eV) at the two electrodes can

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b15730. Detailed description of the photodetector characterization, including responsivity, detectivity, and singlecarrier mobility; figures showing J−V characteristics of PPDs under AM 1.5 light illumination, J−V characterG

DOI: 10.1021/acsami.7b15730 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

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istics of SSP-10% PPDs with different swelling time from 0.5, 1, 2, to 3 min under dark and light conditions, UV− vis absorption spectra of pristine and PC71BM removed P3HT films, and XPS spectra of the S 2p peaks for pristine P3HT, SSP P3HT/PC71BM, and PC71BM removed P3HT films, AFM height images (1 μm × 1 μm) of SSP-10% P3HT under layers with different soaking times of 0.5, 1, 2, and 3 min, schematic energylevel diagrams of BHJ and SSP single-carrier devices; tables showing Jph, Jd, and on/off current ratio of SSP10% PPDs with different swelling time at −0.5 V, and EQE, responsivity, as well as detectivity of PPDs to UV, blue, green, and red lights at −0.5 V (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.Y.). *E-mail: [email protected] (A.D.T.). ORCID

Hanyu Wang: 0000-0003-2170-4536 Jaemin Kong: 0000-0002-6496-9879 Junsheng Yu: 0000-0002-7484-8114 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was funded by the Foundation for Innovation Research Groups of the National Natural Science Foundation of China (NSFC) (Grant No. 61421002), the NSFC (Grant Nos. 61675041 and 61177032), the Project of Science and Technology of Sichuan Province (Grant No. 2016HH0027). The authors gratefully acknowledge the China Scholarship Council (No. 201606070043), the National Science Foundation (DMR-1410171), NSF-PECASE award (CBET-0954985), and Yale West Campus Materials Characterization Core for partial support of this work. The author also thanks the assistance of Dr. Min Li (Yale West Campus Materials Characterization Core) for the XRD measurement. Parts of this research was carried out at the Center for Functional Nanomaterials, Brookhaven, National Laboratory, which is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886.

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DOI: 10.1021/acsami.7b15730 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX