Structural and electronic properties of 2,2′,6,6′-tetraphenyl-dipyranylidene and its use as a hole-collecting interfacial layer in organic solar cells

Article abstract of DOI:10.1016/j.dyepig.2017.03.002

The accumulation of positive charges at the anodic interface considerably limits the efficiency of photovoltaic solar cells based on polymer/fullerene bulk heterojunctions (BHJs). Interfacial layers (IFLs) such as PEDOT:PSS improve charge injection but ha

Full text of DOI:10.1016/j.dyepig.2017.03.002

Accepted Manuscript
Structural and electronic properties of 2,2′,6,6′-tetraphenyl-dipyranylidene and its use
as a hole-collecting interfacial layer in organic solar cells
M. Courté, M. Alaaeddine, V. Barth, L. Tortech, D. Fichou
PII:
S0143-7208(17)30120-1
10.1016/j.dyepig.2017.03.002
DYPI 5828
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 19 January 2017
Revised Date: 17 February 2017
Please cite this article as: Courté M, Alaaeddine M, Barth V, Tortech L, Fichou D, Structural and
electronic properties of 2,2′,6,6′-tetraphenyl-dipyranylidene and its use as a hole-collecting interfacial
layer in organic solar cells, Dyes and Pigments (2017), doi: 10.1016/j.dyepig.2017.03.002.
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Structural and electronic properties of 2,2’,6,6’-tetraphenyl-
dipyranylidene and its use as a hole-collecting interfacial layer in
organic solar cells
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M. Courté,‡a M. Alaaeddine,‡b,c,d V. Barth,b,c,d L. Tortech,b,c,d and D. Fichou a,b,c,*
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a School of Physical and Mathematical Sciences, Nanyang Technological University,
637371, Singapore.
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b Sorbonne Universités, UPMC Univ Paris 06, UMR 8232, Institut Parisien de Chimie
Moléculaire, F-75005, Paris, France.
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c CNRS, UMR 8232, Institut Parisien de Chimie Moléculaire, F-75005, Paris, France
d CEA Saclay, IRAMIS, NIMBE, LICSEN, F-91191 Gif-sur-Yvette, France
‡ These two authors have contributed equally to the work.
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* Corresponding author
E-mail address: denisfichou@ntu.edu.sg
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Abstract
The accumulation of positive charges at the anodic interface considerably limits the
efficiency of photovoltaic solar cells based on polymer/fullerene bulk heterojunctions (BHJs).
Interfacial layers (IFLs) such as PEDOT:PSS improve charge injection but have no effect on
the unbalanced electron/hole transport across the BHJ. We report here the use of 2,2’,6,6’-
tetraphenyl-dipyranylidene (DIPO-Ph4), a planar quinoïd compound, as an efficient anodic IFL
in organic solar cells based on BHJs made of poly(3-hexylthiophene)/[6,6]-phenyl-C61-
butyric-acid-methyl-ester (P3HT:PCBM). When deposited under vacuum onto a glass
substrate, DIPO-Ph4 thin films are constituted of densely packed and vertically aligned
crystalline needles. Current-sensing atomic force microscopy (CS-AFM) reveals a
considerable increase of the hole-carrying pathways in DIPO-Ph4 thin films as compare to
PEDOT:PSS, thus revealing their hole transporting/electron blocking properties. Inserting a
10 nm thick IFL of DIPO-Ph4 in combination with a 5 nm thick PEDOT:PSS between the ITO
electrode and the P3HT:PCBM film leads to photocurrent densities up to 11.5 mA/cm2 under
AM 1.5G and conversion efficiencies up to 4.6%, that is substantially higher than PEDOT:PSS-
only devices.
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Keywords: Organic solar cells; interfacial layers; hole collection; bulk heterojunction; atomic
force microscopy.
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1. Introduction
A strategy to improve the performance of organic solar cells (OSCs) consists of tailoring
the interfaces between the organic active materials and the metallic electrodes. To this end
interfacial layers (IFLs) are inserted between the active layer and the respective electrodes in
order to reduce the energy barriers which promote charge carrier diffusion.[1-8] IFLs
operate as selective charge extraction materials for either holes or electrons, while blocking
the opposite charge carriers. Conducting polymer-based IFLs are commonly used to modify
the ITO surface because they ensure an electrical ohmic contact and minimize the
roughness. Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is the
ubiquitous hole transporting layer that is used in OSCs.[9] A thin PEDOT:PSS IFL increases the
open-circuit voltage (Voc) and improves hole collection at the anodic interface between ITO
and the photoactive material.
However, it is well-known that PEDOT:PSS suffers from several drawbacks. Its acidic
character (pH=1-2) etches the ITO electrode and considerably shortens the long-term
stability of the devices. Furthermore, PEDOT:PSS films are hygroscopic and exhibit weak
electron blocking properties.[10-12] Another drawback is the weak visible light absorption of
PEDOT:PSS that does not contribute to light harvesting in the devices.[13] Finally, PEDOT:PSS
thin films often possess important structural and electrical inhomogeneities that are not
compatible with reproducible device performances.
Therefore, alternative IFL materials have been investigated to substitute PEDOT:PSS in
OSCs and other organic devices. Various organic materials have been explored as IFLs with
the aim to improve charge collection, such as for example self-assembled monolayers of 4-
nitrophenylthiolate[14] or halogenated propysilanes,[15] and graphene oxide.[16-18] In a
previous work, we reported that 2,2’,6,6’-tetraphenyldithiapyranylidene (DITPY-Ph4) acts as
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a hole collecting IFL when introduced in OSCs based on P3HT:PCBM.[19] Inorganic materials
have also been studied as hole collecting IFLs, in particular transition metal oxides such as
MoO3,[20-22] NiOx,[23,24] VOx, [25] copper based solutions [26,27] or
polyoxometalate.[28] Note that these IFLs can be used either as single layers or in
combination with a PEDOT:PSS layer.
In this work, we describe the structural and electronic properties of 2,2’,6,6’-tetraphenyl-
dipyranylidene (DIPO-Ph4, Fig. 1) and report on its use as an efficient hole-transporting IFL in
organic solar cells based on bulk heterojunctions BHJs made of poly(3-hexylthiophene) and
[6,6]-phenyl-C61-butyric-acid-methyl-ester (P3HT:PCBM). DIPO-Ph4 is a large planar quinoïd
compound made of two pyranylidene heterocycles linked together by an exocyclic C=C
double bond in 4-4’positions relative to the two O-atoms. DIPO-Ph4 is isoelectronic with
tetrathiafulvalene and behaves as a strong π-donor with various acceptors.[29] We reveal
here the remarkable needle-like morphology of vacuum-deposited crystalline DIPO-Ph4 thin
films and show that it behaves as a more efficient anodic IFL in solar cells as compared to its
sulfur analog DITPY-Ph4.
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Fig. 1 Left: Chemical structure of 2,2’,6,6’-tetraphenyldipyranylidene (DIPO-Ph4).
Right: Deep-red shiny crystalline powder of DIPO-Ph4 after purification by Soxhlet
extraction in acetonitrile.
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2. Experimental Section
Synthesis and purification of DIPO-Ph4
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The synthesis of DIPO-Ph4 has been reported previously.[30] However, we
modified the synthetic protocol in order to substantially increase the yield and to
improve the purity of the final compound (Scheme 1). Briefly, glutaryl chloride is
functionalized by two phenyl groups using a Friedel-Craft acylation, and further oxidized by
BF4.Et2O into a pyrylium salt. The latter compound is then reduced by tributylphosphine
P(C4H9)3 instead of zinc which is difficult to remove from the final product. The intermediate
ylide is finally condensed in basic medium using EtN(i-Pr)2 (DIPEA) to form 2-2’-6-6’-
tetraphenyldipyranylidene (DIPO-Ph4) in 85% yield. The raw product is a brownish
powder that is purified by Soxhlet extraction in acetonitrile during several days. The
final compound is a highly crystalline deep red powder (mp=325°C) with shiny
metallic reflections (Fig. 1).
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Scheme. 1. Synthetic pathway to 2,2’,6,6’-tetraphenyldipyranylidene (DIPO-Ph4).
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OPV device fabrication
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ITO-coated glass substrates (Sigma-Aldrich; 2.5×2.5 cm²; R=70-100 Ω/cm2) are
patterned by chemical etching with HCl (5 % aq.) and sonicated in successively
ethanol and acetone during 10 minutes. The samples are finally exposed to UV-ozone
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during 10 minutes in order to remove organic impurities and improve the
hydrophilicity of the substrates. The crystalline DIPO-Ph4 is finely divided in a mortar
and placed in a quartz boat. DIPO-Ph4 is evaporated at a slow rate (0.1 Å/s) in a
thermal vacuum chamber (P0∼
10-4 Pa) and the thickness is monitored by a quartz
microbalance (Inficon X TM/2). A 5 nm layer of PEDOT:PSS (Sigma-Aldrich; water
solution 1.3 % in weight) is spin-coated on top of DIPO-Ph4 thin film. P3HT/PCBM bulk
layer is prepared by spin-coating from a solution of P3HT (Sigma-Aldrich; purity >98.5
%) and PCBM (Sigma-Aldrich; purity 99.5 %) dissolved in dichlorobenzene (ratio of
1:0.8 P3HT-PCBM) and sonicated beforehand during 10 minutes to obtain a
homogenous blend solution. The spin-coating conditions are adjusted to obtain 120
nm P3HT/PCBM thick films. The organic films undergo post-annealing at 110 °C under
argon during 10 min. The devices are completed by successive thermal evaporation of
LiF (7 Å) and Al (100 nm) through shadows mask with a diameter of 7 mm2. The
reference cells were prepared by deposition of 20 nm film of PEDOT:PSS on ITO
followed by an annealing at 120°C under argon for 30 min. Successive layers are
deposited under the same conditions.
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3. Results and discussion
The UV-visible absorption spectrum of DIPO-Ph4 as recorded in
dichloromethane (Fig. 2) shows a structured absorption peak in the visible range
(λ
max=456 nm) with a high molar extinction coefficient 9×104 l/mol.cm. An absorption
tail of weak intensity is observed from 500 to 600 nm. Vacuum-deposited DIPO-Ph4
thin films exhibit a much broader absorption over almost the entire visible range, with
an absorption edge at 640 nm and several structures assigned to absorption bands.
Photoluminescence spectra in solution shows a sharp peak at 606 nm. Whereas DIPO-
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Ph4 thin film presents two peaks at the 636 nm and 686 nm. The bandgap of DIPO-Ph4
(
Eg=1.9 eV) is determined from the absorption spectrum. X-ray diffraction performed
on DIPO-Ph4 single crystals grown from solution reveals a parallel arrangement
between neighbouring planar molecules with an intermolecular distance of 3.49
Å.[31] Such a crystal structure is compatible with a π−stacking as well as a widening of
the optical absorption spectrum over the entire visible range as compared with that
in solution (Fig. 2b).
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Cyclic voltammetry (Fig. S1) shows two reversible oxidation waves at respectively 0.24
V and 0.5 V, corresponding to the successive formation of the radical cation and
dication. The HOMO energy level is estimated at −4.7 eV from the first oxidaꢀon peak
at 0.24 V. DIPO-Ph4 has a slightly lower oxidation potential (Eox=0.24 eV) as compared
to that of its sulfur analog DITPY-Ph4 (Eox=0.30 eV). Therefore the highest occupied
molecular orbital (HOMO) of DIPO-Ph4 intercalates adequately between ITO and P3HT
(EHOMO=−5.0 eV) to allow hole transfer from the acꢀve layer.
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Fig. 2 a) Optical absorption (blue) and photoluminescence (red) spectra of DIPO-Ph4 a)
in solution in dichloromethane and b) as a 50 nm thin film vacuum-deposited on
quartz. Inserts: photographs of the respective DIPO-Ph4 solution and thin film.
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DIPO-Ph4 thin films are prepared by slow vacuum-deposition (0.1 Å/s) onto
glass/ITO substrates. Atomic force microscopy (AFM) reveals a highly homogenous
surface made of monodisperse grains having an elongated needle shape (Fig. 3). A
distribution of the number of grains as a function of their height shows an average
height in the range 20-40 nm while the highest needles observed are 70 nm high. The
root mean square (RMS) roughness is measured at 13 nm which is much higher than
PEDOT:PSS film (RMS=0.9 nm). This particular nanomorphology of DIPO-Ph4
considerably increases the contact area between the active material and the anode,
thus greatly enhancing hole extraction. However, in order to protect the
nanostructure from partial dissolution by the organic solvent during spin-coating
deposition of the P3HT:PCBM active layer, a 5 nm thin layer of PEDOT:PSS is spin-
coated (from an aqueous solution) on top of the DIPO-Ph4 films. AFM images (Fig. S2)
confirm that the DIPO-Ph4 morphology is preserved (RMS=11.3 nm). A practical
advantage of PEDOT:PSS film is its robustness towards organic solvents during
fabrication process, making it compatible with most of organic photoactive materials.
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Fig. 3 (a) Tapping-mode AFM images of a 10 nm thin DIPO-Ph4 film vacuum-deposited
on ITO. (b) 3D lateral view of a typical AFM image of a 10 nm DIPO-Ph4 thin film. (c)
Distribution of the needles height in a 10 nm thin film of DIPO-Ph4.
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Current-sensing atomic force microscopy (CS-AFM) images of DIPO-Ph4 films are
recorded in contact mode using either positive or negative tip voltages (Fig. 4). For a
−0.3 V tip voltage the CS-AFM images exhibit high and homogeneous hole conduction
with current intensities up to 10 nA. The hole conduction have preferential pathways
through the grain boundaries. In comparison, CS-AFM images recorded on PEDOT:PSS
films reveal insulating region and the holes are only diffused in some localized
pathways which correspond to individual PEDOT domain (Fig. S3). Current mapping
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recorded at +0.3 V shows a much weaker conduction of electrons as compare to
holes.
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Fig. 4 CS-AFM images (5×5 µm²) of a thin film of DIPO-Ph4 (10 nm) vacuum-deposited
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on an ITO/glass substrate recorded at tip voltages of –0.3 V (left) and +0.3 V (right).
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We inserted a vacuum-deposited DIPO-Ph4 film (10 nm) as an anodic IFL in
combination with a PEDOT:PSS layer (5 nm) into P3HT:PCBM-based OSC devices
(Figure 5) and compared the PV performance with those of reference cells using a
PEDOT:PSS-only IFL. All devices have an open circuit voltage (Voc) of ≈0.6 V in
agreement with usual values reported for P3HT:PCBM-based devices. The short-
circuit current density of the reference cells is Jsc=7.6 mA.cm-2, resulting in a power
conversion efficiency (PCE) of 2.8% (Fig. 6, blue curve). With a 10 nm thin layer of
DIPO-Ph4, the organic solar cell exhibit higher current density. Typical devices exhibit
an average current density of Jsc=9.5 mA.cm-2 and an average PCE of 3.8% (Fig.6, red
curve). These characteristics correspond to a PCE increase of 35% as compare to the
reference cell using a PEDOT:PSS-only IFL. In the course of our experiments several
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double-IFL devices exhibit higher current densities up to Jsc=11.5 mA.cm-2 with a PCE
of 4.6% (Fig.6, light blue curve) corresponding to a 64% increase as compare to OSCs
using a PEDOT:PSS-only IFL.
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Fig. 5 left) Scheme of a solar cell using a double IFL: DIPO-Ph4 (10 nm, red needles)
and PEDOT:PSS (5 nm, dark grey coating). Right) Band diagram of a solar cell using a
DIPO-Ph4 (10 nm)/PEDOT:PSS (5 nm) double IFL.
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We assign the enhanced Jsc to the high hole conduction of the DIPO-Ph4 films, as
revealed by CS-AFM (Fig. 4). However, these higher characteristics are sample-
dependent due to the difficulty in controlling rigorously the nanomorphology of the
DIPO-Ph4 films, in particular the precise height, shape and density of the crystalline
needles. These parameters are extremely sensitive to slight variations in the
deposition rate, quality of the substrate and purity of the starting materials. Indeed,
devices using higher DIPO-Ph4 thicknesses (15 nm) show lower Voc (Fig. 6, green
curve) and thus a decrease in efficiency (PCE=2.5%).
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a/
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b/
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Fig. 6 (a) J-V curves measured in the dark and under white light illumination (AM 1.5G;
70 mW/cm²) of two P3HT:PCBM-based solar cells using DIPO-Ph4 (10 nm; red and
cyan lines)/PEDOT:PSS (5 nm) or DIPO-Ph4 (15 nm; green line) /PEDOT:PSS (5 nm) as a
double hole injecting IFL. A typical reference cell using a PEDOT:PSS-only IFL is shown
for comparison (blue line). b) EQE spectrum (red) of a PV cell using a DIPO-Ph4 (10
nm)/PEDOT:PSS (5 nm) double IFL. The dashed blue line represents the integrated
current density corresponding to the red J-V curve of Fig. 6a. The PV characteristics of
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our devices are summarized in Table 1.
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Table 1
IFL
Voc
(V)
Jsc
FF PCE
(%)
(mA/cm2)
PEDOT:PSS
Average values
0.60
0.58
7.6
0.44 2.8
DIPO-Ph4 (10nm)/PEDOT:PSS (5nm) Average values
9.5
11.5
8.1
0.50 3.8
0.47 4.6
0.40 2.5
Maximum values 0.60
DIPO-Ph4 (15nm)/PEDOT:PSS (5nm) Average values 0.55
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4. Conclusions
In summary, highly crystalline thin films of 2,2’,6,6’-tetraphenyldipyranylidene (DIPO-Ph4)
have been prepared by vacuum-deposition and used as hole injecting interfacial layers in
organic solar cells. The DIPO-Ph4 thin films are made of vertically-aligned nano-needles.
Current-sensing atomic force microscopy (CS-AFM) reveals a considerable increase of the
hole-carrying pathways in DIPO-Ph4 thin films as compare to PEDOT:PSS, thus revealing their
hole transporting/electron blocking properties. Inserting a 10 nm thick IFL of DIPO-Ph4 in
combination with a 5 nm thick PEDOT:PSS between the ITO electrode and the P3HT:PCBM
film leads to photocurrent densities up to 11.5 mA/cm2 under AM1.5G and conversion
efficiencies up to 4.6%, that is substantially higher than PEDOT:PSS-only devices. We are
currently extending this work by using DIPO-Ph4 as a HTM in various types of devices,
including perovskite solar cells and field-effect transistors.[32] We also engineer the band
alignment of DIPO-Ph4 by chemical design and synthesize a series of dipyranylidene analogs
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bearing electron-donating or electron-accepting peripheral substituents around the quinoïd
core.
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Acknowledgements
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The authors wish to thank the support from the Ministry of Education in Singapore under
the AcRF Tier 2 (MOE2014-T2-1-132).
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Appendix A. Supplementary Data
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Supplementary
data
related
to
this
article
can
be
found
at
http://dx.doi.org/10.1016/j.dyepig
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Highlights
Structural and electronic properties of 2,2’,6,6’-tetraphenyl-
dipyranylidene and its use as a hole-collecting interfacial layer in
organic solar cells
M. Courté,‡a M. Alaaeddine,‡b,c,d V. Barth,b,c,d L. Tortech,b,c,d and D. Fichou a,b,c,*
• Dipyranylidene acts as an efficient anodic interlayer in organic solar cells.
• It possesses excellent hole transporting/electron blocking properties.
• Photocurrent densities up to 11.5 mA/cm2 are recorded under AM 1.5G
• Conversion efficiencies up to 4.6% are attained, higher than PEDOT:PSS-only
devices
• The origin is also to be found in the high crystallinity of dipyranylidene films.