Achieving Highly Sensitive Near‐Infrared Organic Photodetectors using Asymmetric Non‐Fullerene Acceptor

Organic photodetectors (OPDs) based on non‐fullerene acceptors (NFAs) have received considerable attention because of their potential for use in various commercial applications as near‐infrared (NIR) light sensing platforms. However, recent OPDs suffer from low NIR photoresponse and large dark/noise currents with narrow bandgap organic photoactive materials. Herein, a π‐bridge molecular engineering strategy replacing alkoxythienyl with benzothiadiazole for ultra‐narrow bandgap (ultra‐NBG) NFAs is designed to achieve simultaneously high photoresponse at NIR region and low noise current density, thereby leading to excellent NIR (≈1050 nm) detectivity (D*). The newly synthesized ultra‐NBG NFAs, namely COB and CBT with optical bandgaps below 1.14 eV, present high responsivity (R) with 0.369 and 0.080 A W−1, respectively, at a wavelength of 1050 nm. Especially, with effectively suppressed noise current density, COB‐based OPD exhibits a high NIR (≈1050 nm) D* value of 2.18 × 1011 cm Hz1/2 W−1 at −0.5 V bias. The obtained R and D* values for these NFAs exceed or are comparable to those of a commercial Si photodetector at 1050 nm. This work provides important insight into the π‐bridge molecular engineering strategy for ultra‐NBG NFAs, which facilitate achieving highly sensitive NIR OPDs with high NIR photoresponse and low dark/noise current.

acceptors based on only a few NIR polymer donors. [16][17][18][19][20] From the perspective of molecular design, nonfused NFAs with strong noncovalent intramolecular interactions present a highly feasible design rationale, which can lead to structural diversity and tailorable optoelectronic properties with a synthetic simplicity that is not found for fused-ring electron acceptors. [21][22][23] For example, in the previous report, ultra-narrow bandgap (ultra-NBG) nonfused NFAs of COTIC-4F, CO1-4F, and CTIC-4F, which were synthesized with cyclopentadithiophene (CPDT) derivative as the strong electron-rich central donor (D), alkoxythienyl (D′) and alkylthienyl (D′′) as the -bridge spacer, and 2-(5,6-difluoro-3oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (IC-2F) as the strong electron-poor acceptor (A), demonstrated a clear transition of molecular orbital energy levels, optical energy bandgap (E g opt ) and spectral window of optoelectronic devices. [24] By replacing strong electron-donating alkoxythienyl of D′ with weak electron-donating alkylthienyl of D′′, the CO1-4F (A-D′-D-D′′-A) and CTIC-4F (A-D′′-D-D′′-A) show deep-lying highest occupied molecular orbital (HOMO) energy levels of −5.3 and -5.4 eV with E g opt of 1.2 and 1.3 eV, while the COTIC-4F (A-D′-D-D′-A) has a HOMO energy level of −5.2 eV with E g opt of 1.1 eV. Also of note is that the CO1-4F and CTIC-4F-based OPDs exhibit more sensitive detection of injected light with the optimization of responsivity (R) and dark current density (J dark ) characteristics. Nevertheless, in comparison with COTIC-4F, the absorption maxima of CO1-4F and CTIC-4F molecules were significantly blue-shifted from ≈1000 to 830 nm, and therefore, the NIR detection capability of CO1-4F and CTIC-4F based OPDs seriously deteriorated. From the molecular design and device engineering point of view, the deepened HOMO energy level with introduction of an alkylthienyl unit on -bridge of NFA can contribute to the suppression of J dark as a result of the limited hole injection from the cathode to acceptor HOMO. [25,26] However, with a weak intramolecular charge transfer (ICT) character, the increase of E g opt of CO1-4F and CTIC-4F make the OPD devices lose the responses to NIR light over 1050 nm. These findings promoted us to consider a new molecular design of NFAs with a particular interest in achieving high responses while retaining longwavelength NIR detections over 1050 nm.
We herein report the strategic -bridge engineering of ultra-NBG nonfused NFAs based on the COTIC-4F conjugated framework with the aim of achieving highly sensitive NIR (≈ 1050 nm) detection of OPDs. The molecular structure of the NFAs, abbreviated as COT, COB, and CBT, were designed based on a strong electron-rich central donor CPDT (D) and alkoxythienyl (OT) or benzo[c]- [1,2,5] thiadiazole (BT) as -bridges ( = D′ or A′) flanked with two strong electron-poor termini IC-2F (A), resulting in an A--D--A-type molecular skeleton (Figure 1a). The -bridge engineering mainly serves the purpose of retaining strong ICT characters and downshifting HOMO energy levels of resultant NFA by replacing OT with BT. The BT-based conjugated materials generally feature narrow-bandgap properties with long-wavelength absorption capabilities, which can be readily realized by donor-acceptor alternation. In addition, due to the strong electron-withdrawing nature of BT unit, the relevant molecules possess lower-lying frontier molecular orbitals relative to the counterpart molecules with electron-rich units. [27][28][29][30][31][32][33] In this regard, the rationale of our molecular design is to retain strong ICT of NFA (e.g., COB) as well as to downshift HOMO energy level of NFA for simultaneously achieving high photoresponse and detectivity over 1050 nm through the BT-based -bridge engineering. Indeed, the COB displays strong NIR absorption ranging from 700 nm to 1100 nm with an E g opt of 1.08 eV, which is comparable to E g opt of COT. Furthermore, with an excellent NIR R of 0.369 A W −1 at 1050 nm and low J dark of 5.22 × 10 −8 A cm −2 at −0.5 V bias, the COB-based OPD exhibits a high NIR (≈ 1050 nm) D * value of 2.68 × 10 12 cm Hz 1/2 W −1 . In a detailed physical and electrical investigation, with a loosely packed -stacking structure and well-mixed phase morphology, the COB-based photoactive layer exhibit reduced charge carrier mobility and relatively low trap states, which can effectively suppress the J dark with minimal loss in the R and external quantum efficiency (EQE). [34,35] Therefore, based on the presented results, it is assumed that the tailoring effect of -bridge engineering on the optoelectronic properties of ultra-NBG NFA facilitates the cooptimization of NIR absorption and J dark characteristics, thereby leading to excellent NIR R and D * performance of OPDs. Finally, to provide accurate noise estimation, the noise power spectral density is further investigated and the COB-based OPD demonstrate the noise-based D * value of 2.18 × 10 11 cm Hz 1/2 W −1 at a wavelength of 1050 nm, presenting a promising potential of our -bridge engineering strategy of ultra-NBG NFA for the development of high D * NIR OPDs. This work provides a feasible approach to modify energy level of novel NFAs while obtaining NIR absorption for highly sensitive OPDs and semitransparent devices.

Results and Discussion
The synthetic and characterization details for the prepared compounds are provided in the Supporting Information. Briefly, symmetric acceptors COT and CBT were synthesized according to a previously reported method with slight modifications. [24] The preparation of asymmetric acceptor, COB, begins by coupling a mono-stannylated precursor of CPDT (D) and a monobrominated precursor of BT (A′) via a Stille cross-coupling reaction. Subsequent C-H activated arylation was conducted to produce an asymmetric bis-aldehyde derivative and finally, a Knoevenagel condensation with IC-2F afforded COB under mild conditions at 45°C to avoid the occurrence of side reactions. In our molecular design, the ethylhexyl groups on the thienyl -bridges were replaced by butyloctyl groups to improve the solution processability compared to COTIC-4F.
Density functional theory (DFT) calculations at B3LYP/6-31G** level have been carried out to rationalize the molecular design strategies ( Figure 1b). As shown in Figure S9 (Supporting Information), COT shows coplanar -backbone geometry, while COB and CBT have slightly twisted conformations with a dihedral angle of ≈20°between BT and IC-2F due to their steric repulsion. Molecular orbital distributions of the calculated COT model in the HOMO and lowest unoccupied molecular orbital (LUMO) levels were spread over the entire conjugated backbone. Compared with the COT model, the molecular orbital of BT in COB model was highly localized in the LUMO level owing to the strong electron-withdrawing character of BT, whereas BT partially contributed to HOMO level ( Figure 1b). It indicates that a new ICT character in the COB model has occurred owing to the replacing the -bridge of COT with the BT. Similarly, the introduced BT in CBT model also showed the tendency of molecular orbital distributions like the COB model, on the other hand, the ICT strength would be expected relatively weak due to the absence of strong donating skeleton (OT-CPDT). To gain deeper insight into the detailed molecular orbital distribution and electronic transition of the three models, we conducted charge transfer (CT) analysis based on the density difference grid as shown in Supporting Information, Figure S10. The details about S 0 , S 1 transitions are tabulated in Table S1 and all the molecules show strong S 0 → S 1 transition with large oscillator strengths ranging from 2.50 to 2.69. The large oscillator strengths are attributed to well-distributed HOMOs and LUMOs over the -backbones. Also, it is our interest to explore how the incorporation of BT unit affects the ICT by charge density difference (CDD) maps. The CDD is estimated as overlapped two centroids of charge associated with the region of density decrease and increase. [36][37][38] The COT and CBT, as a symmetric OT or BT -bridged NFA, show virtually identical centroid positions for positive (C + ) and negative (C -) densities attributed to their symmetrical structure. On the other hand, the COB as an asymmetric OT and BT -bridged NFA shows slightly separated C + and C − , signifying the transition of ICT character (Supporting Information, Figure S10). The calculated bandgap of Table 1. Optical and electrochemical properties of COT, COB, and CBT. COB could retain an energy bandgap (E g ) of 1.69 eV with a deeplying HOMO energy level of −5.44 eV compared with that of COT (E g of 1.64 eV and HOMO energy level of -5.24 eV). However, the calculated bandgap of CBT was a considerable wide value as 1.78 eV owing to weak ICT character ( Figure 1b). The optical and electrochemical properties of each acceptor film were measured, as shown in Figure 1c-d (see also Table  1 and Supporting Information Figure S11). More specifically, a strong NIR absorption can be observed at 600-1200 nm for the NFAs (Figure 1c), wherein the longest wavelength maxima ( max ) of ≈1000 nm were obtained for COT and COB, as confirmed by the simulated absorption spectra (Supporting Information, Figure S11). This indicates that the COB molecules retain strong ICT characteristics, which could be attributed to the appropriate push-pull arrangement and thereby efficient electronic communication through the conjugated backbone. COT exhibits an absorption profile with a vibronic shoulder peak (i.e., representing a 0-1 transition) at ≈870 nm, indicating that COT molecules could form close -stacking interactions between their end groups in the films. [27,39] Notably, a distinct 0-1 shoulder peak of COB was also observed at a similar wavelength. In contrast, CBT shows featureless absorption from 600 to 1100 nm, probably because its torsions between BT (A') and IC-2F (A) on both sides hinder close packing. Cyclic voltammetry (CV) measurements were carried out in order to estimate orbital energy levels (Supporting Information, Figure  S12). The HOMO levels of NFAs, estimated from the onset of oxidation, are ≈ −5.22, −5.30, and −5.41 eV for COT, COB, and CBT, respectively, which is consistent with a tendency observed by the DFT result. This indicates that the incorporation of the BT -bridge downshifts HOMO levels and such shifts would lead to the increment of ΔE HOMO values relative to PTB7-Th as expected. LUMO levels are calculated by the following equation: The resulting HOMO energy level estimates of −4.14, −4.22, and −4.27 eV for COT, COB, and CBT, respectively.
To investigate the photodetector characteristics of COT, COB, and CBT, we fabricated OPDs devices with inverted structures based on the ITO/ZnO/PTB7-Th: NFAs (≈80 nm)/MoOx/Ag architecture (Figure 2a). The external quantum efficiency (EQE) spectra are presented in Figure 2b, wherein it can be seen that COT, COB, and CBT exhibited gradual blue shifts in their EQE maxima from 990 to 940 nm and then to 850 nm. These trends in the EQE spectra were expected based on the differences in the absorption spectra between the various NFAs. However, the COB OPD demonstrated higher EQE values between 300 and 970 nm than the COT OPD, whereas the CBT OPD presented a lower EQE value in the overall spectral region. Figure 2c shows the semilog plots of J-V characteristics of the three OPD devices under dark conditions. J dark , which is defined as the current generated in the absence of light, must be suppressed to ensure a low noise and a high D * for sensitive detection of weak light signals. As shown in Figure 2c and Table 2, the OPD devices comprising PTB7-Th:COT, PTB7-Th:COB, and PTB7-Th:CBT exhibited J dark values of 2.29 × 10 −6 , 5.22 × 10 −8 , and 2.26 × 10 −7 A cm −2 at −0.5 V, respectively. The statistical data obtained from more than 14 devices at −0.5 V demonstrated a sufficient reliability for not only the determined J dark values but also shunt resistance for these OPD devices (Supporting Information, Figure S13 and S14). Furthermore, a comparison of J-V characteristics and electrochemical impedance spectroscopy (EIS) analysis of OPD devices with or without ZnO under dark conditions was carried out and make sure the bulk property of photoactive layer, rather than that of the device interface, is a dominant factor for the difference of J dark between OPD devices. (Supporting Information, Figure  S15, S16, S17 and Table S2). Generally, it is challenging to achieve NIR OPDs with low J dark values because their relatively low E g values promote charge injection from the metal electrodes or thermal generation within the photoactive layer. [27] In this sense, it is noteworthy that COB and CBT, which are -bridges engineered with consideration of their symmetry and intermolecular interactions, exhibit J dark values that are one or more orders of magnitude lower than that of COT.
To examine the responses of the OPDs with variation in the incoming photon energy, the R of each OPD device was evaluated as a function of the incident light wavelength. The specific value of R was calculated using the relationship: Where is the wavelength of the incident light and the EQE is measured as a function of incident light. It was found that the optimal R values for COT, COB, and CBT were 0.465 A W −1 at 1010 nm, 0.455 A W −1 at 970 nm, and 0.360 A W −1 at 880 nm, respectively, while their R values at a constant wavelength of 1050 nm were 0.429, 0.369, and 0.080 A W −1 , respectively (Figure 2d and Supporting Information, Figure S18 and Table S3). The above results, therefore, confirm that the COB system exhibits a relatively small blue shift and a superior performance in terms of R compared to the CBT system. In addition, at a wavelength of 1050 nm, the COB system achieved an R-value that was comparable to that of commercialized Si-photodiode detectors (i.e., ≈0.38 A W −1 ). [40] Based on the J dark -V characteristics and the R values, the value of D * which represents an achievable signal-to-noise ratio for a 1 A signal from a detector with an active area of 1 cm 2 and an electrical bandwidth of 1 Hz, is defined as follows:  where A is the effective area of the photodetector (cm 2 ), Δf is the detection bandwidth (Hz), and i noise is the noise current (A). In OPDs, the shot noise (i shot ) and the thermal noise (i thermal ) are the dominant sources of the noise current and called "white" noise (i white ). [43] Accordingly, the noise current can be estimated as follows [42,43] : where q is the elementary charge (≈1.602 × 10 −19 C), I dark is the dark current, k B is the Boltzmann constant (≈1.38 × 10 −23 J/K), T is the temperature (K), and R shunt is the shunt resistance (Ω) (Supporting Information, Figure S19). As a result of this calculation, the COB OPD was found to exhibit a significantly higher whitenoise-limited specific detectivity (D * white ) of 2.68 × 10 12 cm Hz 1/2 W −1 at a wavelength of 1050 nm compared to the COT and CBT OPDs (i.e., 4.75 × 10 11 and 2.84 × 10 11 cm Hz 1/2 W −1 ), respectively; this value for the COB OPD is also higher than previously reported literature values (Figure 2e,f and Supporting Information, Table S3). Furthermore, despite the lower D * white of the CBT OPD in 1000-1100 nm wavelength range, this OPD exhibits a higher D * white than the COT OPD in all wavelength ranges under 1000 nm. Therefore, it can be assumed that engineering of the -bridge of the CPDT backbone is responsible for the variation in J dark , wherein the significantly lower values obtained for COB and CBT lead to their superior D * white values. To explore the correlation between the morphology of the photoactive layer and the device characteristics, atomic force microscopy (AFM) and transmission electron microscopy (TEM) measurements were performed (Figure 3a,b). The PTB7-Th:COT and PTB7-Th:COB blends formed well-mixed, uniform BHJ films with low root mean square (RMS) roughness values of <1.2 nm. However, evidence of large phase segregation with a high RMS value of ≈4.8 nm was observed for the PTB7-Th:CBT blends film. We, therefore, considered that these large domains result from the lower solubility of CBT induced by a reduction in the number of solubilizing groups, which leads to early demixing of the blend components during the film evolution period. [15] As a result, the high surface roughness can increase the interfacial resistance, causing detrimental effects in terms of charge separation and collection. [44,45] For a more detailed structural analysis considering the molecular arrangement and crystallinity of each blend film grazing incidence wide-angle X-ray scattering (GIWAXS) was employed. As can be seen from Figure 3c, the diffraction patterns of the blend films of PTB7-Th with COT, COB, and CBT show a clear preference for the face-on orientation of the molecular backbone on the substrate, with both (100) peaks in the in-plane (IP) and (010) peaks in the out-of-plane (OOP) direction being observed. In addition, the two-dimensional (2D) GIWAXS patterns of the PTB7-Th:COB and PTB7-Th:CBT blend films showed features characteristic of an overall weakened crystallinity, with a reduced number of diffraction peaks compared to that of the PTB7-Th:COT blends film. In the case of the face-on oriented structure, the (010) peak in the OOP direction corresponds to the diffraction of the conjugated backbone -stacking. Interestingly, in accordance with the result of DFT calculation for the conformation of NFA molecular backbone, diffraction patterns of the COT, COB, and CBT neat films exhibit a gradual increase in -stacking distance from 3.41 to 3.44 nm and then to 3.48 nm (Supporting Information, Figure S20 and Table S5). In addition, the q z values of the (010) peaks of the PTB7-Th:COT, PTB7-Th:COB, and PTB7-Th:CBT blend films in the OOP direction were ≈ 1.785, 1.770, and 1.775 Å −1 (d = 3.52, 3.55, and 3.54 Å, respectively) (Figure 3d and Supporting Information, Table S6), which indicates that the PTB7-Th:COB and PTB7-Th:CBT blend films contain more loosely packed -stacking structures than PTB7-Th:COT. Furthermore, estimation of crystal coherence length (CCL) and the full width at half maximum (FWHM) of the azimuthal intensity angle plot for the (010) peak in OOP direction indicated that the PTB7-Th  Figure S21 and Table S6). These results reveal that introduction of the benzothiadiazole A' unit into the CPDT backbone can reduce the intermolecular interactions between the conjugated backbone of the blend film, ultimately resulting in a reduction in the overall crystallinity with an enlarged packing distance and misorientation of the -stacking structures.
It is widely accepted that a deterioration in the charge carrier mobility of a photoactive layer can lead to a decrease in J dark because J dark depends linearly on the charge carrier mobility. [25] To calculate the effective mobilities (μ) of the prepared blend films, we considered the photocharge-carrier extraction method with a linearly increasing voltage (photo-CELIV). More specifically, this is a powerful technique for investigating the effective mobilities of photoinduced charge carriers in optimized devices for real applications in terms of the device configuration and thickness of the component layers. Thus, the effective mobility can be calculated as follows: where d is the photoactive layer thickness, A is the slope of the linearly increasing voltage pulse, A = d(V t −V 0 )/dt), t max is the time corresponding to the highest value of the extraction current, Δj is the photo-induced current of the photoactive layer, Δj = j(t max ) − j(0), and j(0) is the displacement current. The effective mobilities for the charge carriers in the PTB7-Th blend with COT, COB, and CBT were therefore determined to be 3.21 × 10 −5 , 2.85 × 10 −5 , and 1.66 × 10 −5 cm 2 V −1 s −1 , respectively (Figure 4a-c and Supporting Information, Table S7). Consequently, it can be observed that a larger dihedral angle on the CPDT backbone resulting from the introduction of a benzothiazole-based A' unit not only leads to a larger packing distance and misorientation of the -stacking structures, it also reduces the effective mobility, as determined by the photo-CELIV technique. However, it should be noted that the photo-CELIV results obtained for our three blend systems (i.e., μ COT > μ COB > μ CBT ) were not linearly consistent with the J dark results (i.e., J dark, COT > J dark, CBT > J dark, COB ). Thus, to obtain further insight into the charge recombination characteristics of these blends, the dependence of the shortcircuit current density (J sc ) and open-circuit voltage (V oc ) on the light intensity (P light ) was investigated (Figure 4d,e). The correlation between J sc and P light follows the power law (J sc ∝ P light , where is the exponential factor). [46,47] In the plot of J sc as a function of P light (ranging from 1 to 100 mW cm −2 ) with neutral density filters, the value of the COB device (0.974) was higher than those of the COT (0.929) and CBT (0.895) devices, indicating a decreased space charge limitation and reduced bimolecular recombination in the COB device compared with those of the COT and CBT devices (Figure 4d and Supporting Information Table  S7). The ideality factor, n, is the factor that describes the dependence of V oc on light intensity. In theory, n is defined as follows, n = q Tk B dV oc dln (P) (6) where q is the elementary charge, k B is the Boltzmann constant, T is the temperature, and P is the light intensity. [48,49] The value of n can be estimated from the slope of light intensity dependence of V oc curve. The n value closer to unity indicates that bimolecular recombination is the dominant recombination process, while n > 1 indicates that trap-assisted recombination is the dominant recombination process in the bulks. And, recent reports show that n < 1 can be obtained due to the influence of traps at surface. [50,51] In the semilogarithmic plot of V oc as a function of light intensity (ranging from 1 to 100 mW cm −2 ) with neutral density filters, the n for the PTB7-Th:COT, PTB7-Th:COB, and PTB7-Th:CBT OPDs were 1.22, 1.06, and 1.25, respectively; this indicates a reduction in trap-assisted recombination in the PTB7-Th:COB device compared with that in the PTB7-Th:COT and PTB7-Th:CBT devices (Figure 4e). The exciton dissociation probability (P diss ), which is calculated as P diss = J ph J sat −1 under short-circuit conditions (where J ph is the photocurrent density and J sat is the saturated J ph at a high effective voltage (V eff )), was also investigated using the plot of J ph (J ph = J illumination − J dark ) as a function of V eff (V eff = V 0 − (V app -JR s ), where V 0 is the voltage at which J ph = 0, V app is the applied voltage, and R s is series resistance). [52][53][54][55] In the plot of J ph − V eff , the calculated P diss values of the COT, COB, and CBT devices are 71.3, 74.4, and 63.9%, respectively (Supporting Information, Figure S22 and Table S7). These outcomes indicate that the degree of exciton dissociation in the COB device is higher than those in the COT and CBT devices, ultimately suppressing severe charge recombination at the donor-acceptor interface. [56][57][58][59] Based on these results, both a reduction in the charge carrier mobility and the suppression of bimolecular and trap-assisted recombination in the PTB7-Th:COB blend film can be associated with the effective suppression of J dark , thereby leading to excellent R and D * values at a wavelength of 1050 nm.
For a more detailed investigation of the noise characteristics, the noise current with a noise power density of −0.5 V was further measured, as shown in Figure 4f and Supporting Information Figure S23. In the low-frequency region, the noise power density spectra exhibited a ≈1/f relationship. Upon increasing the applied frequency, the noise power density gradually decreased and finally reached a frequency-independent regime. Compared with the COT and CBT devices, the COB device was found to exhibit frequency-independent characteristics in the relatively lowfrequency region, in addition to presenting a noise power density that was 1-2 orders of magnitude lower throughout the overall frequency range. Since the 1/f noise characteristic in the lowfrequency region indicated the occurrence of carrier trapping and de-trapping processes in the photoactive layer with numerous traps, it can be assumed that the lower trap density of the photoactive layer of the COB device contributes to the suppression of J dark . [60][61][62] Furthermore, from the value of D * attributed to the noise current (D * noise ), it was observed that although the D * noise demonstrated relatively lower value than the D * white obtained using i shot and i thermal , the COB device continued to exhibit a D * noise value that was an order of magnitude larger than those of the COT and CBT devices at 1050 nm (i.e., 2.18 × 10 11 cm Hz 1/2 W −1 for the COB device, c.f., 2.26 × 10 10 and 1.99 × 10 10 cm Hz 1/2 W −1 for the COT and CBT devices), respectively; this D * noise value for the COB OPD is also greater than previously reported literature values at the wavelength of over 1000 nm (Figure 4g and Supporting Information Figure S24). We also performed -3 dB frequency (f -3 dB ) and linear dynamic range (LDR) measurements at -0.5 V reverse bias for examining speed and reliable light intensity of the fabricated OPDs (Figure 4h,i). The f -3 dB values for COT-, COB-, and CBT-based OPDs were determined to be 62.5, 52.1, and 26.9 kHz, respectively, the level of which is well corre- lated with the charge carrier mobility in Figure 4a-c. In addition, LDR values for COT-, COB-, and CBT-based OPDs were 72, 97, and 82 dB, respectively, implying that COB device exhibited the largest reliable light detection range due to their lowest J dark level.

Conclusion
In summary, new nonfused NFAs with ultra-narrow bandgaps of <1.14 eV were successfully designed and synthesized throughbridge engineering for lower HOMO level and effective NIR photoresponse. The blends of NFA with PTB7-Th were found to possess adequate bandgaps for use in near-infrared (NIR) applications, with high R of 0.429, 0.369, and 0.080 A W −1 at 1050 nm for COT, COB, and CBT, respectively. The incorporation of benzothiadiazole as a -bridge was considered to lower the HOMO energy levels and release -stacking interactions, affecting dark current densities of the relevant devices. In particular, COB exhibited a remarkably high D * white of 2.68 × 10 12 , and D * noise of 2.18 × 10 11 cm Hz 1/2 W −1 with a −0.5 V bias at 1050 nm. Notably, COB is one of few electron acceptor materials featuring an ultranarrow bandgap (<1.1 eV) and high sensitivity beyond 1000 nm. These results can provide new insights into the material design and thus are expected to play a role as a material that can be commercialized in the OPD application market in the future.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.