Controlling Tamm-plasmons for organic narrowband near-infrared

Here, we achieve narrowband enhancement of the below-gap near-infrared ... A possible solution to achieve a narrowband response is provided by placing...
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Controlling Tamm-plasmons for organic narrowband near-infrared photodetectors Andreas Mischok, Bernhard Siegmund, Dhriti Sundar Ghosh, Johannes Benduhn, Donato Spoltore, Matthias Böhm, Hartmut Fröb, Christian Koerner, Karl Leo, and Koen Vandewal ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b00427 • Publication Date (Web): 08 Aug 2017 Downloaded from http://pubs.acs.org on August 9, 2017

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Controlling Tamm-plasmons for organic narrowband near-infrared photodetectors Andreas Mischok,∗,†,‡,¶ Bernhard Siegmund,∗,†,‡,¶ Dhriti Sundar Ghosh,†,‡ Johannes Benduhn,†,‡ Donato Spoltore,†,‡ Matthias Böhm,‡ Hartmut Fröb,†,‡ Christian Körner,†,‡ Karl Leo,†,‡ and Koen Vandewal†,‡ †Dresden Integrated Center for Applied Physics and Photonic Materials (IAPP) ‡Institute for Applied Physics, Technische Universität Dresden, 01062 Dresden, Germany ¶A. Mischok and B. Siegmund contributed equally to this work. E-mail: [email protected]; [email protected]

Abstract Organic spectrometers are attractive for biomedicine and industrial process monitoring but are currently limited in terms of spectral selectivity and the accessible wavelength range. Here, we achieve narrowband enhancement of the below-gap near-infrared response of charge-transfer (CT) excitations in organic photodiodes by introducing them into a high-quality microcavity. The device architecture includes a non-conductive distributed Bragg reflector and thin metal electrodes, leading to the formation of sharp Tamm-plasmon-polariton resonances. We demonstrate how to tailor the arising multimode spectra for spectroscopic photodetectors and present efficient single-resonance devices with remarkable line widths below 22 nm, which are partially transparent for visible wavelengths. Taking advantage of the spectrally broad CT band, we vary the resonator thickness to provide a proof of concept that benefits from the spectral selectivity of our high-quality microcavities. Finally, utilizing transfer-matrix-calculations,

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we propose further improvements on the cavity architecture towards single digit line widths.

Keywords near infrared, microcavity, Tamm-plasmon-polariton, charge-transfer state, organic photodetector

A multitude of biomedical diagnosis and industrial process monitoring methods benefit from spectroscopy in the near infrared (NIR) which allows resolving chemical compositions in a fast, remote, and non-destructive manner. 1,2 In recent years, organic photodetectors proved their potential as a low-cost technology with high device integration, scalability, and spatial resolution. 2,3 Up to now, organic monochromatic detectors for spectroscopic applications mostly suffer from a lack of narrow absorption bands with suitable extinction yields, a moderate spectral selectivity, and the absence of absorption beyond 1500 nm. 2,4 A possible solution to achieve a narrowband response is provided by placing an absorber inside an optical microcavity, where its optical quality controls the detector line-width. Here, absorbers with low extinction coefficients are beneficial, enabling high optical quality factors due to the back-and-forth reflection of resonant photons inside the cavity. 5 In turn, the quantum efficiency strongly increases in a narrow wavelength region around the resonance. Moreover, weak absorption bands extending over several 100 nm allow for miniaturized spectrometers, as the detected wavelength is directly controlled via the resonator optical thickness. 6 Both these requirements are fulfilled for intermolecular charge-transfer (CT) states, which arise from blending donor and acceptor molecules in an organic bulk heterojunction (BHJ). 7 The CT absorption process, that is, the excitation of electrons from occupied donor states into unoccupied acceptor states, occurs at photon energies below the optical gaps of the neat absorbers. While this transition is only weakly allowed, it can extend the absorp-

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tion spectrum of the neat materials by several hundreds of nanometers. 8 Hence, CT states enable high quality resonances and spectroscopy in the NIR regime, circumventing the lower absorption limit of the neat organics. Recently, we have introduced CT states for photodetection in mechanically flexible metalmetal microcavities. 9 Given the low CT absorption yield, the obtained spectral widths of around 40 nm are expected to be drastically narrowed when replacing a metal mirror by a highly reflective distributed Bragg reflector (DBR). To benefit from the low production costs of DBRs comprising metal oxides, we combine such a low loss mirror with a thin metal electrode providing lateral conductivity. The resulting sharp Tamm-plasmon-polariton (TP) resonances 10 have been exploited up to now in microcavity lasers, 11–15 exciton-polariton devices, 16–18 and to tune emission spectra in organic light emitting diodes. 19 Meanwhile, DBR-metal interfaces have rarely been demonstrated in optical sensors, 20 and only recently were utilized for NIR photodetection at inorganic Schottky junctions. 21,22 In this work, we improve organic photodetectors by integrating them into high quality cavities based on DBRs with ultra-thin metal electrodes. Within the CT absorption band of ZnPc:C60 , we achieve remarkably narrow full widths at half maximum below 22 nm, external quantum efficiencies (EQE) up to 17 %, and spectral tunability by varying the resonator thickness. Furthermore, the use of DBRs with optimized reflectivity in the NIR allows detectors with approx. 30 % transparency in the blue and green spectral region. Utilizing transfer-matrix simulations, we finally present further detector concepts, expected to achieve line widths below 10 nm and further improve EQE.

Results and Discussion Device Architectures for Cavity-Enhanced CT Absorption As photo-active organic blend, we use the small molecule donor zinc-phtalocyanine (ZnPc) mixed with fullerene (C60 ) as acceptor in a volume ratio of 1:1. Strong absorption occurs 3 ACS Paragon Plus Environment

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mainly in the blue-green (C60 , optical gap Egap = 1.8 eV) and green-red (ZnPc, Egap = 1.5 eV) spectral regions. In conventional organic photodetectors, photocurrent is obtained from donor or acceptor excitons diffusing to a heterojunction, where one of charges transfers to the adjacent material. Due to its low binding energy, 23,24 the resulting charge-transfer state can be dissociated thermally to generate photocurrent. 9,25 With a CT state energy of 1.2 eV (i.e. the interfacial gap for ZnPc:C60 (1:1)), NIR photons can also directly populate these interfacial states via a weak optical transition. As shown in Figure 1 (b), donor electrons on the highest occupied molecular orbital (HOMO) are promoted into the lowest unoccupied molecular orbital (LUMO) of the neighbouring acceptor molecule. 7,26 The extinction coefficient of CT absorption is typically 1-3 orders of magnitude lower than for above gap excitation, 9 and therefore highly suited for high quality, narrowband photodetection when embedded into a microcavity. A low extinction coefficient is in fact crucial when aiming for spectral selectivity. As the number of roundtrips photons take in a cavity, given by the quality factor Q, increases, the linewidth ∆λ (∆E) of the corresponding resonance at λ (E) decreases. Consequently, Q relates to the spectral width as Q = E/∆E ≈ λ/∆λ. As bottom mirror, we use a DBR comprising alternating layers of high (TiO2 ) and low (SiO2 ) refractive index material, where each optical layer thickness corresponds to a quarter of the design wavelength λD . The photonic stopband of the DBR exhibits a high reflectivity of at least 99.9 % at λD (compare transmission measurement for λD =900 nm in SI, Fig. S2 (a)) . In addition, the mirrors exhibit a transmittivity above 75% throughout the whole visible spectrum (see Fig. S2), enabling a visible transparent device. A thin silver layer with a thickness of 25 nm or less is deposited on top of the DBR, functionalized by (in)organic seed layers (details see SI Section III and Table S1). An organic electron transport layer (ETL), photo-active layer, and hole transport layer (HTL) are subsequently deposited. The energetic positioning of ETL and HTL allows for efficient extraction of the separated charges to the respective electrode (see simplified scheme in Fig. 1 (b)). As counter reflector/electrode, we utilize silver of at least 25 nm thickness, such that light can enter the device from this side,

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while its neat reflectivity of ≈90 % still enables high quality factors. The thicknesses of the transport layers are tuned to facilitate a cavity resonance in the NIR absorption band of the CT state. For this purpose, transfer-matrix simulations 27 are employed, which take into account TP formation (see next section). Using the transfer-matrix method, the transport layers are then individually optimized to position the BHJ in the optical field maximum inside the cavity. On top of the resonator, an optional sequence of organic neat absorbers (such as those from the photoactive blend) is processed to filter out the photoresponse in the visible range. Further details on sample composition and processing are found in the Methods and Supporting Table S1.

Formation of Tamm-Plasmon-Polaritons To understand the formation of resonances in our device, we derive the optics of the arising coupled TP modes. The alternating refractive index of the DBR layers create a photonic lattice which can exhibit surface states at its edge, in analogy to electronic surface states in a crystal lattice. 28 Combining such a photonic lattice with a plasmonic layer now leads to the formation of a narrow, confined TP resonance at the DBR-Ag interface as a result of the interaction between a photonic surface state and plasmons in a metal. 10 In contrast to surface plasmons, the TP propagates perpendicular to the metal layer, giving easy optical access to the resonance. Here, this allows the combination of high-Q vertical cavities with direct electrical readout. Adding another reflective layer (such as a second metallic electrode) inside a microcavity leads to the formation of two coupled resonators, one comprising the DBR and bottom Ag contact, the other inside the photodiode (indicated as TP2 and TP1 in Fig. 1 (a)). The formation of TP states in such a cavity can be described by solving the

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coupled resonator equation: 29

−

t2Bot =(1 − (rDBR rBot e2iφDBR )−1 )× 2 rBot (1 − (rTop rBot e2iφTop )−1 ),

(1)

where rBot and tBot describe the reflection and transmission coefficients of the bottom metal layer, rDBR and φDBR describe the reflectivity and phase of light at the DBR and rTop and φTop the reflectivity and the phase change of the top reflector. In the most general case, the solution describes the formation of two coupled TP states that each split into transverse magnetic (TM) and transverse electric (TE) polarization under large angles, respectively. The field distribution of these states under normal incidence is presented in Figure 2 (a), pointing out the formation of two resonances at 950 nm and 1025 nm and their respective non-zero phases at the contacts. In turn, we can describe the optical microcavity with an effective thickness dC,eff (indicated with dashed lines in Fig. 2 (a)):

dC,eff = dC +

λC (|φBot | + |φTop |) , 2πnC,eff

(2)

with the physical thickness and (effective) refractive index of the cavity dC and nC,eff , as well as the phase change of the bottom mirror (DBR and bottom contact) φBot due to the high extinction coefficient of silver. Here, nC,eff consists of the mean of refractive indices of the individual organic layers. A resonant mode will appear at the wavelength λC , when nC,eff dC,eff = m × λC /2, where m ∈ N is the order of the resonator, solving to: λC nC,eff dC = . 2 m − (|φBot | + |φTop |) /π

(3)

As the phase change at the top mirror does not differ much for small TP-splittings (see Fig. 2 (a)), the TP states are mainly differentiated by their respective phases φBot at the 6 ACS Paragon Plus Environment

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wavelengths for TP1 and TP2. Utilizing transfer matrix calculations, 27 we determine the electric field distribution of both resonances in the photoactive layer (see Fig. 2 (a)) and in the whole device (see SI Fig. S3 (a), optical constants in Fig. S4) for our model detector with 25 nm bottom and 30 nm top Ag contact (Device 1, for entire layer sequence see SI Table S1). We observe a significant phase shift of the cavity modes when interacting with the bottom Ag layer, leading to the formation of the coupled Tamm resonances described above. Interestingly, both resonances show a maximum inside the photoactive layer of the device, leading to an absorption enhancement at two distinct wavelengths. Fig. 2 (b) shows the simulated TP states under variation of the cavity thickness, where their pronounced anti-crossing highlights the coupling between the resonances. At a photoactive layer thickness of 50 nm, we observe highly coupled cavity modes with optimized quality and the strongest field enhancement in this simulation. The predicted optical resonances coincide well with the experiment as is shown in the supporting information (SI, Figure S5). Here, we observe a strong EQE enhancement in the CT absorption band, below the optical gap of donor and acceptor. Resolving the detector absorption and EQE by incident angle allows confirming several features expected from TPcavities, such as a blue shift accompanied by a polarization splitting at higher angles. Further discussion is detailed in Section VI and Fig. S5 of the SI.

Controlling the Resonances Via Equation (3), we identify the phase change and thus the layer thickness of the bottom Ag (dAg,bot ) as critical parameter for both TP resonances. As illustrated in Fig. 2 (c), changing dAg,bot allows tuning the phase shift φBot , and in turn influences both resonances. In absence of a bottom Ag contact, only a single TP resonance between DBR and top mirror is observed. When dAg,bot is increased to more than 5 nm, the second TP resonance appears. Here, TP1 shifts towards the red with increasing dAg,bot , due to an increased transmitted phase shift at 7 ACS Paragon Plus Environment

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the cavity-metal interface (compare Equation (3) and transmission through thin metal films, e.g. in Born and Wolf 30 ). TP2 emerges from the red sideband of the DBR and shifts to the blue. Here, the decreasing phase shift (towards 180◦ ) in the reflected phase is the crucial factor. Up to a bottom Ag thickness of approx. 60 nm, we see the coexistence of coupled TP modes, where they merge and form a single resonance for an even thicker contact layer. It is immediately obvious that when increasing dAg,bot even further, the underlying DBR does not play an important optical role any more, owing to the low transmission through the thick metal. While at least 5 nm are required for a conductive bottom contact, a thinner silver layer will typically lead to a better optical performance and higher Q factor of the cavity. The devices we realized represent on the one hand a TP cavity with two highly coupled resonances, where dAg,bot =25 nm (Device 1) and on the other hand a device with an ultrathin contact (dAg,bot =5 nm, Device 2), showing a vanishing TP2 resonance due to a weak DBR-metal resonator (see SI, Fig. S3 (b) and (c)). The optical simulations provide a clear understanding on how to control TP resonances. However, technological challenges arise when depositing such thin metal contacts. A detailed description on processing silver films on functionalized surfaces can be found in the Supporting information, Section III. While the single-resonance device seems more promising for an application in NIR spectroscopy, by carefully controlling both metal and cavity thickness (compare Fig. 2 (a) and (c)), two-resonance devices with almost arbitrary spectral distance can be realized.

Device Performance In the following, we study how well the absorption yield ηabs of both TP resonances converts, via internal losses (given by the internal quantum efficiency ηIQE ), into an external quantum efficiency EQE = ηabs ·ηIQE . 31,32 As ratio of extracted electron-hole pairs per incoming photons, the EQE is measured via the excitation power Pin and photocurrent jphoto via EQE(λ) =

hc jphoto eλ Pin

with h as Planck’s constant, c as vacuum speed of light, e as elementary

charge, and λ as wavelength. 2,32 8 ACS Paragon Plus Environment

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As seen from the measured EQE spectra in Fig. Figure 3 (a), both TP absorption resonances translate well into a spectral photoresponse. For Device 1, we clearly see two distinct peaks at 950 nm and 1025 nm in the EQE (solid lines), corresponding well to the simulated resonance wavelengths (dashed lines). For TP1 at 950 nm (TP2 at 1025 nm), we obtain a maximum EQE of 9 % (2 %) with a full width at half maximum (FWHM) of only 18 nm (40 nm), determined by a Lorentz-fit to the measured signal. One has to note, that the ratio of TP1 to TP2 differs slightly when comparing experiment and simulation. Here, this effect is related to inaccuracies in the determination of these very low extinction coefficients, as no further internal losses are expected during CT dissociation. 25 By comparing total device absorption (Fig. 2) to the photocurrent-generating CT absorption in the BHJ, plotted here, we can quantify parasitic absorption in the other layers of the device. Typically, both electrodes contribute by far the largest portion of parasitic absorption, followed by the weakly NIR-absorbing doped HTL. 9 The EQE contribution in the visible is filtered by an organic filter sequence (see SI, Table S1), making the device visible-blind. For Device 2, as shown in Fig. 3 (b), we are able to tailor the TP-charge-transfer (TPCT) modes to create a single-resonance device exhibiting an EQE of 17 % at 880 nm and a FWHM of only 21 nm determined by a Lorentz-fit of the measured resonance signal (see SI, Figure S6). Furthermore, by varying the thicknesses of the organic cavity layers in accordance with optical simulations (see SI, Table S1), the detection wavelength can be shifted, allowing for narrowband spectroscopic NIR detection. Fig. 3 (c) shows three different sensors with resonances at 880 nm, 930 nm, and 970 nm, respectively. Further tunability is possible, albeit sacrificing peak EQE. In previous reports 9 we have shown ZnPc:C60 based detectors up to 1100 nm, as well as detectors based on a tetraphenyl dipyranylidene:fullerene blend (TPDP:C60 ) up to 1550 nm. Since the gap of donor HOMO to acceptor LUMO is of major significance for the CT state absorption wavelength range, further material combinations are expected to increase the wavelength range beyond that.

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Fig. 1 (c) shows a photograph of Device 2. Most notably, we observe a semitransparent top metal contact (25 nm thick) as well as an almost invisible bottom contact (5 nm), forming a semitransparent microcavity. While the blue and green spectral regions are partially transmitted through the device, the red part is completely absorbed in the filter sequence mentioned before. Supplementary Fig. S2 shows measured transmission spectra for a DBR (a) as well as for Device 2 (c) with corresponding photographs. Despite absorber, filter, and metal layers, a device transmission of ≈30 % in the blue-green is achieved. Future donor:acceptor blends with high band gap for each semiconductor will make the usage of filter sequences obsolete. This step would further improve the visible transparency and opens the pathway for the realization of low-cost, visible-transparent organic NIR spectrometers.

Alternative TP Resonator Concepts With the achieved understanding of the formation of TP resonances, further design concepts for narrowband photodetectors can now be envisioned. A significant portion of optical losses stems from the parasitic absorption in both metal electrodes 9 even when combined with a DBR. Using transfer-matrix simulations, we explore alternative concepts to further enhance the optical quality. A natural advancement would be to combine DBRs with ultra-thin metal electrodes for both cavity mirrors, as shown in Fig. 3 (d) (and in more detail in the SI, Fig. S7 (a) and (b)). To preserve the partial transmission of the upper reflector, we choose the top DBR only to consist of 11 layers. 5 In such a configuration, our simulation predicts a narrowing of the EQE line width down to a remarkable value of 7 nm and a peak photoactive layer absorption of more than 40% for the resonance TP1 at ≈875 nm. While the resonances TP1 and TP2 couple again, interestingly the TP2 resonance does not reside within the organic stack. Fig. S7 (b) shows a significant field intensity in the photoactive layer only for TP1, while TP2 is completely located in the mirror layers, reducing the EQE peak of TP2 to virtually zero. In addition to increasing EQE and lowering FWHM, this design thus facilitates only a single 10 ACS Paragon Plus Environment

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CT absorption resonance (Fig. 3(d)), making it desirable for application. Alternatively, transparent conductive oxides (TCOs), as often applied in organic solar cells and light emitting diodes, are considered to further improve the resonator quality. As the deposition of TCOs involves high temperature steps, such as sputtering, they can only be processed before sensitive organic layers, i.e. as bottom electrode. In a DBR-based microcavity, TCOs such as indium tin oxide (ITO) can therefore replace the metal contact and even act a final high-index layer (n=1.55) of the dielectric mirror. Fig. 3 (e) shows such a configuration in red (with further details in Supplementary Figure S7 (c) and (d)), comprising 144 nm of ITO as 21st DBR layer and low-ohmic contact (sheet resistance Rs ≈ 20Ω/), and 25 nm of silver as top mirror and contact. Here, due to the missing Ag layer on the bottom, the resonator in the DBR vanishes, translating to a single TP resonance. The ITO itself does not provide a strong enough reflection (or plasmonic interaction strength) to facilitate the appearance of a TP2 state. Our simulation predicts a narrow linewidth of only 12 nm and a peak photoactive layer absorption comparable to Device 2, at a resonance of 875 nm. While both concepts trigger technological challenges to the device fabrication, they can lead to miniature spectrometers with an even higher spectral resolution and are therefore worth pursuing in future work. As the internal quantum efficiency is not dependent on the thickness of efficiently doped transport layers, 33,34 the simulated CT absorption lines are expected translate well into the EQE of real devices and constitute no limitation to spectral tunability and optimization of the design. Finally, it has to be noted that parasitic absorption in the transport layers still plays and important role and limits the optical quality. Alternative high-gap transport layers (with suitable dopands) however can overcome this final limitation.

Conclusion We have demonstrated a novel near infrared (NIR) detector concept by utilizing a Tammplasmon-polariton-based microcavity to optically enhance the charge-transfer (CT) state 11 ACS Paragon Plus Environment

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absorption in the bulk heterojunction of an organic photodiode. A careful choice of the donor:acceptor blend allows redshifting the CT energies, far below the gap of both neat absorbers. Due to the use of a high quality resonator, we facilitate NIR photodetection in the well-known material system ZnPc:C60 and achieve remarkable spectral line widths of 20 nm while reaching an external quantum efficiency of 17 % at 880 nm. Carefully controlling the growth of the bottom silver contact, we are able to switch between one- and two-resonance detectors with tunable spectral distance and finally provide a proof of concept for narrowband NIR spectroscopy by varying the resonator length. Additional resonator structures are suggested which are expected to further enhance the spectral selectivity. From a fundamental perspective, this novel architecture allows observing and tuning the coupling of CT states with Tamm-plasmon-polaritons. Finally, the nature of the resonator and organic system facilitates NIR detection at low cost while staying transparent in the visible, something which is not trivial to achieve with inorganic semiconductors.

Methods The TP device comprises an n-i-p organic photodiode embedded into a microcavity resonator made of a DBR bottom and a silver top mirror as depicted in Figure 1 (a). The DBR consists of 21 alternating layers of high (TiO2 , nTiO2 = 2.1) and low (SiO2 , nSiO2 = 1.45) refractive index layers with corresponding optical thicknesses of λD /4, where λD is the design wavelength. This mirror is deposited by electron beam physical vapor deposition at a base pressure of 10−6 mbar and an additional partial oxygen pressure of 2×10−4 mbar to ensure favorable optical conditions of the TiO2 layers. 35 We functionalize the DBR surface either with a 3 nm MoO3 |1 nm Au seed layer or a 5 nm polyethylenimine (PEI, Sigma Aldrich) layer 36 to ensure favourable growth conditions for thin silver bottom contacts. On top of the DBR, we fabricate an organic photodiode by thermal (co-) evaporation of organic small molecules and electrode materials. The electron transport layer (ETL) comprises 4,7-diphenyl-1,10-

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phenanthroline (BPhen, Lumtec) doped 1:1 with Cs, followed by a bulk heterojunction (volume ratio 1:1) of the donor zinc-phtalocyanine (ZnPc, TCI Europa) and the acceptor C60 (CreaPhys), and the hole transport layer N,N’-((Diphenyl-N,N’-bis)9,9,-dimethyl-fluoren-2yl)-benzidine (BF-DPB, Synthon) doped with the proprietary dopand NDP9 (Novaled Ag). The cavity is finished with a top electrode consisting of 3 nm MoO3 |1 nm Au|25 nm Ag, which also acts as the cavity top mirror. The cross-section of upper and lower electrode define a photoactive area of 6.4 mm2 . To improve stability, layers of MoO3 and tris (8hydroxy-quinolinato)-aluminium (Alq3 , Sigma Aldrich) are sequentially evaporated on top of the device followed by additional, electrically inert, absorber layers (ZnPc and 2,3,10,11tetrapropyl-1,4,9,12-tetraphenyl-diindeno[1,2,3-cd:1’,2’,3’-lm]perylene (P4-Ph4-DIP, synthesized in-house)) facilitating an optical filter to reduce the detector signal in the visible without reducing the NIR transmission below their respective optical gap. The samples are finally sealed under nitrogen atmosphere with a cover glass by means of an UV-hardened epoxy glue. Besides PEI, all organic materials were purified at least once by vacuum gradient sublimation. To monitor both layer thicknesses and blending ratios while thermal evaporation, quartz crystal microbalances are used. The base pressure for evaporating small molecules, fullerene, and inorganics is 10−7 mbar. The polyethylenimine layer, in contrast, is spin-coated from aqueous solution followed thermal annealing, as described in further detail in reference. 36 The layer thicknesses for different samples are collected in the SI, Table S1.

To determine the external quantum efficency (EQE), either the sample or a silicon reference diode are excited with monochromatic light from a xenon arc lamp, modulated by a chopper wheel. From the known spectral response of the Si reference diode, the wavelength dependent excitation intensity Pin is obtained. The photocurrent jphoto is recorded with a lock-in amplifier under short-circuit conditions, preamplified with a transimpedance amplifier. 31 To measure angle-resolved absorption spectra, a goniometer (spectrometer Maya2000 Pro, Ocean Optics) is utilized while transmission measurements of the mirrors are carried

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out in a two-beam VIS-NIR spectrometer (Shimadzu SolidSpec-3700).

Acknowledgements We thank the "Bundesministerium für Bildung und Forschung" for funding within the scope of InnoProfile 2.2 (03IPT602X) as well as the "Deutsche Forschungsgemeinschaft" within the scope of SPP1839 (LE747/53-1). Furthermore, we acknowledge the graduate academy of the TU Dresden for support, financed by the excellence initiative of the German federal and state governments. We thank Andreas Wendel for finalizing the sample preparation and Frederik Nehm for helpful discussions on thin electrodes. K.L. acknowledges his support as fellow of the Canadian Institute for Advanced Research (CIFAR).

Supporting Information Supporting Information Available: Table S1 and Figures S2-S7 containing further details on sample composition, transmission spectra of DBR and final devices, details on growth of metal thin films, supporting optical simulations and optical constants, fitting of the detection line, and angle-resolved EQE and absorption measurements. This material is available free of charge via http://pubs.acs.org

References (1) Ozaki, Y. Near-Infrared Spectroscopy – Its Versatility in Analytical Chemistry. Anal. Sci. 2012, 28, 545–563. (2) Baeg, K. J.; Binda, M.; Natali, D.; Caironi, M.; Noh, Y. Y. Organic light detectors: Photodiodes and phototransistors. Adv. Mater. 2013, 25, 4267–4295. (3) Ng, T. N.; Wong, W. S.; Chabinyc, M. L.; Sambandan, S.; Street, R. A. Flexible image

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