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Page 2 of 31 Kim et al. Soft Sci 2024;4:33 https://dx.doi.org/10.20517/ss.2024.28

INTRODUCTION
The continuous development of large area electronic devices requires innovations of materials in form
factors, optical transparency, and multifunctionality for emerging applications such as flexible/stretchable
displays, smart windows, transparent energy harvesters, and computing systems on plane . Conventional
[1-4]
silicon-based semiconductors cannot satisfy these needs, making the development of advanced
semiconductors necessary. n-Type metal oxide semiconductors are highly transparent (T > 90%),
vis
2
physically robust, and exhibit high electrical performances [electron mobility (μ ) > 50 cm ·V ·s and
-1 -1
e
conductivity > 5,000 S·cm ] . However, the lack of available p-type semiconductors limits the potential for
-1 [5,6]
new forms of complementary large area electronic devices .
[7]
Considerable efforts have been directed towards realizing large area compatible flexible and transparent p-
type semiconductors. Although the conduction band maximum (CBM) of n-type metal oxides composed of
spherical and largely dispersed metal ns orbitals resulting in small effective masses and suitable pathways for
electrons, the valance band minimum (VBM) of typical metal oxides mainly comprises oxygen (O) 2p
orbital. The O 2p orbital is anisotropic and localized, leading to relatively large effective masses and low hole
mobility (μ ). Moreover, while the O vacancies in n-type oxide semiconductors act as facile donors, the
h
metal vacancy in p-type oxide semiconductors usually results in a deep level trap rather than acting as facile
acceptors. To relieve the anisotropic and localized characteristics of the O 2p orbital in metal oxide
semiconductors, the hybridization concept at the VBM was proposed by Kawazoe et al. The hybridization
of the metal (n-1)d orbital with the energy levels of an anion (such as O) at the VBM can enhance orbital
overlap, reduce hole effective mass, and subsequently facilitate hole transport [8-10] . Following this concept,
metal oxides such as Cu O, SnO, zinc oxide (ZnO), and NiO were developed . However, compared to the
[11]
x
n-type counterparts, the p-type oxides still struggle with high effective mass, low transmittance, unstable
chemical states, or poor electrical conductivity.

Copper iodide (CuI) is a promising alternative for p-type metal oxides, having high electrical properties,
optical transmittance, and even flexibility. Iodine has smaller electronegativity and size than O, resulting in
[12]
more delocalization of pseudo-spherical 5p orbitals at the VBM . Consequently, CuI could possess low
* [13,14]
hole effective mass (m ) and high μ h . Metal oxides do not inherently satisfy flexible properties, but
h
strategies such as creating thin metal oxide layers [15,16] , multi-layers [17-20] , nanostructures [21-23] , or mixing with
other flexible materials such as organics and polymers [24-26] , making them suitable for flexible electronic
devices. However, with a relatively lower Young’s modulus of 36.6 GPa for CuI than the very rigid Young’s
modulus of 118 GPa for Sn-doped In O (ITO), CuI has demonstrated flexible characteristics in various
3
2
applications [27-31] . Moreover, the low processing temperature of CuI thin films and their adaptability to
various fabrication methods make them easy to deposit on flexible substrates. In addition, the μ of CuI is
h
reported as 43.9 cm ·V ·s in the single grain, and carrier concentration ranges from 10 to 10 cm ,
-3
17
-1 -1
19
2
attributed to copper vacancies [14,32] . The direct bandgap of CuI is 3.1 eV, resulting in transmittance in the
visible region of over 80% [12,33] . CuI can be fabricated using various methods such as solution process [33-35] ,
vacuum process [36-38] , and iodination process [28,39] , making it suitable for industry. It also exhibits reasonable
[40]
stability compared to other halide materials . Therefore, we suggest CuI as a promising p-type
semiconductor counterpart to n-type metal oxides such as ITO and In-Ga-Zn-O (IGZO) for next-
generation electronic devices, offering high electrical properties, high transmittance, and flexibility.
In this review, we discuss the recent advances of CuI as a transparent, flexible, and high-performance p-type
semiconductor, focusing on its physical and electrical properties. First, we briefly summarize the various
fabrication methods for CuI. Then, we overview theoretical and experimental studies to understand the
fundamental electrical characteristics of CuI, such as limitations of mobility, nature of defects, and possible