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Kim et al. Soft Sci 2024;4:33 https://dx.doi.org/10.20517/ss.2024.28 Page 13 of 31

photodiodes. The photoluminescence quenching effect and the formation of an effective interface between
CuI and MoS facilitated efficient exciton dissociation and charge separation, thereby enhancing the
2
[102]
optoelectronic performance [Figure 5J] . Zhang et al. reported a self-powered UV photodetector utilizing
a p-CuI/n-CsPbBr heterojunction, which exhibited a high photocurrent of nearly 100 nA, high
3
photosensitivity with an on/off ratio of 1.5 × 10 , excellent wavelength selectivity between 565-525 nm, and a
3
fast response speed with rise and decay times of 0.04 and 2.96 ms at 540 nm under zero bias. These results
indicate that the CuI layer can form a good interface and achieves excellent optoelectronic performance,
combined with various kinds of n-type materials . Table 3 indicates the properties of published data of
[103]
CuI-based photodetectors.

TRANSPARENT CONDUCTIVE ELECTRODES
Transparent conductive electrode
As a TCE, CuI has been reported to have a record-high electrical conductivity of 596 S·cm , with over 80%
-1
transmittance in the visible region . Consistent with computational reports, sulfur and selenium doping is
[28]
promising for enhancing carrier concentration, although further studies are needed. In intrinsic CuI
without any dopant atoms, a record-high electrical conductivity of 283 S·cm has been achieved with
-1
temporal iodine doping at the film surface . To enhance mobility, decreasing defects through annealing or
[38]
doping is an effective strategy. Here, we explore various methods to enhance electrical and optical
properties, such as doping, fabrication methods, and encapsulation, which guide future efforts to further
improve electrical conductivity.

Sulfur-doped CuI has been reported to exhibit the highest electrical conductivity with high optical
transmittance. In 2022, Ahn et al. reported that sulfur-doped CuI had a high electrical conductivity of
511 S·cm (carrier concentration of 3.2 × 10 cm and mobility of 9.8 cm ·V ·s ) and a high transmittance
20
2
-3
-1 -1
-1
of over 80%. Moreover, with O doping by hydrogen peroxide treatment on the surface of the CuI thin film,
the electrical conductivity increased to 596 S·cm without any change in transmittance. The fabrication
-1
method for sulfur-doped CuI was liquid iodination, and computational experiments confirmed that this
method has a lower doping formation energy compared to vapor iodination [Figure 6A]. The CuI thin film
also underwent a bending test, bending 1,000 times with a 2 cm radius, showing a resistivity increase of only
2.6% compared to before bending. Additionally, it was applied as a source/drain electrode, demonstrating a
fully transparent TFT, and as an anode electrode layer for organic light emitting diodes (OLEDs) . In 2023,
[28]
Mirza et al. reported sulfur-doped CuI using the PLD method. The CuI film exhibited a high carrier
concentration of up to 9 × 10 cm and a high electrical conductivity of 435 S·cm . The transmittance was
-3
-1
20
[46]
over 75%. They confirmed that the doped sulfur forms amorphous Cu S phases at GBs in CuI thin films .
x
In 2024, Son et al. reported on sulfur-doped CuI using solution processes with thiourea derivatives. N-
ethylthiourea was identified as an optimal dopant, allowing control of the carrier concentration from 9 ×
10 to 2.52 × 10 cm , and achieving a conductivity of 390.7 S·cm -1[34] .
-3
20
18
Selenium-doped CuI on c-sapphire substrates was reported by Storm et al. in 2023. The selenium content
varied from 0.1 at.% to 10 at.%. They confirmed that when the selenium content is higher than 1 at.%, the
phase was changed due to selenium doping at iodine sites, leading to lower transmittance and electrical
15
17
-3
conductivity. With less than 1 at.% of Se, the carrier concentration increased from 10 to 8 × 10 cm .
However, with selenium content between 1 at.% to 3 at.% in CuI, the carrier concentration decreased to
-3
16
10 cm and then increased again to 8 × 10 cm up to 10 at.% Se [Figure 6B]. The hole mobility reached
-3
17
21 cm ·V ·s at around 1 at.% Se doping (carrier concentration of 8 × 10 cm ). The transmittance also
17
-3
-1 -1
2
remained over 80% up to 1 at.% Se-doped CuI .
[109]