Page 79 - Read Online

Kim et al. Soft Sci 2024;4:33 https://dx.doi.org/10.20517/ss.2024.28 Page 3 of 31

dopants. There have been intensive research efforts on tuning of carrier concentration for target
applications, such as reduction, increase, and optimization of hole concentration for thin-film transistors
(TFTs), transparent conducting electrodes (TCEs), and thermoelectric generators (TEGs), respectively. As
interest in CuI increases as a transparent p-type semiconductor, we summarize recent progress on diverse
applications, such as diodes, channel layers for TFTs, TCEs, hole transport layers (HTLs) for optoelectronic
devices, and thermoelectric devices, as well as its flexibility and stability of each device. Finally, we will
discuss current challenges and outlook for the successful industrial implementation of CuI as a p-type
transparent semiconductor.

FABRICATION METHODS OF COPPER IODIDE
[41]
In 1907, Bädeker reported CuI with vapor iodination as the first transparent electrode . As a promising
transparent p-type semiconductor, extensive research has been conducted for enhancement of carrier
mobility, control of surface morphology and crystallinity, optimization of carrier concentration, cost-
effective fabrication methods, and realization of various electronic devices [Figure 1]. CuI can be fabricated
with various methods, making it suitable for industrial applications. These methods include solution
process, vacuum process, and iodination process, allowing CuI to be easily formed into polycrystalline thin
films. For the solution process, techniques such as spin-coating and spray-coating are used. Spin-coated CuI
has shown high transistor performance by controlling film thickness through annealing or doping to
decrease carrier concentration . The selection of an appropriate solvent, such as acetonitrile (ACN),
[42]
dimethyl sulfoxide (DMSO), or a co-solvent mixture of ACN and ethanolamine, is also crucial [34,43] . The
mechanical and chemical vulnerability of CuI with weak ionic bonding between Cu and I hinder facile
-
+
patterning of CuI thin-film with standard photolithography processes. To overcome the challenges, direct
[44]
pattering of CuI thin-films was demonstrated with inkjet printing and spray-coating over stencil mask .
[45]
The conventional deposition method of CuI thin-films in industry is vacuum processes, such as sputtering,
thermal evaporation, and pulsed laser deposition (PLD), which are beneficial for large-scale manufacturing
with facile control of film thickness, large area uniformity, and compatibility to other layers without damage
from processing solvents. PLD is a favorable method for depositing high-purity films and maintaining
stoichiometry due to the high power laser and its ability to vaporize CuI with high energy in a short time .
[38]
Sputtered CuI thin films exhibit high electrical conductivity and transmittance . To control the carrier
[39]
concentration of sputtered CuI thin-film, dopants can be introduced by using alloy target or ion
implantation into the CuI films [46,47] . Including reactive evaporation of Cu with I vapor, thermal evaporation
2
[48]
of CuI is favored for its ease of operation .
Iodination method is a straightforward way to fabricate CuI films by converting Cu or precursor films to
[39]
[49]
CuI. When Cu, Cu N , or Cu S thin-film is exposed to an iodine-rich environment, a spontaneous
2
3
iodination process occurs, regardless of an iodine state (vapor, solid, or dissolved in solvent). Vapor
iodination involves placing Cu precursor films and small iodine particles in a closed system until the
process is complete, usually within 1 h. This method is simple but controlling iodine concentration can be
challenging, especially at low temperatures, although heating the container with iodine particles helps. Solid
iodination has been reported to convert Cu N to CuI with higher transmittance than vapor iodination by
3
Yamada et al. . In liquid iodination, non-polar solvents such as hexane result in good morphology and
[39]
high electrical conductivity. Ahn et al. found that liquid iodination reduces doping formation energy
compared to vapor iodination and allows facile control of iodine concentration by adjusting the solution’s
[28]
concentration .