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Polarized Ferroelectric Field-Enhanced Self-powered Perovskite Photodetector Fengren Cao, Wei Tian, Meng Wang, and Liang Li ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b00770 • Publication Date (Web): 30 Jul 2018 Downloaded from http://pubs.acs.org on August 2, 2018
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Polarized Ferroelectric Field-Enhanced Self-powered Perovskite Photodetector Fengren Cao, Wei Tian, Meng Wang, and Liang Li* College of Physics, Optoelectronics and Energy, Center for Energy Conversion Materials & Physics (CECMP), Jiangsu Key Laboratory of Thin Films, Soochow University，Suzhou 215006, P. R. China
ABSTRACT: The hybrid organic-inorganic perovskites have been considered as promising candidates for high-performance photodetectors. However, how to fabricate self-powered broadband perovskite photodetector with tunable photoresponse is still challenging. Herein, we design, for the first time, a ferroelectric semiconductor SrTiO3 (STO) and perovskite film hybrid structure-based photodetector. After applying appropriate poled bias, the hybrid photodetector exhibits higher responsivity and faster response speed, as compared to unpoled device and the device without STO ferroelectric interlayer. By optimizing the STO interlayer density, the positively poled photocurrent and response speed are enhanced to 0.956 mA and 0.1 s at zero bias, respectively. The corresponding responsivity is as high as 0.73A/W at the wavelength of 550 nm, which is much higher than that of the device without STO interlayer. The improved performance is attributed to the fact that positively polarized STO layers can provide a built-in potential and promote the band bending at the semiconductor surface to facilitate the separation and transportation of the photogenerated charge carriers. Our work provides a promising route to design high-performance ferroelectric-perovskite photodetector.
KEYWORDS: perovskite, ferroelectric polarization, SrTiO3, spin-coating, photodetector
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Photodetectors that convert incident light into electric signals play an important role in various fields including optical communication, environmental pollution monitoring, sensors, and water sterilization.1-4 Recently, hybrid organic-inorganic perovskites, CH3NH3PbX3 (X = Cl, Br, I), have been proved as promising candidates for high-performance photodetectors due to their suitable direct band gap (~1.5 eV), large absorption coefficients, broad absorption range, high carrier mobility, and long charge diffusion length.5-9 These merits enable perovskite to construct photodetectors with high responsivity and fast response speed, as well as broad response ranging from ultraviolet (UV) to visible light. Up to date, tremendous efforts have been devoted to building perovskite photodetectors, including the fabrication of low-dimensional nanostructures, band engineering, and construction of heterojunction by coupling transition metal oxides.10-12 For example, our group reported electrospun ZnO nanofiber-solution processed perovskite hybrid structure to decrease charge recombination, significantly enhancing the detectivity to 1.4 x 1013 Jones at 740 nm.12 However, most of perovskite photodetectors require an external power source to separate the photogenerated carriers. The self-powered photodetectors that operate at zero bias is of great importance, which can reduce the power consumption, decrease the weight, and scale down the size, thus expanding the scope of application. A common approach of achieving self-powered device is to construct p-i-n or
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Schottky junction based on their photovoltaic effects. For example, our group designed a Cu2O/CuO/perovskite/TiO2 double-twisted fibrous perovskite photodetector, which showed an ultrahigh detectivity of 2.15 x 1013 Jones under the illumination of 800 nm light at zero bias.13 To simultaneously broaden the spectra response range, we reported a self-powered perovskite/TiO2/Si trilayer photodetector with the photoresponse up to 1150 nm.14 As an alternative approach, photodetectors can also be integrated with an energy harvester unit, such as piezoelectric nanogenerators and solar cells, or be combined with an energy storage unit to achieve self-powered integrated system. For example, Yan’s group configured a digital versatile disc (DVD)-based triboelectric nanogenerator to drive the CH3NH3PbI3 single crystal photodetector.15 Our group designed a self-powered all-perovskite based photodetector-solar cell nanosystem, in which the solar cell drives well the operation of photodetector.16
Recently, ferroelectric materials have been intensively studied as potential photovoltaic materials. Ferroelectric semiconductors can be polarized by an external electric field and produce a built-in potential through the whole bulk, possessing stronger separation capability of photogenerated electron-hole pairs.17,18 The band alignment at the ferroelectric/semiconductor interface can be modulated by changing the direction of applied electric fields, which provides a great opportunity to enhance the performance of ferroelectric optoelectronic devices. For instance, Singh et al. introduced ferroelectric semiconductor poly(vinylidene fluoride) (PVDF)-NaNbO3 on PVDF/Cu substrate as a photoanode and achieved a remarkable photocurrent enhancement (~121%) in a photoelectrochemical cell with electric field polarization.19 Hu et al. fabricated a
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ferroelectric polymer poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) film gated MoS2 phototransistor. The remnant polarization can depress the dark current of MoS2 semiconducting channel and significantly enhance the sensitivity at zero gate voltage.20 Among various ferroelectric materials, SrTiO3 (STO) semiconductor has considerable electric polarization (~1.5 µC cm-2 at 1.4 K) and high room-temperature electron mobility (~6 cm2 V-1 s-1).21-23 Several studies have demonstrated that STO is a suitable material to construct ferroelectric photovoltaic devices. Wu et al. introduced polarized STO shell onto TiO2 nanowire array to increase the charge separation and hole transportation in photoelectrochemical cell.24 In spite of the great achievements in the field of ferroelectric devices, it is still scarce to manipulate the ferroelectric effect for improving the performance of perovskite photodetectors.
In this work, for the first time, the ferroelectric semiconductor is integrated with perovskite to manipulate the transfer behavior of carriers. Based on the polarization effect, we design and construct a high-performance self-powered photodetector using a sandwich structure of STO ferroelectric layer/perovskite film/Spiro-OMeTAD hole transport layer. The optoelectronic measurements reveal that both the responsivity and the response speed of devices strongly depend on the direction of polarized field. After the positive poling (+1.0 V), the optimum device exhibits a responsivity as high as 0.73 A/W and response time of 0.1 s under 550 nm light illumination. These parameters are comparable and even higher than some of previously reported perovskite based self-powered photodetectors (Table S1). Our investigations provide a platform to understand the effect of polarization on optoelectronic devices and open up a door to design ferroelectric field-assisted self-powered
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photodetectors in the future.
EXPERIMENTAL SECTION Preparation of STO Film on FTO Substrate. The STO film was deposited on FTO glass by a facile spin-coating process. Before use, FTO substrates were etched using Zn powder and diluted HCl, and then cleaned by acetone, ethanol, and deionized water sequentially in an ultrasonic bath. 0.817 g tetrabutyl titanate (C16H36O4Ti) was added in 8 mL 2-methoxyethanol (C3H8O2) and heated to boiling under continuous stirring. 0.537 g stontium acetate hemihydrate (Sr(CH3COO)2·1/2H2O) was dissolved in 4 mL glacial acetic acid (C2H4O2) to form a transparent solution. The above two solutions were mixed and stirred continuously for 2 hours to form a uniform precursor, followed by aging in a fridge for 2 weeks. The above solution was spin-coated on etched FTO at 3000 rpm for 40 s. Afterwards, the as-prepared film was heat-treated in air at 600 °C for 15 min with a heating rate of 20 °C/min to obtain crystallized STO film. STO films with different thicknesses were deposited on FTO substrates by adjusting the cycle number of spin-coating process.
Fabrication of FTO/STO/Perovskite/Spiro-OMeTAD/Ag Photodetectors. In the first step, STO film was deposited onto the etched FTO glass using the aforementioned procedure. Subsequently, the perovskite layer was coated onto the STO/FTO substrate. In brief, 0.461 g PbI2 was added into 1mL DMF to prepare PbI2 precursor solution. 0.500 g MAI was added into 10 mL isopropanol (99.5%, Aladdin) to prepare MAI precursor solution. Then 50 µL PbI2 precursor solution was spin coated on the substrate at 3000 rpm
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for 30 s. At the spin coating time of 10 s, 50 µL MAI precursor solution was immediately added to the substrate, followed by stewing on a plate for 30 min at room temperature to remove the residual DMF and then heating on the 150 °C hot plate to get high-crystalline perovskite layer. After cooling down to room temperature, spiro-OMeTAD was spin coated on the perovskite film at 2000 rpm for 30 s. The spiro-OMeTAD solution was prepared by dissolving 72.3 mg of spiro-OMeTAD, 28.8µL of 4-tert-butylpyridine, and 17.5 µL of lithiumbis-(trifluoromethanesulfonyl)imide (Li-TFSI) (520 mg Li-TFSI in 1 mL acetonitrile, 99.8%, Sigma-Aldrich) in 1 mL of chlorobenzene(99.9%, Alfa Aesar). All the above procedures were conducted in the glove box in nitrogen atmosphere. Finally, a 100 nm thick Ag electrode was deposited by thermal evaporation with a shadow mask to obtain the FTO/STO/Perovskite/Spiro-OMeTAD/Ag (Labeled as STO-x, where the x denotes the cycle number of spin-coated STO) photodetector (the active area is defined as 0.09 cm2). As a reference, the device without STO interlayer (FTO/Perovskite/Spiro-OMeTAD/Ag (STO-0)) was also fabricated.
Material Characterization. The structure and morphology of the as-synthesized samples were characterized by field-emission scanning electron microscope (FE-SEM, Hitachi, SU8010) combined with energy dispersive X-ray spectroscopy (EDX). The surface statistics was analyzed by Atomic Force Microscope (AFM, Bruker, Dimension Icon) with tapping mode in air. The phase was checked by the X-ray diffractometer (XRD, D/MAX-III-B-40KV, Cu Kα radiation, λ = 0.15418 nm). The chemical analysis was recorded by an X-ray photon spectroscopy (XPS) system (ESCALAB, 250 Xi). The energy band structure was evaluated by the ultraviolet photoemission spectroscopy (UPS) (Thermo
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Scientific, Escalab 250Xi). The absorbance spectra were collected by a UV-vis spectrophotometer (Shimadzu, UV-3600). Ferroelectric hysteresis loops (P-V) for STO was recorded using a ferroelectric tester (Radiant 609B-3).
Measurements. All the photoresponse performance under white and different monochromatic light, including photocurrent, spectra responsivity, and time-dependent character, were recorded by a Source Meter (Keithley 4200) using a four-probe station (Advanced, PW-800) in ambient condition. The humidity and temperature in the measurement process were about 30% and 20 oC, respectively. Monochromatic light with various wavelength was produced by simulated solar light system (Newport, 94043A) combined with a monochromator (Zolix, Omni-λ 3009) using order sorting filters. The light intensity was modulated through an aperture and calibrated by using a power meter (Newport, 1936-R). Stability measurement was also recorded by the same test system. In the measurement process, devices were illuminated by light from the glass side. The poling treatment was directly conducted on the photodetector using Ag as top electrode and FTO substrate as bottom electrode at room temperature for 5 min. The poling voltage was set as ±1 V. Positive poling was applying a positive potential to the bottom FTO electrode.
RESULTS AND DISCUSSION
Figure 1a presents the schematic device structure of the hybrid STO-x photodetector. Firstly, STO films with different thicknesses were deposited on cleaned FTO substrate by a simple spin-coating process followed by spin-coating perovskite layers. The coverage of STO film
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can be adjusted by the cycle number of spin-coating. Finally, Spiro-OMeTAD and 100 nm Ag film were used as hole transport layer and conductive electrode, respectively. SEM images of STO films in Figure S1 (Supporting Information) show that nanoparticles distributed on the surface of FTO become denser as the cycle number increases (1, 2, and 3 cycle). AFM images illustrate the root-mean-square (rms) roughness decrease from 31.9 nm to 25.3 nm, 17.8 nm and 16.7 nm as the spin-coating cycle of STO increases from 0 to 1, 2 and 3 (Figure S2, Supporting Information). This indicates the thickness of STO increases with the increasing of cycle number. The variation of STO films has negligible effect on morphology of perovskite layer, including film thickness and crystal size (Figure 1b, Figure S3 and Figure S4. Supporting Information). The typical cross-sectional SEM image of the as-fabricated photodetector device is shown in Figure 1c. Unfortunately, it is hard to identify the spin-coated STO layer due to the ultrathin thickness. Other layers are clearly defined. The thicknesses of perovskite and spiro-OMeTAD layers are measured to be 300 and 200 nm, respectively. Figure 1b presents the top-view SEM image of perovskite layer, indicating the formation of a compact perovskite film with high coverage.
The composition and chemical state of STO was confirmed by XPS, as seen in the full scan spectrum (Figure 2a). Only Sr, Ti and O signals can be identified without any other impurities, suggesting the high purity of as-fabricated STO film. In the Sr XPS survey (Figure 2b), the peaks at 134.5 eV and 132.7 eV are indexed to Sr2+.25,26 For the Ti survey spectrum (Figure 2c), the peaks at 463.9 eV and 458.2 eV correspond to Ti4+.25,26 The peaks at 531.0 eV and 529.3 eV in Figure 2d are assigned to adsorbed oxygen on the STO surface and O2- state in the STO lattice, respectively.26,27 The above results verify that the
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as-prepared film is indeed STO. The P-V curve of pristine STO at room temperature shows intrinsic ferroelectric property (Figure S5, Supporting Information). The typical energy-dispersive X-ray (EDX) spectrum in Figure S6 (Supporting Information) further confirms the existence of Sr, Ti, and O elements in the as-fabricated film. Elemental mapping in Figure S6 (Supporting Information) reveals the uniform element distribution of Sr, Ti, and O on FTO substrate. The element ratio between Sr and Ti is 1:1.1, which is close to the stoichiometric ratio of STO. The higher O element ratio is attributed to the effect of FTO substrate.
To investigate the phase and crystallinity of perovskite film on FTO and STO/FTO, XRD patterns were recorded, as shown in Figure S7a (Supporting Information). Besides the peaks of FTO glass, the other diffraction peaks can be indexed to the perovskite, indicating the presence of STO layer has no obvious influence on crystal growth of perovskite layer, in accordance with SEM results. The absence of STO peaks is due to the fact that the amount of STO is low and its top surface is covered by a thick perovskite layer. Thus, we directly recorded the XRD pattern of STO film deposited on FTO glass obtained under the same experimental condition (Figure S7b, Supporting Information). All the diffraction peaks are attributed to STO (JCPDS No. 35-0734) without any detectable impurities. It should be noted that there is also no typical PbI2 peak at 12.8o after polarization treatment, indicating the stability of samples and excluding the influence of PbI2 phase on the photodetection performance.
Figure 3 illustrates the time-dependent photoresponse (I-t) curves of the devices with unpolarized STO under white light illumination at 0 V. The photocurrent decreases from
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0.32 mA to 0.25 mA, 0.16 mA and 0.05 mA as the spin-coating cycles of STO increases from 0 to 1, 2 and 3. In comparison, the photocurrent is very small (only ~100 nA) for the STO/Spiro-OMeTAD device without perovskite, indicating the STO layer and Spiro-OMeTAD layer have negligible effect on the light response. The reduction in photocurrent is ascribed to the unmatched band energy between STO and perovskite, and the accumulation of electrons at the STO/perovskite interface may result in the downward band bending of perovskite,28 which hinders the transfer of photogenerated carriers between two layers. The I-V curves in Figure S9 (Supporting Information) also show that the dark currents are suppressed to lower values as the spin-coating cycle of STO increases, indicating the STO can reduce the leakage current. The energy band alignment of the device will be plotted and discussed in the following part. Once the positive poling (+1 V) is applied, the devices with STO interlayers (1, 2 and 3 cycles) show increased photocurrent, whereas their photocurrents obviously decline as the negative poling is performed. The change of the photoresponse after poling is attributed to the polarization-induced internal electric field formed in the ferroelectric STO layer, which changes the state of the electrons accumulation at the STO/perovskite interface and thus modulates the band bending tendency and affects the separation and transport of photogenerated charge carriers. On the contrary, after the positive or negative poling treatment, there is no obvious change for the photocurrent of the STO-0 device, suggesting that the poling treatment has no observable effect on the device without ferroelectric STO. Furthermore, the rise time (the time taken for the current increasing from 10% to 90% of the maximum value) and the decay time (the time taken for the current decreasing from 90% to 10% of the maximum value) also exhibit
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a remarkable change, with faster response speed after positive poling and slower speed after negative poling. The STO-1 device shows the highest performance with the response speed of 0.3 s (rise time) and
