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Page 6 of 31 Kim et al. Soft Sci 2024;4:33 https://dx.doi.org/10.20517/ss.2024.28

acts as compensation for the hole carrier. Darnige et al. reported various intrinsic defects in CuI, such as
iodine and copper Frenkel and Schottky defects, noting that O can easily fill these defect sites when exposed
[56]
to air . Graužinytė et al. conducted a comprehensive study on CuI doping, examining 64 elements. They
found S and Se to be promising p-type dopants that can significantly increase carrier concentration by
replacing iodine sites. Although chalcogen impurities generally exhibit self-compensating behavior due to
deep iodination energy, S and Se have low formation energy. In contrast, O, while easily doped on the
surface, does not enhance carrier concentration effectively due to its high formation energy. For n-type
doping, alkaline earth metals, group III and IV-B transition metals, tungsten, and most group VI-A
elements were identified as potential candidates. However, due to the low formation energy of V , only
Cu
beryllium and magnesium are suitable for n-type dopants. Despite these results, many n-type dopants such
as Br, Zn, Ag, and Sn have been reported . Yamada et al. calculated that Cu Br I alloys can control carrier
[30]
x
1-x
[57]
17
concentration from 10 to 10 cm without significantly affecting transmittance and bandgap . In 2023,
20
-3
Willis et al. reported the limitations of μ of CuI. They examined the phonon- and impurity-limited carrier
h
mobility as a function of carrier concentration. The μ is limited to 162 cm ·V ·s in the phonon-limited
-1 -1
2
h
-1
region and 32.6 cm ·V ·s in the ionized impurity-limited region [Figure 2C]. In the high carrier
2
-1
concentration region, where degenerately doped conditions prevail, carrier mobility is dominated by
ionized impurities. They also calculated that the intrinsic copper vacancy can lead to a carrier concentration
of up to 2 × 10 cm . Moreover, they asserted that while S and Se can act as p-type dopants, those dopants
-3
19
may not directly affect the carrier concentration due to the high degree of charge localization and relatively
high formation energy compared to V . They also mentioned the indirect effects of doping on the
Cu
crystallinity and morphology of CuI, which may enhance its electrical properties .
[14]
ACTIVE LAYERS
Diode
CuI is well compatible with various n-type semiconductors, particularly metal oxides and metal halides such
as ZnO, titanium oxide (TiO ), silver iodide (AgI), and a-IGZO [58-61] . The n-ZnO/p-CuI heterojunction
2
6
diode was reported by Schein et al. in 2013, which exhibited a current rectification ratio of 6 × 10 under
± 2 V and an ideality factor of η = 2.14 . In 2016, they further improved the n-ZnO/p-CuI heterojunction
[58]
diode by using a heteroepitaxial method at room temperature, enhancing the film crystallinity. This device
showed an improved rectification of up to 2 × 10 and a low saturation current density of 5 × 10 A·cm -2[59] .
-9
9
In 2021, Kim et al. reported an enhancement in rectification by incorporating a ZnS interfacial layer, which
improved the rectification ratio from 2.38 × 10 to 1.71 × 10 . As shown in Figure 3A, the ZnS interlayer
7
2
enhances carrier transport between n-ZnO and p-CuI by lowering potential barriers and reducing O
[62]
vacancy defects .
Lee et al. reported an n-BaSnO /p-CuI heterojunctions diode with a high rectification ratio of 6.75 × 10 5
3- δ
under ± 2 V and an ideality factor of η = 1.5. The CuI layer was fabricated by thermal evaporation, which
can generate iodine vacancy when the deposition process takes a long time for thick films. The increases in
iodine vacancies and the decrease in built-in potential in the heterojunction led to an improved rectification
[63]
ratio .
n-IGZO/p-CuI heterojunction diodes were reported by Yamada et al., demonstrating a rectification ratio of
6
6 × 10 and an ideality factor of η = 1.6 [Figure 3B]. Polycrystalline CuI film was fabricated by solid-phase
iodination of sputtered Cu N film under low temperatures . The following year, the group reported an
[64]
3
n-IGZO/p-CuI Br heterojunction diode, controlling the composition of I and Br to adjust the hole density
1-x
x
[57]
[Figure 3C] . A (111) oriented p-CuI film was grown by thermal evaporation, and an amorphous SiZnSnO
(SZTO) film was grown using radio frequency (RF) magnetron sputtering method. The rectification ratio