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S. Nair and J. V. Gohel
4.1 CuI (Copper Iodide)
CuI was the first inorganic HTM to be deployed by Christians et al. (2014). A PCE
of 6% was achieved with good stability and higher electrical conductivity (J sc =
17.8 mA/cm
2 , V oc = 0.55 V); this was comparable with results obtained with SpiroOMeTAD (J sc = 16.1 mA/cm
2 , V oc = 0.79 V, PCE = 7.9%). The lower efficiency
was attributed to the lower value of V oc using CuI. A gas–solid reaction method
was used for the deposition of CuI film on perovskite which yielded a very high
J sc of 32.72 mA/cm
2 and an efficiency of 7.4% (Gharibzadeh et al. 2016). It was
also reported in this study that there was low hysteresis observed due to high hole
mobility of CuI. As observed in the previous study, this study also reported a lower
value of V oc (0.73 V) due to recombination effects at the CuI–perovskite interface.
This solid–gas reaction was further used by Yu and co-workers to use CuI as a hole
transport material. Yu et al. managed to achieve an efficiency of 14.7% with a J sc
of 20.9 mA/cm
2 and V oc of 1.04 V (Wang et al. 2017). A low-temperature solution
process method was adopted by Bian and co-workers to deposit CuI on perovskite
layer. This led to an efficiency of 16.8%, the highest reported so far for CuI-based
HTMs (Sun et al. 2016). It was observed by the authors that CuI had better air stability
than polymer-based HTM like PEDOT:PSS under similar conditions.
Recently, CuI has also been used in conjunction with PEDOT:PSS as a double
hole transport layer by Wang and co-workers (2018). The CuI acted as a buffer layer
which greatly increased the stability of the device and hole extraction. The average
exciton lifetime was significantly reduced to 2.7 ns. A PCE of 14.3% was achieved
for this device.
4.2 CuSCN
Copper thiocyanate (CuSCN) is another p-type HTM that has gained popularity among researchers because of its high mobility of 0.01–0.1 cm
2 V
−1 S
−1 ,
high conductivity (10
−2 –10
−3 S cm
−1 ), and high chemical stability. There are
different fabrication methods for the preparation of CuSCN like doctor blading technique, chemical bath deposition, electrochemical deposition, and ionic
layer absorption reaction. Chavhan et al. (2014) investigated the first use of
CuSCN as an inorganic hole transport material in a planar heterojunction-based
glass/FTO/TiO 2 /CH 3 NH 3 Pb 3–x Cl x /CuSCN/Au with power efficiency of 6.4%. The
devices were characterized, and it was found that the main limiting photovoltaic
parameter was a low V oc (0.727 V), which was attributed to low diffusion length.
Qin et al. used a doctor blading technique for depositing CuSCN. The device structure
had the configuration of FTO glass/ TiO 2 / CH 3 NH 3 PbI 3 /CuSCN/Au (Qin et al. 2014).
The presence of CuSCN gave a solar performance of PCE 12.4%. An inverted planar
structure was used by Bian and co-workers (2015) by one-step deposition crystallization method to fabricate CH 3 NH 3 PbI 3 films on top of CuSCN. This PSC prepared
S. Nair and J. V. Gohel
4.1 CuI (Copper Iodide)
CuI was the first inorganic HTM to be deployed by Christians et al. (2014). A PCE
of 6% was achieved with good stability and higher electrical conductivity (J sc =
17.8 mA/cm
2 , V oc = 0.55 V); this was comparable with results obtained with SpiroOMeTAD (J sc = 16.1 mA/cm
2 , V oc = 0.79 V, PCE = 7.9%). The lower efficiency
was attributed to the lower value of V oc using CuI. A gas–solid reaction method
was used for the deposition of CuI film on perovskite which yielded a very high
J sc of 32.72 mA/cm
2 and an efficiency of 7.4% (Gharibzadeh et al. 2016). It was
also reported in this study that there was low hysteresis observed due to high hole
mobility of CuI. As observed in the previous study, this study also reported a lower
value of V oc (0.73 V) due to recombination effects at the CuI–perovskite interface.
This solid–gas reaction was further used by Yu and co-workers to use CuI as a hole
transport material. Yu et al. managed to achieve an efficiency of 14.7% with a J sc
of 20.9 mA/cm
2 and V oc of 1.04 V (Wang et al. 2017). A low-temperature solution
process method was adopted by Bian and co-workers to deposit CuI on perovskite
layer. This led to an efficiency of 16.8%, the highest reported so far for CuI-based
HTMs (Sun et al. 2016). It was observed by the authors that CuI had better air stability
than polymer-based HTM like PEDOT:PSS under similar conditions.
Recently, CuI has also been used in conjunction with PEDOT:PSS as a double
hole transport layer by Wang and co-workers (2018). The CuI acted as a buffer layer
which greatly increased the stability of the device and hole extraction. The average
exciton lifetime was significantly reduced to 2.7 ns. A PCE of 14.3% was achieved
for this device.
4.2 CuSCN
Copper thiocyanate (CuSCN) is another p-type HTM that has gained popularity among researchers because of its high mobility of 0.01–0.1 cm
2 V
−1 S
−1 ,
high conductivity (10
−2 –10
−3 S cm
−1 ), and high chemical stability. There are
different fabrication methods for the preparation of CuSCN like doctor blading technique, chemical bath deposition, electrochemical deposition, and ionic
layer absorption reaction. Chavhan et al. (2014) investigated the first use of
CuSCN as an inorganic hole transport material in a planar heterojunction-based
glass/FTO/TiO 2 /CH 3 NH 3 Pb 3–x Cl x /CuSCN/Au with power efficiency of 6.4%. The
devices were characterized, and it was found that the main limiting photovoltaic
parameter was a low V oc (0.727 V), which was attributed to low diffusion length.
Qin et al. used a doctor blading technique for depositing CuSCN. The device structure
had the configuration of FTO glass/ TiO 2 / CH 3 NH 3 PbI 3 /CuSCN/Au (Qin et al. 2014).
The presence of CuSCN gave a solar performance of PCE 12.4%. An inverted planar
structure was used by Bian and co-workers (2015) by one-step deposition crystallization method to fabricate CH 3 NH 3 PbI 3 films on top of CuSCN. This PSC prepared