Organic Cavity Photodetectors Based on Nanometer-Thick Active

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Organic Cavity Photodetectors Based on Nanometer-Thick Active Layers for Tunable Monochromatic Spectral Response Jing Wang, Sascha Ullbrich, Ji-Ling Hou, Donato Spoltore, Qingwei Wang, Zaifei Ma, Zheng Tang, and Koen Vandewal ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.9b00471 • Publication Date (Web): 16 May 2019 Downloaded from http://pubs.acs.org on May 16, 2019

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Organic Cavity Photodetectors Based on Nanometer-Thick Active Layers for Tunable Monochromatic Spectral Response Jing Wang1, Sascha Ullbrich2, Ji-Ling Hou3, Donato Spoltore2, Qingwei Wang1, Zaifei Ma1,* Zheng Tang1,* Koen Vandewal3* 1. Center for Advanced Low-dimension Materials, State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai, 201620, P. R. China 2. Dresden Integrated Center for Applied Physics and Photonic Materials (IAPP) and Institute for Applied Physics, Technische Universität Dresden, Dresden, 01187, Germany 3. Institute for Materials Research (IMO-IMOMEC), Hasselt University, Wetenschapspark 1, 3590 Diepenbeek, Belgium Keywords: resonance microcavity; tunable spectral; narrowband response; organic photodetector; visible light photodetector Abstract Application of spectroscopic photo-detecting technologies in future innovations such as wearable or integrated electronics will require miniaturized spectrometers. This can be achieved by using an array of small-area, wavelength-selective photodetectors. Here, filterless narrowband photodetectors based on a novel device concept are demonstrated. The narrowband photoresponse is realized by utilizing nanometer-thick 2,2-((3,4-dimethyl-[2,2:5,2:5,2:5,2quinquethiophene]-5,5-diyl)bis (methanylylidene))-dimalononitrile (DCV5T-Me):C60 photoactive layers (3-6 nm) in a Fabry-Perot cavity. By varying the cavity thickness, achieved by adjusting the transport layer thicknesses, we realize continuously tunable detection wavelengths, spanning the entire visible region (400-700 nm). Most importantly, because the active layer is only nanometer-thick, position of the active layer can be adjusted within the cavity. Thus, with an optimized position of the active layer, the photodetectors exhibit an overtone free, monochromatic spectral response with a full-width-at-half-maximum value of 25 nm and an external quantum efficiency over 50%. Although the absorber layers were kept thin, we realize peak specific detectivities over 1012 Jones, which is comparable to that of the most efficient narrowband organic photodetectors lacking spectral tunability over a broad wavelength range. 1 ACS Paragon Plus Environment

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Main text Spectroscopic photodetection is commonly used in modern science for identifying substances and characterizing materials, as well as in technologies such as imaging, surveillance, and agricultural or industrial process monitoring.1 The required spectrometers are often constructed using broadband photodetectors and bulky wavelength discriminating components. However, future innovative spectroscopic applications, such as wearable and dermal devices or implants for health monitoring, demand low cost, simple and compact devices.2-4 Spectrometer miniaturization can be realized by using a set of narrowband photodetectors with gradually varying spectral response. Although narrowband photodetectors are possible to fabricate,5-7 for instance, by employing novel device concepts,8-11 tailored photoactive materials,12–14 or optical filters,15, 16 a wide range spectral tunability is still very difficult to realize. Besides, efficient filters with a narrowband transmission are often expensive and increase the architectural complexity of the photodetector device. An elegant solution to realize a filterless and narrowband photodetector, is to place the photoactive material inside a resonant optical microcavity. This has been demonstrated for both organic17–20 and inorganic materials.21-26 For the latter, near-infrared spectral responses with a full-width-at-half-maximum (FWHM) less than 10 nm have been achieved using two multilayer dielectric mirrors, deposited using epitaxy techniques.21 ,22 However, the detection range, which is proportional to the distance between the mirrors of the cavity, was limited to a narrow wavelength range of less than 100 nm, due to the rather small free spectral range (FSR, i.e., the wavelength spacing between adjacent resonance peaks). The FSR of a cavity decreases quadratically with decreasing resonance wavelength,22 and in general, Fabry-Perot cavities yield a polychromatic, overtone containing response spectrum. Therefore, widely tunable and true monochromatic photodetection, especially difficult in the visible range from 400 to 700 nm, is yet to be realized.

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In this work, we demonstrate cavity based organic photodetectors with a monochromatic spectral response in the visible wavelength range. The high monochromaticity is realized by employing ultra-thin vacuum deposited organic photo-absorbing layers (3 or 6 nm). This allows us to optimize the position of the active layer within the cavity, avoiding resonance overtones. Even though the light absorbing layer of the cavity detector is ultra-thin (3-6 nm), we achieve a device performance comparable to the best reported narrowband photodetectors lacking wavelength tunability. We obtain peak external quantum efficiencies (EQEs) at the resonance wavelengths over 50%, a FWHM as narrow as 25 nm, and shunt resistances on the order of 1 MΩ cm-2 at short-circuit, resulting in calculated photodetector specific detectivities over 1012 Jones (cm∙Hz0.5∙W-1) at the resonances wavelengths. The cavity devices consist of silver mirrors, and ultra-thin vacuum deposited organic bulkheterojunction (BHJ) films, sandwiched between thicker n- and p-doped organic transport layers (20 - 230 nm). A schematic picture of the cavity photodetector architecture with an ultrathin active layer is shown in Figure 1a. When the optical depth (unitless) of the materials in between the two metal mirrors (electrodes) is sufficiently low, light absorption in the thin active layer is enhanced in a narrow region around the resonance wavelength λm22 𝝀𝒎 =

𝟐𝒏𝑳𝒎

(1)

𝒎

where n is the effective refractive index of the cavity and m is a natural number, representing the order of resonance. Therefore, λm can be tuned by adjusting the distance between the electrodes – the cavity thickness Lm. However, higher order resonance overtones, for m > 1, will appear at shorter wavelengths. To achieve narrow spectral response with a small FWHM, the effective absorption coefficient of the cavity, α, must be low, since19 FWHM ≈

𝛼𝜆2𝑚

(2)

𝑛𝜋

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(a) cavity thickness Lm

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top mirror (Ag) transparent electron transport layer absorber layer

transparent hole transport layer semi-transparent mirror (Ag)

substrate

(b)

(c)

(d)

(e)

Figure 1. (a) A schematic picture of the device architecture of an organic resonant cavity photodetector. The bottom mirror, simultaneously acting as the bottom electrode, is partially transparent, and the charge transport layers are highly transparent and conductive. (b) A typical spectral response spectrum of a resonant cavity photodetector based on a broadband photo-absorbing active material. The parameters determining the FWHM and resonance wavelength of the photodetector are indicated. (c) TMM simulated EQE spectra of a cavity detector with a first order resonance wavelength at 540 nm and different active layer thicknesses. The device IQE is assumed to be 100%. Simulated device architecture: Ag (22 nm)/Bphen:Cs/DCV5T-Me:C60/BPAPF:NDP9/Ag (120 nm). The active layer is placed at the center of the device. (d) TMM simulated EQE spectra of a cavity photodetector with an ultra-thin active layer placed at the center or close to one of the contacts in the cavity. The total cavity thickness, i.e., the sum of the thicknesses of the active layer, the electron transport layer (ETL) and the

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hole transport layer (HTL), is 145 nm, resulting in a first order resonance at 700 nm. (e) TMM simulated optical field distribution of the cavity photodetector.

when absorption losses in the interlayers and the mirrors are negligible, the effective absorption coefficient can be approximated by19 α=

𝛼𝑎𝑐𝑡𝐿𝑎𝑐𝑡

(3)

𝐿𝑚

where Lact is the thickness and αact is the absorption coefficient of the active layer. Since αact of a photoactive material is generally high, we thus must use thin active layers for a small FWHM. In addition, using a larger Lm for a higher order resonance will also lead to a smaller FWHM. To evaluate the optimum thickness of the active layer for a sufficiently narrow first order resonance, transfer matrix model (TMM) simulations,27-29 based on the real dielectric functions of the materials used in the device, are performed for a device architecture as depicted in Figure 1a. We use 4,7-diphenyl-1,10-phenanthroline (Bphen):Cs (1:1), with a reported conductivity of 2.5x10-5 S cm-1, as the electron transport material, and 9,9-bis[4-(N,N-bis-biphenyl-4ylamino)phenyl]-9H-fluorene (BPAPF):NDP9 (9:1), with a conductivity of 4.5x10-5 S cm-1, as the hole transport material. 30, 31 The active layer (3-6 nm) is based on a broadband absorber methyl-substituted dicyanovinyl-capped quinquethiophene (DCV5T-Me)f mixed with C60 (2:1), previously used in organic bulk-heterojunction BHJ solar cells,32 with an absorption spectrum covering the entire visible range (Figure S1, SI). Chemical structures and energy levels of the materials used in this work are shown in Figure S2 (SI). TMM simulation results for devices with a first order resonance wavelength at 540 nm are shown in Figure 1c: These devices have a constant total cavity thickness (100 nm), but different active layer thicknesses (3-24 nm). The simulation results suggest that in order to achieve a FWHM of less than 100 nm, the thickness of the active layer must be smaller than 10 nm. Furthermore, the position of the active layer within the cavity also matters, especially for avoiding undesired overtones. For instance, Figure 1d shows the predicted EQE spectra in a cavity device with a detection wavelength designed to be close to 700 nm. With the active layer 5 ACS Paragon Plus Environment

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placed out of the cavity center, a clear overtone signal at 400 nm, due to a higher order resonance, appears. This overtone signal is similar to those in cavity devices with thick active layers, and can only be eliminated using external filters. However, we find that such a polychromatic spectral response induced by resonance overtones can in fact be avoided, by placing the ultra-thin active layer at the position where the second order resonance has a field minimum. As suggested by the TMM (Figure 1e): placement of 3 nm of active layer at the device center allows it to be confined within the optical electric field minimum of the higher order resonance, but at the field maximum of the first order resonance. In this case, power dissipation of the second order wavelength is very weak in the active layer and the cavity photodetector only produces photocurrent at first order resonance wavelength, leading to an overtone free, monochromatic spectral response. This highlights the advantage of using ultra-thin active layers: whereas undesired polychromatic spectral response due to resonance overtones is unavoidable for cavity devices with thick active layers, monochromatic response can be realized in the cavity devices with ultra-thin active layers, enabling truly filterless narrowband photodetection. The required active layer thickness of a few nanometer and precision in layer position are easily achieved experimentally using thermal evaporation of organic small molecules. However, due to the use of ultra-thin active layers, the devices are expected to be prone to internal shunt. Therefore, we fabricate devices with minority charge carrier blocking materials inserted at the interfaces between the active layer and the doped transport layers. BPAPF (5 nm, undoped) is used to block electrons,33 and C60 (3 or 6 nm, undoped) is used to block holes. Experimental and simulated EQE for the cavity devices based on a 3 nm thick DCV5TMe:C60 (2:1) layer are shown in Figure2a and Figure S3c (SI), respectively. We achieve EQE values ranging from 20% to 40% depending on the resonance wavelength. The FWHM is in the range of 60-75 nm, higher than the values (30-60 nm) predicted by Equation 2 which assumes no parasitic absorption in the electrodes/mirrors. Indeed, for the used Ag electrode, 6 ACS Paragon Plus Environment

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parasitic absorption losses are rather high and limit the device EQE, as indicated by TMM simulations shown in Figure S4 (SI). (a)

(b)

Figure. 2. Experimental EQE spectra, measured FWHM values, and calculated device specific detectivities for resonant cavity photodetectors with (a) 3 nm and (b) 6 nm thick active layers. For each device, the position of the active layer within the cavity is optimized to be in the optical field maximum. The detection wavelength is tuned to cover the entire visible range via varying the thicknesses of the charge transport layers. Their corresponding responsivity spectra and EQE spectra simulated using a transfer matrix model are shown in Figure S3.

Using a thicker active layer (6 nm) results in a higher EQE of over 50% (Figure 2b). However, due to the higher optical depth (αLm) of the cavity system, the FWHM values slightly increase. Furthermore, the EQE is lower for shorter wavelengths (< 500 nm) in both simulations and experiments. The reason is partially due to parasitic absorption of the BPAPF:NDP9 transport layer as well as a lower internal quantum efficiency (IQE) at shorter wavelengths. See SI, for a more detailed discussion.

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From current density-voltage (JV) characteristics (Figure S5, SI), we extract the shunt resistance at 0 V to be up to 1 MΩ cm-2, with the most reproducible value ranging from 0.1 to 1 MΩ cm-2. However, within this series, we observe no clear dependence of the shunt resistances on the active layer or the transport layer thickness. Neglecting noise sources such as 1/f noise, a highest limit for the specific detectivity D* is calculated based on the J-V characteristics. Although the calculated D* is expected to be higher than the real value, it represents the highest achievable D* of the cavity photodetectors based on ultra-thin active layers. According to equation 4:34 𝜆 ∙ 𝐸𝑄𝐸

𝐷 ∗ = ℎ𝑐 ∙ 𝑖𝑛𝑜𝑖𝑠𝑒

(4)

where h is the Planck constant, c is speed of light and inoise is the noise current (SI) 𝒊𝒏𝒐𝒊𝒔𝒆 = 𝟐𝒒𝒊𝒅 +

𝟒𝒌𝑻 𝑹𝒔𝒉

(5)

we obtain the calculated D* of over 1012 Jones at short circuit conditions (Figure 2), which is a respectable number for organic photodetectors,35 and among the highest for recently reported filterless narrowband photodetectors.8–11 (a)

(b)

Figure 3. (a) Linear Dynamic range (130 dB) and (b) response time (90%-10% fall time) of a cavity photodetector with an ultra-thin active layer based on DCV5T-Me:C60, and a cavity thickness of 85 nm, excited at the resonance wavelength (530 nm), at short circuit conditions. The 3dB cut-off-frequency is given in Figure S7, in SI.

Furthermore, due to low bimolecular recombination losses,36 the device response is linear for over 7 orders of magnitudes of light intensities, as shown in Figure 3a. We do not observe 8 ACS Paragon Plus Environment

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strong deviation from linearity at the highest illumination intensity of 100 mW cm-2. In order to investigate the frequency response of the cavity devices, a 25 picoseconds laser pulse at its 530 nm resonance wavelength is used. The current response is shown in Figure 3b. Although the transit time of the carriers within the ultra-thin active layer is expected to be short (10 ns range),37 the use of a large device active area (6.44 mm2) limits the response time to its capacitance (RC) limited value. Nevertheless, we obtain a 90% to 10% fall time value of 1.2 μs (Figure 3b) which is sufficiently fast for spectroscopic applications. (a)

(b)

Figure 4. (a) TMM simulated optical field distribution for a cavity photodetector with an ultra-thin active layer based on DCV5T-Me:C60 (6 nm) and a second order resonance wavelength at 550 nm. The total cavity thickness, i.e., the sum of the thicknesses of the active layer, the electron transport layer (ETL) and the hole transport layer (HTL), is 258 nm. (b) TMM simulated EQE spectra of the cavity detector with the active layer placed at three different positions in the cavity.

To exploit the full potential of the proposed cavity device concept for monochromatic spectral response, we further reduce the spectral line-width by moving to higher order resonances, using thicker transport layers. Figure 4a depicts the TMM simulated optical field distribution of a cavity device with an expected second order resonance wavelength of 550 nm, at which two distinct field maxima are observed: one is close to the transparent Ag (bottom) mirror, and the other one is close to the reflecting Ag (top) mirror. The third order resonance is visible at 400 nm, containing 3 optical field maxima. To achieve a photocurrent only at the second order resonance, the position of the active layer can no longer be at the center of the 9 ACS Paragon Plus Environment

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cavity. At this position (see position 1 in Figure 4a) the optical field is at a maximum for both the first and third order resonance, but at a minimum for the second order resonance. Therefore, the predicted EQE for the cavity device with the active layer placed at the device center is almost 0 at 550 nm, but high at 400 nm (Figure 4b, position 1). Note that the field maximum of the second order resonance is neither an optimum position for the absorber layer: Although a high EQE at 550 nm is predicted when the active layer is placed there (position 3 in Figure 4a), an additional response peak at 400 nm will also occur, due to the presence of high optical field of the third order resonance. Consequently, the optimum position is therefore slightly off the second order resonance field maximum, as shown in Figure 4a (position 2). (a)

(b)

Figure 5. Measured EQE spectra and FWHM values of resonant cavity photodetectors with (a) a 6 nm active layer and second order resonances and (b) a 3 nm active layer and third order resonances. Position of the active layer in the cavity is optimized for the optical electric field to avoid overtones for all resonance wavelengths.

Based on the TMM simulations, we optimized the active layer position and built cavity photodetectors operating at second, and also third order resonance wavelengths. Results are shown in Figure 5. The lowest FWHM value is further reduced to a value close to 30 nm for

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devices with second order resonances, and 25 nm for devices with third order resonances, while overtone signals within in the visible range are avoided. We note that the EQE of devices with higher order resonances are generally lower as compared to devices with first order resonances. This is partially caused by a lower IQE due to much thicker transport layers. Nevertheless, with measured shunt resistances comparable to that of the devices with first order resonances and D* of these devices is also on the order of 1012 Jones. It should be noted that higher order resonances can be clearly identified in the reflectance spectra of the devices, regardless of the active layer thickness, as shown in Figure S6 (SI). It should be further stressed that such overtones are always present when using cavity detector concept or when using cavity filters integrated directly on top of broadband photodetectors18, 19, 22

The use of ultra-thin active layers, at well-chosen positions within the cavity, as reported

here, is so far the only method reported to successfully avoid a photocurrent response to resonance overtones. In summary, we present a combination of ultra-thin organic active layers with thick transparent charge transport layers in a cavity device architecture for monochromatic, visible light detectors. By optimization of the position of the active layer within the cavity, only possible for devices with ultra-thin active layers, we avoided undesired resonance overtone signals. We showed that the monochromatic spectral response is tunable over the entire visible spectrum (400 – 700 nm) with a FWHM as low as 25 nm, and we demonstrated peak EQEs of over 50%. Although the active layers were kept a few nanometers thick, the devices maintain shunt resistances on the order of 1 MΩ cm-2. This led to calculated photodetector specific detectivity over 1012 Jones, comparable to organic photodetectors with thick active layers but lacking wavelength selectivity. The presented organic cavity photodetectors are based on active materials absorbing in the visible region. However, the device concept is also applicable in the

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UV and NIR. Therefore, we believe that this simple device architecture is highly promising for innovative areas of spectroscopic applications that are incompatible with existing technologies. Materials and Methods Device fabrication: The complete device structure is glass/ MoO3 (3nm)/ Au (1nm)/ Ag (22nm)/ BPhen:Cs (1:1, varied thickness)/ BPhen (5nm)/ C60 (3nm or 6nm)/ DCV5T-Me:C60 (v:v 2:1, 3nm or 6nm/ BPAPF (5nm)/ BPAPF:NDP9 (10%, varied thickness)/ MoO3 (3nm)/ Ag (120nm). In this device structure, MoO3 and Au layer are used as a seed layer to obtain a highly smooth Ag layer, 38, 39 which is important for the microcavity structure. Through tuning the thicknesses of BPhen:Cs and BPAPF:NDP9 layer, the photo-response of the visible detectors is easily tuned in the visible region. The bottom and the top Ag electrodes also act as microcavity mirrors. The devices were constructed via thermal evaporation in vacuum with a pressure under 10-8 mbar (K. J. Lesker, UK) at room temperature. The layer thickness, evaporation rates and mixing/doping ratios were controlled by quartz crystal microbalances. The evaporation rates for all the deposited layers did not exceed 1 Å/s. The photoactive area of the visible photodetectors was determined by the geometrical intersection of the bottom Ag and the top Ag electrode, which is 6.4 mm2. After evaporation, all samples were encapsulated using ultraviolet-cured epoxy glue (XNR 5592; Nagase ChemteX, Japan) with glass covers on top. Device Characterization: EQE of the devices were recorded under a short-circuit condition, using a EQE robot setup with a xenon lamp (Apex Illuminator), a monochromator (Newport Oriel, USA), a chopper (at frequency 230 Hz) and a lock-in amplifier (Signal Recovery, USA). The illuminated area was 2.89 mm2. The spectral and intensity of the lamp was calibrated by a Hamamatsu S1337-33BQ Si-photodiode (Hamamatsu-Photonics, Japan). The current-voltage characteristics were measured using a source meter (2400 SourceMeter, Keithley Instruments, USA). The transient photocurrent response was measured using a pulsed laser (PL2210 Ekspla) with a 25 ps pulse length and a repetition rate of 1 kHz. Excitation wavelength was at 532 nm. Current signals were recorded using an oscilloscope DPO7354C from Tektronix under a short12 ACS Paragon Plus Environment

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circuit condition. The linear dynamic range of the devices was measured under a short-circuit condition by using a set of neutral density filters, an LED lamp, and a Keithley 2400 SourceMeter. For low intensity illumination regime, the dynamic range was measured using a pre-amplifier and a lock-in amplifier. Optical transfer matrix simulations were done using optical constants of the materials determined using Beer-Lambert’s Law and Ellipsometry, see reference27-29 for more details. Supporting Information Energy levels and absorption coefficients of the materials used, electric properties of the cavity devices, optical simulation results, internal quantum efficiency of the cavity devices. Corresponding Authors Email: [email protected]; [email protected]; [email protected] Acknowledgements Z. T. acknowledges a starting grant from Donghua University, Z. M acknowledges an Alexander-von-Humboldt fellowship. The work was further supported by the German Federal Ministry for Education and Research (BMBF) through the InnoProfile projekt "Organische pi-n Bauelemente 2.2". References (1) Arquer, F. P. G. de; Armin, A.; Meredith, P.; Sargent, E. H. Solution-Processed Semiconductors for next-Generation Photodetectors. Nat. Rev. Mater. 2017, 2, 16100. (2) Bacon, C. P.; Mattley, Y.; DeFrece, R. Miniature Spectroscopic Instrumentation: Applications to Biology and Chemistry. Rev. Sci. Instrum. 2003, 75 (1), 1–16. (3) Wang, S.; Xu, J.; Wang, W.; Wang, G.-J. N.; Rastak, R.; Molina-Lopez, F.; Chung, J. W.; Niu, S.; Feig, V. R.; Lopez, J.; et al. Skin Electronics from Scalable Fabrication of an Intrinsically Stretchable Transistor Array. Nature 2018, 555 (7694), 83–88. (4) Miao, J.; Zhang, F.; Du, M.; Wang, W.; Fang, Y. Photomultiplication Type Organic Photodetectors with Broadband and Narrowband Response Ability. Adv. Optical Mater. 2018, 6 (8), 1800001. (5) Miao, J.; Zhang, F. Recent Progress on Highly Sensitive Perovskite Photodetectors. J. Mater. Chem. C 2019, 7 (7), 1741–1791. (6) Wang, W.; Du, M.; Zhang, M.; Miao, J.; Fang, Y.; Zhang, F. Organic Photodetectors with Gain and Broadband/Narrowband Response under Top/Bottom Illumination Conditions. Adv. Optical Mater. 2018, 6 (16), 1800249. (7) Miao, J.; Zhang, F. Recent Progress on Photomultiplication Type Organic Photodetectors. Laser & Photonics Reviews 2019, 13 (2), 1800204. (8) Armin, A.; Vuuren, R. D. J.; Kopidakis, N.; Burn, P. L.; Meredith, P. Narrowband Light Detection via Internal Quantum Efficiency Manipulation of Organic Photodiodes. Nat. Commun. 2015, 6, 6343. (9) Fang, Y.; Dong, Q.; Shao, Y.; Yuan, Y.; Huang, J. Highly Narrowband Perovskite Single-Crystal Photodetectors Enabled by Surface-Charge Recombination. Nat. Photonics 2015, 9 (10), 679–686. 13 ACS Paragon Plus Environment

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For Table of Contents Use Only Organic Cavity Photodetectors Based on Nanometer-Thick Active Layers for Tunable Monochromatic Spectral Response Jing Wang, Sascha Ullbrich, Ji-Ling Hou, Donato Spoltore, Qingwei Wang, Zaifei Ma,* Zheng Tang,* Koen Vandewal* Filterless organic photodetectors with a monochromatic narrowband spectral response are achieved using a new optical cavity device concept. A tunable spectral response is realized for the entire visible range, resulting in the depicted colored devices, gradually changing from violet to red.

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