Solar-blind hybrid organic-inorganic photodetectors fabricated via additive manufacturing on crystalline or amorphous Ga2O3 films

The early detection of threats such as wildfires and corona discharges in power lines, along with the control of radiation emitted from hydrogen combustion or afterburners of fast-moving aerial objects, requires spectrally selective photodetectors in the UV-C region of the electromagnetic spectrum. These are advantageous over detectors that are sensible to infrared radiation since the ozonosphere effectively blocks the totality of UV-C light coming from the sun, resulting in a reduced rate of false alarms thanks to the virtual absence of a background signal.1 Gallium oxide (Ga2O3), a spontaneously n-type semiconductor that presents five possible polymorphs,2 is a promising material for the fabrication of UV-C photodetectors due to its absorption coefficient greater than 104 for photons of energy above its bandgap value of about 4.9 eV.3 Recently, we demonstrated that the metastable orthorhombic κ-phase is a valid candidate for the realization of solar-blind detector devices based on the metal-semiconductor-metal photo-resistor structure,4,5 while other research groups explored the possibility of making self-powered p/n heterojunction photodiodes based on n-type Ga2O3 and p-type organic conductive polymers 6 or small molecules 7 deposited by spin-coating. The advantages of this cost-effective organic-inorganic hybrid structure lie in the fact that the devices can work without any external voltage bias applied thanks to the photovoltaic effect. In this work, four hybrid organic-inorganic planar solar blind photodetectors based on a p++/n heterojunction were fabricated, each of them on a different Ga2O3 film, namely on unintentionally doped (UD) β-Ga2O3, UD κ-Ga2O3 and Si-doped κ-Ga2O3 epilayers grown by metal-organic vapour phase epitaxy (MOVPE) and on a layer of amorphous Ga2O3 deposited by radio-frequency magnetron sputtering (RFMS) at room temperature. By comparing their electrical and optoelectronic properties, it was possible to establish that the detector fabricated on the thermodynamically stable monoclinic UD β-phase presented the best photodetection performance in self-powered mode. In that device, the obtained values for photodetection figures of merit such as the value, position in wavelength and rejection ratio with respect to visible light of the spectral responsivity peak, along with the specific detectivity, are equal to 45 mA/W at 240 nm, 1.6∙103 and 2.4∙1012 Jones respectively. The study also allowed us to clarify fundamental aspects of the electronic conduction through the devices based on different crystallographic phases. Moreover, for the first time aerosol-jet printing, an additive manufacturing technique, has been employed to deposit with a predetermined interdigitated geometry an ink based on PEDOT:PSS PH1000, a highly conductive p-type organic polymer. Finally, the very cheap amorphous Ga2O3 film emerged as a more sustainable alternative to β-Ga2O3 in operative contexts near room temperature, thanks to the better compatibility with the organic layer printing process, resulting in the best sharpness of the deposited lines of conductive ink, along with the lower cost of RFMS deposition with respect to MOVPE, while still maintaining relatively good performances, including faster response times than β-Ga2O3-based photodiodes. References: [1] E. Monroy et al., Wide-bandgap semiconductor ultraviolet photodetectors, Semicond. Sci. Technol. 2003, 18, R33 [2] R. Roy et al., Polymorphism of Ga2O3 and the System Ga2O3-H2O, J. Am. Chem. Soc., 1952, 74, 3, 719–722 [3] S.J. Pearton et al., A review of Ga2O3 materials; processing; and devices, Appl.Phys.Rev. 2018, 5, 011301/1-55 [4] C. Borelli et al., Electronic properties and photo-gain of UV-C photodetectors based on high-resistivity orthorhombic κ-Ga2O3 epilayers, Materials Science and Engineering: B, 2022, 286, 116056 [5] M. Pavesi et al., ε-Ga2O3 epilayers as a material for solar-blind UV photodetectors, Mat. Chem. Phys., 2018, 205, 502-507 [6] S. Li et al., A self-powered solar-blind photodetector with large: Voc enhancing performance based on the PEDOT:PSS/Ga2O3 organic-inorganic hybrid heterojunction. J. Mater. Chem. C, 2020, 8, 1292-1300 [7] C. Wu et al, A general strategy to ultrasensitive Ga2O3 based self-powered solar-blind photodetectors, Materials Today Physics, 2022, 23, 100643 Acknowledgments: This work was supported by PRIN Project 2022A4AN2F “UV-C Sensors based on Gallium Oxide (USE GAO)” - MUR-PNRR-M4C2- I1.1 – Funded by NextGenerationEU, and by University of Parma through Bando FIL di Ateneo 2021 project “Ga2O3-based diodes for power electronics” MUR_DM737_A_MAFI_PARISINI DM 737 25.06.2021 co-funded by MUR - D.M. 737/2021 - PNR - PNRR - NextGenerationEU. This work has benefited from the equipment and framework of the COMP-HUB and COMP-R Initiatives, funded by the “Departments of Excellence” program of MIUR, 2018-2022 and MUR, 2023-2027. The authors would like to thank Dr. Abderrahim Moumen, Dr. Anna Sacchi and Dr. Tarek Hidouri for the useful and insightful discussions.