Organic near‐infrared photodetectors with photoconductivity‐enhanced performance

Organic near‐infrared (NIR) photodetectors with essential applications in medical diagnostics, night vision, remote sensing, and optical communications have attracted intensive research interest. Compared with most conventional inorganic counterparts, organic semiconductors usually have higher absorption coefficients, and their thin active layer could be sufficient to absorb most incident light for effective photogeneration. However, due to the relatively poor charge mobility of organic materials, it remains challenging to inhibit the photogenerated exciton recombination and effectively extract carriers to their respective electrodes. Herein, this challenge was addressed by increasing matrix conductivities of a ternary active layer (D–A–D structure NIR absorber [2TT‐oC6B]:poly(N,N′‐bis‐4‐butylphenyl‐N,N′‐bisphenyl)benzidin [PolyTPD]:[6,6]‐phenyl‐C61‐butyric acid methyl ester [PCBM] = 1:1:1) upon in situ incident light illumination, significantly accelerating charge transport through percolated interpenetrating paths. The greatly enhanced photoconductivity under illumination is intrinsically related to the unique donor–acceptor molecular structures of PolyTPD and 2TT‐oC6B, whereas stable intermolecular interaction has been verified by systematic molecular dynamics simulation. In addition, an ultrafast charge transfer time of 0.56 ps from the NIR aggregation‐induced luminogens of 2TT‐oC6B absorber to PolyTPD and PCBM measured by femtosecond transient absorption spectroscopy is beneficial for effective exciton dissociation. The solution‐processed organic NIR photodetector exhibits a fast response time of 83 μs and a linear dynamic range value of 111 dB under illumination of 830 nm. Therefore, our work has opened up a pioneering window to enhance photoconductivity through in situ photoirradiation and benefit NIR photodetectors as well as other optoelectronic devices.


INTRODUCTION
High-performance photodetectors have been widely applied to various areas, such as radiographic measurement, automatic detection control, photometric measurement, and optical communication. [1]Most commercially available photodetectors typically employ traditional inorganic materials such as Si, InGaAs, InP, CdS, InSb, etc., [1][2][3] which generally have limited optical tunability, low absorption coefficients, and comprehensive device fabrication processes.Due to the rigid nature of these thick inorganic thin films and substrates, their potential application for flexible and wearable devices is inhibited.[6] Especially, organic semiconductors own a high absorption coefficient of typically 10 5 cm −1 , and the very thin active layer of only 100 nm is sufficient enough to absorb most of the incident light.[13] Therefore, idealized device architecture with new organic materials has been explored to meet these innovative applications. [14]specially sensitive NIR OPDs are desirable for regular medical diagnostics such as measuring the pulse and blood oxygenation levels, which require necessary noninvasive penetration depth through skin or biotissues and low background noise from the ambient condition.In this regard, many aggregation-induced luminogens (AIEgens) [15] exhibit excellent photophysical properties in the NIR region, such as good biocompatibility and high sensitivity, and are successfully employed in bioimaging, [16][17][18] photodynamic therapy, [19][20][21] which is a promising candidate for NIR photodetectors. [22,23]owever, two critical drawbacks of organic semiconductors need to be addressed for effective NIR photodetection.First, due to the low relative permittivity of organic materials (ε r ≈ 3-4), photogenerated excitons usually have a high binding energy of about 0.35-0.5 eV, and large surface interfaces with energy favorable offsets are desirable for exciton dissociation.This drawback could be addressed by rationally designed bulk heterojunction (BHJ) architecture and optimized for interpenetrated percolation paths.Up to now, organic solar cells have achieved over 18% efficiency and an ultralong lifetime using ternary BHJ by newly designed molecules, regulating the morphology/domain of the film and reducing energy loss. [23]Second, due to the relatively low charge mobilities of organic materials (typically less than 1 cm 2 /V⋅s), [1] they are almost at the lower end of semiconductors. [24,25]Therefore, although organic materials exhibit strong absorptivity to generate sufficient exciton upon the light incident, bicontinuous networks of separated donor and acceptor phases need to be formed for efficient electron-hole pair separation and charge transport to their respective electrodes. [26]It is urgent to improve carrier mobility of the employed organic matrix and inhibit the excitons' recombination for NIR OPDs, maximizing effec-tive charge transport for sensitive photoresponse and device high-performance devices.
Although numerous efforts have been explored to increase charge mobilities of organic materials, such as adding more conductive polymers or dopants, [9,26,27] these strategies are ineffective.For example, the added polymers could improve active film formation.However, they could unexpectedly disrupt the necessary percolated donor and acceptor phases, causing an uncontrollable disordered domain and reduced charge collection efficiency. [28,29]Meanwhile, these doping engineering methods undoubtedly could cause complexity in material synthesis and affect the absorption of active layers. [25,30]Therefore, developing a more realistic strategy to increase charge mobilities without affecting device operation is still urgent.Surprisingly, the intrinsic photoconductivity of organic active thin films could be inherently improved upon light irradiation, [31] which suits perfectly for optoelectronic devices.Heeger and coworkers reported a novel conjugated polymer with photoconductive properties and proved its spectral sensitivity in the NIR region. [32]hoto-enhanced conductivities benefit charge transport of in situ photogenerated carriers in the optoelectronic devices, significantly reducing nonradiative recombination processes.
In this work, one D-A-D structure NIR absorber (labeled as 2TT-oC6B), poly(N,N′-bis-4-butylphenyl-N,N′bisphenyl)benzidin (PolyTPD), and [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM) were employed as the active layer of the OPDs.The pure individual films of Poly-TPD and 2TT-oC6B demonstrated one order of magnitude enhancement of measured current under incident light illumination by conductive atomic force microscopy (C-AFM), indicating interest photoconductivity.A self-driven OPD with 2TT-oC6B:PolyTPD:PCBM (1:1:1) ternary blend film has a response time as fast as 83 μs and a linear dynamic range (LDR) value of 111 dB under the illumination of 830 nm monochromatic NIR light.The ternary device shows a significant performance improvement compared to the binary device (without PolyTPD), with a rise time 55 times faster and the responsivity enhanced by 10 times.According to the femtosecond transient absorption test results, the charge separation time in the BHJ is significantly reduced from 3.78 ps (binary) to 0.56 ps (ternary).The photoconductivity of Poly-TPD and 2TT-oC6B is verified by C-AFM, and the enhanced photoconductivity has greatly contributed to NIR sensitivity.In addition, the molecular dynamics (MD) simulations verify that the improved photoconductivity is attributed to the donor-acceptor (D-A) structure of PolyTPD and 2TT-oC6B and the stable intermolecular interaction in the blend film.Therefore, this work has opened up a pioneering window to enhance inherent photoconductivity of organic materials by in situ photoirradiation, which could benefit photodetectors as well as other optoelectronic devices.

Design strategy for the organic photodetector
We fabricated the OPD employing the PolyTPD:2TT-oC6B:PCBM ternary blend BHJ as described in the experimental section.Figure 1A shows the chemical structures of the three components.2TT-oC6B displays typical aggregation-induced emission characteristics: dim emission in solution but bright fluorescence in the aggregate state due to restriction of intramolecular motion. [19]2TT-oC6B with NIR-I absorption (600-900 nm) and NIR-II emission (930-960 nm) has the quality of an excellent NIR OPD donor (Figures 1B and S1a), which could be prepared by facile solution process (Figures S1b and S3).The as-obtained PolyTPD:2TT-oC6B:PCBM ternary blend film has excellent absorption properties from 600 to 900 nm, which is inherently attributed to 2TT-oC6B (Figure S1c).The energy level of 2TT-oC6B was measured by ultraviolet photoelectron spectroscopy as shown in Figure S2.A vertical OPD device structure was employed and the energy levels of the blend film are shown in Figure 1D, indicating a favorable offset for exciton separation.As shown in Figure 1C,D, PCBM acts as an electron transport layer due to its lowest unoccupied molecular orbital (LUMO) and the deep highest occupied molecular orbital.PolyTPD can enhance the hole transportability and optimize the morphology of thin films.In addition, PolyTPD can also function as the electron blocking layer and suppress the exciton recombination contributing to its high LUMO.Furthermore, 3-(6-(diphenylphosphinyl)-2-naphthalenyl)-1,10-phenanthroline and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate are adopted as the electron and hole transport layer materials, respectively, to optimize charge transport. [33]

Device performance
The ternary device performance is significantly enhanced compared to the binary 2TT-oC6B:PCBM blend.Compared to binary OPD with 2TT-oC6B:PCBM blend film, the dark current of the as-obtained PolyTPD:2TT-oC6B:PCBM ternary device is substantially suppressed, while photocurrent is significantly improved due to PolyTPD participation, which is beneficial for device sensitivity enhancement (Figure S4). Figure 2A shows the semilog plot of the currentvoltage (J-V) characteristics of the ternary OPDs, which offers a low dark current of nearly 10 −11 A at 0 V bias.And under the illumination of 830 nm monochromatic NIR light intensities of 0.07, 0.38, and 2.60 W/cm 2 , the photocurrent density keeps increasing and reaches a magnitude of 10 −6 A.
The ternary OPDs exhibit a good linear response to the light intensities varying from 1 × 10 −6 to 1 mW/cm 2 at 830 nm (Figure 2B), which corresponds to an LDR of 111 dB.LDR illustrated the range within which the detector output scales linearly with the input signals, and its calculation method is shown in the experimental section.When monochromatic light changes to 771 and 680 nm, the LDR increases to 118 and 120 dB (Figure S5).These performances are praiseworthy compared with some traditional inorganic devices and excellent OPDs, such as GaN photodetectors (PDs) (50 dB) and InGaAs PDs (66 dB), and comparable with commercial Si PDs (120 dB) (Figure 2C). [9,34]The LDR of commercial silicon detectors is also characterized, as shown in Figure S6.
The detectivity of the wavelength and noise spectral density of ternary OPD are shown in Figure S7.
As shown in Figure S8, the binary OPD barely shows effective sensitivity, while ternary OPD has an obvious photoresponse from 400 to 900 nm at 0 V bias under 5 mW/cm 2 light illumination, indicating PolyTPD is necessary to be integrated into the active layer for effective hole transport.Otherwise, the ternary OPDs show a maximum R of 2.0 mA/W under no bias.The responsivity (R value) keeps increasing when employing a sufficiently large reverse bias and reaches 27 mA/W at −3 V bias voltage (Figure 2D), which should be attributed to more efficient photogenerated charges being extracted at the contacts driven by an increased applied electrical field.In addition to the high photoresponsivity, the ternary device shows an ultrafast photoresponse.The transient photocurrent of ternary devices was measured under no bias conditions at 680 nm (light-emitting diode) light intensity of 5 mW/cm 2 , achieving a fast transient response of rise time (T R ) at 83 μs (output signal increasing from 10% to 90% of the peak output value), and delay time (T D ) of 95 μs (output signal decrease from 90% to 10% of the peak output value) (Figure 2F).In contrast, the T R and T D of binary OPDs are 4.5 ms, almost 55 times slower than that of the ternary OPDs (Figure S9), demonstrating the superior charge transmission morality of the ternary OPDs F I G U R E 2 (A) J-V curves of the ternary organic photodetectors (OPDs) in the dark and under the illumination of 830 nm with different power densities.(B) Power-dependent photocurrent density.(C) Linear dynamic ranges (LDRs) of different photodetectors. [8,9,12,35,36,39,40](D) Bias-dependent responsivity.(E) Temporal response times at 0 V with a power density of 5 mW/cm 2 at 830 nm; inset: I-t curve.38]

DISCUSSIONS
Multi-scale characteristics, including film conductivity, intermolecular interaction, and exciton dynamics, were implemented to explore the mechanism of exciton dynamics and carrier transport.Hence, the performance of ternary and binary devices was systematically tested and the mechanism for the performance improvement was carefully investigated.

Thin film conductivity
C-AFM provides direct information about the specific nanoscale morphological changes in nanoscale and their impact on optical and electrical properties.As shown in Figures 3, S10, and S11, all the spin-coating films gain a smooth surface morphology with a height and amplitude varying less than the nanometer scale, as well as a uniform current density distribution.To note, all three singlecomponent films exhibited very weak currents.However, it is clear that the currents of 2TT-oC6B and PolyTPD under light are significantly improved compared to those under dark conditions, which is attributed to their D-A structure.
The binary system can slightly enhance the conductivity.2TT-oC6B:PCBM blend film shows almost no current (less than 1 pA) when measured in the dark.However, the current increase to around 5 pA after adding PolyTPD to the blend film.Under illumination, the current in the binary film changed only slightly, while the current in the ternary film was significantly increased to about 10 pA, indicating its high conductivity.That is to say, adding PolyTPD not only increases the intrinsic conductivity of the organic thin film matrix (conductivity in darkness) but, more importantly, enhances the photoconductivity of the matrix under light.For photodetector devices, increasing the photocurrent without consuming the dark current is beneficial for improving detection capability.Higher conductivity helps enhance the carrier transport and inhibit the recombination of carriers, resulting in a larger linear working region (LDR value) and faster response.

Exciton dynamics
The femtosecond transient absorption test was implemented to explore the exciton relaxation and charge transfer process upon photon excitation.As shown in Figure 4, in the 2TT-oC6B and PCBM binary films, relatively strong absorption appears at about 500 and 1030 nm.Among them, 520 nm is caused by terephthalic acid (TPA) radicals, [41] and 1030 nm is due to the absorption of PCBM radicals. [42]The attenuation of the absorption peak is related to the separation and coincidence of excitons.After photoirradiation, the 2TT-oC6B was excited, excitons were generated (labeled as *[PolyTPD:2TT-oC6B:PCBM]), and then the exciton separated, followed by transporting electron and hole to PCBM and PolyTPD, respectively (labeled as [PolyTPD ⋅+ :2TT-oC6B:PCBM ⋅− ]).Once electron and hole charges recombine, films could be recovered to the ground state.In the binary system, excitons have separation and recombination times of 3.78 and 393 ps, respectively.However, after adding Poly-TPD, the separation time of exciton is shortened to 0.56 ps, while the recombination time of excitons is increased to 527 ps.This is due to PolyTPD's good hole transportability and high LUMO value to block electrons.Therefore, faster exciton separation, as well as suppressed recombination, contributes to quick photoresponse and enhanced photocurrent.

Intermolecular interaction
The device performance dramatically varied with the different aggregation states of the BHJ.High-performance OPD requires sufficient exciton generation, effective exciton separation and efficient carrier transport.As shown in Figures S12 and 5A, the photocurrent of as-obtained ternary OPDs is dramatically increased with the amount of Poly-TPD + PCBM component in the blend film and reaches the maximum peak, followed by a quick drop.The continuous and interpenetrating percolation path formed by enough PolyTPD and PCBM components plays a dominant role in facilitating charge collection.Therefore, even though there is pure 2TT-oC6B in the matrix for sufficient NIR photon absorption, photocurrent is barely measurable since there is no effective exciton dissociation without any surficial interface.After adding PolyTPD + PCBM with a critical threshold point, the percolation path for electron and hole transport could be dynamically formed.Until the concentration of PolyTPD + PCBM reaches 66.7% (wt%) (or PolyTPD:2TT-oC6B:PCBM = 1:1:1), sufficient exciton generation, effective exciton separation and efficient carrier transport reach to a balanced point, contributing to the maximum collected photocurrent.With further increase in PolyTPD + PCBM components, less and less excitons are generated due to loss of the 2TT-oC6B NIR absorber, so the photocurrent begins to decrease.Meanwhile, MD simulations are implemented to study the interaction between these three components of PolyTPD:2TT-oC6B:PCBM (1:1:1) at the molecular level.Figure 5B shows the mixture after 50 ns of relaxation time, and the root-mean-square error of the three components proves that the system is stable (Figure S13).The 2TT-oC6B could be uniformly dispersed between PolyTPD and PCBM networks, and the long-chain PolyTPD could stretch out and wrap around to form a continuous channel for hole transport.Simultaneously, PCBM could be scattered with each other and creates a virtual channel for electron transport.Figures 5C and S14 exhibit the configurations and radial distribution functions between different components, quantitatively showing the interaction and molecular distance between molecules.2TT-oC6B, PCBM, and PolyTPD could all form relatively sharp peaks in radial distribution functions, indicating the stable molecular interaction force.The sharp peaks of 2TT-oC6B-2TT-oC6B show up at around 2 nm, indicating the most stable space gap for the interaction of two 2TT-oC6B molecules.Considering that the size of one 2TT-oC6B molecule is about 1 nm, [19] the 2TT-oC6B has been sufficiently separated without aggregation.2TT-oC6B and PolyTPD share a smaller distance of less than 1 nm, suggesting that the long-chain polymer wraps around the small molecule 2TT-oC6B due to the stable D-A interaction between the donor TPD group on PolyTPD and the acceptor core of 2TT-oC6B.At the same time, the distance between 2TT-oC6B and PCBM is around 1 nm, as the TPA group on 2TT-oC6B can form hydrogen bonds with the terminal O on PCBM, creating D-A interaction.Therefore, the 2TT-oC6B is surrounded by PCBM and PolyTPD, so that photogenerated electrons can hop to PCBM while holes hop to PolyTPD after exciton dissociation at desirable heterojunction at the interfaces, followed by efficient charge collection through percolated pathways.
Most interestingly, the average current estimated by C-AFM shows that both 2TT-oC6B and PolyTPD have demonstrated about one order of magnitude enhancement under the same incident light illumination compared with a dark condition, indicating inherently enhanced photoconductivity for hole transport, as shown in Figure 5D.In contrast, the PCBM as an electron transport layer remains quite similar.To note, the dark current of as-obtained NIR OPDs is substantially suppressed due to their extremely low mobilities, which is beneficial for low noise current.As a result, this unique ternary device could form an idealized architecture for sensitive high photocurrent upon incident light with suppression of dark current, as well as ultrafast photoresponse, exhibiting outstanding photoresponse properties.The fundamental operation mechanism is schematically illustrated in Figure 5E.For the device operated under light, both electron and hole carriers could be transported swiftly on their respective vehicle once separated from excitons.

CONCLUSION
A self-driven OPD with a rationally designed AIEgen BHJ (2TT-oC6B:PolyTPD:PCBM = 1:1:1) has successfully demonstrated a fast LDR response, which is comparable to silicon-based devices.The ternary device shows a significant performance improvement compared to the binary device (without PolyTPD); the rise time is greatly improved from 4.5 ms to 83 μs, and the responsivity is enhanced by 10 times.According to the results of the femtosecond transient absorption test and MD simulations, the charge separation time in the BHJ is improved from 3.78 ps (binary) to 0.56 ps (ternary).The device enhancement originates from the photo-enhanced conductivity by photoconductive materials (PolyTPD and 2TT-oC6B), facilitating effective charge separation and accelerating charge transfer along percolation pathways.The dynamic operation mechanism of this highly sensitive NIR OPD can be summarized as follows: first, the ultrafast charge transfer time of 0.56 ps from 2TT-oC6B to PolyTPD and PCBM potentially ensures effec-tive electron-hole pair dissociation, pumping up sufficient free electron and hole charges.Second, the ordered interpenetrating percolation paths formed in the ternary phase at threshold point facilitate charge transport, substantially reducing nonradiative recombination.Third, the suddenly enhanced photoconductivity upon light illumination accelerates charge transport and collection, greatly contributing to the photocurrent.Given the universality of poor mobility in organic semiconductors, enhancing the matrix conductivity by in situ photoirradiation benefits photodetectors as well as other optoelectronic devices such as organic photovoltaics.

F
I G U R E 5 (A) Dependence of photocurrent on content of (poly(N,N′-bis-4-butylphenyl-N,N′-bisphenyl)benzidin [PolyTPD] + [6,6]-phenyl-C 61butyric acid methyl ester [PCBM]) (1:1 by weight) in the PolyTPD:D-A-D structure near-infrared absorber (2TT-oC6B):PCBM ternary system.(B) Percolation pathways in the OPD are simulated by molecular dynamics.(C) Configurations and radial distribution functions between different components.(D) Average current value estimated by conductive atomic force microscopy (C-AFM) image.(E) Schematic diagram of the comparison of the carrier transport capability of the device in dark and light conditions.

A
U T H O R C O N T R I B U T I O N S Siwei Zhang analyzed the experimental data, implemented the molecular dynamics simulation, and wrote the manuscript.Zhenlong Li, Jingzhou Li, and Xubiao Li fabricated and measured the device.Shunjie Liu synthesized the 2TT-oC6B.Bingzhe Wang and Guichuan Xing performed the femtosecond transient absorption test.Fang Chen and Jiangyu Li performed the C-AFM measurements.Ben Zhong Tang, Guodan Wei, and Siwei Zhang discussed the manuscript outline.Jacky W.Y. Lam, Zheng Zhao, and Feiyu Kang revised the manuscript.All authors discussed the results and commented on the manuscript.