An Alternating D1-A-D2-A Conjugated Ternary Copolymer Containing [1,2,5]selenadiazolo[3,4-c]pyridine Unit With Photocurrent Response Up to 1,100 nm

Two narrow band gap conjugated ternary copolymers comprising two electron-rich (donor, D) and one electron-deficient (acceptor, A) moieties regularly alternating along the polymer backbone were designed and synthesized. The polymers with the repeating unit in a D1-A-D2-A manner were constructed by copolymerizing a bisstannyled-D1 (D1 = n-alkyl-substituted cyclopentadithiophene) and a dibromo-monomer (Br-A-D2-A-Br, D2 = branched-alkyl-substituted cyclopentadithiophene, A =[1,2,5]selenadiazolo[3,4-c]pyridine or 5-fluorobenzo[c][1,2,5]selenadiazole) through a palladium-catalyzed Stille polymerization. This approach that enables variations in the donor fragment substituents can not only control the polymer regiochemistry but also the solubility. Two ternary copolymers exhibited absorbance up to near-infrared region along with relatively narrow band gap in the range of 1.02–1.26 eV. The polymeric photovoltaic cells based on CDTPSE/PC61BM show the short circuit density of 1.45 mA cm−2, open current voltage of 0.53 V, and photocurrent spectra response from 300 to 1,150 nm under AM 1.5 simulator (100 mW cm−2). It is indicated that it can be potentially applied to near infrared photodetectors.

Two narrow band gap conjugated ternary copolymers comprising two electron-rich (donor, D) and one electron-deficient (acceptor, A) moieties regularly alternating along the polymer backbone were designed and synthesized. The polymers with the repeating unit in a D1-A-D2-A manner were constructed by copolymerizing a bisstannyled-D1 (D1 = n-alkyl-substituted cyclopentadithiophene) and a dibromo-monomer (Br-A-D2-A-Br, D2 = branched-alkyl-substituted cyclopentadithiophene, A =[1,2,5]selenadiazolo [3,4-c]pyridine or 5-fluorobenzo[c][1,2,5]selenadiazole) through a palladium-catalyzed Stille polymerization. This approach that enables variations in the donor fragment substituents can not only control the polymer regiochemistry but also the solubility. Two ternary copolymers exhibited absorbance up to near-infrared region along with relatively narrow band gap in the range of 1.02-1.26 eV. The polymeric photovoltaic cells based on CDTPSE/PC 61 BM show the short circuit density of 1.45 mA cm −2 , open current voltage of 0.53 V, and photocurrent spectra response from 300 to 1,150 nm under AM 1.5 simulator (100 mW cm −2 ). It is indicated that it can be potentially applied to near infrared photodetectors.
-Broad photocurrent response up to 1,100 nm was realized.

INTRODUCTION
Conjugated polymers have attracted both industrial and academic interests due to the potential for large-area, flexible, and low-cost applications in recent years. Great progress has been made in related research areas based on conjugated polymers including polymer light-emitting diode (PLED), polymeric field-effect transistor (PFET), polymer solar cell (PSC) (Zhao et al., 2016). The conjugated polymers with narrow band gap are promising for the development of polymer photodetectors (PPDs) due to the broad absorption and photoresponse (Gong et al., 2009). Photodetectors from the ultraviolet (UV) to near-infrared (NIR) wavelength is critical for multi-functional applications including chemical/biological sensing, image sensing, environmental monitoring, remote control (Chen et al., 2016). Conventional photodectors exhibited limited absorption ranges through inorganic materials including GaN, Si, InGaAs (Hu et al., 2013). Polymer photodetectors (PPDs) achieve broad spectral response and high sensitivity through photoinduced electron transfer from donor materials (conjugated polymers) to acceptor materials (fullerene derivatives) (Chen and Cao, 2009). For polymer photodetectors applications, donor materials with a narrow band gap are desirable because of their contribution to obtaining photocurrent at long wavelength including NIR region (Hendriks et al., 2014). The common methods to reduce the band gap of conjugated polymers such as extension of conjugation length or introduction of strong donor and acceptor groups can often lead to a rise in the highest occupied molecular orbital (HOMO) energy level . High-lying highest occupied molecular orbital (HOMO) energy level of conjugated copolymers is essentially unfavorable for ambient stability of devices (Miao et al., 2018). The open circuit voltage (V oc ) of OPVs including photodetectors is directly related to the bang gap between the HOMO of donor materials (conjugated polymers) and the lowest unoccupied molecular orbital (LUMO) energies of the acceptor (fullerene). To promote the V oc , the D-A type of conjugated copolymers containing weak donor unit (D, donor) and strong acceptor (A, acceptor) along polymer backbone are synthesized (Han et al., 2017). The "weak donor-strong acceptor" strategy which is widely used in the synthesis of donor materials in OPVs can not only maintain low-lying HOMO energy levels to promote the V oc but also effectively reduce band gap through intramolecular charge transfer (ICT) (Zhou et al., 2010). It is difficult to balance between selecting weak donor to promote the V oc value and selecting strong donor to broad spectral range to the NIR. Bazan et al. reported a conjugated polymer containing pyridyl[2,1,3]thiadiazole (PT) acceptor unit with very low E g -eV oc loss in OPVs by controlling conjugated polymer regioregularity (Wang et al., 2014). It is possible to fabricate promising PPDs materials with strong donor unit in conjugated polymer by controlling the precise orientation of the donor or acceptor units relative to the backbone vector (Qin et al., 2014).
Compared with pyridyl[2,1,3]thiadiazole (PT), [1,2,5]selenadiazolo[3,4-c]pyridine (PSe) has lower band gaps and the wavelength region has longer in near-infrared (NIR) wavelength than the sulfur based analogs (Hou et al., 2008). In fact, some Se-containing electron-acceptor building blocks, such 2,1,3-benzoselenadiazole and the derivatives are generally poorly soluble. In order to overcome the poor solubility and improve the regularity of the polymer, we proposed regular conjugated ternary copolymers consisting of ternary components with the repeating units denoted as D1-A-D2-A (Huang et al., 2015). This means that the solubility of the D1-A-D2-A structure polymer can be altered by changing of the solubilizing groups on D1 or D2 fragment, respectively. Compared with other well-known donor units, the CDT unit SCHEME 1 | Synthetic route of D1-A-D2-A ternary copolymers.
The two polymers were purified by Soxhlet extraction by using methanol, acetone, and hexane successively to remove the oligomer and catalyst. The number-average molecular weight (M n ) and polydispersity index (PDI) of the two polymers were determined by gel permeation chromatography (GPC) at 150 • C using 1,2,4-trichlorobenzene as the eluent and polystyrene standards. The number-average molecular weight (M n ) was determined to be 21.9 and 46.1 kDa for CDT-PSE, CDT-FBSE, respectively, with PDI values of 2.7 and 2.3, respectively. Thermal properties of the two polymers were detected by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) measurements. No discernable phase transition was realized in DSC characteristics up to 300 • C. TGA measurements demonstrated that the decomposition temperatures (corresponding to 5% weight-loss temperatures) were 354.3 and 365.2 • C for CDT-PSE and CDTF-BSE, respectively, implying good thermal stabilities of the two polymers (Supplementary Figure 8).

Optical Properties
Absorption spectra in the 300-1,400 nm region of CDT-PSE and CDT-FBSE in o-dichlorobenzene solution with concentration of ca. 1 × 10 −5 g mL −1 as well as in thin films are shown in Figure 1. Two polymers show two absorption bands. The high energy absorbance band located in the range of 300-500 nm can be attributed to the π-π * transition of D1-A-D2-A backbones in dilute solutions, while the low energy absorbance peak displayed typical dual band features of D-A copolymers with the intramolecular charge transfer (ICT) effects between the donors and the acceptor moieties located at long wavelength up to NIR regions. Broader and further red-shifted bands arise in the solid-state absorption spectra. The maximum absorption edge of CDT-PSE and CDT-FBSE was at 1,200 and 1,000 nm, respectively. Relative to CDT-FBSE, the absorption peak of CDT-PSE as a thin film was broader in width, which is consistent with the greater degree of structural order due to the PSe nitrogen atoms. The optical band gaps (Eopt g) of the two D1-A-D2-A polymers as estimated from the onset of the absorbance of solid films are in the range of 1.02-1.26 eV.

Electrochemical Properties
To investigate the influence of regioregularity on the frontier energy levels of the D1-A-D2-A structure polymer, cyclic voltammetry (CV) measurements were performed. The measurement was carried out in tetra(n-butyl)ammonium hexafluorophosphate (n-Bu 4 NPF 6 , 0.1 M in acetonitrile) solution by using a ITO glass electrode as working electrode and saturated calomel electrode (SCE) as a reference electrode at a scan rate of 50 mV s −1 . The films of CDT-PSE and CDT-FBSE were cast from o-DCB solution with a concentration of 10 mg mL −1 and all measurements were performed under an inert atmosphere. The CV characteristics of two polymers are demonstrated in Figure 2. It is assumed that the redox potential of Fc/Fc+ was measured as a standard, which has an absolute energy level of −4.80 eV to vacuum. The onset of oxidation (E ox ) of the two polymers was estimated to be 0.14, and 0.37 eV for CDT-PSE and CDT-FBSE in the same experimental conditions, respectively (Supplementary Figure 9). Therefore, the highest occupied molecular orbitals (E HOMO ) of the two polymers were calculated according to the following equation of E HOMO = -e(E ox +4.80) (eV). Consequently, the HOMO values are −4.94 and −5.17 eV for CDT-PSE and CDT-FBSE, respectively. According to the equation of E LUMO =-E HOMO + Eopt g, the E LUMO are calculated to be −3.92 and  PC 61 BM as the photoactive layer, which showed a moderate PCE = 0.49% (V oc =0.63 V, J sc = 2.37 mA cm −2 , and FF = 33.2%). We speculated that the moderate value in V oc can be attributed to the decreased HOMO level. The decreased short circuit current (J sc ) in CDT-PSE can be attributed to the lack of driving force for the charge separation in the interface of polymer and PC 61 BM due to the relatively low-lying LUMO energy level of CDT-PSE. The external quantum efficiency (EQE) curves are shown in Figure 4. Both the polymers show a broad EQE response from 300 nm to NIR region. The photosensitivity response of CDT-FBSE is higher over a wide range compared to CDT-PSE. But it is worth noting that CDT-PSE exhibits prolonged response beyond 1,100 nm, which reveals its great potential for the application in NIR photodetector.

Morphology
Tapping mode atomic force microscopy (AFM) was investigated the surface topography of the bulk heterojunction polymer: PC 61 BM (1:1 in wt:wt) blend films (Figure 5). The blend film of CDT-PSE: PC 61 BM showed comparatively smooth surface topography with a root-mean-square roughness (RMS) value of 0.47 nm. From Figure 5B one can clearly observe the rough morphology of CDT-FBSE: PC 61 BM with a rootmean-square (RMS) roughness of 0.42 nm. The smooth surface topography of the polymer/PC 61 BM components may give rise to this non-optimal nanostructure, which is essentially not favorable for charge carrier transporting to the corresponding electrodes and would lead to relatively low J sc of the device.

CONCLUSION
In conclusion, two novel D1-A-D2-A ternary copolymers containing asymmetric reactivity unit ([1,2,5]selenadiazolo[3,4c]pyridine or 5-fluorobenzo[c][1,2,5]selenadiazole) was synthesized by stepwise approach. Incorporating the branchedalkyl-substituted cyclopentadithiophene (D2) to the A-D-A intermediates is of great benefit to solubility and purification of the PSe or FBSe units. We have shown that the D1-A-D2-A repeating structure can control the regiochemistry of the polymer backbone and indicate a much lower E g -eV oc loss than the D-A analogs. We have presented the n-alkyl-substituted cyclopentadithiophene (D1) to enhance the intermolecular π-stacking interactions. The polymers  CDT-PSE and CDT-FBSE show wide light absorption ranges of 300-1,000 and 300-1,100 nm in solid thin film, respectively. The photoresponse wavelengths of the CDT-PSE based on CDT-PSE/PC 61 BM blends extend to about 1,100 nm, illustrating its great potential for the application in NIR photodetector.

Materials
All reagents were purchased from commercial sources such as from Aldrich, Acros and TCI Chemical Co. The 1 H and 13 C NMR spectra were measured on a Bruker AV-300 (300 MHz) in a deuterated chloroform solution with tetramethylsilane(TMS) as the internal standard.

Measurement and Characterization
1 H and 13 C NMR spectra were measured on Bruker AVANCE Digital 300 MHz NMR workstation or Bruker AVANCE Digital 400 MHz NMR workstation. The number-average molecular weights (Mn) were determined by Polymer Laboratories PL220 Chromatograph (150 • C in 1,2,4-trichlorobenzene) with linear polystyrene as the standard. Cyclic voltammograms (CV) were recorded on CHI 600D electrochemical workstation at a scan rate of 50 mV s −1 . The tapping-mode atomic force microscopy images were performed on a Nano-Scope NS3A system (Digital Instrument) to observe the surface morphologies of the ITOcoated glass substrates. UV-vis spectra were obtained by a HP 8453 spectrophotometer. Thermogravimetric analyses (TGA) measurements were carried out with a NETZSCH TG 209 under a heating rate of 10 • C min −1 and a N 2 flow rate of 20 mL min −1 . Differential scanning calorimetry (DSC) were conducted on a Netzsch DSC 204 under N 2 flow at heating and cooling rates of 10 • C min −1 .