In this review, the primary focus is the recent advances in the development of freestanding inorganic crystalline semiconductors and their manipulation technology for flexible optoelectronic applications. We firstly cover the details of the growth and processing techniques of freestanding inorganic crystalline semiconductors in various dimensions and their material property under strain condition. Finally, fabrication processes and opto-electrical properties of flexible optoelectronics are introduced. Future research directions are also discussed, including further enhancement of device performance, building more types of optoelectronic devices on flexible substrates, and process integration for the advanced optoelectronic circuits and systems. GRAPHICAL ABSTRACT

Amorphous InGaZnO (a-IGZO) UV photodetectors with a metal-semiconductor-metal structure were fabricated on polyethylene terephthalate (PET) substrates, and the effects of surface fluorine plasma treatment on a-IGZO were investigated. With this treatment, the oxygen vacancies (O _vac 's) at the a-IGZO surface were effectively passivated, thus reducing the shallow-level donor states and electron traps. Accordingly, the direct and/or trap assisted tunnelings through the metal/a-IGZO Schottky barrier were suppressed, leading to a significant reduction in the dark current. On the other hand, the photo current was nearly unchanged if the treatment time was less than 20 min. It is due to the simultaneous reductions in hole trap and recombination center, both of which are related to the Ovac's at deep levels. As a result, a high detectivity of 4.36 × 10 ¹³ Jones was realized with an increase of nearly 10 times. Moreover, a highly uniform trap distribution was achieved, and it could be beneficial to the photoconductive reliability of the device.

In this paper, the impact of O ₂ postdeposition annealing treatment on the material and UV detection characteristics of amorphous InGaMgO (IGMO) thin films was intensively studied. With the replacement of Zn by Mg, amorphous InGaMgO possesses an optical bandgap larger than that of the conventional amorphous InGaZnO by over 0.5 eV. Furthermore, it was found the post-deposition annealing in O ₂ effectively suppressed the oxygen vacancies in the amorphous IGMO thin film, resulting in a significant reduction in its dark current. Due to strong hole trapping, all the samples except the 480-min annealed one exhibited large photocurrent and accordingly high responsivity over 1 × 10 ³ A/W @ 10 V. Moreover, both photocurrent and responsivity improve with increasing annealing time up to 240 min. As a result, the Metal-Semiconductor-Metal photodetector based on the 240-min annealed amorphous InGaMgO thin film exhibited a very prominent performance, including a high responsivity of 1.6 × 10 ⁴ A/W and a large photo-to-dark current ratio of 1.3 × 10 ³ . We attribute such excellent properties to the improved carrier mobility as well as the reduction of recombination centers in the O ₂ -annealed film. However, some degradations in device performance were observed when the annealing time reached 480 min, which can be explained by the suppression of localized tail states, as demonstrated by the abruptly reduced Urbach energy, and accordingly the inhibition of related extrinsic excitation. This work has provided a promising candidate for the application in transparent contact-free interactive display.