Received: 3 May 2020 Revised: 22 July 2020 Accepted: 23 July 2020

DOI: 10.1002/er.5876

REVIEW PAPER

Recent progress of graphene-based materials for efficient

charge transfer and device performance stability in
perovskite solar cells

Nur E. Safie | Mohd A. Azam | Mohd F.A. Aziz | Mashasriyah Ismail

Fakulti Kejuruteraan Pembuatan (Faculty

of Manufacturing Engineering), Universiti
Summary
Teknikal Malaysia Melaka, Melaka, The relevance of graphene-based materials throughout the niche area of perov-
Malaysia skite solar cells (PSCs) is indeed the main focus of this review. Specific proper-
Correspondence ties of two types of solar cell materials, namely hybrid perovskites and
Mohd A. Azam, Fakulti Kejuruteraan graphene-based materials are at the core of significant breakthroughs in a wide
Pembuatan (Faculty of Manufacturing
range of applications. The specific features of graphene-based materials, along
Engineering), Universiti Teknikal
Malaysia Melaka, Hang Tuah Jaya, 76100 with their unique properties, have been utilized mainly in the development of
Durian Tunggal, Melaka, Malaysia. photovoltaic devices. PSCs are known to have promising device performance
Email: asyadi@utem.edu.my
and surpass other third-generation solar cells, including organic photovoltaics,
quantum dot solar cells, and dye-sensitized solar cells. However, PSCs address
several limitations in the device mechanism and material stability. PSCs per-
formance tends to deplete over time due to several factors such as the degrada-
tion of materials used in the device caused by exposure of thermal and
moisture, as well as an undesired chemical reaction in the interfaces. Several
experimental studies, especially on the integration of carbon materials, includ-
ing the graphene, have been extensively explored. The integration of
graphene-based materials is one of the potential methods for altering and mod-
ifying components in PSCs, including perovskite structure and charges trans-
port layers. Therefore, this review gives an overview of recent progress in the
development of PSCs with the integration of graphene-based materials. The
emphasis will be on the influence brought by graphene-based materials on
the charge transport mechanism at the interfaces and perovskite morphology
toward the improvement of photovoltaic performance and stability.

KEYWORDS
charge transportation, device stability, film morphology, graphene-based materials, photovoltaic
performance

1 | INTRODUCTION factors of fast-growing solar cell technology is the inven-

tion of various materials that meet the device require-
Photovoltaic technology has undergone rapid develop- ments, including low-cost synthesis or fabrication
ment ever since solar energy could be harvested abun- procedure, and abundantly available materials. Gener-
dantly with the breakneck growth of various types of ally, solar cell devices have been classified into three gen-
solar cells that hold promising efficiency. One of the vital erations, where the first generation is established and

Int J Energy Res. 2020;1–28. wileyonlinelibrary.com/journal/er © 2020 John Wiley & Sons Ltd 1
2 SAFIE ET AL.

dominate the market of the solar cell industry, which is a with the concept of third-generation solar cells, where
crystalline silicon solar cell.1 The second generation is the emphasis is on providing low-cost processing mate-
the thin-film solar cells made up of microlayers of semi- rials, secure fabrication procedures with high mechanical
conductor materials and categorized as amorphous flexibility. Hence, in this review, the integration of
silicon,2,3 cadmium telluride,4 and copper indium gal- graphene-based materials including pristine graphene,
lium diselenide.5 Meanwhile, the solar cells that fall graphene oxide (GO), reduced GO (rGO), and graphene
within the third generation include organic solar cells quantum dots (GQDs) in PSCs is addressed significantly.
(OSCs) (also known as polymer solar cells),6,7 dye- Throughout this review, the focus is on the role of
sensitized solar cells (DSSCs),8-10 quantum dot solar graphene-based materials in the modification of the
cells,11 and recently, perovskite solar cells (PSCs).12,13 perovskite morphology toward enhancement of charge
The main aim of looking for silicon-based solar cell transportation dynamics, performance, and device stabil-
replacement is to provide a cost-efficient scalable fabrica- ity, consequently reported in recent research works.
tion procedure along with consistent device stability and
efficiency. With such aspirations, researchers are inventing
more assuring materials that are available abundantly and 1.1 | Perovskite solar cells
environmentally friendly. Tandem solar cells also open up
broad diversity of photovoltaic applications.14-16 Nonethe- 1.1.1 | Device architecture
less, the third-generation solar cells, especially PSCs, have
exhibited significant advantages as they offer simple device Generally, the architecture of PSCs can be categorized as
fabrication techniques, various choices of synthesized direct (n-i-p) and inverted (p-i-n) with either planar or
material and are cost-effective for commercial-scale pro- mesoscopic structure, as depicted in Figure 1. The
duction.17-20 Among the devices, the hybrid PSCs is con- mesoscopic structure mostly contains a mesoscopic oxide
sidered the rising star in the photovoltaic industry as it scaffold, including titanium dioxide (TiO2), which has
showed a significant increment of power conversion effi- been reported to obtain high device PCE,32 which is
ciency (PCE) in less than a decade. The rapid PCE strongly related to the thickness of TiO2 typically in the
improvement reported increased from 3.8% in 200921 to an range 300-1000 nm.33 However, the mesoscopic oxide
initial steady-state value of 27% of tandem perovskite/sili- often needs high heat treatment,34,35 which results in det-
con solar cell structure as certified by the National Renew- rimental effects on device performance and limits the
able Energy Laboratory. However, there are several factors industrial manufacturing roll-to-roll process toward the
that it lags behind the other generations, for instance, less implementation of flexible devices. Hence, researchers
stability, reproducibility, and scalability for industry pro- are interested in exploring PSCs with planar structures,
duction.22 Therefore, to enhance the stability and perfor- especially in inverted architecture. Inverted planar PSCs
mance of the device demands further research. is a promising device as it offers excellent advantages
To date, researchers are seeking a solution to counter such as owing low-temperature procedures that are com-
device instability. Research has been conducted to pro- patible with large-scale production and suitable for flexi-
duce chemically stable perovskite material by altering the ble and wearable devices.36
chemical structure inducing mixed halide23 and mixed For the direct (n-i-p) architecture, the electron trans-
cation.24 Furthermore, the strategy to encapsulate perov- port material (ETM) is fabricated above transparent con-
skite material from surrounding conditions has also been ductive oxides (TCO) such as fluorine-doped tin oxide
well explored. One of the smart strategies is employing (FTO) or indium tin oxide (ITO) followed by perovskite
carbon materials, including graphene-based materials, as layer, hole transport material (HTM), and top contact
interfacial materials in the PSCs system. Their excellent electrode. Meanwhile, in the inverted (p-i-n) configura-
properties, such as ambipolar charge mobilities, large tion, HTM is deposited onto the TCO while ETM is lay-
specific surface area, and high thermal conductivity, ered onto the perovskite material before implement the
make it a great candidate to enhance PSCs device stabil- top contact electrode. The energy alignment is crucial in
ity as well as photovoltaic performance.25 Carbon mate- order to follow the device architecture, for instance, the
rials including the carbon nanotubes as well as graphene conduction band (CB) of electron transport layer (ETL)
are broadly used with their promising properties in differ- should be closer to that of perovskite in order to accept
ent fields including supercapacitors and batteries,26-28 the photogenerated electron, and the CB should be mat-
sensors,29 catalysts,30 and solar cells.31 ched with top electrode for inverted device architecture
Aside from the excellent electrical and mechanical whereby the CB of ETL should be matched with TCO for
properties, the carbon-based nanomaterials are compara- direct device architecture for efficient electron
tively low-cost and abundant materials that are parallel transport.37
SAFIE ET AL. 3

F I G U R E 1 Illustration of PSC components comprises of a perovskite active layer sandwiched between ETL and HTL with
corresponding electrodes in direct and inverted architecture (vertical) as well as planar and mesoscopic structure (horizontal). ETL, electron
transport layer; HTL, hole transport layer; PSC, perovskite solar cell [Colour figure can be viewed at wileyonlinelibrary.com]

FIGURE 2 Energy levels of various common HTMs and ETMs together with iodide and bromide-based halide-perovskites.37 ETM,
electron transport material; HTM, hole transport material [Colour figure can be viewed at wileyonlinelibrary.com]

1.1.2 | Working principle and device hole transport layer (HTL) to separate the hole from perov-
components skite material; and (4) corresponding electrodes for the
electron transmission throughout the circuit. Perovskite
The essential components of PSCs contain (1) a perovskite active layer plays an essential role in light harvesting and
active layer to capture light and initiate generation of the to generate the dynamics of the charges and charge trans-
electron; (2) an ETL to collect the generated electron; (3) a portation to the respective contact.38 HTL is responsible
4 SAFIE ET AL.

for the hole collection in perovskite/HTL interface, where hydrophobic that limits the moisture absorption which is
specific criteria should be met to be selected as HTL. For beneficial in device stability.50 Their tunable work function
efficient device performance, HTL should provide (i) high is also beneficial as they can meet the needs of different
hole mobility and low electron affinity which is beneficial device structures. Moreover, PCBM is widely used as an
for electron blocking to hinder charges recombination, electron acceptor in inverted planar architecture as it offers
(ii) compatible energy band matching where the position a minimal-temperature solution process51 and therefore is
of the valence band (VB) or highest occupied molecular compatible with a flexible substrate.
orbital should be closer to perovskites for efficient hole
mobility, (refer Figure 2), (iii) high thermal stability and
moisture resistant to prohibit device degradation, and 1.2 | Graphene-based materials
(iv) low cost which is compatible for industrialization.
The most common HTM used in the development of Graphene exists as a two-dimensional (2D) sheet of
PSCs is organic material such as 2,20 ,7,70 -tetrakis(N,N-di-p- single-atom thickness, which is a synthesis from the
methoxyphenylamine)-9,90 -spirobifluorene (spiro-OMeTAD)39 three-dimensional graphite. The layer resembles a
and poly(3,4-ethylenedioxythiophene) polystyrene sulfo- honeycomb-like structure made up of sp2 hybridization
nate (PEDOT:PSS).40 Nevertheless, the organic HTL faces of carbon atoms. Chemical vapor deposition (CVD) so far
some instability issues, for instance, employing spiro- is the best technique for synthesizing high-quality
OMeTAD in PSCs often requires the incorporation of graphene thin film.52-55 However, since the method
p-type dopant like Li-bis(trifluoromethanesulfonyl) imide requires expensive equipment, researchers are looking
(Li TFSI),41 tert-butylpyridine (tBP),42 and co-complexes43 for another alternative way of producing graphene-like
to enhance its conductivity which in turn reduces the perfor- material. From works of literature, the most commonly
mance of a device. The dopant reported tends to introduce studied graphene-related materials are GO and rGO.
moisture to the perovskite material due to its corrosive and Both materials can be synthesized with a more straight-
hygroscopic nature. Similarly, PSCs with PEDOT:PSS as forward method compared with pristine graphene and
HTM often prepared in high acidity conditions results in incorporation in additive resulting in tunable properties
device distortion.44 ITO substrate was reported to be such as tunable bandgaps56 that open up to its broad
degrading when in direct contact with hygroscopic PSS that application. GO is often synthesized by using several
shows the capability to absorb moisture from the atmosphere, methods such as conventional Hummer's method,57 mod-
which limits the hole extraction through ITO/PEDOT:PSS ified Hummer's method,58 simplified Hummer's
interface.45 This limit has resulted in the use of inorganic method,59 and improved Hummer's method.60 In Hum-
HTM; for instance, copper (I) thiocyanate (CuSCN) due to its mer's method, exposure is to highly acidic and oxidizing
stability against moisture and heat.46 agents such as potassium permanganate and concen-
Another charge selective layer that plays crucial roles trated sulfuric acid applied to the graphite powder via dif-
for electron extraction from the CB of perovskite materials ferent treatments and washing processes.
is ETL. The significant criteria needed in ETL layer are Meanwhile, there are various methods for the reduc-
(i) high electron mobility for a substantial electron- tion process of GO to form rGO. However, the primary
accepting property, (ii) matching energy band where the process is mainly via thermal reduction,61 and chemical
CB or lowest unoccupied molecular orbital should be posi- reduction usually involves a strong reductant agent such
tioned slightly lower than that of perovskite materials so as hydrazine hydrate62 or a combination of both pro-
that the electron could be injected efficiently within cesses.63 Also, some research conducted has proposed a
ETL/perovskite interface, (refer Figure 2) (iii) hydrophobic- fast and green reduction process by using L-ascorbic
ity and chemical stability in order to enhance moisture- acid64,65 and lemon extract66 as a safer and less acidic alter-
resistant and prohibit unnecessary chemical reactions with native. Furthermore, GO and rGO properties are not simi-
other components in the device architecture. Metal oxide lar to graphene due to atomic structure modification as
such as TiO2 and zinc oxide (ZnO) is the common anode depicted in Figure 3. Graphene itself has been proven to
materials used as an ETL in mesoscopic PSCs architecture. have high electrical conductivity (106 S cm−1).68 However,
Although they have been extensively studied with excellent GO becomes insulating material with low conductivity
device performance due to the capability of preventing leak- caused by the disruption of the sp2 conjugation with the
age current,47 high thermal treatment48 and poor crystallin- presence of carboxylic, hydroxyl, epoxide, and carbonyl
ity, as well as carrier mobility,49 resulted in its limited group.69 Conversely, the presence of functional groups
usage. Furthermore, fullerene-based materials, including makes it hydrophilic, which facilitates more substantial
[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and C60, contact with polar substances for various applications.
is the alternative solution since it shows high mobility and Nevertheless, GO can restore electrical conductivity via
SAFIE ET AL. 5

chemical reduction and thermal annealing processes to these graphene-based materials after incorporated in the
become rGO, which shows better conductivity.70 rGO is PSC device components and how it influenced the device
the alternative form of graphene where both graphene and stability, performance and charges transportation dynam-
rGO are having electrical conductivity properties, but rGO ics in perovskite material as well as in HTL/perovskite
obtained much lower conductivity as compared with pris- and ETL/perovskite interface based on studies reported
tine graphene. Above all, GO and rGO widely explored in in recent years summarized in Table 1.
photovoltaic devices, especially in PSCs.
Apart from that, GQDs is a zero-dimensional
(0D) (refer Figure 4), which generally employ as the light 2 | P E R O V SKI T E M A T E R I A L S
absorber72 and electron acceptor73 in a photovoltaic
device, owing to its significant optical properties, high Metal halide perovskite material that acts as an active light-
electrical conductivity, chemically stable and tunable absorbing layer in PSCs consists of a chemical structure
energy level.74 Possess strong quantum confinement known as ABX3 (A is cation typically methylammonium
effect making GQDs distinct from other 2D graphene- [MA], formamidinium [FA], cesium [Cs], rubidium [Rb]; B
based materials. GQDs consist of few-layer chopped frag- is cation such as lead [Pb2+], tin [Sn2+], germanium [Ge2+];
ment graphene and possess a smaller size, but it has a X is a halide such as iodide [I−], bromide [Br−], chloride
large surface to mass ratio. GQDs are easily dispersed due [Cl−]) as shown in Figure 5. The most notable single-halide
to the presence of the functionalized group at the edge perovskite materials are MAPbI3,115 MAPbBr3,116
make it favorable candidates to be integrated into the FAPbI3,117 CsPbI3,118 and RbPbI3.119 The origin of the PSCs
PSC device. The graphene-based materials become a started with the introduction of perovskite material in solid-
mover in the application of solar cells as an alternative to state DSSCs as a sensitizer. Kojima et al established the first
pristine graphene, which undergoes some chemical struc- perovskite material in solar cells with two different composi-
ture changes but still promotes high mechanical, ther- tions: methylammonium lead iodide (CH3NH3PbI3) and
mal, and electrical properties. Within graphene-based methylammonium lead bromide (CH3NH3PbBr3).21 Perov-
materials, rGO has shown significant advantages being skite is the active layer that acts as a light-harvesting mate-
used in the application of solar cells. Researchers are rial in the PSCs mechanism. In this study, CH3NH3PbI3
eager to study the effect of rGO since it has been reported shows impressive light absorption ability as it is capable of
to have high conductivity and carrier mobility compared capturing a broad spectrum of wavelengths up to 800 nm
to GO,75 in each of the components in PSCs system, compared with CH3NH3PbBr3. As such, comprehensive
including electrodes, ETL, HTL, and the absorber layer. research has been focused on CH3NH3PbI3, which recorded
Nevertheless, there is still numerous research exploiting a PCE close to 20%.120
GO and GQDs in the architecture of PSCs. In the follow- The organic-inorganic hybrid perovskite includes
ing sub-sections, this article discusses further the role of semiconductor of CH3NH3PbI3, CH3NH3PbBr3, and the

F I G U R E 3 A schematic
illustration of possible ways for the
preparation of graphene and rGO.67
rGO, reduced graphene oxide
[Colour figure can be viewed at
wileyonlinelibrary.com]
6 SAFIE ET AL.

F I G U R E 4 Schematic illustration of synthesis techniques to produce no-oxidation GQDs (no-ox-GQDs), low-oxidation GQDs (low-ox-
GQDs), medium-oxidation GQDs (med-ox-GQDs), and high-oxidation GQDs (high-ox-GQDs).71 GQD, graphene quantum dot [Colour figure
can be viewed at wileyonlinelibrary.com]

mixed halide form CH3NH3PbI3–xClx or CH3NH3PbI3– that can transport both electron and holes simulta-
xBrx are the most common perovskite material frequently neously, perovskite's film quality is also essential,
used for solar cell applications. The benefits of this class whereby morphology, grain size, and thickness are the
of materials as an active absorber layer include minimiz- factors that could determine the charges dynamics and
ing recombination losses,121 low costs of material,122 long device performance. Larger grain size and thicker perov-
charge carrier diffusion distances,123 and the potential of skite layers are desirable for the maximum light
cation and anion replacement for tuning the energy harvesting. Employing graphene-based materials into the
band.124 Despite having promising advantages, PSCs also perovskite solution is one strategy for controlling the
encounter several drawbacks to meet high efficiency, perovskite crystallinity toward larger grain size for effi-
large-scale production, low-cost materials and proce- cient charge transportation. Li et al76 introduced
dures, and also high stability devices.125 For instance, the graphene nanofibers into the perovskite (MAPbI3), which
standard known limit of PSCs is the perovskite material leads to improving nucleation and crystallization of the
itself. The crystallinity of perovskite was reported to have nanofibers. The crystal grain size of the perovskite layer
chemical instability under several conditions such as increases over two as depicted in Figure 6, which com-
exposure on moisture, light, heat, and oxygen from the prehends to higher PCE of 19.83%. The device stability
environment, which leads to depletion of photovoltaic shows that the presence of graphene nanofibers-based
performance.126 only reduces by 10.5% after 300 hours exposure in ambi-
ent conditions with relative humidity without the same
period. Besides, Chung et al77 emphasized the role of GO
2.1 | Integration of graphene-based embedded with perovskite wherein the optimum concen-
materials in perovskite material tration, GO is beneficial in improving the charge separa-
tion as it served as a hole acceptor and prohibit charge
A perovskite material is an essential layer in PSC device recombination, leads to higher device perfor-
architecture. Aside from having ambipolar properties mance (15.2%).
TABLE 1 Summary of the graphene-based PSCs in terms of photovoltaic performance and device performance stability

Photovoltaic performance
SAFIE ET AL.

Jsc
Device architecture PCE (%) (mA cm−2) Voc (V) FF Device performance stability Year Ref.
Perovskite
FTO/TiO2/MAPbI3 + G (nanofibers)/ 19.83 24.38 1.08 76 Retained 94% after 150 h and 89.5% of initial PCE after 300 h 2018 76
spiro-OMeTAD/Au under the relative humidity of 85 RH% and room temperature.
ITO/GO/ MAPbI3 + GO/PCBM/Ag 15.20 20.71 0.96 76 Retained nearly 80% of the initial device value on storage over 2017 77
2000 h under a relative humidity of 50%.
FTO/b-TiO2/m-TiO2/FA0.85MA0.15Pb 18.73 21.80 1.15 74 — 2016 78
(I0.85Br0.15)3 + N-rGO/spiro-
OMeTAD/Au
FTO/c-TiO2/m-TiO2/MAPbI3 17.62 22.49 1.03 76 — 2017 79
+ GQD/spiro-OMeTAD/Au
FTO/α-Fe2O3/MAPbI3 + NSGQDs/ 19.20 23.5 1.03 79 Humidity stability enhancement by 78% after 400 h and the 2020 80
HTL/Au thermal stability is increased by 84% after 300 h from the initial
PCE
ETM
TiO2
FTO/c-TiO2/m-TiO2 + G/perovskite/ 18.19 22.48 1.08 75 Retained more than 88% of its initial device efficiency value in the 2016 81
GO/spiro-OMeTAD/Au 16 h test period under prolonged exposure to light at the
maximum power point but losing more than 15% of the initial
PCE in prolonged heating test at 60 C in the oven.
FTO/c-TiO2/m-TiO2 + G/GO-Li/ 16.20 22.85 1.03 69 — 2017 82
MAPbI3/spiro-OMeTAD/Au
FTO/c-TiO2/m-TiO2 + G/MAPbI3/ Actual value is not stated Retained 93% of the initial performance after 1 week of aging 2019 83
spiro-OMeTAD/Gold under dry condition (HR = 30 C) and in the dark
FTO/c-TiO2/ m-TiO2 + G/ZrO2/ 13.60 22.84 0.98 62 — 2020 84
MAPbI3/C
FTO/c-TiO2/rGO + Li-m-TiO2/ 19.54 21.98 1.11 80 — 2016 85
(FAPbI3)0.85(MAPbBr3)0.15/spiro-
OMeTAD/Au
FTO/c-TiO2/rGO + TiO2/ 17.66 22.16 1.07 75 — 2019 86
(FAPbI3)0.85(MAPbBr3)0.15/spiro-
MeOTAD/Au
(Continues)
7
 8

TABLE 1 (Continued)

Photovoltaic performance

Jsc
Device architecture PCE (%) (mA cm−2) Voc (V) FF Device performance stability Year Ref.
FTO/c-TiO2/m-TiO2/GO-Li/MAPbI3/ 11.80 19.61 0.86 70 Improved stability shown by GO-Li ETL-based device under prolonged 2016 87
spiro-MeOTAD/Au illumination condition (100 mW cm−2) provided by a white light
emitting diode (LED) at room temperature and 60 h of aging test
FTO/NiO/GO/Perovskite/GO-Li/ 11.20 18.60 0.97 62 Investigated cells lost only 30% of their efficiency after a period of 2018 88
TiOx/Al 15 days under ambient conditions
ZnO
FTO/ZnO/G + ZnO/MAPbI3/spiro- 10.34 19.97 0.93 56 — 2017 89
MeOTAD/Au
FTO/NG + ZnO NR NCs/MAPbI3/ 16.82 21.98 1.02 75 2020 90
spiro-MeOTAD/Ag.
FTO/ZnO/MLG/MAFA 21.03 23.42 1.15 78 Only 7% PCE reduction under continuous illumination within 2019 91
Methylammonium- 300 h
Formamidinium-perovskite/
spiro-MeOTAD/Au
SnO2
FTO/G-SnO2/MAPbI3/spiro- 18.11 23.06 1.09 72 Retained 90% of the original PCE under 300 h exposure without 2018 92
OMeTAD/Au. encapsulation in ambient condition with humidity levels of
40 ± 5%.
EMMBF4/Ag/GNs/SnO2/C60-SAM/ 13.36 18.39 1.10 66 — 2018 93
MAPbI3/spiro-OMeTAD/Au.
ITO/SnO2:NGO/ 16.54 18.87 1.17 75 The addition of NGO does not affect the stability of the device 2020 94
Rb0.05(FA0.83MA0.17)0.95Pb(I0.83 measured under an ambient conditions.
Br0.17)3 + CsI/spiro-OMeTAD/Au
FTO/G@SnO2/CsFAMA-perovskite/ 19.60 23.50 1.08 77 — 2019 95
spiro-OMeTAD/Au
PEN/ITO/G5@SnO2/CsFAMA- 17.70 22.10 1.07 75 — 2019 95
perovskite/spiro-OMeTAD/Au
PCBM
ITO/PEDOT:PSS/ MAPbI3-xClx/rGO: 14.51 23.52 0.94 66 Retained more than 50% of its original efficiency after 50 h of 2017 96
PCBM/PEN/Ag prolonged solar illumination at a high rate of relative humidity
(RH) (>50%)
ITO/GQD:PCBM/MAPbI3/spiro- 17.56 22.03 1.09 73 The unpackaged cells can keep >80% of the initial PCE under 2017 97
OMeTAD/Au simulated sunlight with the full UV component present after
300 h
SAFIE ET AL.
TABLE 1 (Continued)

Photovoltaic performance
SAFIE ET AL.

Jsc
Device architecture PCE (%) (mA cm−2) Voc (V) FF Device performance stability Year Ref.
APTES-GR/PCBM:GQDs/Au 15.03 19.66 1.06 72 — 2019 98
HTM
Spiro-OMeTAD
ITO/TiO2/MAPbI3Cl3 − x/rGO/spiro- 18.75 21.50 1.11 79 Device with rGO shows higher stability while stored under 2017 99
OMeTAD/Au constant 1-sun illumination in a N2-filled glovebox
ITO/SnO2/MAFA-perovskite/rGO 18.13 23.05 1.11 71 Preserved 75% of its initial efficiency after storing 500 h in ambient 2020 100
+ spiro-OMeTAD/Au condition (40%-60% humidity)
PEDOT:PSS
ITO/ PEDOT:PSS:G/MAPbI3/PCBM/ Actual value is not stated Improved stability at room operation conditions 2020 101
Yb/Al
ITO/GO + PEDOT:PSS/ 14.20 20.01 0.90 79 — 2017 102
(FAPbI3)0.85(MAPbBr3)0.15/
PC61BM/BCP/Ag
ITO/PEDOT:PSS/GO/MAPbI3/ 15.34 21.92 0.94 75 Remained at 83.5% of the initial PCE values after aging for 39 days 2017 103
PCBM/Ag in air.
ITO/PEDOT:PSS/SrGO/MAPbI3/ 16.01 19.39 1.04 80 improved device stability under ambient environment condition 2020 104
PCBM/BCP/Ag according to the ISOS-D-1 protocol
CuSCN
ITO/rGO/CuSCN/MAPbI3/PCBM/ 14.28 18.21 1.03 76 Maintained 90% of its initial efficiency measured under continuous 2018 105
BCP/Ag AM 1.5 sun illumination for about 100 h
FTO/TiO2/CsFAMAPbI3–xBrx/ 19.22 22.65 1.09 75 Restrained up to 95% of its initial efficiency after aging for 1000 h 2017 106
CuSCN/rGO/Au under the full-sun illumination at 60 C
FTO/TiO2/CsFAMAPbI3-xBrx/ 15.20 21.30 1.01 71 Maintained more than 94% of its initial PCE under a relative 2020 107
CuSCN/G/Au humidity of 50% in dark condition
Graphene-based material as HTM
ITO/GO/MAPbI3/C60/Bphen/Ag 6.62 13.68 0.94 52 — 2017 108
ITO/a-GO/MAPbI3 − xClx/PCBM/ 14.14 18.40 1.00 77 Maintained 90% the initial PCE in a 30 days period under dry N2 2019 109
BCP/Ag atmosphere
ITO/rGO/MAPbI3/PCBM/BCP/Ag 10.80 15.40 0.98 72 Retained 62% from its original PCE after 140 h of exposure without 2015 110
encapsulation to ambient conditions with a humidity of
approximately 50%.
(Continues)
9
10 SAFIE ET AL.

Ref.
111

112

113
Year
2017

2016

2020
Retained 72% of the original PCE within 30 days at ambient conditions
without device encapsulation in a condition of 25 C and relative

Retained 50% of its original PCE without any encapsulation in

maintained half of its original PCE after 1000 h with device

(humidity of 50%, temperature of 25 C).

ambient atmosphere for 30 days

Device performance stability

FIGURE 5 ABX3 perovskite structure114 [Colour figure can be

viewed at wileyonlinelibrary.com]
humidity of 30%

Also, Hadadian and team78 introduced nitrogen-

doped rGO (N-rGO) into mixed-cation lead mixed-halide
(FA0.85MA0.15Pb(I0.85Br0.15)3 perovskite solution where
the nitrogen group from the N-rGO interact with the FA
cations from the perovskite materials providing enlarge
FF
77

76

51

grain size and thicker perovskite layer for better light-

harvesting properties. The N-rGO also act as surface pas-
Voc (V)

sivation of perovskite by improving the hole selection

0.96

1.01

0.80

and inhibit the charge recombination within perovskite

material by discovering the increment of Voc. Kim et al127
simplified the method by implementing pristine rGO into
Photovoltaic performance

(mA cm−2)

mixed-cation lead mixed-halide (included cesium, Cs)

perovskite solution also discovered high Voc in the pres-
22.10

19.10

8.00

ence of rGO by preventing the charge recombination

Jsc

within perovskite material.

Furthermore, Fang et al79 have fabricated mesoscopic
PCE (%)

PSC and incorporated GQD with perovskite solution to

16.40

14.70

3.28

passivate the electron trap in the perovskite surface

and attained PCE 17.62%. The impedance spectroscopy
results indicate a lower charge transfer resistance (Rct)
for the GQD-perovskite-based device, which means
ITO/F-rGO/MAPbI3/PC61BM/BCP/

that the GQD incorporation can promote the efficient

ITO/rGO/MAPbI3/PCBM/Ag

electron transfer of the perovskite/ETL interfacial

FTO/TiO2/MAPbI3/rGO/Au
(Continued)

layer. Faster electron extraction and slower recombina-

tion rate illustrate the significant function of GQDs in
Device architecture

the surface passivation in perovskite material. Alterna-

tively, Chen et al80 modifying N, S co-doped graphene
quantum dots (NSGQDs) into the perovskite and
TABLE 1

achieved humidity stability enhancement by 78% after

Ag

400 hours and the thermal stability is increased by 84%

after 300 hours from the initial PCE attained is 19.2%.
SAFIE ET AL. 11

F I G U R E 6 (A) and (B) Pristine perovskite layer before annealing. (C) and (D) Perovskite with graphene nanofibers before annealing.
(E) Pristine perovskite layer after annealing. (F) Perovskite layer with graphene nanofibers after annealing76

The role of NSGQDs is vital in facilitating perovskite 3 | ELECTRON TRANSPORT

crystalline growth, improving the charge transfer in MATER IAL
ETL/perovskite/HTL interface, and stimulating the
surface defect passivation by minimizing the charge The primary role of the ETL is to collect photogenerated
recombination. electron from the perovskite layer in PSCs efficiently. One of
12 SAFIE ET AL.

the main criteria suitable for acting as an ETM is one mate- be suitable candidates for this application. Moreover, to
rial that needs to have closer and lower conduction band enhance the properties of ETM, graphene-based mate-
minimum (CBM) to that CBM position of perovskite mate- rials were immediately introduced as part of the passiv-
rial for excellent electron injection. The most common ation and doping method. Graphene-based materials can
ETM used is mesoscopic TiO2, which is initially employed be either implemented as a buffer layer in both of the
as an electron collector in DSSCs128 and OSCs.129 Eventu- charges transporting components since they are capable
ally, pristine TiO2 with a compact layer is introduced to of transferring both electron and holes or known as hav-
match with planar device architecture. Despite well-known ing bipolar properties. The function of the buffer layer is
efficient injection rates in perovskite/TiO2 interface, low as a mediator that could help to enhance charge selectiv-
electron mobility (0.1-4.0 cm2 V−1 s−1), instability under ity, hole blocking capacity, and as well as inhibit the
ultra-violet (UV) illumination exposure, and high- charge recombination in PSC devices. Graphene-based
temperature treatment procedure have limited the used of materials are capable candidates for charge transporta-
pristine TiO2 in ETL.130 Hence, few inorganic materials tion materials, especially for mass-scale industrial pro-
explored to encounter the TiO2 drawbacks such as ZnO,131 duction concerning the other ETM with several proven
tin oxide (SnO2),132 zinc stannate (Zn2SnO4, ZSO),133 and as higher carrier injection capabilities in previous works.
well as organic materials including PCBM134 which gener-
ally offered higher electron mobilities than pristine TiO2.
Having slightly higher electron mobility (5-8 cm2 V−1 s−1) 3.1 | Integration of graphene-based
and higher CBM as compared with TiO2, make SrTiO3 materials in ETM
another potential ETM. In addition, SrTiO3 has high dielec-
tric constant,135 which could inhibit the recombination at 3.1.1 | TiO2
the ETL/perovskite interface, but the reported device effi-
ciency is still low and needs to be improved. Besides bulk Despite being the most favored ETM due to its capabil-
electron mobility (240 cm2 V−1 s−1), SnO2 shows better sta- ity in hole blocking with its deep VB position, closer CB
bility because of its high transparency in the UV light edge positions to the perovskite, and longer electron
range.136 However, high quality in the mesoscopic structure lifetime, TiO2 is reported to be unstable and tend to
of SnO2 still needs to be explored to reduce recombination decompose under full radiation spectrum exposure.
that will cause hysteresis in the J–V curve. ZnO, on the other Once exposed to UV light, photogenerated holes in TiO2
hand, promotes high structural quality with solution- react with oxygen adsorbed at surface vacancies thus
processed in low temperature,137 which is favorable for the acting as deep traps, which further leads to recombina-
large scale required in the industry. Even so, passivation and tion limiting the device efficiency and stability upon UV
or doping method is crucial for the ZnO-based device to light exposure. Agresti et al81 have tested the impact of
achieve stability and excellent efficiency since the graphene (G) adsorb onto TiO2 to the stability of PSC
ZnO/perovskite interface would suffer from the unwanted under certain conditions. The insertion of G prepared by
reaction between ZnO and protons in the CH3NH3+, which liquid-phase exfoliation of graphite flakes in the meso-
lead to decomposition of the perovskite material. porous TiO2 (mTiO2) scaffold exhibits significant long-
Alternatively, ZSO offers excellent chemical stability term stability of the device under prolonged exposure to
concerning polar organic solvent and acid/base solution light at the maximum power point. Adsorption of G in
for solution processing.138 The highly porous ZSO offers mTiO2 + G ETL-based device proving favorable
better nucleation and crystallization of perovskite mate- mTiO2 + G/perovskite interface interactions after suc-
rial by improving its grain size uniformly, which leading cessful retention of more than 88% of its initial device
to negligible hysteresis. On the other hand, PCBM is one efficiency value in the 16 hours test period. The positive
of the fullerene derivatives commonly used as organic stability impact may occur because, pristine G in contact
ETM, especially in inverted planar architecture. As com- with perovskite could lead to interfacial ferroelectricity,
pared with the inorganic ETM, PCBM is promising ETM which offers efficient electron collection. It also pushes
due to its capability to passivate the interfacial defect and the hole wave functions away from G to inhibit
reduce the density of trap states of the perovskite layer, electron-hole recombination across the mTiO2 + G/
which leads to the hysteresis-free device. Despite that, perovskite interface. The Jsc improvement observed by
PCBM has intrinsic shortcomings that need to be the increasing trend in incident light intensity (Pinc)
resolved because its deficient adhesion in PCBM/perov- concerning the pristine TiO2 ETL-based device revealed
skite interface may cause molecular aggregation.139 an improvement in the selection of charge injections
Graphene and its derivatives have exhibited high would enhance the device system's stability under pro-
mobility and conductivity and, therefore, considered to longed light exposure (see Figure 7).
SAFIE ET AL. 13

F I G U R E 7 Normalized (A) Voc, (B) Jsc, (C) FF, and (D) PCE trends versus time under prolonged 1 sun illumination at maximum power
point (MPP) polarization provided by a calibrated white light emitting diode (LED) for devices A-D (A: without graphene; B: GO-based
device; C: G-doped TiO2-based device; D: GO/G-doped TiO2-based device).108 GO, graphene oxide; PCE, power conversion efficiency; TiO2,
titanium dioxide [Colour figure can be viewed at wileyonlinelibrary.com]

In 2017, Biccari et al82 also employed graphene thereby beneficial in slowing down the degradation reac-
flakes-doped TiO2 (G + mTiO2) in ETL with the same tions of perovskites, which could eventually inhibit the
preparation procedure of liquid-phase exfoliated device efficiency. In contrast, Agresti81 reported
graphene, discovered an increment of electron injection mTiO2 + G ETL-based device showed the fastest degra-
efficiency from the perovskite layer to the G + mTiO2 dation rate after losing more than 15% of the initial
ETL. The value of decay time constant, τ analyzed from device efficiency value compared with pristine TiO2-
photoluminescence (PL) intensity, decreasing from 25 to based device in prolonged heating test at 60 C in the
15 nanoseconds after employing the G-doped mTiO2, oven. The results obtained indicate that prolonged ther-
which convey electron collection was improved through mal stress may permanently degrade the favorable inter-
G + mTiO2/perovskite interface as compared with a actions between graphene and perovskite by possessing a
device without G inserting prepared in this work. Having burn-in effect on the performance of the device.
low electron mobility could result in charge accumula- Understanding the ultrafast dynamics of hot-carriers,
tion in the pristine TiO2/perovskite interface, and degra- especially in the ETL/perovskite interface, is also crucial
dation of perovskite will occur on the aged device to choose great ETM for highly stable PSCs. O'Keeffe
because the active layer has been altered with the reac- et al83 reported employing graphene flakes-doped TiO2 as
tion of TiO2. Whereas after the mTiO2 layer is doped with an ETL with a device configuration: Glass + FTO/c-
G flakes, the enhanced load injection/transport at the TiO2/m-TiO2 + Graphene/CH3NH3PbI3/Spiro-OMeTAD/
perovskite/ETL interface has significantly reduced the Gold (PSC-G) could preserve the stability and hence
charge accumulated at the TiO2/perovskite interface, improve device efficiency through hot-carriers'
14 SAFIE ET AL.

temperature properties. The effect of the carrier tempera- Agresti et al87 constructed lithium-neutralized GO
ture stability is analyzed based on femtosecond transient (GO-Li) deposited in the mesoscopic structure of PSCs.
absorption (TA) measurements, which deliver informa- The GO-Li ETL devices showed a slight increment in Jsc
tion on the carrier dynamics within perovskite nano- (19.61 mA cm−2) and device PCE (11.8%) than the refer-
crystals by differentiating the small ETL crystals from the ence device (without GO-Li) (Jsc: 17.7 mA cm−2; PCE:
large capping layer crystals spectroscopically. Both as- 10.3%). However, the Voc reading (0.859 V) showed a
prepared graphene-free and graphene-containing PSCs reduction, which indicates that the electron transport in
undergo an aging test for a week initially shows identical the TiO2/GO-Li interface facilitated the mismatching of
values of hot-carrier temperatures; however, the aged energy levels between TiO2 (4.23 eV) and GO-Li (4.3 eV).
graphene-free PSC obtained lower carrier temperatures The presence of Li was expected to tune the work func-
as opposed to the aged graphene-containing PSC. The tion of GO closer to the CB of TiO2, but the work func-
carrier temperature stability reflects the nanocrystals' sta- tion obtained was not correctly matched in the device
bility in the perovskite embedded in the graphene-TiO2 system. Moreover, the PL intensity of TiO2/GO-Li/perov-
layer of mesoporous. Through using TiO2/graphene skite showed better quenching effect interpreted as the
nanocomposites as a mesoporous ETL, the goal is to efficient electron-hole transportation from the perovskite
enhance the transfer of charges and the aggregation of layer to the respective charges collector, which is parallel
electrons, thus reducing the load trapping and recombi- to the increment of Jsc value obtained by the GO-Li ETL
nation that can occur on the TiO2 surface. devices. However, Nouri et al88 employed lithium modi-
Recently, Yang et al84 discovered that an overloaded fied GO (GO-Li) in inverted planar PSCs. The maximum
ratio of GO in TiO2 shows a reduction in device perfor- device PCE achieved was 11.2% by adding a thin layer of
mance. The optimum ratio acquired in this study is 1 wt a transparent sol made of TiOx (Ti-based sol) to match
% GO loaded in TiO2, where the PCE obtained is 13.60% the energy level between GO-Li and perovskite layer. The
with the highest Jsc (22.84 mA cm−2) compared to other charges transfer dynamic, and device stability was not
ratios studied in this work. The GO-TiO2 nanocomposites discussed further in this work. Even though the efficiency
prepared by the sol-gel method, in this case, provide bet- achieved is lower compared with the reference device
ter interaction with perovskite materials as the grain size (where hole and transport layers are PEDOT:PSS and
of crystallite perovskite enhanced after being deposited PCBM, respectively), the investigated device shows lon-
on the GO-TiO2 nanocomposites film. The quenching ger time device stability.
ability was not studied further in this work in order to
discuss the charge dissociation. Nonetheless, the perfect
match energy level obtained by GO-TiO2 nanocomposites 3.1.2 | Zinc oxide
(4.0 eV) closer to the CB of perovskite (3.8 eV) summa-
rized the excellent interaction between GO-TiO2/perov- Researchers have started operating on ZnO as an ETL for
skite interface to provide efficient charger transfer as well PSC applications as an alternative to the TiO2 in which
as improved the Jsc in the device.
Furthermore, Cho and co-workers85 suggested the
implementation of rGO in lithium-treated mesoporous
TiO2 (Li-mTiO2). The lithium-treated is beneficial as its pas-
sivated surface traps in the TiO2 layer, improving electron
injection significantly. The best device performance
recorded by incorporation of rGO in Li-mTiO2 yielded a
higher PCE of 19.54%, which pointed out the beneficial
effect of rGO in enhancing the electron extraction from the
perovskite layer. Parallel with this fact, the PL decays
showed better quenching ability with the incorporation of
rGO in the device imputed efficient and faster electron
transfer from the perovskite interfaces. However, further
device stability and film morphology are not discussed in
this case. Similarly, Patil et al investigated the incorporation F I G U R E 8 Schematic illustration of PSC having an
of annealed rGO with the same architecture but further architecture of FTO/N-doped graphene-ZnO NR composite/
study on perovskite morphology showed the crystal grain of CH3NH3PbI3/Spiro-OMeTAD/Ag.90 FTO, fluorine-doped tin oxide;
perovskite material enhanced after being deposited on the PSC, perovskite solar cell; ZnO, zinc oxide [Colour figure can be
rGO-TiO2 composites yielding a high PCE of 17.66%. viewed at wileyonlinelibrary.com]
SAFIE ET AL. 15

ZnO retains UV exposure stability. In addition, ZnO has enhancement of conductivity. According to the PL
few advantages to offer including lower temperature quenching, reduction of PL intensity achieved by G/ZnO
treatment, which is applicable for flexible devices as well and NG/ZnO nanocomposites compared to pristine ZnO
as better electron mobility as compared with TiO2. depicted efficient charge carrier extraction in the
Despite this, ZnO-based PSCs still possess lower PCE ETL/perovskite interface. Strong quenching is reflected
than TiO2. ZnO often overlooked owing to chemical in increased electron density of state, providing efficient
instability when in interaction with perovskites material. high electron extraction from the perovskite film, thereby
It is deplorable that ZnO prepared using a solution pro- inhibiting charge carrier recombination.
cess could eventually degrade perovskite crystal structure In both of these works, the optimum concentration
into insulating lead halide due to deprotonation of for graphene doped ZnO is 0.75% and 0.8% to achieve bet-
methylammonium with a hydroxyl group on the surface ter PCE. Higher concentration, however, shows decre-
of ZnO. Along with TiO2, interfacial defects of the ZnO ments in all of the photovoltaic performance which may
layer also contribute to the charge accumulation, which be due to the accumulation of G in the ETL/perovskite
leads to a higher recombination rate that inhibits the interface that would induce direct contact of the perov-
device performance. Introducing doping to the ZnO skite film with overabundance G and results in low elec-
structure could be beneficial to inhibit the defects and, at tron collection efficiency, hence, led to a slight decrease
the same time, improve the electron collection efficiency. in short-circuit current density (Jsc). Tavakoli et al91 dem-
Chandrasekhar et al89 incorporating different onstrated monolayer graphene (MLG) as an interfacial
graphene concentrations onto ZnO as ETL in MAPbI3- layer on top of the ZnO ETL to impede the chemical reac-
based device via a simple spray deposition method and tion within the ZnO/mixed-halide perovskite interface.
obtained the highest PCE of 10.34% for the optimum con- The inclusion of MLG at the ZnO/perovskite interface
centration of 0.75 wt% G-ZnO nanocomposites. Later in was found to be enhanced not only with carrier extrac-
2020, Chandrasekhar et al90 modified integration of G in tion and photovoltaic properties but also in prohibiting
as-prepared ZnO nanorod ETL by introducing nitrogen- the perovskite film from decomposition at high tempera-
doped graphene (refer Figure 8) using a hydrothermal tures. The device performance shows negligible hysteresis
treatment which produced PCE 16.82% with optimum with a PCE of 21.03% and improved stability with only
concentration of 0.8 wt% NG-ZnO NR NCs. Along with 7% PCE reduction under continuous illumination within
the increment of graphene's concentration adsorbs in 300 hours. Such findings strongly highlight the role of
ZnO, the perovskite crystal grain size improved, which MLG on the interface in suppressing light-activated reac-
indicates that the graphene is playing an essential role in tions over time.
the better growth of perovskite thin film. The high sur-
face area of the graphene sheets in the nanocomposite
improved the perovskite anchoring around the graphene, 3.1.3 | Tin oxide
which aided excellent grain growth and enhanced
absorption. Achieving a larger grain size for the active As compared to other metal oxides, including ZnO and
layer is crucial in order to enhance charge collection effi- TiO2, SnO2 offers excellent properties to be a better ETL
ciency. Significantly larger perovskite crystals on G-ZnO such as bulk electron mobility and low trap density due to
and NG-ZnO nanocomposites efficiently disperse the wide optical bandgap (3.6-4.0 eV) which results in better
incoming light, thereby increasing the efficiency of pho- stability under illumination exposure. Despite this, SnO2-
ton recycling. Large crystals can divert the light from off- based PSCs encounter a stability issue whenever FTO is
normal and thus improve the valid optical path of the used as a substrate, because the transfer of fluorine to
device. SnO2 may significantly reduce the electron selectivity of
As a result, efficient charges collections and improved SnO2, hence, make it favorable ETL candidate for planar
mobility with the presence of the graphene network in and flexible PSC device. However, the elevated annealing
the ETL and, to some extent, forward light scattering temperature at 150 C to 200 C for crystalline SnO2 films is
with the nanocomposites are responsible for improving still a task-driven to their application on flexible transpar-
the device parameter such as Jsc and PCE. Besides, the ent substrates, such as PET and polyetherimide (PEI) as
presence of nitrogen as an n-type dopant in the graphene well as ITO substrate for planar architecture to replace
structure will fill in carbon vacancies, which are a com- FTO. Hence, considerable attention at improving other
mon possible defect that could exist in the graphene solution methods with lower-temperature procedures is
sheet. The NG-ZnO nanocomposites successfully reduce necessary. Nevertheless, there are still some surface con-
the series resistance across the ETL/perovskite interface flicts between the SnO2 and the perovskite layers linked to
due to efficient electron injection, which leads to the interface charge's recombination. While many of the
16 SAFIE ET AL.

F I G U R E 9 (A) J−V curves of the best-performing PSCs based on SnO2 and SnO2-graphene measured at the reverse, and forward scans;
(B) EQE spectra of the best-performing cells based on SnO2 and SnO2-graphene; (C) stabilized PCEs measured as a function of time for the
SnO2 and SnO2 graphene-based devices biased at their respective Vmp 0.925 V and 0.872 V, respectively, and (D) normalized time-resolved
photoluminescence response of the perovskite film fabricated on SnO2 and graphene-SnO2 ETLs.84 ETL, electron transport layer; PCE,
power conversion efficiency; PSCs, perovskite solar cells; SnO2, tin oxide [Colour figure can be viewed at wileyonlinelibrary.com]

SnO2 ETLs produced with a low-temperature method, exposure without encapsulation in ambient condition
there are often some trap states in the SnO2 developed by with humidity levels of 40 ± 5%.
oxygen vacancy, which may lead to the hysteresis of the Liu et al,93 on the other hand, have fabricated flexible
device photovoltaic performance. PSC devices and modified incorporation of weakly oxidized
Zhu and co-workers92 demonstrated graphene pre- graphene nanosheets (GNs) as a dopant into SnO2 (SnO2/
pared by mechanical exfoliation from graphite flakes as GNs) as ETL and obtained PCE 13.36% with negligible hys-
the dopant in SnO2 ETL for planar device architecture teresis. Since the SnO2 annealed with a low temperature in
and achieved over 18% PCE with attenuated hysteresis. flexible PSC fabrication, the tendency to cause charge
The calculated hysteresis index showing only 0.02 for G- recombination at ETL/perovskite interface by electron
SnO2 ETL compared to 0.08 for pristine SnO2-based transport obstruction is high due to the poor SnO2 crystal-
devices. Efficient electron transfers within the G-SnO2/ linity. The insertion of GNs with a better conductive net-
perovskite interface are correlated to the J–V hysteresis work into SnO2 promotes efficient electron mobility in the
(refer Figure 9) as the electron transfer rate in G-SnO2 ETL to ensure faster electron injection. Also, the CB of
ETL-based device obtained is significantly faster than SnO2 (−4.20 eV) and GNs (−4.22 eV) are satisfying the
that of pristine SnO2 ETL-based device. Besides, the inte- energy alignment of the device structure, which could hin-
gration of G-SnO2 nanocomposites is not only beneficial der the charge recombination and contributes to the selec-
for efficient charge transportation, but the PSC device tion, injection, and transport of photogenerated charges.
also shows improvement in device stability after success- The oxygen vacancy in the SnO2 layer must be con-
fully retaining 90% of the original PCE under 300 hours trolled, as the electrical and optical properties vary
SAFIE ET AL. 17

depending on the oxidation states of Sn in order to lightweight and thin ETLs for highly effective PSCs. The
encounter trap state issues, which could cause charges introduction of GQDs, which have a smaller size than
recombination and device hysteresis. Hong and team94 graphene sheets, is favorable to fill the electron trap in
proposed the doping method via incorporation of nitro- SnO2 structure, which eventually could improve the con-
gen GO (NGO) to SnO2 ETL as an oxidizing agent for ductivity of SnO2.140 Zhou et al95 fabricate GQD/SnO2 com-
controlling the SnO2 oxidation state to increase its electri- posites (G@SnO2) as an efficient ETL and achieved PCE
cal conductivity. NGO is capable of passivating the oxy- 19.6% for planar and 17.7% for flexible mixed-cation lead-
gen vacancies in SnO2 by switching the oxidation state of halide perovskite-based device. Besides increment of film
Sn in SnO2 from Sn2+ to Sn4+. The attenuation of the conductivity upon optimizing concentration and size of the
photogenerated excitation lifetime of the perovskite layer GQD, tunable CB closer to perovskite and G@SnO2 ETL
conveyed efficient electron injection to the SnO2:NGO film homogeneity tremendously propitious for inducing the
ETL due to enhanced electrical properties. SnO2:NGO electrons mobility and limiting the recombination within
ETL device obtained high PCE by exceeding 16% with ETL/perovskite interfacial, resulting in a substantial
almost no hysteresis with optimum concentration of improvement in photovoltaic efficiency.
NGO (5 vol%). Despite showing beneficial efficient char-
ges transportation, the perovskite film morphology and
device stability under ambient condition showing no sig- 3.1.4 | [6,6]-Phenyl-C61-butyric acid
nificant difference with the integration of NGO into SnO2 methyl ester
compared to pristine SnO2 ETL-based device.
Substituting massive graphene flakes with smaller PCBM is fullerene-based electron acceptors that require
graphene nanodots is highly desirable to achieve simple solution and low-temperature procedure shows

F I G U R E 1 0 Performances of the devices with PCBM and PCBM:GQDs ETL under continuous solar illumination in the glove box and
measured in air (45% humidity).90 ETL, electron transport layer; GQD, graphene quantum dot; PCBM, [6,6]-phenyl-C61-butyric acid
methyl ester [Colour figure can be viewed at wileyonlinelibrary.com]
18 SAFIE ET AL.

UV-light stability, make it attractive alternative ETL in attenuation of charge recombination. However, concen-
PSC device to replace inorganic transport materials often tration higher than that is reported to reduce the electron
used for the planar device architecture. PCBM, which has mobility as the excessive amount of GQDs may eventu-
small molecules, is effectively passivating deep traps at ally increase the defect in the PCBM layer, leads to ineffi-
the surface and grain boundaries of perovskite material, cient charges transportation and inhibit the device
thereby effectively collect photogenerated electrons from performance.
the perovskite material, thus inhibiting the possible
charge recombination and device hysteresis. However,
PCBM also encounters few drawbacks that could limit 4 | H O L E T R A N S P O R T MA T E R I A L
device performance, including low electron mobility
(6.1 × 10−2 cm2 V−1 s−1) and low conductivity meanwhile The primary function of the HTM in the PSC device is to
under thermal annealing, PCBM which unveil weak selectively extract holes after separated from the
adhesion with perovskite film may cause molecular photogenerated electron within perovskite materials and
aggregation and induced degradation of device transfer the holes to the counter electrode. One of the
performance. criteria to be selected as HTM is to have high conductiv-
Alternatively, Kakavelakis et al96 revealed the incor- ity to suppress device series resistance (Rs) and increase
poration of rGO with PCBM in inverted planar architec- the fill factor (FF) to improve the photovoltaic perfor-
ture yielding a device PCE of 14.51%. The quenches PL mance. Generally, HTMs can be either inorganic and
intensity, which indicates improved charge injection organic molecules, and the integration of graphene-based
from the perovskite layer to the ETL, showed better materials are tremendously discovered as an alternative
quenching effect on the rGO doped PCBM ETL compared to the low cost and less acidic charge selective materials.
to pristine PCBM ETL. The efficient charges transfer
proved the conductivity increment measured in this work
where the rGO-doped PCBM film obtained higher con- 4.1 | Integration of graphene-based
ductivity (0.495 ± 0.001 m S cm−1) compared to pristine materials in ETM
PCBM (0.109 ± 0.005 m S cm−1). Besides, the rGO-doped
PCBM ETL device was capable of retaining more than 4.1.1 | Spiro-OMeTAD
50% of its original efficiency after 50 hours of prolonged
solar illumination at a high rate of relative humidity Spiro-OMeTAD is organic HTM with small molecules
(RH) (>50%) indicate that incorporation of rGO as a dop- that are remarkably the most common HTM used and
ant in PCBM ETL is possible to improve the device stabil- remains one of the great HTM that contributes high PCE,
ity. On the other hand, Yang and team97 proposed especially in mesoscopic device architecture. However,
integrating GQDs with PCBM reported PCE 17.56% with pristine Spiro-OMeTAD, which has a basic triangular
free hysteresis in planar device architecture. The overall pyramid configuration, may cause vast intermolecular
photovoltaic performance of PCBM:GQDs ETL-based distances that result in low charge mobility and poor con-
device shows improvement compared to pristine PCBM ductivity. Therefore, Spiro-OMeTAD often modified by
ETL-based device due to increment of PCBM:GQDs con- adding the additive such as 4-tert-butylpyridine (TBP)
ductivity from 0.151 to 0.422 m S cm−1 and electron or/and bis(trifluoromethane)sulfonamide lithium salt
mobility. (Li-TFSI) as a dopant to improve its conductivity, but the
Besides, the device stability is well monitored in this presence of these additives facilitates degradation to the
work under UV irradiation and ambient condition show- perovskite material that leads to reduced stability device.
ing stability device with GQDs doping as depicted in One suitable approach for improving the reliability of
Figure 10. Upon exposure to light, the PCBM structure PSCs while retaining high performance is integrating
tends to dimerize. It would initiate deep traps whereby functional materials into the HTM. Li et al99 demon-
the incorporation of GQDs beneficial to fill in the traps to strated the passivation surface method by implementing
suppress the negative effect of PCBM dimerization in functionalized rGO interfacial layer in between perov-
device performance, leads to high stability device either skite and Spiro-OMeTAD to inhibit recombination within
in ambient condition or under UV light exposure. Shin perovskite/HTL interface, therefore beneficial for
et al98 also discovered the importance of studying the enhancing hole extraction kinetics. The device yielded
optimized concentration of GQD:PCBM composites ETL. PCE 18.75% with Voc improvement. The functionalized
Higher PCE obtained is 15.03% on a flexible PSC device group may passivate the traps on the surface of perov-
for an optimized concentration of GQDs into PCBM, skite material and reduce the Voc losses that inhibit
which is 2.5 mg/L due to enhance ETL conductivity and device performance.
SAFIE ET AL. 19

Kim et al127 and Suragtkhuu et al100 have studied the slower rate of recombination and better charges mobility
device stability effect by integrating rGO as a dopant in in the perovskite/HTL interface. Luo et al,103 however,
the Spiro-OMeTAD HTL. Kim127 investigated the roles of employing GO interfacial layer between PEDOT:PSS and
rGO in device stability under thermal stress. The PbI2 perovskite material as a strategy to improve the moisture
peak from perovskite material is less shown in the rGO- resistance and reduce the contact barrier at the surface of
doped HTL device, which indicates that the incorpora- PEDOT:PSS. The distribution of GO layer may prohibit
tion of rGO successfully suppressed the perovskite degra- the hydrophilicity of PSS material within the PEDOT:
dation by acting as a passivation layer. Besides, rGO PSS/perovskite interface as well as enhancing the wetta-
presence in the spiro-MeOTAD layer also protected spiro- bility of GO/PEDOT:PSS HTL in order to provide better
MeOTAD from being crystallized at high temperatures. crystallinity of perovskite film with attenuate pinholes,
Meanwhile, Suragtkhuu100 observed a significant thereby improving charge transportation and material
enhancement in the device stability under the ambient stability in the PSC device. GO/PEDOT:PSS HTL device
condition for 500 hours where rGO + Spiro-OMeTAD- achieved better PCE 15.34% and 39 days aged device suc-
based HTL retained 75% of its initial PCE due to rGO cessfully retains 85% of its initial PCE compared to pris-
hydrophobic properties that may promote protection for tine PEDOT:PSS HTL device.
perovskite materials from the surrounding moisture Mann and coworkers104 suggested a different
which could lead to degradation. As a result, the PSCs' approach by treating the rGO with sulfonic acid and for-
moisture and thermal stability were significantly ming a bilayer HTL with PEDOT:PSS. The PEDOT:PSS/
improved by the use of rGO in the perovskite and spiro- SrGO interface sandwiched in the inverted planar PSCs
MeOTAD layers, given that the overall morphology of shows an increment in device performance with a PCE of
perovskite and spiro-MeOTAD was not significantly 16.01%. The SrGO integration is successfully tuning the
altered by rGO implementation. energy level of PEDOT:PSS single HTL from 5.02 to
5.34 eV which is closer to the VB of perovskite material
(5.4 eV) concludes the enhancement of charge transfer
4.1.2 | Poly(3,4-ethylenedioxythiophene) and reduction of recombination in the device interface.
polystyrene sulfonate The hydrophobic properties of SrGO are also acknowl-
edged to improve device stability. In comparison with
3,4-Ethylenedioxythiophene-polystyrene sulfonic acid is PEDOT:PSS single HTL, the PEDOT:PSS/SrGO bilayer
polymer-based organic HTM that offers excellent proper- HTL appeared to show better device stability and provide
ties including high work function (5.2 eV), high trans- protection for the PEDOT:PSS, known to be highly acidic
parency, and possess low-temperature (<100 C) method. and tends to decompose ITO electrode. In the year 2020,
PEDOT:PSS is mainly used for an inverted planar device Redondo-Obispo et al101 modified a simple low-
as it provides a smooth surface on the ITO electrode and temperature method for the distribution of graphene-
is compatible with a flexible PSC device. Nevertheless, doped PEDOT:PSS HTL in an inverted planar device. As
PEDOT:PSS solution possesses high acidity, which will a result of this dopant method, not only the PEDOT:PSS
limit PSC device performance and stability due to corro- layer improves its conductivity, but also the perovskite
sion of ITO electrodes and perovskite material, which surface is affected by increasing the size of the perovskite
suffers from sensitivity to the PEDOT:PSS acidity. Also, a crystalline layer and reducing the presence of PbI2. This
more substantial energy barrier between perovskite and technique eliminates the adverse reactions of PEDOT:
PEDOT:PSS layer makes it at a disadvantage where the PSS, the hygroscopic and acid aspect of the PSS material.
charge extraction and electron blocking ability are
prohibited. Incorporation graphene-based materials into
PEDOT:PSS are tremendously investigated in order to 4.1.3 | Copper (I) thiocyanate
enhance PSC device stability and charges mobility.
Conversely, Niu et al102 implemented GO doping in CuSCN is the cheapest inorganic HTM that is abundantly
PEDOT:PSS HTL in inverted planar mixed-halide PSCs. available and exhibit great properties such as high hole
The authors prepared different concentrations of GO- mobility as well as good thermal and moisture stability.
doped PEDOT:PSS and reported that the best fit concen- Nevertheless, CuSCN have permeable properties, which
tration of GO is 500 μL, which yields PCE 14.20%. In this somehow limit the device stability as it could not hinder
case, the GO-doped rose on PEDOT:PSS hole mobility the moisture diffusion and ion migration. Chowdhury
from 5.55 × 10−5 cm2 V−1 s−1 to 1.57 × 10−4 cm2 V−1 s−1 et al105 implemented the bilayer HTL concept using
due to better quenching effect with improved average untreated rGO with CuSCN via low-temperature process
decay time, τ2 (67.85 nanoseconds) which indicates a in the inverted planar PSCs architecture and yields PCE
20 SAFIE ET AL.

F I G U R E 1 1 Scanning electron microscope (SEM) top view images of MAPbI3 films on: (A) ITO/rGO; (B) ITO/rGO/CuSCN;
(C) steady-state PL measurement of the CH3NH3PbI3 on top of glass, ITO/rGO, and ITO/rGO/CuSCN; and (D) energy level diagram of the
fabricated PSC.105 CuSCN, copper (I) thiocyanate; ITO, indium tin oxide; PL, photoluminescence; PSC, perovskite solar cell; rGO, reduced
graphene oxide [Colour figure can be viewed at wileyonlinelibrary.com]

14.28%. According to this study, the rGO incorporation the inverted planar PSCs. The optimum thickness of
influenced the device stability by maintaining 90% of its CuSCN reported in this work appeared to be 10 nm, which
initial efficiency measured under continuous AM 1.5 sun influences the reduction of series resistance, Rs to 4.9 Ω
illumination for about 100 hours. Initially, Arora et al106 from 9.7 Ω obtained by rGO single HTL. The rGO/CuSCN
first discovered the stability enrichment of PSCs device bilayer HTL provides good interaction with the perovskite
with the addition of rGO spacer layer in between inor- material which allowed growing larger crystal grain size
ganic HTL, CuSCN and gold (Au) electrode but in the and shows a perfect quenching effect on the PL intensity
mesoscopic architecture. The device performance with compared to the rGO single HTL, that can be seen in
CuSCN/rGO/Au interface was reported to restrain up to Figure 11. In addition, the perfect match of energy align-
95% of its initial efficiency after aging for 1000 hours ment provided by rGO/CuSCN bilayer HTL device utilizes
compared to CuSCN/Au interface device that showed the device performance. In this case, the photovoltaic
poor photostability by losing 50% of its initial efficiency parameters show improvement correlates with the posi-
in a day under the full-sun illumination at 60 C. This tive effects on the perovskite morphology that contributes
study revealed that thiocyanate anions were forming to the efficient charges transfer and reduces recombina-
undesired interaction with gold electrode layer that leads tion possibilities in the HTL/perovskite interface.
to device degradation. However, CuSCN/rGO interface Moreover, Lee et al107 developed a new transfer
behavior in the device system was not fully analyzed in method for CVD-synthesis graphene to be implemented
this study. as an interfacial layer between perovskite material and
Chowdhury and the team105 discovered the impact of CuSCN in mixed-cation lead-halide perovskite-based
the rGO integration with CuSCN to form bilayer HTL in device and achieved PCE 15.2%. The atomically thin
SAFIE ET AL. 21

impermeable graphene acts as a barrier layer that would graphene-based materials as an interfacial layer between
impede the Au electrode diffusion, I− ion migration, and perovskite and electrode in HTL-free device where the
moisture from degrading the device stability, thereby graphene-based materials could act as a stand-alone
CuSCN/G/Au-based device successfully maintaining HTM. Sun et al108 employed GO sheets in between ITO
more than 94% of its initial PCE under a relative humid- and perovskite layer for an HTL-free inverted planar
ity of 50% in dark condition. Also, graphene possesses device and achieved PCE 40% higher than without GO-
high hole mobility act as an excellent conductive barrier based device. The presence of the GO layer also acts as a
where the CuSCN/G/Au-based device can enable selec- nucleation site that provides better crystalline growth for
tive hole transport by the carefully aligned energy band the perovskite material, leads to efficient charge mobility.
within the perovskite/HTL interface. With this, it is a Wang et al109 proposed a different approach by treating
fundamental criterion to understand the nature of the the GO with ammonia to produce non-corrosive HTL
material used in the device architecture in order to fabri- denoted by a-GO in inverted planar PSCs and achieved
cate high efficiency and stability PSC device. PCE 14.14%. The presence of ammonia was expected to
suppress the acidic nature of pristine GO which is
destructive to the stability of the device. The ammonia-
4.1.4 | Graphene-based material acts treated GO-based device showed <10% PCE loss mean-
as HTM while the pristine GO HTL reported >20% PCE loss
under dry N2 atmosphere in 30 days. Also, the energy
Other than integrating the graphene-based materials as a level measured for a-GO (5.3 eV) ideally tuned closer to
dopant or passivation material in HTL/perovskite inter- the VB of perovskite material (5.4 eV) which postulates
face, tremendous studies suggested implementing the effective hole transfer in perovskite/a-GO interface.

F I G U R E 1 2 SEM images of CH3NH3PbI3 films on: (A) glass/ITO/PEDOT:PSS; (B) glass/ITO/rGO; (C) time-resolved PL characteristics
of the CH3NH3PbI3 films based on different HTMs; (D) photoluminescence responses of CH3NH3PbI3 films on glass/ITO/PEDOT:PSS, glass/
ITO/GO, and glass/ITO/rGO.110 HTM, hole transport material; ITO, indium tin oxide; PEDOT:PSS, poly(3,4-ethylenedioxythiophene)
polystyrene sulfonate; rGO, reduced graphene oxide [Colour figure can be viewed at wileyonlinelibrary.com]
22 SAFIE ET AL.

Yeo et al110 has synthesis rGO via p-hydrazinobenzen- low device performance. From this analysis, a fast decay
esulfonic acid hemihydrate under a simple solution proce- lifetime with quenching competency is required to pro-
dure in room-temperature condition as a novel HTL in planar duce efficient devices as it stimulates efficient charge
perovskite architecture of glass/ITO/rGO/CH3NH3PbI3/ transport across the interfaces in the PSCs system. The
PCBM/bathocuproine (BCP)/Ag. In this study, PSC device rGO HTL also improved device stability by retaining 62%
with rGO as the HTL shows better device performance with from its original efficiency after 140 hours of exposure
champion cells achieving an PCE of 10.8% with the highest without encapsulation to ambient conditions with a
Jsc and Voc obtained as compared to GO and (poly(ethylene humidity of approximately 50%. However, PEDOT:PSS
dioxythiophene):poly(styrene sulfonate)) PEDOT:PSS pre- HTL device took only 120 hours to degrade fully due to
pared in the same work. From the SEM images shown in highly acidic properties.
Figure 12, it shows that the grain size of perovskite materials Jokar et al111 investigated the effect of using different
increased when deposited on the rGO layer as compared to reducing agents, including hydrazine (N2H4), sodium
the PEDOT:PSS layer which proves that the perovskite mate- borohydride (NaBH4), and 4-hydrazino benzenesulfonic
rial growth is better on glass/ITO/rGO interface. The enlarge- acid, to produce three rGO, labeled rGO-NH, rGO-BH,
ment of crystal grain size contributes to the reduction of trap and rGO-4-hydrazino benzenesulfonic acid (HBS), respec-
density, which enhances the charge transport efficiency. The tively. In this study, the treated rGO single HTL was also
research team clarifies charge transport efficiency by investi- compared with GO and PEDOT:PSS single HTL. All the
gating the time-resolved photoluminescence (TRPL) analysis treated rGO HTL device efficiency surpassed the perfor-
where the decay process from rGO HTL (average decay life- mance of GO (at 13.8%) and PEDOT:PSS (at 14.8%) HTL
time, τ is 22.93 nanoseconds) was enhanced compared to the devices in inverted planar PSCs architecture where rGO-
reference PEDOT:PSS HTL (average decay lifetime, τ is HBS shows higher PCE of 16.4%. However, the PL inten-
45.55 nanoseconds). sity and transient PL (TRPL) decay show that the GO HTL
In addition, the rGO HTL also shows better quenching device has a better quenching effect and faster time decay
ability from the PL intensity, which indicates that the hole compared to others, which indicated that the rapid hole
generated in the perovskite layer has been efficiently extraction happened within GO HTL/perovskite inter-
transferred to the respective collector, thus inhibiting the faces. The authors suggested that the hole localization in
excite electrons from undergoing a recombination process the ITO/GO HTL interface causes rapid recombination
known to emits fluorescence. A faster decay lifetime rev- and inhibits device performance meanwhile better device
ealed that electrons spend less time in the excited state performance was seen in rGO HTL device even though it
hence restraining the recombination process that causes showed that slower hole injection might happen because

F I G U R E 1 3 Transmission electron microscope (TEM) images for perovskite films deposited on: (A) PEDOT:PSS; and (B) MFGO
HTLs; and (C) contact angles of water droplets on the different ITO/HTL substrates.112 HTL, hole transport layer; ITO, indium tin oxide;
MFGO, fluorinated reduced graphene oxide; PEDOT:PSS, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate
SAFIE ET AL. 23

of delocalized hole nature in benzene rings hindered the inhibited the recombination possibilities to give a bet-
charge recombination process. ter photovoltaic performance.
In this case, the stability of the device was analyzed
without device encapsulation in a condition of 25 C and
relative humidity of 30% for 1000 hours. The GO and 5 | C O N C L U S I O N A N D OU T LO O K
rGO single HTL successfully maintained half of their
original efficiency after 1000 hours which pointed out In summary, the relevance roles of graphene-based mate-
that the presence of GO and rGO offered long-term sta- rials in the specific HTL/perovskite and ETL/perovskite
bility to the device while PEDOT:PSS single HTL pre- interfaces in the PSCs system have been discussed. The
pared in the same work showed rapid degradation and interaction of materials in each interfacial mainly
device failure after 650 hours. Similarly, Kang and affected the crystal growth kinetics, which was attributed
team113 also compared the role of GO, rGO, and PEDOT: to the perovskite film growth resulting in a high-quality
PSS single HTL in device stability with mesoscopic n-i-p film morphology that is favorable in order to provide
architecture. The rGO single HTL device retained 50% of excellent dynamics of charge transportation, especially in
its original efficiency without any encapsulation in ambi- HTL/perovskite and ETL/perovskite interfaces. Since
ent atmosphere for 30 days and surpassed the perfor- perovskite is most likely a vital component in PSCs
mance stability of PEDOT:PSS single HTL prepared in mechanism, having poor crystallinity and smaller crystal
the same work. grain size asserted inhibition of reducing trap density.
Alternatively, Yeo et al112 have fabricated planar PSCs Smaller crystal grain size will negatively affect charges
devices with tailored graphene derivative through the dissociation and transportation throughout the device
introduction of fluorinated reduced GO (MFGO) as an and external load. From the previous works, it can be
HTL via simple solution processing. The GO undergoes concluded that the incorporation of graphene-based
treatment with 4-(trifluoromethyl)phenylhydrazine materials has significantly improved film morphology,
reductant to introduce CF3 functional groups on the where they ideally reduce the electron trap density at the
rGO-basal planes and edges. The presence of CF3 func- surface of perovskite material, thereby enhance the crys-
tional groups was reported to help slightly in the increase tal size growth. The high charges mobility properties pos-
of hydrophobicity properties of the ITO/HTL interfaces. sess by these graphene-based materials help in facilitates
Figure 13 shows the differences in the contact angle of the charges extraction efficiency in the device. Neverthe-
the water droplets on the respective ITO/HTL interfaces less, the tunable work function properties and high dis-
studied in this work. The hydrophobicity properties persion compared to pristine graphene are beneficial
needed to encapsulate the perovskite material known to characteristics as GO, rGO, and GQDs acknowledged as
quickly degrade when exposed to air, especially in a superior additives and materials fit in any of the interfa-
highly humid environment. The device stability has cial materials to enhance photovoltaic performance and
proven together with the efficiency reading after a few device stability. Considering the numerous reports that
days that the device with MFGO was 72% capable of described the relevance of photovoltaic technologies'
retaining the original reading within 30 days compared innovative application, GO, rGO, and GQDs are likely to
to PEDOT:PSS, which needed only 9 days to show a com- remain a significant alternative to pristine graphene as
plete failure of the device performance. the capable dopant and passivator to enhance the perov-
In this case, TEM analysis was used to investigate skite and charge transports materials' properties in order
the differences in perovskite morphologies upon crys- to produce efficient and stable PSCs. The unique proper-
tallization on MFGO and PEDOT:PSS HTL, as depicted ties of these graphene-based materials can also be impor-
in Figure 13. A few nanometers of small crystal grain tant reasons to promote them as the significant
with various orientations were observed in perovskite components in PSCs as they have the potential to provide
crystal grain deposited on PEDOT:PSS. Meanwhile, low-temperature, simple, and less acidic procedures use-
the larger grain size of perovskite crystallite on MFGO ful for industrialization and green energy purpose.
can be seen highly in ordered without defect due to the
low surface energy of fluorinated MFGO. The com- ACKNOWLEDGEMENT
pactly distributed crystal grain appeared to enhance Authors are grateful to Universiti Teknikal Malaysia
the charge transfer distribution within the MFGO/ Melaka for the facilities support and UTeM Zamalah
perovskite interface. The efficient charges extraction Scheme for PhD support of NE Safie.
supported by the shorter PL lifetime observed in the
MFGO layer, which indicated that charge carriers were ORCID
rapidly transported from the perovskite layer and Nur E. Safie https://orcid.org/0000-0003-4360-785X
24 SAFIE ET AL.

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