A Cross-Linkable Electron-Transport Layer Based on a Fullerene?Benzoxazine Derivative for Inverted Polymer Solar Cells

Article abstract of DOI:10.1002/cplu.202000354

The synthesis, optoelectronic characterization and device properties of a cross-linkable fullerene derivative, [6,6]-phenyl-C₆₁-butyric benzoxazine ester (PCBB) is reported. PCBB shows all the basic photophysical and electrochemical properties

Full text of DOI:10.1002/cplu.202000354

A Multidisciplinary Journal Centering on Chemistry
Accepted Article
Title: A Cross-Linkable Electron Transport Layer Based on a
Fullerene–Benzoxazine Derivative for Inverted Polymer Solar
Cells
Authors: Sandeepa Kulala Vittala, Remya Ravi, Biswapriya Deb, and
Joshy Joseph
This manuscript has been accepted after peer review and appears as an
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the VoR from the journal website shown below when it is published
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content of this Accepted Article.
To be cited as: ChemPlusChem 10.1002/cplu.202000354
Link to VoR: https://doi.org/10.1002/cplu.202000354
01/2020

10.1002/cplu.202000354
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A Cross-Linkable Electron Transport Layer Based on a Fullerene–
Benzoxazine Derivative for Inverted Polymer Solar Cells
Sandeepa Kulala Vittala,[a] Remya Ravi,[a] Biswapriya Deb*[a] and Joshy Joseph*[a]
Abstract: Herein, the synthesis, optoelectronic characterization and
device properties of a new cross-linkable fullerene derivative, [6,6]-
phenyl-C61-butyric benzoxazine ester (PCBB) is reported. The
PCBB inherits all the basic photophysical and electrochemical
properties of parental [6,6]-phenyl-C61-butyric methyl ester (PCBM).
Thermal cross-linking of the benzoxazine moiety in PCBB resulted in
the formation of cross-linked, solvent resistive adhesive films (C-
PCBB). Atomic force microscopy (AFM) and optical microscopic
studies showed dramatic reduction in the roughness and
aggregation behaviour of P3HT-PCBM polymer blend film upon
incorporation of C-PCBB interlayer. An inverted bulk hetero junction
solar cell based on the configuration ITO/ZnO/C-PCBB/P3HT-
PCBM/V2O5/Ag achieved 4.27% power conversion efficiency (PCE)
compared to the reference device ITO/ZnO/P3HT- PCBM/V2O5/Ag
(PCE=3.28%). This 25% increment in the efficiency is due to the
positive effects of C-PCBB on P3HT/C-PCBB and PCBM/C-PCBB
heterojunctions.
the presence of P3HT donor shown improved PCE compared to
reference PCBM acceptor.
On the other hand, it has been demonstrated that the
nanoscale morphology of BHJ active layer tend to undergo
macrophase
separation
due
to
the
high
aggregation/crystallization tendency of fullerene molecules.[2a]
When solar cells are working under natural conditions, such an
aggregation will enhance with time due to continuous heat-cool
cycles, which will hamper the device performance.[10] Therefore,
to create stable and high life time PSCs for practical applications,
stable BHJ film morphology is highly essential.[11] One of the
strategies is to “freeze” the active layer nano-structure obtained
after spin coating.[12] In this line, cross-linking approach have
been developed using cross-linkable groups such as epoxide,[13]
halide,[14] azide,[15] oxetane,[16] and styrene.[17] These moieties
were successfully substituted into the side chains of the polymer
or fullerene and have shown promising results in terms of stable
nanostructures and devices.[18] Also, many cross-linking
reactions namely, click reaction,[19] disulfide bond formation,[19a]
20]
olefin metathesis,[19a,
have been introduced to retain the
Introduction
supramolecular nanostructure. For example, Yongfang Li et al.
reported that the P3HT-ICBA based inverted device with C-
PCBSD electron extracting interlayer improves the PCE to a
remarkable 6.2% through electronic and orbital interactions at
the interface.[21] Later the same group fabricated vertically
aligned C-PCBSD fullerene derivative as ETL instead of cross-
linked film using the same P3HT-ICBA polymer blend system
and achieved record high PCE of 7.4% due to the high charge
transport across the vertical nanorods of ETL.[22] Using similar
strategy many cross-linkable fullerenes has been used as ETL in
BHJ-PSCs[11a, 16, 23] and perovskite solar cells.[24] These highest
efficiencies achieved using modified fullerene derivatives in the
presence of P3HT electron donor shows the requirement and
role of fullerene functionalization towards simple and stable PSC
device with improved efficiencies.
The functionalization of cross linkable fullerenes can be
achieved, either covalently attaching cross-linkable units to
fullerene monomer or by external addition of reactive species
during solution processing which could undergo cross-linking
upon thermal annealing[18] or oxidative polymerisation.[25] The
first method maintains the organization of fullerenes along the
polymerizable groups facilitating a possible stable nanostructure
via self-assembly through non-covalent interactions.[26] On the
other hand, the second approach sometimes fails in controlling
the desired nano-morphology.[27] Recently, cross-likable
benzoxazine derivatives gather tremendous attention due to
their excellent properties including, low water absorption, near-
zero volumic shrinkage upon curing and low coefficient of
thermal expansion,.[28] Moreover, they can be easily
synthesized from conventional cheap precursors and cross-
linked under heat triggered reaction, without the aid of external
initiators or catalysts.[29] In the present work, we have
undertaken covalent functionalization of fullerene with
benzoxazine cross-linkable group, explored the morphological
In the recent decades, polymer solar cells (PSCs) received
widespread interest in the scientific community due to their
potential application in fabrication of light weight and flexible
electronics by low-cost solution processing techniques.[1] In
order to increase the donor-acceptor interfacial area in the
polymer blend for efficient exciton dissociation and charge
transport, the concept of bulk heterojunction (BHJ) composing
bicontinuous interpenetrating nanonetwork of polymer blend
containing p-type donor and n-type acceptor architecture have
been adopted.[2] The poly(3-hexylthiophene) (P3HT) and [6,6]-
phenyl-C61-butyric acid methyl ester (PCBM) donor-acceptor
combination was used as a standard to understand the role of
active layer morphology towards efficient BHJ-Polymer Solar
Cell (BHJ-PSC) device which reached power conversion
efficiency (PCE) >5% and is being used as a reference system
till today.[3] Previously, many groups have shown improved
efficiency by introducing innovative polymer donors,[3b] fullerene
acceptors[4] and incorporating new interfacial electron/hole
transport layers.[5] Also, the inverted PSC configuration is most
suited to realize stable and long lasting device than conventional
architecture.[6] Modified fullerene derivative such as indene C60-
bisadduct,[7]
(DMPCBA),[8] bis-o-quino-dimethane C60 (Bis-QDMC)[9] etc. in
fullerene-di(4-methyl-phenyl)methano-C60
[a]
Dr. S. K. Vittala, Dr. R. Ravi, Dr. B. Deb and Dr. J. Joseph
Photosciences and Photonics Section
CSIR-National Institute for Interdisciplinary Science andTechnology,
Thiruvananthapuram 695 019 (India) and Academy of Scientific and
Innovative Research (AcSIR), Ghaziabad 201002, India
E-mail: joshy@niist.res.in; biswapriya.deb@niist.res.in
Supporting information for this article is given via a link at the end of
the document.
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processes. This method is advantageous over conventional
voltammetry due to its fast analysis and high sensitivity with a
wide dynamic range of electroactive materials. Moreover, in
SWV deoxygenation of the solution is not required unless it
interferes with the electrode reaction. The disadvantage of SWV
is the lack of information regarding the reversibility of redox
species. Measurements were carried out using the thin films of
fullerene derivatives on glassy carbon electrode with Ag/AgCl as
a reference electrode, calibrated by ferrocene (E1/2(ferrocene) = 0.45
V
vs. Ag/AgCl). Both compounds exhibited identical first
reduction potential of -0.57 V vs Ag/Ag+ (Figure 1b). The LUMO
energy levels were calculated relative to the reference energy
level of ferrocene (4.8 eV below the vacuum level) using the
equation (1).[31]
EHOMO/LUMO = - [4.8 + Eox/red – 0.45] eV ………… (1)
The HOMO/LUMO energy levels of both compounds were found
to be -5.5 eV and -3.78 eV, respectively indicating negligible
perturbation in electronic properties upon substitution. The
reduction potentials and LUMO energy levels of fullerene
derivatives were summarized in Table S1. Further, DFT
calculations of the neutral PCBB and PCBM were performed at
the B3LYP/6-31G (d, p) level and optimized their HOMO and
LUMO energies in vacuum conditions. The geometry
optimization provided HOMO and LUMO energy levels of -5.65
eV and -3.09 eV for both PCBB and PCBM (Figure S1). The
comparable HOMO/LUMO energy values of PCBB and PCBM
indicate that benzoxazine substitution does not perturb the
electronic properties of the PCBB derivative.
We have investigated fluorescence quenching of P3HT
donor in the presence of PCBB and PCBM fullerene derivatives
to assess the electron transport properties. The fluorescence
intensity of P3HT at 585 nm (λmax) decreased with increasing
concentration of both PCBB and PCBM (Figure S2a and S2b).
This is mainly attributed to the intermolecular electron transfer
from P3HT donor to the fullerene acceptor due to their
Scheme 1. Synthetic scheme for cross-linkable fullerene derivative, PCBB
under study. EDC: 1-ethyl-3-(3-(dimethylamino)propyl) carbodiimide, DMAP:
4-(dimethylamino)-pyridine, ODCB: ortho-dichlorobenzene.
and optoelectronic properties and demonstrated their potential
application as ETL in BHJ-PSCs.
Results and Discussion
Synthesis and Characterization of PCBB
The synthesis of [6,6]-Phenyl-C61-butyricbenzoxazine ester,
PCBB with benzoxazine cross-linkable group was carried out as
shown in Scheme 1. The starting materials [6,6]-phenyl C61-
butyric acid (PCBA), obtained by the ester hydrolysis of PCBM
and methylol benzoxazine (MB) were synthesized following
29]
reported literature procedures.[28a,
Subsequently, the final
PCBB derivative was synthesized via EDC-DMAP coupling of
PCBA and MB in ortho-dichlorobenzene (ODCB) to obtain the
desired final product PCBB. The structural characterization of
PCBB was accomplished through FT-IR, 1H NMR, 13C NMR,
UV-visible and MALDI-TOF mass spectral analysis.
Photophysical Properties of PCBB
Since, the core structure of PCBB resembles its parental
derivative PCBM, we have studied the photophysical properties
of PCBB in comparison to PCBM using UV-Visible absorption
spectroscopy,
square
wave
voltammetry
(SWV),
thermogravimetric analysis and P3HT fluorescence quenching
experiments. Typical UV-Visible absorption spectra of PCBB in
chloroform showed the characteristic peak at 328 nm, which is
similar to that of PCBM (Figure 1a).[30]
The electrochemical properties of PCBB and PCBM were
compared by using SWV method. SWV is pulse voltammetry
technique which is carried out by monitoring the current using
staircase-shaped voltage ramp over a range of set values. The
current measured at the end of forward/reverse pulse provides
information in the form of a peak for each oxidation/reduction
Figure 1. (a) Normalized absorption spectra of PCBB and PCBM in
chloroform. (b) Square wave voltamogram of PCBB and PCBM vs Ag/AgCl at
a scan rate of 100 mV s-1. (c) Stern-Volmer plot of PCBB and PCBM obtained
from P3HT fluorescence quenching studies. (d) TGA profiles of PCBB and
PCBM; the black lines in thermogram indicate the Td value.
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favorable HOMO-LUMO energy levels.[32] In order to understand
the quenching efficiency of PCBB fullerene derivative compared
to parental PCBM the Stern-Volmer constant was calculated
using the equation
Fo/F = 1 + Ksv [Q] ………… (2)
where Fo and F are the fluorescence intensities in the absence
and presence of the quencher, respectively, Ksv is the
Stern−Volmer quenching constant, and [Q] is the concentration
of the quencher. The Stern-Volmer plots revealed linear
increment for both P3HT-PCBM and P3HT-PCBB which is
characteristic of dynamic quenching (Figure 1c). The Ksv for
P3HT-PCBM and P3HT-PCBB was calculated to be 5.63 x 103
M-1 and 4.35 x 103 M-1, respectively. The Ksv value for P3HT-
PCBM is quite similar to that reported in the literature.[33] Ksv, is
indicative of the quenching interactions within the donor-
acceptor system and PCBB derivative has slightly lower value
compared to PCBM due to the presence of bulkier benzoxazine
cross-linking group. Therefore, we have used PCBB as cross-
linkable electron-transport layer along with P3HT-PCBM donor-
acceptor systems.
Further, the thermal stability of PCBB was measured and
compared with PCBM through thermogravimetric analysis (TGA)
before carrying out thermal cross-linking experiments. The TGA
profile of both PCBB and PCBM showed thermal stability up to a
temperature of 400 C and exhibited 5% weight loss (Td) at 416
C and 488 C, respectively (Figure 1d). The relative lower Td
value of PCBB is attributed to the bulkier benzoxazine moiety at
the ester linkage compare to methyl group in PCBM. The
Figure 3. Absorption coefficient of the (a) C-PCBB film before (red) and after
(blue) rinsing with chloroform and non-cross-linked PCBB layer after rinsing
with chloroform (black); (b) PCBM film before rinsing (red) and after heating
followed by rinsing with chloroform (blue). (c) Photograph of glass coated
PCBB film (thickness ca. 135 nm) dipped in chloroform, before and after
cross-linking at 200 C for 15 min.
analysis (Figure 2a). It exhibited a broad exothermic peak with a

Tmax value around 210 C, indicating the thermal cross-linking of
benzoxazine groups.[28b] The functional group transformation

before and after heat cross-linking at 200 C for 15 min were
monitored using FT-IR spectroscopy (Figure 2b). The FT-IR
spectrum of PCBB after heating showed disappearance of peak
at 948 cm-1, characteristic to the out of plane bending vibrational
mode of benzene –C-H to which oxazine ring was attached.[34]
Moreover, heat cross-linking results in the appearance of a new
peak at 3315 cm-1, which is attributed to the phenolic –OH
stretching frequency. This confirms thermal cross-linking of
PCBB molecule. Based on the reported mechanism of
benzoxazine cross-linking in the literature, we propose the heat-
triggered ring opening polymerisation of benzoxazine group in
PCBB, as shown schematically in Figure 2c.[28a] From these
photophysical and cross-linking experiments, we can consider
PCBB as
a cross-linkable PCBM derivative with similar
optoelectronic properties.
To get further insight into the cross-linking behaviour, solid
state absorption spectra of PCBB film cured at 200 C for 15 min
on a glass plate was measured before and after rinsing with
chloroform (Figure 3a). For comparison, we also used PCBM
coated film processed under similar conditions (Figure 3b).
Profilometer measurement for the PCBB and PCBM films
showed film thickness of 137 nm and 134 nm, respectively
(Figure S3). The cross-linked PCBB (C-PCBB) film showed
similar absorption properties before and after rinsing, whereas
PCBM film completely washed away upon rinsing irrespective of
temperature and heating time. As expected, the absorbance of
non-cross-linked PCBB film also completely disappeared after
rinsing with solvent indicating the importance of curing, without
which the material can get washed out by the solvent. This
confirms that PCBB forms adhesive films exhibiting excellent
solvent resistive property after heat cross-linking. The
Figure 2. (a) DSC thermogram of PCBB at a heating rate of 10 oC min-1. (b)
Solid state FT-IR spectra of the PCBB and C-PCBB. (c) Chemical structures
of PCBB and C-PCBB indicating the functional group transformation upon
thermally triggered cross-linking.
thermal characterization studies indicate that the benzoxazine
derivative, PCBB is thermally stable under the experimental
conditions and can be used for thermal curing studies.
Cross-linking and Solvent Resistive Properties
The thermal transition properties of PCBB monomer were
investigated through Differential Scanning Calorimetry (DSC)
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photograph in Figure 3c represents the glass plate with C-PCBB
and PCBB films immersed in beaker containing chloroform
which indicates the insoluble nature of C-PCBB film and
dissolution of non-cross-linked PCBB film. These results
ascertain that when the temperature is about 200 C
benzoxazine groups of PCBB molecule provide adequate
flexibility to react in solid state and forms insoluble cross-linked
films as proposed in Figure 2c.
is same, in case of P3HT-PCBM film there is a decrease in the
amount of P3HT with equimolar increase in PCBM. These
opposing trends of decreased P3HT absorption and increased
PCBM absorption result in net similar absorption compare to
P3HT film. But in case of C-PCBB/P3HT-PCBM, there is an
additional C-PCBB interfacial layer along with P3HT-PCBM
which exhibits increased absorption due to the added absorption
from C-PCBB. Also, the shoulder band at 603 nm in the
absorption spectrum of the blend indicates the crystallinity of
P3HT which remained intact even after blending with PCBM and
in the presence of C-PCBB layer. Moreover, the emission
maximum of P3HT at 715 nm was quenched to 67% in the
presence of PCBM (Figure 5b). For polymer blend with C-PCBB
also same amount of quenching was observed. This experiment
revealed that incorporation of C-PCBB interlayer increases the
absorption of the polymer blend in the visible region without
compromising crystallinity of the P3HT polymer.
Another issue with the P3HT-PCBM polymer blend film is
their tendency to undergo macrophase separation as a result of
crystallization/aggregation of PCBM over the time. The direct
contact with the hydrophilic ZnO layer and interfacial erosion is
the primary concern. Therefore, the aggregate growth of P3HT-
PCBM polymer blend with and without C-PCBB layer was
studied using optical microscope. For these studies, the samples
of P3HT, P3HT-PCBM and C-PCBB/P3HT-PCBM were
analyzed before (10 min annealing) and after ageing for 48 h. It
was quite clear that as-prepared films of all three samples were
smooth, without exhibiting any kind of aggregation (Figure 5c-e).
However, both P3HT and P3HT-PCBM films after 48 hours of
ageing showed aggregated crystal like structure indicating
macroscopic phase separation of nanomorphology (Figure 5f
and 5g). On the other hand, P3HT-PCBM in the presence of C-
PCBB interlayer exhibited smooth and homogeneous film under
the similar experimental conditions (Figure 5h). This is mainly
because of the C-PCBB layer, providing smooth hydrophobic
contact to the polymer blend which otherwise in contact with the
hydrophilic ZnO, tends to undergo faster crystallization. This
further indicates the indirect effect of in situ cross-linking PCBB
on the morphological stabilization of polymer blend and inhibition
of time dependent macrophase separation.
Roughness and Crystallization Control
To investigate the applicability of C-PCBB as ETL in BHJ-
PSC, the inverted configuration of ITO/ZnO/C-PCBB/P3HT-
PCBM/V2O5/Ag was adopted. The propensity of C-PCBB layer
on the morphology of polymer blend was studied before carrying
out device fabrication. Initially ZnO nanoparticle solution
prepared using sol-gel process was coated on ITO. The surface
of ITO/ZnO showed root mean square (RMS) roughness of 4.68
nm (Figure 4a). Upon coating of C-PCBB layer, the ITO/ZnO/C-
PCBB surface exhibited relatively less roughness of 0.31 nm
(Figure 4b). The polymer blend was coated without and with C-
PCBB layer on ITO/ZnO(Figure 4c and 4d). The surface of
polymer blend resulted 2.58 nm roughness with C-PCBB layer
and 3.06 nm without C-PCBB. The decreased roughness in the
presence of C-PCBB is due to the reduction in the voids of
unevenly distributed ZnO by PCBB rendering smoother and
more hydrophobic surface. With the view to study solid state
properties, the solid state absorption
Photovoltaic properties
BHJ-PSC devices were fabricated with inverted
configuration without and with C-PCBB layer to understand the
propensity of C-PCBB as ETL. Devices A and B were prepared
by spin-coating a mixture of P3HT-PCBM (1:1, w/w) from
ODCB to form a ~130 nm thin film on ZnO coated ITO
substrates without and with C-PCBB interlayer, respectively.
Figure 6 and Table 1 represent the J-V characteristics and
summary of photovoltaic performance of the devices A and B,
respectively. Under identical fabrication condition reference
device A showed a PCE of 3.28% which is in agreement with
average reported values for P3HT-PCBM systems.[21a] In the
Figure 4. AFM tapping mode height images of (a) the bare ZnO surface and
(b) C-PCBB thin film on top of the ZnO. P3HT-PCBM polymer blend films (c)
without and (d) with C-PCBB interlayer after thermal annealing at 120 °C for
10 min, (1.0 ×1.0 µm). The root mean square roughness was indicated in the
insets.
and emission spectra of P3HT, P3HT-PCBM and C-
PCBB/P3HT-PCBM were investigated. In the presence of
PCBM, the absorption spectrum of P3HT was unchanged in the
region of 400-600 nm while there was an increase in the
intensity from 300-400 nm (Figure 5a). Since the film thickness
presence of C-PCBB ETL, the PCE of device
substantially improved to 4.27%. This value signifies a ~25%
improvement over device and is mainly because of
simultaneously enhanced open circuit voltage (Voc), short-circuit
B was
A
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which may give rise to easy charge recombination at the
P3HT/ZnO interface in device A. The n-type C-PCBB acting as
a hole-blocking layer in device B with energy offset of 1.4 eV
relative to 0.8 eV at P3HT/ZnO is still useful for reducing its
probability. Moreover, the device B showed higher Voc (0.64 V)
relative to the reference device A (0.56 V). This indicate the
introduction of C-PCBB in device B act as a hole-blocking layer
which prevents the leaky paths by suppressing the back electron
transfer from ZnO to P3HT in the active layer. This was reflected
by the higher shunt resistance (Rsh) of device B (2068.4 cm2)
than device A (1177.7 cm2). Hence, in device B the electron
transport can occur very efficiently in both heterojunctions of
P3HT/C-PCBB and PCBM/C-PCBB while in device A it can
happen only at PCBM/ZnO heterojunction. The overall 25%
increase in the PCE of the reference device using C-PCBB
interlayer confirms that the PCBB act as an efficient cross-
linkable electron transport layer in inverted PSCs with efficient
exciton separation, decreased charge recombination and
interface contact resistance compare to the reference device.
The stability of the unencapsulated inverted solar cell devices A
and B were periodically monitored by measuring PCEs for
Figure 5. Solid state (a) absorption and (b) emission spectrum of (i) P3HT
alone (24 mg/mL), (ii) P3HT-PCBM blend (1:1, w/w) and (iii) C-PCBB/P3HT-
PCBM, λexc = 600 nm. Optical microscope images of thin films formed from i, ii
and iii (c-e) after annealing for 10 min and (f-h) ageing for 48 h (thickness ca.
135 nm).
14
7
0
current (Jsc), and fill factor (FF) relative to device A. Both P3HT
donor and PCBM acceptor domains at the interface of BHJ
active layer undergo contact with the ZnO bottom layer to form
two types of localized nanoscale heterojunctions (Figure 7).
Hence, device A contains P3HT/ZnO and PCBM/ZnO localized
heterojunctions at the interface, whereas device B has P3HT/C-
PCBB and PCBM/C-PCBB contact.
Careful analysis of the P3HT domain in the interface we
can have following advantages in the case of device B by
employing extra C-PCBB layer. In device B, the C-PCBB
-7
Device A
-14
Device B
-21
-0.2
0.0
0.2
0.4
0.6
Voltage (V)
provides an extra P3HT/C-PCBB interface for
exciton
Figure 6. J-V curves of devices A and B under AM 1.5 G illumination at 100
mV/cm2.
dissociation that is more efficient than the P3HT/ZnO interface in
device A. Also, the LUMO of C-PCBB (3.8 eV) is situated
between the conduction band of ZnO (4.4 eV) and the LUMO of
P3HT (3.3 eV). Therefore, the C-PCBB can act as an
intermediate energy gradient where electrons can be easily
transported to the ZnO layer through cascade pathway.
Additionally, we found that the series resistance (Rs) of the
device B has a lower value (94.8 cm2) compare to the device A
(160.1 cm2). This clearly indicate that the ohmic contact in
device B lowers the contact resistance due to the enhanced
electrical coupling of PCBB, facilitating better electron transport
through the interface. As a result, there might be an efficient
electron extraction from the PCBM domain by the C-PCBB
interfacial layer. This is in line with the decreased roughness of
P3HT-PCBM polymer blend in the presence C-PCBB due to the
formation of smooth bicontinuous network (Figure 4c and 4d).
These advantages might be associated with the improvement of
Jsc, from 11.68 mA/cm2 (device A) to 12.75 mA/cm2 (device B).
Further, the energy difference between the conduction
band of ZnO and the HOMO of P3HT (5.2 eV) is only 0.8 eV,
Table 1. Summary of photovoltaic performance of different devices.
Jsc
Voc [V]
a)
FF
[%]c)
 [%]d)
Rsh
[ cm2] e)
Rs
Device
[mA/cm2]
[ cm2] f)
b)
A
B
0.56
0.64
11.68
12.75
50.20
52.41
3.28
4.27
1177.7
2068.4
160.1
94.8
a) Open-circuit voltage, Voc; b) short-circuit current, Jsc; c) fill factor, FF; d)
PCE, η; The performances provided in the table are the champion ones; The
average PCE values for five devices of A and B were 3.01% and 4.13%,
respectively; The standard deviations were calculated to be 0.27 and 0.15,
respectively; e) shunt resistance, Rsh; f) series resistance, Rs. Configurations:
device A, ITO/ZnO/P3HT-PCBM (1:1, w/w)/V2O5/Ag; device B, ITO/ZnO/C-
PCBB/P3HT-PCBM (1:1, w/w)/ V2O5/Ag.
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(Baytron P VP AI 4083) was purchased from H. C. Stark and passed
through a 0.45 µm syringe filter before spin-coating. Patterned ITO was
obtained from Ossila Ltd. All other reagents were purchased from Sigma-
Aldrich and used as received. PCBA and Methylol benzoxazine were
synthesized according to the reported procedures.
3.3
3.8
5.5
3.8
5.5
4.4
7.6
3.8
4.4
5.2
Synthesis of PCBB: To a solution of [6, 6]-Phenyl-C61-butyric acid (50
mg, 0.056 mmol) in ODCB (5 mL) was added 4-dimethylaminopyridine (7
mg, 0.056 mmol), (16 mg, 0.084 mmol) and stirred for 10 min at 0 oC.
Methylol benzoxazine (17 mg, 0.067 mmol) was added directly to the
reaction mixture and stirred overnight at room temperature. The crude
mixture was purified by column chromatography using toluene (Rf = 0.6)
afforded brown colour solid in 24% yield (15 mg). 1H NMR (500 MHz,
CDCl3), δ (ppm): 7.91 (t, 2H), 7.52 (m, 3H), 7.43 (m, 3H), 7.09 (t, 2H),
6.97 (m, 2H), 6.91 (t, 1H), 5.35 (s, 2H), 4.94 (s, 2H), 4.61 (s, 2H), 2.90 (t,
2H), 2.54 (t, 2H), 2.19 (m, 2H); 13C NMR (125 MHz, CDCl3), δ (ppm):
172.96, 154.50, 150.69, 148.80, 148.26, 147.78, 145.84, 145.20, 145.15,
145.07, 145.04, 144.79, 144.67, 144.51, 144.40, 144.01, 143.76, 143.10,
143.00, 142.94, 142.91, 142.21, 142.18, 142.13, 142.11, 140.98, 140.75,
138.03, 137.57, 136.72, 132.11, 129.05, 128.45, 128.26, 128.14, 127.30,
121.62, 120.92, 118.99, 118.31, 79.86, 66.21, 51.84, 34.11, 33.62,
22.37; HRMS (m/z): [M]+ calculated for C86H25NO3+, 1119.18; found,
1120.21 (M+H).
5.5
7.6
Figure 7. Schematic configuration of inverted device architecture,
ITO/ZnO/P3HT-PCBM (1:1, w/w)/V2O5/Ag without (Device A) and with (Device
B) C-PCBB layer showing energy diagrams of the two localized
heterojunctions at the interface.
8 days (Figure S4). The Device B (ITO/ZnO/C-PCBB/P3HT-
PCBM/V2O5/Ag) retained >85% of its original PCE value after 8
days of exposure to ambient conditions. In sharp contrast, the
PCE of device A (ITO/ZnO/P3HT-PCBM/V2O5/Ag) decayed very
rapidly in just 2 days. This explicitly demonstrates the crucial
role of C-PCBB interfacial layer in increasing the stability and
lifetime of device B over device A. However, detailed device
optimization and stability measurements under accelerated
conditions are required, which is part of ongoing research in our
lab.
Device fabrication: The ZnO sol was prepared using
a sol-gel
procedure by dissolving zinc acetate dihydrate (C4H6O4Zn.2(H2O),
99.9%, 1.6 g) and monoethanolamine (HOCH2CH2NH2, 99%, 0.045 g)
in anhydrous 2-methoxyethanol (>99.8%, 0.96 mL) under vigorous
stirring for hydrolysis reaction and aging for 3 h. The solar cell devices
were fabricated under optimized conditions according to the modified
reported procedure. The patterned indium tin oxide (ITO)-coated
glass substrate was first cleaned with detergent, ultrasonicated in DI-
water, chloroform and isopropyl alcohol for 10 min, respectively and
subsequently dried in an oven overnight. After UV-ozone treatment for
15 min, nanosized ZnO thin films with a thickness of ca. 40-50 nm were
spin-coated using the sol-gel precursor solution at 3000 rpm on top of the
ITO substrate. The films were sintered at 200 C for 30 min in air. During
this process the precursor converts to ZnO gel and forms a transparent
thin film of ZnO nanoparticle. For device fabricated with C-PCBB
interlayer (device B), an ODCB solution containing PCBB (5 mg/mL) was
spin-cast on the ZnO film to form a thin film with a thickness of ca. 10 nm.
Subsequently, the as-cast film was heated at 200 oC for 15 min for
thermal cross-linking in the glove box. For devices A and B, a ODCB
solution containing a mixture of P3HT-PC61BM (1:1, w/w) was then spin-
cast to form a 130 nm thin film on top of the ZnO (Device A) and C-
PCBB (Device B) thin film, respectively. Both the devices were
thermally annealed at 120 C for 10 min in the glove box followed by
spin-coating vanadium pentoxide with a thickness of ca. 40 nm and then
heated at 120 C in the glove box. Finally, the top electrode of Ag film
(100 nm-thick), was evaporated thermally at a pressure below 10-6 torr.
Devices without encapsulation were characterized in ambient condition.
The active area used for the measurement was 0.1 cm2.
Conclusions
We have successfully synthesized
a
cross-linkable
fullerene derivative, PCBB and demonstrated its application as
ETL in P3HT-PCBM based inverted polymer solar cells. PCBB
act as ‘cross-linkable PCBM’ inheriting all the photophysical and
electrochemical properties of PCBM. Thermal cross-linking of

PCBB at 200 C for 15 min directed the formation of solvent
resistive films, which otherwise will get washed away during
solution processing. Additionally, incorporation of C-PCBB
interfacial layer reduced both roughness and aggregate growth
in the active layer morphology. The performance of device
fabricated with inverted configuration of ITO/ZnO/P3HT-
PCBM/V2O5/Ag with C-PCBB interfacial layer showed ~25%
improvement in PCE (4.27%) relative to the reference device
(PCE = 3.28%). Our results confirm that incorporation of PCBB
ETL facilitates cascade electron transport towards effective
charge collection in PSCs. The strategies adopted in this work
could be used in device fabrication for the construction of stable
and efficient solar cells.
Characterization and measurements: 1H (500 MHz) and 13C NMR (125
MHz) spectra were measured on a Bruker Avance DPX spectrometer.
Chemical shifts are reported in parts per million (ppm) using
tetramethylsilane (TMS) (δH = 0 ppm) or the solvent residual signal
(CDCl3: δC = 77.00 ppm) as an internal reference. The resonance
multiplicity is described as s (singlet), d (doublet), t (triplet) and m
(multiplet). High resolution mass spectral (HRMS) analysis was
Experimental Section
Materials: The materials and reagents for synthesis were purchased
from Sigma-Aldrich, Merck, and Spectrochem chemical suppliers.
Regioregular P3HT (55 kDa) was purchased from Rieke Metal Inc. and
PCBM (>99.5%) was obtained from Nano-C. These chemicals were used
as received without further purification. Vanadium Pentoxide (V2O5)
performed on
a Thermo Scientific Q Exactive Hybrid Quadrupole-
Orbitrap electrospray ionization mass spectrometer (ESI-MS) instrument.
Infrared spectra were recorded in the diffused reflectance mode in the
solid state (KBr) using Shimadzu IR Prestige-21 Fourier Transform
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Infrared Spectrophotometer. All experiments were carried out using
spectroscopic grade solvents at room temperature (25± 1 oC) unless
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Keywords: cross-linking • electron transport • fullerenes •
photovoltaics• solar cells
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Table of Contents
FULL PAPER
Sandeepa Kulala Vittala,
A cross-linkable fullerene derivative
Remya Ravi, Biswapriya Deb*
and Joshy Joseph*
PCBB,
benzoxazine
functionalized
moiety
with
resulted
crosslinked, solvent resistive adhesive
film upon heat triggered ring opening
polymerisation. The fabrication of
inverted bulk hetero junction solar cell
using cross-linked PCBB as electron
Page No. – Page No.
A Cross-Linkable Electron Transport
Layer Based on a Fullerene–
Benzoxazine Derivative for Inverted
Polymer Solar Cells
transport
layer
achieved
25%
enhancement in the power conversion
efficiency compare to the reference
device.
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