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Abstract
We demonstrated an organic upconversion device (UCD) successfully converted input NIR light (850–1310 nm) into 524 nm green emission. The UCD under 980 nm light irradiation exhibits a high photon to photon conversion efficiency of 12%. In addition, the linear dynamic range reaches 72.1 dB and the maximum on/off ratio of luminance reaches 4.4×104, which guarantee the clarity of imaging from 850 to 1310 nm. The UCD in this work has the characteristics of high efficiency and long wavelengths detection, and it makes some senses for long wavelengths NIR bio-imaging in further researches.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Near-Infrared (NIR) up-conversion devices (UCDs) are the devices that can convert invisible infrared light to visible light, which enable the detection and processing of NIR light more convenient and easier [1–3]. Because of this, NIR UCDs have great potential applications in night vision, military, security and noninvasive/pixel-less biomedical imaging, which have increasingly gained a lot of attention in recent years [4–14]. NIR UCDs using inorganic materials first came to the attention of researchers, and have achieved conversion from NIR light to visible light [15–18] by using inorganic semiconductors such as GaAs, AlGaAs and many other lanthanide materials. However, these inorganic NIR UCDs are almost using epitaxial growth techniques and as a result, they require strict lattice matching and are expensive to prepare. In addition, inorganic materials cannot be prepared in large areas and on flexible substrates, which is the main challenge for inorganic materials UCDs.
Organic materials have the advantages in low cost, flexibility and large area preparation with substrate-independent growth, which gives the above problems a possibility. As a result, the researches about pure organic NIR UCDs become a new focus [19–21]. For the detection of UCD, there are three structures of organic photodetector (OPD). Although in recent year photomultiplier type OPD has high external quantum efficiency (EQE) and good detection performance from 350–850 nm [22–24], the energy level and device structure mismatch with UCD, so more photodiode type devices are chosen as the detection part. Early in 2002, M. Chikamatsu and colleagues [20] achieved the conversion from 780 nm NIR to blue emission by using TiOPc as a photosensitive layer (PSL). With the development of organic optoelectronics, the detectable wavelength of organic NIR UCDs has increased from 830 to 1100 nm in 2020 [21,25–32]. Although some progress has been made in wavelength detection, the photon to photon conversion efficiency (P-P efficiency), which is an important parameter can reflect the NIR UCDs ability to convert a NIR photon into a visible photon, keeps in low level: about 25% at 780 nm [28] but lower than 1% at 900 nm [30]. At the same time, the low linear dynamic range (LDR) also hinders the imaging application. LDR is closely related to the imaging quality, and the higher LDR means the better contrast, which enables the images more clearly. LDR correlates to both wavelength and power density of the laser. The UCD published by D. Y. Kim and et al [21] has the highest LDR of 62.9 dB under 14.1 mW/cm2 at 830 nm, and S. J. He and et al [30] got the UCD with a LDR of 62.6 dB at 980 nm under 84.6 mW/cm2, so that it is difficult for them to use the UCD to apply in complicated imaging. In 2021, C. J. Shih reported a new UCD with a deduced resolution of 1588 dpi at 3 V bias [28]. In order to get longer detectable wavelength, using nanoparticles or other materials has become a new solution. K. Strassel has reported a synthesis and the UCD device integration of short-wave infrared (SWIR) squaraine dyes which has a long detect wavelength of 800–1200 nm [33]. In 2020, H. Li used a temporal approach to realize multiplexed UCD (at 808 nm) in vivo imaging [34]. In fact, because of the longer detecting wavelength required, nanoparticles doped device used more in bio-imaging [18,32,34,35]. However, the doped nanoparticles, some of which are doped with metals that are harmful to the environment, are not friendly enough to the environment compared to all-organic devices. Therefore, to achieve a wider spectral range as well as higher conversion efficiency and LDR become urgent problems for the current organic NIR UCDs.
In this work, blended organic film of poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b’]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)] (PTB7-Th): 2,2'-((2Z,2'Z)-(((4,4,9,9-tetrakis(4-hexylphenyl)-4,9-dihydro-sindaceno[1,2-b:5,6-b’]dithiophene-2,7-diyl)bis(4-((2-ethylhexyl)oxy)thiophene-5,2- diyl))bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1- diylidene))dimalononitrile (IEICO-4F): 2,2'-((2Z,2'Z)-(((4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b’]dithiophene-2,6-diyl)bis(4-(heptan-3-yloxy)thiophene-5,2-diyl))bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (COTIC-4F) is employed as the PSL for UCD to absorb broad NIR light reaching far to 1300 nm. Combining with a familiar phosphorescent emissive layer (EML) of bis(2-phenylpyridinato-C2,N) (acetylacetonate)iridium(III) (Ir(ppy)2acac) doped 4,4-N,N-dicarbazole-biphenyl (CBP), the photon to photon conversion efficiency (P-P efficiency) of the UCD reached 12% at 980 nm under the power density of 0.48 mW/cm2. At the same time, when the input power density is under 0.02 mW/cm2, the UCD still has response. Furthermore, the turn-on voltage (Von, the bias value at the luminance of 0.1 cd/m2) of this UCD is 2.05 V at 980 nm (208 mW/cm2), while under the dark condition, the Von is 7 V. This assures a good contrast, and the maximum LDR reaches 72.1 dB at 9 V under 980 nm, which is higher than current UCDs. The organic NIR UCD of this work successfully achieves the bio-imaging application in NIR region from 850 nm to 1310 nm. Under all these different wavelengths of radiation, the details of the imaged biological samples can be clearly distinguished. The above good device performance makes some sense to future research about longer detecting and imaging in the NIR region.
2. Materials and methods
ITO-coated glass substrates were purchased form South China Science & Technology Company Limited. Organic materials used in the UCD were all purchased from other firms, including Nanjing Zhi-yan for PTB7-Th, Organrec Ltd. for IEICO-4F and COTIC-4F, and Lumtec. for TAPC, CBP, Ir(ppy)2acac and TPBi. The ZnO was prepared by the sol-gel process. The solution was mixed up with 110 mg zinc acetate, 31 mg ethanol amine, and 1 ml 2-methoxyethanol and stirred over 8 hours. All devices were fabricated onto patterned ITO substrates with a sheet resistance of approximately 17 Ω and an active area of 10 mm2. All commercially available reagents and solvents were used without further purification. The standard device structure for the UCD was ITO/ZnO(20 nm)/PTB7-Th:IEICO-4F:COTIC-4F(1:0.5:1 wt%)(doped 2% CN in the CB solution)(150 nm)/TAPC(30 nm)/CBP:3%Ir(ppy)2acac (30 nm)/TPBi(50 nm)/LiF(1 nm)/Al(100 nm).
The ITO substrate was washed by the washing detergent and dried in the oven at 120 °C for 2 hours. After ultraviolet treating for 30 minutes, coating with the ZnO solution on the substrate with 5000 rpm, 30 seconds. And then annealed in an ambient atmosphere at 200 °C for an hour. Following this, transferring it to the nitrogen glove box to coat the photosensitive layer, with 800 rpm for 40 seconds and annealed in the nitrogen atmosphere at 100 °C for 10 minutes. Then transferred it into the evaporation facility, and the rest of layers were all vacuum deposited at a pressure under 5×10−4 Pa. The deposition rates were less than 1.5 Å/s for organic materials, and 0.1 Å/s and 10 Å/s for LiF and Al, respectively.
3. Results and discussion
The organic NIR UCD in Fig. 1(a) has a structure of ITO/ZnO(20 nm)/PTB7-Th: IEICO-4F: COTIC-4F (1:0.5:1 wt%) (150 nm) (doped 2% chlorine naphthalene (CN) in chlorobenzene(CB) solution)/TAPC (30 nm)/ CBP: Ir(ppy)2acac (7% wt%) (30 nm)/TPBi (50 nm)/LiF (1 nm)/Al(100 nm). The chemical structures of other used materials were shown in Fig. S1. The thickness of the device is tested by the profile meter. ZnO is the electron transport layer (ETL) facilitating the transport of dissociated electrons, which also acts as a barrier for holes, and 20 nm is the optimal thickness for the high Von [36].
Fig. 1. Structures and work mechanisms of UCD in this work. (a) UCD structure and PSL used materials’ chemical structures. (b) Normalized absorption spectrum of the PSL film and EL spectrum of the UCD. (c) Energy levels diagram and work mechanism in the dark condition of the UCD. (d) Work mechanism under NIR light illumination of the UCD.
The bulk heterojunction (BHJ) structure of PTB7-Th: IEICO-4F: COTIC-4F is used as the photosensitive layer (PSL), in which PTB7-Th is used as donor, and two non-fullerene small molecule materials are served as acceptors. In an effort to assess the stability and large-area uniformity of the PSL, using atomic force microscope (AFM) and transmission electron microscope (TEM) to test morphology characteristics of the blend film, as shown in Fig. S2. The root mean square (RMS) of the surface roughness of the layer is found to be 2.93 nm. The smooth surface leads to an easier growth for the next layer [37]. Simultaneously, the different scales of TEM also show the low aggregation, which is helpful to the transition of the carriers [38]. The absorption spectra of single materials in the PSL were shown in Fig. S3, and it can be seen that the absorption peaks of PTB7-Th, IEICO-4F and COTIC-4F are 639/698, 850 and 1010 nm, respectively. While the absorption of blended film which has a wide absorption from 700 to 1300 nm with three peaks of 724, 863 and 965 nm, as shown in Fig. 1(b) (red curve), has a certain degree of deviation because of the aggregation of the mixed materials. There is a sandwich-like structure above the PSL, with a hole transport layer (HTL) of TAPC, EML of Ir(ppy)2acac doped CBP, phosphorescent emissive layer (ETL) of TPBi and cathode from the bottom up. Ir(ppy)2acac is a high-efficiency phosphorescent emitter, which makes NIR UCD have a high performance. The emission spectrum of the UCD was also shown in Fig. 1(b) green curve, with an emitting peak at 524 nm. The energy levels and work mechanism diagrams of UCD were drawn in the same graph to help analyze the working mechanism better. Two acceptors with band gaps both less than 1.2 eV are selected, and it can theoretically detect wavelengths beyond 1000 nm, which is corresponding to the absorption spectrum.
Since photoexcitons cannot be generated under dark conditions (Fig. 1(c)), meanwhile the high valence band of ZnO hinders the migration of holes from anode, the device is hard to give luminance which means it has a high Von theoretically. On the other side, when the NIR light is irradiating (Fig. 1(d)), the molecules in PSL are excited by photons and generate excitions, which then produce holes and electrons. The matched energy levels of each material also assure the efficient carriers transport in UCD. Under the action of built-in electric field and bias voltage, the holes migrate to EML and combine with electrons from the cathode to give luminance. Accordingly, UCD has a rigorous theoretical basis for the conversion of NIR to visible light.
In order to better analyze the features of UCD, we first tested its detection part, an organic photodetector (OPD), by the configuration of: ITO/ZnO/PTB7-Th: IEICO-4F: COTIC-4F (1:0.5:1 wt%)/MoO3/Ag. Firstly, the EQE with different negative bias under the dark condition were tested, reflecting the performance of the entire absorption band device for photon utilization. Since the EQE testing instrument required a detector replacement for testing over 1100 nm, two graphs were drawn. One was the EQE from 300 to 1100 nm shown in Fig. 2(a), the other was from 900 to 1340 nm shown in Fig. S4 (a). From Fig. 2(a), there is a broad range from 700 to 1000 nm that EQE keeps at the same level, which matches with the absorption curve in Fig. 1(b) well. When the device is biased at 0 V (black curve), the EQE is 31.87% at 980 nm. With the increase of bias, the EQE increases to 34% and 35.6% under -0.5 and -1 V, respectively. And from Fig. S4 (a), the EQE is about 6.02% at 1100 nm, and 0.14% at 1300 nm. With the bias increasing from -0.5 V to -1 V, the EQE increases to 1.3% and 2.7% at 1300 nm respectively. After that, we tested the current density-voltage (J-V) characteristics of the detection unit under different wavelengths, which was shown in Fig. 2(b). Without NIR illumination, the dark current density is 5.29×10−8 A/cm2 at -1 V. When the input power at the same level, with the wavelength increases the current density decreases, which has the corresponding trend as the EQE. At the same time, when wavelength increases from 850 to 1064 nm, the response maintained at similar levels. Due to the high-power of 1310 nm laser, it is not convenient to analysis at the same graph, its J-V curves under different powers were shown in Fig. S5. Another important parameter of OPDs is photoresponsivity (R) calculated by the equation of $R = EQE \times q/h\upsilon $, where q, h and υ are one electron charge, Planck’s constant, and the frequency of light. The calculated values of R under different wavelengths at -1 V and -0.5 V were listed in Table 1. The R is 281.69 mA/W under 980 nm at -1 V. To further understand the OPD performance under different powers, the detectivity (D*) is calculated by: ${D^\ast } = {{\sqrt {A\Delta f} } / {NEP}}$, where A is active area, Δf is bandwidth and NEP is noise equivalent power. Assuming that shot noise due to the dark current is the primary contribution [39] in the total noise current of the photodetector (Δf is 1 Hz), D* is calculated by: ${D^\ast } = R \times \sqrt {{1 / {2q{J_{dark}}}}} $, where Jdark is the current density under dark condition. From the equation, D* was anti-correlated with Jdark. When the applied bias decreased from -1 V to -0.5 V, Jdark decreased (black curve in Fig. 2(b)) from 5.67×10−8 to 1.83×10−8 A/cm2. As a result, D* increased with the applied bias decreasing. The curves of D* dependent to wavelengths drawn from this equation was shown in Fig. 2(c). And the D* under different voltages were also listed in Table 1.
Fig. 2. Performances of the detection unit of the UCD. (a) EQE under different voltages from 300 to 1100 nm. (b) J-V curves in the dark and under 850, 940, 980, 1064 nm. (c) D* as a function of wavelength under different voltages. (d) D* as a function of frequency and wavelength at -1 V.
Table 1. The R and D* under different wavelengths at -0.5 V and -1 V
D* has a decrease with the bias voltage increasing, which is related to the increase of Jdark. And from 700 to 1000 nm, D* keeps at a stable level of 1012 Jones. This means the detection part has a good detecting performance and guarantees the detecting capacity of the UCD. Fig. S4 (b) showed the noise spectrum under different voltages of the detection unit. The noise spectrum linearly decreases with 1/f, and under minus bias, it has a low level of 10−23 - 10−28 A2/Hz from 1 to 104 Hz, which is beneficial to the low power density detection. With the bias increasing, the Jdark increases so that the noise increases. As for the high-speed photodiodes, the bandwidth-dependent D* can be reflected by, where In is dark current related with frequency. By using the noise spectral density $(S_n=I_n/\sqrt{f})$, we drew the two-dimensional intensity plot of the D* at -1 V bias related to wavelength and frequency (Fig. 2(d)). From the intensity graph, it is easy to get that the maximum D* is achieved in the region of 700-1000 nm, which absolutely matches with the absorption and EQE curves. Because the noise decreases with 1/f, under low frequency, the D* is at low level. With the increase of frequency, D* increases and reaches 1.61×1012 Jones. And the same intensity graph at -0.5 V bias was shown in Fig. S4 (c), and has the same tendency compared to -1 V. Besides, the response time at different voltages under 980 nm (75.2 mW/cm2) was shown in Fig. S4 (d). By giving the voltage pulse signal under 30% duty cycle with time step of 5×10−6 s, the response time (τr), tested by the analysis tools, are 1.07×10−5 s, 8.51×10−6 s and 7.43×10−6 s at 0 V, -0.5 V and -1 V, respectively. The fall time (τf) is at the same level with τr of 8.56×10−6 s, 9.48×10−6 s and 3.64×10−6 s at 0 V, -0.5 V and -1 V, respectively. Through these tests, the detection part has response to NIR light, which has a significance for the preparation of NIR UCD.
The emitting part of UCD used a traditional OLED structure of ITO/TAPC (30 nm)/CBP: Ir(ppy)2acac (7% wt% doped) (30 nm)/TPBi (50 nm)/LiF (1 nm)/Al (100 nm) and then its performance was tested. The center emitting wavelength is bright green light at 524 nm. The current density-voltage-luminance curves were shown in Fig. S6 (a). The highest luminance reaches 5.94×104 cd/m2 and the highest current density reaches 230 mA/cm2 at 10 V. Because there is a change of exciton density at different voltages, a roll-off causes at high luminance [40]. In addition, current efficiency (CE) (black curve) and power efficiency (PE) (blue curve) were calculated and drawn in Fig. S6 (b). The maximum CE and PE are 84 cd/A and 61 lm/W at the luminance of 1490 and 138 cd/m2, respectively. The highly efficient emitter of Ir(ppy)2acac leads to high CE and PE and furthermore, contributes to the high-efficiency UCD. EQEe-p of the emitting part was calculated and drawn in Fig. S6 (c). The maximum EQEe-p is 22.1% at the luminance of 1490 cd/m2, and has a same tendency with CE and PE curves. Therefore, the high-efficiency emitting part ensures the performance of the UCD.
As the step described in the Materials and methods section, we prepared the UCD and tested its performance. The testing sketch map was showed in Fig. S7. The device and spectrometer are fixed on the same line, and the distance between the lens and the UCD is 10 cm. After aligning the optical path, recording the angle of the attenuator and the maximum power of laser. Different powers of laser are achieved by rotating the attenuator for an angle and are tested by the power meter. Figure 3(a) showed the relation between luminance and voltage under different input power density of 980 nm. The Von (the bias value when the luminance is at 0.1 cd/cm2) under dark condition is 7 V, and the luminance is less than 1 cd/m2 at 11 V. A part of pioneering works also adopted 0.1 cd/m2 as the Von luminance [17,27,29]. Additionally, the luminance of 0.1 cd/m2 is a significant parameter to evaluate an imaging system. The low Von can detect the luminance at lower power (such as Fig. S8, 940 nm of 0.0024 mW/cm2, and 1064 nm of 0.02 mW/cm2), and if 1 cd/m2 is taken as the Von luminance at this time, it will not be able to detect the lower power of input NIR. At the same time, from the dark luminance (Fig. 3(a), black curve), it can be seen that it is less than 1 cd/m2 at 11 V. If 1 cd/m2 is taken as the Von luminance, The contrast before 11 V is meaningless. As the result, the choice of 0.1 cd/m2 as the Von luminance has practical significance. When the UCD is illuminated by NIR light, it emits green light and has a low Von of 2.05 V (208 mW/cm2). With the input power density decreasing to 0.02 mW/cm2, Von increases to 2.5 V. This is because low power density irradiation can only excite a small part of the excitons, resulting in the reduction of combination in EML. Under the highest power density of 208 mW/cm2, the maximum luminance is over 104 cd/m2. The on/off ratio of luminance is a measurement to reflect the contrast and the higher value the clearer images and is calculated by Lmax/Lmin, where Lmax and Lmin are the maximum and minimum luminance, respectively. Its value reaches the maximum of 4.4×104 at 6 V under 980 nm (208 mW/cm2), and then decreases to a certain extent with the increase of dark luminance.
Fig. 3. The characteristics of the UCD. (a) L-V curves in the dark and under illumination of 980 nm with different input power densities. (b) L-Power Density curves under illumination of 980 nm with different input power densities. (c) LDR under different voltages at 980 nm. (d) Intensity graph of P-P efficiency under different voltages and illumination of 980 nm with different input power densities.
Figure 3(b) illustrated the correlation between luminance and power density under different voltage bias. There is a certain deviation within the normal error range in the fitting lines, which is caused by the instability of the laser. And the fitting lines have good linearity from 0.01 to 100 mW/cm2. Under low voltage bias, the luminance is easy to get saturation, and is still at low luminance even when UCD is irradiated by high power density. This is because the migration of electrons is a combination effect of bias and input NIR power. With low voltage, electrons are hard to migrate from cathode to EML, so the luminance depends by the amount of electrons in EML. With the bias increasing, the amount of electrons in EML increase and combine with the holes from PSL, and the luminance increases lineally. In order to further analyze the feasibility of imaging application, LDR is a good measure parameter, which is computed by the equation:$LDR = 20\log ({{{{L_{\max }}} / {{L_{\min }}}}} )$. The higher LDR, the more details. The calculated voltage related LDR curve was shown in Fig. 3(c). As the voltage increasing from 3 V to 9 V, the LDR increased from 28.07 to 72.1 dB, and over 9 V, it stays at the same level. Therefore, 9 V is chosen to be the bias in the imaging application. Furthermore, by using the tested data including luminance and current, we calculated the P-P efficiency and drew the intensity graph shown in Fig. 3(d). The P-P efficiency is calculated by the equation of:$P - P\textrm{ }efficiency = {{\int {({{{\lambda {I_{photo}}(\lambda )} / {R(\lambda )hc}}} )d\lambda } } / {({{{{\lambda_{in}}{P_{NIR}}} / {hc}}} )}}$, where λ, Iphoto, R(λ), and PNIR are the wavelengths of the photons, the photocurrent of the photodetector collecting the extracted photons, the responsivity of the photodetector collecting the extracted photons, the incident NIR wavelength, and the incident NIR power, respectively. And the equation can be simplified to the product of two components EQE of UCD, one is EQEp-e of detection part, and the other is EQEe-p of emitting part. And the P-P efficiency can be calculated by: $\textit{EQ}{\textit{E}_{\textit{p - e}}}\mathit{\ \times EQ}{\textit{E}_{\textit{e - p}}}$. The EQEp-e can be also calculated by:$EQ{E_{p - e}} = ({{{({{J_{Light}} - {J_{Dark}}} )} / {{P_{in}}}}} )\times ({{{h\nu } / q}} )$, where Jlight, Jdark and Pin are the current density under illumination, the current density under dark condition and the input power density, respectively. From the illustration, the highest intensity is in the high voltage bias and low power density and the P-P efficiency has a tendency to increase and then decrease, and the maximum P-P efficiency reaches 12% under the power density of 0.48 mW/cm2 at 11 V. In this region, EQEe-p keeps in the same level, while EQEp-e changes significantly and has a peak, so that P-P efficiency in this part is the largest. With increasing in applied bias, there can be more cathode injected electrons to reach the EML to participate in light emitting. However, the incident NIR does not change, so the number of excitons that can be excited do not change. Despite the increase in injected electrons, there are no more holes to compound with, luminance become a saturation (luminance curves in Fig. 3(a) and Fig. S8). P-P efficiency reflects the conversion of NIR to visible light, since the incident NIR did not change, the luminance of the UCD does not change either. As the result, the P-P efficiency does not change after bias further increasing. We also analyzed the optical characteristics of this UCD under other wavelengths, including 850, 940, 1064 and 1310 nm (Fig. S8, S9). Under different wavelength lasers irradiation, the UCD has the same tendency compared to 980 nm. Because the absorption of UCD has a drop after 1000 nm, the Von of other wavelengths is higher than 980 nm. For 850 and 940 nm, the maximum luminance and Von are similar to 980 nm, which corresponds to the EQE shown in Fig. 2(a). As for 1310 nm, the luminance is 10 cd/m2 under 50 mW and the Von is close to 3 V. And L-P graphs of 850, 940 and 1064 nm show a good linear fitting, which means the UCD has a good performance of its LDR. Limited to the power of 1310 nm laser, its LDR is relatively narrow. So far, we have manufactured a high response and efficiency UCD with long detecting wavelength.
UCD in this work has an excellent detection performance for low power density and long wavelength NIR detection with low Von and high P-P efficiency. Simultaneously, the high LDR and on/off ratio of luminance enable a great prospect for the application of complicated bio-imaging. As we mentioned before, LDR reaches 72.1 dB at 9 V bias, so under 9 V bias the image quality can be the best. We used this UCD for imaging applications on masks and biological sections separately under different wavelengths. The schematic testing path was shown in Fig. 4(a). The NIR laser illuminates the UCD vertically, and then the samples, including mask figure. and biological sample, are placed between the UCD and laser, finally CCD is placed above the UCD and laser. By adjusting the position of CCD and changing different wavelengths of laser, we can see the visual light images on the computer screen. In the imaging application, the optical path diagram is shown in Fig. 4(a), from which it can be seen that the laser illuminates the UCD vertically through the sample. The distance between the laser and the sample, the distance between the sample and the UCD and the distance between the CCD and the sample is 3-5 cm, 1 cm and 15 cm, respectively.
Fig. 4. NIR imaging applications schematic and imaging results. (a) Optical testing path of NIR imaging. (b) Mask images of letter “e” under CCD camera and UCD. (c) The biological sample images of fly leg under CCD and UCD.
The left of Fig. 4(b) showed CCD direct images of the slices used, and the right panels were the mask images irradiated by 850, 895, 940, 980, 1064 and 1310 nm laser. The letter of “e” through the UCD can be seen clearly, and under different wavelength, there are same distinguishable images. The image under 980 nm has the best quality, with the high LDR. Simultaneously, the images at 1064 and 1310 nm are also clear. The bright and dark spots in the image at 1310 nm are due to the uneven luminescence intensity of the laser. In the case of UCDs, the imaging resolution is mainly determined by the imaging system, and according to our pre-work [41], it achieved 18 lp/mm. And the luminance signal-noise ratio was shown in Fig. S10. At 9 V bias, the signal-noise ratio was 104-105 from 850 to 1064 nm, which guaranteed the imaging quality.
Also, we used the thin biological slide sample of fly leg as the second imaging application. The slide sample was bought from the firm of SAGA and its visible light image was shown in the left of Fig. 4(c). The villi on the fly's leg (in red square) are simply identifiable by the CCD camera, while the rest of the internal tissue details (in yellow square) are not detectable with only an outline present. The visible images obtained by the UCD under different wavelengths of NIR illumination are all have a distinct outline. The image of villi is clear to see under 980 nm, and as well as under 1310 nm. Be similar to villi, the imaging in the tissue has clear image quality under 980 nm. Because of the penetration at long wavelengths, the 1310 nm NIR light can easily penetrate its thicker shell. The images under 1310 nm could tell more details. This indicates that long wavelengths are important for non-invasive performances of the UCD in this work are comparable to the majority of in biological imaging.
The reported works by comparing the devices performances and maximum imaging wavelength, as shown in Table 2. So far, we have successfully prepared UCD with high R to 850-1300 nm, and it is capable of imaging the corresponding wavelengths.
Table 2. Performance summary of reported organic NIR UCDs
4. Conclusion
By combing the non-fullerene based NIR detector and high-performance OLED, we successfully prepared a UCD with a broad NIR detection region from 760 to 1300 nm, which has a good performance. The UCD has a good detection performance capability of weak light detection (down to 0.02 mW/cm2) and P-P efficiency up to 12% under 980 nm irradiation. The low Von of 2.05 V and high luminance over 104 cd/m2 are realized at higher power and bias. Meanwhile, the UCD has a high LDR of 72.1 dB at 9 V and a high luminance on/off ratio of 4.4×104. With the low power consumption and wide optical power recognition of UCD, we show the application in mask and sample biological imaging, and successfully get the clear visual images from 850 to 1310 nm. So far, our work has shown a high-efficiency and long wavelength response UCD in NIR detecting and given a new possibility for noninvasive bio-imaging applications.
Funding
National Natural Science Foundation of China (52130304, 61922022, 62075029, 62105055); International Cooperation and Exchange Project of Science and Technology Department of Sichuan Province (2020YFH0063); China Postdoctoral Science Foundation (2020TQ0058, 2021M7006); Fundamental Research Funds for the Central Universities (ZYGX2021J017).
Disclosures
The authors declare that they have no conflicts of interest.
Data availability
Data underlying the results presented in this paper are available from the corresponding authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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