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Revealing the Impact of F4-TCNQ as Additive on Morphology and Performance of High-Efficiency Nonfullerene Organic Solar Cells


Revealing the Impact of F4-TCNQ as Additive on Morphology and Performance of High-Efficiency Nonfullerene Organic Solar Cells

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© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1806262 (1 of 9)
Revealing the Impact of F4-TCNQ as Additive on Morphology
and Performance of High-Efficiency Nonfullerene Organic
Solar Cells
Yuan Xiong, Long Ye,* Abay Gadisa, Qianqian Zhang, Jeromy James Rech, Wei You,
and Harald Ade*
Fluorinated molecule 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane
(F4-TCNQ) and its derivatives have been used in polymer:fullerene solar cells
primarily as a dopant to optimize the electrical properties and device
performance. However, the underlying mechanism and generality of how
F4-TCNQ affects device operation and possibly the morphology is poorly
understood, particularly for emerging nonfullerene organic solar cells. In this
work, the influence of F4-TCNQ on the blend film morphology and photovol-
taic performance of nonfullerene solar cells processed by a single halogen-free
solvent is systematically investigated using a set of morphological and elec-
trical characterizations. In solar cells with a high-performance polymer:small
molecule blend FTAZ:IT-M, F4-TCNQ has a negligibly small effect on the
molecular packing and surface characteristics, while it clearly affects the
electronic properties and mean-square composition variation of the bulk. In
comparison to the control devices with an average power conversion efficiency
(PCE) of 11.8%, inclusion of a trace amount of F4-TCNQ in the active layer has
improved device fill factor and current density, which has resulted into a PCE
of 12.4%. Further increase in F4-TCNQ content degrades device performance.
This investigation aims at delineating the precise role of F4-TCNQ in non-
fullerene bulk heterojunction films, and thereby establishing a facile approach
to fabricate highly optimized nonfullerene solar cells.
DOI: 10.1002/adfm.201806262
Y. Xiong, Dr. L. Ye, Dr. A. Gadisa, Prof. H. Ade
Department of Physics
Organic and Carbon Electronics Lab (ORaCEL)
North Carolina State University
Raleigh, NC 27695, USA
Dr. Q. Zhang, J. J. Rech, Prof. W. You
Department of Chemistry
University of North Carolina at Chapel Hill
Chapel Hill, NC 27599, USA
The ORCID identification number(s) for the author(s) of this article
can be found under
printing technology using low-cost mate-
rials and eco-friendly solvents.[2–8] In a
typical bulk-heterojunction OPV, a binary
mixture of donor and acceptor molecule
constitutes the photoactive layer, where
charge generation and transport occur.
In the quest of achieving high-efficiency
OPVs, extensive efforts have been devoted
to the design and synthesis of new con-
jugated polymers and nonfullerene mole-
cule acceptors.[9–12] In addition to the
materials innovations, manipulating the
multiple morphology parameters such
as molecular packing, molecular orienta-
tion, domain size, miscibility, and vertical
composition distribution is also critical
for the development of OPVs,[13,14] in par-
ticular for those based on nonfullerene
The performance of binary systems is
usually limited by the intrinsic proper-
ties (absorption, charge mobility) of the
constituent materials. To overcome the
intrinsic limitation of a binary mixture, a
third component[15,16] is often introduced
to enhance certain properties of OPVs. For
instance, adding an absorber or sensitizer
could significantly improve the light absorption and form an
energetic cascade structure for more efficient charge creation.
Unlike absorber materials, a fluorinated molecule 2,3,5,6-tetra-
fluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) with poor
optical absorption (Figure 1) has been recently used to opti-
mize the electrical properties of polymer:fullerene films. It is
noteworthy that the effects of small molecule F4-TCNQ gen-
erally depend on the systems and processing conditions and
the precise role of F4-TCNQ is still under debate. For instance,
a previous study[17] by Moulé and co-workers demonstrated
that the F4-TCNQ additive had the tendency to remain in
polar polymer S-P3MEET due to the strong binding force with
poly(ethylene oxide) (PEO) side-chain of the polymer. Never-
theless, F4-TCNQ can diffuse out of the nonpolar polymer
P3HT. Moreover, another study demonstrated that F4-TCNQ
preferred to segregate to the air/liquid interface during the
solvent drying process in P3HT:PCBM system.[18] In addition,
Salleo group[19] suggested that the behavior of F4-TCNQ in
unannealed and annealed P3HT:PCBM devices was different,
Solar Cells
1. Introduction
Organic photovoltaic (OPV) has emerged as a promising
technology for renewable energy generation, due to its via-
bility for large-area manufacturing[1] through roll-to-roll
Adv. Funct. Mater. 2019, 29, 1806262
... A common approach to charge organic semiconductors is electrical p-doping with strong oxidizing agents such as 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F 4 TCNQ). [25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42] In this work, we investigate the stabilization of organic nanoparticle dispersions by electron transfer from the comprised polymer to F 4 TCNQ. To present the principal case and for best comparability with the literature, we study the effect of F 4 TCNQ on dispersions of P3HT or P3HT:ICBA. ...
... [24,57] Earlier investigations on solution-processed polymer:fullerene light-harvesting layers demonstrated no detrimental effects of small amounts of F 4 TCNQ on the OSC performance. [28,32,37,42] We prepared solutions of P3HT:ICBA (1:1 w/w) in chloroform (8 g L −1 ) and added different amounts of F 4 TCNQ (w F TCNQ 4 between 0.1 and 1 wt% with respect to the mass of P3HT) to the solutions by injecting the respective amount of F 4 TCNQ/ acetonitrile solution (10 g L −1 ). Then we carried out the nanoprecipitation as described in Section 2.1. ...
... Our observations are well in accordance with previous recombination studies on the influence of dopants on the performance of OSCs. [28,32,65] While small amounts of dopants can reduce SRH recombination by trap filling, the incorporation of large numbers of trap states either intentionally or by impurities can lead to an increased SRH recombination. [63,66,67] In summary, the incorporation of small amounts of F 4 TCNQ into the nanoparticulate solar cells hardly affects the device performance. ...
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Organic semiconductor nanoparticle dispersions are electrostatically stabilized with the p‐doping agent 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F4TCNQ), omitting the need for surfactants. Smallest amounts of F4TCNQ stabilize poly(3‐hexylthiophene) dispersions and reduce the size of the nanoparticles significantly. The concept is then readily transferred to synthesize dispersions from a choice of light‐harvesting benzodithiophene‐based copolymers. Dispersions from the corresponding polymer:fullerene blends are used to fabricate organic solar cells (OSCs). In contrast to the widely used stabilizing surfactants, small amounts of F4TCNQ show no detrimental effect on the device performance. This concept paves the way for the eco‐friendly fabrication of OSCs from nanoparticle dispersions of high‐efficiency light‐harvesting semiconductors by eliminating environmentally hazardous solvents from the deposition process.
... For example, Xiong et al. reported that the addition of F 4 -TCNQ to FTAZ:IT-M films does not alter the crystal structure of the blend components but enhances the purity of mixed domains. 470 A systematic understanding with regard to how a dopant modifies the nanostructure of organic semiconductors is still emerging. However, the structure−property relationships described in this section appear to be general and not specific to a particular class of molecules. ...
... Overall, doping of the BHJ with different molecular dopants has been found to increase the carrier concentration and enhance carrier transport, which in certain systems appears to be one of the limiting factors. 470,503,504 Optimal extrinsic doping was also shown to induce balanced carrier transport and improved charge extraction, ultimately leading to suppression of adverse processes such as space-charge effect and recombination losses, resulting in the enhanced J SC and FF (Figure 27). To this end, recent work showed that addition of a tiny amount of dopant was enough to enhance both the J SC and FF (Table 1) by improving the carrier photogeneration efficiency and suppressing bimolecular recombination while simultaneously affecting the morphology of the BHJ (refs 118, 129, 508−517, 226, 518, 244, 264, 470, 503, and 505−507). ...
... The highest PCE of 12.4% was achieved with F4TCNQ-doped FTAZ:IT-M owing to higher charge carrier density, charge carrier mobility, and relatively lower charge recombination rate. 51 For the FTAZ:IT-M blend, it was observed that the optimized concentration of F4TCNQ leads to higher mean-square composition variation, which was further extended to another NFA system comprising of PTB7-Th:EH-IDTBR. At present, the majority of reports on doped OPVs are based on binary blends. ...
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Organic solar cells (OSCs) have been dominated by donor:acceptor blends based on fullerene acceptors for over two decades. This situation has changed 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 coexistence 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.
The current work reports a high power conversion efficiency (PCE) of 9.54% achieved with nonfullerene organic solar cells (OSCs) based on PTB7-Th donor and 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene) (ITIC) acceptor fabricated by doctor-blade printing, which has the highest efficiency ever reported in printed nonfullerene OSCs. Furthermore, a high PCE of 7.6% is realized in flexible large-area (2.03 cm²) indium tin oxide (ITO)-free doctor-bladed nonfullerene OSCs, which is higher than that (5.86%) of the spin-coated counterpart. To understand the mechanism of the performance enhancement with doctor-blade printing, the morphology, crystallinity, charge recombination, and transport of the active layers are investigated. These results suggest that the good performance of the doctor-blade OSCs is attributed to a favorable nanoscale phase separation by incorporating 0.6 vol% of 1,8-diiodooctane that prolongs the dynamic drying time of the doctor-bladed active layer and contributes to the migration of ITIC molecules in the drying process. High PCE obtained in the flexible large-area ITO-free doctor-bladed nonfullerene OSCs indicates the feasibility of doctor-blade printing in large-scale fullerene-free OSC manufacturing. For the first time, the open-circuit voltage is increased by 0.1 V when 1 vol% solvent additive is added, due to the vertical segregation of ITIC molecules during solvent evaporation.
With chlorinated solvents unlikely to be permitted for use in solution-processed organic solar cells in industry, there must be a focus on developing non-chlorinated solvent systems. Here we report high efficiency devices utilising a low-bandgap donor polymer (PffBT4T-2DT) and a non-fullerene acceptor (EH-IDTBR), from hydrocarbon solvents and without using additives. When mesitylene was used as the solvent, rather than chlorobenzene, an improved power conversion efficiency (11.1%) was achieved without the need for pre- or post- treatments. Despite altering the processing conditions to environmentally friendly solvents and room temperature coating, grazing incident X-ray measurements confirmed that active layers processed from hydrocarbon solvents retained the robust nano-morphology obtained with hot-processed chlorinated solvents. The main advantages of hydrocarbon solvent processed devices, besides the improved efficiencies, were the reproducibility and storage lifetime of devices. Mesitylene devices showed better reproducibility and shelf-life up to 4000h with PCE dropping by only 8% of its initial value.