Solution-Processed High-Detectivity Near-Infrared Polymer

Mar 14, 2013 - End-Group Engineering of Low-Bandgap Compounds for High-Detectivity Solution-Processed Small-Molecule Photodetectors. Ji Qi , Jinfeng H...
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Solution-Processed High-Detectivity Near-Infrared Polymer Photodetectors Fabricated by a Novel Low-Bandgap Semiconducting Polymer Xiaowen Hu,† Yang Dong,† Fei Huang,*,† Xiong Gong,*,†,‡ and Yong Cao† †

Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, P. R. China ‡ Department of Polymer Engineering, College of Polymer Science and Polymer Engineering, The University of Akron, Akron, Ohio 44325, United States ABSTRACT: High-detectivity near-infrared (NIR) polymer photodetectors (PDs) fabricated by a novel low-bandgap semiconducting polymer blended with fullerene derivatives are reported. Operating at room temperature, the polymer PDs have a spectral response from 400 to 1100 nm. By incorporation of an alcohol/water-soluble polymer as a cathode interlayer in bulk heterojunction polymer PDs, the polymer PDs exhibit a high detectivity of 1.75 × 1013 cm•Hz1/2/W at 800 nm. These results demonstrated that the NIR polymer PDs are comparable to Si-based PDs.

1. INTRODUCTION Photodetectors (PDs) have been the subject of extensive research due to industrial and scientific applications such as remote control, chemical/biological sensing, optical communication, and spectroscopic and medical instruments.1−5 Recently, Konstantatos and coworkers fabricated near-infrared (NIR) PDs by spin-coating colloidal quantum dots from solution onto gold interdigitated electrodes.6 These devices showed a large photoconductive gain and high detectivity at 1.3 μm; however, 3-dB bandwidth was only ∼18 Hz, and the driving voltage for achieving that high detectivity was as high as 40 V. These requests extremely limited the application of these NIR PDs. Polymer PDs have attracted much attention due to their unique features: operating at room temperature, low-cost manufacturing, and high performance. All of these features allow polymer PDs to have a huge potential for a great variety of applications.7 However, there are few reports on NIR polymer PDs.8,9 With the development of new narrow bandgap-conjugated polymers, better control of the nanoscale morphology of the interpenetrating electron donor/acceptor networks, and utilizing both hole extraction and electron extraction layers in the cells, we have reported that the detectivity of solution-processed polymer PDs has reached 1013 cm•Hz1/2/W (1 Jones = 1 cm•Hz1/2/W) and the spectral response covered from ultraviolet (UV) to NIR region.10 However, there is a strong need for the development of high detectivity and low operational voltage NIR polymer PDs while simultaneously maintaining the benefit of low-cost solution process. Here we reported the synthesis of a low-bandgap semiconducting polymer and further fabrication of NIR polymer PDs by this novel low-bandgap semiconducting polymer blended with fullerene derivatives. By using an © 2013 American Chemical Society

electron-withdrawing building block to modify [1,2,5]thiadiazolo[3,4-f ]benzotriazole (TZBTTT),11 as the electron acceptor unit in the donor−acceptor (D-A) copolymer structure, a novel D−A copolymer, poly[2,6-(4,8-bis(2ethylhexyloxy)benzo[1,2-b:4,5-b′]dithiophene)-alt-5,5-(4′,8′di-3-hexylthiophen-2-yl)-6-(2-ethylhexyl)-[1,2,5]thiadiazolo[3,4-f ]benzotriazol (PTZBTTT-BDT), was synthesized. PTZBTTT-BDT possesses an absorption ranged from 300 to 1100 nm, which covers from UV to NIR. We further investigated the performance of polymer PDs made by PTZBTTT-BDT blended with [6,6]-phenyl C61-butyric acid methyl ester (PC61BM). The polymer PDs exhibits a spectral response from 400 to 1100 nm. By incorporation of an alcohol/ water-soluble conjugated polymer, poly[(9,9-bis(3′-(N,Ndimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN), as an electron extraction layer, the polymer PDs possess a detectivity of 1.75 × 1013cm•Hz1/2/W at wavelength of 800 nm.

2. EXPERIMENTAL SECTION 2.1. Materials. All of the starting materials, unless otherwise specified, were obtained from Guangzhou Chemical Reagent Factory, Aldrich, Acros, and TCI Chemical and used as received. Thereinto, acetic acid, ethyl acetate, acetone, tetrahydrofuran (THF), and N,N-dimethylformamide (DMF) were purchased from Guangzhou Chemical Reagent Factory; toluene, methanol, and chlorobenzene were purchased from Received: January 4, 2013 Revised: March 9, 2013 Published: March 14, 2013 6537

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Scheme 1. Synthetic Routes for PTZBTTT-BDT

Aldrich; and NaNO2, potassium t-butoxide, 2-bromoethylheptane, tributyl-(4-hexyl-2-thienyl)-stannane, Pd(PPh3)4Cl2, and N-bromosuccinimide (NBS) were purchased from TCI Chemical. All of the solvents were further purified under a nitrogen flow. 2.2. Cyclic Voltammetry. Cyclic voltammetry (CV) measurement was carried out using a CHI800 electrochemical workstation. The measurement was done in a solution of tetrabutylammonium hexafluorophospate (Bu4NPF6) (0.1 M) in acetonitrile using saturated calomel electrode (SCE) and a platinum wire as reference and counter electrode, respectively, at a scan rate of 50 mV/s at room temperature. A platinum electrode coated with thin copolymer film was used as the working electrode. 2.3. Device Fabrication and Characterization. The device architectures of polymer PDs are ITO/PEDOT:PSS/ PTZBTTT-BDT:PC 61 BM/Al, ITO/MoO 3 /PTZBTTTBDT:PC61BM/Al, ITO/PEDOT:PSS/PTZBTTTBDT:PC 61 BM/PFN/Al, and ITO/MoO 3 /PTZBTTTBDT:PC61BM/PFN/Al, where ITO is indium tin oxide and PEDOT:PSS is poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate). PTZBTTT-BDT mixed with PC61BM (1:2, by weight) was dissolved into o-dichlorobenzene with a concentration of 10 mg/mL. For polymer PDs with PEDOT:PSS as the buffer layer, a ∼40 nm of PEDOT:PSS layer was spin-cast onto the ITO surface and then thermal annealed at 140 °C for 10 min under ambient conditions. For polymer PDs with MoO3 as the buffer layer, a ∼10 nm MoO3

was thermally deposited onto the cleaned ITO glass at an evaporation rate of 0.1 Å/s. After that, a ∼70 nm of polymer active layer was deposited by spin coating of PTZBTTTBDT:PC61BM solution onto the top of either PEDOT:PSS or MoO3 layer. For polymer PDs with PFN electron extraction layer, a ∼5 nm thin PFN layer was casted onto top of polymer active layer from PFN alcohol/water solution before deposition top Al electrode. We thermally deposited ∼80 nm of Al top electrode onto the top of either polymer active layer or PFN electron extraction layer through a shade mask. The active area of the device was measured to be 0.16 cm2. The current density−voltage (J−V) curves were measured using a Keithley 2400 source-measurement unit. A solar simulator (Oriel model 91192) was used as a light source. The external quantum efficiency (EQE) measurement was performed using a DSR100UV-B spectrometer with a SR830 lock-in amplifier. A bromine tungsten lamp is used as the light source.

3. RESULTS AND DISCUSSION PTZBTTT-BDT was synthesized by Stille polycondensation reaction between 4,8-bis(5-bromo-4-hexylthiophen-2-yl)-6-(2ethylhexyl)-[1,2,5]thiadiazolo[3,4-f]benzotriazole and 2,6-bis(trimethyltin)-4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b′]dithiophene. The synthetic routes are described in Scheme 1. Starting materials, 4,7-dibromo-5,6-dinitro-benzo[1,2,5]thiadiazole (1), 4,7-dibromo-5,6-diamino-benzo[1,2,5]thiadiazole (2), and 2,6-bis(trimethyltin)-4,8-bis(26538

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ethylhexyloxy)benzo[1,2-b:4,5-b′]dithiophene (7) (0.193 g, 0.25 mmol), and Pd(PPh3)4 (3 mg) were dissolved in 7 mL of toluene in a 50 mL two-necked round-bottomed flask. The mixture was refluxed with vigorous stirring in the dark for 48 h under an argon atmosphere. After cooling the above solution to the room temperature, the mixture was poured in 300 mL of methanol. The precipitated material was collected by filtration through a funnel. After being washed with acetone for 24 h in a Soxhlet apparatus to remove oligomers and catalyst residues, the resulting polymer was dissolved in 30 mL of chlorobenzene. The solution was filtered with a PTFE filter, concentrated, and precipitated from methanol to yield a dark-green polymer (225 mg, yield = 81.5%). 1H NMR (300 MHz, CDCl3, δ): 8.52 (br, 2H), 7.62 (br, 2H), 4.95 (br, 2H), 4.20 (br, 4H), 3.05 (br, 4H), 2.36 (br, 3H), 0.88−1.79 (br, 66H). Anal. Calcd for (C60H83N5O2S5)n: C, 67.56; H, 7.54; N, 6.57; S, 15.03. Found: C, 70.23; H, 8.08; N, 5.98; S, 15.34. Mn =7.8 kDa, Mw = 13.6 kDa, PDI = 1.74. Because alkyl side chains are linked on the triazole rings, PTZBTTT-BDT has good solubility in common organic solvents, such as toluene, chloroform, chlorobenzene, and 1,2-dichlorobenzene. This feature is good for solution processing of the polymer active layer in polymer solar cells. Figure 1 shows the normalized UV−visible absorption spectra of thin films of pristine PTZBTTT-BDT, pristine

ethylhexyloxy)benzo[1,2-b:4,5-b′]dithiophene (7), were synthesized according to the literature12,13 and characterized by 1H NMR before being used for the synthesis of PTZBTTT-BDT. 4,8-Dibromo-5H-[1,2,5]thiadiazolo[3,4-f]benzotriazole (3). 4,7-Dibromo-5,6-diamino-benzo[1,2,5]thiadiazole (2) (10 g, 31 mmol) was added to the mixture of acetic acid (150 mL) and ethyl acetate (250 mL); then, the temperature was increased to 80 °C until the solid was totally dissolved. NaNO2 (2.58 g, 37 mmol) was added to the solution in several batches and participated in the reaction for another 4 h. The solvent was removed and the crude product was obtained and dried under reduced pressure without further purification. 4,8-Dibromo-6-(2-ethylhexyl)-[1,2,5]thiadiazolo[3,4f ]benzotriazole (4). 4,8-Dibromo-5H-[1,2,5]thiadiazolo[3,4f ]benzotriazole (3) (3 g, 7.3 mmol) was added to the distilled DMF solvent, potassium t-butoxide (1.12 g, 10 mmol) and 2bromoethylheptane (1.92 g, 10 mmol) were added, and the mixture was stirred under reflux for 24 h. After solvent was removed by evaporation, the column chromatography on silica gel was performed to obtain the product. The product was an orange solid (1.08 g, yield: 27%). 1H NMR (300 MHz, CDCl3, δ): 4.85 (dd, 2H), 2.45 (m, 1H), 1.23−1.50 (m, 8H), 0.88− 1.00 (m, 6H). 13C NMR (75 MHz, CDCl3, δ): 151.3, 145.4, 98.6, 62.4, 40.5, 30.3, 28.2, 23.9, 22.8, 13.9, 10.4. Anal. Calcd for C14H17Br2N5S: C, 37.60; H, 3.83; N, 15.66; S, 7.17. Found: C, 37.18; H, 3.52; N, 15.21; S, 7.43. 4,8-Bis(4-hexylthiophen-2-yl)-6-(2-ethylhexyl)-[1,2,5]thiadiazolo[3,4-f ]benzotriazoe (5). 4,8-Dibromo-6-(2-ethylhexyl)-[1,2,5]thiadiazolo[3,4-f ]benzotriazole (4) (1g, 2.23 mmol) and tributyl-(4-hexyl-2-thienyl)-stannane (2.25 g, 4.96 mmol) were dissolved in dry toluene (50 mL), 20 mg Pd(PPh3)4Cl2 was added, and the solution was protected by argon for 15 min. The reaction was heated to 120 °C with stirring overnight. After evaporating the solvent, the residue was purified by chromatography. The product is a dark-blue powder (1.11 g, yield: 78%). 1H NMR (300 MHz, CDCl3, δ): 8.66 (s, 2H), 7.21 (s, 2H), 4.90 (d, 2H), 2.78 (t, 4H), 2.37 (m, 1H), 1.80 (m, 4H), 1.20−1.50 (m, 23H), 0.88 (t, 9H). 13C NMR(75 MHz, CDCl3, δ): 150.0, 143.9, 142.7, 136.9, 132.3, 124.6, 111.9, 60.7,40.7, 31.7,30.8, 30.6, 30.5, 29.1, 28.6, 24.2, 22.9, 22.6, 14.1, 14.0, 10.7. Anal. Calcd for C34H47N5S3: C, 65.66; H, 7.62; N, 11.26; S, 15.47. Found: C, 65.32; H, 7.31; N, 11.78; S, 15.70. 4,8-Bis(5-bromo-4-hexylthiophen-2-yl)-6-(2-ethylhexyl)-[1,2,5]thiadiazolo[3,4-f ]benzotriazole (6). 4,8-Bis(4hexylthiophen-2-yl)-6-(2-ethylhexyl)-[1,2,5]thiadiazolo[3,4-f ]benzotriazole (5) (2.7 g, 4.4 mmol) was dissolved in 50 mL of THF, and NBS (1.78 g, 9.68 mmol) in THF solution was added dropwise to the above solution. The solution was continuously stirred at room temperature overnight. After evaporating the solvent, the product was purified with column chromatography. The dark-blue powder was obtained (3.1g, yield: 91%). 1H NMR (300 MHz, CDCl3, δ): 8.30 (s, 2H), 4.76 (dd, 2H), 2.70 (t, 4H), 2.54 (m, 1H), 1.85 (m, 4H), 1.20−1.60 (m, 20H), 1.05 (t, 3H), 0.88 (t, 9H). 13C NMR (75 MHz, CDCl3, δ): 149.2, 142.6, 141.9, 136.7, 131.5, 115.1, 110.7, 60.7, 40.7, 31.7, 30.8, 30.6, 30.5, 29.1, 28.6, 24.2, 22.9, 22.6, 14.1, 14.0, 10.0. Anal. Calcd for C34H45Br2N5S3: C, 52.37; H, 5.82; N, 8.98; S, 12.34. Found: C, 52.77; H, 6.18; N, 8.88; S, 12.53. PTZBTTT-BDT. 4,8-Bis(5-bromo-4-hexylthiophen-2-yl)-6(2-ethylhexyl)-[1,2,5]thiadiazolo[3,4-f ]benzotriazole (6) (0.194 g, 0.25 mmol), 2,6-bis(trimethyltin)-4,8-bis(2-

Figure 1. UV−visible absorption spectra of thin films: pristine PTZBTTT-BDT, pristine PC61BM, and PTZBTTT-BDT:PC61BM bulk heterojunction composites. Inset shows the absorption spectra from 300 to 500 nm.

PC61BM, and bulk heterojunction (BHJ) composite of PTZBTTT-BDT:PC61BM. The pristine PTZBTTT-BDT film has a strong absorption from 300 to 1100 nm with two absorption peaks. One is peaked at ∼470 nm; another is peaked at ∼810 nm. Both absorption peaks originated from the π* to π transition of different moieties.14 The absorption onset is at ∼1100 nm, and consequently the optical band gap (Eopt g ) of PTZBTTT-BDT is 1.1 eV. The addition of PC61BM into PTZBTTT-BDT did not substantially alter the absorption spectrum of PTZBTTT-BDT; the absorption spectrum of PTZBTTT-BDT:PC61BM is superposition of absorption spectra of PTZBTTT-BDT and PC61BM. A new spectral feature that appears between 300 and 500 nm in the spectrum of the PTZBTTT-BDT:PC61BM composites is attributed to the absorption from PC61BM. 6539

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The energy level of the highest occupied molecular orbital (HOMO) energy level of PTZBTTT-BDT was estimated by CV.15 The HOMO level values of the PTZBTTT-BDT were calculated according to the following empirical formulas16

Scheme 2. (a) Energy Diagrams of PTZBTTT-BDT, PC61BM, and MoO3 and Workfunctions of PEDOT:PSS and Al and (b) Molecular Structures of PC61BM and PFN

E HOMO = −e(Eox + 4.40) (eV)

where the Eox is the onset oxidation and reduction potential versus SCE. Figure 2 shows theCV spectrum of PTZBTTT-

Figure 2. Cyclic voltammetry curves of PTZBTTT-BDT.

BDT. The Eox of PTZBTTT-BDT is 0.74 V. As a result, the HOMO level of PTZBTTT-BDT was calculated to be −5.14 eV. The lowest unoccupied molecular orbital (LUMO) energy level of PTZBTTT-BDT was obtained from optical bandgap (see Figure 1) and HOMO energy level. Therefore, the LUMO energy level of PTZBTTT-BDT is −4.04 eV. Scheme 2 depicts the energy level diagrams of PTZBTTTBDT, PC61BM, and MoO3 and the workfunctions of PEDOT:PSS and Al. LUMO offset and HOMO offset between the PTZBTTT-BDT and PC61BM are both close to 0.3 eV, which implies that charge transfer between PTZBTTT-BDT and PC61BM is efficient.17 Moreover, the barriers for charge collection at the electrodes are small. Therefore, in the cells with an architecture of ITO/PEDOT:PSS(MoO3)/PTZBTTTBDT:PC 61BM/Al, the charge-separated carriers can be efficiently generated by photoinduced charge transfer and subsequently transported via the BHJ nanomorphology to opposite electrodes.18 Figure 3a shows the J−V characteristics of polymer PDs measured in the dark and under illumination. In the dark, the J−V curve shows the rectification behavior, indicating that the polymer PDs show the asymmetry characteristics. Under an illumination of 2.81 mW/cm2 at λ = 500 nm and 3.28 mW/cm2 at λ = 800 nm, respectively, the photogenerated charge carriers greatly increased the reversed current; however, there was not much change in the forward current. The ratios of photocurrents at λ = 500 nm and λ = 800 nm to dark current density at −1 V are 0.5 × 102 and 0.6 × 102, respectively. These confirmed that charge carriers can be efficiently generated by photoinduced electron transfer and subsequently transported via the BHJ nanomorphology to opposite electrodes.18,19 The EQE measured under short-circuit condition using lockin amplifier was presented in Figure 3b. A similar profile of absorption (Figure 1) and EQE indicates that photons absorbed in the NIR range by PTZBTTT-BDT do contribute

to the photocurrent. At λ = 800 nm, the EQE is 13% electron per photon. Accordingly, R, the responsivity, is calculated to be 80 mA/W. The detectivity (D*) expressed as D* = R/(2qJd)1/2 (cm•Hz1/2/W; 1 Jones = 1 cm•Hz1/2/W) (where R is responsivity, q is the absolute value of electron charge (1.6 × 10−19 Coulombs), and Jd is the dark current density (A/ cm2))10,20 was calculated to be D* = (8.01 and 8.22) × 1012 cm•Hz1/2/W at λ = 500 nm with light intensity of 2.81 mW/ cm2 and λ = 800 nm with light intensity of 3.28 mW/cm2, respectively. To optimize performance of polymer PDs, we further investigated the performance of the polymer PDs with an architecture of ITO/PEDOT:PSS/PTZBTTT-BDT:PC61BM/ PFN/Al. Because a thin PFN interlayer can avoid Fermi level pinning between the metal cathode and the electron acceptor material, PC61BM, in the active layer, resulting in an electronselective electrode in nature,21 PFN was chosen as an electron extraction layer for optimizing performance of polymer PDs. Figure 4a shows the J−V characteristics of the polymer PDs with and without PFN cathode interlayer in the dark and under illumination. It was found that the reversed dark leakage current for the polymer PDs was considerably suppressed when incorporating the PFN as the cathode interfacial layer in the polymer PDs. The superior diode performance was also observed from the polymer PDs with PFN cathode interlayer as compared with those without PFN cathode interlayer; for example, the rectification ratio at ±2 V for polymer PDs with PFN layer is 1.1× 105, which is much higher than 1.0× 103 observed from the polymer PDs without PFN layer. Moreover, the HOMO of PFN is −5.61 eV,22 implying that the interlayer is efficient in blocking photogenerated holes from moving into 6540

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Figure 3. (a) Current−voltage (J−V) characteristics of the polymer PDs measured in dark and under illumination with λ = 800 and 500 nm. (b) Corresponding EQE versus wavelength. The device structure of polymer PDs is ITO/PEDOT:PSS/PTZBTTT-BDT:PC61BM/Al.

Figure 4. (a) Current density−voltage (J−V) characteristics of the polymer PDs with and without PFN cathode interlayer in dark and under illumination. (b) EQE profile of polymer PDs with and without PFN interlayer.

As in Figure 4b, the EQE of the polymer PDs was shown from 400 to 1100 nm. By combining the detectivity at 800 nm with the EQE data, the polymer PDs detectivity values were obtained over the entire spectral range. Operating at room temperature, the polymer PDs exhibited spectral response from 400 to 1100 nm, with the detectivity greater than 1013 Jones at wavelengths from 400 to 950 nm and greater than 1010 Jones from 950 to 1100 nm. Figure 5 compares the detectivity of polymer PDs with the detectivity of Si PDs. The detectivity of the polymer PDs is comparable to those from Si PDs. For polymer PDs application, whereas the responsivity and the detectivity are important parameters, the stability is another important factor that should not be neglected. To enhance the stability of the polymer PDs, we used MoO3 as the hole extraction layer to replace PEDOT:PSS buffer layer because PEDOT:PSS possesses strong acidity that etches the ITO and degrades the polymer active layer.26 Figure 6 shows the EQE of polymer PDs with MoO3 and with PEDOT:PSS as the anode interfacial layer, respectively. The EQE of polymer PDs with MoO3 as the anode interfacial layer remained at ∼85% of the original value (from 13 to 11%) after the polymer PDs were exposed to air for 15 days; the corresponding responsivity

the Al cathode. As a consequence of reduced leakage current, a markedly lower dark current can be expected according to the thermionic emission theory.23 To restrain the dark current is one of the approaches to get high detectivity in polymer PDs.7,10,20,24,25 By incorporating the PFN as the electron extraction layer, a dark current of 1.25× 10−10 A/cm2 was observed. This low dark current indicated that the detectivities of current NIR polymer PDs are much better than those reported in our previous papers.24,25 In addition to the significant reduction in dark current, a slightly higher photocurrent under illumination λ = 800 nm was observed from the polymer PDs with a PFN interlayer. The EQE spectra of the polymer PDs illuminated by monochromatic light are shown in Figure 4b. It can be seen that the EQE has been improved almost over the whole detectable wavelength range. At λ = 800 nm, the EQE increased from 13 to 16% (by an extra increase of 23%), and the corresponding responsivity increased from 80 to 100 mA/W, which is in good agreement with enhanced photocurrent. The detectivity at λ = 800 nm was calculated to be 1.75 × 1013 Jones, which is two times higher than that without the PFN cathode interlayer. 6541

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fabrication, it is convincing that the polymer PDs are comparable to or even better than Si-based PDs.

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AUTHOR INFORMATION

Corresponding Author

*(X.G.) E-mail: [email protected]. Phone: +330-972-4983. Fax: 330-972-3406. (F.H.) E-mail: [email protected]. Phone: +86-20-87114609. Fax: +86-20-87110606. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was financially supported by the Ministry of Science and Technology (nos. 2009CB623601 and 2009CB930604), the Natural Science Foundation of China (nos. 21125419, 50990065, 51010003, and 51073058), and Guangdong Natural Science Foundation (grant no. S2012030006232).

Figure 5. Detectivities of Si photodetector and polymer photodetector versus wavelength.

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remained at 88% of the original value (from 80 to 70 mA/W). The EQE of polymer PDs used PEDOT:PSS as the anode interfacial layer degraded to 56% of the original value (from 16 to 9%) after being exposed to air for 15 days; the responsivity decreased from 100 to 60 mA/W, which is ∼40% degradation. It is clear that the polymer PDs with a thin MoO3 layer as the anode interfacial layer have better stability than those with PEDOT:PSS as the anode interfacial layer.

REFERENCES

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4. CONCLUSIONS In summary, we have synthesized a novel low-bandgap semiconducting polymer with a 1.1 eV bandgap. We further demonstrated solution-processed high-detectivity NIR polymer PDs by this novel low-bandgap semiconducting polymer blended with [6,6]-phenyl C61-butyric acid methyl ester. The polymer PDs exhibited a high responsivity of 100 mA/W and corresponding detectivity of 1.75 × 1013 cm•Hz1/2/W at wavelength λ = 800 nm. In addition, polymer PDs based on MoO3 as the anode interfacial layer showed superior long-term air stability as compared with those fabricated with PEDOT:PSS as the anode interfacial layer. With the high performance of the NIR polymer PDs operated at room temperature and the potential of low-cost and large-area

Figure 6. (a) EQE of polymer PDs with an architecture of ITO/PEDOT:PSS/PTZBTTT-BDT:PC61BM/PFN/Al. (b) EQE of polymer PDs with an architecture of ITO/MoO3/PTZBTTT-BDT:PC61BM/PFN/Al. Blue curve is the original test, black curve is the second test after 15 days. 6542

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp4001237 | J. Phys. Chem. C 2013, 117, 6537−6543