High-Performance Organic Photodetectors by Introducing a Non-Fullerene Acceptor to Broaden Long Wavelength Detective Spectrum
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We demonstrate the broadband visible organic photodetectors (OPDs) by introducing a non-fullerene acceptor of 3,9-bis(2-methylene-(3-(1,1dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3d:2,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene (ITIC) into the bulk heterojunction (BHJ) based on a conventional system of poly(3-hexylthiophene-2,5-diyl) (P3HT):[6,6]-phenyl C71-butyric acid methyl ester (PC71BM) .The resultant OPDs exhibit a specific detectivity beyond 1012 Jones in the whole visible region ranged from 380 nm to 760 nm, and the highest detectivity reaches 2.67 × 1012 Jones at 710 nm. UV-Vis absorption spectrum, steady-state photoluminescence, atomic force microscopy, and space-charge-limited current property were applied to analyze the film characteristics of obtained OPDs. Owing to the long-wavelength absorption band of ITIC, the spectral photodetection range has been broadened effectively, and better film morphology, more effective energy transfer, and the reduced electron mobility in the active layer are responsible for the excellent photodetection capability. The proposed scheme provides a reliable strategy for implementing high-performance broadband visible OPDs.
KeywordsOrganic photodetectors Non-fullerene acceptor Surface morphology UV-Vis absorption Full visible light photodetection
Atomic force microscope
External quantum efficiency
The highest occupied molecular orbital
Indium tin oxide
Dark current density
The current density-voltage
The lowest unoccupied molecular orbital
Organic solar cells
[6,6]-Phenyl C71-butyric acid methyl ester
Root mean square
Visible light, as part of electromagnetic spectrum that can be directly perceived by human vision (380–780 nm), plays an important role in daily life and industrial production . Visible light remote sensing is the most commonly used in aerial photographic reconnaissance. Color image sensing is also mostly based on visible light, etc. . As a bridge between the optical signal and electrical signal, photodetector plays an irreplaceable role in the above applications, thus causing extensive and continuous attention . Therefore, the research of high-performance visible photodetector is imperative and of great significance. Compared with traditional inorganic photodetectors, organic photodetectors (OPDs) have attracted tremendous attention for applications in flexible and portable electronic applications due to their flexibility, tunable absorption, lightweight, large-area detection, and low cost of preparation . In recent years, although OPDs have made some achievements in such aspects as high-external quantum efficiency , low dark current density  and high detectivity , there are only a few research attempts to investigate high-performance broadband OPDs with full visible photodetection until now.
The efficient light harvesting and broad absorption range are of crucial importance in broadband OPDs. Therefore, many donor and acceptor materials with different band gaps have been developed and many classical donor/acceptor heterojunction systems have been constructed in the course of past research . Among them, poly(3-hexylthiophene) (P3HT):phenyl-C71-butyric acid methyl ester (PC71BM) bulk heterojunction (BHJ) has been widely studied in organic photovoltaic devices, on account of its relatively high-carriers mobility, stable performance, simple structure, low cost, and mature preparation process [9, 10]. Nevertheless, although the spectral response of P3HT:PC71BM covers 400–600 nm, it is not wide enough to constitute full visible photodetection, because of the absence of the long-wave region. Therefore, it is necessary to find an effective method to expand the spectral response range of P3HT:PC71BM conventional system. Similar to organic solar cells (OSCs) [11, 12], introducing a third material into the active layer is one of the most efficient and simple methods to fulfill the broadband OPDs with extended photodetection range and excellent performance . For example, Rauch et al. developed the P3HT:PC71BM BHJ where PbS quantum dots as the introducing component, which successfully extended the detective range of OPDs to 1800 nm . Mario Caironi et al. developed the T1:P3HT:PC71BM OPDs with broadband response of 360–680 nm by introducing a middle-wavelength-absorption electron donor T1 .
Recently, a new class of non-fullerene electron acceptors has shown high absorption coefficients and excellent electrical properties, yielding widespread concern in the research of photovoltaic devices [16, 17]. Compared with conventional fullerene derivatives acceptors, non-fullerene acceptors have diversified and strong absorption, so they are the better options to introduce into the traditional system as the third component . For example, Tan et al. developed a ternary acceptor blending device by doping 3,9-bis(2-methylene-(3-(1,1dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3d:2,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene (ITIC) in the PBDTBDD:PC60BM blend to achieve perfect complementary absorption and high PCE of 10.36% . Furthermore, the distinctive feature of ITIC is the long-wave spectral response of 600–800 nm, compared with the short and medium wave response inherent in traditional fullerene derivatives. Therefore, ITIC may be suitable for combination with P3HT:PC71BM BHJ with the response of 400–600 nm, which can extend the photodetection range to the long-wave range to realize the effective photodetection of full visible spectrum continuously.
Hence, in this work, ITIC is firstly introduced into P3HT:PC71BM conventional system to form broadband OPDs. Compared with the control P3HT:PC71BM OPDs, the ternary blends system achieves a wider spectral response. Meanwhile, by tuning the ratios of ITIC and PC71BM respectively, the broadband OPDs covering the full visible band from 380 nm to 760 nm are obtained, compared with the original photodetection band of 380–620 nm. Moreover, due to the wider light harvesting region, better film morphology, more effective energy transfer, and the lower dark current, the optimizing OPDs exhibited a high detectivity of 2.12 × 1012 Jones and 2.67 × 1012 Jones at 560 nm and 710 nm, respectively.
Before starting the OPDs fabrication, ITO substrates were consecutively cleaned in ultrasonic bath for each 10 min with water-detergent solution, acetone solvent, deionized water, and IPA solvent, respectively . After dried in the oven, these ITO substrates were treated with oxygen plasma for 20 min. Then, PEDOT:PSS was spin-coated at 3000 rpm for 60 s on ITO substrates. After thermal annealing at 150 °C for 20 min, the substrates were moved into a high-purity glovebox (O2, H2O < 1 ppm). P3HT, PC71BM, and ITIC were dissolved in chlorobenzene with different mass ratios. The total concentration of these materials was fixed at 30 mg ml−1, and the blend mass ratio of donor (P3HT) and acceptors (PC71BM, ITIC) was fixed at 1:1. Active layer solutions were spin cast on the top of PEDOT:PSS layer at 2000 rpm for 60 s. Subsequently, the blend films were annealed at 120 °C for 10 min. Followed by the deposition of Ag as anode at a deposition speed of 5 Å S−1. The active area of these OPDs was 0.02 cm2.
The ultraviolet-visible (UV-Vis) absorption was measured by using a Shimazu UV1700 UV-Vis spectroscopy system. The steady-state photoluminescence (PL) was measured by using a Hitachi F-7000 PL spectroscopy. Surface morphologies of active layers were characterized by atomic force microscope (AFM, AFM 5500, Agilent, Tapping Mode, Chengdu, China). A light source was used as an AM 1.5 G solar simulator with an illumination power of 100 mW cm−2. The current density-voltage (J-V) curves of OPDs in the dark and under illumination were measured with a Keithley 4200 programmable voltage-current source. The EQE spectra were obtained under a xenon lamp light passing through a monochromator. All parameters were measured at room temperature (T = 300 k).
Results and Discussion
Characterization of Active Layers
where ε0 is the vacuum permittivity, ε is the relative permittivity of the organic materials, μ is the charge carrier mobility, V is the applied voltage, and d is the thickness of the active layers. J-V characteristics in dark condition for the electron-only devices with different active layers are shown in Fig. 3b. According to Eq. (1), the electron mobility of devices with different ratios are 1.48 × 10−3 cm2 V−1 s−1, 8.92 × 10−4 cm2 V−1 s−1, 7.89 × 10−4 cm2 V−1 s−1, 4.75 × 10−4 cm2 V−1 s−1, and 4.43 × 10−4 cm2 V−1 s−1, respectively. With the increase of the proportion of ITIC, the electron mobility of device decreases significantly since the electron mobility of ITIC is lower than PC71BM , which may cause the dark current of the OPDs to decrease after the introducing of ITIC .
According to the absorption spectra of active layers, the long-wavelength absorption band of introduced ITIC should be able to broaden the long wavelength photodetection range of OPDs effectively. Furthermore, the introduction of ITIC also changes the electrical properties and surface morphology of active layers. From the perspective of SCLC, the introduction of ITIC reduces the electron mobility of the active layer, which obviously would reduce the carrier transport capacity of the devices. This would have the same adverse effect on dark current and photocurrent. However, the introduction of ITIC also allows the active layer to capture more photons from long wavelength to contribute photocurrent, which overcomes the adverse effect of low electron mobility on photocurrent under light condition. Better film morphology and more effective energy transfer in the ternary active layer are also beneficial to the excellent photocurrent. In conclusion, dark current will decrease with the addition of ITIC, while photocurrent will change regularly under the influence of various factors. Therefore, it is necessary to prepare OPDs constructed by active layers with different ratios to determine the high photocurrent and low dark current, so as to achieve excellent photodetection performance.
Performance of OPDs
Additionally, to make sure the OPDs have a stable and recoverable response ability, the current density as a function of time is shown in Fig. 5c for the broadband OPDs with various ratios. The cyclical current signals were recorded upon the on/off modulation of the light illumination. Each cycle is 20 s with an exposure time of 10 s and the total duration is 120 s. The results show that the current of each OPD increases significantly under illumination and returns to the original level after the light is turned off. It is obvious that these OPDs have stable and repeatable response/recovery characteristics, which is desirable for practical applications .
To further investigate the influence of the ITIC ratio on recombination of OPDs in light condition, JSC as a function of light intensity is plotted. In general, a power law dependence between JSC and I can be expressed as JSC∝Iα. When α approaches 1, bimolecular recombination is relatively weak [28, 29]. As shown in Fig. 5d, the OPDs with the ratio of 1:1:0, 1:0.7:0.3, and 1:0.5:0.5 have the similar α values, which are 0.817, 0.797, and 0.803, respectively. This means that these three OPDs have a similar level of bimolecular recombination. However, due to the introduction of ITIC, more long-wave photons are absorbed in ternary active layers, so that the photocurrent of the OPDs with moderate doping ITIC is greater than that of the P3HT:PC71BM OPDs. As further changing the ternary ratios to 1:0.3:0.7 and 1:0:1, the α values drop to 0.713 and 0.680, respectively. This indicates that the large amount of ITIC doping intensifies the recombination and significantly reduces the photocurrent.
Photodetective performance of obtained OPDs
EQE (%) @
R (A W−1) @
D* (× 1012 Jones) @
where EQE is external quantum efficiency, q is the electron charge, λ is the wavelength of incident light, h is the Planck constant, and v is the frequency of light. According to Eq. (2), the trend of R is dependent on the EQE and λ when the other parameters are constant. The calculated results of R values are shown in Fig. 6b and Table 1. Similar to the EQE curves, 1:0.5:0.5 based OPDs obtain higher R than other OPDs in both long wavelength and short wavelength range. The R values of optimizing broadband OPDs reached 0.21 A W−1 and 0.25 A W−1 at 560 nm and 710 nm, respectively. The wide R curve indicates that the broadband OPDs doped with appropriate amount of ITIC can absorb the incident light of the full visible spectrum evenly and convert it into photocurrent efficiently.
The calculated results of D* are shown in Fig. 6c. For the control OPDs based on P3HT:PC71BM, the detectivity exceeds 1.0 × 1012 Jones from 380 nm to 600 nm and reaches 1.67 × 1012 Jones at 560 nm. For comparison, OPDs doping by ITIC have extended the effective photodetection range to the full visible spectrum of 380–760 nm. Specifically, the detectivity of obtained OPDs with ratio of 1:0.5:0.5 reached 2.12 × 1012 Jones and 2.67 × 1012 Jones at 560 nm and 710 nm, respectively. On the one hand, the photodetection range of OPDs have been broadened by the addition of ITIC. On the other hand, the detectivity of optimizing OPDs in the full visible spectrum is higher than that of other OPDs, which is caused by high photocurrent and low dark current at the optimizing ratio of active layer.
In summary, the high-performance OPDs with full visible light photodetection are fabricated by introducing a non-fullerene acceptor of ITIC into the P3HT:PC71BM control system. The three materials form the complementary spectrum, which together effectively realize a broadband photodetector covering whole visible spectrum. Moreover, the OPDs with appropriate ratio of P3HT:PC71BM:ITIC exhibit a better photon-harvesting ability, lower dark current, more efficient energy transfer, and more favorable film morphology to improve detectivity. Remarkably, our approach is concise, highly reproducible, and scalable. Our work indicates that choosing suitable non-fullerene electron acceptor and binary system to construct the active layer of complementary light absorption spectrum is an effective method to achieve high-performance broadband OPDs, which will be widespread applicable in the future research.
GY designed and carried out the experiments. GY, ZW, YD, and DZ participated in the work to analyze the data and prepared the manuscript initially. JY gave materials and equipment supporting. All authors read and approved the final manuscript.
This work was financially supported by the National Key R&D Program of China (Grant No. 2018YFB0407102), the Foundation of National Natural Science Foundation of China (NSFC) (Grant Nos. 61421002, 61675041, and 51703019), and Sichuan Science and Technology Program (Grant Nos. 2019YFH0005, 2019YFG0121, and 2019YJ0178).
The authors declare that they have no competing interests.
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