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Dedicated to Prof Francis Garnier on the occasion of his 75
th
birthday
Molecular Materials for Organic Photovoltaics:
Small is Beautiful
Jean Roncali , * Philippe Leriche , and Philippe Blanchard
1. Introduction
The steady increase of energy demand associated with the
emergence of environmental concerns has created a situation
propitious to an intensifi cation of research on the conversion
of solar energy. In this context organic photovoltaics (OPV) is
attracting increasing interest motivated in part by the light-
ness, plasticity, and fl exibility of organic materials. However,
the major reason to develop OPV is clearly an expected drastic
reduction of the cost of PV electricity.
Although the active materials are of course key components
of solar cells and represent a substantial part of the overall cost
of the devices, a quasi exclusive focus and harsh competition
aiming at the improvement of the power conversion effi ciency
( PCE ) of OPV cells has contributed to leave in the background the
problems related to the cost, stability, environmental impact and
scalability of the synthesis of active OPV
materials. In recent years, donor materials
based on molecular chromophores have
gained considerable interest in the various
branches of research on OPV. In this con-
text, the aim of this article is to discuss
some recent developments of the chemistry
of donor materials based on small molec-
ular structures combining structural versa-
tility, simplicity and low molecular weight,
features that are generally associated with
higher overall yield, lower environmental
impact and easier up-scaling of the syn-
thesis. It is hoped that emphasis on such
structures will stimulate further synthetic
chemistry oriented towards the reduction of
the cost and environmental impact of active
materials and thus contribute to a future
industrial development of OPV. In order
to clearly place the objectives of this short
survey, special attention will be focused on
donors with molecular weights ≤500. After
a brief survey intended to situate present
OPV research in a historical perspective,
the main classes of molecular structures
that fulfi l the above exposed conditions and with already estab-
lished potential for PV conversion will be discussed with the aim
of identifying possible directions for future research.
OPV cells are basically developed along two main lines.
Thermal evaporation of active materials under vacuum was the
initial method of fabrication.
[ 1–10 ]
This technique presents several
specifi c advantages such as possible use of insoluble materials
often more stable than their soluble analogues, precise control
of the thickness and deposition rate of the active layers and easy
fabrication of multi-layered devices. The second simpler, cheaper
and more recent approach involves the solution-processing of the
active fi lm. This method offers the possibility to work on large
area of (eventually fl exible) substrates by low energy-demanding
technologies such as printing or roll-to-roll processes. As illus-
trated in this article, these two technologies developed in parallel
have deeply infl uenced the chemistry of active OPV materials.
Since the pioneering work of the seventies and until recently
vacuum-deposited cells have been based on dyes and pigments
like e.g., phthalocyanines, squaraines or merocyanines ini-
tially developed for other applications such as tainting, paint or
xerography.
[ 1–7 ]
Although the enhancement of the photo-gener-
ation of charges in a phthalocyanine in the presence of an elec-
tron-acceptor had been reported in 1980,
[ 2 ]
the fabrication of
An overview of some recent developments of the chemistry of molecular
donor materials for organic photovoltaics (OPV) is presented. Although
molecular materials have been used for the fabrication of OPV cells from
the very beginning of the fi eld, the design of molecular donors specifi cally
designed for OPV is a relatively recent research area. In the past few years,
molecular donors have been used in both vacuum-deposited and solution-
processed OPV cells and both fi elds have witnessed impressive progress with
power conversion effi ciencies crossing the symbolic limit of 10 %. However,
this progress has been achieved at the price of an increasing complexity of
the chemistry of active materials and of the technology of device fabrica-
tion. This evolution probably inherent to the progress of research is diffi cult
to reconcile with the necessity for OPV to demonstrate a decisive economic
advantage over existing silicon technology. In this short review various
classes of molecular donors are discussed with the aim of defi ning pos-
sible basic molecular structures that can combine structural simplicity, low
molecular weight, synthetic accessibility, scalability and that can represent
possible starting points for the development of simple and cost-effective OPV
materials.
DOI: 10.1002/adma.201305999
Dr J. Roncali, Prof. P. Leriche, Dr P. Blanchard
Group Linear Conjugated Systems
CNRS MOLTECH-Anjou
University of Angers
2 Bd Lavoisier 49045 , Angers , France
E-mail: jeanroncali@gmail.com
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the fi rst donor-acceptor (D/A) heterojunction by Tang in 1986
[ 8 ]
is generally considered as the beginning of the “modern era”
of OPV. However, research carried out in the next few years
basically involved the same molecular donors.
[ 9,10 ]
C o n j u g a t e d p o l y m e r s ( C P s ) , w i d e l y i n v e s t i g a t e d s i n c e t h e e n d
of the seventies, form the second big class of donors for OPV.
[ 11 ]
The fabrication of the fi rst organic light-emitting diodes (OLED)
based on CPs in 1990 has generated a considerable interest for
the semiconducting properties of these materials.
[ 12 ]
H o w e v e r ,
it took a few more years before CPs entered the fi eld of OPV. A
fi rst example of solar cell based on poly(thiophene) was reported
in 1984 by Garnier et al.,
[ 13 ]
b u t t h e r e a l d e b u t o f C P - b a s e d O P V
started ca. ten years later triggered by two simultaneous discov-
eries. The fi rst is the demonstration of photo-induced electron
transfer between CPs and fullerene C
60
b y S a r i c i f t c i e t a l .
[ 14 ]
a n d
the second is the invention of the concept of bulk heterojunc-
tion (BHJ) solar cells.
[ 15,16 ]
I n B H J c e l l s t h e h u g e e x t e n s i o n o f
the area of the heterojunction interface resulting from the for-
mation of interpenetrated networks of D and A materials allows
to circumvent a major limitation of planar heterojunctions
(PHJ) related to the short exciton diffusion-length in organic
materials. The fi rst BHJ cells were based on soluble derivatives
of poly( p - p h e n y l e n e v i n y l e n e ) ( P P V ) .
[ 15,16 ]
T h e i n i t i a l v a l u e s o f
PCE o f ∼1.0 % raised rapidly to 2.50 %,
[ 17 ]
w h i l e s o m e t i m e l a t e r
PPVs were supplanted by poly(3-hexylthiophene) (P3HT)
[ 18 ]
which became the new standard material and led, after inten-
sive of optimization, to PCE o f 4 . 5 – 5 . 0 % i n 2 0 0 5 .
[ 19,20 ]
Although low band gap CPs have been investigated for a long
time,
[ 21 ]
the synthesis of the fi rst low band gap CPs specifi cally
designed for OPV started at the turn of the millenium,
[ 22,23 ]
and
it was only in 2008 that low band gap polymers capable of sur-
passing P3HT were synthesized.
[ 24 ]
In the past fi ve years new
classes of low band gap polymers have generated impressive
progress and recently PCE of ∼9.0 % have been reported for
single junction BHJ cells.
[ 25,26 ]
In spite of these remarkable results, CPs pose some prob-
lems related to the reproducibility of their synthesis, purifi ca-
tion, and hence electronic properties. In fact the crude mate-
rials contain conjugated chains of various lengths and require
fractioning with different solvents in order to reduce polydis-
persity. Furthermore, it is often necessary to remove remnant
terminal groups by appropriate chemical treatment. These
additional processes contribute to increase the cost and envi-
ronmental impact of the material.
A possible alternative consists in the replacement of CPs by
soluble molecular donors. Besides reproducible synthesis and
purifi cation, molecules allow more direct and reliable analyses
of structure-properties relationships. Initiated in 2005,
[ 27 ]
this
research area has rapidly grown and led to the synthesis of many
classes of chromophores
[ 28 ]
s u c h a s 3 D o l i g o t h i o p h e n e s
[ 29 ]
t r i -
phenylamines,
[ 30 ]
( 1 ) , d i k e t o p y r r o l o p y r r o l e s ( 2 ) ,
[ 31 ]
b o r o n d i p y r -
romethenes ( 3 ) ,
[ 32 ]
i n d i g o s ( 4 ) ,
[ 33 ]
d i c y a n o p y r a n e d e r i v a t i v e s ( 5 ) ,
[ 34 ]
( Figure 1 ) a n d m a n y o t h e r t a i l o r e d
π
- c o n j u g a t e d m o l e c u l e s .
[ 28 ]
The
fi eld has progressed rapidly and several groups recently reported
PCE o f ∼8.0% comparable to those obtained with the best poly-
mers.
[ 35–37 ]
Thus, Bazan et al. have synthesized hybrid D-A-D-A-D
oligothiophenes containing two benzothiadiazole groups ( 6 ) .
[ 35 ]
Chen et al. have developed A-D-A oligothiophenes with different
rigid median blocks like e.g., dithienobenzene ( 7 )
[ 36 ]
while Li et al.
Jean Roncali received his
Ph.D. in 1984 in Paris under
the supervision of Francis
Garnier. After successive
positions as engineer and
researcher in the Laboratory
of Molecular Materials of
CNRS, in 1991 he moved to
the University of Angers to
create the Linear Conjugated
Systems Group as Research
Director. Among others, Jean
Roncali has received the Grammatikakis-Neuman Prize
of the French Academy of Sciences in 2002 and the Pierre
Süe Great Prize of the French Chemical Society in 2008.
His research deals with the design of tailored functional
π
-conjugated systems for applications in electrode mate-
rials photonics, molecular electronics, energy conversion
and storage, and organic photovoltaics.
Philippe Leriche was born in
Bretagne and studied chem-
istry in Rennes and Nantes.
After a PhD in organic
chemistry under the supervi-
sion of Pr Gorgues (Angers,
France) and Becher (Odense,
Denmark) in 1996, he was a
postdoctoral fellow with Pr
Cava (Tuscaloosa, USA) and
became assistant professor
(1998) and professor (2009).
He was head of chemistry department from 2009 to
2012 when he became assistant director of the Faculty of
Sciences of Angers). His main research interests cover
design, synthesis, and characterization of molecular mate-
rials for organic electronics and photonics.
After a Ph.D. thesis on TTF-
based molecular materials in
1994 under the supervision
of Professors G. Duguay
(Nantes) and A. Gorgues
(Angers), a postdoctoral
fellowship on electroactive
TTF-based macrocycles in the
group of J. Becher (Odense,
Denmark), P. Blanchard
joined the group of J. Roncali
as CNRS Researcher in 1995.
He became CNRS Research Director in 2007. His current
interests concern the design, synthesis and characteriza-
tion of pi -conjugated systems for molecular and organic
electronics.
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synthesis and processing of active materials and of the archi-
tecture and fabrication of OPV cells. In this context it could be
interesting and timely to reconsider the problem of the synthesis
of OPV materials from a different viewpoint taking into account
the criteria required by a possible industrial development.
[ 41,42 ]
2. Molecular Donors Based on Low Molecular
Weight Chromophores
2.1. Oligothiophenes
O l i g o t h i o p h e n e s ( n T s ) f o r m a n i m p o r t a n t c l a s s o f o r g a n i c s e m i -
conductors widely used for the realization of OFETs. Already in
1974, Khun et al. published a fi rst study of the photoelectrical
behavior of a monolayer of quinquethiophene ( 9 ) ( Figure 4 )
sandwiched between a semi-transparent aluminium layer and
a mercury drop and measured the photo-current as a function
of applied bias voltage.
[ 43 ]
A fi rst prototype of bilayer D/A het-
erojunction cell was reported in 1995 by Shirota et al. who used
used the same median unit to create a 2D conjugated system ( 8 )
( Figure 2 ) a n d o b t a i n e d h i g h PCE without resorting to additives,
in contrast to the two previous record examples.
[ 37 ]
V a c u u m -
deposited OPV has also witnessed rapid progress and recently
the groups of Wong
[ 38 ]
a n d B ä u e r l e
[ 39 ]
reported stack multi-layer
devices with PCE o f 6 . 8 0 – 6 . 9 0 % . H o w e v e r , d o n o r s f o r v a c u u m -
deposited cells have generated less effort in synthetic chemistry
than soluble molecules while advances in the technology of multi-
layer cells have strongly contributed to the progress of the fi eld.
D u r i n g a p e r i o d o f i n t e n s i v e r e s e a r c h f o c u s e d o n t h e s y n -
thesis of new materials, the cost of the target compounds is
generally not taken into consideration. However, now that the
symbolic limit of 10% PCE
[ 40 ]
h a s c l e a r l y d e m o n s t r a t e d t h e s c i -
entifi c credibility of OPV, questions such as cost, environmental
impact and scalability of the synthesis of active materials will
play a key role in the industrial destiny of OPV. Although it is
widely accepted that the major motivation for developing OPV
is an expected drastic reduction of the cost of PV electricity com-
pared to silicon technology, the progress accomplished in recent
years has involved a considerable complication of the structure,
Figure 1. Chemical structures of compounds 1–5 .
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Conversion of one or more thiophene rings into the corre-
sponding S-S d i o x i d e s l e a d s a t t h e s a m e t i m e t o a n e n h a n c e d
π
- e l e c t r o n d e l o c a l i z a t i o n d u e t o t h e d e - a r o m a t i z a t i o n o f t h e r i n g
and to a strong increase of electron affi nity. On this basis, Cama-
ioni et al. have reported one of the fi rst studies of the effect of
structural modifi cation on the PV properties of nTs. S-S d i o x -
ides ( 12 , 13 ) ( F i g u r e 4 ) w e r e u s e d a s e l e c t r o n a c c e p t o r m a t e r i a l
in BHJ cells with P3HT as donor. While the overall PCE o f t h e
devices remained low, a net effect of the presence of the acceptor
was demonstrated compared to a device based on pure P3HT,
with in particular an increase of V
oc
from 0.40 V to ca. 1.0 V.
While precise structure-properties relationships were diffi cult to
establish, the largest effects on V
oc
w e r e o b s e r v e d w i t h c o m -
pounds possessing an internal S,S d i o x i d e g r o u p s u c h a s ( 13 ) .
[ 48 ]
I n t r i g u i n g r e s u l t s h a v e b e e n p u b l i s h e d i n 2 0 0 6 b y L i u e t a l .
who synthesized quater- and quinque-thiophene with a ter-
minal vinyl group and used them as donor in bilayer cells
with perylenetetracarboxilic dianhydride (PTCD) as acceptor.
The devices gave moderate J
sc
o f 1 . 9 8 a n d 1 . 9 2 m A c m
−2
f o r
14 a n d 15 r e s p e c t i v e l y a n d PCE o f 2 . 3 0 a n d 2 . 6 7 % . U n u s u a l l y
high V
oc
o f 2 . 0 7 a n d 1 . 8 7 V w e r e r e p o r t e d f o r c e l l s b a s e d o n 14
and 15 w h i l e a c o n t r o l e x p e r i m e n t w i t h u n s u b s t i t u t e d 4 T g a v e
vacuum deposited octithiophene ( 11 ) a s d o n o r a n d a p e r y l e n e -
based pigment as acceptor. Under white light illumination at
105 mW cm
−2
t h e c e l l d e l i v e r e d a s h o r t - c i r c u i t c u r r e n t d e n s i t y
( J
sc
) o f 2 . 9 0 m A c m
−2
, a n o p e n c i r c u i t v o l t a g e ( V
oc
) o f 0 . 4 2 V
and a fi ll-factor ( FF ) o f 0 . 5 0 l e a d i n g t o a c o n v e r s i o n e f fi ciency
of 0.59%.
[ 44 ]
V i d e l o t e t a l . h a v e f a b r i c a t e d S c h o t t k y d i o d e s b a s e d
on thin fi lms of 11 a n d a n a l y z e d t h e e f f e c t o f t h e o r i e n t a t i o n
of molecules on the substrate on the performances of the cells.
The horizontal orientation was obtained by a rubbing technique.
A blue shift of
λ
max
w a s o b s e r v e d f o r t h e v e r t i c a l l y o r i e n t e d m o l -
ecules. No PV effect was observed in this case while a small
PCE w a s o b s e r v e d f o r t h e h o r i z o n t a l l y o r i e n t e d m o l e c u l e s .
[ 45 ]
More recently, Sakai et al. have reported a fi rst example of
hybrid heterojunction cell (HHJ) based on sexithiophene 10 .
[ 46 ]
HHJ cells involves a co-evaporated layer of donor and acceptor
sandwiched between layers or pure donor and acceptor. A PHJ
bilayer cell 10 /C
60
gave a PCE of 0.75% and rather low V
oc
of
0.35 V. Then by optimizing the 10 /C
60
ratio in a HHJ device
a PCE of 1.50% was obtained for a 4 mm
2
multi-layer device
involving BCP ( Figure 3 ) as exciton blocking layer.
O l i g o t h i o p h e n e - S-S d i o x i d e s h a v e b e e n e x t e n s i v e l y i n v e s -
tigated as emitters for OLEDs by the group of Barbarella.
[ 47 ]
Figure 2. Chemical structures of compounds 6–8 .
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multi-layer devices involving optimized hole and electron
transporting layers. In a fi rst paper, a 5T with four butyl groups
( 20c) was used as donor with C
60
as electron acceptor. Two
different multi-layer devices were fabricated involving ITO/
Au/hole-transporting layer (HTL)/Donor/C
60
/BPhen/Al. Two
types of HTL combinations were used namely p-doped Meo-
TPD/4P-TPD in the fi rst case and p-doped Di-NPB/Di-NPB in
the second type of device. The dopant was either F4-TCNQ or
a proprietary material. Cells of 3-4 mm
2
active area gave high
J
sc
values ∼ 12 mA cm
−2
and PCE of 2.00 and 2.20 % however,
the FF values remained low despite the presence of the doped
HTL system. The increase of the doping level of the HTL signif-
icantly improved the FF raising PCE up to 3.40 %.
[ 51 ]
Replace-
ment of the four butyl substituents R
2
by four ethyl groups
( 20b ) reduces PCE values to 1.20 and 2.50 % (depending on the
HTL system) due in particular to a lower FF despite the fact
that 20b presents a hole mobility ca. three times higher than
20c .
[ 52 ]
A crystallographic study of 4Ts 19a-c has revealed the key
role of the length of the alkyl substituent R on the molecular
packing and on the PV performances of the resulting cells.
Multi-layer HHJ cells with a co-evaporated donor:C
60
active
layer showed that the best results were obtained with 19b (R =
Me) with a J
sc
of 6.50 mA cm
−2
and a PCE of 3.80 %.
[ 53 ]
a V
oc
o f 0 . 9 0 V .
[ 49a ]
U n t i l n o w t h e s e e x p e r i m e n t s h a v e n o t b e e n
reproduced nor contradicted by other groups but if confi rmed
these results would certainly open interesting new directions for
the molecular design of donors for OPV. The same group has
reported PHJ cells based on vacuum deposited cyano-substituted
ter- and quaterthiophenes ( 16 , 17 ) ( Figure 4 ) a n d P T C D .
[ 49b ]
Simple devices ITO/ 16 o r 17 / P C D T/ A l o f u n s p e c i fi ed active
area gave J
sc
o f 7 . 6 0 a n d 9 . 6 8 m A c m
−2
f o r 16 a n d 17 r e s p e c -
tively with V
oc
o f 0 . 6 5 a n d 0 . 8 3 V a n d PCE o f 1 . 5 1 a n d 2 . 1 0 % .
However, the cells show poor FF v a l u e s ( 2 4 a n d 2 0 % ) , s u g -
gesting much room for improvement. The rather large values of
J
sc
c o u l d b e n e fi t form the large absorption of the PTCD acceptor
in the 400-600 nm region namely beyond that of the donors.
[ 49b ]
Another example of modifi cation of nTs in view of PV con-
version was reported by Yassar et al. who synthesized a 4T
with a terminal dicyanovinyl group ( 18 ) ( Figure 5 ). A large
bathochromic shift of the absorption onset was observed com-
pared to unsubstituted 4T, however the Schottky diode fabri-
cated by sandwiching the vacuum-deposited molecule between
ITO and aluminium electrodes delivered only a low J
sc
.
[ 50 ]
Bäuerle et al. have extended the use of the dicyanovinyl
end-group to a large library of nTs ( 19-21 ).
[ 51–58 ]
In collabora-
tion with the group of Leo they have investigated the PV prop-
erties of these molecules in sophisticated vacuum-deposited
Figure 3. Auxilliary compounds for OPV cells.
Adv. Mater. 2014, 26, 3821–3838
