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Journal ArticleDOI

13.9% Efficiency Ternary Nonfullerene Organic Solar Cells Featuring Low-Structural Order

06 Sep 2019-ACS energy letters (ACS Publications)-Vol. 4, Iss: 10, pp 2378-2385

AbstractThe insufficient phase separation between polymer donors and nonfullerene acceptors (NFAs) featuring low structural order disrupts efficient charge transport and increases charge recombination, con...

Topics: Organic solar cell (52%)

Summary (1 min read)

Jump to: [Introduction] and [ACKNOWLEDGMENTS]

Introduction

  • Acceptors (NFAs) featuring with low-structural orders disrupts efficient charge transport and increases charge recombination, consequently limits the maximum achievable power conversion efficiency (PCE) of organic solar cells (OSCs).
  • The appreciatively enlarged length scale of phase separation induced by the presence of 10 wt% IT-M facilitates increased and balanced charge mobilities with minimized trap-assisted recombination.
  • Adding 10 wt% IT-M into binary blend leads to the growth of both PBDB-T and m-INPOIC domains, which are enlarged to 14.8 and 35.0 nm respectively.
  • A slope close to unity signifies that bimolecular recombination dominates in these devices.
  • The optimized morphology via tuning of the phase separation of the ternary system leads to increased light absorption and charge mobility, balanced charge transport and suppressed carrier recombination.

ACKNOWLEDGMENTS

  • This work was supported by the National Natural Science Foundation of China (Grant No. 21774097) and the Natural Science Foundation of Hubei Province (Grant No. 2018CFA055) of China.
  • All authors thank the beamline BL16B1 at Shanghai Synchrotron Radiation Facility for providing the beam time and help during experiment.
  • The authors also thank the Diamond Light Source (UK) beamline I07 where GIWAXS measurements were performed (via beamtime allocation SI22651-1).
  • The authors also thank the U.K. EPSRC for funding studentships for R.C.K. (DTG allocation), M.E.O’K.

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This is a repository copy of 13.9% efficiency ternary nonfullerene organic solar cells
featuring low-structural order.
White Rose Research Online URL for this paper:
http://eprints.whiterose.ac.uk/156045/
Version: Accepted Version
Article:
Du, B., Geng, R., Li, W. et al. (11 more authors) (2019) 13.9% efficiency ternary
nonfullerene organic solar cells featuring low-structural order. ACS Energy Letters, 4 (10).
pp. 2378-2385. ISSN 2380-8195
https://doi.org/10.1021/acsenergylett.9b01630
This document is the Accepted Manuscript version of a Published Work that appeared in
final form in ACS Energy Letters, copyright © American Chemical Society after peer review
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1
13.9% Efficiency Ternary Nonfullerene Organic
Solar Cells Featuring Low-structural Order
Baocai Du
a,b
, Renyong Geng
c
, Wei Li
a,b
, Donghui Li
a,b
, Yuchao Mao
a,b
, Mengxue Chen
a,b
, Xue
Zhang
a,b
, Joel A. Smith
d
, Rachel C. Kilbride
d
, Mary E. OÕKane
d
, Dan Liu
a,b
, David G. Lidzey
d
,
Weihua Tang
c*
, Tao Wang
a,b,
*
a
School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070,
China
b
State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology,
Wuhan 430070, China
c
School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094,
China
d
Department of Physics and Astronomy, University of Sheffield, Sheffield, S3 7RH, UK
ABSTRACT: The insufficient phase separation between polymer donors and non-fullerene
acceptors (NFAs) featuring with low-structural orders disrupts efficient charge transport and
increases charge recombination, consequently limits the maximum achievable power conversion
efficiency (PCE) of organic solar cells (OSCs). Herein, an NFA IT-M has been added as the third
component into the PBDB-T:m-INPOIC OSCs, and is shown to effectively tune the phase
separation between donor and acceptor molecules, although all components in the ternary system
exhibit low degrees of structural orders. The incorporation of 10 wt% IT-M into a
PBDB-T:m-INPOIC binary host blend appreciably increases the length scale of phase separation,
creating continuous pathways which increase and balance charge transport. This leads to an
enhanced photovoltaic performance from 12.8% in the binary cell to 13.9% for the ternary cell with
simultaneously improved open-circuit voltage, short-circuit current and fill factor. This work
highlights the beneficial role of ternary components in controlling the morphology of the active
layer for high performance OSCs.

2
Over the past few years, the development of non-fullerene acceptors (NFAs) has driven the
impressive progress of organic solar cells (OSCs).
1Ð4
The tunable energy levels and absorption
spectra of NFAs can allow for control of complementary absorption and low voltage loss which are
critical for high power conversion efficiency (PCE)
5Ð8
, with over 16% PCE achieved for
single-junction binary non-fullerene OSCs.
11
Whilst the emergence of new electron donor and
acceptor materials is the primary motivation to further advance OSCs, compositional and
morphological optimization within the photoactive layer is vital to realize closer to the theoretical
maximum PCE.
10Ð12
The desired morphology of the photoactive layer should resemble nanoscale
phase separated domains for efficient exciton diffusion and dissociation, of the order of the limited
exciton diffusion lengths which are usually not more than 10 nm.
15Ð17
Furthermore bicontinuous
networks are favorable for charge carrier transport, collection and suppression of bimolecular
recombination.
12,18,19
Most of the polymer donors, e.g. PTB7-Th and PBDB-T, exhibit low-structural order in the
form of p-p stacking, due to the confinement of bulky conjugated repeating units in a
macromolecular structure.
20,21
The versatile chemical structures of NFAs endow this class of
fascinating electron acceptors with very different molecular packing behaviors. For instance,
COi8DFIC and INPIC-4F show a high tendency to crystallize into lamellae
22Ð25
, whilst ITIC and
IEICO series materials exhibit p-p stacking only
26,27
. NFAs with similar chemical structures to
conjugated polymers leads to good miscibility between donors and acceptors, especially with those
of low structural orders; however this commonly results in insufficiently separated phases after
solution casting, which impacts charge generation, transport and recombination.
28Ð30
Whereas the
fine phase separation and intimate contact between donors and acceptors benefits exciton
dissociation, these morphologies are not ideal for charge transport.
Although good efficiencies have been achieved in those photovoltaic systems featuring low
structural orders, e.g. PBDB-T:ITIC and PBDB-T:IT-M, further enhancement of performance has
proved rather difficult. For example, thermal annealing of PBDB-T:IT-M blend films barely
increases the structural orders of PBDB-T and IT-M, explained by their intrinsic low ability to
self-organize. Consequently less than 10% PCE improvement has been obtained for annealed

3
devices compared with as-cast devices.
31
Solvent vapor annealing (SVA) is another effective
approach that has been demonstrated to reorganize molecular packing within blends and improve
the efficiency of many fullerene-based OSCs.
32,33
However, in non-fullerene OSCs featuring
low-structural orders e.g. PTB7-Th:ITIC, only minor enhancement of molecular packing has been
observed using a range of solvent or solvent mixture vapors to anneal devices, as such a PCE
increase of only 10% can be achieved.
34
Ternary photovoltaic solar cells prepared by incorporating a third component into conventional
binary solar cells have emerged as a promising strategy for realizing further improvements in
efficiency.
35Ð37
This method is favourable as it removes the time-consuming and expensive process
of synthesizing new conjugated polymers. Whilst the primary advantage of the ternary strategy is to
achieve complementary light absorption
37Ð39
, it can also effectively regulate the morphology.
40,41
Although literature reports have demonstrated reduced trap density and recombination in ternary
systems compared to binary systems,
42Ð45
less attention has been paid to tuning the phase separation
and efficiency of non-fullerene OSCs featuring low structural orders.
In this work, we employ the non-fullerene acceptor IT-M as the third component to tune the
domain size in PBDB-T:m-INPOIC blends, which have until now been inhibited by insufficient
phase separation, as all components exhibit low-structural orders. The presence of an intermediate
amount of IT-M enhances photon absorption in the ternary device. The appreciatively enlarged
length scale of phase separation induced by the presence of 10 wt% IT-M facilitates increased and
balanced charge mobilities with minimized trap-assisted recombination. As a result, the ternary
OSC achieves a maximum PCE of 13.9% compared with 12.8% for the PBDB-T:m-INPOIC binary
OSC, with the simultaneously increased device metrics of V
oc
of 0.86 V, J
sc
of 22.2 mA/cm
2
and FF
of 71.3%. This work highlights the beneficial role of ternary components in mediating morphology
of the active layer to improve device performance.
The chemical structures, energy levels of materials and schematic of the device structure used
in this work are shown in Figure 1a-b.
46,47
Figure 1c clearly shows the complementary absorption
of the different components in the ternary system. To examine the possible Fšrster resonance energy
transfer (FRET) between m-INPOIC and IT-M, we have measured the photoluminescence (PL)

4
spectra of the individual components and their mixtures with different weight ratios. As shown in
Figure 1d, IT-M and m-INPOIC exhibit distinct emission peaks at 765 and 873 nm respectively.
The broad overlap between the emission spectrum of IT-M and the absorption spectrum of
m-INPOIC (Figure 1c) should enable efficient energy transfer from IT-M to m-INPOIC. In the
m-INPOIC:IT-M blend, the emission signal of IT-M is markedly quenched with a single emissive
peak observed at 873 nm that is associated with m-INPOIC, suggesting efficient energy transfer
from IT-M to m-INPOIC which is favorable for photovoltaic performance.
48
From this efficient
energy transfer process we can also imply there is good miscibility between these two acceptors,
with close mixing in the blend film.
35,41,49
Figure 1. (a) Chemical structures of polymer donor and non-fullerene acceptors, and the device
architecture used in this work. (b) Schematic energy diagrams of PBDB-T, IT-M and m-INPOIC,
here the lightning bolt indicates the related energy transfer process. (c) Optical absorption spectra of
neat films of PBDB-T, m-INPOIC and IT-M. (d) PL spectra of m-INPOIC, IT-M and their mixtures
at the weight ratios of 9:1 and 8:2, excited with a 532 nm laser.
Transmission electron microscopy (TEM) was performed to understand the phase separated
domain size within the binary and ternary blends. As shown in Figure 2a-c, the dark and bright

Figures (8)
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Abstract: There has been an intensive search for cost-effective photovoltaics since the development of the first solar cells in the 1950s. [1–3] Among all alternative technologies to silicon-based pn-junction solar cells, organic solar cells could lead the most significant cost reduction. [4] The field of organic photovoltaics (OPVs) comprises organic/inorganic nanostructures like dyesensitized solar cells, multilayers of small organic molecules, and phase-separated mixtures of organic materials (the bulkheterojunction solar cell). A review of several OPV technologies has been presented recently. [5] Light absorption in organic solar cells leads to the generation of excited, bound electron– hole pairs (often called excitons). To achieve substantial energy-conversion efficiencies, these excited electron–hole pairs need to be dissociated into free charge carriers with a high yield. Excitons can be dissociated at interfaces of materials with different electron affinities or by electric fields, or the dissociation can be trap or impurity assisted. Blending conjugated polymers with high-electron-affinity molecules like C60 (as in the bulk-heterojunction solar cell) has proven to be an efficient way for rapid exciton dissociation. Conjugated polymer–C60 interpenetrating networks exhibit ultrafast charge transfer (∼40 fs). [6,7] As there is no competing decay process of the optically excited electron–hole pair located on the polymer in this time regime, an optimized mixture with C60 converts absorbed photons to electrons with an efficiency close to 100%. [8] The associated bicontinuous interpenetrating network enables efficient collection of the separated charges at the electrodes. The bulk-heterojunction solar cell has attracted a lot of attention because of its potential to be a true low-cost photovoltaic technology. A simple coating or printing process would enable roll-to-roll manufacturing of flexible, low-weight PV modules, which should permit cost-efficient production and the development of products for new markets, e.g., in the field of portable electronics. One major obstacle for the commercialization of bulk-heterojunction solar cells is the relatively small device efficiencies that have been demonstrated up to now. [5] The best energy-conversion efficiencies published for small-area devices approach 5%. [9–11] A detailed analysis of state-of-the-art bulk-heterojunction solar cells [8] reveals that the efficiency is limited by the low opencircuit voltage (Voc) delivered by these devices under illumination. Typically, organic semiconductors with a bandgap of about 2 eV are applied as photoactive materials, but the observed open-circuit voltages are only in the range of 0.5–1 V. There has long been a controversy about the origin of the Voc in conjugated polymer–fullerene solar cells. Following the classical thin-film solar-cell concept, the metal–insulator–metal (MIM) model was applied to bulk-heterojunction devices. In the MIM picture, Voc is simply equal to the work-function difference of the two metal electrodes. The model had to be modified after the observation of the strong influence of the reduction potential of the fullerene on the open-circuit volt

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This work highlights the beneficial role of ternary components in controlling the morphology of the active layer for high performance OSCs.