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4.4 Graphene Oxide (GO)
Graphene and its derivates have been used both in electron transport layer (ETL)
and hole transport layer (HTM). Graphene oxide (GO) is the most commonly used
graphene derivative as HTM because of its high thermal conductivity (600 Wm
−1
k
−1 ), appreciable valence band (5.2 eV), and high charge mobility. Wu and coworkers (2014) first time employed graphene oxide (GO) as a conductor. They found
that perovskite films observed on graphene exhibited much better crystallization and
efficient hole extraction. An efficiency of about 12.4% was observed. Moreover,
GO was employed as a dual functional buffer layer by Li.et al. 2014 to address the
issues with wettability of the HTM solution on perovskite surface and recombination of charge carriers. This led to an efficiency of about 15.1% with a high J sc of
20.2 mA/cm
2 , V oc of 1.04 V, and a film factor of 0.73. The highest reported efficiency
was 18.1% by Agresti et al. (2016) in which they used graphene in both ETL and
HTM. Graphene oxide was used as a buffer layer between perovskite and HTM layer
and doped graphene flakes in their mesoporous structure. This structure showed a
good stability of 88% of PCE after 16 h under one sun illumination (Table 3).
5 Outlook
In this review, focus was given on recent research strides in hole transport materials
(HTM) based on organic, inorganic, and polymer-based substances. As progress
continues in the field of photovoltaics pertaining to perovskite, hole transport layers
play an indispensable part in charge transportation. It is one of the three main layers
constituting the perovskite cell and has a huge impact on efficiency and stability of
the cell.
Whilst costly, Spiro-OMeTAD still dominates the role as an HTM for perovskite
cells among researchers. With its costly synthetic production, researchers are developing and incorporating different alternatives using novel materials. Among the
organic-based small molecules, triphenylamine, thiophene, carbazole, triptycene,
and triazine have garnered interest among researchers. It has been found through
literature that doped organic-based HTMs may lead to high efficiency but are more
susceptible to degradation with time. A proper HTM must have good film-forming
properties, must be chemically stable and should have higher mobility. A good HTM
should also have a suitable HOMO energy level for efficient transport of holes from
the perovskite to the hole-transporting layer. Also, HOMO energy level is not the only
limiting factor in choosing an efficient HTM; it should also have proper recombination of charge carriers, because recombination of charge carriers in perovskite–HTM
interface plays an important role in the efficiency and stability of the cell.
Small organic molecule-based HTM possess proper interface contact, good filmforming ability, and more processing solvent compared to polymeric HTM. Polymeric HTMs like P3HT and PTAA have tunable HOMO level, high conductivity, and
S. Nair and J. V. Gohel
4.4 Graphene Oxide (GO)
Graphene and its derivates have been used both in electron transport layer (ETL)
and hole transport layer (HTM). Graphene oxide (GO) is the most commonly used
graphene derivative as HTM because of its high thermal conductivity (600 Wm
−1
k
−1 ), appreciable valence band (5.2 eV), and high charge mobility. Wu and coworkers (2014) first time employed graphene oxide (GO) as a conductor. They found
that perovskite films observed on graphene exhibited much better crystallization and
efficient hole extraction. An efficiency of about 12.4% was observed. Moreover,
GO was employed as a dual functional buffer layer by Li.et al. 2014 to address the
issues with wettability of the HTM solution on perovskite surface and recombination of charge carriers. This led to an efficiency of about 15.1% with a high J sc of
20.2 mA/cm
2 , V oc of 1.04 V, and a film factor of 0.73. The highest reported efficiency
was 18.1% by Agresti et al. (2016) in which they used graphene in both ETL and
HTM. Graphene oxide was used as a buffer layer between perovskite and HTM layer
and doped graphene flakes in their mesoporous structure. This structure showed a
good stability of 88% of PCE after 16 h under one sun illumination (Table 3).
5 Outlook
In this review, focus was given on recent research strides in hole transport materials
(HTM) based on organic, inorganic, and polymer-based substances. As progress
continues in the field of photovoltaics pertaining to perovskite, hole transport layers
play an indispensable part in charge transportation. It is one of the three main layers
constituting the perovskite cell and has a huge impact on efficiency and stability of
the cell.
Whilst costly, Spiro-OMeTAD still dominates the role as an HTM for perovskite
cells among researchers. With its costly synthetic production, researchers are developing and incorporating different alternatives using novel materials. Among the
organic-based small molecules, triphenylamine, thiophene, carbazole, triptycene,
and triazine have garnered interest among researchers. It has been found through
literature that doped organic-based HTMs may lead to high efficiency but are more
susceptible to degradation with time. A proper HTM must have good film-forming
properties, must be chemically stable and should have higher mobility. A good HTM
should also have a suitable HOMO energy level for efficient transport of holes from
the perovskite to the hole-transporting layer. Also, HOMO energy level is not the only
limiting factor in choosing an efficient HTM; it should also have proper recombination of charge carriers, because recombination of charge carriers in perovskite–HTM
interface plays an important role in the efficiency and stability of the cell.
Small organic molecule-based HTM possess proper interface contact, good filmforming ability, and more processing solvent compared to polymeric HTM. Polymeric HTMs like P3HT and PTAA have tunable HOMO level, high conductivity, and