Enhanced Photoresponse in Metasurface-Integrated Organic

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Enhanced Photoresponse in Metasurface-Integrated Organic Photodetectors Xin Xu, Hoyeong Kwon, Brian Gawlik, Nasim Estakhri, Andrea Alu, S.V. Sreenivasan, and Ananth Dodabalapur Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b05261 • Publication Date (Web): 30 Apr 2018 Downloaded from http://pubs.acs.org on April 30, 2018

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Enhanced Photoresponse in Metasurface-Integrated Organic Photodetectors Xin Xu†, Hoyeong Kwon†, Brian Gawlik‡, Nasim Mohammadi Estakhri†§, Andrea Alù*†, S.V. Sreenivasan‡, and Ananth Dodabalapur*† †Department of Electrical and Computer Engineering, The University of Texas at Austin, Austin TX 78712, USA ‡Department of Mechanical Engineering, The University of Texas at Austin, Austin TX 78712, USA TOC GRAPHIC

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Abstract—In this work, we experimentally demonstrate metasurface-enhanced photoresponse in organic photodetectors. We have designed and integrated a metasurface with broadband functionality into an organic photodetector, with the goal of significantly increasing the absorption of light and generated photocurrent from 560 nm up to 690 nm. We discuss how the metasurface can be integrated with the fabrication of an organic photodiode. Our results show large gains in responsivity from 1.5X to 2X between 560 nm and 690 nm.

KEYWORDS: metasurface, organic photodiode, efficiency enhancement, nanofabrication, nanophotonics, bulk heterojunction Very often, photodetector efficiencies and responsivities need to be enhanced over a range of wavelengths over which the absorbing material is not strongly absorbing because either the absorbing material has a weak natural absorption at those wavelengths, or more recently as we move towards ultra-thin or even 2D materials, there is simply not enough material to achieve strong absorption1–5. To counteract this issue, techniques to increase the absorption have been developed such as the incorporation of Fabry-Perot cavities, plasmonic structures, photonic crystal architectures, and others6–10. In this work, to the best our knowledge, we demonstrate the first experimental use of phase-gradient metasurfaces to achieve relatively broadband enhancement of the efficiency of organic photodiodes.


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Our metasurface design applies the concept of the generalized Snell’s law of reflection and refraction11. Recent studies have shown that metasurfaces designed based on generalized Snell’s law can be applied underneath a photodetector to enhance light matter interactions11–19. By steering light propagation from normal incidence towards a bent angle, the metasurface offers extended light paths inside the thin photodetector and increases light sensitivity and photodetector efficiency. Following previous works11–19, the surface admittance distribution profile is calculated to deflect the beam inside the organic material into the desired angle. As shown in Fig. 1(a), the period D is discretized into 4 unit cells, dragging the phase of incident light from 0 to 1.5π in steps of 0.5π. The metasurface period is defined as D = λo/|sinθr|, where λo is the design wavelength in the organic material, and θr is the angle of anomalous reflection in

Figure 1. (a) 3D model of the metasurface with the inset being a scanning electron microscope image of four repeating units of the fabricated metasurface. (b) The efficiency of the metasurface at coupling normally incident light into the desired diffraction channel. At the design wavelength, 40% of the light is reflected into the anomalous reflection angle of 20º.


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the organic material. Fig. 1(a) shows a schematic of the optimized metasurface, composed of Au and ZnO, and the inset of Fig. 1(a) shows a scanning electron microscope image of four periods of the fabricated sample. The metasurface is designed to send normal incident light to a deflection channel θr = 20º at λ = 700 nm inside the stacked structure, whose geometry is shown in Fig. 2. The presence of a thick Si substrate made the optimization difficult, because of the large simulation domain. Therefore, the structure was first optimized with a reflective electric ground plane, for which the optimized metasurface provided an efficiency of anomalous reflection into the desired diffraction order of |S+1|2 = 76%. We then simulated the optimized metasurface under a half space of Si, as shown in Fig. 1(b), providing an anomalous reflectivity of 40% towards the desired direction.

Figure 2. Schematic layer structure of the photodetectors. The top Au contact is not to scale with the rest of the structure. The photodetectors are designed to be optically simple by reducing the number of transport layers to only those necessary for diode performance. (a) Photodetector structure with metasurface with period D. The period D is defined at the free space wavelength λ = 700 nm and reflects the normally incident light to θ = 20º inside the organic material P3HT:PC71BM. (b) Photodetector without metasurface.


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The metasurface performs best at the design wavelength λ = 700 nm, but it offers relatively high performance over a wide bandwidth20, as shown in Fig. 1(b). This implies that the response of the metasurface is rather different from conventional approaches to light trapping using resonance phenomena, with significant advantages in terms of bandwidth of operation, which translates into good flexibility on the type of photodetector that we can use. Based on the results in Fig. 1(b), the metasurface was optimized and integrated within the photodetector in Fig. 2(a) and Fig. 2(b) shows the case of a regular photodetector without metasurface. Au and ZnO are the materials used for the implementation of the metasurface. The full integrated structure with Si substrate was simulated, and the electric field distribution

Figure 3. Simulated scattered E-field in the z-direction. (a) Simulation of the metasurface integrated photodetector at the design wavelength of λ = 700 nm. The wavefront is tilted into the desired angle with high efficiency. (b) Simulation of the metasurface integrated photodetector at λ = 600 nm. From this simulation, a metasurface response at wavelengths besides the design wavelength can be observed. (c) Simulation of a photodetector without a metasurface at λ = 700 nm. ACS Paragon Plus Environment

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is shown in Fig. 3 for three scenarios. Specifically, Fig. 3(a) and 3(b) shows the metasurface integrated structure [Fig. 2(a)] with 700 nm and 600 nm light respectively, while Fig. 3(c) shows the scenario without the metasurface. Comparing the field profiles between Fig. 3(a) and (c), we can see that the E-field is bent towards the left (near θr) in the case with metasurface, which increases the propagation path inside the photodetector and, in turn, results in high absorption. There is natural dispersion expected from the fixed periodicity of the metasurface, but the metasurface offers high quality of deflection even for other frequencies as shown in Fig. 3(b), confirming the results in Fig. 1(b). Based on the design in Fig. 2(a), the metasurface fabrication was performed using e-beam lithography and incorporated into organic photodiodes. We note that the fabrication of metasurfaces over large areas is compatible with higher throughput and lower cost (compared to e-beam lithography) fabrication processes such as nanoimprint lithography21–24. In order to make a straightforward comparison between photodiodes with and without the inclusion of the metasurface, the substrate was patterned so that the two types of photodiodes would be fabricated side by side. Our design uses the Au metasurface as the bottom contact of the photodiode, and its performance is compared to the one with a flat and continuous Au bottom contact. Other than this modification, the two devices are identical and are fabricated on the same substrate. The metasurface was fabricated using e-beam lithography and metal deposition and lift-off. On a substrate of silicon with 150 nm thick thermally grown silicon oxide, ZEP 520A was spun and patterned using a JEOL 6000 FSE E-beam. The pattern was developed using a bath of amyl acetate for 12 seconds. 40 nm of Au with a titanium adhesion layer of 3.5 nm was deposited with a Lesker Thermal Evaporator, and lift-off was performed by soaking samples in a


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Remover PG bath at 80ºC for four hours followed by 10 seconds of sonication. Additional fabrication details can be found in supporting information. Incorporation of a metasurface into an organic photodiode requires a modification of the standard inverted photodiode structure. The standard organic photodiode is bottom illuminated, and is built on top of a transparent conducting substrate, such as ITO coated glass25,26. However, this standard design does not work with our metasurface, since it changes the reflection angle of incident light. In the standard inverted organic photodiode stack, the metasurface would have to be fabricated after the deposition of the organic layer, but the fabrication process of the metasurface would destroy the organic absorption layer. In our modified photodiode structure, as seen in Fig 2, the metasurface is fabricated on a substrate of thermally grown silicon oxide (SiO2) on a silicon wafer. Our structure is also much simpler than the more standard organic photodiodes, eschewing additional transport layers, to make the electromagnetic simulations through the structure easier. Deposited above and around the metasurface is an electron transport layer of ZnO, which was spin-coated in a nitrogen glove box from a precursor of diethylzinc dissolved in toluene from Sigma-Aldrich and annealed at 150ºC27 in ambient. This method for depositing ZnO uses a low annealing temperature which protects the metasurface from possible damage via thermal expansion. The organic active layer is the polymer-fullerene blend of poly(3hexylthiophene):[6,6]-phenyl C71 butyric acid methyl ester (P3HT:PC71BM), which was spincoated from a precursor of the blend dissolved in dichlorobenzene (DCB) in a ratio of 1 mg: 1 mg: 1 mL25. The P3HT was purchased from Rieke Metals, the PC71BM was purchased from American Dye Source, and the DCB was from Sigma-Aldrich. An energy band diagram can be found in the supporting information. This organic blend was selected because it is well studied and understood, and absorbs strongly in the green but absorbs less at longer wavelengths, where


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the metasurface enhancement would be most obvious1,28–32. The thickness and optical properties of the thermal SiO2, ZnO, and the organic blended film were measured using a Woollam Ellipsometer M-2000D. Lastly, 50 nm thick finger shaped Au top contacts were deposited by thermal evaporation through shadow masks which are patterned to leave gaps for light to pass through. The photodiodes were measured in a vacuum environment to prevent degradation of the organic film. The current of each photodiode was measured while sweeping the voltage from 1 V to -1 V, under light and under dark conditions. A xenon lamp white light source with narrow band-pass filters (FWHM = 10 nm) was used as the light source with filters from 710 nm down to 600 nm in 10 nm increments and then down 560 nm in 20 nm increments. The intensity of light with each individual filter was measured using a ThorLabs PM100D Power Meter with a silicon photodiode with sensitivity from 400 nm to 1100 nm. The measured light intensity was then used to calculate the responsivity33. Photodiodes with metasurface back contact show a large increase in photocurrent compared to the photodiodes with unpatterned metal contact. This trend held across multiple devices and samples. Fig. 4 shows the photocurrent, responsivity, and detectivity of photodiodes with and without the metasurface. It can be seen that there is an increase in responsivity of 2X from 560 nm up to 660 nm at which point the performance increase gradually drops down to 1. Also, it is important to note that across this entire range from 560 nm to 710 nm there is no loss in performance when comparing metasurface devices to devices without metasurfaces. The detectivity is within the range of values reported by Heeger et al. for polymer photodiodes33. Additional figures of merit can be found in the supporting information.


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(a)

(b)

(c)

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Figure 4. Results from experimental organic photodiodes. (a) Photocurrent of photodiodes with and without metasurfaces. (b) Responsivity of photodiodes with and without metasurfaces. (c) The multiplicative enhancement of responsivity of photodiodes with metasurfaces compared to photodiodes without metasurfaces for experimental devices and simulated devices. (d) The detectivity of the diodes with and without metasurfaces. The general trend of the experimental results match those in simulations, however the extent of increase in the measured photocurrent in metasurface containing devices is slightly less than that predicted numerically. This can be attributed to several factors. First, the fabrication of the metasurface will not result in perfectly rectangular Au lines. Second, mismatches in film thickness are expected to impact the performance. Lastly, the simulation makes the assumption of perfectly smooth flat interfaces, but actual fabricated devices have rough interfaces as the films conform to the patterned metasurface. Interestingly enough, from 560 nm to 610 nm the


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experimental results actually outperform the theoretical predictions. This is due to the simulation predicting an internal electromagnetic resonant effect due to the arrangement of all constituent layers of the photodiode. However, the experimental devices are never perfectly flat or smooth, leading to a weakening of the resonance that lowers the actual measured photocurrent, thus improving the comparison between metasurface and non-metasurface device. In order to validate the enhancement of photocurrent, and confirm that it is indeed due to increased light absorption, samples without top contacts were exposed to light of varying wavelengths and imaged with an imaging spectrophotometer. The imaged reflectance of an aluminum mirror was taken as a baseline to calculate the reflectance of the photodiodes. Fig. 5 shows the reflectance of the metasurface area compared to the reflectance of the unpatterned (a)

(b)

(c)

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Figure 5. Measured reflectance at varying frequencies of light of the photodiode stack with an aluminum mirror used as a reference reflection of 1. The left area is the metasurface, while the right area is unpatterned Au. (a) 560 nm, (b) 600 nm, (c) 650 nm, (d) 700 nm. The clearly darker metasurface regions show lower reflectance which means increased absorption.


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metal area at various wavelengths from 560 nm up to 700 nm. The clearly darker metasurface region means that there is less reflectance compared to the unpatterned metal area, and we can safely deduce that the metasurface area is indeed absorbing more strongly than the area with no metasurface. We have demonstrated the benefit of the integration of a metasurface to increase organic photodiode performance from 560 nm to 690 nm. There are several key advantages of this technique. One is its inherent flexibility: the metasurface geometry can be designed to increase absorption of a wide range of weakly absorbing thin films over a broad range of wavelengths. Another advantage is the potential of the metasurface with improved lithography and metal patterning. By making finer metal patterns we can increase the degree to which we bend light to go beyond 20º, which would lead to a further enhancement in performance. This is particularly beneficial to weakly absorbing materials, such as graphene. We stress that this technique is not limited to making photodetectors. There are many parallels between photodetectors and solar cells, and metasurfaces would be appropriate to increase the absorption in thin film solar cells, particularly over wavelengths where absorption is weak19. Metasurface patterning can also greatly benefit from improved lithography techniques, such as nanoimprint lithography, which is a mechanical nanofabrication technique that reproduces a pattern from a master21 . If nanoimprint lithography were used in the fabrication of the proposed metasurfaces, we may be able to manufacture patterns with a higher throughput, over larger areas, and even flexible substrates24 . Thus, this metasurface technique is applicable to scaling to an industrial fabrication level. In summary, we demonstrated the experimental realization of metasurface-enhanced organic photodetectors with significantly increased absorption of light and generated photocurrent over a


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large range of wavelengths, from 560 nm to 690 nm. We described how the metasurface can be integrated with the fabrication of an organic photodiode. Our results show large gains in responsivity from 1.5X to 2X between 560 nm and 690 nm. This general approach of using metasurfaces is very promising for a number of photodetector and photoabsorbing materials with low absorption. ASSOCIATED CONTENT Supporting Information The supporting information contains: • • •

Energy band diagram of the organic photodiode Detail fabrication steps More figures of merit for photodiodes such as external quantum efficiency, noise equivalent power, and rectification ratio

AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected] Present Addresses §University of Pennsylvania, Philiadelphia, PA 19104, USA Author Contributions X.X. is responsible for the fabrication of the metasurface and the photodiode and all the electrical measurements and preparation of the paper. H.K. and N.E.M. designed and simulated the metasurface. H.K. also worked to prepare the paper. B.G. is responsible for the measurements with the spectrophotometer. S.V.S. guided the spectrophotometer work. A.A. is


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responsible for the overview of the design and simulation work. A.D. coordinated the overall research and designed and provided guidance for the experimental work. All authors contributed to the editing of the manuscript. Funding Sources National Science Foundation under Cooperative Agreement No. EEC-1160494 Welch Foundation with grant No. F-1802 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work is based upon work supported in part by the National Science Foundation under Cooperative Agreement No. EEC-1160494 and the Welch Foundation with grant No. F1802. Any opinions, findings and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. REFERENCES (1)

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Enhanced Photoresponse in Metasurface-Integrated Organic

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