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S. Nair and J. V. Gohel
The overall performance of both the triazine derivatives was low compared to that
of Spiro- OMeTAD. Moreover, an appreciable stability of 250 h was obtained for
Triazine-Th-OMeTPA (Do Sung et al. 2014) (Table 1).
3 Polymer-Based HTM
3.1 P3HT Poly(3-Hexylthiophene)
In the past few years, P3HT-based polymer HTMs have garnered popularity among
the researches. Snaith et al. used PSC based on P3HT as the HTM. They used
a fullerene C60SAM-based organic monolayer to induce electron transfer from the
polymer HTM and the perovskite layer of CH 3 NH 3 PbI 3 − x Cl x I (Abrusci et al. 2013).
But it only gave an efficiency of about 6.7% as compared to the PCE of 11.7% for
Spiro-OMeTAD. This paved the way for future research for P3HT as HTM.
The thickness of P3HT plays a good role in photovoltaic measurements of the
PSC as demonstrated by Abbas et al. (2015). Their research entailed the use of P3HT
with the three types of different HTM thicknesses. They found out that the thinnest
layer (20 nm) of P3HT lead to loss of voltage and a very thick layer (45 nm) of P3HT
lead to increase in shunt resistance, thus decreasing the overall performance of the
cell. The intermediate thickness of 30 nm gave the highest PCE of 13.7%.
Dopants have also been extensively used in P3HT-based HTM. Xiao et al. (2015)
have reportedly used a graphdiyne dopant in a P3HT HTM. The GD dopants have
found to increase the scattering of light, which in turn increases the absorbance rate of
the PSC. The dopants have found to influence the HOMO levels through interaction
with P3HT. The HOMO level is decreased due to π–π stacking of dopant and P3HT.
This decrease in HOMO level causes an ease in the movement of the charge carrier
between perovskite and HTM. The PCE of these devices was found to be up to
14.58% (Conings et al. 2015).
Habisreutinger et al. (2014) focussed on the use of single-walled carbon nanotubes (SWNT) in P3HT to use as HTM. The intent was to tune the functionalizing
polymer around SWNT into a selective p-type hole collection layer in PSC. The
P3HT/SWNT-PMMA (poly(methylmethacrylate) showed no degradation even up to
96 h compared with other HTMs (Li-Spiro-OMeTAD, P3HT, PMMA, PTAA) under
the same condition and resulted in an efficiency of 15.3%.
Recently, Kundu and Kelly (2018) synthesized P3HT nanowire and deposited it
in a PMMA poly(methylmethacrylate) matrix to use it as an HTM layer. Different
device HTMs of same configuration PMMA:P3HT 80:20, 85:15, 90:10, and 95:05
were prepared using P3HT and PMMA by varying their ratios. It was found that
the device performance was heavily impacted by the ratios of PMMA:P3HT. The
blended configuration of P3HT/PMMA gave an efficiency ranging from 5.6% (for
PMMA:P3HT 95:05) to 9.1% (for PMMA:P3HT 90:10). Interestingly, P3HT-only
S. Nair and J. V. Gohel
The overall performance of both the triazine derivatives was low compared to that
of Spiro- OMeTAD. Moreover, an appreciable stability of 250 h was obtained for
Triazine-Th-OMeTPA (Do Sung et al. 2014) (Table 1).
3 Polymer-Based HTM
3.1 P3HT Poly(3-Hexylthiophene)
In the past few years, P3HT-based polymer HTMs have garnered popularity among
the researches. Snaith et al. used PSC based on P3HT as the HTM. They used
a fullerene C60SAM-based organic monolayer to induce electron transfer from the
polymer HTM and the perovskite layer of CH 3 NH 3 PbI 3 − x Cl x I (Abrusci et al. 2013).
But it only gave an efficiency of about 6.7% as compared to the PCE of 11.7% for
Spiro-OMeTAD. This paved the way for future research for P3HT as HTM.
The thickness of P3HT plays a good role in photovoltaic measurements of the
PSC as demonstrated by Abbas et al. (2015). Their research entailed the use of P3HT
with the three types of different HTM thicknesses. They found out that the thinnest
layer (20 nm) of P3HT lead to loss of voltage and a very thick layer (45 nm) of P3HT
lead to increase in shunt resistance, thus decreasing the overall performance of the
cell. The intermediate thickness of 30 nm gave the highest PCE of 13.7%.
Dopants have also been extensively used in P3HT-based HTM. Xiao et al. (2015)
have reportedly used a graphdiyne dopant in a P3HT HTM. The GD dopants have
found to increase the scattering of light, which in turn increases the absorbance rate of
the PSC. The dopants have found to influence the HOMO levels through interaction
with P3HT. The HOMO level is decreased due to π–π stacking of dopant and P3HT.
This decrease in HOMO level causes an ease in the movement of the charge carrier
between perovskite and HTM. The PCE of these devices was found to be up to
14.58% (Conings et al. 2015).
Habisreutinger et al. (2014) focussed on the use of single-walled carbon nanotubes (SWNT) in P3HT to use as HTM. The intent was to tune the functionalizing
polymer around SWNT into a selective p-type hole collection layer in PSC. The
P3HT/SWNT-PMMA (poly(methylmethacrylate) showed no degradation even up to
96 h compared with other HTMs (Li-Spiro-OMeTAD, P3HT, PMMA, PTAA) under
the same condition and resulted in an efficiency of 15.3%.
Recently, Kundu and Kelly (2018) synthesized P3HT nanowire and deposited it
in a PMMA poly(methylmethacrylate) matrix to use it as an HTM layer. Different
device HTMs of same configuration PMMA:P3HT 80:20, 85:15, 90:10, and 95:05
were prepared using P3HT and PMMA by varying their ratios. It was found that
the device performance was heavily impacted by the ratios of PMMA:P3HT. The
blended configuration of P3HT/PMMA gave an efficiency ranging from 5.6% (for
PMMA:P3HT 95:05) to 9.1% (for PMMA:P3HT 90:10). Interestingly, P3HT-only