Organic solar cells based on non-fullerene
acceptors
Jianhui Hou, Olle Inganäs, Richard H. Friend and Feng Gao
The self-archived postprint version of this journal article is available at Linköping
University Institutional Repository (DiVA):
http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-144871
N.B.: When citing this work, cite the original publication.
Hou, J., Inganäs, O., Friend, R. H., Gao, F., (2018), Organic solar cells based on non-fullerene
acceptors, Nature Materials, 17(2), 119-128. https://doi.org/10.1038/NMAT5063
Original publication available at:
https://doi.org/10.1038/NMAT5063
Copyright: Nature Publishing Group
http://www.nature.com/
1
Organic solar cells based on non-fullerene acceptors
Jianhui Hou
1
, Olle Inganäs
2
, Richard H. Friend
3
, Feng Gao
2
1
Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Polymer Physics and Chemistry,
Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
2
Biomolecular and organic electronics, Department of Physics, Chemistry and Biology (IFM), Linköping
University, Linköping SE-58183, Sweden
3
Cavendish Laboratory, J J Thomson Avenue, Cambridge CB3 0HE, United Kingdom
Abstract
Organic solar cells (OSCs) have been dominated by donor:acceptor blends based on
fullerene acceptors for over two decades. This situation has changed very recently, with
non-fullerene (NF) OSCs developing very quickly. The power conversion efficiencies of
NF OSCs have now reached a value of over 13%, which is higher than the best fullerene-
based OSCs. NF acceptors show great tunability in absorption spectra and electron energy
levels, providing a wide range of new opportunities. The co-existence of low voltage losses
and high current generation indicates that new regimes of device physics and
photophysics are reached in these systems. This Review highlights these opportunities
made possible by NF acceptors, and also discuss the challenges facing the development of
NF OSCs for practical applications.
2
Light absorption in organic semiconductors generates strongly bound excitons. Donor (D):
acceptor (A) bulk-heterojunction (BHJ) structures, suitable for low-cost solution processing,
provide an efficient approach to split the excitons into free carriers.
1,2
Among different acceptor
materials, fullerene derivatives attracted the most attention and gave the highest power
conversion efficiencies (PCEs) for almost two decades. Unique to fullerene derivatives is their
ball-like fully conjugated structure, which provides strong electron-accepting and isotropic
electron-transport capabilities and facilitates electron delocalization at the D:A interfaces.
3
As
such, fullerene derivatives were believed to be a critical component for efficient operation of
organic solar cells (OSCs). Indeed, acceptor materials based on molecules other than fullerene
derivatives, generally categorized as non-fullerene (NF) acceptors, usually resulted in low
PCEs,
1,4
which were mainly attributed to the difficulties in the morphological control.
5
However, this situation has changed recently, with quick development of NF OSCs. The
PCEs of NF OSCs have increased dramatically since 2015, now reaching a high value of
13.1%.
6
Such a high value is better than 11.7% in the best fullerene-based OSCs.
7,8
The quick
development of NF OSCs during the past two years has benefited a lot from the synthetic
methods, materials design strategies and device engineering protocols developed during the
past two decades for fullerene-based OSCs. The wide range of donor molecules developed for
fullerene-based OSCs provides a rich library for immediate use in NF OSCs. In addition,
various design strategies originally developed for donor molecules are readily available to tune
the absorption spectra and energy levels of NF acceptors, allowing better flexibility in realizing
donor-acceptor systems with complementary absorption and optimized energy band diagram.
The development of a few high-performing molecules, discussed in the next section, has also
contributed to attracting the interest of the research community on NF OSCs.
From the device point of view, a feature that contributes to high PCEs in NF OSCs is that
excitons can separate efficiently upon negligible driving energies.
9–14
As a result, NF OSCs
often show high photocurrent and low voltage losses at the same time. In contrast, charge
separation in fullerene-based OSCs usually becomes problematic under low driving energies,
presenting a trade-off between high photocurrent and high photovoltage.
15,16
Currently, the
device physics and photo-physics investigations are lagging behind the rapid developments of
3
materials and device engineering. A fundamental question open to the community is how the
excitons split into free carriers in these NF OSCs with low driving energies.
Along with opportunities, one of the key challenges for NF acceptors might lie in their
anisotropic structures. It is well acknowledged that D:A π-π interactions, which depend on the
molecular orientation between the donor and acceptor materials, are very important for the
charge transfer and transport in the devices.
17
Compared with the isotropic ball-like conjugated
backbones in fullerene derivatives, the anisotropic conjugated structures of NF acceptors make
it more challenging to ensure efficient π-π interactions.
18–21
Therefore, it becomes critically
important to pair the donors with right acceptors in NF OSCs. From this aspect, the great
diversity in chemical structures of NF acceptors also brings challenges for morphological
control in devices, not only due to the well-known requirement for fine-tuning the phase
separation but also because of the demand for molecular orientation control.
The development of NF acceptors presents opportunities which are otherwise not possible
in fullerene-based OSCs, and also opens up challenges which are waiting to be tackled for future
applications of this promising technology. In addition, the operation of NF OSCs also implies
new working mechanisms, which could be fundamentally different from those in fullerene-
based OSCs. This Review aims to discuss these opportunities, challenges, and working
mechanisms, hoping to foster further advances in this field. The PCEs of NF OSCs are
promising for future improvement, through both materials chemistry and device engineering.
As such, NF OSCs provide a promising technology for practical applications in the near future,
especially considering that they have also shown excellent thermal stability.
22,23
State-of-the-art non-fullerene acceptors
Based on chemical structures, state-of-the-art NF acceptors can be categorized into two
types: acceptors based on fused aromatic diimides, and acceptors based on strong
intramolecular electron push-pulling effects. These high-performance NF acceptors share two
features in common (Figure 1a). First, their conjugated backbones are modified with π-
conjugated functional groups involving highly electronegative elements, e.g. oxygen (in the
form of a carbonyl group) and/ or nitrogen (in the forms of a cyano group or nitrogen-containing
4
hetero-aromatic segments). Second, π-electrons in these functional groups can be well
delocalized into the backbones. The first feature provides strong electron accepting abilities,
and the second feature ensures a relatively low reorganization energy so that the accepted
electrons can be transported easily without being trapped. In addition, for solution-processed
OSCs, appropriate functional groups also need to be carefully designed to satisfy the solubility
requirement ─ this is different from vacuum-deposited OSCs. We note that in vacuum-
deposited solar cells, NF acceptors also demonstrate great potential in enhancing light
absorption and hence attract considerable attention,
24,25
consistent with recent development in
solution-processed OSCs.
Acceptors based on fused aromatic diimides
Fused aromatic diimide derivative was the first acceptor material used in heterojunction
OSCs ─ it was used as the electron acceptor in the pioneering bilayer devices in 1986 (Figure
1a).
26
Since then, this type of materials have been considered as promising candidates for NF
OSCs, and have attracted continuous attention. Among various derivatives of fused aromatic
diimides, perylene diimides (PDIs) and naphthalene diimides (NDIs) (Figure 1b), widely used
in the traditional dye industry, are two of the most intensively studied acceptor molecules
because of their advantages in strong light absorption, low synthesis cost, and excellent stability.
PDIs have been frequently used in small molecular acceptors.
27
The key considerations for
the molecular design of PDI-based acceptors are to restrain their aggregation effects and
improve their miscibility with polymer donors. PDIs have rigid and planar conjugated
backbones. As a result, the PDI derivatives in solid states tend to form large-sized crystals,
which are undesirable for the nanoscale morphology in the BHJ structures.
4
In addition to the
universal approaches (for instance, side chain engineering and solvent engineering) for
controlling the aggregation effects, an effective method to solving this problem is to link PDI
units, forming dimeric or multilinked PDIs (Figure 1b).
28–30
These linked PDI derivatives are
twisted, helping to break the aggregations of PDIs. For example, an efficient OSC based on a
PDI derivative was obtained by using a dimer named as SF-PDI
2
, where two N-alkyl-substituted
PDIs were linked by an unsubstituted spirofluorene (Figure 1c).
9