Letter Cite This: ACS Macro Lett. 2018, 7, 395−400
pubs.acs.org/macroletters
High Performance All-Polymer Photodetector Comprising a Donor− Acceptor−Acceptor Structured Indacenodithiophene− Bithieno[3,4‑c]Pyrroletetrone Copolymer Petri Murto,†,‡ Zewdneh Genene,†,‡,§,∥ Cindy Montenegro Benavides,‡,⊥,# Xiaofeng Xu,† Anirudh Sharma,□,△ Xun Pan,□ Oliver Schmidt,⊥ Christoph J. Brabec,#,○ Mats R. Andersson,□ Sandro F. Tedde,*,⊥ Wendimagegn Mammo,*,§ and Ergang Wang*,† †
Department of Chemistry and Chemical Engineering/Applied Chemistry, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden § Department of Chemistry, Addis Ababa University, P.O. Box 33658, Addis Ababa, Ethiopia ∥ Department of Chemistry, Ambo University, P.O. Box 19, Ambo, Ethiopia ⊥ Siemens Healthineers, Technology Center, Günther-Scharowsky-Str. 1, 91058 Erlangen, Germany # Friedrich-Alexander Universität Erlangen-Nürnberg, Department für Material Science, i-MEET, Martensstrasse 7, 91058 Erlangen, Germany ○ ZAE Bayern, Renewable Energies, Immerwahrstrasse 2, 91058 Erlangen, Germany □ Flinders Centre for Nanoscale Science and Technology, Flinders University, Sturt Road, Bedford Park, Adelaide, SA 5042, Australia △ University of Bordeaux, Laboratoire de Chimie des Polymères Organiques (LCPO), UMR 5629, B8 allée Geoffroy Saint Hilaire, 33615 Pessac Cedex, France S Supporting Information *
ABSTRACT: The synthesis of an acceptor polymer PIDT2TPD, comprising indacenodithiophene (IDT) as the electron-rich unit and an interconnected bithieno[3,4-c]pyrrole-4,4′,6,6′-tetrone (2TPD) as the electron-deficient unit, and its application for all-polymer photodetectors is reported. The optical, electrochemical, charge transport, and device properties of a blend of poly(3-hexylthiophene) and PIDT-2TPD are studied. The blend shows strong complementary absorption and balanced electron and hole mobility, which are desired properties for a photoactive layer. The device exhibits dark current density in the order of 10−5 mA/ cm2, external quantum efficiency broadly above 30%, and nearly planar detectivity over the entire visible spectral range (maximum of 1.1 × 1012 Jones at 610 nm) under −5 V bias. These results indicate that PIDT-2TPD is a highly functional new type of acceptor and further motivate the use of 2TPD as a building block for other n-type materials.
A
Photoactive layers based on conjugated polymers can provide strong and complementary light absorption for broad spectrum detection, as well as highly tunable electronic properties and better stability compared to the fullerene blends.14−17 Despite these promising aspects, to date only a few examples have been reported where functional polymer/polymer blends have been used in all-polymer photodetectors (all-PPDs). A blend using P3HT as the donor and a ladder-structured polypyrrone as the acceptor, was reported to deliver an EQE of ca. 20% and a detectivity of 1.3 × 1011 Jones at 610 nm.18 Recently, detectivities in the order of 1012 Jones and spectral responses
bulk-heterojunction (BHJ) comprising both p-type donor and n-type acceptor polymers is an alternative approach to the well-established polymer/fullerene blends in organic photodetector (OPD) applications.1−5 Devices based on a conjugated polymer, e.g. poly(3-hexylthiophene) (P3HT), as the donor and a fullerene derivative, e.g. [6,6]-phenyl-C61butyric acid methyl ester (PC61BM), as the acceptor can deliver satisfactory external quantum efficiency (EQE) up to 80% and detectivity in the order of 1012 cm Hz1/2/W (Jones).6−10 Unfortunately, fullerene derivatives exhibit rather weak light absorption in the visible spectral range, especially toward the low-energy region and low long-term stability of the BHJ morphology, both of which intrinsically limit the performance of the photoresponse devices.11−13 © XXXX American Chemical Society
Received: January 3, 2018 Accepted: March 8, 2018
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DOI: 10.1021/acsmacrolett.8b00009 ACS Macro Lett. 2018, 7, 395−400
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ACS Macro Letters
Figure 1. (a) Chemical structures of P3HT and PIDT-2TPD and (b) HOMO/LUMO energy levels (CV) of P3HT donor and PIDT-2TPD acceptor in comparison to PC61BM.
decrease that energy level, both of the 2TPD monomer and the resulting PIDT-2TPD copolymer, thus, making it suitable as an acceptor. P3HT is a highly crystalline donor polymer, which can have large phase separation with the acceptor material in BHJ devices.31−33 We introduced the bulky hexylphenyl substituents on the IDT and the branched 2-octyldodecyl side chains on the 2TPD moiety to ensure good miscibility with P3HT and to suppress strong π−π stacking in the blends. Besides, the lipophilic side chains will impart good solubility and easy processability from common solvents, such as chlorobenzene (CB) used in this work. PIDT-2TPD exhibited a relatively high number-average molecular weight (Mn) of 55.7 kg/mol and a narrow dispersity of 2.4, which is desirable for good film-forming property and charge transport in the BHJ layer. Figure 2a shows the absorption coefficient versus wavelength curves of P3HT and PIDT-2TPD in CB solution. The acceptor polymer PIDT-2TPD showed slightly stronger absorption peaking at 609 nm, as compared to the donor polymer P3HT with an absorption maximum at 458 nm. In the spectrum of the thin film (Figure 2b), the absorption profile of
extending up to 1100 nm were obtained by using naphthalene diimide (NDI)- or perylene diimide (PDI)-based acceptor polymers in combination with different donor polymers.2−5 Although such values are comparable to the fullerene-based devices, the NDI- and PDI-based acceptors exhibit relatively weak light absorption, which partially limit the EQEs of the allPPDs below 25%.2,3,16,19−22 Therefore, there is a strong need to develop polymer acceptors with high absorption coefficients at different wavelengths. Herein, we report the application of poly[4,4,9,9-tetrakis(4hexylphenyl)-4,9-dihydro-s-indaceno[1,2-b:5,6-b’]dithiophene2,7-diyl-alt-5,5′-bis(2-octyldodecyl)−4H,4’H-[1,1′-bithieno[3,4-c]pyrrole]-4,4′,6,6’(5H,5′H)-tetrone-3,3′-diyl] (PIDT2TPD) (Figure 1) as the acceptor and P3HT as the donor for all-PPDs. From the molecular design point of view, the acceptor should ideally have (i) complementary absorption with the donor and (ii) low-lying lowest unoccupied molecular orbital (LUMO) energy level to obtain sufficient LUMO− LUMO offset and driving force for charge transfer in blends with the donor. We used the so-called donor−acceptor− acceptor (D−A−A) design strategy to optimize the LUMO energy of the acceptor polymer.23 Moreover, PIDT-2TPD was chosen as the acceptor because indacenodithiophene (IDT)and indacenodithieno[3,2-b]thiophene (IDTT)-based small molecules have been shown to exhibit strong light absorption and high power conversion efficiencies in organic photovoltaics.24−27 The IDT- and IDTT-based acceptors have also shown good miscibility with different donor polymers in the BHJ, which have resulted in stable device performance.12,13,28 On the other hand, thieno[3,4-c]pyrrole-4,6-dione (TPD) and bithieno[3,4-c]pyrrole-4,4′,6,6′-tetrone (2TPD) have been recently introduced as electron-deficient units in acceptor polymers with the merit of having tunable electronic properties for all-polymer BHJ solar cells.29,30 In this work, a blend of P3HT/PIDT-2TPD in 2:1 ratio showed strong photoresponse reaching 680 nm and EQE broadly above 30%. We calculated a maximum detectivity of 1.1 × 1012 Jones at 610 nm. The detectivity was maintained nearly planar over the visible range, thanks to the complementary absorption from the donor and acceptor. The dark current density remained low and did not exceed the value of 6.42 × 10−5 mA/cm2 at −5 V bias. These results were obtained without using any additive in the processing of the BHJ layer. The synthesis of PIDT-2TPD is described in detail in the Supporting Information (SI) and illustrated in Scheme S1 and Figure S1, respectively. We combined the IDT donor with the 2TPD acceptor by using the Pd-catalyzed Stille polycondensation polymerization. Connecting two TPD acceptor units together is a versatile method to stabilize the LUMO and
Figure 2. (a) Absorption coefficients of P3HT and PIDT-2TPD in chlorobenzene solution. (b) Absorption coefficients of the neat polymers and the polymer/polymer blends in thin films. 396
DOI: 10.1021/acsmacrolett.8b00009 ACS Macro Lett. 2018, 7, 395−400
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ACS Macro Letters Table 1. SCLC Mobilities and Device Characteristics of Different Blend Ratios P3HT/PIDT-2TPD ratioa 2:1 1:1 1:2 a
μhb (cm2/(V s)) −5
9.23 × 10 5.97 × 10−5 6.43 × 10−6
μec (cm2/(V s))
Jd (−5 V) (mA/cm2)
−5
−5
3.97 × 10 9.07 × 10−5 9.81 × 10−6
6.42 × 10 6.83 × 10−6 2.85 × 10−6
b
Jph (−5 V) (mA/cm2)
Ion/Ioff (−5 V)
4.57 × 10−2 1.56 × 10−2
7.1 × 102 2.3 × 103
d
d
c
Measured by w/w-% of the polymers. Hole-only device structure: ITO/PEDOT:PSS/active layer/MoO3/Al. Electron-only device structure: ITO/ZnO/active layer/LiF/Al. dNo photocurrent response.
Figure 3. (a) J−V characteristics of two different polymer/polymer blend ratios in the dark and under green light at 532 nm (intensity 780 μW/ cm2). (b) External quantum efficiency, (c) responsivity, and (d) specific detectivity of the all-PPD based on P3HT/PIDT-2TPD 2:1 blend at different bias voltages.
be the two most promising for efficient exciton separation in the photoactive layer. The highest occupied molecular orbital (HOMO) and LUMO energy levels of the donor and acceptor polymers were measured experimentally by cyclic voltammetry (CV). As shown in Figure 1, the HOMO−HOMO and LUMO−LUMO offsets between P3HT and PIDT-2TPD were both 0.8 eV, which was deemed sufficient for exciton separation in the devices and supports the observed PL quenching (Figure S2). Moreover, both the neat polymers and the blend films showed good electrochemical stability and reversible or semireversible oxidation and reduction waves (Figure S3), which would potentially indicate balanced charge transport in the BHJ. To get a better understanding of the electron and hole transport, we carried out space-charge-limited current (SCLC) mobility measurements for the P3HT/PIDT-2TPD 2:1, 1:1, and 1:2 blends (Figure S4). The SCLC results are summarized in Table 1. All three blends showed somewhat balanced charge transport, but best mobilities were achieved with the 2:1 and 1:1 blends. Although the SCLC method might underestimate the mobility under light,39 we expected that the 2:1 and 1:1 blends would deliver superior photocurrent in the devices. The all-PPDs were fabricated by using the device structure: ITO/TIPS pentacene/active layer/Al, where the active layer comprised P3HT/PIDT-2TPD blend in the ratio of 2:1, 1:1, or 1:2. TIPS pentacene was selected as an electron-blocking layer because it was recently found to reduce the dark current density (Jd) of P3HT-based devices up to an order of magnitude.40 The
PIDT-2TPD remained similar to that in solution, with an absorption maximum at 610 nm. P3HT had a red-shifted and broadened absorption window peaking at 516 nm in going from solution to the solid state, which can be attributed to its characteristic π−π stacking.31−34 As a result, the absorption coefficient of P3HT increased in comparison to PIDT-2TPD in the solid state. We noted that the absorption from PIDT-2TPD was significantly stronger than what has been reported previously for NDI- and PDI-based acceptors (peak maximum between 1−4 × 104 cm−1).2,16,19−22,35 Both polymers exhibited optical band gaps of 1.9 eV, which were estimated from the low-energy onsets of the solid-state absorption bands. The absorption coefficient versus wavelength plots of P3HT/PIDT2TPD 2:1, 1:1, and 1:2 blend films are shown in Figure 2b for comparison. All blends exhibited strong complementary absorption from 350 to 680 nm, but the absorption from P3HT dominated in the 2:1 and 1:1 blends, as expected from the neat polymer films. The photoluminescence (PL) spectra of the polymers are shown in Figure S2 (SI). PIDT-2TPD exhibited a relatively high PL intensity in CB solution, as compared to P3HT. In thin film, P3HT showed only a weak PL, which can be attributed to aggregation quenching of the emission due to the strong π−π stacking.36−38 Significantly, the high intensity emission from PIDT-2TPD was completely quenched in the P3HT/PIDT2TPD 2:1 and 1:1 blends, whereas the increased amount of PIDT-2TPD in the 1:2 blend resulted in weak residual emission. This indicated that the 2:1 and 1:1 blends should 397
DOI: 10.1021/acsmacrolett.8b00009 ACS Macro Lett. 2018, 7, 395−400
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ACS Macro Letters active layer was spin-coated from CB solution to obtain a desired thickness of 400 nm (see the SI for details). Figure 3a shows the current density−voltage (J−V) characteristics of the devices from −5 to 5 V bias. The best device performance was obtained with the 2:1 blend, which exhibited a Jd of 6.42 × 10−5 mA/cm2 under −5 V bias voltage. This is not only among the lowest dark currents reported for all-PPDs at −5 V, but also comparable to the devices based on fullerene derivatives.3,5,8,41 Under 780 μW/cm2 illumination at 532 nm (green light), the photocurrent density (Jph) was 4.57 × 10−2 mA/cm2 at −5 V. This corresponded to an on/off ratio of 7.1 × 102 (here the on/ off ratio is defined as the ratio of photocurrent and dark current under −5 V bias). We noted that the Jd further decreased to 6.83 × 10−6 and 2.85 × 10−6 mA/cm2 for the 1:1 and 1:2 blends, respectively, but the Jph was three times lower for the 1:1 blend as compared to the 2:1 blend. The 1:2 blend did not show any photocurrent response under the green light (Table 1). Therefore, the Jd and Jph of the 1:2 blend has not been included in Figure 3a. Based on these results, we focused on the performance of the P3HT/PIDT-2TPD 2:1 blend as the optimal active layer. One of the key factors defining the performance of OPDs is the EQE, which is the ratio between the amount of extracted photogenerated charges and the incident photons.1 Typical EQE curves for the donor-rich device (2:1 blend) are shown in Figure 3b. The active layer showed high electrochemical stability as the EQE was systematically increased with increasing bias and reached over 30% at −5 V. The responsivity (R), which is the ratio between the photocurrent and the incident light intensity, increased almost linearly from 0 V to −5 V bias and reached 0.16 A/W at 610 nm (Figure 3c). The EQE and R graphs resembled the absorption of the blend film (Figure 2b), which indicated that both the donor and acceptor polymers had strong contribution to the photoresponse, and thus validated our design motif. Specific detectivity (D*) allows the comparison of the performance of devices with different surface areas and operating bandwidths. Assuming that the noise is dominated by the shot noise (dark current noise), the detectivity was calculated using the equation D* = R/(2qJd)1/2, where q is the absolute value of electron charge (1.6 × 10−19 C) and Jd is the dark current (A/cm2).2 Due to the low dark current at −5 V, the D* was 1.1 × 1012 Jones at 610 nm, which is in the same order of magnitude with the best-performing all-PPDs and, to the best of our knowledge, higher than the values reported so far for all-PPDs comprising P3HT as the donor.2−5,18 This clearly confirmed that PIDT-2TPD is a promising acceptor material for polymer/polymer BHJ devices. Moreover, the D* of the P3HT/PIDT-2TPD 2:1 blend remained nearly planar from 370 to 660 nm, which is essentially over the entire visible range. Achieving the best performance at high reverse bias (−5 V) is desired for the integration of the photodetector as the light-sensitive component into real-life imaging and communication applications.6,42 We studied the frequency response of this blend at −5 V bias and the 3 dB cutoff frequency was recorded at 1.5 × 103 Hz (Figure S5). The speed was probably limited by the moderate, yet balanced, electron and hole mobilities in the active layer. Tapping mode atomic force microscopy (AFM) was employed to study the morphology of the blend films via surface topography. All three blends exhibited smooth surfaces but the root-mean-square (RMS) roughness was highest for P3HT/PIDT-2TPD 2:1 blend (2.11 nm, Figure 4a). The RMS
Figure 4. Tapping mode AFM topography (5 × 5 μm) images of P3HT/PIDT-2TPD blends in (a) 2:1, (b) 1:1, and (c) 1:2 ratio.
roughness decreased to 1.44 and 1.06 nm for the 1:1 and 1:2 blends, respectively (Figure 4b,c). This makes sense as the more amorphous PIDT-2TPD is expected to be highly miscible and interfere with the P3HT crystal domains. To get further insight into the polymer/polymer miscibility, we carried out dynamic mechanical thermal analysis (DMTA) for the neat polymers and the blends (Figure S6). The two tan δ peaks of P3HT at −82 and 37 °C were in line with the previously obtained values43 and attributed to the relaxation of the side chains and glass transition (Tg) of the polymer, respectively. Significant drop in the storage modulus of PIDT-2TPD was observed between −20 and 50 °C, accompanied by a tan δ peak at 8 °C, which we attribute to its Tg. The 2:1 and 1:1 blends exhibited two tan δ features close to the Tgs of the neat polymers, which clearly indicate the presence of partially separated P3HT-rich and PIDT-2TPD-rich phases. In contrast, the 1:2 blend showed a dominant tan δ feature of the PIDT2TPD-rich phase, suggesting that P3HT was effectively dispersed in the blend. The general observation was that the two tan δ features of the blends shifted at different temperatures according to the blend ratio. The increased amount of PIDT-2TPD (in 1:2 blend) resulted in a less structured morphology without desirable phase separation, which partially explains the decreased mobility and the lack of photocurrent response (Table 1, Figure 3a). However, the donor-rich blend (2:1 ratio) showed a continuous and interpenetrating network structure of P3HT-rich and PIDT2TPD-rich domains, which is ideal for exciton separation and charge carrier transport in the BHJ, and is in conformity with the best device performance obtained from the 2:1 blend. In summary, we introduced PIDT-2TPD as a new type of acceptor polymer with strong light absorption for all-PPD application. A blend of P3HT/PIDT-2TPD in 2:1 ratio exhibited EQE over 30% and nearly planar detectivity over the entire visible spectral range under −5 V bias. The photoresponse corresponded to the absorption profile of the blend film, thus confirming a strong light absorption from PIDT-2TPD in the all-PPD device. This is an important step forward to achieve improved photoresponse from the acceptor polymer. The results obtained in this study are highly motivating for the development of future polymer/polymer BHJ materials utilizing the D−A−A design strategy, for example, with the strong light absorption extending to the near-infrared region.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.8b00009. Experimental section, material synthesis and characterization, NMR, absorption and PL, cyclic voltammetry, 398
DOI: 10.1021/acsmacrolett.8b00009 ACS Macro Lett. 2018, 7, 395−400
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ACS Macro Letters
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SCLC mobility, AFM, all-PPDs characterization, and DMTA temperature scans (PDF).
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Anirudh Sharma: 0000-0003-4841-0108 Ergang Wang: 0000-0002-4942-3771 Author Contributions ‡
P.M., Z.G., and C.M.B. contributed equally to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge financial support from the European Community’s Seventh Framework Programme (FP7/20072013) under Grant Agreement No. 607585 (OSNIRO), the Swedish Research Council, the Swedish Research Council Formas, and Chalmers Area of Advance Materials Science and Energy. W.M. and Z.G. acknowledge financial support from the International Science Program (ISP), Uppsala University, Sweden. A.S, X.P, and M.R.A. acknowledge funding from the Australian Research Council (DP170102467).
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DOI: 10.1021/acsmacrolett.8b00009 ACS Macro Lett. 2018, 7, 395−400
Letter
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DOI: 10.1021/acsmacrolett.8b00009 ACS Macro Lett. 2018, 7, 395−400
