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Abstract
In this work, we have reported a vertical CsPbBr3/ZnO heterojunction photodetector for photo-sensing lights from UV to visible band. The ZnO thin film is deposited on the c-sapphire substrate through a molecular beam epitaxy (MBE) technique, and then the CsPbBr3 thin film is synthesized on the as-prepared ZnO film layer by using a solution processing method. The as-prepared CsPbBr3/ZnO heterostructure presents type-II energy band structure induced by the energy band offset effect, which can promote the separation and extraction efficiencies of the photo-generated electron-hole pairs. Compared with the CsPbBr3 based metal-semiconductor-metal (MSM) structure photodetector, the heterojunction photodetector presents higher responsivity and detectivity of 630 µA/W and 7 × 109 Jones. While compared with the ZnO based MSM structure photodetector, the heterojunction device reveals much faster response speeds of 61 µs (rise time) and 1.4 ms (decay time). These findings demonstrate that the CsPbBr3/ZnO heterojunction photodetector is promising for constructing next generation perovskite based optoelectronic devices.
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1. Introduction
Since the first report of the dye-sensitized solar cells incorporated with APbX3 (A = CH3NH3; X = Cl, Br, or I) as sensitizer [1], hybrid organic-inorganic perovskite emerges as a promising material for optoelectronic devices including solar cells [2–4], lasers [5], light emitting diodes [6], and photodetectors [7–9] due to its large light absorption coefficient, high tolerance to defects, and long carrier diffusion length. Nevertheless, the instability of the hybrid perovskite in ambient environment strongly restricts its practical applications [10,11]. Alternatively, all inorganic perovskite CsPbX3 (X = Cl, Br, I) has been considered as an ideal substitution owing to its environmental stability. Moreover, compared with other two counterparts (CsPbI3 and CsPbCl3), CsPbBr3 reveals more stable in phase structure.
Among those mentioned optoelectronic devices, photodetector is a photo-sensor which converts light signal into electric signal with numerous applications such as image sensing, optical communication, and environmental monitoring [12]. CsPbBr3 based MSM structure photodetector is naturally proposed because of the simple preparation process by depositing metals onto the CsPbBr3 as contact electrodes [13,14]. However, the CsPbBr3 based MSM structure device normally faces the issue of narrow band response. Compared with the MSM structure one, the heterostructure photodetector with specific energy band alignment can not only tremendously promote the photoelectric performance of the device, but also broaden the response spectra band of the device. Generally, three kinds of hetero-band structures including type-I (straddling gap), type-II (staggered gap), and type-III (broken gap) are classified [15]. The straddling gap is usually employed in the multi-quantum wells (MQWs) of the light emitting diodes or laser diodes, in which the electrons and holes are confined in the well layers with narrower bandgap. For a photodetector, the staggered gap structure is preferred owing to the effective separation of the photo-generated electron-hole pairs. In order to fabricate heterojunction with CsPbBr3, traditional semiconductors are considered because of their mature preparation methods and excellent photoelectric characteristics. Fan et al. reported an one dimensional (1D) CsPbBr3/CdS heterostructure prepared by a physical vapor deposition method [15]. The heterojunction photodetector with type-II band alignment reveals responsivity of 24.5 mA/W, and response speeds of 1.5 ms (rise time) and 14.9 ms (decay time). Wide bandgap semiconductor GaN with controllable p-doping is also proposed by our group to fabricate a hetero-structure photodetector with CsPbBr3 [16]. However, the energy band alignment between CsPbBr3 and GaN is belong to type-III band structure, thus resulting to the poor photo-response performance under the small external bias voltages (-2.8 V < V < 3.8 V). ZnO, a wide bandgap semiconductor with high electron mobility and low preparation cost, is regarded as another excellent candidate for constructing heterojunction with CsPbBr3, and the energy band alignment between CsPbBr3 and ZnO is calculated as type-II band structure. In fact, ZnO has already proved its great potential as electron transportation layer in perovskite solar cells [17,18]. To date, CsPbBr3 based photodetectors decorated with ZnO nanoparticles/nanorods [19–21] and sandwiched with ZnO compact layers [22] are reported and achieve preliminary progresses. However, to our best knowledge, a vertical structure CsPbBr3 based heterojunction photodetector with ZnO thin film as hetero-counterpart is still rare.
In this work, we have demonstrated a vertical CsPbBr3/ZnO heterostructure by spin dip-coating the perovskite onto the ZnO film layer. The crystal structure, optical characteristics, and morphologies of both materials are systematically studied. Owing to the large light absorption coefficient, long diffusion length in CsPbBr3, and type-II band alignment within CsPbBr3/ZnO hetero-interface, the heterojunction photodetector shows a resonsivity of 630 µA/W, a detectivity of 7 × 109 Jones, and a response speeds of 61 µs (rise time) and 1.4 ms (decay time). In addition, the response spectra of the CsPbBr3/ZnO heterojunction photodetector presents a broad response band from UV to green band, which is quite different from the CsPbBr3 MSM structure photodetector.
2. Experimental
2.1 Synthesis of materials
The ZnO film layer was deposited onto the c-sapphire substrate through a plasma enhanced molecular beam epitaxy method. Before the substrate was loaded into the growth chamber, it was ultrasonic cleaned in the ethanol for 30 min and deionized water for 15 min, and then it was dried by the nitrogen gun. The growth temperature was set as 400 °C, the temperature of the high purity Zinc source (5N) was steadied at 320 °C, the flow rate of the oxygen was 80 sccm, and the radio frequency power of the oxygen plasma generator was maintained at 380 W, respectively. The growth time for the ZnO epi-layer was 2 hours and its thickness was estimated as ∼160 nm. The CsPbBr3 film layer were prepared by using a saturated solution with 5.5 millimole CsBr and 2.5 millimole PbBr2 dissolved into 5 mL dimethyl sulfoxide (DMSO) solution. The mixed materials were continuously stirred for 2 hours until they were completely dissolved. After being stirred for 2 hours, the supernatant of the mixed materials was filtered through an organic filter head with a diameter of 0.45 µm. Methylbenzene was employed as the antisolvent for the precipitate of CsPbBr3. Subsequently, the saturated solution was spin dip-coated onto the glass substrate and half covered ZnO thin film. After that, the as-coated saturated solution was annealed at 100 °C and then the CsPbBr3 film layer were precipitated.
2.2 Characterization
The crystal structure and quality of the as-prepared CsPbBr3 and ZnO thin films were studied by X-ray diffraction (XRD) with Cu α-line (1.54 Å) as light source. Raman spectra with back-scattering configuration were performed to investigate the phonon frequencies of the as-prepared CsPbBr3 and ZnO thin films. UV-Visible absorption spectra of the CsPbBr3 thin film, ZnO thin film, and CsPbBr3/ZnO heterostructure were studied to determine their absorption edges. The inter-band transition characteristics of the as-prepared CsPbBr3 and ZnO thin films were studied by the photoluminescence (PL) emission and excitation spectra excited by a 400 W Xe-lamp light, and the PL decay traces excited by a 360 nm nano-second pulse laser. The chemical states of different elements were investigated through the X-ray photoelectron spectroscopy (XPS). Scanning electron microscopy (SEM) was utilized to clarify the surface morphologies of the as-prepared CsPbBr3 and ZnO thin films.
2.3 Device fabrication and photoelectric measurement
The as-prepared perovskite solution was spin dip-coated onto the half covered ZnO thin film, and then the hybrid structure was annealed at 100 °C for 10 min. The coating and annealing processes were repeated for several times in order to get the targeted thickness. After that, the Indium metal was employed as the Ohmic contact electrodes of the CsPbBr3 and ZnO thin films. The steady-state photo-response performances of the CsPbBr3 MSM structure photodetector, ZnO MSM structure photodetector, and CsPbBr3/ZnO heterojunction photodetector were investigated by employing a 150 W Xe-lamp as light source and a semiconductor parameter instrument as electric signal collector. The transient-state response speeds of the device was determined by using a 355 nm nano-second pulse laser as excitation source and an oscilloscope as electric signal collector.
3. Results and discussion
Figure 1(a) presents the XRD patterns of the as-prepared ZnO thin film, CsPbBr3 thin film, and CsPbBr3/ZnO heterostructure. Only two peaks located at 34.4° and 41.66° are detected on the ZnO thin film, which can be ascribed to the diffraction signals from the (002) facet of the wurzite ZnO and the (006) facet of the sapphire substrate, respectively [23]. The full width at half maximum (FWHM) of the (002) XRD peak of the ZnO film layer is 0.17°. Five peaks centered at 15.02°, 15.17°, 21.4°, 30.38°, and 30.7° are detected on the as-prepared CsPbBr3 thin film, which are originated from the diffraction signals from the (002), (110), (111), (004), and (220) facets of the orthorhombic CsPbBr3 [24]. The XRD result of the CsPbBr3/ZnO heterostructure contains both diffraction signals from ZnO and CsPbBr3. However, the signal from ZnO is much stronger than that of CsPbBr3, which may be owing to the c-axis growth preference of the ZnO. The Raman spectra are employed to study the phonon vibration frequencies of the as-prepared samples. As indicated in Fig. 1(b), besides the Raman peaks from sapphire substrate (denoted as “*”), three peaks located at 96.2 cm-1, 329.1 cm-1, and 435.7 cm-1 are denoted as the E2 (low), E2 (high) - E2 (low), and E2 (high) modes of the ZnO thin film. Compared with the vibration frequencies of ZnO single crystal, the red shifts of the E2 (low) and E2 (high) modes indicate the compressive stress in the ZnO host lattice [25,26]. For CsPbBr3 thin film, five Raman peaks located at 39.6 cm-1, 68.4 cm-1, 121.7 cm-1, 141 cm-1, and 309 cm-1 can be clearly observed. All the signals can be denoted as the vibration eigenmodes of the orthorhombic CsPbBr3, agreeing well with previous reports [24,27]. Fig. 1(c) presents the absorption spectra of the as-prepared ZnO thin film, CsPbBr3 thin film, and CsPbBr3/ZnO heterostructure. Apparently, the absorption edges of the ZnO and CsPbBr3 thin films are ∼375 nm and ∼546 nm, and two absorption edges corresponding to the individual materials are observed on the CsPbBr3/ZnO heterostructure. The PL emission spectra of the ZnO and CsPbBr3 thin films are shown in Fig. 1(d). The near band edge emission peak of the ZnO thin film is located at ∼376 nm. Except that, a broad green band emission is clearly collected and can be ascribed to the donor like defects such as oxygen vacancies [28,29]. Two strong emission peaks located at ∼512 nm and ∼533 nm can be observed and ascribed to the inter-band emission of the CsPbBr3 thin film. The PL excitation spectra are also investigated to explore the photon excited behavior of the samples. The exited optical grating for ZnO thin film is set at 380 nm, and that for CsPbBr3 thin film is set at 512 nm and 533 nm. The PL intensity of the 380 nm peak monotonously increases as the excited wavelength increases from 260 nm to 360 nm, which is owing to the increase of the incident light intensity of the Xe-lamp. The excitation tendency for CsPbBr3 thin film also corresponds well with the spectra distribution of the Xe-lamp. For example, the spectra of the Xe-lamp distributes a sharp peak at 467.5 nm, which agrees well with the excitation spectra of the CsPbBr3 thin film. The findings indicate the excitations by the Xe-lamp have not yet reached the saturation of the materials. The PL decay traces of the ZnO and CsPbBr3 thin films are shown in Fig. 1(f), the PL life times of the CsPbBr3 thin film are 120 ns (512 nm peak) and 133 ns (533 nm peak), while the PL life time of the ZnO thin film is determined as long as 7.5 µs. The much longer PL life time of the ZnO thin film maybe owing to the photogenerated carriers are trapped by the defect energy, which will slowly release and recombine as the incident laser is off.
Fig. 1. (a) The XRD patterns of the as-prepared ZnO thin film, CsPbBr3 thin film, and CsPbBr3/ZnO heterostructure; (b) The Raman spectra of the as-prepared ZnO and CsPbBr3 thin films; (c) The absorption spectra of the as-prepared ZnO thin film, CsPbBr3 thin film, and CsPbBr3/ZnO heterostructure; (d-f) The PL emission spectra, excitation spectra, and time-resolved PL decay trace of the as-prepared ZnO and CsPbBr3 thin films, respectively.
The chemical states of different elements are studied through the XPS measurements. The C-1s peak (as shown in Fig. 2(a)) located at 284.5 eV is employed as the standard calibration. Figure 2(b) exhibits the XPS spectrum of the Zn-2p core electron, symmetry peaks centered at 1022 eV and 1045 eV are originated from the signals of the Zn-2p3/2 and Zn-2p1/2 orbitals. The spin-orbital splitting of ∼23 eV is agreeing well with previous reports [30,31]. The XPS peak of O-1s core electron (shown in Fig. 2(c)) can be fitted with two Gaussian peaks, one centered at 531.16 eV is ascribed to the signals from the lattice-oxygen and the other one located at 532.7 eV is originated from the oxygen vacancies [32,33]. The XPS finding with regard to the oxygen vacancies also agrees well with the green band emission in the PL spectrum. The valence band scanning for the ZnO thin film is presented in Fig. 2(d). The position of the valence band maximum (VBM) with respect to the Fermi energy level can be calculated as ∼2.6 eV through the intersection of linear fits to the valence band leading edge and the background [34]. The XPS spectra of the Cs-3d, Pb-4f, and Br-3d core orbitals are revealed in Fig. 2(e-g), all of three XPS spectra can be well fitted by two Gaussian peaks. The spin-orbital splitting between Cs-3d5/2 and Cs-3d3/2 orbitals, Pb-4f7/2 and Pb-4f5/2 orbitals, Br-3d5/2 and Br-3d3/2 orbitals are determined as ∼13.9 eV, ∼4.86 eV, and ∼0.78 eV, respectively. From Fig. 2(h), the valence band maximum (VBM) with respect to the Fermi energy level of the CsPbBr3 thin film is calculated as ∼1.76 eV.
Fig. 2. (a-c) The XPS spectra of the C-1s, Zn-2p, and O-1s core electrons, respectively; (d) The valence band scanning spectrum of the as-prepared ZnO thin film; (e-g) The XPS spectra of the Cs-3d, Pb-4f, and Br-3d core electrons, respectively; (h)The valence band scanning spectrum of the as-prepared CsPbBr3 thin film.
The morphologies of the as-prepared CsPbBr3 and ZnO thin films are investigated by SEM and presented in Fig. 3. From the top view SEM images of the CsPbBr3 thin film at different magnifications (Fig. 3(a-c)), dense and square shape crystals with sizes of 4∼10 µm are clearly observed. The square shape morphology also verifies the orthorhombic crystal structure of the CsPbBr3 micro-crystals, agreeing well with the XRD results. Figure 3(d) reveals the top view morphology of the as-prepared ZnO thin film, dense crystals with sizes of hundreds nanometers are determined owing to the low growth temperature of 400 °C, which normally results in three dimensional (3D) island growth mode.
Fig. 3. (a-c) The SEM images of the as-prepared CsPbBr3 thin film at different magnifications; (d) The SEM images of the as-prepared ZnO thin film.
Figure 4(a) and 4(b) present the I-V curves of the In-CsPbBr3-In and In-ZnO-In devices under dark condition. Apparently, both I-V curves reveal linearly characteristics, which indicates the contacts between Indium metal and semiconductors (CsPbBr3 and ZnO) are Ohmic contacts. In addition, under the same bias voltage, the dark current of the In-ZnO-In device is larger than that of the In-CsPbBr3-In device. This can be ascribed to the high donor concentration in ZnO, which is induced by O vacancies and proofed by the PL and XPS measurements. Figure 4(c) shows the I-V curve of the In-CsPbBr3/ZnO-In device under dark, and the obvious rectification behavior is originated from the formation of the heterojunction. The energy band diagram of the heterojunction device is also provided in Fig. 4(d). According to previous reports [16,35] and the PL result shown in Fig. 1(d), the electron affinities of CsPbBr3 and ZnO are 3.43 eV and 4.6 eV, the bandgaps of CsPbBr3 and ZnO are 2.25 eV and 3.3 eV. Apparently, type-II energy band structure between CsPbBr3 and ZnO is determined. The valence band maximum difference (ΔEV) and conduction band minimum difference (ΔEC) of two different materials are calculated as 2.22 eV and 1.17 eV. Therefore, the type-II band structure can naturally enhance the collection efficiency of the photogenerated electron-hole pairs.
Fig. 4. (a-c) The I-V curves of the In-CsPbBr3-In photodetector, In-ZnO-In photodetector, and In-CsPbBr3/ZnO-In heterojunction photodetector under dark condition, insets are the schematic structures of corresponding devices; (d) Schematic energy band diagram of the CsPbBr3/ZnO heterojunction.
Responsivity, a key parameter represents the photo-electronic conversion efficiency of a photodetector, is determined through the following equation: [36]
where Id is the dark current, Iph is the photocurrent, S is the effective irradiated active area, Pλ is the power density, respectively. The responsivity of the In-CsPbBr3-In, In-ZnO-In, and In-CsPbBr3/ZnO-In photodetectors are provided in Fig. 5(a), 5(c), and 5(e). The In-CsPbBr3-In device presents narrow response band from 520 nm to 560 nm, which agrees well with previous reports on the CsPbBr3 single crystals [37,38]. The In-ZnO-In MSM photodetector shows obvious response from UV to blue band. The blue band response is caused by the band tail state induced by the defects, which can be verified by the PL and absorption spectra. The In-CsPbBr3/ZnO-In heterojunction photodetector extends the response range from UV to green band through combining the two different active materials. At the same external bias voltage, the responsivity of the In-CsPbBr3/ZnO-In heterojunction photodetector is much larger than that of the In-CsPbBr3-In device and smaller than that of the In-ZnO-In device. Under -5 V bias voltage, the maximum responsivity of the heterojunction device is ∼626 µA/W. Generally, the detectivity (D*) demonstrates the noise equivalent power (NEP) in a photodetector and can be calculated as:[36] where Jd is the dark current density and q is the elementary charge constant. The detectivity of the In-CsPbBr3-In, In-ZnO-In, and In-CsPbBr3/ZnO-In photodetectors are provided in Fig. 5(b), 5(d), and 5(f). Different from the responsivity of three devices, the In-CsPbBr3/ZnO-In heterojunction photodetector shows the largest detectivity at the same bias voltages. Under -5 V bias voltage, the maximum detectivity of the heterojunction device is 7 × 109 Jones.Fig. 5. (a-b) The responsivity and detectivity of the In-CsPbBr3-In MSM structure photodetector under -3 V and -5 V, respectively; (c-d) The responsivity and detectivity of the In-ZnO-In MSM structure photodetector under -3 V and -5 V, respectively; (e-f) The responsivity and detectivity of the In-CsPbBr3/ZnO-In heterojunction photodetector under -3 V and -5 V, respectively.
Figure 6(a) presents the steady-state time-resolved response curve of the In-CsPbBr3/ZnO-In heterojunction photodetector under 360 nm light (∼0.332 mW/cm2) illumination at -5 V bias voltage. Apparently, as the illuminated UV light is in “on state”, the current of the device rapidly boosts to ∼22.4 nA and then recoveries to ∼0.31 nA when the UV light is in “off state”. The photoresponse curve also reveals weak pyro-phototronic effect, which is related to the polarization potential induced by the incident light field and normally observed in ZnO low dimensional structure photodetectors [37–41]. However, the pyro-phototronic effect occurs much weaker in ZnO thin film structure than that in ZnO nano/micro structures. From the one single period I-T curve of the device shown in Fig. 6(b), the rise time and the decay time of the heterojunction device are determined as less than 0.1 s, which is known as the time-resolved limitation of our semiconductor parameter instrument. In order to determine the accurate response speeds of the device, a 355 nm nano-second pulse laser is utilized as the irradiated light source and an oscilloscope with resolution in nano-second scale is employed as the electric signal collector. As presented in Fig. 6(c), the CsPbBr3/ZnO heterojunction photodetector reveals excellent stability and repeatability even under the illumination of the strong UV laser radiation. From the one single period transient I-T curve of the device shown in Fig. 6(d), the accurate rise time and the decay time are determined as 61 µs and 1.4 ms, respectively. The 3 dB bandwidth of the CsPbBr3/ZnO heterojunction photodetector is determined as ∼8.3 kHz. Compared with recent reports on CsPbBr3 based heterojunction photodetectors (Table 1), our device also reveals relatively good performance.
Fig. 6. (a) 4-cycle steady-state time-resolved response of the In-CsPbBr3/ZnO-In heterojunction photodetector under 360 nm light (∼0.332 mW/cm2) illumination at -5 V bias voltage; (b) One single period I-T curve of the device derived from Fig. 6(a); (c) 5-cycle transient-state time-resolved response of the In-CsPbBr3/ZnO-In heterojunction photodetector under 355 nm pulse laser illumination at -5 V bias voltage; (d) One single period transient-state I-T curve of the device derived from Fig. 6(c).
Table 1. Comparison of the characteristic parameters of CsPbBr3 based heterojunction photodetectors.
4. Conclusions
In conclusion, a vertical CsPbBr3/ZnO heterostructure is prepared for harvesting light from UV to green band. The ZnO thin film is prepared on the c-sapphire substrate by using a MBE technique. Then the perovskite solution is spin dip-coated onto the half-covered ZnO film layer to construct the heterojunction. According to the energy band theory, type-II energy band alignment is determined within the hetero-interface. Owing to the effective inter-facial charge separation by the staggered gap, the In-CsPbBr3/ZnO-In heterostructure photodetector exhibits low dark current of ∼0.5 nA, resonsivity of 630 µA/W, detectivity of 7 × 109 Jones, and rapid response speeds of 61 µs (rise time) and 1.4 ms (decay time). These results indicate that the all-inorganic perovskite based heterojunction photodetector shows great potential in future optoelectronic devices.
Funding
National Natural Science Foundation of China (52172149, 61705043, 51872132, 52122607).
Acknowledgments
The authors thank Dr. Qiushi Liu for his helpful discussion and improvement on this manuscript.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
Data availability
The data that supports the findings of this study are available from the corresponding author upon reasonable request.
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