A Review on Contemporary Hole Transport Materials …
149
(2015) has found that triphenylamine-based HTM containing vinyl derivatives, 3,6di (2-(4-(N,N-di(p-tolyl)amino)phenyl)vinyl)-9-ethylcarbazole (apv-EC) and 3,6-di
(2-(4-(N,N-di(p-tolyl)amino)phenyl)vinyl)-2-thiophene (apv-T) had a PCE of about
12% without the use of any dopant material. Their overall efficiency was reported to
be less than Spiro-OMeTAD. Triphenylamine has low solubility in organic solvents
and low ionic potential which would be conducive to PSCs (Lv et al. 2015). It is
found that devices with HTM with no dopant offered comparable efficiency as that
of Spiro-OMeTAD but had lower stability. Also, the nonplanar TPA compound lead
to lower hole mobility due to increased long intermolecular distance. To counter this
effect, additives like lithium are added to increase hole mobility and stability.
Choi and Park synthesized triphenylamine-based two star-shaped HTM (FAMeOPh) and TPA-MeOPh were by that had a fused triphenylamine and a triphenylamine core. FA-MeOPh showed a comparable efficiency of 11.86% to that of
Spiro-OMeTAD-based cell (12.75%) (Choi et al. 2014). It also showed good relative stability of 250 h under the sun (Choi et al. 2014). It is easy to synthesize, and
inexpensive materials would make it a good alternative to spiro-based HTM (Fig. 4).
Zhang and Graetzel reported a novel butterfly-shaped HTM (Z1011) based on
triphenylamine. The PSC based on this configuration managed to get an efficiency
of about 16.3% with no doping compared to an efficiency of 16.5% in p-doped SpiroOMeTAD. Moreover, the stability of these materials was better than Spiro-OMeTAD
with no encapsulation for about 1000 h (Zhang et al. 2016).
To improve the charge transfer, Park et al. prepared HTM based on three molecules
triphenylamine (TPA) and [2,2]-paracyclophane. The three molecules varied in their
number of TPA groups and thus named Di-TPA, Tri-TPA, and Tetra-TPA. It was
found that with the increase in TPA groups, the efficiency, JV characteristics, and fill
factor increased. Tetra-TPA had the highest PCE of 17.9% with V oc of 1.05, J sc of 22
mAcm
−2 , and fill factor of 78. This enhanced photovoltaic performance is attributed
to pronounced charged transport in the HTM film (Park et al. 2016) (Figs. 5 and 6).
2.3 Thiophene-Based Organic Hole Transport Material
Li and Co-workers first reported a thiophene-based heterocyclic molecule 3,4
ethylenedioxythiophene (H101) as a hole-transporting material (HTM) to achieve an
efficiency of about 10.8% with no doping (Li et al. 2014a). This result was compared
to H101 with varied dopings of 5% and 15% chemical FK102 and Spiro-OMeTAD
with 15% doping. The results showed that H101 with 15% of doping achieved a PCE
of about 13.8 compared to that of Spiro-OMeTAD’s 13.7 (Li et al. 2014a). This gave
a promising start for thiophene-based HTM.
Based on this study later on, Li synthesized two more molecules based on H101,
H111, and H112 which had slightly better PCEs (14.7 and 14.9%). The Tg values for
the new HTMs were high, which indicated that it had better stability and it was also
reported that it had better HOMO levels, which consequently accounted for higher
V oc (Li et al. 2014b).
149
(2015) has found that triphenylamine-based HTM containing vinyl derivatives, 3,6di (2-(4-(N,N-di(p-tolyl)amino)phenyl)vinyl)-9-ethylcarbazole (apv-EC) and 3,6-di
(2-(4-(N,N-di(p-tolyl)amino)phenyl)vinyl)-2-thiophene (apv-T) had a PCE of about
12% without the use of any dopant material. Their overall efficiency was reported to
be less than Spiro-OMeTAD. Triphenylamine has low solubility in organic solvents
and low ionic potential which would be conducive to PSCs (Lv et al. 2015). It is
found that devices with HTM with no dopant offered comparable efficiency as that
of Spiro-OMeTAD but had lower stability. Also, the nonplanar TPA compound lead
to lower hole mobility due to increased long intermolecular distance. To counter this
effect, additives like lithium are added to increase hole mobility and stability.
Choi and Park synthesized triphenylamine-based two star-shaped HTM (FAMeOPh) and TPA-MeOPh were by that had a fused triphenylamine and a triphenylamine core. FA-MeOPh showed a comparable efficiency of 11.86% to that of
Spiro-OMeTAD-based cell (12.75%) (Choi et al. 2014). It also showed good relative stability of 250 h under the sun (Choi et al. 2014). It is easy to synthesize, and
inexpensive materials would make it a good alternative to spiro-based HTM (Fig. 4).
Zhang and Graetzel reported a novel butterfly-shaped HTM (Z1011) based on
triphenylamine. The PSC based on this configuration managed to get an efficiency
of about 16.3% with no doping compared to an efficiency of 16.5% in p-doped SpiroOMeTAD. Moreover, the stability of these materials was better than Spiro-OMeTAD
with no encapsulation for about 1000 h (Zhang et al. 2016).
To improve the charge transfer, Park et al. prepared HTM based on three molecules
triphenylamine (TPA) and [2,2]-paracyclophane. The three molecules varied in their
number of TPA groups and thus named Di-TPA, Tri-TPA, and Tetra-TPA. It was
found that with the increase in TPA groups, the efficiency, JV characteristics, and fill
factor increased. Tetra-TPA had the highest PCE of 17.9% with V oc of 1.05, J sc of 22
mAcm
−2 , and fill factor of 78. This enhanced photovoltaic performance is attributed
to pronounced charged transport in the HTM film (Park et al. 2016) (Figs. 5 and 6).
2.3 Thiophene-Based Organic Hole Transport Material
Li and Co-workers first reported a thiophene-based heterocyclic molecule 3,4
ethylenedioxythiophene (H101) as a hole-transporting material (HTM) to achieve an
efficiency of about 10.8% with no doping (Li et al. 2014a). This result was compared
to H101 with varied dopings of 5% and 15% chemical FK102 and Spiro-OMeTAD
with 15% doping. The results showed that H101 with 15% of doping achieved a PCE
of about 13.8 compared to that of Spiro-OMeTAD’s 13.7 (Li et al. 2014a). This gave
a promising start for thiophene-based HTM.
Based on this study later on, Li synthesized two more molecules based on H101,
H111, and H112 which had slightly better PCEs (14.7 and 14.9%). The Tg values for
the new HTMs were high, which indicated that it had better stability and it was also
reported that it had better HOMO levels, which consequently accounted for higher
V oc (Li et al. 2014b).