Nanoimprinted Perovskite Nanograting Photodetector with Improved Efficiency Honglei Wang,† Ross Haroldson,‡ Balasubramaniam Balachandran,§ Alex Zakhidov,∥ Sandeep Sohal,∥ Julia Y. Chan,¶ Anvar Zakhidov,‡,⊥ and Walter Hu*,†,# †
Department of Electrical Engineering, ‡Department of Physics, §Department of Mechanical Engineering, and ¶Department of Chemistry and Biochemistry, The University of Texas at Dallas, Richardson, Texas 75080, United States ∥ Department of Physics, Texas State University, San Marcos, Texas 78666, United States ⊥ Energy Efficiency Center, National University of Science and Technology, MISiS, Moscow 119049, Russia # ASIC and System State-Key Laboratory, Institute of Microelectronics, Fudan University, Shanghai 200433, China S Supporting Information *
ABSTRACT: Recently, organolead halide-based perovskites have emerged as promising materials for optoelectronic applications, particularly for photovoltaics, photodetectors, and lasing, with low cost and high performance. Meanwhile, nanoscale photodetectors have attracted tremendous attention toward realizing miniaturized optoelectronic systems, as they offer high sensitivity, ultrafast response, and the capability to detect beyond the diffraction limit. Here we report high-performance nanoscale-patterned perovskite photodetectors implemented by nanoimprint lithography (NIL). The spin-coated lead methylammonium triiodide perovskite shows improved crystallinity and optical properties after NIL. The nanoimprinted metal−semiconductor−metal photodetectors demonstrate significantly improved performance compared to the nonimprinted conventional thin-film devices. The effects of NIL pattern geometries on the optoelectronic characteristics were studied, and the nanograting pattern based photodetectors demonstrated the best performance, showing approximately 35 times improvement on responsivity and 7 times improvement on on/off ratio compared with the nonimprinted devices. The high performance of NIL-nanograting photodetectors likely results from high crystallinity and favored nanostructure morphology, which contribute to higher mobility, longer diffusion length, and better photon absorption. Our results have demonstrated that the NIL is a costeffective method to fabricate high-performance perovskite nanoscale optoelectronic devices, which may be suitable for manufacturing of high-density perovskite nanophotodetector arrays and to provide integration with state-of-the-art electronic circuits. KEYWORDS: perovskite, nanoimprint lithography, nanogratings, photodetector, nanoscale, crystallinity
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via relatively expensive methods such as vapor−liquid−solid or epitaxy, which have difficulty in system level integration.13 The development of their practical applications has thus been hampered. Recently, hybrid organolead halide perovskites, such as CH3NH3PbI3 type (lead methylammonium triiodide: MAPbI3), have emerged as a promising material for advanced optoelectronic devices due to their advantages of long carrier diffusion length, high carrier mobility, and large optical absorption coefficient over a broad wavelength range.14−17
hotodetectors that can convert optical signals into an electrical signal play an important role in a variety of applications, such as optical communication, digital imaging, and environment monitoring.1−4 Nanoscale photodetectors enable the opportunity to integrate high-density devices with state-of-the-art integrated circuits, while simultaneously offering high sensitivity, ultrafast response due to high surface-to-volume ratio, and reduced conductive channel dimensions.5−7 Imaging systems with nanoscale pixels are capable of achieving resolution beyond the diffraction limit.8 Materials compatible with conventional silicon electronics or flexible substrates are especially attractive. Most of the nanostructured photodetectors reported to date are inorganicbased materials such as carbon nanotubes,9 group II−VI,10,11 and group III−V compounds.12 These materials are synthesized © 2016 American Chemical Society
Received: August 16, 2016 Accepted: November 15, 2016 Published: November 15, 2016 10921
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Figure 1. Schematic of NIL process with (a) Si flat mold, (b) Si nanopillar mold, and (c) Si nanograting mold.
butyrolactone (GBL) and N,N′-dimethyl sulfoxide (DMSO) as solvents of methylammonium lead halide perovskite for spincoating followed with a toluene drip while spinning, which allows formation of a homogeneous perovskite layer after thermal annealing. In this study, (100) Si with 100 nm thick thermal oxide was used as the substrate. After spin-coating and toluene-dripping, the sample was annealed at 100 °C for 10 min, which subsequently drives out the solvents and forms a perovskite thin film with a thickness of 265 nm. One of the samples was imprinted, as illustrated in Figure 1. Three different Si molds were used to compare different structures. A flat surface (Figure 1a), nanopillar (Figure 1b), and nanograting (Figure 1c) structures were first treated with an antiadhesion monolayer of 1H,1H,2H,2H-perfluorodecyltrichlorosilane (FDTS). The molds were then placed on the perovskite thin film on different areas in a single process to ensure the same conditions applied to all molds. The samples were imprinted with a profile of conditions illustrated in Figure S1. The NIL was done at a temperature of 100 °C and a pressure of 7 MPa for 20 min. Reference samples with as-spin-coated perovskite thin films without NIL were also prepared. The morphology of both the nonimprinted (Figure 2a) and imprinted films (Figure 2b−d) was studied by scanning
Moreover, these hybrid organic−inorganic perovskites are solution processable for cost-effective fabrication. Among currently reported perovskite photodetectors,4,18−28 the planar metal−semiconductor−metal (MSM) structure has attracted interest due to its simplicity and integration advantages. The first perovskite MSM photodetector was reported by Hu et al.,24 who utilized an ITO−perovskite−ITO structure and achieved a photoresponsivity of 3.49 A/W and 0.0367 A/W at 365 and 780 nm respectively, with a 3 V bias. Since then, effort has been made to further improve the MSM photodetector performance, which is greatly influenced by the charge carrier diffusion length and mobility and could be improved with better morphology and higher crystalline quality. Lian et al.4 and Saidaminov et al.25 have reported a single-crystalline perovskite photodetector with significantly improved responsivity compared to polycrystalline counterparts. Toward the study of nanoscale hybrid organic−inorganic perovskite, Horvath et al.29 reported the synthesis of MAPbI3 nanowires with a mean diameter of 50 and 400 nm and length up to 10 μm using a simple slip-coating process. The asfabricated MSM photodetector, however, shows a low responsivity of 5 mA/W. Zhuo et al.30 also reported the synthesis of porous perovskite nanowires using Pb-containing precursors. However, these methods lack precise control of the nanowire position and dimensions. Meanwhile, organolead halide perovskite is not compatible with conventional photolithography that is widely used to fabricate nanoscale electronic and optoelectronic devices and circuits.31 Therefore, creating nanoscale perovskite optoelectronic devices by well-controllable, low-cost fabrication methods while simultaneously maintaining high optoelectronic performance remains a major challenge. In this work, we report high-performance nanoscale perovskite photodetectors based on highly crystalline nanograting structures implemented by nanoimprint lithography (NIL). NIL has been previously applied to pattern organic materials to obtain highly ordered nanostructure morphology while simultaneously improving optoelectronic properties by inducing polymeric chain alignment32,33 and thus enhancing carrier mobility.34 However, to our best knowledge, NIL has not been reported on organohalide perovskites to date. Here we demonstrate the use of NIL processes to successfully define micro- and nanostructures on perovskite thin films into active device components, while simultaneously improving the perovskite film crystalline quality and corresponding optoelectronic performance.
RESULTS AND DISCUSSION Characterization of Nanoimprinted and Nonimprinted Perovskite Thin Films. A modified solvent-engineering method reported by Jeon et al.35 was used for MAPbI3 perovskite deposition. This method utilizes a mixture of γ-
Figure 2. Scanning electron micrograph of (a) nonimprinted perovskite thin film and perovskite thin film imprinted with (b) a Si flat mold, (c) a Si nanopillar mold, and (d) a Si nanograting mold. (e) SEM image of freestanding perovskite nanorods peeled off from the surface. 10922
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Figure 3. (a) UV−vis transmission and (b) reflectance spectra of nonimprinted, flat-imprinted, nanohole, and nanograting MAPbI3 films. (c) Steady-state photoluminescence and (d) time-resolved photoluminescence of MAPbI3 thin film, nanogratings, and nanoholes.
electron microscopy (SEM). The well-defined perovskite nanohole structures (Figure 2c) and nanograting structures (Figure 2d) were negative replications of the Si nanopillar mold and nanograting mold, respectively. The nanoholes have a patterned diameter and pitch of 275 and 600 nm, respectively, while the patterned width and pitch of nanogratings are 270 and 600 nm, respectively. The SEM cross-section image reveals a depth of 315 nm with almost no residual layer for the nanoholes and a structure depth of 300 nm and residual layer thickness of 130 nm for the nanogratings. No obvious perovskite volume change is observed after NIL. The results have demonstrated that although perovskite is an ionic solid and does not have a glass transition behavior like polymers, it can be successfully patterned by NIL, as it can deform and fill in mold cavities upon applied heat and pressure. According to the mechanical properties study by Jing,36 the hybrid perovskite layer has a low shear modulus and is suitable for fabrication of a flexible device or compliant device with large deformation demand. Our results here demonstrate that NIL is a convenient and cost-effective technique to fabricate perovskite nanostructures. Morphologic improvement by NIL is also revealed. From both top the view and cross-section view, the nonimprinted film (Figure 2a) shows small crystal grains and clear grain boundaries, while the nanoimprinted film (Figure 2b−d) shows a larger grain size and smoother surface. The nanogratings, particularly, have shown almost invisible grain boundaries. Figure 2e demonstrates free-standing nanorods stripped during mold release, and the nanorods appear to have a dramatic morphologic difference compared with the residual layer. It is likely that with multidimensional confinement during nano-
imprinting the nanorods have been formed with significantly better crystallinity than the underlying residual layer. To confirm the improved structural order and crystallinity, another set of nonimprinted perovskite and imprinted perovskite samples were analyzed with X-ray diffraction (XRD), and the results are presented in Figure S2. The sharp (220) reflection indicates the crystallinity of the imprinted MAPbI3 films. The crystallite sizes of the nonimprinted and nanoimprinted films were determined to be 68 and 188 nm, respectively. Both microscopic images and diffraction analysis have confirmed the positive effect of NIL on perovskite crystallinity. A simple hypothesis is that during nanoimprinting with elevated temperature and pressure the perovskite small grains slide toward the mold cavities and collide with each other, which forms bigger grains and defects such as dislocations, disclinations, and vacancies are reduced. Another explanation is that the grain boundaries have been pushed to the perovskite and mold interface during nanoimprinting and therefore crystal grains are redefined based on the mold structures. Previously, improved crystallinity was also reported by Xiao et al.37 using a hot-pressing process as the post-treatment of a perovskite thin film, and the treated perovskite showed improved photoluminescence lifetime and solar cell efficiency. Matsushima et al.38 reported a similar perovskite morphologic treatment process by using a hot isostatic pressure method. The temperature was also found to have an influence on perovskite morphology.39 Therefore, we expect that with an optimized NIL process by carefully choosing temperature, pressure, and time, the morphology and crystallinity of the perovskite thin film could be further improved. 10923
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Figure 4. Structure and characteristics of perovskite metal−semiconductor−metal photodetectors. (a) Schematic illustration of the nanograting-PSPD. (b) I−V characteristics of nanograting-PSPD with 0.11 to 7.27 mW/cm2 halogen light illumination. The inset shows the current as a function of irradiance. (c) Temporal current characteristics of nanograting-PSPD at 7.27 mW/cm2 halogen light illumination with a bias voltage of 1 V. The light was switched on and off for 10 cycles for the repeatability test. (d) I−V characteristics of tf-PSPD, flat-PSPD, nanohole-PSPD, and nanograting PSPD at 7.27 mW/cm2 halogen light illumination in logarithmic scale. The inset shows the same curves in linear scale.
fabricated metal−semiconductor−metal photodetectors by evaporating 300 nm thick gold electrodes with a 25 μm electrode gap on both imprinted and nonimprinted perovskite samples. The width of the electrode gap is 100 μm. On the basis of the film morphology, four types of devices were studied: the conventional nonimprinted thin-film perovskite photodetector (tf-PSPD), flat mold imprinted perovskite photodetector (flat-PSPD), nanohole perovskite photodetector (nanohole-PSPD), and nanograting perovskite photodetector (nanograting-PSPD), as shown in Figure S4. The photoelectrical characteristics of four different types of photodetectors were obtained under the same test configuration at room temperature in air. The schematic illustration is shown in Figure 4a with a nanograting-PSPD. One should note that for a nanograting-PSPD, the electrode pairs were deposited so that the current flow is along the gratings under applied electrical field. Figure 4b shows an example of an I−V curve of the nanograting-PSPD in the dark and under halogen light illumination with irradiance varying from 0.11 to 7.27 mW/ cm2. The linear current−voltage behavior indicates a good ohmic contact between gold and perovskite. In the dark state, the device has a resistance of tens of gigaohms. Under illumination, large amounts of electron−hole pairs are generated due to photon absorption and are subsequently extracted by the electrical field, which causes a dramatic increase of conductance. At the same voltage, the photocurrent increases gradually with incident light density, as illustrated in the inset graph of Figure 4b. The corresponding I−V curve for the tf-PSPD is presented in Figure S5. Figure 4c shows the temporal current of the nanograting-PSPD under 7.27 mW/
We then investigated the optical properties of both nonimprinted and nanoimprinted films by characterizing the transmission (Figure 3a), reflectance (Figure 3b), photoluminescence (PL, Figure 3c), and PL lifetime (Figure 3d) of perovskite films on glass substrates. The imprinted films with nanoholes and nanogratings show significantly reduced reflectance for the whole spectrum and reduced transmission in the wavelength range of 550 to 800 nm. The results indicate with proper mold nanostructure design for NIL favorable photon trapping in perovskites for higher optical absorption is feasible. Figure S3 shows iridescence of the patterned nanohole sample due to 2D photonic crystal effects, suggesting reasonable uniformity of NIL patterning across the 1 cm2 imprinted area, while the area appearing dark represents incomplete NIL or defects. The photoluminescence emission peak is located around 780 nm (1.59 eV) (Figure 3c). The nanoimprinted perovskites demonstrate better spontaneous emission properties with approximately 3 times improvement for nanogratings and 4 times improvement for nanoholes compared with the nonimprinted thin films. The time-resolved photoluminescence acquired using a time-correlated single photon counting method (excitation laser wavelength 435 nm, pulse width 100 fs, repetition rate 1 MHz), as presented in Figure 3d, demonstrates improved lifetime by NIL: lifetime increases from a perovskite thin film (35 ns) to imprinted nanogratings (42 ns) and nanoholes (50 ns). Characterization of Nanoimprinted and Nonimprinted Perovskite Photodetectors. To evaluate the optoelectronic performance of perovskites by NIL, we 10924
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fabricated. One sample was annealed at 100 °C for 10 min, while another was annealed for 30 min at the same temperature during the perovskite thin-film preparation process. Their photodetector performance was characterized and is presented in Figure S7. The devices with 30 min annealing show relatively worse performance, suggesting possible degradation due to long-time thermal treatment. The results indicate that the improvement of nanoimprinted devices were caused by the combined effect of elevated pressure and temperature during NIL, especially with confined nanostructures. We believe the improved crystallinity of the imprinted perovskite film is one of the primary causes of the performance enhancement for the nanoimprinted photodetectors. Several processes including photon absorption, electron−hole generation, carrier transport, and recombination would determine the photodetector performance.40 The results of SEM (Figure 2) and XRD (Figure S2) have shown that NIL has induced the formation of larger and ordered grains, and thus the crystallinity of perovskite has improved, which plays a positive role in multiple processes. First, the carrier transport and mobility would increase,25 as the charge carriers encounter less scattering at the grain boundaries or defects. Therefore, both the illuminated current and dark currents increase significantly in the NIL imprinted samples. Second, it will result in a longer electron−hole recombination lifetime,37 which is verified through PL lifetime tests (Figure 3d). Improvements in both mobility and carrier lifetime contribute to longer diffusion length. These effects contribute to the dramatically improved photocurrent and thus responsivity for the nanoimprinted photodetectors. The comparable on/off ratio between the flatPSPD and nanohole-PSPD indicates that charge carrier transport might be the primary cause of the inferior performance of the nanohole-PSPD. The vertical nanohole arrays could hinder the carrier transport since the charge carriers suffer from severe surface scattering driven by the electrical field. The gratings, on the other hand, largely enhanced the carrier transport due to well-aligned conductive channels along the electrical field and ordered crystal grains to reduce surface and grain boundary scattering. The nanograting structure is also suitable for photon management due to the one-dimensional photonic band gap effect.41,42 Therefore, the nanograting-PSPDs deliver the best performance in these samples with 35-fold higher responsivity and 7-fold higher on/ off current ratio than the tf-PSPD. It is noted that due to the limitation of the mold depth, the nanograting-PSPD has a residual layer of 130 nm that also contributes to the total device current. From the SEM images of Figure 2d and e, the residual layer may not have a high crystallinity as compared to the gratings. Therefore, it is reasonable to assume that the residual layer has a similar performance to that of the flat-PSPD, and its contribution to the total current should be less than 20%. By further optimizing the mold depths, we expect the residuallayer-free nanogratings to have even higher performance. High-Performance Perovskite Nanograting Photodetectors. To further evaluate the nanograting-PSPDs, they were tested along with the tf-PSPD under monochromatic LED illumination. The currents of the nanograting-PSPD, tf-PSPD, and a commercial Si photodiode were measured and plotted against the LED forward current in Figure S8. Both the tfPSPD and nanograting-PSPD were biased at 1 V, while the Si photodiode was reverse biased at 10 V. The nanograting-PSPD shows much higher current than the tf-PSPD. One should note that the Si photodiode has the largest current, as its effective
cm2 halogen lamp illumination with a bias of 1 V. The light was switched on and off for 10 cycles during the test. An on/off current ratio of more than 1000 was achieved with a dark current as low as 30 pA while illuminated with a current of more than 40 nA. To compare the optoelectronic performance between the tf-PSPD, flat-PSPD, nanohole-PSPD, and nanograting-PSPD, their I−V characteristics under 7.27 mW/cm2 halogen light illumination were plotted in Figure 4d. One could clearly see that under the same conditions the flat-PSPD, nanohole-PSPD, and nanograting-PSPD all exhibit a large improvement in photocurrent compared with the tf-PSPD. The nanograting-PSPD has the highest photocurrent, over 35 times that of the tf-PSPD at a bias of 1 V. To obtain a more reliable performance comparison, multiple devices were tested for each type. The corresponding I−V curves under 7.27 mW/cm2 halogen light illumination are shown in Figure S6. Responsivity is widely used to evaluate the efficiency of a photodetector responding to an optical signal. It is defined as the ratio of the photocurrent to the illumination power, as expressed by Iph R= L light × S (1) where Llight is the incident light power density, S is the effective area, and Iph represents the photocurrent as given by Iph = Iilluminated − Idark
(2)
where Iilluminated and Idark are the current with and without illumination, respectively. Besides responsivity, the on/off ratio that is represented by illuminated current divided by the dark current is another important parameter for photodetectors. The calculated photodetector parameters along with their geometries are summarized in Table 1. All the nanoimprinted Table 1. Performance and Geometry Comparison of tfPSPD, Flat-PSPD, Nanohole-PSPD, and Nanograting-PSPD Devices parameter
tfPSPD
flatPSPD
nanoholePSPD
nanogratingPSPD
thickness (nm) pattern width (nm) pattern pitch (nm) responsivitya on/off ratioa dark currenta
265 N/A N/A 1 1 1
265 N/A N/A 15.4 5.2 3.0
315 275 600 11.9 5.0 2.4
300, residue 130 270 600 34.7 6.9 5.0
a
Values of responsivity, on/off ratio, and dark current are normalized to the results of the tf-PSPD. Responsivity and on/off ratio under 7.27 mW/cm2 illumination were extracted from raw data as shown in Figure S6 and Table S1.
devices showed significantly enhanced performance; that is, the responsivity R and on/off ratio are over 10 times and 5 times higher than those of the nonimprinted devices, respectively. Particularly, in the dark environment, Idark has also increased by 3 times for the flat-PSPD, 2.4 times for the nanohole-PSPD, and 5 times for the nanograting-PSPD, which should be attributed to the improvement of charge carrier transport in the NIL films under an applied electrical field. To study whether thermal annealing during NIL was the primary cause of performance enhancement of nanoimprinted devices, another set of perovskite thin-film samples was prepared and the corresponding MSM photodetectors were 10925
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Figure 5. Irradiance-dependent responsivity plot at (a) λ = 466 nm and (b) λ = 635 nm. Red curve represents the nanograting-PSPD and blue curve represents the tf-PSPD. The right y-axis illustrates the relative responsivity normalized to the Si photodiode. Both perovskite photodetectors were biased at 1 V, while the Si photodiode was reverse biased at 10 V.
nm and 58.5 A/W at 4.5 nW/cm2 illumination of λ = 635 nm with a bias voltage of 1 V. Such performance is ∼30 times better than the tf-PSPD and more than 100 times better than the commercial Si photodiode. The performance enhancement is likely due to NIL-induced higher crystallinity; particularly, the nanograting structure is favorable for better photon absorption and charge carrier transport. Further improvement on the nanograting-PSPD performance is expected by optimizing the nanograting geometries and the nanoimprinting conditions. Additionally, nanograting-based photodetectors and solar cells with vertical P−I−N structures are in preparation. Our study demonstrated that NIL is a simple yet effective way to fabricate high-performance nanoscale optoelectronic devices using emerging hybrid perovskite materials, which are suitable for electronic circuit integration and manufacturing.
radiant-sensitive area is over 3000 times that of our perovskite photodetectors, while the current per effective illuminated area under the same irradiance is of concern here. The irradiance was evaluated with the Si photodiode, and the corresponding photodetector current versus irradiance is plotted in Figure S9. Figure 5 shows the calculated photodetector responsivity versus irradiance at λ = 466 nm (Figure 5a) and λ = 635 nm (Figure 5b) with a bias voltage of 1 V. It is observed that generally with a decrease in the light intensity the responsivity increased. The results were in agreement with the literature.4,26,27 The performance of the nanograting-PSPD was superior to that of the tf-PSPD, similar to the results of the halogen light illumination tests. At λ = 466 nm (Figure 5a), the tf-PSPD has only R = 0.16 A/W, while the nanograting-PSPD has R = 3.23 A/W under 1 μW/cm2 illumination. With 2 nW/cm2 irradiance, the nanograting-PSPD has R = 24.1 A/W, which is 100 times that of the commercial Si photodiode (0.12 A/W). Similarly, at an illumination of λ = 635 nm (Figure 5b), the nanograting-PSPD has R = 6.2 A/W, which is over 30 times that of the tf-PSPD and 20 times that of the commercial Si photodiode under 1 μW/cm2 irradiance. At 4.5 nW/cm2 irradiance, the responsivity of the nanograting-PSPD has increased to 58.5 A/W, which is 100 times more than that of the commercial Si photodiode (0.3 A/W). Both devices show better response at λ = 635 nm than at 466 nm. The imprinted nanograting-PSPDs also outperform the previously reported hybrid perovskite nanowire29,30,43,44 and thin-film photodetectors.24,26,45,46
METHODS Perovskite Thin-Film Preparation. The perovskite solution was prepared by dissolving a 1:1 molar ratio of PbI2 and CH3NH3I in a 7:3 volume ratio of γ-butyrolactone:N,N′-dimethyl sulfoxide solvent mixture in a N2 glovebox. The resulting concentration was 1.2 M. The solution was heated for 24 h at 60 °C. The solution was then spincoated onto the Si substrates with 100 nm thick thermal SiO2 that was previously ultrasonically cleaned with acetone and treated by oxygen plasma. A two-step spin-coating process was performed for 22 s at 1000 rpm and then 22 s at 5000 rpm. A 350 mL amount of anhydrous toluene was dropped on the film after 12 s in the second spin-coating step. The sample was then annealed on a hot plate at 100 °C for 10 min, during which solvents were evaporated and a dense and uniform MAPbI3 film was formed with a thickness of about 265 nm. Nanoimprinting of Perovskite Films. The Si flat, nanopillar, and nanograting molds were first treated with FDTS in n-heptane solvent for 5 min and then cleaned with acetone and blow dried with N2. The molds were then annealed at 100 °C for 20 min. Monolayer FDTS was formed on the Si molds, which served an antiadhesive purpose in the NIL process. The Si flat mold, nanopillar mold, and nanograting mold were then placed on the perovskite thin-film-coated substrate at different areas in a single process. The imprint utilized a multistep process: 90 s at a temperature of 35 °C and a pressure of 2 MPa, 180 s at a temperature of 55 °C and a pressure of 4 MPa, 180 s at a temperature of 75 °C and a pressure of 6 MPa, and then, importantly, 1200 s at a temperature of 100 °C and a pressure of 7 MPa. The pressure was kept at 7 MPa while the chamber was cooled to a temperature of 35 °C. The nanoimprinting process was then finished, and perovskite nanostructures were formed as a negative replication of the Si molds. Fabrication of a Metal−Semiconductor−Metal Photodetector. SiO2 (100 nm) was thermally grown on a (100) Si wafer.
CONCLUSION In summary, we report the use of nanoimprint lithography to define ordered perovskite nanostructures as active device areas, while NIL simultaneously improves their crystallinity and optoelectronic performance. NIL was conducted on perovskite thin films with flat, nanopillar, and nanograting molds. Planar metal−semiconductor−metal photodetectors were fabricated on the perovskite films with different morphologies, and their optoelectronic performance was characterized. All of the nanoimprinted devices demonstrated significantly improved performance compared to nonimprinted devices, while the nanograting devices are the best with an average of 35 times improvement in responsivity and 7 times improvement in on/ off current ratio under 7.27 mW/cm 2 halogen light illumination. The nanograting-PSPD has a high responsivity value of 24.1 A/W at 2 nW/cm2 LED illumination of λ = 466 10926
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ACS Nano Perovskite thin films and nanoimprinted samples were prepared as described previously. A 300 nm thick gold film was deposited on the perovskite samples in an e-beam evaporator using a shadow mask. The gap between the gold electrode pairs was 25 μm in length and 100 μm in width. The effective photodetector area was 2.5 × 10−5 cm2. Characterization of Photodetector Devices. A Keithley 4200 and a Cascade probe station were used to characterize the perovskite photodetectors. The devices under a dark environment and different illumination conditions were tested. A 150 W halogen lamp was used for illumination for all the devices. A blue LED with a peak wavelength of 466 nm and a red LED with a peak wavelength of 635 nm were used for the responsivity test of the nanograting-PSPD and tf-PSPD devices. The illumination light intensity was calibrated with a commercial Si photodiode.
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ASSOCIATED CONTENT S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b05535. Nanoimprint process profile, X-ray diffraction, optical image, photodetector 3-D schematics, and optoelectric characteristics (PDF)
AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. ORCID
Honglei Wang: 0000-0002-5079-625X Julia Y. Chan: 0000-0003-4434-2160 Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS This work is partially supported by the National Science Foundation (Nos. ECCS-0901759 and ECCS-0955027). Financial support of the Welch Foundation under grant AT1617 is also highly appreciated. W.H. acknowledges support from the 1000 Talent Program of Shanghai, China. Partial financial support from the Ministry of Education and Science of the Russian Federation in the framework of Increase Competitiveness Program of NUST “MISiS” (No. K2-2015014) is also appreciated. The authors acknowledge D. Saranin and A. Ishteev for help with preparation of perovskite films, Y. Ren for X-ray data collection, and Y. Yang and M. Aryal for helpful discussions on nanoimprint lithography. REFERENCES (1) Tang, L.; Kocabas, S. E.; Latif, S.; Okyay, A. K.; Ly-Gagnon, D.S.; Saraswat, K. C.; Miller, D. A. B. Nanometre-Scale Germanium Photodetector Enhanced by A Near-Infrared Dipole Antenna. Nat. Photonics 2008, 2, 226−229. (2) Xia, F.; Mueller, T.; Lin, Y.-m.; Valdes-Garcia, A.; Avouris, P. Ultrafast Graphene Photodetector. Nat. Nanotechnol. 2009, 4, 839− 843. (3) Lopez-Sanchez, O.; Lembke, D.; Kayci, M.; Radenovic, A.; Kis, A. Ultrasensitive Photodetectors Based on Monolayer MoS2. Nat. Nanotechnol. 2013, 8, 497−501. (4) Lian, Z.; Yan, Q.; Lv, Q.; Wang, Y.; Liu, L.; Zhang, L.; Pan, S.; Li, Q.; Wang, L.; Sun, J.-L. High-Performance Planar-Type Photodetector on (100) Facet of MAPbI3 Single Crystal. Sci. Rep. 2015, 5, 16563. (5) Hayden, O.; Agarwal, R.; Lieber, C. M. Nanoscale Avalanche Photodiodes for Highly Sensitive and Spatially Resolved Photon Detection. Nat. Mater. 2006, 5, 352−356. 10927
DOI: 10.1021/acsnano.6b05535 ACS Nano 2016, 10, 10921−10928
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ACS Nano
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DOI: 10.1021/acsnano.6b05535 ACS Nano 2016, 10, 10921−10928
