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Page 16 of 31 Kim et al. Soft Sci 2024;4:33 https://dx.doi.org/10.20517/ss.2024.28

2
-1 -1
mobility slightly grew from 2.75 to 3.5 cm ·V ·s . Moreover, TiO suppressed recrystallization and grain
2
growth in CuI, and also suppressed iodine loss and oxidation of CuI. The resistivity of CuI without TiO
2
was 56 mΩ cm (approximately 18 S·cm ), whereas that of CuI with TiO was 14 mΩ cm (approximately
-1
2
-1
71 S·cm ). Additionally, the stability over six months in the air was impressively enhanced with the
introduction of TiO . The resistivity and transmittance of CuI without TiO increased more than four times
2
2
and decreased to below 50%, respectively. However, the resistivity changed only by 5% and the
transmittance was maintained in the presence of TiO 2 [111] . Xue et al. fabricated CuI-Cu O thin films in 2023.
2
The surface morphology and electronic properties were enhanced with Cu O because Cu O suppressed the
2
2
migration through CuI GBs during iodination, reducing grain size. They controlled the O flow during Cu
deposition using sputtering. Under optimized conditions, the CuI-Cu O had a carrier concentration of
2
-1
18
-3
-1 -1
6.16 × 10 cm , hole mobility of 25.4 cm ·V ·s , and electrical conductivity of 18.57 S·cm . Regarding
2
stability in ambient conditions over six months, the electrical conductivity of CuI without Cu O decreased
2
by 53%, whereas that of CuI with Cu O decreased by 32% .
[112]
2
Inagaki et al. reported a high-quality CuI thin film on an Al O substrate in 2020. They adopted a 2 nm CuI
3
2
buffer layer to achieve high crystalline and low defect CuI layers. A relatively low deposition temperature
(160 °C) for the buffer layer was effective in mitigating defects. The electrical conductivity ranged from 0.1
-1
17
18
-3
to 1 S·cm , the carrier concentration increased from 10 to 10 cm , and mobility increased from 1-5 to
10 cm ·V ·s after adopting the buffer layer .
-1 -1
2
[113]
In addition to research into doping, many studies on CuI using different fabrication methods have been
reported. These CuI films generally exhibit good transmittance around 80% and flexible properties. CuI thin
films have been made by thermal evaporation, solution processing, PLD, sputter, iodination methods, and
more. The PLD process can introduce various dopants during deposition, and the sputtering process is also
conducive to doping by adding gas during iodination. In the case of iodination, solid-iodinated CuI films
showed higher transmittance than the vapor-iodinated ones. Most of the results mentioned above are
summarized in Figure 7 [28,32,34,36-39,46,109-118] . The colored lines represent the electrical conductivity according to
the carrier concentration and mobility. The electrical conductivity was calculated using σ = n·e·μ, where the
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σ, n, e, and μ are electrical conductivity, carrier concentration, the charge of an electron (1.602 × 10 C), and
-1
mobility, respectively. When the electrical conductivity exceeds 1,000 S·cm , CuI can be compatible with n-
-1
type TCEs such as ITO. S-doped or I-rich CuI thin films can exceed the 100 S·cm line. To achieve
industrially viable optoelectrical performance of CuI, further innovations are needed for effective doping
and high carrier mobility. Additionally, we compared the electrical conductivity and transmittance of CuI
films across different fabrication methods. Regardless of the method, CuI films generally exhibit optical
transmittance of around 80% in the visible region. However, the CuI films from liquid iodination and PLD
methods with chalcogen doping demonstrate superior electrical conductivity [Figure 8]. The electrical
conductivity result corresponds with the result in Figure 7.
For further enhancement of the conductivity of CuI, it is crucial to overcome trade-off between hole
concentration and hole mobility or even to achieve synergetic improvement in both. In 2020, Crovetto et al.
reported on the hygroscopic nature of CuI under ambient humidity. The electrical conductivity doubled
when exposed to ambient humidity for 5 h, and the work function decreased by 1 eV. This is because
hydration at the GB may be beneficial for overall hole mobility . In 2023, Stralka et al. reported on the
[114]
conductive properties of grains and GBs in CuI using current probe force microscopy (cp-AFM). They
measured current densities across voltage ranges, finding that current primarily flows through GBs at low
voltages. Above that voltage (roughly 50 to 100 mV), current begins to flow through the grain surface as
well. However, the grain surface is susceptible to oxidation under ambient conditions, which causes surface