Visible-Light-Responsive High-Detectivity Organic Photodetectors with

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Visible-light-responsive high-detectivity organic photodetectors with a 1-µm-thick active layer Jong Baek Park, Jong-Woon Ha, Sung Cheol Yoon, Changjin Lee, In Hwan Jung, and Do-Hoon Hwang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b13550 • Publication Date (Web): 17 Oct 2018 Downloaded from http://pubs.acs.org on October 20, 2018

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ACS Applied Materials & Interfaces

Visible-light-responsive high-detectivity organic photodetectors with a 1-μm-thick active layer Jong Baek Parka‡, Jong-Woon Haa‡, Sung Cheol Yoonb,c, Changjin Lee b,c, In Hwan Jungd*, and Do-Hoon Hwanga* a Department

of Chemistry, Pusan National University, 2 Busandaehak-ro, Geumjeong-gu, Busan 46241, Republic of Korea

b Division

of Advanced Materials, Korea Research Institute of Chemical Technology (KRICT), 141 Gajeong-ro, Yuseong-gu, Daejeon 34114, Republic of Korea

c

Department of Chemical Convergence Materials, University of Science and Technology (UST), 217 Gajeong-ro, Yuseong-gu, Daejeon 34113, Republic of Korea d Department

of Applied Chemistry, Kookmin University, 77 Jeongneung-ro, Seongbuk-gu, Seoul 02707, Republic of Korea

KEYWORDS: TPD-based polymer, organic photodetector, organic photodiode, low dark current, high responsivity

ACS Paragon Plus Environment

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ABSTRACT

Organic photodetectors (OPDs) are attracting attention for use in flexible and portable electronic applications such as image sensors, remote sensing, optical communications, and medical sensors because of their strong photon responsivity in thin films over a broad range of wavelengths. In particular, the efficient photon-to-current conversion of OPDs under visible light allows their use in indirect X-ray detectors using scintillators to convert X-rays to visible light. The polymer PBDTT-8ttTPD shows strong absorption bands in the region of 500–650 nm, as well as high hole mobility, which provides excellent photoresponsivity and photon-to-current conversion efficiency. A p-n junction photodetector was fabricated by blending PBDTT-8ttTPD and PC71BM and varying the thickness of the active layer (260–1100 nm). The PBDTT8ttTPD:PC71BM-based OPDs show promising photodetecting properties having a low dark current of 3.72 × 10-9 A cm-2 and high responsivity of 0.39 A W-1 because of the well-controlled morphology, high molar absorption coefficient, and excellent carrier mobility of the PBDTT8ttTPD:PC71BM layer. Consequently, the specific detectivity of the PBDTT-8ttTPD-based OPD devices was 1.13 × 1013 Jones at -2 V on irradiation with a light emitting diode (530 nm wavelength) with a power density of 55.6 μW cm-2.


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INTRODUCTION In the last few decades, organic-semiconductor-based electronic devices such as organic photovoltaics (OPVs), organic light emitting diodes (OLEDs), organic photodetectors (OPDs), and organic thin film transistors (OTFT) have been developed for use in portable and flexible devices.1–4 Recently, OPDs have drawn attention because they could replace heavy and thick silicon materials and have unique advantages, such as low production costs based on solution processing, high yields of photogenerated charge carriers, and easily tunable optical gaps for light absorption.5–8 OPDs can be divided into photodiode and phototransistor types. The photodiode type of OPD is similar to an OPV, being composed of two electrodes (bottom and top), a photoactive layer, and interfacial layers. The photodiode type of OPD has a fast response time to light but gives a low output signal. In contrast, the phototransistor type of OPD is similar to an OTFT, consisting of three electrodes (source, drain, and gate), photoactive layer, and dielectric. The phototransistor has a relatively slow response time to light but provides an amplified output signal.1,9,10 OPDs have broad applications in optical communications, image sensors, biomedical sensors, machine vision, remote controls, and control circuits.11–19 Importantly, OPDs can be utilized effectively as portable X-ray detectors in emergency or rescue situations because of their light weight and flexibility. X-ray detectors mostly use indirect detection, which involves the conversion of X-rays to visible light using a scintillator and the detection of the converted light using photodiodes and a thin-film transistor (TFT) backplane. In the detection process, the X-ray beam passes through the scintillator, such as GOS:Tb and CsI(Tl), producing green light (530– 560 nm), and the photodiode array detects the amplified light.5 Thus, the development of widebandgap organic semiconductors showing excellent photosensitivity in this wavelength region is


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urgently required.20–24 The photodiode array generates electrons and holes under reverse bias or driving bias. For high-performance OPDs, the OPD devices should have a suitable spectral bandwidth, low dark current, high responsivity (R), high frequency response, and reasonable linear dynamic range.25–28 However, it is challenging to maximize all the OPD properties by device engineering because there is a trade-off relationship between the R and dark current with film thickness.21,22,29,30 The photocurrent and external quantum efficiency (EQE) are related to the R, which is optimized at a specific active layer thickness. In contrast, the dark current is reduced as the thickness of the active layer is increased. Therefore, the development of organic semiconductors having both excellent charge current densities and low dark currents, even in thick films, is crucial. In this study, we have developed high R and high detectivity (D*) OPDs via synthesizing thieno[3,4-c]pyrrole-4,6-(5H)-dione (TPD)-based wide bandgap polymers which can replace the commonly used OPD materials such as Poly(3-hexylthiophene-2,5-diyl) (P3HT) or Poly[N-9'heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT)20,22. Previously, we reported TPD-based wide bandgap polymers with high power conversion efficiency for organic photovoltaic devices.31,32 TPD is a heterocyclic electron-withdrawing core with rigid and planar backbone structure and the alkyl side groups on TPD enable to possess good solubility. Thus, the incorporation of TPD in the polymer backbone allows a high carrier mobility and charge transport ability owing to the strong inter-chain interactions, and TPD-based polymers having superior EQEs and photocurrent performances are regarded as one of the best OPD polymers. However, few TPD-based polymers have been developed for OPD application3335.

The discovery of high R and high D* TPD-based polymers for OPDs is timely important and

will contribute to the study of high-performance OPD materials.


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The TPD-based wide bandgap polymer, poly(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2b:4,5-b']dithiophene-co-5-(2-hexyldecyl)-1,3-bis(6-octylthieno[3,2-b]thiophen-2-yl)-4Hthieno[3,4-c]pyrrole-4,6(5H)-dione) (PBDTT-8ttTPD)32, reported by our group was used as the donor material and [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) as an acceptor material for bulk heterojunction (BHJ) OPD devices (Figure 1). We compared the OPD performance of the PBDTT-8ttTPD:PC71BM-based OPD with that of P3HT:PC71BM-based control device at different film thicknesses. The P3HT:PC71BM device exhibited a R of 0.31 A W-1 and a D* of 3.65 × 1012 Jones, whereas the PBDTT-8ttTPD:PC71BM device showed a superior R of 0.39 A W-1 and a D* of 1.13 × 1013 Jones, even at the thick film thickness of 940 nm. The impressive OPD performance of the PBDTT-8ttTPD:PC71BM devices can be attributed to the face-on preferred morphology and excellent hole/electron mobility of the blend film. Detailed OPD studies have been carried out to investigate the optical, electrochemical, morphological, and charge transport properties.

Figure 1. Structure of the photoactive materials used in this experiment.


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EXPERIMENTAL Fabrication of OPDs The OPD devices were fabricated using the following inverted structure: indium tin oxide (ITO)/ZnO/polymer:PC71BM/MoO3/Ag. The ZnO sol-gel solution was prepared as reported previously.36 The PBDTT-8ttTPD and PC71BM were dissolved in chlorobenzene with 3 vol% of 1,8-diiodooctane under ambient conditions (1.0:1.5 w/w, various total concentrations from 30 to 60 mg mL-1) and stirred overnight at 100 ℃. The reference materials, P3HT and PC71BM, were dissolved in 1,2-dichlorobenzene (1.0:0.7 w/w) with various total concentrations from 51 to 102 mg mL-1 at ambient condition and stirred overnight at 100 ℃. The ITO substrates were washed, sequentially, using an ultrasonic bath in detergent solution, distilled water, acetone, and 2-propanol for 10 min each. The, the cleaned ITO substrate was treated with UV–ozone plasma for 20 min. A ZnO layer was fabricated on the treated ITO by spin coating at 2000 rpm for 30 s, and the substrates were baked at 200 ℃ for 60 min on a hotplate under ambient conditions. The active layer was formed by spin coating at various rotational speeds for 30 s in a glove box filled with argon. After drying the active layer, the reference substrate, P3HT:PC71BM, was baked at 160 ℃ for 10 min. MoO3 (8 nm) and Ag (100 nm) were thermally evaporated under a base pressure of 10-7 torr. The active area of the OPD devices was 0.09 cm2.

RESULTS AND DISCUSSION Synthesis and characterization of polymers PBDTT-8ttTPD polymer was synthesized as previously reported.32 The synthesized polymer was purified by Soxhlet extraction with hexanes, acetone, and chloroform. The number-average


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molecular weight (Mn) and dispersity (Đ) were measured to be 179 000 g mol-1 and 2.84, respectively, by gel permeation chromatography (GPC, Figure S1). The thermal properties of PBDTT-8ttTPD and P3HT were investigated by thermogravimetric analysis (TGA), and the decomposition temperatures (Td, at 5% weight loss) of the polymers were found to be 444 and 436 ℃, respectively (Figure S2). Both polymers showed excellent thermal stability. The absorption spectra of the thin films are shown in Figure 2(a), and the data are summarized in Table 1. P3HT, the reference polymer, absorbed photons in the region of 400–650 nm; however, the synthesized PBDTT-8ttTPD polymer showed stronger and sharper absorption peaks at 500–650 nm,37,38 which is a more suitable wavelength range for image sensing. The optical bandgaps (Egopt) of PBDTT-8ttTPD and P3HT were estimated from the absorption band edges to be 1.86 and 1.91 eV, respectively. PBDTT-8ttTPD clearly showed a shoulder peak at 616 nm, implying that PBDT-8ttTPD has stronger intermolecular interactions than P3HT.37,39 The maximum absorption coefficient of PBDTT-8ttTPD is higher (6.9 × 104 cm-1 at 616 nm) than that of P3HT film (5.7 × 104 cm-1 at 535 nm), indicating that PBDT-8ttTPD could produce a better photocurrent under illumination. The electrochemical properties of the polymers were measured by cyclic voltammetry (CV). The highest occupied molecular orbital (HOMO) energy level was estimated from the first onset potential of the oxidation graph, and the lowest unoccupied molecular orbital (LUMO) energy level was calculated in two ways: from the first onset potential of the reduction graph (ELUMOCV) and from the optical band gap of the polymer (ELUMOOPT). These results are displayed in Figure 2(b) and Table 1.


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Figure 2. (a) UV/Vis absorption spectra of PBDTT-8ttTPD and P3HT thin films, (b) cyclic voltammograms of the PBDTT-8ttTPD and P3HT films on Pt electrodes in 0.1 M tetrabutylammonium tetrafluoroborate (TBABF4) and CH3CN solution, and (c) energy level diagram of PBDTT-8ttTPD, P3HT, and PC71BM.

Table 1. Molecular weight properties and optical and electrochemical properties of the synthesized polymers. Materials

Film a

Egopt (eV) b

EHOMO (eV) c

LUMOopt (eV) d

ELUMO (eV) d

λmax (nm)

λedge (nm)

PBDTT-8ttTPD

352, 575, 616

665

1.86

-5.23

-3.37

-3.49

P3HT

525

648

1.91

-4.93

-3.02

-3.41

a

Polymer film on a glass by spin-casting from chloroform solution at 2000 rpm for 30 s. b Calculated from the absorption band edge of the polymer films, Eg = 1240/λedge. c EHOMO = e(Eoxonset + 4.72). d ELUMO = -e(Eredonset - 4.71)

OPD device performance with polymer:PC71BM The

BHJ

OPD

devices

were

made

with

the

following

inverted

structure:

ITO/ZnO/polymer:PC71BM/MoO3/Ag. Because the dark current (Jd) of the devices is highly affected by the thickness of the active layer, we controlled the thickness to have the optimum


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weight ratio.31 The current-voltage (J-V) characteristics of the OPDs were measured under various light intensities (55.6–2777.8 μW cm-2) and thicknesses (200–1000 nm) of the active layer, and the results are summarized in Figures S3–S15 and Tables S1–S13. For a detailed analysis, a comparison of the photodetecting properties of PBDTT-8ttTPD:PC71BM and P3HT:PC71BM devices was carried out at -2 V and 55.6 μW cm-2 or the dark current, as shown in Figure 3. As the thickness of the PBDTT-8ttTPD:PC71BM blended film increased, Jd decreased gradually from 2.48 × 10-7 to 3.72 × 10-9 A cm-2, whereas the photocurrent (Jph) reached a maximum of 2.18 × 10-5 A cm-2 at a thickness of 350 nm, and then slightly decreased to 2.00 × 10-5 A cm-2 at a thickness of ca. 1 μm. The maximum R was 0.42 A W-1, and the R was as high as 0.39 A W-1, even at a thickness of 940 nm. The small deviation in R with thickness implies that the PBDTT-8ttTPD:PC71BM film has excellent charge transport properties and few defect sites, thus minimizing charge recombination. In the case of the P3HT:PC71BM-based devices, a higher Jd was generated than that of the PBDTT-8ttTPD:PC71BM devices under the same conditions, approximately 10 times greater. This indicates that the PBDTT8ttTPD:PC71BM film has a better nanomorphology, which can reduce the leakage current in the active layer. The Jph of the P3HT:PC71BM-based devices was also lower than that of the PBDTT-8ttTPD:PC71BM film, and, thus, the resultant R of the P3HT:PC71BM-based devices was ca. 0.3 A W-1. The dynamic characteristics of OPDs was measured under the identical green-light source (22.1 μW cm-2) through a Keithley 2600 to conform the on-off switching properties of OPD device in various active layer thickness (Figure S16). Similar with static characteristic shown in Figure 3, PBDTT-8ttTPD:PC71BM devices showed the decreased Jd and consistent Jph as the thickness of the active layer is increased, whereas P3HT:PC71BM devices showed minor


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improvement depending on the active layer thickness. In particular, the on/off ratio of PBDTT8ttTPD:PC71BM devices was more than 10-fold improved compared to the P3HT:PC71BM devices. The detectivity (D*) of the OPDs was obtained from eq (1) using R and Jd at the same bias. The general physical/chemical constants were obtained from the NIST Reference on Constants, Units, and Uncertainty40: q is the elementary charge (1.602 × 10-19 C), ħ is the Planck constant (6.626 × 10-34 m2 kg s-1), c is the velocity of light (3.00 × 108 m s-1), and D* is usually expressed in Jones (cm Hz0.5 W-1). The D* values of the PBDTT-8ttTPD:PC71BM and P3HT:PC71BM devices were calculated to be 1.13 × 1013 and 0.36 × 1013 Jones, respectively, at a thickness of ca. 940 nm and a bias of -2 V (Figure 4 and Table 2). As a result, higher R and D* values were obtained for the PBDTT-8ttTPD:PC71BM-based devices, implying better photodetection. Compared to the other TPD-based polymers33,34 (D*: 0.1 – 1 × 1013 Jones), PBDTT8ttTPD:PC71BM devices also shows promising D* values over 1013 Jones.

𝐷 ∗ (𝑑𝑒𝑡𝑒𝑐𝑡𝑖𝑣𝑖𝑡𝑦) =

𝐴∆𝑓 𝑁𝐸𝑃

=

𝑅 2𝑞𝐽𝑑

[cm Hz0.5 W ―1, 𝐽𝑜𝑛𝑒𝑠]

𝜆𝑞

𝐽𝑝ℎ

𝑅 (responsivity) = 𝐸𝑄𝐸ℏ𝑐 = 𝑃𝑙𝑖𝑔ℎ𝑡 [A W ―1]

(1) (2)

The EQEs of the OPDs were measured under reverse bias and 0 V, and the responsivities (REQE) were calculated from the EQE using eq (2). The detectivity (D*EQE) was obtained from eq (1) using REQE and Jd, similar to the calculation of D*. As shown in Figure 4(d), at the short-circuit voltage (0 V), the theoretical limit of REQE at 530 nm is 0.43 A W-1. The REQE values of the PBDTT-8ttTPD:PC71BM and P3HT:PC71BM devices at 530 nm were 0.35 and 0.28 A W-1, respectively. Notably, REQE of the PBDTT8ttTPD:PC71BM devices reached 80.9% of the theoretical limit between 380 and 640 nm; thus,


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the PBDTT-8ttTPD polymer could be used for photodetectors in a broad range of wavelengths. The direct measurement of OPDs using a 530-nm LED lamp at the short-circuit voltage yielded an R of 0.37 A W-1 for the PBDTT-8ttTPD:PC71BM devices and an R of 0.26 A W-1 for the P3HT:PC71BM devices. The similar trend between R and REQE confirms that the PBDTT-8ttTPD polymer possess superior photodetecting properties compared to the P3HT polymer. Figure 4(c), obtained from Figure 4(b), shows the D*EQE spectra of the PBDTT8ttTPD:PC71BM and P3HT:PC71BM-based devices. The PBDTT-8ttTPD:PC71BM devices showed much higher D*EQE values than those of the P3HT:PC71BM devices over the entire wavelength range because of the low dark current and high EQE of the PBDTT-8ttTPD:PC71BM devices. The light power dependence of the current density upon light illumination is shown as 𝐽𝑝ℎ ∝ 𝐼𝛼. Here, I is the light intensity and α is the exponential factor. The lower rate of bimolecular recombination makes α closer to 1 during charge sweep-out.41,42 As shown in Table S7 and S13, the α values of the PBDTT-8ttTPD:PC71BM device (0.99) was slightly higher than that of P3HT:PC71BM device (0.98), indicating the minimized bimolecular recombination in the PBDTT-8ttTPD:PC71BM OPD.


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Figure 3. (a, c) Current density, (b, d) responsivity, and detectivity at various active layer thicknesses of the PBDTT-8ttTPD:PC71BM and P3HT:PC71BM devices at 55.6 μW cm-2 at -2 V.


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Figure 4. The best OPD performance of the PBDTT-8ttTPD:PC71BM and P3HT:PC71BM devices at an active layer thickness of over 900 nm. (a) J-V curve under 55.6 μW cm-2 green LED light (530 nm), (b) EQE and REQE at -2 V, and (c) D*EQE at -2 V. (d) At the short-circuit voltage, the theoretical limit of REQE, experimental REQE, and direct measurement of R under 530 nm LED light.


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Table 2. Photodetecting properties of the PBDTT-8ttTPD:PC71BM and P3HT:PC71BM-based OPDs. Device

Jd (A cm-2) c

Jph (A cm-2) c, d

On/off REQE current ratio (A W-1)

D* (Jones)

PBDTT-8ttTPD :PC71BM a

3.72 × 10-9

2.03 × 10-5

5.46 × 103

0.39

1.13 × 1013

P3HT :PC71BM b

2.27 × 10-8

1.43 × 10-5

6.30 × 102

0.31

3.65 × 1012

Thickness of a 940 and lamp.

b

920 nm, c at -2 V, d Light source: 55.6 μW cm-2 from 530-nm LED

To evaluate the OPD performance further, the shunt resistance was calculated from the dark current at various active layer thicknesses (Figure S17). Because the film is thicker, the shunt resistances in both the PBDTT-8ttTPD:PC71BM and P3HT:PC71BM-based devices were increased. However, the shunt resistance in the PBDTT-8ttTPD:PC71BM-based devices are larger than those in the P3HT:PC71BM-based devices at a similar film thickness, which suggests that the low Jd of the PBDTT-8ttTPD:PC71BM-based devices is attributed to its high shunt resistance. To analyze the charge transport properties of the OPDs, the hole mobility was measured using the space charge limited current (SCLC) method. The J-V curves of hole-only devices are displayed in Figure S18, and the field-dependent hole mobilities for various film thicknesses are shown in Figure 5. In the case of the PBDTT-8ttTPD:PC71BM blended films, the hole mobilities at an electric field of 317 (V cm-1)1/2 are nearly constant, regardless of the film thickness: 6.96 × 10-4 cm2 V-1 s-1 at 256 nm, 6.99 × 10-4 cm2 V-1 s-1 at 469 nm, and 8.31 × 10-4 cm2 V-1 s-1 at 843 nm. In the case of the P3HT:PC71BM blended films, the hole mobilities at an electric field of 409 (V cm-1)1/2 also showed a similar trend; that is, 2.57 × 10-4 cm2 V-1 s-1 at 280 nm, 2.65 × 10-4 cm2


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V-1 s-1 at 431 nm, and 2.28 × 10-4 cm2 V-1 s-1 at 735 nm, but the overall hole mobilities of the P3HT:PC71BM blended films are lower than those of the PBDTT-8ttTPD:PC71BM blended films. The high hole mobility of the PBDTT-8ttTPD:PC71BM blended films, even with a thick layer of 843 nm, is responsible for the excellent R value of the PBDTT-8ttTPD:PC71BM devices.

. Figure 5. Thickness and electric field dependence of the hole mobility The thickness dependence of the morphology of the PBDTT-8ttTPD:PC71BM and P3HT:PC71BM blend films was studied using atomic force microscopy (AFM) and twodimensional grazing-incidence wide-angle X-ray scattering (2D-GIWAXS). The AFM images are shown in Figure S19. The root-mean-square (RMS) roughness, ca. 2.5 nm, of the PBDTT8ttTPD:PC71BM film surface indicates a uniform surface, regardless of the film thickness, whereas that of the P3HT:PC71BM film surface increased from 13.9 to 17.2 nm as the film thickness increased. A smooth surface results in better contact with the electrode. Thus, the PBDTT-8ttTPD:PC71BM blend film can improve the charge injection and photocurrent in the OPDs.38


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The molecular orientation was obtained from the 2D-GIWAXS measurements. The 2DGIWAXS images and line-cut graph of the polymer:PC71BM blended film are displayed in Figure 6 and Table S14. All the PBDTT-8ttTPD:PC71BM blend films exhibited intermolecular π-π stacking peaks (qz(010)) at around 1.74 Å-1 in the out-of-plane axis. This corresponds to a dspacing of 0.36 nm. However, in the in-plane axis, there was only azimuthally uniform PC71BM ordering. This implies that PBDTT-8ttTPD mainly forms a face-on orientation in the PBDTT8ttTPD:PC71BM blend films, which can improve the vertical charge transport in the OPDs. In the case of the P3HT:PC71BM blend films, all the films showed strong edge-on orientation, as shown by the average interval of the (h00) reflections along the qz axis of 0.40 Å-1 and the intermolecular π-π stacking peak of 1.69 Å-1 along the in-plane (qxy (010)) axis. However, the qz (010)

peak along the out-of-plane axis was not observed in the P3HT:PC71BM blend films. The

ratio between edge-on and face-on (Aedge-on/Aface-on) orientations of the polymer:PC71BM blend films was quantitatively calculated from the azimuthal angle scan of the (100) peak (Table S14). In the case of the PBDTT-8ttTPD:PC71BM blend film, Aedge-on/Aface-on is 1.46, whereas Aedgeon/Aface-on

of the P3HT:PC71BM film is 4.78, implying dominant edge-on orientation. As a result,

the face-on orientation of PBDTT-8ttTPD polymer, rather than the strong edge-on orientation of P3HT, is beneficial for efficient charge transport in OPDs. 43,44


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Figure 6. GIWAXS images of the PBDTT-8ttTPD:PC71BM blended films at various active layer thickness values: (a) 250, (b) 650, (c) and 1080 nm and those of P3HT:PC71BM blended films with thicknesses of (d) 250, (e) 440, and (f) 690 nm. The line cut graphs and the azimuthal angle


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scans of the BHJ film obtained by GIWAXS for (g, i) PBDTT-8ttTPD:PC71BM and (h, j) P3HT:PC71BM.

CONCLUSIONS High-performance OPDs should have a suitable absorption range for detecting the target wavelength, a low dark current, and a high carrier mobility and R under reverse bias. We have developed OPDs showing promising photodetecting properties in a broad range of visible light wavelengths (380–640 nm) using the PBDTT-8ttTPD polymer. The PBDTT-8ttTPD:PC71BMbased OPDs showed excellent REQE and R values of 0.35 and 0.37 A W-1, respectively, at the short-circuit current condition, which is approximately 80.9% of the theoretical maximum R value of 0.43 A W-1. A high D* of 1.13 × 1013 Jones at -2 V at a irradiated light power density of 55.6 μW cm-2 was obtained by increasing the thickness of the PBDTT-8ttTPD:PC71BM blend films. Notably, even when using an active layer as thick as ca. 1 μm, the R value of the PBDTT8ttTPD:PC71BM-based OPD was maintained as high as 0.42 A W-1. This high-performance is a result of the high absorption coefficients, superior hole mobility, and smooth film surface of the thick film (ca. 900 nm), as well as the face-on preferred ordering of the PBDTT-8ttTPD polymer in the PBDTT-8ttTPD:PC71BM blend films.

ASSOCIATED CONTENT Supporting Information. Detailed experimental information, GPC and TGA results for PBDTT-8ttTPD and P3HT, performance of PBDTT-8ttTPD:PC71BM blended OPDs and P3HT:PC71BM blended OPDs with various active layer thickness, shunt resistances of OPD


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devices with various thickness values, J-V characteristics of the hole-only devices, AFM images and GIWAXS graphs of blended active layer films. AUTHOR INFORMATION Corresponding Author *Prof. D.-H. Hwang ([email protected]) *Prof. I.H. Jung ([email protected]) Author Contributions ‡These authors contributed equally. ACKNOWLEDGMENT This work was supported by the National Research Council of Science & Technology (NST) grant

by

the

Korea

government

(MSIT)

(No.

CAP-15-04-KITECH,

NRF-

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