Inkjet-Printed Photodetector Arrays Based on Hybrid Perovskite

Mar 14, 2017 - Morphology-Tailored Halide Perovskite Platelets and Wires: From Synthesis, Properties to Optoelectronic Devices. Zhixiong Liu , Yang Mi...
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Inkjet-printed Photodetector Arrays Based on Hybrid Perovskite CH3NH3PbI3 Microwires Yang Liu, Fushan Li, Chandrasekar Perumal Veeramalai, Wei Chen, Tailiang Guo, Chaoxing Wu, and Tae Whan Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b01379 • Publication Date (Web): 14 Mar 2017 Downloaded from http://pubs.acs.org on March 15, 2017

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Inkjet-printed Photodetector Arrays Based on Hybrid Perovskite CH3NH3PbI3 Microwires Yang Liu, Fushan Li*, Chandrasekar Perumal Veeramalai, Wei Chen, Tailiang Guo Institute of Optoelectronic Technology, Fuzhou University, Fuzhou 350002, People’s Republic of China Chaoxing Wu, Tae Whan Kim* Department of Electronic Engineering, Hanyang University, Seoul 133-791, Korea Abstract Hybrid perovskite CH3NH3PbI3 has attracted extensive research interests in optoelectronic devices in recent years. Herein, inkjet printing method has been employed to deposit a perovskite CH3NH3PbI3 layer. By choosing the proper solvent and controlling the crystal growth rate, hybrid perovskite CH3NH3PbI3 nanowires, microwires, network and islands were synthesized by means of inkjet printing. Electrode-gap-electrode lateral-structured photodetectors were fabricated with these different crystals, of which hybrid perovskite microwires-based photodetector would balance the uniformity and low defects to obtain switching ratio of 16000%, responsivity of 1.2 A/W and normalized detectivity of 2.39×1012 Jones at the light power density of 0.1 mW/cm2. Furthermore, the hybrid perovskite microwires-based photodetector arrays were fabricated and applied in imaging sensor, from which the clear mapping of the light source signal was successfully obtained. This work paves a new way for the realization of low-cost, solution-processed and high-performance hybrid perovskite-based photodetector arrays. Keywords: Perovskite; Photodetector; Inkjet printing; Microwires; Arrays. *Corresponding authors: [email protected] (F. Li); [email protected] (T.W. Kim) 1

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1. Introduction Photodetectors that can convert incident photon flux density into an electrical signal are attractive in various applications including optical communications, environmental monitoring, security surveillance, and chemical/biological sensing.1-4 The overall performances of photodetectors including low power consumption, easy fabrication, broad-band detection, fast response, and a large detectivity, are essential for a practical application prospect. In the past few decades, various types of semiconductor materials have been applied in photodetectors, such as Si, ZnO, MoS2, quantum dots and polymers5-10. Recently, low-cost solution-processed hybrid perovskite has attracted extensive attention in solar cell11, 12, light-emitting diodes13, phototransistors14, 15 and laser16, due to its excellent photoelectric properties including high absorption coefficient, extended diffusion length of excitation and high external quantum efficiency (EQE) in the visible wavelength range17-20, which are essential properties for photodetectors21-24. Yang et al. has reported the multilayer photodetectors with a large detectivity approaching 1014 Jones25. Bao et al. synthesized the high-quality 2D perovskite crystals by chemical vapor deposition method, which were utilized for photodetectors.26 Yan and co-workers fabricated a self-powered halide perovskite single crystal photodetector showing the responsivity of 7.92 A/W to white light27. In recent years, inkjet printing has attracted increasing attention because it offers a mask-free, material-effective, and patterned-deposition method to form functional patterns in optoelectronic and electronic devices28, 29, which is clearly superior to 2

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other solution-processing methods, such as spin-coating18, blade coating19, and so on. The inkjet printing of hybrid perovskite solar cells was reported by Monojit Bag et al., who fabricated perovskite solar cells in air by inkjet printing of alkyl-ammonium cations onto PbI2 thin film deposited by spin coating30. Song et al. fabricated solar cells based on the hybrid perovskite CH3NH3PbI3 layer on a mesoscopic TiO2 film by inkjet printing technique

31

. These findings demonstrated that the inkjet printing

method could be used to deposit hybrid perovskite, however, the devices fabricated without pattern do not use this technique to a maximum of advantage and the quality of continuous multilayer film deposited by inkjet printing is not as good as that by traditional solution-processed methods. Here, for the first time, we report the synthesis of hybrid perovskite CH3NH3PbI3 nanowires, microwires, network and islands by means of inkjet printing via choosing proper solvent and controlling the crystal growth rate. Hybrid perovskite was deposited as an irradiation absorber for photodetector with an electrode-gap-electrode lateral structure. CH3NH3PbI3 microwires-based photodetectors that balanced the uniformity and low defects achieved a switching ratio of 16000%, responsivity of 1.2 A/W and normalized detectivity of 2.39×1012 Jones at light power density of 0.1 mW/cm2, which is much better than that of their spin-coated counterparts fabricated at the same conditions. The hybrid perovskite microwires-based photodetector arrays were further fabricated and applied in imaging sensor, and clear mapping of the light source signal was successfully obtained. This work might provide a novel route for the realization of low-cost, solution-processed and high-performance hybrid 3

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perovskite-based photodetector arrays.

2. Results and discussion.

Figure1 (a)Stable single droplets obtained with different ink compositions. Optical images of hybrid perovskite CH3NH3PbI3 crystals by using inks with solvent at the temperature of 25 °C (b) GBL, (c) GBL and DMF mixed solvent (with a volume ratio of 1:1), and (d) DMF. The scale bar is 10 µm.

Optimizing the experimental parameters like temperature and ink composition (single or mixed solvent) is useful to obtain the stable single droplet and tune the film profile. Figure 1a demonstrated that stable single droplets could be obtained by controlling the driving voltage waveforms for the three kinds of inks. Figure 1b−1d showed three different morphologies of hybrid perovskite CH3NH3PbI3 crystals by using the inks with different solvents such as GBL, mixed solvent (GBL and DMF 4

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with a volume ratio of 1:1) and DMF (40 wt%). Interestingly, for the ink with solvent GBL, the ITO-coated glass substrate surface was covered intricately with discontinuous perovskite crystallites with distinct edges and corners (not the guided growth), which is consistent with the results of previous work

32, 33

. While for the

perovskite precursor solution with mixed solvent of GBL and DMF, some uniform round and fattened crystal plates surrounded by a lot of small perovskite crystallites were resulted on the substrate surface, however, the perovskite crystallites did not connect with each other well. On contrary with above results, for the ink with the solvent DMF, long nanowires that could stretch across the gap between ITO electrodes (30 µm) were formed on the substrate. The absence of nanowires for the solution with mixed solvent of GBL and DMF could be attributed to that DMF is evaporated first due to higher evaporation rate and the residual GBL (high boiling temperature or slow evaporation rate) mainly controlled the morphology of the final perovskite crystallites on the substrate. From the above results, no anisotropic growth was observed during the inkjet printing process with GBL solution. The polar aprotic solvent DMF plays a key role in the anisotropic growth of perovskite material. The results suggested that the perovskite crystallites morphology would rely heavily on the solvent used for preparation of inks, and the solvent DMF would be an optimal solvent for inkjet printing photodetector. The evaporation rate of solvent in hybrid perovskite solution, as an essential parameter in the morphology of the crystal, has been reported in spin coating34-36, during which most of the solvent in the deposited film could be removed rapidly by 5

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centrifugal force and the rate of evaporation could be tuned by spin speed and spin duration. Instead, in inkjet printing, the film deposited on the substrate are usually still in a fluid state, containing a large amount of solvent. Thus, in-situ heat treatment was applied to adjust the solvent evaporation by tuning the substrates temperature from 25 °C to 75 °C during the printing to control the morphology of the perovskite crystallites. After the crystallites were formed, the samples were heated on a 100 °C hotplate to enhance the crystallinity of perovskite. Figure 2a−2f showed the morphology and surface structure of the perovskite examined by SEM. The ink used here consists of the perovskite precursor dissolved in DMF with a concentration of 40 wt%. Higher concentration was not chosen in order to avoid the blocking of nozzle due to the solvent evaporation during inkjet printing. The morphology of the nanowires as shown in Figure 2a was formed in a sporadic distribution when the inks were dropped onto the substrate by natural evaporation in the ambient environment (about 25 °C). The inset shows that the surface of nanowires are smooth and the nanowires growth keeps the direction even at the cross intersection, indicating the few defects and pinholes.

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Figure 2 SEM images showing different perovskite morphologies by tuning the substrates temperature of (a)25 °C, (b)35 °C, (c)45 °C,(d)55 °C, (e)65 °C and (f)75 °C

during the inkjet printing process. (g)Schematics showing the deposition

process of inkjet printing on ITO substrates. (h)Photocurrent statistical box plots of photodetectors based on nanowires, microwires, network, and islands.

The result implies that the most of the solvent still remains in the liquid phase after the inks drop on the substrate, and the precursor has enough space to flow and time to crystallize and self-assemble at the low drying temperature, favoring the large but sporadic perovskite crystallization. As shown in Figure 2b, when the substrate 7

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temperature increased to 35 °C, microwires were formed with the width of the perovskite crystals growing up to 3 micrometers and the further growth along with the axial was terminated at the junction. Compared to nanowires, perovskite microwires show better coverage and uniformity in distribution (the inset in Figure 2b). When the temperature increased to 45 °C as in Figure 2c, the morphology of perovskite crystals was changed into network, a quite different distribution from nanowires and microwires. At low temperatures, few crystal nuclei appeared randomly, then with the nanowires or microwires growth, the inks were diluted to an under-saturated condition and flowed to aggregate along the crystal axis. However, when temperatures rise to a critical value that the evaporation rate reaches equilibrium with the crystallization rate along the crystal axis, the under-saturated condition derived from the crystal growth turned into super-saturation quickly. Under this situation, the super-saturated inks formed more crystal nuclei locally and the crystals connect with each other nearby to form the networks rather than self-assembled 1D crystals such as nanowires or microwires due to lack of sufficient time to flow and crystallize along the axis. When the substrate temperature rose up to 55 °C and 65 °C (Figure 2c-2d and the corresponding insets), the morphology of the perovskite crystals remains nano-network, but the width gets thinner with more pinholes and defects appearing on the crystals owing to the high solvent evaporation ratio. Increasing the evaporation temperature further up to 75 °C, perovskite material scattered like the islands consisting of a large number of small crystalline grains. When we set the substrate 8

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temperature even higher, the nozzle was blocked by the perovskite crystals and no inks dropt on the substrates. In order to demonstrate these structures are perovskite, the XRD spectra of nanowires, microwires, network and islands are shown in Figure S1. The main diffraction peak at 14.2º (110) and 28.3º (220) were identified as the characteristic peaks of perovskite, similar to the works reported before33, 37. The presence of the low intensity peak at 12° (the (001) peak in PbI2 phase) could be attributed to the humidity-induced partial decomposition of CH3NH3PbI3, considering the whole processes in ambient environment. The low intensity peak at the (001) peak in PbI2 phase and strong characteristic peaks of the perovskite indicates efficient conversion and high crystalline quality of CH3NH3PbI3 microwires. Schematics of inkjet printing process to form the hybrid perovskite CH3NH3PbI3 layers on the substrate with different heating temperature are shown in Figure 2g. It should be noted that, in the spin-coating process, most of the perovskite redundant solution was spun off by centrifugal force during the deposition, which not only waste solution but also is hard to control the morphology and size of the perovskite crystal. However, as a deposition technology of economization and environmental protection, the volume of the perovskite solution and the process of the perovskite crystal could be readily tuned. The box charts of photocurrent ranges were plotted to show the photoelectric characteristic of the hybrid perovskite-based photodetectors with different morphology in Figure 2h. The nanowire-based photodetectors show the worst photocurrent uniformity owing to the sporadic distribution of nanowires. The narrow and high photocurrent ranges of microwire-based photodetectors revealed the 9

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relatively good uniformity, low defects and high coverage. The poor photocurrent uniformity of network and islands could be ascribed to the unstable inkjet drops in high heating temperature, considering small interval distance (below 0.7 mm) between the nozzle and the substrate. The results suggested that hybrid perovskite microwires, combining both the low defect density of nanowires and high coverage of network, are a better choice for photodetectors. Thus, all the devices without additional explanation in the following are based on hybrid perovskite microwires.

Figure 3 (a)Schematic of the photodetector array consisting of 25 pixels and the electrode-gap-electrode lateral structure of a single pixel. (b)Energy level diagram and working principle of the hybrid perovskite microwires photodetector. (c)Current characteristics versus voltage of the device under different illumination power intensities at the wavelength of 630 nm. Inset: Absorption spectrum of the 10

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CH3NH3PbI3 microwires. (d)Current characteristics versus voltage of the sample 1 (S1) by inkjet printing and the sample 2 (S2) by spin coating. Inkjet printing was performed with substrates heated at 35 °C and spin coating was performed at ambient environment (about 25 °C). Figure 3a shows the schematic of the microwires photodetector array consisting of 5×5 pixels. The active channels between two adjacent ITO pre-patterned pads were 30 µm in length and 1000 µm in width with the simple electrode-gap-electrode lateral structure which is convenient for inkjet printing and the electrical measurement. The energy level and the working principle of hybrid perovskite microwires photodetector are shown in Figure 3b. As the photosensitive material, CH3NH3PbI3 layer generated electrons and holes with the irradiation of light source, which were separated under the applied electric field and collected by electrodes. The key figure-of-merit parameters of photodetectors are switching ratio (SR), responsivity (R), detectivity (D*) and response speed18. The photodetector switching ratio was calculated by SR=(Ip/Id), where Ip is photocurrent, Id is dark current. Responsivity was defined as R=((Ip−Id)/(P·S)), where P represents light power intensity and S stands for the effective sensitive area. Considering that the shot noise dominates the total noise in photoconductive photodetectors, normalized detectivity can be given by D*=((Ip− Id)/(P(2·q·Id·S)1/2)), where q represents elementary charge. Evidently, in order to improve the properties of the photodetector, the dark current of the diode should be depressed as low as possible to distinguish from very weak optical signals, and the photo current should be as high as possible to transfer photons to electric signal 11

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sensitively. On the account of above fact, hybrid perovskite CH3NH3PbI3 microwires with good surface coverage, low traps and pinholes were chosen as the best candidate for photodetector. Absorption spectrum of the CH3NH3PbI3 microwires (inset of Figure 3c) shows that the device responded sensitively in the visible light spectral regions. Here, the monochromatic light at the wavelength of 630 nm was chosen as the power source in the measurement system. Figure 3c depicts the current characteristics versus voltage of the device under different light illumination power intensities. The photodetector current-voltage (I-V) characteristics were given at different power intensities from 0.1 to 7 mW/cm2 under the voltage from -10 to 10 V. The linear curves indicate the ohmic contact formed between microwires and ITO electrodes, which could be attributed to the energy adjustment after ITO electrodes were processed by plasma. In addition, the current increases gradually with the increasing of light intensities when the same voltage is applied. At a bias of 10 V, the dark current was 1.1 nA and the photocurrent increased to 180 nA when the device was illuminated at light power density of 0.1 mW/cm2. A switching ratio of 16000%, responsivity of 1.2 A/W and normalized detectivity of 2.39×1012 Jones were achieved. When the illumination intensity increased to 7 mW/cm2, photocurrent could be more than 1000 nA, producing a photocurrent on/off ratio of ~1000. To the best of our knowledge, this is one of the best results among the previous reports for the perovskite-based lateral photodetectors33,

37

. Unlike the other efficient way to improve properties of

photodetectors by creating high speed carrier channel between the nanoparticles and 12

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graphene, 38, 39 we achieved extraordinary performance by tailoring the morphology of perovskite. To compare the performance of the microwires with the film deposited by spin coating under the same conditions, a spin-coated photodetector was fabricated as the control device. Figure 3d shows the comparison of the current of the sample 1 (S1) by inkjet printing and the sample 2 (S2) by spin coating. The S2 exhibits higher dark current and lower photocurrent, with a switching ratio of ~1400%, responsivity of 0.87 A/W and normalized detectivity of 0.97×1012 Jones at light power density of 0.1 mW/cm2. The inferior properties could be attributed to the large overlapping besom-shaped crystal and poor surface coverage as is shown in Figure S2, when compared with the microwires-based device. The photo response of perovskite-based photodetectors was evaluated under light illumination at 630 nm with an intensity of 3 mW/cm2. From the current-time curves in Figure 4a, the photocurrent and dark current were 140 nA and 0.18 nA respectively, indicating the stable and abrupt response of sequential photodetector to the switch of light. Also, the SR exceeded 770 at a bias voltage of 2 V, which might result from better crystallization and interface passivation between the microwires and ITO. The photo response speed was tested as shown in Figure 4b by taking into account one on-off cycle with time resolution. Both the rise time that photocurrent changes from 0 to 70% of the peak photocurrent and decay time are shorter than 10 ms. It should be pointed out that the response time was limited by our photocurrent measurement instrument. The results indicated the capability of the perovskite microwires 13

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photodetector arrays to follow fast changing optical signal.

Figure 4 (a)Current-time curve illuminated by 630 nm light (3 mW/cm2) at a bias voltage of 2 V. (b)Photo response time tested by high resolution scan to one cycle of current-time curves. The heating temperature of CH3NH3PbI3 microwires is 35 °C.

In order to verify the promising application of photodetector arrays as imaging sensors, the photodetectors arrays including 5×5 pixels of CH3NH3PbI3 microwires were fabricated. Figure 5a showed the light fringes with varied intensities that were utilized as light source to illuminate on the photodetector arrays. The output photocurrent of each pixel was measured and the detecting image calculated from the results is shown in Figure 5b. The detecting image could represent the distribution of the

signal

of

light

resource

accurately.

The

implementation

of

present

two-dimensional imaging might be attributed to the high uniformity, complete crystallization and the stability of the inkjet printing at ambient conditions.

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Figure 5 (a)Spatial distribution mapping of light power intensity. (b)Output photocurrent mapping calculated from the results of each pixel. The heating temperature of CH3NH3PbI3 microwires is 35 °C.

3. Conclusions. To conclude, we succeeded to synthesize hybrid perovskite CH3NH3PbI3 nanowires, microwires, network and islands by means of inkjet printing in ambient environment via controlling the crystallization in terms of solvent used for preparation of inks. The as-fabricated hybrid perovskite microwires-based photodetector, with balanced uniformity and low defects, exhibited a switching ratio of 16000%, responsivity of 1.2 A/W and normalized detectivity of 2.39×1012 Jones at light power density of 0.1 mW/cm2, which is much better than the performance of the spin-coated device fabricated under the same conditions. The hybrid perovskite microwires-based photodetectors arrays were further fabricated and applied in imaging sensor, and clear mapping of the light source signal was successfully obtained. This work paves a new way for the realization of low-cost, solution-processed and high-performance hybrid perovskite-based photodetector arrays. 15

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4. Experimental Section Material: All the materials mentioned in the following parts were obtained from commercial suppliers and used without further purification. The hybrid perovskite CH3NH3PbI3 precursor solution (5 mL, 40 wt%) was prepared by mixing PbI2 and CH3NH3I at 1:1 equimolar ratio dissolved in N,N-dimethylformamide (DMF) solvent,gamma-butyrolactone (GBL) and mixed solvent (GBL and DMF with a volume ratio of 1:1), heating at 60 °C for 12 h inside a nitrogen-filled glovebox with oxygen and moisture levels less than 1 ppm. Device

Fabrication:

ITO-coated

glass

substrates

pre-patterned

by

photolithography were cleaned by ultrasonication successively in acetone, isopropanol and deionized (DI) water. The substrates were dried by nitrogen stream, followed by 30 s oxygen plasma treatment to increase the work function of ITO. The active channels were 30 µm in length and 1000 µm in width. The inkjet printing on substrates was accomplished by using a Microfab JETLAB 2 equipped with an 80 µm diameter piezoelectric-driven inkjet nozzle and a motorized stage with the accuracy of 5 µm. For comparison, a control device was fabricated by spin-coating with CH3NH3PbI3 precursor solution (40 wt% in DMF) on the substrate. Characterization: The CH3NH3PbI3 morphology was characterized by laser confocal fluorescence microscope (Olympus, BX51M) and scanning electronic microscope (SEM, FEI, Nova Nano SEM 230). The UV−Vis absorption spectra were tested with a UV/Vis/NIR spectrophotometer (Shimadzu , UV-3600). X-ray diffraction spectra was collected with an X'Pert PRO diffractometer (PANalytical). All 16

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the electrical parameters of the devices were measured with a semiconductor characterization system (Keithley, Model SCS-4200) assisted with a probe station covered with a Faraday Box to provide optical and electromagnetic shielding. Monochromatic light was obtained by a xenon lamp (NBT, HDX-F300) and light-emitting diode (LED) at a wavelength of 630 nm, and a power meter (NBT, FZ400) was used to measure the light intensity.

Acknowledgements This work was supported by the National Natural Science Foundation of China (61377027, U1605244), and the National Key Research and Development Program of China (2016YFB0401305). This research was also supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2016R1A2A1A05005502).

Supporting Information. Supporting Information is available free of charge on the ACS Publications website. The XRD spectra of perovskite nanowires, microwires, network and islands, and the SEM image of perovskite deposited by spin coating in ambient environment are shown in Figure S1-S2 (PDF).

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