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Page 22 of 31 Kim et al. Soft Sci 2024;4:33 https://dx.doi.org/10.20517/ss.2024.28

performance of the device might be attributed to the harsh annealing conditions (100 °C for 1 h), which
[136]
induced many defects . Aliyaselvam et al. reported a CuI HTL using a green solvent, monoethanolamide
(MEA), applied for the first time as a CuI solvent. The optimized annealing temperature was 80 °C,
resulting in high-density and uniform grains. Additionally, the crystallinity was higher than at other
annealing temperatures, with an electrical conductivity of 28.38 S·cm . The performance of the solar cell
-1
[137]
-2
with the CuI HTL showed a PCE of 21.37%, an FF of 82.77%, a J of 22.74 cmA·cm , and V of 1.14 V .
SC
OC
CuI HTLs are also used for organic solar cells, which are highly favorable in terms of flexibility, lightweight,
environmental friendliness, and transparency. In 2015, Das et al. reported a CuI HTL in organic solar cells
and compared various concentrations of CuI layers with PEDOT:PSS layers. The PCE was 2.33% for the
[43]
PEDOT:PSS HTL, and with a 0.08 M CuI HTL, the PCE of the device was 2.25% . Peng et al. conducted a
similar experiment. They also compared the PCEs of solar cells with a CuI HTL and a PEDOT:PSS HTL.
The CuI was dissolved in ACN, spin-coated, and annealed at 100 °C. The PCE with CuI was 5.54%, which
[138]
was similar to the PCE with the PEDOT:PSS (5.50%) .

In 2021, Khatum et al. reported simulation results for a WS -based photovoltaic device. The PCE of the
2
device without a CuI HTL was 22.09%, but with a CuI HTL, the PCE was enhanced to 29.87%. This
improvement was attributed to the reduction of carrier recombination at the back surface with the CuI
-2
11
layer. They optimized the defect density between the absorption layer and HTL to 10 cm . Additionally,
they asserted that increasing the doping density in the HTL could minimize carrier recombination . The
[139]
detailed solar cell properties according to the CuI HTL layer are summarized in Table 5.
THERMOELECTRIC DEVICES
Thermoelectric device
As a green energy source, CuI-based TEGs have garnered significant interest, particularly due to their
transparent and flexible properties [141-143] . TEG is created using pairs of p-type and n-type semiconductors,
and its performance is evaluated using the figure of merit (zT). The zT is calculated based on electrical
conductivity, Seebeck coefficient, and thermal conductivity. These parameters are interrelated, making it
challenging to control and enhance each value simultaneously. CuI has emerged as a promising p-type
material for transparent and flexible TEGs due to its reasonable price and non-toxic nature compared to
existing TEG materials. In this chapter, we will investigate strategies to enhance the thermoelectric
properties of CuI.

Mulla et al. investigated the thermoelectric properties of CuI bulk pellets. The CuI powder was synthesized
through a solution iodination process with iodine in ethanol, followed by different annealing conditions,
ranging from no annealing to annealing temperatures between 373 and 573 K. They found that the electrical
resistivity increased with higher annealing temperatures, and the Seebeck coefficient also rose from 431 to
1,490 μV·K -1[144] . Yang et al. produced CuI thin-film by sputtering with iodine vapor during deposition. They
measured the electrical properties, Seebeck coefficient, thermal conductivity, and even transmittance and
flexibility. The carrier concentration of CuI was controlled by varying the iodine pressure during
deposition. Generally, thermoelectric properties are optimized when the trade-off is balanced; as the carrier
concentration increases, electrical conductivity rises while the Seebeck coefficient decreases. The zT value
20
-3
was optimized to 0.21 at a carrier concentration of approximately 10 cm . As shown in Figure 12A, the
thermal conductivity ranged from 0.5 to 0.56 W·m ·K with increasing carrier concentration. The relatively
-1
-1
low thermal conductivity value of CuI was attributed to phonon scattering and the presence of heavy iodine
[27]
atoms . Coroa et al. fabricated 300 nm CuI thin films through iodination of thermally evaporated Cu thin
films, achieving a zT of 0.29. The electrical conductivity was 110 S·cm with a carrier concentration of 1.7 ×
-1