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We report a prototypical device of CH3NH3PbCl3 film ultraviolet photodetectors that were fabricated with a coplanar metal-semiconductor-metal Au interdigital electrode configuration. Pure phase CH3NH3PbCl3 films with a good crystallinity were formed by a hybrid sequential deposition process featured with inter-diffusion of PbCl2 and CH3NH3Cl upon annealing. The CH3NH3PbCl3 film photodetector exhibits a high responsivity of 7.56 A /W at 360 nm, a ultraviolet/visible rejection ratio (R360 nm/R500 nm) was about two orders of magnitude and fast response speed with a rising time of 170 μs and a decay time of 220 μs. All the above results demonstrate CH3NH3PbCl3 film photodetector as a competitive candidate in the application of visible blind UV detectors.
© 2016 Optical Society of America
1. Introduction
Organic–inorganic hybrid perovskites such as methylammonium lead halide (MAPbX3, MA = CH3NH3, X = Cl, Br, I) have aroused great interest among scientists and technologists as a new class of photovoltaic materials [1–3]. These materials have been widely used as light harvesters because of their broad absorption spectra, high carrier mobility, long-range balanced electron and hole diffusion lengths [4–6]. Especially, halide perovskite films with the good crystallinity and low defect density can be prepared using the facile and low-cost deposition methods. These advantages make organolead halide perovskites the forefront of modern optoelectronic semiconductor devices such as solar cells, photodetectors and light emitting diode [7]. For the application in the photodetectors, most works now focus on MAPbI3 based devices with two kinds of device architectures including p-n junction type and the metal-semiconductor-metal (MSM) structure [8–13]. The MAPbI3 perovskite film photodetector, based on a p-n junction structure, exhibits a large detectivity approaching 1014 Jones [8], the high responsivity of 208 A W−1 under 550 nm illumination [9], and a low noise and high average EQE approaching 90% [10]. On the other hand, photodetector was much more easily fabricated. The MSM structure photodetector based on the MAPbI3 perovskite films also exhibited excellent photoconductive properties, such as high photosensitivity of 14.5 A/W [11], excellent stability [11], high Ilight / Idark ratio of 104 [12] and rapid response speed of <50 ms [12], due to the excellent properties of perovskite films mentioned above.
Note that all the photodetectors based on MAPbI3 is sensitive to a broadband wavelength from the ultraviolet (UV) to the entire visible range [9–13]. However, for visible-blind ultraviolet (UV) detectors, MAPbCl3 rather than MAPbI3 should be much more suitable due to the wide band gap of 3.11 eV [14], although a sharp increase of the spectral response below 400 nm was also reported by Hu et al for MAPbI3 film detector with a MSM configuration. Recently, the visible-blind UV detector based on MAPbCl3 single crystal grown by the inverse temperature crystallization has firstly been demonstrated by Maculan et al [15] The MAPbCl3 detector with exceptional long-term stability under ambient conditions and high on-off ratio suggests a potential for the deployment of MAPbCl3 single crystals in practical applications. However, for this visible blind UV photodetector, neither the spectral response nor the UV/visible rejection ratio has been reported. Moreover, a low responsibility of 46.9 mA/W was observed for this single crystal detector. Very recently, MAPbCl3 single crystal detector with a similar vertical MSM structure presents a novelty narrow spectral response with a full-width at half-maximum of <20 nm at the center of 430 nm [16]. Also, a low external quantum efficiency (EQE) of 0.5-5% was obtained for this single crystal detector. The low responsibility or the low EQE is supposed to arise from the 0.1-1 mm thick crystal which cannot be thinned by further polishing. In addition, although the coplanar MSM structure with finger electrode distance of several micrometers can efficiently collect carriers, it is difficult to adopt this technology in the single crystal MAPbCl3 detector due to the detrimental effect of general photoresist materials on MAPbCl3 during lithography process. In contrast to single crystal photodetector, the MAPbCl3 film with hundreds of nanometer can completely absorb the UV photons, and a low applied voltage can be applied to collect carriers due to a strong electric field between electrodes. More importantly, the lithography process could be applied on the substrate rather than the perovskite film itself. So, the coplanar MSM structure photodetector can be easily processed for a photodetector based on film process. The advantage of film detector also includes low consumption of materials for film photodetector, an easy-control and low-cost deposition method, easy preparation of high quality films on different substrates and the choice of various device architectures like field effect transistor. So, the MAPbCl3 film photodetector may become a more competitive candidate for the application as UV detectors.
In this work, we report a prototypical device of MAPbCl3 films ultraviolet photodetectors that were fabricated with a coplanar MSM Au interdigital electrode configuration. The MAPbCl3 films were prepared by a hybrid sequential deposition process (HSDP), including firstly the deposition of PbCl2 film by thermal evaporation, the upper MACl film by spin-coating, and finally the formation of MAPbCl3 films upon annealing. This MAPbCl3 film photodetector exhibits a considerably better performance than detector based on single crystal, with a high responsibility of 7.56 A/W at 360 nm, the UV/visible rejection ratio of nearly two orders of magnitude, the photo-to-dark current ratio of 64 and fast response speed with a rising time of 170 μs and a decay time of 220 μs.
2. Experimental
2.1 Synthesis of MAI
The CH3NH3Cl powder was synthesized using the similar method of CH3NH3I as reported previously by reacting methylamine (33 wt % in ethanol, Sigma-Aldrich), with hydrochloric acids (45 wt % in water, Sinopharm) in a 100 mL round-bottom flask at 0°C for 2 h with stirring. After reaction, the white precipitate of MACl was collected using a rotary evaporator through carefully removing the solvents at 50 °C. The white precipitate was re-dissolved in absolute ethanol and precipitated with the addition of diethyl ether, and this procedure was repeated twice. The final MACl was collected and dried at 60 °C in a vacuum oven for 24 h. To form CH3NH3Cl precursor solution, the as-prepared MACl powder was dissolved in the isopropanol.
2.2 Perovskite photodetector fabrication
To fabricate the photodetector, the Au electrodes (50 nm) were firstly patterned by photolithography on a silicon wafer covered with an 82 nm thick SiO2 layer. Both the separated spacing and width of this finger electrode are 5 µm and effective area of 3 × 105 µm2. The PbCl2 film was then deposited by thermal evaporation on the top of the finger electrode. To avoid oxygen and moisture, the samples were subsequently transferred into a N2-filled glove box, where the thin-film MACl layers were spin-coated from a homogeneous 0.15 mol/L precursor isopropanol solution at 3000 rpm for 60s. Finally, the as-deposited films were annealing at 60 °C for 30 minutes. The MAPbCl3 thin film appears as a highly transparent and colorless film on the quartz substrate. The photodetector based on a 202 nm-thick MAPbCl3 film is finally fabricated, as shown in the schematic process flow of Fig. 1.
Fig. 1 Schematic illustration of a hybrid sequential deposition process used in the preparation of MAPbCl3 perovskite thin films.
2.3 Characterizations
The crystalline structure of the films was evaluated by X-ray diffraction (XRD, Rigaku D MAX-3C, Cu-Kα, λ = 1.54050Å). The UV-Vis transmittance spectra were measured with HITACHI U-2910 spectrophotometer with scan speed 300 nm/min and slit width quantitative 4nm in the wavelength range 200-900 nm at room temperature. Morphology of the perovskite films were investigated by a scanning electron microscope (SEM, JEOL JSM-6700F). Photoluminescence (PL) was measured at room temperature using the 514 nm Ar+ ion laser beam. X-ray photoemission spectroscopy analyses were performed using a PHI-Quantera SXM TM surface analysis system equipped with a monochromatic Al-Ka X-ray gun. X-ray photoemission spectroscopy (XPS) core level spectra of Pb 4f, Cl 2p and N 1s are fitted according to a Voigt peak shape along with a Shirley background. The current density–voltage (J-V) curves were measured (2400 Series SourceMeter, Keithley Instruments Inc.) by using a Xe lamp. The spectral responsivity was measured using a monochromator combined with an optical chopper and a lock-in amplifier. The light intensity was calibrated by a UV-enhanced silicon photodetector in the wavelength range of 320–600 nm. The time dependent photoresponse signal was recorded by a 1GHz digital oscilloscope with a Keithley 428 current amplifier. All the electronic properties were measured under the atmosphere environment.
3. Results and discussion
Briefly, the MAPbCl3 films were prepared by HSDP process as described in Fig. 1. The PbCl2 films (83 nm) were firstly deposited by thermal evaporation on the lithography interdigital electrode SiO2/Si substrates. The as-deposited PbCl2 films exhibit diffraction peaks at 19.6°, 22.9°, 24.9°, 32.2° and 39.7, corresponding to the (020), (120), (111),(211) and (002) plane of PbCl2 orthorhombic structure, respectively, as shown in Fig. 2(a). Weak peaks indicate a weakly crystalline nature of PbCl2 films, which can be confirmed by small grains with size of 20-50 nm shown from the surface SEM images in Fig. 2(b). This weak crystalline PbCl2 film can facilitate the diffusion of the upper MACl into PbCl2 film just as weak crystalline feature of PbI2 films have been proved to be a key factor to achieve a complete transformation of PbI2 into MAPbI3 due to easy inter-diffusion of MAI [17]. Actually, the MAPbCl3 films can be achieved by spin-coating the MACl film on the PbCl2 film using a 0.15 mol/L isopropanol precursor, followed by a post annealing at 60 °C for 30min. The pure phase MAPbCl3 films then formed via the inter-diffusion of MACl and PbCl2 upon post-annealing process, which can be evidenced by the XRD pattern of MAPbCl3 film in Fig. 2(a). Strong diffraction peaks at 15.55°, 22.14°, 31.58°, 35.34°, and 48.55°, corresponding to the (100), (110), (200), (210) and (300) planes of a cubic perovskite structure with a = 5.65Å close to the reported value of 5.67 Å for single crystalline MAPbCl3, indicate a good crystallinity of the as-deposited perovskite films, which seems to be a general case in organolead halide perovskite films by inter-diffusion method, such as high crystalline quality of MAPbI3 film [1,17,18]. The absence of characteristic peaks of PbCl2 verifies the complete reaction of all the PbCl2 films into MAPbCl3. In addition, the residual MACl on the surface are also evaporated completely after post annealing. Note also that a (100) preferred orientation of MAPbCl3 films with cubic phase is shown in Fig. 2(a). This is consistent with the case in the corresponding tetragonal MAPbI3 films with a (100) or (011) preferred orientation, prepared by a similar sequential process using PbI2 and MAI [1]. A small surface energy of this (100)-nonpolar plane is responsible for the (100) preferred orientation growth of perovskite films, which can benefit to carrier transport and thus device performance.
Fig. 2 (a) X-ray diffraction patterns of the MAPbCl3 film (red line) and PbCl2 film (black line) on the glass substrate. (b) A top-view SEM image of the PbCl2 film. (c) A top-view SEM image of the MAPbCl3 film. (d) The cross-sectional view of a representative MAPbCl3 film photodetector. (e) The XPS core level spectra of Pb 4f, Cl 2p and N 1s for the MAPbCl3 thin film
The surface of the as-deposited MAPbCl3 films prepared by HSDP is covered by dense and homogenous grains and there are no voids or pinholes on the surface of the perovskite film, as seen by the surface SEM image of Fig. 2(c). The dense perovskite thin films can be attributed to the dense starting PbCl2 seed layer. The cross-sectional SEM image in Fig. 2(d) clearly confirms the full reaction of PbCl2 with the upper MACl film into MAPbCl3 with absence of any remnant PbCl2 layer. The thickness of the perovskite thin film can be estimated from the cross-sectional SEM image to be about 202 nm, showing a ratio of 2.4:1 proportional to the thickness of the PbCl2 layer (83nm). This value is very close to the theoretical value of 2.3:1 for the MAPbCl3 to PbCl2 thickness ratio using the lattice parameters obtained from the above powder XRD results, which is similar to the case in the MAPbI3 film prepared from an inter-diffusion of PbCl2 and MAI by a full vacuum-based methods [19].
The surface composition and chemical state of MAPbCl3 films are then studied by X-ray photoemission spectroscopy (XPS). All the peaks are fitted by a Voigt type with a Gaussian/Lorenz ratio of 0.7/0.3 using a Shirley baseline. The Pb 4f7/2 and 4f5/2 doublet peak with binding energies of 138.16 eV and 143.02 eV, corresponding to Pb2+ state, can only be seen in Fig. 2(e). Note that the absence of metallic Pb component at lower binding energy, which are often observed in MAPbI3 films [20], suggests a pure MAPbCl3 phase obtained. In addition, except the Cl-Pb bonding and N-H bonding, neither the Cl 2p nor the N1s core level spectra show any other peaks, indicating there is no new chemical bond formation. The calculated atomic ratio of N:Pb:Cl for the as-deposited MAPbCl3 films is around 1.04:1:3.08, very close to the ideal stoichiometry of MAPbCl3. In addition, the films show a slight lead deficiency while an iodine deficiency is most often observed in the MAPbI3 films due to the easy loss of iodine by sublimation [21]. This most likely comes from the stronger bonding of Pb-Cl than that of Pb-I, leading to the difficulty of the loss of Cl elements.
After the achievement of pure phase MAPbCl3 films with a good crystallinity, the corresponding film photodetector was fabricated with a coplanar interdigital MSM configuration shown in Fig. 1. The typical dark I-V curve of the MAPbCl3 device is enlarged to show clearly a nonlinear behavior in Fig. 3(a), indicating a Schottky contact formed. Since the barrier of Schottky contact is very sensitive to the interface states, an asymmetrical I-V behavior can be found in MAPbCl3 photodetectors even with the symmetry electrode structure. In contrast, the MAPbI3 photodetectors with a similar configuration of coplanar interdigital Au electrodes show an Ohmic contact rather than Schottky contact [11, 22]. This behavior likely arises from completely different band alignments between MAPbI3/Au and MAPbI3/Au interface, as shown in Fig. 3(b). For the MAPbI3/Au interface, the valence band maximum is close to the Fermi energy of Au, leading to the free hole inter-transport from both sides. In contrast, there exists a large barrier for both electron and hole at the interface of MAPbCl3/Au. The difference of the band alignment between MAPbCl3/Au and MAPbI3/Au comes from the difference of valence band maximum of lead halide perovskite consisting mainly of the halide p valence electron. A high binding energy of Cl 3p leads to a low valence band maximum as compared with I 5p [23]. Under the illumination with 360 nm UV light with the irradiance of 2.2 mW/cm2, a photocurrent of 6.4 μA was measured at an applied voltage of 4V. The current ON/OFF ratio is around 64. Note that almost no hysteresis under forward and reverse bias can be observed in the dark and light current shown in Fig. 3(a), although a hysteresis can be observed clearly in the MAPbCl3 single crystal photodetector, seemingly coming from the interface between MAPbCl3 single crystal and metal electrode involving many mobile ions due to polishing process used in the single crystal detector [15]. Rather, for a film photodetector, the perovskite film is deposited directly on a cleaned SiO2/Si substrate without any additional processes, leading to a smaller amount of mobile ions.
Fig. 3 (a) I-V curves of the MAPbCl3 thin film photodetector in dark (red and light magenta lines) and under light illumination at 360 nm. The dark I-V curves are multiplied by ten to show a nonlinear relationship clearly. (b) Energy band diagrams of Au/MAPbCl3/Au and Au/MAPbI3/Au. (c) Photocurrent and responsivity versus the illuminated light irradiance.
The responsivity (R) can be calculated as follows,
where Ilight is the photocurrent (A), Idark is the dark current (A), P is the incident light intensity (W/cm2), and S is the effective illuminated area (cm2). The relationship of photocurrent and responsivity with incident irradiance is shown in Fig. 3(c). The values of the photocurrent increases from 2.2 μA to 6.4 μA, while the responsivity value decreases from 7.56 to 0.97 A/W, as the irradiance increases from 0.1 to 2.2 mW/cm2, The non-linear relationship of responsivity with irradiance strongly implies that the Schottky barrier at interface and field distribution in films changes with a illumination-determined active defect density. The highest responsivity reaches 7.56 A/W at the wavelength of 360 nm at 4V for a low irradiance of 0.1 mW/cm2. The value of responsivity of MAPbCl3 film detector with a MSM configuration is the same magnitude as 1.2 A/W of MAPbI3 film detector with the same configuration throughout 750-350 nm at 2V [11, 22]. As compared to the film UV photodetector based on perovskite oxides, such as BaTiO3, SrTiO3, SrZrTiO3 film detector [24–27], which often presents a low responsivity in the range of around 0.1 W/A, the organolead halide perovskite film photodetector shows the excellent responsivity due to a high carrier transport property and long carrier life time, just as that of the ZnO-based film detector most often adopted in the UV detector [28].The spectral responsivity of the photodetector with 5 μm finger width at 4 V bias under the ambient environment is shown in Fig. 4(a). A very sharp cutoff in the spectra responsivity wavelength can be observed at around 400 nm, corresponding to a photon energy of 3.1 eV. This result agrees well with the absorbance and photoluminescence (PL) spectra of MAPbCl3 films shown in Figs. 4(a) and 4(b). The UV(360nm) to visible (500nm) rejection ratio is about 92, nearly two orders of magnitude, which demonstrates an intrinsic visible blind characteristics of the MAPbCl3 UV detector. The as-deposited MAPbCl3 film has a sharp absorption edge at 399 nm and PL peak at 404 nm, which is consistent with literatures reported [14, 16, 29]. However, MAPbCl3 single crystals show an absorption edge at 435 nm and PL peak at 440 nm, with red shift of bandgap in comparison to the MAPbI3 film. The same situation of red shift of bandgap is also found for the MAPbI3 single crystal as compared to MAPbI3 film. Lower trap densities due to higher dimensional structurally coherent units that are tight in the single crystal compared to their polycrystalline counterparts, were suggested to be responsible for the narrowing of bandgap in MAPbX3 single crystals [6,7,15]. Notably, the first observation of an exciton peaks at 400nm in the absorbance spectra of Fig. 4(a) means that stable exciton can exists at room temperature for MAPbCl3, which is well in accord with the reported exciton binding energies of 3750 meV in the three-dimensional hybrid halide perovskite materials [30]. Very recently, a similar case that a prominent absorption band at 2.35 eV obtained from a room temperature ellipsometry measurement for the MAPbBr3 single crystal is attributed to excitonic absorption is also reported [31].
Fig. 4 (a) Responsivity (R) and absorbance versus the illumination wavelength. (b) Transmittance and PL spectra of the MAPbCl3 thin film. (c) Time response of the photodetector with a light irradiance of 0.1 mW/cm2. The inset shows the schematic of the measurement circuit. (d) Single photocurrent response cycle with light irradiation ON and OFF, showing rise (170 μs) time and decay (220 μs) time of the photodetector measured at a bias of 4 V and at a light intensity of 0.1 mW/ cm2.
Finally, we analyzed the time response behaviors of the as-prepared photodetectors, as shown in Fig. 4(c). The I-V characteristics of the photodetector are measured under illumination of UV light (λ = 360 nm) with a light irradiance of about 1 mW/cm2 at a bias of 4V. The film photodetector shows a good cycling response of under an ON/OFF interval of 5 ms illumination. Herein, the rise time and decay time of the photodetector are defined as the time taken for the initial current to increase or decrease to 90% of the peak value, respectively. As shown in Fig. 4(d), the rise time is 170 μs after triggering and the decay time is 220 μs after termination of irradiation. These response times are much faster than those for ZnO and TiO2 film photodetectors suffering from a well-known oxygen-deficiency related problem [32,33]. Surprisingly, the response time for a film detector is much better than that of MAPbCl3 single crystal [15], showing a rising time of 24 ms and a decay time of 62 ms. The reason for this most likely comes from the slow response of the mobile ions at interface between electrode and single crystal.
4. Conclusion
In summary, we have successfully fabricated the first solar-blind UV photodetectors based on a MAPbCl3 film via a low-cost hybrid sequential deposition process with interdigitated electrodes. The MAPbCl3 film photodetector exhibits a much better performance than single crystal detector [15,16]. The photodetectors are very sensitive to UV light, with a high responsivity reaching 7.56 A/W at the wavelength of 360 nm. The spectral response has a sharp cutoff wavelength at 400 nm, which agrees well with the bandgap of as-prepared MAPbCl3 film (3.1 eV), indicating that the developed MAPbCl3 film photodetector is a promising candidate for applications in UV light detection. In addition, our photodetectors show high ON-OFF ratio and fast rise and fall times. All the results demonstrate MAPbCl3 film photodetector as a competitive candidate in the application of visible blind UV detectors.
Acknowledgments
This work was supported by National Natural Science Foundation of China (No.11375112 and No. 51272159), and Natural Science Foundation of Zhejiang province (LY15A040001). The authors also thank Instrumental Analysis and Research Center of Shanghai University for the XRD, SEM and XPS work carried out.
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