Single-material organic solar cells with fully conjugated electron-donor alkoxy-substituted bithiophene units and electron-acceptor benzothiadiazole moieties alternating in the main chain

Article abstract of DOI:10.1039/d0tc00541j

Main chain conjugated linear polymers, constituted by alternating electron-donor (D) and-acceptor (A) moieties, have been prepared with the aim of testing their performances as photoactive components in single material organic solar cells (SMOSCs). The D moiety is constituted by bithiophene co-units bearing in position 3 of the thiophene ring a hexyloxy, a hexyloxymethyl or a hexyl group, while the A moiety is represented by the benzothiadiazole group. The D-A polymers were obtained in high yield through the poorly demanding oxidative FeCl₃ polymerization process-starting, respectively, from the related precursors 4,7-bis(3-hexyloxythiophen-2-yl)benzo[c][2,1,3]thiadiazole, 4,7-bis[3-(6-methoxyhexyl)thiophen-2-yl]benzo[c][2,1,3]thiadiazole and 4,7-bis(3-hexylthiophen-2-yl)benzo[c][2,1,3]thiadiazole-with low dispersity indexes, close to the monodisperse state, after fractionation with methanol. The materials have been thoroughly characterized for their physical and structural properties and then tested for photoconversion efficiency in SMOSCs by using different deposition procedures of the photoactive component. In agreement with Kelvin probe force microscopy (KPFM) measurements, the best photovoltaic performance was observed for the polymer based on conjugated 3-alkoxythiophene and benzothiadiazole moieties, achieving significant photocurrents for this type of fully conjugated alternating D-A structure (J_sc = 2.63-3.72 mA cm^-2).

Full text of DOI:10.1039/d0tc00541j

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ISSN 2050-7526
PAPER
Nosang V. Myung, Yong-Ho Choa et al.
A
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Single-material organic solar cells with fully
conjugated electron-donor alkoxy-substituted
bithiophene units and electron-acceptor
benzothiadiazole moieties alternating in the main
chain
Martina Marinellia, Massimiliano Lanzia, Andrea Lisciob, Alberto Zanellic, Mattia Zangolic,
Francesca Di Mariac,d*, Elisabetta Salatellia,*
a
Dpt. of Industrial Chemistry Toso Montanari, University of Bologna, Viale del
Risorgimento 4, I-40136 Bologna, Italy
b
CNR-IMM, Via del Fosso del Cavaliere 100, I-00133 Roma, Italy
c
CNR-ISOF, Via P. Gobetti 101, I-40129 Bologna, Italy
d
Mediteknology srl, Via P. Gobetti 101, I-40129, Bologna, Italy
*
Corresponding authors: elisabetta.salatelli@unibo.it (E. Salatelli)
francesca.dimaria@isof.cnr.it (F. Di Maria)
KEYWORDS Single-material organic solar cells • alternating D–A polymers • spray coating
method • photo-charges • alkoxy alkyl substituents.
1

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ABSTRACT: Main chain conjugated linear polymers, constituted by alternating electron-donor
(D) and -acceptor (A) moieties, have been prepared with the aim of testing their performances as
photoactive components in single material organic solar cells (SMOSCs). The D moiety is
constituted by bithiophene co-units bearing in the position 3 of the thiophene ring an hexyloxy, a
hexyloxymethyl or a hexyl group, while the A moiety is represented by the benzothiadiazole
group. The D–A polymers were obtained in high yield through the poorly demanding oxidative
FeCl3 polymerization process - starting, respectively, from the related precursors 4,7-bis(3-
hexyloxythiophen-2-yl)benzo[c][2,1,3]thiadiazole,
4,7-bis[3-(6-methoxyhexyl)thiophen-2-
yl]benzo[c][2,1,3]thiadiazole and 4,7-bis(3-hexylthiophen-2-yl)benzo[c][2,1,3]thiadiazole - with
low dispersity indexes, close to the monodisperse state, after fractionation with methanol. The
materials have been thoroughly characterized for their physical and structural properties and then
tested for photoconversion efficiency in SMOSCs by using different deposition procedures of the
photoactive component. In agreement with Kelvin probe force microscopy (KPFM)
measurements, the best photovoltaic performance was observed for the polymer based on
conjugated 3-alkoxythiophene and benzothiadiazole moieties, achieving significant photocurrents
for this type of fully conjugated alternating D–A structures (Jsc = 2.63-3.72 mA cm-2).
2

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1. INTRODUCTION
Polymeric solar cells (PSCs) have been widely investigated in the past decades for their
attractive combination of unique properties such as flexibility, lightweight, low cost and
production easiness.1 PSCs are generally constituted by a bicontinuous interpenetrating network -
made of a conjugated polymer acting as electron donor (D) and an electron acceptor moiety (A) -
capable of absorbing sunlight, producing excitons and generating electrical charges that can
migrate to the electrodes.1-3 A blended structure, namely bulk hetero-junction (BHJ), is currently
adopted as the standard architecture for organic PSCs. Regioregular poly(3-alkylthiophene)s
(P3AT)s thanks to their high solubility, film-forming ability and structural versatility have been
the most studied and promising D materials in BHJ PSCs, with [6,6]-phenyl C61 methyl butyrate
(PCBM) as the A counterpart.3,4 However, their relatively large band gap limits the amount of
solar light that can be absorbed. Therefore, researchers are actively synthesizing polymers with a
narrower band gap, higher carrier mobility and a wider absorption up to the near infrared region.5-10
It has been proved that the synthesis of conjugated copolymers combining D and A units within
the same macromolecule is a powerful strategy to obtain low band gap materials, since the
intramolecular charge transfer mechanism in D–A structure allows the extension of the
conjugation length and affects the band gap values leading to the formation of new high-lying
HOMO and low-lying LUMO energy levels.5-11 Recently, D–A polymers were also employed as
photoactive materials for the construction of single-material organic solar cells (SMOSCs), i.e.
cells in which the photon-to-electron conversion process is ensured by one material in which D
and A units are covalently linked.12,13 The development of solar cells that employ a single material
is highly promising since it could significantly reduce the complexity and cost of cell fabrication
while overcoming the issue related to the morphological instability of multicomponent BHJs.14-16
3

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For this purpose, several materials have been designed in the last years that differ essentially for
the nature of D and A moieties, and, in particular, in their connection mode which determines the
different intra- and intermolecular interactions.13,17-22 The D and A moieties can be covalently
linked by a flexible insulating spacer, such as in “double-cable” systems, or by different conjugated
bridges such as acetylenic, aromatic, olefinic (D––A) or fully conjugated (D–A), as in “in chain”
D–A systems.17-22 We have recently reported that “in chain” D–A oligomers and polymers,
alternating “sulfur-overrich” bis(3,4′-S-alkyl)-2,2′-bithiophenes as D units and 2,1,3-
benzothiadiazoles, as A units, display photovoltaic behavior in SMOSCs devices with power
conversion efficiencies in the 0.24−0.54% range.23 Up to now, these materials are among the few
examples of fully conjugated D–A structures that lead to SMOSCs with significant photocurrents
(Jsc = 1.70-2.40 mA cm-2), a most interesting result since in these systems it was expected to be
difficult to achieve an efficient nanophase separation between D and A moieties, with a consequent
fast charge recombination and consequential low photocurrents.13 It is important to note that only
two other examples with a similar architecture are reported to work in SMOSCs, i.e., a
triphenylamine-oligothiophene conjugated system and a DPP-thieno-TTF copolymer.24,25 At
present, polymers with a D–A–D–A sequence display lower efficiencies compared to block
copolymers (DDDD–AAAA) and double cable polymers (A~DDDD~A).17-22 However, only in
fully conjugated molecular D–A systems it is possible to investigate the structural factors and
minimal size of a D–A system capable of dissociating excitons. We report here a novel family of
fully conjugated “in chain” D–A polymeric systems working as single materials in SMOSCs
capable of reaching efficiencies up to ~ 0.8% with Jsc of 3.72 mA cm-2. We synthesized a polymer
based on conjugated 3-alkoxythiophene and benzothiadiazole moieties, as D and A building blocks
respectively (P3a), and other two polymers that differ in the substitution of the side chain of the D
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group: in one case is present an oxygen atom in a non-conjugated position relative to the thiophene
unit (P3b), thus neutralizing its electronic effect, while in the other it is absent (P3c) (Scheme 1).
The structural, electrochemical and photophysical properties of the polymers were investigated by
differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), 1H-NMR, ultraviolet-
visible (UV-Vis) and photoluminescence (PL) spectroscopy, as well as X-Ray diffractometry
(XRD), cyclic voltammetry (CV), external quantum efficiency (EQE), atomic force microscopy
(AFM) and Kelvin probe atomic force microscopy (KPFM). All polymers were finally tested as
active media in air-processed SMOSCs. Due to charge recombination negatively affecting the
device performance, a proper nanophase separation between D and A in SMOSCs is fundamental
to simultaneously achieve an efficient exciton diffusion, charge separation and transport.12,13 As
this may be conditioned by the deposition process of the photoactive component, the spray coating
method was also adopted for the preparation of the devices, as it shows great potential for large
scale production,26 and the results compared to those obtained by the usual doctor-blade deposition
procedure. In particular, scanning probe measurements provide direct visualization of the
formation of photogenerated charges and clearly show that the presence of clusters in spray-coated
thin film plays a crucial role in increasing the performance of the device, underlining the
importance of the grain boundaries and “disorder”.
2. EXPERIMENTAL
Materials and methods. The synthetic procedure of the intermediates, monomers and polymers
are available in the Supporting Information. 3-bromothiophene, 3-hexylthiophene, 2,1,3-
benzothiadiazole-4,7-bis(boronic acid pinacol ester), sodium bicarbonate, sodium sulphate,
potassium hydroxide, palladium(II)chloride dichloromethane complex (PdCl2dppf), palladium-
tetrakis(triphenylphosphine) [Pd(PPh3)4], 3-methoxythiophene, 1-hexanol, p-toluenesulfonic acid
5

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monohydrate,
p-methoxyphenol,
1,6-dibromohexane,
[1,3-
bis(diphenylphosphino)propane]dichloronickel(II) [Ni(dppp)Cl2] were purchased from Sigma-
Aldrich Co; N-bromosuccinimide and FeCl3 were purchased from Fluka. All reagents and solvents
were used as received. Organic solvents were dried by standard procedures.
TLC was carried out with 0.2-mm thick of silica gel 60 F254 (Sigma). Preparative column
chromatographies were carried out on glass columns with silica gel 60 (particle sizes 0.040-0.063
m, Sigma).
Instruments. Microwave (MW) irradiation was performed in a Milestone Microsynth Labstation
operating at 2450 MHz and equipped with pressure and temperature sensors.
1H-NMR and 13C-NMR spectra were recorded on a Varian Mercury (400-500 MHz) spectrometer
at room temperature in CDCl3 solutions with TMS as the internal standard. Chemical shifts are
given in ppm.
Molecular mass and dispersity of the polymers were determined in CHCl3 by gel permeation
chromatography (GPC) on a HPLC Lab Flow 2000 apparatus equipped with a Rheodyne 7725i
injector, a Phenomenex Phenogel MXL type column and an RI K-2301 KNAUER detector. The
calibration curve was obtained using monodisperse polystyrene standards.
Thermogravimetric analysis (TGA) of the polymers was carried out by a TGA TA Instruments
Q600 apparatus operating under nitrogen atmosphere in the 20÷800°C temperature range at a
heating scan rate of 20°C/min. A DSC TA Instruments Q2000 operating in the –50÷200°C
temperature range at a heating rate of 10°C/min under nitrogen atmosphere was used for the
thermal analyses by differential scanning calorimetry (DSC).
UV-Vis and photoluminescence spectra were run by using a Perkin Elmer Lambda 20 and Perkin
Elmer LS50B spectrophotometer, respectively, at room temperature on 1.3 10–3 M chloroform
6

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solutions in 0.1 and 1 cm quartz cells. Solid state measurements were made on polymer samples
cast from chlorobenzene solutions on quartz slides (5 mg mL-1) by the doctor-blade technique or
spray coating.
X-Ray diffraction analysis (XRD) was performed using a PANalytical X’Pert diffractometer
equipped with a copper anode (λmean = 0.15418 nm) and a fast X’Celerator detector on polymer
samples deposited from chlorobenzene solutions on ITO (5 mg mL-1) by the doctor-blade
technique or spray coating.
Cyclic voltammetry measurements (CV) were performed at 0.1 V/s on an AMEL model 5000
potentiostat/galvanostat controlled by the software Corrwere 2.9 for Windows. A home-made
three- compartments electrochemical cell equipped with a Pt wire auxiliary electrode and an aq.
KCl Saturated Calomel reference Electrode (SCE), both connected to the working compartment
with a liquid bridge, was purged with Ar and maintained under Ar pressure during the
measurements. The supporting electrolyte was 0.1 M (C2H5)4NBF4 (Sigma-Aldrich, puriss.
vacuum dried) in anh. propylene carbonate (Sigma-Aldrich, stored under Ar pressure) where the
oxidation potential of ferrocene resulted 0.50 V vs. SCE. Polymeric thin films were made by
doctor-blade technique or spray coating from chlorobenzene solutions (5 mg mL-1) on indium tin
oxide (ITO) electrodes (2.25 cm2) at room temperature. HOMO, LUMO and band gap (Eg)
energies, expressed in eV units, were estimated on the basis of the following relationships: EHOMO
= – (Eox + 4.68), ELUMO = – (Ered + 4.68), Eg = – (EHOMO – ELUMO) where -4.68 eV is potential of
SCE vs. the electron in the vacuum27-29 that, taking into account the potential of the liquid junction,
corresponds to a ferrocene potential of about -5.1 eV in agreement with Cardona et al.30-33
Atomic force microscopy (AFM) and Kelvin probe force microscopy (KPFM) measurements were
performed in air by employing a commercial digital microscope Bruker MultiMode 8. In order to
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obtain a sufficiently large and detectable mechanical deflection, we used (k = 2.8 N/m) Pt/Ir coated
Si ultra-levers (SCM-PIT, Bruker) with oscillating frequencies in the range between 60-90 kHz.
AFM and KPFM images were acquired in the same measurement; a topographic line scan is first
obtained by AFM operating in semicontact mode and then that same line is rescanned in lift mode
with the tip raised to a lift height of 20 nm. Images were acquired with typical scan-rate 10 ms-
1. Raw AFM data were filtered to remove experimental artifacts by using histogram flattening
procedures.34 The RRMS has been estimated by averaging the values obtained on 5 images acquired
on different regions of each sample (corresponding to about 750 µm2 of sampled surface). The
KPFM technique provides a voltage resolution of about 5 mV and a lateral resolution of a few tens
of nanometres.
External quantum efficiency (EQE) was measured by a 7-SC Spec III Modularized Solar Cell
Spectral Test System (SevenStar Optics, Beijing, PRC). The Jsc values were calculated from the
EQE curves according to Open Photovoltaics Analysis Platform. The thickness of the active layer
was determined using a Burleigh Vista 100 AFM in a non-contact tapping mode. The reported
photoconversion efficiency results were averaged from three different devices prepared under the
same operating conditions with good reproducibility, with a standard deviation of about 5%.
Polymeric solar cells (PSC). PSC were fabricated on commercial ITO-coated glass substrates
(2.5×2.5 cm, surface resistance 20 /sq) according to the following procedure: the ITO layer was
partially etched with acid (aq. HCl 10% wt.) and heated at 60°C for 15 min in order to obtain a
final area of 1.5x1.0 cm covered by ITO. The substrate was then cleaned using distilled water
followed by 2-propanol, and finally dried by a gentle nitrogen flow. A conductive thin layer of
poly(3,4-ethylenedioxythiophene):polystyrene sulfonic acid (Sigma-Aldrich, PEDOT:PSS, 2.8%
wt., dispersion in water), diluted 1:1 v/v with isopropanol, was deposited using the doctor blade
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technique over the previously treated ITO glass and subsequently heated in a Büchi GKR-50 glass
oven at 120°C for 2h under vacuum. The active layer constituted by the synthesized polymer
sample was cast from chlorobenzene solution (5 mg mL-1) by doctor-blade technique or spray
coating and then annealead under vacuum at 120°C for 30 min. The device fabrication was
completed by thermal evaporation of the Al electrode over the active layer through a shadow mask
using an Edwards 6306A coating system operating at 10-6 mmHg (final active area of 1.0 × 1.0
cm). The current-voltage (I-V) characteristics and PCE were measured in air at room temperature
using a Keithley 2401 source meter under the illumination of an Abet Technologies LS150 Xenon
Arc Lamp Source AM 1.5 Solar Simulator (100 mW cm-2), calibrated with an ILT 1400-BL
photometer. The final structure of the device was: ITO (80 nm)/PEDOT:PSS (120 nm)/active layer
(ca. 100 nm)/Al (50 nm).
3. RESULTS AND DISCUSSION
3.1 Synthesis and characterization of polymers P3a-c. The monomers 3a-c, employed for the
synthesis of polymeric derivatives P3a-c, were obtained starting from 3-hexyloxythiophene (1a),10
3-(6-methoxyhexyl)thiophene (1b)35 and 3-hexylthiophene (1c) (Scheme 1). In detail, precursors
1a-c were firstly treated with N-bromosuccinimide36 to give the corresponding 2-bromo
derivatives (2a-c) and then reacted with 2,1,3-benzothiadiazole-4,7-bis(boronic acid pinacol ester)
via Pd catalyzed Suzuki cross coupling. While monomers 3b-c were obtained with good yields
from 2b-c by Pd(PPh3)4 catalyzed Suzuki coupling reaction, and thus through a different synthetic
route with respect to that one reported in literature37 for 3c, the synthesis of 3a from 2a was carried
out following the synthetic protocol based on microwave (MW) assistance,38 recently adopted for
similar benzothiadiazole derivatives,23 with Pd(dppf)Cl2 as catalyst. The implementation of MW
irradiation allowed to obtain 3a in similar yields as 3b-c, but with a remarkable improvement of rate
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(30 min compared to 24 h), accompanied by a significant reduction of the reaction temperature (80°C
vs. 110°C), with a consequent lower production of by-products.
Scheme 1. Synthetic route for the preparation of polymers P3a-c.
(i) 1 equiv of NBS, DMF, 0°C; (ii) 0.5 equiv of 2,1,3-benzothiadiazole-4,7-bis(boronic acid
pinacolester), Pd(dppf)Cl2, NaHCO3, THF/H2O 2/1, MW, 80°C for 3a; 0.5 equiv of 2,1,3-
benzothiadiazole-4,7-bis(boronic acid pinacolester), Pd(PPh3)4, 2M K2CO3, toluene, reflux for 3b-
c; (iii) 4 equiv of FeCl3, CHCl3.
1
The chemical structure of compounds 3a-c was confirmed by H-NMR (Figure S1), which
showed a significant downfield shift of the resonances of the aryl protons for 3a, if compared to
the corresponding protons of 3b and 3c, indicative of an increased electron delocalization in the
benzothiadiazole moiety due to the mesomeric effect of the oxygen atom directly linked to
thiophene ring. The polymerization of 3a-c to obtain P3a-c derivatives was performed in good
yields through a well established and straightforward oxidative coupling using FeCl3. This
procedure is a non-regiospecific method when applied to 3-alkylthiophenes, as it leads to polymers
containing about 75% only of head-to-tail (HT) linkages; however, in the presented cases it allows
to obtain exclusively tail-to-tail (TT) junctions between the thiophene rings, as a consequence of
the symmetrical structure and chemical equivalence of the coupling positions in the precursors. It
is also worth noting that all samples, after purification, show values of the dispersity index close
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to the mono-disperse state, this uniformity in chain length may induce a favorable packing and
1
nanoscale morphology for the separation of hole and electron transport. The H-NMR spectra
(Figure S10) confirm the expected structure of the polymeric samples, with P3a showing
resonances for the aryl protons moved downfield with respect to those of the other samples, as
already observed for the precursors 3a-c.
Table 1. Yields and characterization data of polymeric derivatives.
[c]
[d]
Polymer
P3a
Yield (%)[a]
Mn (g/mol)[b]
8600
Mw/Mn
1.1
xn
Tg (°C)[e]
Td (°C)[f]
238
98
79
54
17.3
29.8
25.3
43
3
P3b
15700
1.1
144
P3c
11800
1.2
1
139
[a] Weight of polymer/weight monomer x 100; [b] Average molecular weight determined by GPC
in CHCl3; [c] Dispersity index; [d] Average polymerization degree; [e] Glass transition
temperature determined by DSC (second heating cycle); [f] Decomposition temperature
determined by TGA.
As far as the thermal properties are concerned, P3a exhibits higher values of Tg and Td than P3b
and P3c despite its lower molecular weight, probably due to a more compact arrangement of the
macromolecules in the solid state (Table 1). Moreover, the behavior of P3b results quite similar to
that of P3c, evidencing that the presence of methoxy groups in the side chains does not
substantially affect the Tg and Td values in the bulk (Figures S11 and S12). In all cases, the presence
of only glass transitions suggests that all the examined polymeric samples are predominantly in
the amorphous state. Accordingly, X-Ray diffraction measurements carried out on P3a-c powders
confirm the absence of any crystallinity in the bulk (Figure S14).
3.2 Optical and electrochemical characterization. As commonly observed in alternating D–
A polymers, in solution the UV-Vis spectra of P3a-c are characterized by broad absorption
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features, with the band at lower wavelength (below 400 nm) assigned to the π–π* electronic
transition of bithiophene donor units, and the band at higher wavelength (453–570 nm) related to
the intramolecular π–π* transition between the ground and the excited states of the donor-acceptor
charge transfer complex (Figure 1).37
Figure 1. Normalized absorption (A) and photoluminescence (B) spectra in CHCl3 of polymers
(green line: P3a; red line: P3b; black line: P3c). Normalized absorption spectra (C) of polymers
(green line: P3a; red line: P3b; black line: P3c) as films cast from chlorobenzene by the doctor-
blade technique (solid line) and spray coating (dashed line).
As shown in figure 1A-B, P3a displays a remarkable bathochromic shift of the maximum of
absorption and emission when compared to P3b and P3c because of the presence of oxygen directly
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linked to thiophene ring. Moreover, the lower Stokes shift of P3a is indicative of a reduced energy
level of the excited state, related to the presence of a more ordered conformation of the
macromolecules in solution promoted by the increased resonance in the main chain. The higher
absorption and emission max values of P3b with respect to P3c may be attributed to a higher
polymerization degree of the former sample instead of an electronic effect by the side-chain
methoxy-group on the aromatic system, since the absorption spectra of their precursors 3b and 3c
are not shifted. (Figure S13). We also analyzed the UV-Vis spectra of thin films of the polymers
deposited by doctor-blade and spray coating techniques from chlorobenzene. The UV-Vis spectra
are reported in Figure 1C and the related data in Table 2. The absorption spectra of the polymers
deposited by the doctor-blade technique appear significantly red-shifted if compared to the
corresponding absorption spectra in solution and exhibit broader bands (Fig. 1C). However, P3a
displays a larger bathochromic effect (74 nm) with respect to the solution than P3b (20 nm) and
P3c (47 nm). On the other hand, the spray coating technique produces thin films slightly red shifted
with some further absorption bands at higher wavelength, in particular for P3a and P3b (Fig. 1C),
probably because of more suitable conformations that better pack in the solid state. It is to be noted
that, X-Ray reflectivity (XRR) and X-Ray diffraction (XRD) measurements, performed on all thin
film polymer samples, indicate the absence of crystallinity (Figure S14). As reported in Table 2,
the lowest optical energy gap values were found for the spray-coated thin films in agreement with
electrochemical measurements, with P3a showing a value of 1.48 eV, which becomes close to 2
eV for P3b and P3c.
Table 2. Maximum absorption (λmax) and emission (λem) wavelengths (nm) and optical energy
gap of polymers P3a-c in CHCl3 solution and in thin film deposited by doctor-blade or spray
coating.
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Thin film
Solution
Doctor-blade
Spray coating
op
op
λmax
(nm)
λem
(nm)
Stokes shift
(nm)
λmax
(nm)
Eg
λmax
(nm)
Eg
Polymer
(eV)
1.53
2.03
2.10
(eV)
P3a
P3b
P3c
570
487
453
677
650
629
107
163
176
644
507
500
629
516
454
1.48
1.93
2.03
Cyclic voltammetry measurements were carried out on thin film of P3a-c prepared both by
doctor-blade and spray coating from chlorobenzene. The results are reported in Table 3 and the
related voltammograms in Figure 2. It can be noted that the oxidation potential of the polymers is
drastically reduced by the presence of oxygen in the side chain of thiophene rings regardless of the
deposition method adopted. This effect is more pronounced for P3a (~0.4 V), in which the oxygen
is conjugated with the thiophene ring, than P3b (~0.15 V) in which the oxygen is at the end of the
side chain. The fact that P3b shows lower oxidation potential than P3c (~0.2 V) cannot be
attributed to an electronic effect of the oxygen atom, as it is too far from the thiophene ring;
however, the presence of oxygen as terminal may influence the intra- and interchain interactions
of the side chains, increasing the polarity of the chains and decreasing the  stacking distance
of the backbone.39,40 Differently, the reduction potential values appear rather insensitive to the
chemical structure of the side chain substituent and strongly dependent on the deposition method.
Indeed, the reduction potentials of polymers P3a and P3b deposited by spray coating method
appear about 0.1 V more negative than those obtained by doctor blade, thus leading to lower energy
gap values, particularly for P3a, in agreement with the results obtained from the absorption spectra.
As highlighted, the method employed for the preparation of the thin films causes a fine tuning of
the energetic levels due to its impact on the resulting film morphology.
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Table 3. Redox potentials, HOMO/LUMO energy levels and electrochemical energy gap values
of the polymers.
onset
onset
(V vs. SCE)
(V vs. SCE)
E
E
red
Polymer
P3a[a]
P3b[a]
P3c[a]
P3a[b]
P3b[b]
P3c[b]
HOMO (eV) LUMO (eV) Eg (eV)
ox
0.66
0.87
1.02
0.62
0.86
1.03
1.17
1.16
1.05
1.05
1.04
1.07
5.34
5.55
5.70
5.30
5.54
5.71
3.51
3.52
3.63
3.63
3.64
3.61
1.83
2.03
2.07
1.67
1.90
2.10
[a] Polymers deposited by doctor-blade; [b] Polymers deposited by spray coating.
Figure 2. Cyclic voltammograms (second scans) in the solid state at 0.1 V s-1 of P3a-c films
(green line: P3a; red line: P3b; black line: P3c) prepared by doctor blade (A) and spray coating
(B) deposition.
3.3 Photovoltaic properties. The photovoltaic properties of the polymers were tested in
SMOSCs with structure ITO/PEDOT:PSS/photoactive polymer/Al. The J−V features for the best
performing solar cells are reported in Figure 3 and the related photovoltaic parameters listed in
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Table 4 (average values collected from ten devices). The film thickness of the three polymers range
between 1.5±0.3 m, as measured by profilometer.
Figure 3. J-V characteristics under white light illumination of P3a-c cells prepared by doctor-
blade (A) or spray coating (B) deposition (the best performing cells are displayed); EQE of the
P3a-c cells prepared by doctor-blade (C) or spray coating (D) deposition. (green line: P3a; red
line: P3b; black line: P3c).
As expected on the basis of energy gap values determined from optical spectra and cyclic
voltammetry, the devices containing the photoactive component P3a and P3b deposited by spray
coating show the best efficiency results, achieving the maximum value of 0.77% with P3a and
0.21% for P3b; differently, P3c shows the lowest PCE value. Indeed, our results are primarily
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originated by the Jsc values, which are dependent on factors related to the efficiencies of each stage
in the photovoltaic process, including the efficiency of light absorption, exciton diffusion, exciton
dissociation, charge transport, and charge collection. They appear enhanced, mainly for P3a, with
the spray coating procedure with respect the doctor-blade deposition. However, even if the Voc
values seem to be independent from the deposition method and the nature of the compound, they
are in good agreement with the trend observed for the electrochemical experiments carried out on
the polymer thin films. Indeed, when the Eg value decreases, the Voc - as being this parameter
dependent to a first approximation on the difference between the HOMO of the donor and the
LUMO of the acceptor - accordingly decreases. The overall cell performances of P3a and P3b
result to be improved by the spray coating deposition, suggesting a more favorable arrangement
of the macromolecules to exciton diffusion and charge transport. Moreover, it is also to be noted
that the presence of oxygen in the side chain of P3b, although not directly involved in the aromatic
conjugation, favours to some extent the photoconversion efficiency with respect to P3c. The
external quantum efficiencies (EQE) of the cells (Figure 3) display maxima ranging from 15%
(doctor-blade) to 25% (spray coating). The EQE values in the examined range were used for
calculation of Jsc, obtaining data (Table 7) consistent with the experimental findings, thus
confirming the best performance of the device prepared with P3a by spray coating. Furthermore,
the EQE profiles follow the trend observed in the UV-Vis spectra of polymers in thin film,
indicating that the whole absorption effectively contributes to the photocurrent. It is also
particularly noteworthy the wide range of absorption obtained for P3a deposited by spray coating,
that could be one of the factors on the basis of the high observed value of Jsc.
Table 4. Properties of organic solar cells prepared with polymeric derivatives P3a-c prepared by
doctor-blade or spray coating technique (average values collected from ten devices).
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Polymer
P3a[e]
P3b[e]
P3c[e]
P3a[f]
P3b[f]
P3c[f]
Jsc (mA/cm2)[a]
2.63±0.09
0.83±0.04
0.11±0.01
3.72±0.15
1.16±0.09
0.11±0.08
Jsc (EQE)
2.59
Voc (V)[b]
0.41±0.01
0.44±0.01
0.45±0.01
0.39±0.01
0.42±0.01
0.45±0.01
FF[c]
PCE (%)[d]
0.55±0.035
0.16±0.012
0.02±0.011
0.77±0.033
0.21±0.016
0.02±0.012
0.51±0.016
0.44±0.014
0.41±0.014
0.53±0.016
0.43±0.012
0.42±0.011
0.81
0.11
3.65
1.07
0.11
[a] Short circuit current; [b] Open circuit voltage; [c] Fill factor; [d] Photovoltaic cell efficiency;
[e] By doctor-blade technique; [f] By spray coating deposition.
3.4 Atomic force microscopy (AFM) and Kelvin probe force microscopy (KPFM)
measurements. Since the electrical properties of the films strongly depend on their structure, we
studied the surface morphology and the corresponding surface photo-voltage by AFM and KPFM
techniques and the values are reported in Table 5. All three polymers deposited by spray coating
exhibit a quite rough surface (RRMS), ranging from ca 40 nm (P3a) to ca 100 nm (P3b, P3c), as
depicted in the AFM images shown in Figure 4. Moreover, micrometric structures are present on
the surfaces with different sizes, in particular, in the range of 0.2–2 µm for P3a, larger than 5 µm
for P3b and smaller than 1 µm for P3c. Maps of the surface density of photogenerated charges
were measured by KPFM.23,41-43 KPFM measurements are performed illuminating continuously
all the samples (i.e. steady-state). We compared two different parameters: surface photovoltage
(SPV) and SPV variance. The first parameter is defined as the difference between the surface
potential under illumination and in the dark: SPV = SPlight − SPdark and, solving the Poisson's
equation for electrostatics, SPV is correlated to the photocharge density.27 Differently, the SPV
variance corresponds to the dispersivity of the measured data (i.e. the higher the SPV variance, the
lower the homogeneity of the photo-charge density). As observed, P3a film displays higher values
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of both SPV and corresponding variance than the other two samples, P3b and P3c. Thus, the
presence of patches with different SPV reflects in larger SPV variance indicating that the single
material can be described as a heterojunction with regions with different charge densities. This
inhomogeneity in the photo-charges reflects in an overall higher SPV value clearly showing a
better intrinsic performances of the system respect to the other two materials.
Table 5. AFM and KPFM data for P3a-c.
Polymer
P3a
RRMS (nm)[a]
SPV variance (mV)
480 ± 40
SPV (V)
1.4 ± 0.2
0.2 ± 0.1
0.1 ± 0.1
40
P3b
100
100
170 ± 20
P3c
100 ± 20
[a] Standard deviation = 10%.
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Figure 4. AFM (left, Z- range: 300 nm) and KPFM (right, Z- range: 1.2 V) images of P3a (A,
B), P3b (C, D) and P3c (E, F).
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4. CONCLUSIONS
Intrinsically regioregular linear D–A polymers - containing in the main chain alternating 4,4’-
disubstituted bithiophene (D) and benzothiadiazole (A) moieties - have been prepared in good
yields, through a straightforward oxidative polymerization process employing FeCl3, and tested as
active materials in SMOSCs. The side-chain substituent linked to the bithiophene moieties plays
an important role, affecting significantly the optical properties, the energy levels and gap values
of the material as well as their function as photoactive single components in solar cells. We show
that the introduction of an oxygen atom in the alkyl side chain determines a widening of the
absorption band with a progressive red-shift, a reduction of the energy gap and a longer life-time
of the generated photo-charges. In particular, we demonstrate that these effects are more
pronounced when oxygen is directly linked to the thiophene unit (P3a). Its high electronegativity
and mesomeric effect strongly influence the charge distribution over the molecule, light harvesting
property and nanophase separation. Finally, the SMOSC fabricated with the polymer P3a confirms
better performances in terms of photoconversion efficiency of sunlight than those fabricated with
P3b, i.e., the polymer with the oxygen atom in a non-conjugated position with respect to thiophene
unit, and P3c, i.e., the polymer without oxygen in the side chain, reaching significant values for
fully conjugated alternating D–A system.13 Moreover, in all cases the deposition of the polymers
through spray coating method allowed to improve the PCE values, if compared to deposition by
doctor blade. Indeed, the former method is likely to favour the formation of substructures on a
nanometer scale that favour a more efficient separation of hole and electron transport. Although
SMOSCs based on fully conjugated D-A polymers lead to much lower efficiencies than double
cable systems, the fact that such structure is capable of producing photocurrents poses many
fascinating fundamental questions that prompt further investigations on this type of systems.
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CONFLICTS OF INTEREST
There are no conflicts to declare.
ACKNOWLEDGMENT
The financial support from the University of Bologna is gratefully acknowledged. F. Di Maria,
acknowledge financial support from the UE project INFUSION (Engineering optoelectronic
INterfaces: a global action intersecting FUndamental conceptS and technology implementatION
of self-organized organic materials, Proposal number: 734834). The authors are deeply grateful to
Prof. Luigi Angiolini of the University of Bologna for helpful discussion. We also thank Dr.
Filippo Pierini for the EQE measurements and Dr. Massimo Gazzano and Dr. Fabiola Liscio for
XRD characterizations.
ASSOCIATED CONTENT
Synthesis of building blocks and polymers; 1H and 13C NMR spectra; TGA and DSC analysis;
UV-PL spectra of monomers; X-Ray diffraction patterns.
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