Study On TiO₂-based Photodetectors

Ultraviolet (UV) photodetectors are becoming increasingly important for their applications in many fields, such as military use, UV communication, environmental monitoring, space research, biological cells canceration monitoring, and so on. As the grown technology developes rapidly, wide band gap semiconductor will replace Si for the fabrication of photodetectors and has enormous potential and broader prospects.Compared with photodetectors based on Si, devices based on wide band gap semiconductors can avoid expensive optical filter and realize visible-blind detection. Wide band gap based photodetectors, which have been used in the fields such as radar, communication and aviation, have attracted lots of attention. In recentyears, wide bandgap semiconductors, such as diamond, GaN, SiC and ZnO, have been studied for wide spreading usage in photodetection, especially in ultraviolet region. However, these materials are either difficult to preparare or have bad stability. Moreover, results from different research groups are sometimes contradictory with each other; therefore, looking for more suitable semiconductor to make photodetectors is crucial.Wide band gap semiconductor TiO2has many advantages, such as good physical and chemical stability, high reactivity, low-cost and good photoelectric characteristics. TiO2has already received enormous attention for photocatalyst, gas sensers and solar cell applications. TiO2has a band gap of3.0-3.2eV and nearly does not absorb visiblelight, which makes TiO2-based photodetectors show high sensitivity to UV/visible. What is more, the growth of TiO2films is simple, which is beneficial to low-cost and large-scale UV photodetectors manufacture.Compared with UV photodetectors based on diamond, GaN, SiC and ZnO, TiO2based UV photodetectors were only reported by several groups. And the reported TiO2UV photodetectors are mostly based on anatase phase. Research on rutile TiO2based UV photodetectors is strongly desired.Immobilization of TiO2as a thin film was a postulate for high performance photodetectors applications. The epitaxial growth of high-quality rutile TiO2films is important for studying its photoelectric property. Untill now, only a fewstudies have dealt with single crystalline, epitaxial rutile TiO2films, most of which deal with Al2O3substrate. In our work, high-quality epitaxial rutile TiO2(110) films have been deposited on MgF2(110) substrate by pulsed laser deposition. Since MgF2has a high optical transmittance in a wide range from UV to near-infrared, it is an important optical material and widely used as window materials in optical elements. During growth, the substrate temperature was maintained at500℃under controlled oxygen partial pressure. Then, photolithography was used to prepare interdigitated electrode. UV photodetector with Ni/TiO2/Ni structure was designed and fabricated to test the UV photoresponse performance. The maximum of the response is4.85A/W under5V bias, which occurred at300nm. As the wavelength became longer than370nm, the device showed no response. As the wavelength became shorter than260nm, the photocurrent descended gradually.Recently, photoelectrochemical cell-based UV photodetectors made by nano-structured TiO2have shown promising potential due to its low-cost, simple fabrication process and fast response. One-dimensional well-aligned nanostrctured semiconductors such as nanorods, nanowires and nanotubes have attracted extensive research interest due to their high surface-to-volume ratio and good electron conductivity. In our work, rutile TiO2nanostructures were grown directly on fluorine-doped tin oxide (FTO) glass by a low temperature hydrothermal method. The crystal structure, surface morphology and optical transmittance of the TiO2nanorod arrays were characterized. Then, self-powered UV detectors based on TiO2nanorod arrays were fabricated and studied. High quality deionized water was injected intothe device as an electrolyte instead of I-/I3-redox couples electrolyte. Water may be the most safe, stable and environmentally friendly electrolyte. Excellent UV-light detection selectivity in a spectral range between310and420nm is observed, which indicates that the device can be used as photodetector for UV-A range(320～400nm) application. Under the illumination of1.25mW/cm2UV light (λ=365nm), the UV detector shows an excellent photovoltaic performance, yielding a short-circuit current of4.67μA and an open-circuit voltage of0.408V. Under the same illumination, the rise time and the decay time of the UV detector are approximately0.15and0.05s.In order to overcome the drawback of a low specific surface area of TiO2thin film and bare nanorod arrays, TiO2nano-branched arrays with a larger surface roughness and surface area were used as an active photoanode for efficient photodetectors. The highest peak responsivityof TiO2nano-branched arrays is approximately0.22A/W at352nm. The corresponding IPCE is higher than77%, which is much higher than TiO2nanorod arrays detectors and commercial silicon detectors at this wavelength. The Jsc and Voc are obviously increased for the TiO2nano-branched arrays based UV photodetectors. The TiO2nano-branched arrays outperform the bare TiO2nanorod arrays film mainly for three reasons:(1) The specific surface area and roughness of TiO2nano-branched arrays are markedly enlarged, leading to enlarged TiO2/electrolyte contact area and flourishing electron-hole pairs;(2) The good electron conductivity is remained in the vertical nanorod trunk, which enables fast transport of photo-generated electrons from the TiO2nanobranches to the collecting FTO substrates through the single-crystalline TiO2nanorods; and (3) The branches can fill the gaps between the nanorods and improve the light harvesting efficiency.A new type of quasi-solid-state UV photodetector was fabricated using liquid crystal (LC)-embedded electrolyte with light-trapping scheme. The photodetecting performance of the device improved significantly, which overcomes leakage of organic solvents and bad stability. Excellent UV-light detection selectivity in a spectral range between310and420nm is observed, which indicates that the device is very suitable for UV-A range application. The maximum responsivity of the spectrumis about0.09A/W, located at the wavelength of383nm. Exposed to365nm UV-light on/off switching irradiation with light intensities of2.5mW/cm2, the photocurrent was observed to be excellent reproducible with fast response (both decay and rise times are less than0.3s).P-type organic semiconductorspiro-MeOTAD was also used as the light absorber and hole transport layer. Solid-state photodetector based on an inorganic/organic heterojunction of TiO2nanorod/spiro-MeOTAD is constructed. Excellent detection selectivity in a spectral range between400and420nm with a full width at half maximum (FWHM) of only26nm is observed, indicating that the device is very suitable for blue range application. The TNAs/spiro-MeOTAD heteroj unction shows reproducible photoresponse with fast respons. Exposed to410nm UV-light on/off switching irradiation with light intensities of75μW/cm2,the rise time and the decay time of the detector are approximately0.12and0.06s respectively.