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Page 10 of 31 Kim et al. Soft Sci 2024;4:33 https://dx.doi.org/10.20517/ss.2024.28

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high capacitance of over 0.5 μF·cm and flexibility, making it suitable for high-performance flexible
electronics. Moreover, the CuI-based TFT on CS with a curvature radius of 2 cm exhibited negligible
current change compared to the unbent state. The high mobility of this TFT is attributed to the dense
arrangement and fine grain size of CuI on the CS dielectric compared to the SiO dielectric, formatting a
2
smoother surface [Figure 4D]. Additionally, the smaller work function of CuI/CS films has a lower intrinsic
hole concentration, contributing to the higher on/off current ratio . Again, although there are various
[84]
reports on high apparent mobility CuI-based TFT devices, it is crucial to note that the mobility of CuI in a
-1 -1[32]
2
single crystal was measured at 43.9 cm ·V ·s . Considering the dielectric with mobile ions, such as ionic
liquid, ion-gel, solid electrolyte or even poorly annealed inorganic dielectric with unintentional H , could
+
result in unreliable field effect mobility estimation, it is always better to report field effect mobility with
reliable dielectric layers, such as thermally grown SiO , atomic layer deposition (ALD) grown Al O , HfO at
2
2
2
3
high temperatures.
In 2022, Lee et al. reported the effect of ALD grown Al O capping layers to passivate solution-processed
2
3
CuI-based TFT. As shown in Figure 4E, annealing under a vacuum atmosphere increased V , enhancing
I
carrier mobility and reducing carrier concentration. Furthermore, annealing in vacuum improved film
density and reduced voids, enhancing crystallinity and electrical properties. Through the synergistic effect of
V generation due to trimethylaluminum (TMA, AlC H ) used to deposit the Al O passivation layer, TMA
2
3
9
3
I
reduces Cu in the CuI thin film, leading to an increase in V . As a result, the TFT device exhibited a field
I
4[40]
effect mobility of 4.02 cm ·V ·s and an on/off current ratio of 1.12 × 10 . In 2024, the same group
-1 -1
2
reported CuI-based TFT fabrication with the CuI channel layer sandwiched structure between both
interfacial and passivation Al O layers, achieving a mobility of 6.54 cm ·V ·s and an on/off current ratio of
-1 -1
2
3
2
1.01 × 10 . By capping the CuI channel with an Al O passivation layer, which was deposited by ALD
5
3
2
process, hydrogen in the capping layer diffused into the channel layer and generated V , reducing hole
I
concentration. Furthermore, the crystallinity and surface morphology of the thin film on the interfacial
Al O were improved, resulting in increased mobility . The properties of CuI-based TFTs are summarized
[85]
2
3
in Table 2.
Photodetector
CuI was used as an ultraviolet (UV) light photodetector with flexibility and transparency similar to organic
photodetectors [64,92-96] . Furthermore, the wide bandgap of CuI helps in reducing the dark current, which
flows through the photodetectors in the absence of light, thereby enhancing the signal-to-noise ratio .
[97]
Typically, photodetectors have p/n heterostructure, metal-semiconductor-metal (MSM) structures, and so
on . Here, we investigate CuI-based photodetectors with heterostructures or MSM structures, and the
[98]
properties of these photodetectors are presented.
In 2022, Huang et al. reported a CuI-based self-powered 365 nm UV photodetector. With an MSM
structure and silver electrode, the CuI layer was fabricated by solid iodination on a polyimide (PI) substrate.
The device exhibits optical switching behavior under alternating light (rise time of 21.1 s and decay time of
23.3 s under 1 V). The average transmittance at visible region (400 to 800 nm) was 90% . Tasy et al. also
[92]
fabricated an MSM structure with Ni- and Zn-doped CuI layers. The Zn-doped CuI layer was fabricated by
the solution process. The Zn substitution improves the crystallinity and increases the grain size, decreasing
resistivity and increasing mobility. Consequently, the Zn-doped CuI photodetector exhibits optimal
performance with a response current of 2.05 × 10 A, responsivity of 722 mA·W , and specific detectivity of
-4
-1
1.51 × 10 Jones. The external quantum efficiency (EQE) also enhanced from 62.2% in the undoped device
8
to 242% in the 8% Zn-doped device .
[93]