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Abstract
Optoelectronic devices based on organic semiconductor materials are on the rise for sensing applications due to their integrability with a variety of substrates – including flexible substrates for wearables. For sensing applications often narrowband absorption is desired with suppression of light at other wavelengths. Here, we investigate narrowband absorption enhancement of organic photodetectors (OPD) with an integrated lateral nanostructure. We show with finite-element simulations, that resonant excitation of low absorbing wavelength regimes allow for up to 3 times the absolute absorption at wavelengths on resonance compared to wavelengths off resonance. We present experimental results for CuPc/C60 OPDs fabricated on grating nanostructures with periods of 350 nm and 400 nm and a grating depth of 140 nm as well as a grating period of 370 nm and grating depths of 30 nm. Angle-resolved transmission spectra clearly show the optical resonance effects. In order to evaluate the electrical resonance effects a measurement system is introduced based on angular laser excitation. An angular resolution of 0.1° is achieved in the analysis of the OPD photocurrent response. Using the measurement setup an increase of the photocurrent by up to 50% is observed for the TE-resonance. It is demonstrated that the resonance wavelength is tuned simply by adjusting the grating period without changes in the layer thicknesses. This opens up new opportunities in realizing pixels of different wavelength response next to each other employing a single active stack design.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Organic photodetectors (OPD), i.e., photodetectors based on organic semiconductor materials, have been investigated as an important component in many applications such as optical communications, imaging and sensing devices [1–3], even combined devices from organic light emitting diodes (OLEDs) and organic solar cells have been realized [4]. Their inherent properties enable lightweight devices while keeping manufacturing costs low which opens up new application fields such as wearable electronics [5–7]. The conversion of an optical signal into an electrical signal is essential for these applications. Thus, a lot of research focused on improving the device efficiency and new materials for the OPDs [8,9]. In recent research efforts a lot of attention was brought towards OPDs in biomedical sensing applications [10,11]. Depending on the material composition of the active layer, they offer a wide range of absorption properties, which allows for appropriate OPD selection for the respective application.
Since the demand for reliable and low-cost sensing devices is high, research now focuses on the implementation of existing OPDs into combined measurement system [12–14]. These systems typically detect emission from fluorescent dyes, so the requirement for a suitable absorption spectrum is high. Most of the commonly used organic compounds show a broadband absorption spectrum which is both beneficial and a hinderance. It allows a large coverage of commonly used fluorescent dyes but at the same time means that the excitation light of the dye is detected as well. Narrowband absorbing materials would solve this problem but are a lot less common and only little research focused on those materials in the past [15–18]. The materials are synthesized for specific applications and wavelength regimes and are not further tunable.
The easiest solutions for narrow absorption involve absorption or interference filters. Both filters have downsides such as increased fabrication complexity or not being compatible with the integrated system design one is aiming for. Different approaches have been followed to alleviate the need for additional filters. The narrowing of the absorption wavelengths was for example accomplished by charge injection narrowing [19] or charge collection narrowing [20]. Even self-filtering photodetectors are designed with depletion layers inside the stack to absorb unwanted wavelengths [21,22]. Alternatively, resonant absorption enhancement may be used to achieve narrowband operation. The general idea is to use a thin active layer that absorbs little of the incident light except for wavelengths that are on resonance in a vertical cavity and experience a high absorption enhancement [23]. Recently used are vertical micro cavity effects where sub bandgap absorption is combined with intermolecular charge transfer processes to narrow the absorption spectrum [24,25]. Figure 1(a) depicts a schematic of an OPD with a vertical cavity of low quality factor. The vertical cavity quality factor may be increased by reducing the absorption and increasing the mirror reflectivity as shown in Fig. 1(b), as shown in previous publications [26]. As the active stack is part of the cavity here, electrical and optical properties are codependent. To realize OPDs sensitive in multiple wavelength regimes, stacked OPDs have been suggested [27].
Fig. 1. Schematic representation of a planar OPD (a), a thin-film OPD with distributed Bragg reflectors (DBR) above and below the organic layers (b) and nanostructured OPD (c). The layer thicknesses are not to scale but ${d_1} \gg {d_2}$. The photocurrent intensity generated inside the absorption layer (red) are depicted in the graphs below, where ${\lambda _{\textrm{res}}}$ is the resonance wavelength for the vertical thin-film resonance inside the layer stack (b) and the lateral waveguide resonance introduced by the grating structure (c). The resonant wavelength is partially trapped inside the organic layers and can couple out of the device in reflection which is depicted by the partially green arrow.
In this work we suggest the integration of nanograting structures in OPDs as shown in Fig. 1(c). The nanograting allows for lateral resonance effects and tailored absorption enhancement to desired wavelength ranges without the need to adjust the vertical cavity. Thus, the electrical stack may be optimized once and the optical resonance wavelength may subsequently be adjusted by changes in the nanograting. In particular different grating periods on a single substrate will allow for OPDs designed for different absorption wavelengths placed next to each other. This is much simpler than a variation of the vertical stack in terms of the fabrication. For different lateral grating periods next to each other a single master with different periods may be realized. All regions of the OPD array are then coated with the same active stack. In comparison the realization of different vertical stacks requires subsequent masked thin-film deposition steps. Thus, the main advantage of the approach proposed in Fig. 1(c) is the potential for realization of an array of OPDs with different absorption bands on a single substrate with an identical active stack.
The angular dependency of the grating resonance allows to further tune the sensitive wavelength regime by changing the angle of incidence. Nanogratings have been studied previously in inorganic photovoltaics for optical light trapping to increase the device efficiency [28,29]. This resonant trapping directs certain wavelengths into the photodetector, increasing the optical path length within the active materials thus increasing the absorption probability and hence photocurrent generation [30]. Previous studies conducted by finite difference time domain (FDTD) simulations show for inorganic photodetectors that high absorption enhancement factors can be generated with such grating structures depending on the aperture size of the incident light [31]. On the other hand, the quality factor of those enhancements is low which results in broadband absorption enhancements. Those broadband enhancements are not suitable for sensing applications as the Stokes shift of commonly used fluorescent dyes is small. To suppress any excitation of the dye and only measure its emission, a highly narrowband absorption enhancement is need.
For organic photovoltaics (OPV) with thin active layers an increase of 8-15% in photocurrent for solar illumination has been predicted in finite-element method (FEM) simulations [32]. Due to the broadband nature of sunlight, the effect is of a nanograting is rather small. Here, we investigate the use of nanogratings for realizing narrowband OPDs. We compare the absorption characteristics of nanostructured OPDs with planar reference device structures in FEM simulations. We find that for a narrow wavelength range increases of up to 300% can be achieved when adding nanostructures with resonant coupling at wavelengths at the absorption minimum of the material. The simulations show narrowband and highly adaptable resonances thus, this concept is ideally suited for narrowband detection enhancement.
The electrical characterization of nanograting effects in organic photodetectors under sunlight conditions is straightforward. Electrical parameters such as the generated photocurrent or the photo conversion efficiency of the nanostructured OPD can directly be compared to planar reference devices. Also, it is standard to measure optical characteristics like transmission or reflection spectra with high wavelength and angular resolution. Analyzing the electrical behavior for narrowband resonance effects is much more challenging, but highly interesting for sensing applications as it determines the sensitivity of the detector at certain wavelengths directly. The most commonly used characterization of electrical resonances is done by measuring the wavelength-integrated external quantum efficiency (EQE). Alternatively, the behavior can be characterized by narrowing the wavelength range of the incident light with a filter or a monochromator. However, this means that the excitation power is too low for the weak absorbing wavelength regime in OPDs to be useful as electrical characterization method. One approach studied before is the excitation with a broadband light source for reaching the OPD working point and then measuring with an additional narrowband excitation in a differential mode [33].
Here, we introduce a measurement system based on collimated laser excitation that allows us to electrically characterize the resonant optical effect by an angular sweep. Due to the lasers’ high power at single wavelengths no additional light-source is needed to generate a current. Furthermore, its narrow-wavelength excitation of the OPD allows to analyze the electrical response with high resolution. We present a high resolution measurement system in the sub nanometer regime that allows to directly compare the optical and electrical effects in semitransparent OPDs. In section 2, the theoretical basis, the fabrication method, the characterization as well as the simulation methodology are described. We then present the results in section 3 and discuss them in section 4.
2. Methods and experimental section
2.1 Resonant light trapping with nanograting structures
Periodic grating structures enable the partial coupling of incident light into quasi guided modes (QGM) inside slab waveguides [34]. Guided light in waveguides is divided into guided modes (GM) and quasi guided modes (QGM); GMs are confined to the waveguide whereas QGM are coupled by the grating structure and therefore may couple to the far field as well. Resonant coupling into a QGM is only achieved if Eq. (1) is fulfilled with ${n_{\textrm{inc}}}$ being the refractive index of the incident medium, $\vartheta $ the incident angle, $\lambda $ the wavelength, and m the mode order [35].
By inserting the wavenumber ${k_0} = 2\pi /\lambda $ into (1) the resonance wavelength can be calculated in dependence of the grating period $\mathrm{\Lambda }$, the effective refractive index of the layer stack ${n_{\textrm{eff}}}$ and angle of incidence using ${k_x} = 2\pi {n_{\textrm{eff}}}/\lambda $:
The modes are divided into transverse electric and transverse magnetic modes, depending on whether the electric (TE) or magnetic (TM) field is perpendicular to the propagation direction of the incident wave. Higher order modes can exist for both TE- and TM-modes but are not considered in this work.
2.2 Fabrication of nanostructured organic photodetectors
All samples in this work are prepared on 25 × 25 mm2 soda-lime glass substrates with a thickness of 1.1 mm. We fabricate the grating structures based on a standard UV nanoimprint lithography process [36]. For the experiments discussed in this paper we use nanostructures with grating periods $\mathrm{\Lambda } = 350/370/400\textrm{nm}$ and grating depths of ${t_{350,400}} = 140\textrm{nm}$ and ${t_{370}} = 30\textrm{nm}$, respectively. The grating periods were chosen from experimentally available gratings such that they match the material stack of the OPD and allow for comparison of different combinations. We aim to cover a wide range of resonant wavelengths and grating depths to study their influence on the device. First, a negative imprint template made of polydimethylsiloxane (PDMS; Dow, Sylgard 184 and curing agent at ratio of 8:1) is prepared from the master template structure. The UV imprint resist Amonil MMS4 (AMO GmbH, Aachen, Germany) is spincoated onto a cleaned glass substrate and the PDMS stamp is manually pressed into the resist. After UV curing, the PDMS is removed and the nanostructure is transferred on the glass substrate. Next, we sputter 140 nm ITO (Kurt J. Lesker, EJUITOX403TK4) onto the nanostructure as the bottom electrode of the OPD. The subsequent organic layers, as well as the upper electrode, are thermally evaporated. The bulk heterojunction comprises of copper phthalocyanine (CuPc; 40 nm; Sigma Aldrich, 410497) as the donor and C60 (30 nm, Sigma Aldrich, 572500-1 G) as the acceptor with bathophenanthroline (BPhen, Sigma Aldrich, 11880-500MG-F) as an additional electron transport layer [37–39]. A stack of molybdenum trioxide (MoO3; 10 nm; Sigma Aldrich, 203815-5 g)/silver (10 nm; Kurt J. Lesker)/MoO3 (10 nm) forms the semitransparent top anode. Finally, the OPD is encapsulated under nitrogen atmosphere with two-component adhesive (UHU Endfest 300) and a glass plate. The fabrication steps are depicted in Fig. 2.
Fig. 2. Visualization of the fabrication steps of the nanostructured OPD. (a), (b) Preparation of the inverted grating structure from PDMS; (c) spin-coating the imprint resist at 3000 rpm for 30s; (d)-(f) transfer of the grating structure on the glass substrate; (g)-(i) deposition of the OPD layers.
2.3 Optical characterization
The fabricated OPDs are characterized by angle-resolved transmission spectroscopy. A white light LED (SUNLIKE COB 2700 K) is used for excitation and the sample is placed on a two-dimensional computer-controlled rotation stage. The transmitted light is fed into the spectrometer (Andor Shamrock 500i) via an optical fiber. A linear polarizer is placed in front of the sample to use either transverse electric (TE) or transverse magnetic (TM) polarized light for excitation.
2.4 Electrical characterization
Similar to the optical characterization, the sample is placed on the rotation stage. For the excitation, a laser diode (Thorlabs CPS532, CPS520, CPS405) is mounted together with a variable pinhole and a polarizer on a xy-stage in front of the sample. This setup allows for a collimated excitation of the OPD under angular tilt as well as TE- and TM-excitation. The photocurrent of the OPD is read-out with a source-measure unit (SMU) connected to the same computer that controls the rotation stages. A schematic visualization and photo can be seen in Fig. 3.
Fig. 3. Schematic (a) and photo (b) of the measurement setup for the electrical characterization; a laser diode excites the OPD through a pinhole and polarizer while the photocurrent is read-out by an SMU connected to a computer; the angular tilt and orientation of the sample is controlled by the computer too.
Before each measurement of a new OPD, a long-term exposure of it is performed to counteract any drift and account for internal reassembling processes of the molecules. When the photocurrent is constant and no longer shows any changes due to switch-off and switch-on processes, the angle-resolved measurement is performed.
The measurement data is collected as current over time and is then mapped to the angle of incidence, controlled by the computer.
2.5 Finite-element method simulations
Theoretical simulations are carried out with COMSOL Multiphysics to investigate the influence of grating structures on the absorption behavior of the OPD. The grating periodicity $\mathrm{\Lambda }$, grating depth t and the layer stack of different materials, i.e., the effective refractive index ${n_{\textrm{eff}}}$, have a decisive influence on the absorption behavior, the resonance position and its shape. For the simulations we use the same semitransparent layer stack as fabricated. The material characteristics for CuPc and C60 are measured via white light ellipsometry; the characteristics for ITO, Bphen, MoO3 and silver are taken from literature [40–43].
We simulate a two-dimensional unit cell with Bloch-periodic boundary conditions to model the periodicity of the structure and add perfectly matched layers on the top and bottom of the stack as depicted in Fig. 4. Using the wave optics module within COMSOL allows us to calculate the electric field distribution $\overrightarrow {\underline{E} } $ inside the stack. We calculate the absorbed power inside the active layers A using Eq. (3).
As an estimate of the potential of absorption enhancement, we calculate the relative enhancement $\eta $ from the quotient of the absorbed power of a nanostructured OPD with a planar reference. The incident electromagnetic wave has transverse electric polarization with a maximum power set to 1W.
Fig. 4. Schematic of the COMSOL model rotated 90°. Periodic boundary conditions are indicated by the blue borders. The excitation is induced via the port and the transmission- and reflection spectra are read-out at the green and red lines. PMLs on the top and bottom of the structure absorb any residual energy.
3. Results and discussion
3.1 Optical resonance effects in OPDs
3.1.1 Simulation results
First, the influence of a nanostructure at normal light incidence ($\vartheta = 0^\circ $) was investigated by varying the grating period and comparing it to a planar reference. The absorption in the active layer, calculated with Eq. (3), is plotted versus wavelength for grating periods $\mathrm{\Lambda } = 300/315/350/370/400\textrm{nm}$ in Fig. 5(a). The black dashed line shows the absorption behavior of the planar OPD without a grating structure as depicted on the left in Fig. 5(b). Its shape is determined by the optical parameters of the active materials used in the simulation. The electric field distribution at the resonance wavelength ${\lambda _{305\textrm{nm}}} = 520\textrm{nm}$ (subscript denotes grating period) for both the planar reference and nanostructured OPD is depicted in Fig. 5(b). It shows how the electric field is highly confined within the active layers and the adjacent ITO electrode only if a nanostructure is included in the stack, which directly leads to higher absorption at that wavelength.
Fig. 5. (a) Absorption characteristics of planar and nanostructured ($\mathrm{\Lambda } = 300/315/350/370/400\textrm{nm},$ $t = 30\textrm{nm}$) organic photodiodes. The inset shows the calculated relative absorption enhancement $\eta $ (left y-axis) and the absolute absorption difference between reference and nanostructured device (right y-axis) over the grating period $\mathrm{\Lambda }$ with a maximum enhancement of ${\eta _{305\textrm{nm}}} = 4.6\%$ and an absorption increase at resonance wavelength of 300%. (b) Simulated electric field distribution for an incidence wavelength of 533 nm and an incidence angle of $\vartheta = 0^\circ $ with a grating period $\mathrm{\Lambda } = 305\textrm{nm}$ and a grating height of $t = 30\textrm{nm}$. ${l_\textrm{T}}$ depicts the integration line for the transmission spectrum calculation.
This resonant enhancement shifts to higher wavelengths with increasing period length according to Eq. (2). It can be seen that the relative difference between the absorption maximum in resonance and its reference, as well as the quality factor of the resonance decreases. An increasing overlap of the materials’ absorption and the resonance position leads to this behavior. The higher the absorption of the material, the lower the effect of the introduced nanostructure. This effect is also shown by the decrease in relative absorption $\eta $ for longer grating periods. For this layer stack the optimal period length is $\mathrm{\Lambda } = 305\; \textrm{nm}$ leading to a relative absorption enhancement of 4.6% over the visible spectrum and an absolute absorption increase at resonance of 300% from 0.1973 up to 0.6127.
The simulations show that the resonance position can be shifted to the desired wavelength with a narrow absorption enhancement bandwidth. To further investigate this effect, simulations under angular excitation were carried out. The angular resolved transmission spectrum is also derived directly from the electric and magnetic field distribution by integration over the integration line ${l_\textrm{T}}$ shown in Fig. 5(b). The transmission of the nanostructured stack is then normalized with the planar reference and shown in Fig. 6. The simulated parameters are the same as for the fabricated samples described in 2.2.
Fig. 6. Transmission spectra of simulated nanostructured OPDs depicted over wavelength (y-axis) and incident angle (x-axis) normalized to the maximum of a planar reference OPD. The scale corresponds to the normalized spectrum with the planar reference. TE0 modes couple into the OPD stack leading to resonances under normal incidence of ${\lambda _{350\textrm{nm},0^\circ }} = 573\textrm{nm},\; {\lambda _{370\textrm{nm},0^\circ }} = 594\textrm{nm},\; {\lambda _{400\textrm{nm},0^\circ }} = 630\textrm{nm}$. TE1 modes are visible but very dim.
The coupling into the OPD is well visible by the cross pattern. Wavelengths that couple into the OPD are partially guided inside the ITO and active layers which leads to higher absorption. This results in lower transmission intensity at these wavelengths that can be seen by the blue lines. The wavelength symmetry occurs when the resonant energy splits in two resonances for higher angles of incidence following the Bragg-Eq. (2). Combined with the angle-dependent symmetry the cross pattern is created and is named resonance cross in the following. We denote the resonance wavelength ${\lambda _{\mathrm{\Lambda },\vartheta }}$ according to their grating period and incidence angle from now on.
3.1.2 Angular transmission measurements
The fabricated samples were characterized by angular transmission spectroscopy from −25° to 5°. The transmission spectra are normalized to the spectrum of the white excitation LED. The spectra for TE polarized excitation of the samples are shown in Fig. 7. Larger positive angles are not used because the sample holder blocks part of the excitation LED, resulting in disturbed images.
Fig. 7. Transmission spectra of fabricated nanostructured OPDs normalized to the excitation spectrum depicted over wavelength (y-axis) and incident angle (x-axis). TE0 modes couple into the OPD stack leading to resonances under normal incident of ${\lambda _{350\textrm{nm},0^\circ \; }} \approx 588\textrm{nm},{\; }{\lambda _{370\textrm{nm},0^\circ \; }} \approx 618\textrm{nm}$ and ${\lambda _{400\textrm{nm},0^\circ \; }} \approx 637\textrm{nm}$.
The resonance patterns follow the expected result, as already shown by the simulations with higher resonance wavelengths for longer grating periods. Likewise, the quality factor of the resonances also varies strongly, showing a much sharper resonance for the shallow 30-nm grating compared to the deeper grating structures. This is to be expected, since it has already been shown that the quality factor of the resonances decreases at higher grating depths [44]. The absolute resonance positions between the measurement and the simulation deviates by $\mathrm{\Delta }{\lambda _{350\textrm{nm}}} = 15\textrm{nm},\; \mathrm{\Delta }{\lambda _{370\textrm{nm}}} = 22\textrm{nm}$ and $\mathrm{\Delta }{\lambda _{400\textrm{nm}}} = 7\textrm{nm}$. A direct comparison under normal incidence between the measurement spectra (solid) and the simulation (dashed) is depicted in Fig. 8.
Fig. 8. Comparison between measurement (red; right y-axis) and simulation data (blue; left y-axis) of three OPDs with different grating periods (solid, dashed, dotted).
This difference can have a number of reasons. The simulation assumes near perfect grating structures, which cannot be produced during manufacturing. The nanoimprint process of the nanostructure is done manually and is therefore susceptible to deviations in grating period and depth. Other publications show that PDMS is known to shrink while curing [45] which leads to a difference between the experimental and the simulated grating period. Additionally, the shape of the nanostructure influences the resonance position, which has already been shown in previous work of our group [46]. Furthermore, the thickness of the individual layers and their material parameters influence the effective refractive index. The ITO layer has the greatest influence, since it has a significantly higher refractive index compared to the organic materials and has to be much thicker than the other layers. A differing refractive index and extinction coefficient between our in-house sputtered ITO and the parameters used for the simulations [40] are likely, as the optical properties of ITO can be changed depending on the exact fabrication parameters and methods [47–49].
Another difference in the absorption spectra can be seen in the quality factors of the resonances. The ones for measured resonances are much lower compared to the simulated quality factors. Although the excitation LED is collimated and refracted by a pinhole, angular components in the excitation light cannot be avoided, whereas perfectly normal incidence is calculated by the simulation.
3.2 Electrical characteristics under angular excitation
First, the resolution and stability of the measurement system is investigated to assure that the narrow resonant absorption can be measured. For this, we monitored the angular photocurrent response of a planar OPD, excited through the substrate side with a 405-nm laser diode. The photocurrent over angle is depicted in Fig. 9(a). The photocurrent shows an oscillating behavior with a changing frequency for higher angles of incidence. The dots in the graph are single measurement points that show the accuracy of the measurement system with a resolution of approximately 0.1° which converts to a spectral resolution of 0.6 nm. The reason behind this oscillating behavior is a thin-film interference with the glass substrate in the vertical resonator, which leads to local excitation maxima and minima. We prove the origin of this effect with Eq. (4) [50].
Fig. 9. (a) Angular photocurrent response for excitation with a 405-nm laser for a planar OPD. (b) Schematic representation of the optical path length difference experienced by the laser at the interface between air ${n_1}$ and glass ${n_2}$. The thickness of the glass $d = 1.1\textrm{mm}$ is not to scale.
For successive angles of incidence (${\vartheta _m} = 1.588^\circ ,$ ${\vartheta _{m + 1}} = 2.098^\circ $) under which constructive interference occurs, we calculate the thickness of the glass $d = 1.06$ mm for $\lambda = 405$ nm and $n = 1.5$. This calculation agrees closely with the manufacturers’ specifications of the glass substrate of 1.1 mm. Considering typical resonance widths of 565 nm to 580 nm from Fig. 8, we now have verified that the measurement systems’ resolution is high enough to detect the resonant absorption.
The following measurements were done with the fabricated nanostructured samples. Figure 10 shows angular photocurrent characteristics of the OPDs excited with the 532-nm laser. The measurement was repeated three times with different polarizations (no polarizer: blue; TE polarizer: red; TM polarizer: yellow) for each sample to verify that a resonant effect due to the nanostructure is detected and not some other effect induced by the measurement and angular sweep itself.
Fig. 10. Angular photocurrent response for the fabricated samples from $\vartheta = 0^\circ $ to $\vartheta = 20^\circ $ and $\vartheta = 30^\circ $, respectively. The OPDs are excited by a $\lambda = 532\textrm{nm}$ laser diode without a polarizer (blue), TE polarized light (red) and TM polarized light (yellow). The TM-resonances are indicated with the dashed vertical line and can be seen at ${\vartheta _{350\textrm{nm},{\; \textrm{TM}}}} \approx 1.5^\circ ,{\; }{\vartheta _{370\textrm{nm},{\; \textrm{TM}}}} \approx 4^\circ {\; \textrm{and}\; }{\vartheta _{400\textrm{nm},{\; \textrm{TM}}}} \approx 12^\circ $; the TE-resonances are indicated with the dotted vertical line and can be seen at ${\vartheta _{350\textrm{nm},\textrm{TE}}} \approx 15^\circ ,{\; }{\vartheta _{370\textrm{nm},\textrm{TE}}} \approx 17^\circ {\; \textrm{and}\; }{\vartheta _{400\textrm{nm},\textrm{TE}}} \approx 24^\circ $.
The blue curve shows for the 350-nm and 370-nm nanograting samples two resonant enhancements in the photocurrent response without the polarizer in the setup. For the 400-nm grating sample one distinct resonance can be seen at 24°. The second resonance is barely visible at 12°. If comparing the unpolarized excitation with the TE- excitation one can see clearly that the big resonance is still visible whereas the small resonance is suppressed and vice versa for the TM-excitation, respectively. This is a clear indication that we are measuring the optical resonant coupling of the TE- and TM-modes electrically via the increase in photocurrent.
As already shown in the simulations and angular transmission measurements, the TE-modes are much more pronounced for the given nanostructures which is also reflected in these measurements. Further, the observed amplitudes between the excitation methods vary. The photocurrent with unpolarized excitation is the highest for all three samples because no light is filtered by the polarizer. The TE resonance has a higher contribution to the enhancement as it seen in the figure. In theory, summing both signals of the polarized excitations equals the unpolarized photocurrent generation without a filter. The observed absolute current values for unpolarized light are a bit higher due to losses in the polarization filter.
Finally, we compare the electrical resonances with the angular transmission spectroscopy. To achieve a higher comparability, we use a second laser wavelength ($\lambda = 520\textrm{nm}$) to excite the samples; no polarizers are used in this setup such that both TE- and TM-modes are excited. This generates different resonance angles which are directly linked to the changed incident wavelength. Figure 11 shows a direct comparison of the electrical (a) and optical (b) resonances of the 370-nm grating device. We compare the electrical photocurrent generation at two wavelengths with the angular transmission spectra of the same sample at different selected wavelengths. The photocurrent generated relates to the absorption of the active material which again leads to a decrease in the transmission. Thus, it is suitable to compare the electrical and optical characteristics.
Fig. 11. (a) Normalized photocurrent response for excitation with 532-nm (red line) and 520-nm (blue line) laser. Increase in photocurrent is observed at resonant angles ${\vartheta _{532\textrm{nm}}} = 17.38^\circ ,\; {\vartheta _{520\textrm{nm}}} = 21.03^\circ $ (TE) and ${\vartheta _{532\textrm{nm}}} = 4.8^\circ $ (TM). (b) Angle-dependent TE transmission of a nanostructured CuPc/C60 OPD at four different wavelengths normalized to maximum. Resonant coupling to the nanostructure is seen at $\vartheta = 17.1^\circ $ for $\lambda = 532\textrm{nm}$, at $\vartheta \approx 10^\circ $ for $\lambda = 567\textrm{nm}$ and at $\vartheta \approx 0^\circ $ for $\lambda = 618\textrm{nm}$.
The coupling by the 532-nm laser is very well visible as the red line in the angular photocurrent response as well as the transmission spectrum with a resonant angle at ${\vartheta _{532\textrm{nm}}} \approx 17^\circ $. The agreement of the resonance angles between the electrical and the optical measurement is very good, diverging less than 0.2° their resonance shapes quality factors of ${Q_{532\textrm{nm},\textrm{e}}} \approx 7.2$ and ${Q_{532\textrm{nm},\textrm{o}}} \approx 8.1$ respectively. The resonant angle of the 520-nm laser excitation is ${\vartheta _{520\textrm{nm}}} = 21^\circ $ which is out of the range of the angular transmission thus cannot be seen in this figure.
The TM-resonance for an excitation with the 532-nm laser has a resonance angle of ${\vartheta _{532\textrm{nm},\textrm{TM}}} \approx 4.8^\circ $ and is only visible in the electrical measurement. The background signal in the transmission measurement is too strong for the TM-resonance to be resolvable. The photocurrent on the other hand is influenced by the coupling of additional light into the active layer. TM modes are typically much better confined within the guiding layers which lead to sharper resonance peaks and here a better sensitivity for the active layer.
4. Conclusions
In this work we presented an approach to selectively enhance the photocurrent generation of organic photodetectors for specific wavelengths. Using the optical light trapping effect of lateral nanostructures, we achieved resonant photocurrent enhancements. We set up a laser-based measurement system to characterize the optical resonance effects electrically. The measurement uses the angular dependency of the resonant coupling into the OPD due to the nanostructure in combination with a narrowband laser excitation. Sweeping the angle of incidence allows to shift the resonant wavelength in- and out of the excitation wavelength which leads to increased absorption in the active material of the OPD. The angular resolution is 0.1° corresponding to a sub nanometer wavelength resolution of 0.6 nm. Due to the measurement at a single wavelength, wavelength-dependent material absorption effects are decoupled from resonant-grating effects.
We fabricated CuPc/C60 OPDs with three different grating structures and characterized them by angular transmission spectroscopy and with our laser excitation setup. The maximum gain by the nanostructure is approximately 50% for the TE-resonance of the OPD with a 350 nm nanostructure. This is a significant photocurrent increase, considering that the grating period resonance wavelength is located on rising edge of the absorption spectrum of the CuPc/C60 OPD and the internal photo conversion efficiency of the OPD is approximately 2%. The resonant angles obtained by the optical measurements are in agreement with the electrical characterization.
Further, we investigated the resonant behavior of nanostructured OPDs with FEM-simulations. The resonant coupling into the OPD is highly adaptable for a desired wavelength. The simulations show that the absorption enhancement is best for resonant coupling close to local absorption minima of the material. The better the material absorbs incident light by itself the lower the quality factor of the resonance and the lower the resonant contribution to the absolute absorption. This technique opens new applications for OPDs as sensing devices, as the detectable wavelength range can be influenced by the layer structure, grating period, active materials and excitation wavelength. Additionally, this resonant enhancement effect is highly angle dependent which can be beneficial for directional detection of regions of interest in sensing applications. The effect of a weak-absorbing OPD combined with a nanograting is similar to the combination of high-absorption OPD with a thin-film bandpass filter.
A significant advantage of the lateral nanostructure is that the resonance wavelength can be adjusted simply by changing the nanostructure underneath the OPD active stack. Material-based approaches require the deposition of different active stacks next to each other. Vertical resonator-structures also require adjustments at least in the layer thicknesses. Using the approach presented here a substrate with different grating periods at different lateral positions can be used to adjust the bandpass wavelength for different OPD pixels using a single design for the active stack. For even better quality factors and suppression ratios than presented here, the layer thickness of the active material and the electrode absorption should be further reduced.
Funding
Gesellschaft für Energie und Klimaschutz Schleswig-Holstein; HORIZON EUROPE European Innovation Council (EIC-Transition project: SOILMONITOR, 101097989); Land Schleswig-Holstein (Open Access Publikationsfonds).
Acknowledgements
The authors acknowledge support by the Energie & Klimaschutz Schleswig-Holstein (EKSH) and the European Innovation Council (EIC-Transition project: SOILMONITOR; 101097989). We acknowledge financial support by Land Schleswig-Holstein within the funding programme Open Access Publikationsfonds.
Disclosures
Figure 5 and 11 were presented in a similar form in a conference abstract for the Photonics North 2023 which will be published by IEEE.
Data Availability
Data underlying the results presented in this paper are not publicly available at this time and can be obtained from the author upon reasonable request.
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