This document is the Accepted Manuscript version of a Published Work that appeared in final
form in J. Mater. Chem. A, 2016, 4, 11009 DOI: 10.1039/c6ta04369k
CuSCN as Selective Contact in Solution Processed Small Molecule Organic Solar Cells
Leads to over 7% Efficient Porphyrin Based Device.
Gabriela Morána, Susana Arrecheaa, Pilar de la Cruza, Virginia Cuesta,a S. Biswasd, Emilio
Palomaresb,c*, Ganesh D. Sharmad*, Fernando Langaa*
a

Universidad de Castilla-La Mancha. Institute of Nanoscience, Nanotechnology and
Molecular Materials (INAMOL), Campus de la Fábrica de Armas, Toledo. Spain. Tel: 34
9252 68843. E-mail: Fernando.Langa@uclm.es
b
Institute of Chemical Research of Catalonia (ICIQ).The Barcelona Institute of Science and
Technology. Avda. Països Catalans, 14. Tarragona. E-43007. Spain..E-mail
epalomares@ICIQ.ES
c
ICREA. Passeig Lluis Companys, 23. Barcelona. E-08010. Spain.
d
Department of Physics, The LNM Institute of Information Technology (Deemed University),
Rupa ki Nagal, Jamdoli, Jaipur (Raj.) 302031, India, E-mail: gdsharma273@gmail.com

Two conjugated acceptor -π-donor -π-acceptor (A-π-D-π-A) small molecules with zinc
porphyrin donor core, 3-ethylrhodanine terminal acceptors connected at meso positions by
ethynylenes and linked by one or two units of thienylenevinylene denoted by 1a and 1b, were
synthesized and their optical and electrochemical properties were investigated. The bulk
heterojunction organic solar cells fabricated with 1a:PC71BM and 1b:PC71BM processed
from THF exhibit power conversion efficiency of 2.75 % (Jsc = 7.96 mA/cm2, Voc = 0.96 and
FF = 0.36) and 3.18 % (Jsc = 8.84 mA/cm2, Voc = 0.90 V and FF= 0.40), respectively.
Moreover, the organic solar cells based on 1a:PC71BM and 1b:PC71BM processed from
pyridine/THF solution showed PCE of 5.27 % ( Jsc = 10.61 mA/cm2, Voc =0.92 V and FF=
0.54) and 5.78 % ( Jsc = 11.58 mA/cm2, Voc = 0.86 V and FF= 0.58), respectively. But most
important is the observation that the PCE was further improved to 7.24 % for devices based
on 1a:PC71BM and 1b:PC71BM active layer processed with pyridine/THF solution by
employing CuSCN as selective contact electrode instead of the most common hole transport
material, namely PEDOT:PSS.
Key worlds: Porphyrin small molecules, bulk heterojunction solar cells, solvent additives,
CuSCN hole transport material
*corresponding authors

Introduction
Solution processed organic solar cells (OSC) is a topic of current scientific interest as
complementary photovoltaic technology to the current Silicon solar cells due to their solution
processability, potential low cost of fabrication, lightweight and high transparency.1 At
present the power conversion efficiency (PCE) of solution processed organic solar cells based
on single BHJ active layer using low bandgap conjugated polymer has exceed 10 %2 and are
expected to be higher than 11 % for tandem configuration3 and more recently about 13.2 %4
for triple junction organic solar cells using evaporated organic thin films. In recent years,
small molecules, defined as organic molecules with low molecular weight, have attracted
much attention due to the well-defined molecular structure, easier purification, and less batch
to batch variations in comparison to polymers5 and the PCE have been improved significantly
up to 10 %, which is comparable to those of polymer counterparts.6
Porphyrins, the synthetic analogues of natural chlorophylls are of special interest,
since they are known to be nature choice as light harvesting antenna systems that are
involved in energy and electron transfer processes.7 Porphyrins exhibit large molar extinction
coefficients,8 tuneable electrochemical and photophysical properties via central metal
modification and/or introduction of suitable substituents at macrocycle peripheral positions.9
As an example, porphyrin dyes has been successfully used as sensitizer for dye sensitized
solar cells and achieved PCE more than 13 %.10 In fact, in recent years, there has been a
great focus of interest in the field of solution processed BHJ organic solar cells using
porphyrin derivatives as donor and resulted PCEs from 2 to 8 %.11 In organic solar cells the
thickness limitation, imposed to obtain good carrier extraction, obliges to use molecules with
outstanding light absorption. Thus, it is clear that porphyrins are a clear target for OSC.
We have previously demonstrated that molecules with a (A-π-D-π-A) structure, where
A is the electron acceptor unit, π is the conjugated moiety and D is the electron donor unit,
and electron withdrawing terminals is a general design rule for molecules that provide
efficient solar cells. Such molecules often present: (i) high mobility with a planar structure
and efficient π-π interactions, (ii) a low bandgap resulting from intramolecular charge
transfer and (iii) good film quality owing to a long conjugated backbone with dispersed alkyl
chains similar to polymers.

Herein we report the synthesis of two new conjugated A-π-D-π-A porphyrin small
molecules, along their photophysical and electrochemical properties. These molecules consist
of same thienyl substituted zinc porphyrin core as donor linked by ethynylenes to one (1a) or
two (1b) units of thienylenevinylene and capped by 3-ethylrhodanine groups as acceptor units
(the chemical structure is shown in Scheme 1). The ethynylenes induces molecular planarity
as well as extend the π-system, moreover, the use of hexyl chains enhance their solubility.

5
Scheme 1 Structure of 1a and 1b.
During preparation of this manuscript, a porphyrin–based small molecule with similar
structure to 1a, but lacking hexyl chains in the thiophene bridge,12 has been described with a
power conversion efficiency up to 4.97% after processing using pyridine followed by thermal
annealing. We want to remark that different alkyl chains provoke remarkable differences in
solid state properties that are crucial for exciton diffusion and charge transport and
consequently for the photovoltaic performance.13
These two porphyrin molecules were used as donor along with PC71BM as acceptor
component for the fabrication of solution processed bulk heterojunction organic solar cells
and showed over all PCE of 2.75 % and 3.18 % for 1a:PC71BM and 1b:PC71BM, respectively
processed with THF solution. When the active layer of device was processed with
pyridine/THF solution, the photovoltaic characteristics were significantly improved resulting
in PCE of 5.27 % and 5.78 %, for 1a:PC71BM and 1b:PC71BM, respectively. The
enhancement in PCE was ascribed to an increase in hole mobility, resulting in better values of
charge transport and charge collection rates due to the optimized crystallinity and the
nanoscale morphology of the active layer, induced by the solvent additive.

Although PEDOT:PSS is widely used as hole transport layer (HTL) in most of the
organic BHJ solar cells based on small molecules and polymers, due the mismatching of its
work function with the HOMO energy level of the donor component in the active layer, form
a barrier for hole extraction and not also a good electron blocking layer and has a tendency to
trap electrons leading to change in device performance over time.14 Moreover, due to the
hygroscopic, acidic and protonation nature of PSS in PEDOT:PSS, influences the device
stability. Copper (I) thiocyanate (CuSCN) as HTL as reported earlier for organic solar cells15
and hybrid perovskite solar cells.16 CuSCN is an inorganic molecular semiconductor, highly
soluble in diethylsulfide and diisopropyl sulphide can be readily processed to yield thin film
with high transparency across the solar spectrum and its hole mobility can be achieved up to
0.01 -0.1 cm2/Vs after the mild thermal treatment (~100° C) and is compatible with a variety
of organic D-A materials. In addition of desirable optical transparency, CuSCN posses a deep
valence band edge ~5.30 -5.35 eV that can help to extract the hole from the active layer as
well as maximize the Voc. Moreover, CuSCN can be deposited from solution using
appropriate solvents and concentrations at room temperature. Inspired by the work reported
in literature, we have used CuSCN as HTL to improve the PCE and achieved over all PCE
around 6.59 % and 7.24 % for devices based on 1a:PC71BM and 1b:PC71BM active layer
processed with pyridine/THF solutions.
RESULTS AND DISCUSSION
Synthesis and characterization

C6H13
S

C6H13
1. TMS

S
N
H
C6H13

S

CHO

CHO
Zn

R

TFA

NH HN

N

N

DDQ/Et3N/BF3—O(C 2H5)2
2. Zn(OAc)2

R
N

N

3

2

5 R = TMS

S

MeOH TBAF
6R=H

C6H13

C6H13

C6H13

S

I

C6H13

C6H13

n

CHO

S

7a: n=0
7b: n=1

Et3N/AsPh3/PD2(dba) 2

C6H13
C6H13
3-Ethylrhodanine

S
C6H13

C6H13

C6H13

OHC

1a / 1b
Piperidine

S

S

N

n
C6H13

N

S

S

CHO
n

Zn
C6H13

N

N

S

C6H13

C6H13

8a: n=0
8b: n=1

C6H13

Scheme 2. Synthetic route to 1a and 1b.
The synthetic approach for porphyrin based molecules 1a and 1b is depicted in
Scheme 2, starting from the Vilsmeier formylation of 2-hexylthiophene (yield: 94%) and
followed by reaction of aldehyde 217 with pyrrole, via a modification of Lindsey’s TFA acid
catalyzed condensation affording 2,2’-((5-hexylthiophen-2-yl)methylene)bis(1H-pyrrole) 3 in
77.5% yield.18 Next, under Ar atmosphere, 3-(trimethylsilyl)-2-propynal was added to a
solution of 3 in CHCl3 and BF3·O(C2H5)2 was added to form the porphyrinogen and this was
oxidized with DDQ; after one hour, Et3N was added, affording 5,15-bis(5-hexylthiopen-2yl)-10,20-bis((trimethylsilyl) ethynyl) porphyrin 4 in 7.3% yield. Porphyrin 4 was treated
with zinc acetate in CHCl3 at room temperature during one night and [5,15-bis(5hexylthiopen-2-yl)-10,20-bis((trimethylsilyl)ethynyl)porphyrinato] zinc (5) was obtained in
quantitative yield.19 The double deprotection of the terminal triple bonds was performed by
treatment of 6 with TBAF, then extracted with CHCl3, and the product was used in the
following step without further purification.
Aldehydes 8a and 8b were obtained by double copper-free Sonogashira coupling of
deprotected 6 with aldehydes 7a and 7b.20 In the presence of Et3N and using as catalyst
Pd2(dba)3 and triphenylarsine; compounds 8a and 8b where obtained in 60% yield (in both

cases) after purification by column chromatography. Finally, the target compounds 1a and 1b
were obtained in 95% and 34% yield respectively by a double condensation of 8a and 8b
with 3-ethylrhodanine and piperidine as base (for details, see the experimental section). All
new compounds were fully characterized by FT-IR, 1H- and 13C-NMR spectroscopy and
MALDI-TOF mass spectrometry of 1a and 1b confirmed the molecular structures showing
the molecular ions at 1597.27 and 2148.16 amu respectively (see ESI). Thermogravimetric
analysis (TGA) results suggest that 1a and 1b have good stability, with decomposition
temperatures (Td) above 360 ºC under an N2 atmosphere (see ESI, Fig. S18).
Optical and electrochemical properties
The normalized optical absorption spectra of 1a and 1b in THF solution and as well
as their thin films are shown in Fig. 1a and 1b and corresponding data were summarized in
Table 1. Compound 1a show a Soret band at 510 nm (lg ε = 5.21) and a broad band at 716
nm (lg ε = 4.95), assigned to a charge transfer transition from the porphyrin core to the 3ethylrhodanine fragment (Fig. 1a). In the other hand, 1b show two new bands at 493 nm (lg ε
= 5.12) and 548 nm (lg ε = 5.19) assigned to the π-π* transition of the conjugated oligomer,
the Soret band at 510 nm (lg ε = 5.15) and the charge transfer band at 722 nm (lg ε = 5.20)
(Fig.1b).
Table 1. UV-Vis Absorption and redoxa data for 1a and 1b
λmax soln

EHOMOd

(ε)

(V)

(V)

511

5.21

0.32

4.95

346

4.71

493

5.17

0.20

510

5.15

548

5.19

723

a

E1red

716
1b

E1oxb, c

(nm)
1a

log

Eg f

5.20

(eV)

ELUMO
e
(eV)

(eV)

-1.48

-5.42

-3.62

1.80

-1.54

-5.30

-3.56

1.74

10-5 M, in THF; b 10-3 M in ODCB-acetonitrile (4:1) versus Fc/Fc+ (Eox = 0.04 V) glassy

carbon, Pt counter electrode, 20 oC, 0.1 M Bu4NClO4, scan rate = 100 mV s-1; c No reversible

processes; d calculated with respect to ferrocene, EHOMO: –5.1 eV; e estimated from E1red; f Eg=
EHOMO–ELUMO.
In thin film, the absorption maxima of both porphyrins showed redshifted and broader
relative to that in solution. The redshift is more pronounced for 1a, and it is attributed to the
extended backbone of 1b resulting in stronger intermolecular π-π stacking interactions than
1a. Such an ordering packing of small molecules is in favour for achieving high charge
mobility.21 The optical bandgaps estimated from the onset of absorption spectra in thin films
are 1.48 and 1.44 eV, for 1a and 1b, respectively. The presence of 3-ethyl-rhodanine in these
molecules effectively decreases the band gaps and extends their absorption into the NIR
region.22 The fluorescence spectra of both porphyrins showed similar emission bands at 748
nm and 750 nm respectively. The transition energy i.e. energy between the lowest vibrational
ground state to lowest vibrational excited state (E0-0) values estimated from the intersection of
absorption and emission spectra (see ESI Fig. S21) are 1.71 eV (724 nm) and 1.66 eV (744
nm) for 1a and 1b, respectively. The lower value of E0-0 for 1b is attributed to the increased
conjugation by the additional thienylenevinylene unit.

Fig. 1 Normalized absorption spectra of 1a and 1b in dilute THF solution and thin film cast
from THF
HOMO and LUMO levels of these porphyrins were determined by cyclic
voltammetry (Fig. S22) and Osteryoung Square Wave Voltammetry (OSWV) (Fig. S23) and
are summarized in Table 1. Due to the more extended conjugation, the HOMO energy of 1b
(-5.30 eV) is higher than that of 1a (-5.42 eV), being both compatible with the LUMO of
[6,6]-phenyl-C71-butyric-acid methyl ester (PC71BM) (-4.00 eV). Comparing with PC71BM
used as acceptor in devices, the energy difference between the HOMO level of porphyrin
compounds 1a and 1b, and the PC71BM LUMO level, is 1.42 eV for 1a and 1.30 eV for 1b,
values which in principle allow high open circuit voltages. The ELUMO values were estimated
from the reduction potential in similar way and are -3.62 eV and -3.56 eV for 1a and 1b,
respectively. The electrochemical bandgap for 1a and 1b are 1.80 eV and 1.74 eV,
respectively and the trend is consistent with the optical bandgap and E0-0. The deeper HOMO

energy levels of these materials are favourable to achieve the high Voc of the corresponding
BHJ OSCs. Moreover, the LUMO energy levels are higher than that for PC71BM (-4.0 eV)
for efficient exciton dissociation, which indicates their suitability for their use as donors and
PC71BM as acceptor in BHJ OSCs. Since the porphyrin core is weak donor and 3-ethylrhodanine is strong acceptor form a weak donor – strong acceptor configuration can provide a
favourable HOMO and LUMO energy levels, as well as suitable energy bandgap, which is
beneficial for the efficient photovoltaic performance of BHJ OSCs.23
Theoretical calculations
Theoretical geometry and HOMO-LUMO properties of 1a and 1b were measured by
(DFT) at the B3LYP 6-31G* (d, p) level, in gas phase, using Gaussian 03W. The most stables
geometries of 1a-b are shown in Fig. 2. Thienylenevinylene fragments are almost coplanar
with the plane described by the porphyrin ring and the triple bond, with dihedral angle of
2.84° for 1a and 5.29° for 1b. This planarity allows an extension of the conjugation between
the porphyrin and the rhodanine fragments. The hexylthiophene rings are linked to the
porphyrin cage with a dihedral angle of 69.5°.

Fig. 2 Geometric structure of 1a and 1b
The theoretical distributions of the orbital coefficients of the HOMO and LUMO
states (Fig. 3), shows that there exists overlapping between the HOMO and LUMO, in both
molecules, favoring the HOMO-LUMO electronic transitions. The charge density of the
HOMO of 1a,b is delocalized over the porphyrin and thienylenevinylene fragments. The
theoretical difference of HOMO-LUMO gap agree with the values experimentally obtained,
1.87 eV and 1.74 respectively for 1a and 1b.

Fig. 3 Electronic density contours and energy levels for HOMO and LUMO calculated for 1a
and 1b
Photovoltaic properties
The solution processed BHJ OSCs were fabricated employing these porphyrins as
donors and PC71BM as acceptor with a conventional device structure of ITO/ HTM/
donor:PC71BM with different weight ratios) /Al. The active layer was spin cast from the
appropriated solution. The current-voltage characteristics under illumination (AM1.5 100
mW/cm2) and incident photon to current conversion efficiency spectra of the devices
processed from 1:2 weight ratio of 1a or 1b and PC71BM blended film are shown in Fig. 4a
and 4b, respectively and the photovoltaic parameters are compiled in table 2.
The device based on 1a:PC71BM and 1b:PC71BM processed without pyridine additive
showed a PCE of 2.75 % (Jsc = 7.96 mA/cm2, Voc = 0.96 and FF = 0.36) and 3.18 % (Jsc =
8.84 mA/cm2, Voc = 0.90 V and FF= 0.40), respectively. Both the devices showed reasonably
high value of Voc, due to their deeper HOMO energy levels. The higher value of value of Voc
for the device based on 1a as compared to 1b, attributed to the relatively deeper HOMO level

of 1a than 1b, since the Voc of the BHJ OSC is directly related to the energy difference
between the HOMO energy level of donor and LUMO energy level of acceptor employed in
BHJ active layer. The higher value of PCE for 1b as compared to 1a mainly attributed to the
superior value of Jsc and FF. As shown in Fig. 4b, the IPCE spectra of the 1a:PC71BM and
1b:PC71BM based OSCs showed broad response covering from 350 nm to 850 nm, consistent
with the absorption spectral properties of 1a and 1b thin films. Moreover, the device based on
1b showed slightly wider response and high values of IPCE, which is consistent with its
absorption spectra. The LUMO offset between the 1b and PC71BM is larger than that for 1a
and PC71BM indicates the stronger driving force for the later than former may also be the
reason for higher Jsc for the solar cell based on 1b.
Table 2 Photovoltaic parameters of organic BHJ solar cells based on 1a:PC71BM (1:2) and
1b:PC71BM (1:2)
Active layer
1a:PC71BMa
1b:PC71BMa
1a:PC71BMb

Jsc
(mA/cm2)
7.86
8.84
10.61

Voc
(V)
0.96
0.92
0.90

0.36
0.40
0.54

PCE
(%)
2.75
3.18
5.27

1b:PC71BMb
1a:PC71BMc
1b:PC71BMc

11.58
10.74
11.67

0.86
0.99
0.94

0.58
0.62
0.66

5.78
6.59
7.24

FF

Rs
Rsh
2
(Ω cm ) (Ω cm2)
52
240
43
256
26
308
18
16
12

378
386
412

CB cast; b Pyridine 3% v / THF cast; c CuSCN HTL and active layer processed with pyridine
3% v / THF
a

Both, 1a and 1b have same chemical structure except for the thienylenevinylene
conjugation length. In order to get information about the influence of conjugation length of π
linker on the charge transport properties, hole mobility (µh) in the active layer was measured
J-V characteristics in dark using a hole only device, i.e. ITO/PEDOT:PSS/active layer/Au
and fitting the curves with space charge limited current model. Fig. 5 shows the J-V
characteristics of the hole only devices using 1a:PC71BM and 1b:PC71BM active layers. The
hole mobility estimated from the Fig. 5 and fitting with SCLC model are 7.32 x10-6 cm2/Vs
and 9.23 x10-6 cm2/Vs for 1a and 1b, respectively. The hole mobility is determined by the
intramolecular backbone, intermolecular packing and π-π stacking. Since both the porphyrins
have same molecular structure except the conjugation length of π-linker, therefore the
difference in the hole mobility may be due to the intermolecular π-π stacking. The two units
of thienylenevinylene in 1b may leads to strong intermolecular π-π stacking, resulted higher

mobility than 1a. In comparison to 1a, 1b showed stronger intermolecular interaction as
evidenced by the redshift in the absorption in both solution and thin film, contributing to its
charge transport ability.24 The stronger π-π stacking can increase the hole mobility due to
better molecular packing.25 These parameters are responsible for higher Jsc and FF resulting
superior overall PCE for OSC based on 1b as compared to 1a.

Fig. 4 (a) Current –voltage characteristics under the illumination of AM1.5 at 100 mW/cm2
and (b) IPCE spectra of the BHJ OSCs based on porphyrin:PC71BM (1:1)

Fig. 5 Current–voltage characteristics of hole-only devices based on active layers cast from
THF and pyridine/THF solvents. The solid lines are SCLC fitting.
The overall PCE of BHJ OSCs based on these porphyrins processed with THF is
smaller than reported in literature for small molecules as donor. Although the Voc of these
devices is reasonably high, the lower PCE is mainly due low values of both Jsc and FF. These
parameters directly related to the light harvesting efficiency of the active layer and the
resulting charge generation as well as the charge transport within active layer and collection
to the respective electrodes and these processes depend upon the morphology of active layer
and charge mobilities. A well-defined morphology and phase separation between the donor
and acceptor components in the active layer within the range of exciton diffusion lengths
(~15-20 nm) are necessary for efficient exciton dissociation and charge transport.26 As
reported in literature, the morphology of the active layer can be controlled by the appropriate
treatments of active layer i.e. thermal annealing,27 solvent annealing28 and solvent additives29
for both polymer and small molecule solar cells. The solvent additive method has been
recently adopted to improve the performance of organic solar cells based on porphyrins as
donor. In order to improve the PCE of our devices we have employed solvent additive
treatment method using a small amount of pyridine as solvent additive, as it is known to be
more effective than other solvent additive in case for porphyrin based solar cells.30 We have
varied the concentration of pyridine in THF host solvent from 0.5 v% to 3.5 v% and found
that 3 v % gives the best result. The J-V characteristics under illumination and IPCE spectra
of the devices processed with pyridine solvent additive are shown in Fig. 4a and 4b,
respectively and photovoltaic parameters are compiled in Table 2. The PCE value of the

devices with solvent additives was significantly enhanced from 2.75 % to 5.27 % (Jsc = 10.61
mA/cm2, Voc =0.92 V and FF= 0.54) and 3.18 to 5.78 % (Jsc = 11.58 mA/cm2, Voc = 0.86 V
and FF= 0.58) for 1a and 1b, respectively. The improvement in the Jsc is consistent with the
IPCE spectra of the devices processed with solvent additive, i.e. the IPCE values are higher
throughout the whole wavelength region. The calculated values of Jsc from the IPCE spectra
were about 10.48 mA/cm2 and 11.43 mA/cm2 for 1a and 1b based devices, respectively in
good agreement, within experimental error, with the values observed in the J−V
characteristics of devices under illumination.
The PCE of BHJ organic solar cells, the Jsc is directly related to the light harvesting
efficiency of the active layer, therefore, therefore, in order to get information about influence
of pyridine additive on the change in light harvesting efficiency of the devices, we have
recorded the UV-visible absorption spectra of the active layers with and without pyridine
additive and shown in Fig. 6 for 1b:PC71BM (1:2) thin films only. Similar results have been
observed for 1a:PC71BM (1:2) thin films. Compared to the film cast from THF, the
pyridine/THF cast film shows a wide absorption and high absorption coefficient particularly
to the absorption bands corresponds to porphyrin, which is related to the enhanced π-π
stacking. Therefore, we believe that the light harvesting efficiency have been improved with
the pyridine additive, which may be one of the reasons for the improvement in Jsc.

Fig. 6 Normalized absorption spectra of 1b:PC71BM thin film cast from THF and
pyridine/THF solutions.

The crystallinity of the active layer plays an important role in charge transport and
collection, thereby affecting the overall PCE of organic bulk heterojunction. The XRD
patterns of pristine 1a and 1b displayed in Fig. 7a and showed significant (100) diffraction
peaks at 2θ = 7.26° and 7.32°, respectively, resulting d-spacing of 1.52 nm and 1.48 nm, for
1a and 1b, respectively. To get information about the change in the crystallinity of porphyrin:
PC71BM active layer thin films with the addition of pyridine solvent into CB, prior to spin
casting, we have recorded the x-ray diffraction (XRD) pattern of the spin cast thin film from
THF and pyridine/THF solutions. The 1a:PC71BM and 1b:PC71BM blends cast from THF
show similar diffraction peaks with weak intensity (as shown in Fig. 7b for 1b:PC71BM),
suggesting effective mixing of PC71BM and porphyrin. However, this diffraction peak
becomes more intense in corresponding blend cast from pyridine/THF solvent, while the
width at half maximum height is decreased. These results shows that the blend film cast from
THF solvent has low crystallinity, while in the case of blend film cast from pyridine/THF
solvent, the crystallinity and π-conjugation of porphyrin in the blend film increase. This is
probably due to the difference in boiling point of THF and pyridine, which makes the film to
dry slowly, assisting in the formation of better self-ordered structure of the blend. The
increase in the crystallinity leads to an enhancement in light harvesting properties of active
layer and higher hole mobility in the blend thin film, leading to increase in Jsc and FF.
Atomic force microscopy (AFM) was used to investigate the change in the surface
morphology of the active layer induced by pyridine solvent additive. The AFM scans were
carried out for the blend films spin coated onto ITO coated substrate using the same
fabrication conditions for device fabrication. Fig. 8 shows the AFM images for 1b:PC71BM
cast with and without solvent additives and corresponding root mean square (rms) roughness
are 0.98 nm and 1.82 nm, for the film cast from THF and pyridine/THF solutions,
respectively. The blend cast from THF shows relatively flat and poor phase separation which
is not favorable for charge transfer from porphyrin to PC71BM thus limited the PCE of
resulting device. However, spin coated film from pyridine/THF showed more aggregated
domains and phase separated domains with larger rms surface roughness. The aggregated
domains may originate from enhanced intermolecular interaction of porphyrin, during the
film formation.31 A higher surface roughness is expected to increase the internal light
scattering and enhance the light absorption.32 The blend film with larger domain size suggests
a good phase separation and well-connected domains, which allow efficient charge
generation and charge transfer within the active layer.

Fig. 7 X-ray diffraction patterns of (a) 1a and 1b thin films cast from THF and (b)
1b:PC71BM

Fig. 8 AFM images of 1b:PC71BM blend films cast from THF and pyridine/THF solutions
(size, 3 µm×3µm).

The charge carrier mobility in the BHJ active layer is crucial for the overall PCE of
the organic solar cell because the photogenerated charge carriers extract by electrodes
depends on the competition between carrier sweeps out, which is limited by the mobility of
charge carriers, their loss by recombination.33 In order to get information about the influence
of the pyridine additive on the charge carrier mobilities, we have measured the hole
mobilities using the J-V characteristics of the hole only devices (Fig. 4). Fitting the
experimental results with SCLC model (solid lines), the hole mobilities are found to be 3.14
x10-5 cm2/Vs and 1.35 x10-5 cm2/Vs for 1a:PC71BM and 1b:PC71BM processed with pyridine
additive. In similar manner, we have also estimated the electron mobilities for the blends
processed with and without pyridine additive and found to be in the range of 2.36- 2.48 x10-4
cm2/Vs. The increase in the hole mobility and almost same electron mobility with solvent
additive treatments results more balanced charge transport in the devices as compared to that
for their counterpart devices processed without additive and enhances the FF and Jsc values,
leading to higher PCE value.
We further investigated the influence of pyridine additive on the photogeneration of
charge carriers in the organic solar cells based on 1a and 1b donor by investigating the
variation of photocurrent (Jph) with effective voltage (Veff). The Jph value is estimated as Jph =
JL-JD, where JL and JD are the current density under illumination and dark, respectively.34
The Veff valve is determined as Veff = Vo-V, where Vo is voltage at which the Jph value is
zero.35 Therefore, the Veff is corresponds to the strength of electric field within the device to
extract the charge carriers. The variation of Jph with Veff is shown on Fig. 9. The log-log plot
of Jph versus Veff shows to distinct regions, i.e. linear and saturation region. As observed in
Fig. 9, the devices based on active layer processed without pyridine additive showed no clear
saturation in Jph with increasing Veff, indicating that the internal electric field is not sufficient
for charge extraction and saturation region is field dependent. However, the devices
processed with pyridine additive showed clear saturation region in Jph-Veff curves indicating
field independent saturation, leading to efficient charge extraction and collection. Therefore
at full saturation, we assume that all the excitons generated after the absorption of photons in
the active layer are dissociated into free electrons and holes and subsequently collected by the
respective electrodes. We have estimated the maximum generation rate of free charge carriers
(Gmax) using Jphsat = qGmaxL, where, Jphsat is the photocurrent density at full saturation point in
Fig. 9, q is the elementary charge and L is the thickness of active layer. The estimated values
of Gmax for the devices based on 1a:PC71BM (THF cast), 1b:PC71BM (THF cast), 1a:PC71BM

(pyridine/THF cast) and 1b:PC71BM (pyridine/THF cast) are 1.50 x1028 m3s-1, 1.64x1028 m3s1

, 2.13 x1028 m3s-1 and 2.34x1028 m3s-1, respectively. The trends observed for the Gmax with

different devices is consistent with the enhancement of Jsc as well as the UV-visible
absorption coefficient, indicating more efficient exciton generation and their dissociation in
the active layer processed with pyridine solvent additive. The charge collection probability
(Pc) of the present solar cells was estimated accordingly Pc = Jsc/Jphsat and found to be 0.62,
0.64, 0.74 and 0.78 for 1a:PC71BM (THF cast), 1b:PC71BM (THF cast), 1a:PC71BM
(pyridine/THF cast) and 1b:PC71BM (pyridine/THF cast), respectively. The greater value of
Pc for the devices processed with pyridine additive based devices as compared to that for
without pyridine additive, is attributed to the better phase separation in the active layer,
increased hole mobility and improved balance in charge transport.

Fig. 9 Variation of photocurrent density (Jph) with the effective internal voltage (Veff) for
devices processed with and without pyridine additives
We have measured the series resistance (Rs) and shunt resistance (Rsh) from the slope
of the J-V characteristics of the devices, under illumination around Voc and Jsc, respectively
and summarized in Table 2. In organic BHJ solar cell, the Rs is composed of the bulk
resistance of active layer depends upon the charge transport ability within active layer and the
morphology of the active layer. It can be seen from the Table 2, the Rs of the devices
processed from pyridine/THF is significantly lower than that processed with THF and can be

attributed to better morphology and balanced charge transport.36 The increase in Rsh is related
to the decrease in leakage current and depends on the intermolecular interaction (or
electronic) coupling between the donor and acceptor materials in the active layer.

Fig. 10 J-V characteristics of devices using CuSCN as hole transport layer for active layers
processed with pyridine/THF solutions.
We have tried CuSCN as HTL as an attempt to improve the PCE of our devices. Since
both 1a and 1b posses HOMO energy levels around in between -5.42 and -5.30 eV, there is a
energy mismatch with the HOMO energy level of PEDOT:PSS (-5.0-5.1 eV) and form a
barrier for hole extraction, therefore there is possibilities to improve the PCE of our present
devices. As active layer we have used pyridine/CB cast film for investigation. The device
fabrication is similar as described in experimental part except HTL. The J-V characteristics of
the devices are shown in Fig. 10. The devices based on 1a:PC71BM and 1b:PC71BM showed
PCE of 6.59 % (Jsc =10.74 mA/cm2, Voc =0.99 V and FF=0.62) and 7.24 % ( Jsc =11.67
mA/cm2, Voc = 0.94 V and FF= 0.66), respectively, which is higher than that for the
PEDOT:PSS HTL. The Jsc values are almost similar that for PEDOT:PSS HTL, since the
both PEDOT:PSS and CuSCN have similar transparency for visible and NIR region of solar
spectrum. Replacing the PEDOT:PSS with CuSCN, the FF increases from 0.54 to 0.62 and
0.58 to 0.66 for 1a and 1b based devices, respectively, may related to the better Ohmic
contact formation between the active layer and CuSCN, due to the deeper work function of

CuSCN than PDEOT:PSS. The improvement in Voc with respect to PEDOT:PSS could be
explained by better alignment between the HOMO level of porphyrin donor and valance band
edge of CuSCN (-5.35 eV) at the active layer/HTL interface, which act as good electron
blocking layer.

Fig. 11 Dark current density-voltage characteristics for the device using PEDOT:PSS and
CuSCN HTL based on 1b:PC71BM active layer processed with pyridine/THF solutions.
The HTL used in the organic BHJ solar cell should have ability to limit the passage of
dark injected minority carriers in reverse bias from anode to cathode through active layer. To
get information about this, we have measured the dark J-V characteristics of the devices
based on 1b:PC71BM active layer (Fig. 11). It can be seen from this figure that the current
density at 0.47 V in reverse bias for PEDOT:PSS and CuSCN HTL is 0.0012 mA/cm2 and
0.00016 mA/cm2, respectively and higher rectification ratio for CuSCN as compared to
PEDOT:PSS, indicating that CuSCN indeed suppress the minority charge carrier injection
into the active layer, thus reducing the recombination process. Moreover, by fitting the
experimental J-V characteristics in dark with Mott-Gurney relationship, we estimated that Vbi
value for CuSCN is 0.098 V higher than that for PEDOT:PSS. The shunt resistance has also
been higher for CuSCN as compared to PEDOT:PSS. The above observations have
synergetic effect on the ability to collect holes and eventually improve the Voc, FF and PCE.

We have also estimated the Pc for these devices using the variation of Jph with Veff
(Figure 12). As shown in Figure 12, the Jph tends to saturate at lower Veff as compared to the
counterpart devices based on PEDOT:PSS. The values of Pc for the devices based on 1a and
1b are about 0.85 and 0.88, respectively. These values are higher than that for devices based
on PEDOT:PSS, indicating that employing CuSCN the better Ohmic contact is formed and
extract the hole more efficiently, leading to suppress the charge recombination and increase
in FF. As the Veff corresponds to the internal voltage to extract the charge carriers, with the
CuSCN low internal voltage is needed to sweep out the charge carriers.
Moreover, the Rs and Rsh has been decreased and increased, respectively for the
CuSCN HTL relative to PEDOT:PSS also an indicative to reduced charge recombination and
efficient charge extraction.

Fig. 12 Variation of photocurrent density (Jph) with the effective internal voltage (Veff) for
devices processed with pyridine/THF solvent and using CuSCN as HTL.
CONCLUSIONS
In this article, we report the synthesis and optical and electrochemical properties of
two new conjugated acceptor-π-donor-π-acceptor (A-π-D-π-A) small molecules with zinc
porphyrin donor core 3-ethylrhodanine terminal acceptors connected at meso positions by
ethynylenes and linked by one (1a) or two (1b) units of thienylenevinylene. The optical and

electrochemical properties of these two porphyrins suggest that blend films these with
PC71BM can effectively harvest the photons from visible to NIR region of solar spectrum and
transfer the electrons from porphyrins to PC71BM, resulting in photovoltaic response. The
BHJ solar cells from 1a:PC71BM and 1b:PC71BM processed from THF displayed PCE of
2.75 % and 3.18 %, respectively. In order to improve the PCE further, we have spin cast the
active layers from pyridine/CB solvent and found that the overall PCE has been significantly
enhanced up to 5.27 % and 5.78 %, for 1a:PC71BM and 1b:PC71BM based devices,
respectively, as a result of higher values of Jsc and FF and are related to the exciton
generation/separation into free charge carrier and mobility/ charge transport which are found
to be improved by the morphology/crystallinity of the active layer, induced by pyridine
solvent additive. Moreover, the PCE has been improved further up to 6.59 % and 7.24 % for
1a:PC71BM and 1b:PC71BM active layer processed with pyridine/THF solution by employing
the CuSCN HTL. The increase in the PCE is due to the suppression on injection of minority
carries (electrons) from anode and increase in Vbi relative to PEDOT:PSS based devices.
Moreover, the low internal field is sufficient to sweep out the charge carriers and reduce the
recombination.
ACKNOWLEDGEMENTS
We are grateful for the financial support of the MINECO, Spain (CTQ2013-48252-P), Junta
de Comunidades de Castilla-la Mancha, Spain (PEII-2014-014-P). SA thanks to the Fundación
Carolina for a grant. EP would like to thank ICIQ, Generalitat Catalunya, and Spanish MINECO

through Severo Ochoa Excellence Accreditation 2014-2018 (SEV-2013-0319) and project
CTQ2013-47183R for the financial support.

EXPERIMENTAL
2,2’-((5-hexylthiophen-2-yl)methylene)bis(1H-pyrrole) (3). At room temperature and
under argon atmosphere, 0.08 mL of TFA (7.71 µL/mmol) was added to a solution of
2Error! Reference source not found. (2.00 g, 10.2 mmol) and pyrrol (4.17 mL, 60 mmol).
The solution was stirred at room temperature for 30 min before quenching with
trimethylamine (1 mL). The organic phase was extracted with chloroform (3 x 50 mL), dried
over MgSO4 and filtered. The solvent was removed under reduced pressure and the crude was
purified by chromatography column (silica gel, hexane:CHCl3, 1:1). The final product was
light brown oil (7.88 mmol, 2.46 g, 77.5% yield). 1H-NMR (400 MHz, CDCl3) δ/ppm: 7.93
(s, 2H), 6.89 (s, 2H) 6.64 (d, 4H, J = 3.3 Hz), 6.21 (dd, 2H, H = 2.8 Hz, J = 5.5 Hz), 6.09 (s,

2H), 5.66 (s, 1H), 2.79 (t, 2H, J = 7.6 Hz), 1.68 (q, 2H, J = 7.6 Hz), 1.42-1.35 (m, 6H), 0.95
(t, 3H, J = 6.5 Hz). FT-IR (ATR) υ/cm-1: 3390, 2920, 2850, 1690, 1560, 1460, 1430, 1250,
1110, 1090, 1030, 968, 883, 760, 714.
5,15-bis(5-hexylthiopen-2-yl)-10,20-bis((trimethylsilyl)ethynyl)-porphyrin

(4).

In

anaerobic conditions, 3-(trimethylsilyl)-2-propynal (0.98 mL, 6.62 mmol) and boron
trifluoride diethyl etherate (BF3O·Et2O, 3.3 mM, 0.27 mL, 2.18 mmol) were added to a
solution of 3 (2.00 g, 6.6 mmol) in 660 mL of CHCl3, and the solution was stirred at room
temperature for 45 min before adding DDQ (1.13 g, 4.96 mmol). The solution was stirred at
room temperature for 1 hour before quenching with 1.3 mL of Et3N (0.4 mL/mmol). After 30
min, the solvent was removed under reduced pressure. The crude was purified by
chromatography column (silica gel, hexane:CHCl3:toluene, 7:2.8:0.2). The final product was
a green-purple solid (0.23 mmol, 200 mg, 7.3% yield). 1H-NMR (400 MHz, CDCl3) δ/ppm:
9.6 (d, 4H, J = 4.6 Hz), 9.11 (d, 4H, J = 4.6 Hz), 7.69 (d, 2H, J = 3.2 Hz), 7.19 (d, 2H, J = 3.2
Hz), 3.14 (t, 4H, J = 7.54 Hz), 2.01-1.93 (m, 4H), 1.6-1.58 (m, 6H), 1.48-1.46 (m, 8H), 1.00
(t, 4H, J = 6.9 Hz), 0.62 (s, 18H), -2.13 (m, 2H). MALDI-TOF MS (m/z): calculated: 834.36;
found: 835.60 (M+). FT-IR (ATR) υ/cm-1: 3310, 2950, 2920, 2850, 2140, 1550, 1470, 1370,
1340, 1250, 1140, 1060, 984, 949, 837, 795, 756, 698, 617.
Compound 5. To a solution of 4 (0.18 mmol, 150 mg) in 15 mL of CHCl3 (85 mL/mmol),
and 0.5 mL of MeOH (2.5 mL/mmol) zinc acetate (165 mg, 0.9 mmol) was added and the
solution was stirred at room temperature during four hours. Then, the organic phase was
extracted with CHCl3 (3 x 25mL), dried and the solvent was removed under reduced pressure.
The crude was purified by chromatography column (silica gel, hexane:CHCl3, 1:1) affording
compound 5 as a green-purple solid (0.17 mmol, 160 mg, 99.1 % yield). 1H-NMR (400 MHz,
CDCl3) δ/ppm: 9.6 (d, 4H, J = 4.6 Hz), 9.18 (d, 4H, J = 4.6 Hz), 7.69 (d, 2H, J = 3.2 Hz), 7.19
(d, 2H, J = 3.2 Hz), 3.14 (t, 4H, J = 7.54 Hz), 2.01-1.93 (m, 4H), 1.64-1.60 (m, 6H), 1.481.47 (m, 6H), 1.00 (t, 6H, J = 6.9 Hz), 0.62 (s, 18H). MALDI-TOF MS (m/z): calculated for
C50H56N4S2Si2Zn: 896.28; found: 896.63 (M+). FT-IR (ATR) υ/cm-1: 2920, 2850, 2140, 1490,
1460, 1340, 1300, 1250, 1210, 1160, 1060, 1000, 972, 837, 791, 756, 710, 621.
Compound 6. At room temperature and under argon atmosphere, TBAF (1M in THF, 0.4
mL, 0.4 mmol) was added to a solution of 5 (150 mg, 0.16 mmol) in 35 mL of CHCl3 (220
mL/mmol). It was added to the solution. The solution was stirred at room temperature for 3
hours and then CaCl2 (222 mg, 2 mmol) was added. After stirring for 30 min, the organic
phase was extracted with CHCl3, (3 x 50 mL). The resultant organic phase was dried, the

solvent was removed under reduced pressure and deprotected 6 was obtained as a blue-green
solid, and used in the next synthetic step without further purification.
Synthesis of 8a.
Using the procedure previously described for Sonogashira condensation,20 to a solution of
deprotected 6 (200 mg, 0.27 mmol) and 7a,20 (337 mg, 0.8 mmol) in 40 mL of THF and 7.5
mL of Et3N, was added a mixture of Pd2(dba)3 (152 mg, 0.16 mmol) and AsPh3, (340 mg, 1.1
mmol). The crude was purified by chromatography column (silica gel, hexane:CHCl3, 2:3)
and recrystallized with CH2Cl2:MeOH. The final product was a turquoise solid (0.16 mmol,
218 mg, 60 % yield). 8a: 1H-NMR (400 MHz, CDCl3) δ/ppm: 9.56 (s, 2H), 9.07 (d, 4H, J =
4.6 Hz), 8.91 (d, 4H, J = 4.6 Hz), 7.76 (d, 2H, J = 3.2 Hz), 7.28 (d, 2H, J = 3.2 Hz), 3.23 (t,
4H, J = 7.6 Hz), 2.47 (s, 2H), 2.30 (s, 2H), 2.10-2.03 (m, 4H), 1.74-1.66 (m, 4H), 1.6-1.52
(m, 20H), 1.38-1.33 (m, 12H), 1.30-1.26 (m, 8H), 1.07-1.03 (t, 4H, J = 6.9 Hz), 0.92-0.87 (m,
18H). MALDI-TOF MS (m/z): calculated for C78H92N4O2S4Zn: 1308.54; found: 1309.01
(M+). FT-IR (ATR) υ/cm-1: 2920, 2850, 2180, 1650, 1610, 1490, 1450, 1410, 1290, 1210,
976, 791, 733, 710, 667.
Synthesis of 8b.
Using the general procedure for Sonogashira condensation previously described, to a solution
of deprotected 6 (125 mg, 0.17 mmol) and 7b20 (300 mg, 0.5 mmol) in 40 mL of THF (230
mL/mmol) and 7.5 mL of Et3N (45 mL/mmol), was added to a mixture of Pd2(dba)3 (92 mg,
0.1 mmol) and AsPh3, (205 mg, 0.7 mmol). The crude was purified by chromatography
column (silica gel, hexane:CHCl3, 2:3) and recrystallized with CH2Cl2:MeOH. The final
product was a turquoise solid (0.10 mmol, 374 mg, 60 % yield). 1H-NMR (400 MHz, CDCl3)
δ/ppm: 9.59 (s, 2H), 9.29 (d, 4H, J = 3.9 Hz), 9.12 (d, 4H, J = 3.9 Hz), 7.75 (s, 2H), 7.25 (s,
2H), 7.05 (d, 2H, J = 16 Hz), 6.76 (d, 2H, J = 16 Hz), 3.21 (t, 3H, J = 7.14 Hz), 2.73 (s, 3H),
2.57 (d, 6H, J = 21.8 Hz), 2.40 (s, 3H), 2.04 (t, 3H, J = 6.9 Hz), 1.77 (s, 2H), 1.65-1.63 (m,
6H), 1.5-1.26 (m, 68H), 1.05-0.99 (m, 16H), 0.94-0.89 (m, 20H). MALDI-TOF MS (m/z):
calculated for C114H148N4O2S6Zn: 1860.92; found: 1862.30 (M+). FT-IR (ATR) υ/cm-1: 2920,
2850, 2170, 1650, 1590, 1490, 1460, 1400, 1290, 1250, 1210, 972, 930, 791, 710, 663.
Synthesis of 1a
3-ethylrhodanine (36 mg, 228 µmol, 3 eq) and four drops of piperidine were added to a
solution of 8a (100 mg, 0.07 mmol) in 10 mL of CHCl3 and it was stirred at room
temperature for 48 h. The crude was purified by chromatography column (silica gel,
hexane:CHCl3:toluene, 7:2.9:0.1). 8a was obtained as a green-red solid (0.06mmol, 115mg,
95.5 % yield), mp: >300°C. 1H-NMR (400 MHz, CDCl3) δ/ppm: 9.09 (d, 4H, J = 3.9 Hz),

8.95 (d, 4H, J = 3.9 Hz), 7.83 (d, 2H, J = 1.9 Hz), 7.33 (d, 2H, J = 1.9 Hz), 6.64 (s, 2H), 3.73
(d, 4H, J = 5.8 Hz), 3.28 (t, 4H, J = 7.6 Hz), 2.3 (s, 2H), 2.12 (t, 4H, J = 7.28 Hz), 1.95 (s,
2H), 1.75 (s, 6H), 1.58 (s, 16H), 1.45 (s, 12H), 1.23-1.19 (m, 16H), 1.10 (d, 14H, J = 3.95
Hz), 0.97 (s, 4H), 0.85 (t, 4H, J = 6.9 Hz.

13

C-NMR (100 MHz, CDCl3) δ/ppm: 192.00,

167.12, 151.68, 151.18, 149.32, 148.50, 147.50, 140.06, 134.01, 133.24, 133.03, 130.58,
126.94, 123.79, 122.77, 119.73, 116.27, 104.54, 101.75, 90.42, 39.86, 32.46, 32.18, 31.86,
31.73, 31.12, 30.95, 30.30, 29.73, 29.60, 29.04, 27.69, 23.24, 23.17, 22.92, 14.63, 14.57,
12.61. MALDI-TOF MS (m/z): calculated for C88H102N6O2S8Zn: 1594.51; found: 1595.30
(M+). FT-IR (ATR) υ/cm-1: 3670, 3470, 2920, 2850, 2160, 1680, 1560, 1500, 1400, 1320,
1230, 1130, 1010, 877, 785, 729.
Synthesis of 1b
3-ethylrhodanine (43 mg, 0.26 mmol) and four drops of piperidine, were added to a solution
of 8b (50 mg, 0.26 mmol) in 10 mL of chloroform and the solution was stirred at room
temperature for 6 hours. Then, the product was purified by chromatography column (silica
gel, hexane:CHCl3:toluene, 7:2.9:0.1). The final product was a green-red solid (0.08 mmol,
20 mg, 34 % yield), mp: >300°C. 1H-NMR (400 MHz, CDCl3) δ/ppm: 9.58 (d, 4H, J = 4.6
Hz), 9.11 (d, 4H, J = 4.6 Hz), 7.79 (s, 2H), 7.72 (d, 2H, J = 3.2 Hz), 7.29 (d, 4H, J = 16 Hz),
7.15 (d, 2H, J = 16 Hz), 4.2-4.14 (m, 4H), 3.2-3.15 (m, 9H), 2.84 (d, 3H, J = 7.3 Hz), 2.67 (t,
6H, J = 6.9 Hz), 2.06-1.98 (m, 9H), 1.56-1.42 (m, 58H), 1.4-1.37 (m, 12H), 1.26 (t, 9H, J =
7.16 Hz), 1.04-0.96 (m, 18H), 0.95-0.89 (m, 12H).

13

C-NMR (100 MHz, THF-d8) δ/ppm:

193.49, 168.14, 153.22, 152.27, 151.43, 149.83, 149.76, 146.08, 143.89, 143.78, 142.19,
139.41, 134.76, 133.78, 132.16, 131.58, 124.87, 123.89, 123.44, 121.06, 120.41, 116.59,
103.27, 103.11, 92.20, 33.66, 33.50, 33.31, 33.21, 33.17, 33.11, 32.98, 32.73, 31.73, 31.39,
30.87, 30.80, 30.75, 30.58, 26.42, 26.29, 26.22, 26.09, 26.02, 25.99, 25.89, 25.80, 25.69,
25.60, 25.49, 25.38, 24.33, 24.21, 24.11, 15.22, 15.20, 15.16, 15.14, 15.04, 13.01. MALDITOF MS (m/z): calculated for C124H158N6O2S10Zn: 2146.89; found: 2146.25 (M+). FT-IR
(ATR) υ/cm-1: 2930, 2860, 1700, 1570, 1490, 1460, 1400, 1320, 1240, 1190, 1140, 1070,
957, 850, 789, 708.
Device fabrication and characterization
The BHJ organic solar cells were fabricated using the glass/ITO/ PEDOT:PSS/1a or
1b :PC71BM with different weight ratios /Al device architecture. The indium tin oxide (ITO)
patterned substrates were cleaned by ultrasonic treatment in aqueous detergent, deionized
water, isopropyl alcohol, and acetone sequentially, and finally dried under ambient
conditions. The anode consisted of glass substrates percolated with ITO, modified by spin

coating with a PEDOT:PSS layer (40 nm) as a hole transport layer and heated for 10 min at
100° C. Mixtures of 1a or 1b with PC71BM (total concentration of 12 mg/mL) with weight
ratios of 1:05, 1 : 1, 1: 2 and 1:2.5 in THF were prepared and then spin cast onto the PEDOT
: PSS layer and dried overnight in an ambient atmosphere. For the 1a or 1b:PC71BM blend
processed with 3 % v/v of pyridine in the THF solvent mixture only the 1:2 weight ratio
mixture was used. The approximate thickness of the active layers was 90 nm. Finally, the
aluminum (Al) top electrode was thermally deposited on the active layer at a vacuum of 10-5
Torr through a shadow mask area of 0.20 cm2. All devices were fabricated and tested in an
ambient atmosphere without encapsulation. The hole-only and electron-only devices with
ITO/PEDOT:PSS/active layer /Au and ITO/Al/ active layer/Al architectures were also
fabricated in a similar way, in order to measure the hole and electron mobility, respectively.
The current–voltage (J–V) characteristics of the BHJ organic solar cells were measured using
a computer controlled Keithley 2400 source meter in the dark and under a simulated AM
1.5G illumination of 100 mW/cm2. A xenon light source coupled with the optical filter was
used to give the stimulated irradiance at the surface of the devices. The incident photon to
current efficiency (IPCE) of the devices was measured illuminating the device through the
light source and the monochromator and the resulting current was measured using a Keithley
electrometer under short circuit conditions.
For CuSCN HTL based devices a thin film of the CuSCN (30 nm) layer was spin
coated at 2500 rpm for 60 seconds, using diisopropyl sulphide as solvent and annealed at
110° C for 20 minutes and then the active layer was deposited as described above.

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