Optoelectronic Properties of MAPbI3 Perovskite/Titanium Dioxide Heterostructures on Porous Silicon Substrates for Cyan Sensor Applications
© Chen and Weng. 2015
Received: 22 September 2015
Accepted: 9 October 2015
Published: 16 October 2015
This work elucidates the optoelectronic properties of graphene/methylammonium lead iodide (MAPbI3)/titanium dioxide (TiO2)/porous Si heterostructure diodes. The porous silicon substrates can accommodate more MAPbI3/TiO2 than the polished silicon substrate such that the MAPbI3/TiO2/porous Si substrate heterostructures have better optoelectronic properties. Photocurrents from 300 to 900 nm were measured. The photocurrent is high in two ranges of wavelength, which are 300–460 nm and 520–800 nm. The photocurrent plateau covers all visible light (360 to 780 nm) except for cyan between 460 and 520 nm. Therefore, the graphene/MAPbI3/TiO2/porous Si heterostructure can be utilized as cyan sensors.
Methylammonium lead iodide (CH3NH3PbI3 or MAPbI3) with the perovskite structure has potential optoelectronic applications, such as solar cells and light-emitting diodes, because of its direct band gap of 1.6 eV, low cost, and ease of production; it has therefore attracted substantial interest [1–5]. MAPbI3 perovskite-based solar cells with a power conversion efficiency of over 20 % have been successfully developed . Such solar cells are promising because of their low cost, simplicity of fabrication, and absorption in the solar spectrum as well as balanced charge transport characteristics with long diffusion lengths [7, 8]. Light-emitting diodes that are based on halide perovskite have also been fabricated . The green light-emitting device with the ITO/PEDOT:PSS/MAPbBr3/F8/Ca/Ag structure has a luminance of 364 cd/m2 at a current density of 123 mA/cm2 and external and internal quantum efficiencies of 0.1 and 0.4 %, respectively.
Last year, several works on perovskite-based photodetectors have been published [10, 11]. The absorption range of the MAPbI3 is very broad. The typical range is from 300 to 800 nm. However, excellent absorption of light cannot transform the absorbed energy into a photocurrent. Therefore, in this study, we developed the graphene/CH3NH3PbI3 (MAPbI3) perovskite/titanium dioxide (TiO2)/porous silicon substrate heterostructure diodes and studied their structure and optoelectronic properties.
Single crystalline (100) p-type boron-doped Si substrates with a resistivity of 10 Ω cm were used in this study. Prior to processing, the wafers were cut into 1 × 1 cm2 and cleaned by ultrasonication in acetone, ethanol, and deionized water, consecutively. The silicon wafers then were immersed in dilute hydrogen fluoride (HF) solution to remove the native oxide layers, yielding a hydrogen-terminated surface. Next, the porous silicon structure was fabricated by metal-assisted chemical etching (MACE) in the freshly prepared dilute solution that contained both HF (48 %) and AgNO3 (0.02 mol/l) (1:1) with different etching times at room temperature. When the Si wafer was dipped into the etching solution, a silver nano-cluster layer was formed on its surface. The wafer was then put in a dilute HNO3 solution (50 wt%) to remove the Ag layer.
The TiO2 layer was coated onto the silicon substrate at a speed of 2000 rpm for 10 s and then annealed at 550 °C for 30 min. Subsequently, MAPbI3 perovskite precursor solution was coated onto the surface of the TiO2/silicon substrates using a spinner at a speed of 5000 rpm for 20 s. In this study, the perovskite layer was deposited by the solvent-engineering technology of 1.2 M Pbl2 and 1.2 M methylammonium iodide (MAI) in a cosolvent of dimethyl sulfoxide (DMSO) and γ-butyrolactone (GBL) (vol. ratio = 1:1) in a glove box filled with highly pure nitrogen. Then, the substrate was annealed at 100 °C for 10 min. The graphene layer was spin-coated using DMF-based graphene suspension (0.3 mg/ml) at 2000 rpm for 20 s. Finally, an indium contact (~2 μm) was evaporated onto the top of the graphene electron-spreading layer to complete the whole diode structure. The morphology and cross section of the resulting structures were examined using field emission scanning electron microscopy (FESEM). Photoluminescence (PL) was measured at room temperature. The excitation source for PL was a 405-nm diode laser. The electronic characteristics were measured using a Keithley 2420 programmable source meter.
Results and Discussion
The optoelectronic characteristics of graphene/MAPbI3/TiO2/Si heterostructure diodes were investigated. As the etching time of the silicon substrate increased, the PL intensity increased, because more TiO2 and MAPbI3 penetrated into the porous silicon substrates, because they accommodated more MAPbI3/TiO2. The MAPbI3/TiO2 on the porous silicon substrate increased the effective contact area of the heterostructure and reduced its series resistance. The photocurrent plateau covered all visible wavelengths (360 to 780 nm), except for those of cyan from 460 to 520 nm. Therefore, as shown in Fig. 6, the graphene/MAPbI3/TiO2/porous Si heterostructure can not only be utilized in a cyan sensor but also for near-IR light (780–900 nm or more) applications.
Financial support of this paper was provided by the Ministry of Science and Technology of the Republic of China under Contract No. MOST 103-2221-E-027-029-MY2.
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