Phthalimide and naphthalimide: Effect of end-capping groups on molecular properties and photovoltaic performance of 9-fluorenone based acceptors for organic solar cells
Graphical abstract
A series of π-conjugated electron deficient small molecules were designed and synthesized with the configuration of central core, π-bridging unit and terminal building block.
Introduction
In recent years, non-fullerene organic solar cells have witnessed significant improvement in power conversion efficiencies (PCEs) due to continuous advancement in molecular engineering, marking an important milestone in the field of organic photovoltaics (OPV) [[1], [2], [3], [4], [5], [6]]. Compared to the conventional acceptor material fullerene and its derivatives, such as [6,6]- phenyl-C61 butyric acid methyl ester (PC61BM) or [6,6]- phenyl C71-butyric acid methyl ester (PC71BM) molecules, non-fullerene acceptors can be advantageous and can exhibit outstanding optoelectronic properties including: (1) a broader absorption in the visible region of the electromagnetic spectrum, resulting in additional photocurrent; (2) that they offer a precise tunability of the energetics with a range of options available in selecting various aromatic building blocks; (3) a facile derivatization and functionalization, and (4) potential increased chemical stability, and (5) last but not least, lower synthesis costs. Owing to recent efforts in exploration of new molecular building blocks and optimization of device processing by tuning blend film morphology, the PCEs of fullerene-free organic solar cells have now achieved 10–11% [[7], [8], [9], [10], [11], [12], [13], [14]] which is competitive with values achieved with fullerene acceptors. Recently, Zhao et al. has reported the world record 13% PCE based on poly{1-(5-(4,8-bis(5-((2-ethylhexyl)thio)-4-fluorothiophen-2-yl)benzo [1,2-b:4,5-b']dithiophen-2-yl)thiophen-2-yl)-5,7-bis(2-ethylhexyl)-3-(thiophen-2-yl)-4H,8H-benzo [1,2-c:4,5-c']dithiophene-4,8-dione} (PBDB-T-SF) polymer as a donor and 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-5,6-difluoro-indanone))-5,5,11,11-tetrakis (4-hexylphenyl)-dithieno [2,3-d:2′,3′-d’]-s-indaceno [1,2-b:5,6-b’]dithiophene (IT-4F) as an acceptor [14].
In order to design new state of the art electron acceptor materials for OPV, there are several standard design steps including the selection of the molecular core, π-bridge, and terminal end-capping groups. Such molecular design can create various molecular models based on the conjugated building blocks used, either electron donating (D) or electron accepting (A) nature. These D and A groups can be either strong or weak based on their ionisation potential (IP) and electron affinity (EA) values respectively. Among various reported D-A type structures for non-fullerene acceptors, the acceptor-donor-acceptor-donor-acceptor structure (A1-D-A2-D-A1) (where A1 and A2 have different electron affinities) is one of the most successful and elegant approaches for achieving new high performance molecules with desired intrinsic optoelectronic properties (such as light-harvesting windows and energy levels) by optimal selection of alternating D and A building blocks [[15], [16], [17], [18], [19], [20], [21]]. Using these design criteria, earlier we designed and reported a new non-fullerene acceptor based on 1,8-naphthaimide as terminal groups, 9-fluorenone as the central core, and thiophenes as π-bridge named NAI-FN-NAI (BO) [22]. Upon blending the earlier reported NAI-FN-NAI (BO) non-fullerene acceptor with P3HT donor in OPV devices, the PCE of the resulting device reached the highest value of 3.6%. In this work, we report of the design and developed of a new compound PI-FN-PI (BO) where we replaced naphthalimide by phthalimide for studying the effect of terminal unit change on optoelectronic properties and photovoltaic performance of the material. Unfortunately, the PI-FN-PI (BO) showed poor solubility in common organic solvent, possibly due to the more rigid structure of phthalimide than that of naphthalimide [23]. In order to enhance the solubility of the novel PI-FN-PI compound, we substituted the 2-butyloctyl alkyl chain with the longer 2-decyltetradecyl (DT) on N-bay position of nitrogen of phthalimide to produce a more soluble version PI-FN-PI (DT) (Fig. 1). In addition, for comparison purpose, a compound NAI-FN-NAI (DT) possessing similar alkyl chain with PI-FN-PI (DT) was synthesized.
There are significant changes observed in the thermal, optical, electrochemical properties, and photovoltaic performance on substitution of the end-capping group from NAI to PI, which are detailed in the discussion below. The OPV devices comprising P3HT as a donor and newly developed highly soluble version of phathalimide end capping group based compound PI-FN-PI (DT) as an acceptor showed the highest performances with a VOC of 0.88 V, a JSC of 4.6 mA cm−2, a FF of 46% and a PCE of 1.9%, which is lower than that of NAI-FN-NAI (BO) (PCE = 3.6%) but higher than that of NAI-FN-NAI (DT) (PCE = 0.6%). The variation is OPV performance clearly indicates the strong impact of end capping groups on OPV performance. This comparative work on molecular engineering provides clear guideline about the appropriate selection of end-capping groups and alkyl chain engineering to achieve optimum performance.
Section snippets
Materials and instruments
All staring materials were purchased commercially as analytical reagents and used directly without any further purification. Compound 2,7-bis-(4,4,5,5-tetramethyl- [[1], [2], [3]]dioxaborolan-2-yl)-fluoren-9-one was prepared according to the literature [24]. Synthesized compounds were characterized by 1H NMR and 13C NMR spectrum, which were obtained with a Varian-400 or Bruker-600 spectrometer. High resolution mass spectra were recorded on an Orbitrap Elite mass spectrometer (Thermo Fisher
New non-fullerene acceptor molecular design and synthesis
The synthetic route to four non-fullerene acceptor compounds NAI-FN-NAI (BO), PI-FN-PI (BO), NAI-FN-NAI (DT), and PI-FN-PI (DT) is shown in Scheme 1. Compound 5 and NAI-FN-NAI (BO) were prepared following our previous reports [22,25]. In order to induce the solution processibility to the phthalimide end capped fluoreneone based compounds, alkyl amines with different chain lengths, made by previously reported methods [22], were reacted with 4-bromo-phthalic anhydride to produce alkylphthalimide
Conclusions
In summary, we have synthesized electron accepting materials based on phthalimide endcapped 9-fluorenone (PI-FN-PI), one with 2-butyloctyl (BO) and another with 2-decyltetradecyl (DT) attached to the end-capping phthalimide moiety. Because of the highly rigid structure of the backbone, PI-FN-PI with short alkyl chains showed poor solubility in organic solvents. Meanwhile the 2-decyltetradecyl (DT) longer alkyl chain substituted compound exhibits better solution processibility. Compared to our
Author contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
Acknowledgment
T.T.D is thankful to QUT for offering here QUTPRA scholarship to conduct her research work. Some of the data reported in this paper were obtained at the Central Analytical Research Facility operated by the Institute for Future Environments (QUT). Access to CARF was supported by generous funding from the Science and Engineering Faculty (QUT). P.S. is thankful to QUT for the financial support from the Australian Research Council for the Future Fellowship (FT130101337) and QUT core funding (
References (30)
- et al.
Effect of side chains on the electronic and photovoltaic properties of diketopyrrolopyrrole-based molecular acceptors
Org. Electron.
(2016) - et al.
Non-fullerene acceptors for organic photovoltaics: an emerging horizon
Mater. Horiz
(2014) - et al.
Non-fullerene electron acceptors for use in organic solar cells
Acc. Chem. Res.
(2015) - et al.
New advances in non-fullerene acceptor based organic solar cells
RSC Adv.
(2015) - et al.
New developments in non-fullerene small molecule acceptors for polymer solar cells
Mater. Chem. Front
(2017) - et al.
Recent progress in non-fullerene small molecule acceptors in organic solar cells (OSCs)
J. Mater. Chem. C
(2017) - et al.
Molecular electron acceptors for efficient fullerene-free organic solar cells
Phys. Chem. Chem. Phys.
(2017) - et al.
Free polymer solar cells with over 11% efficiency and excellent thermal stability
Adv. Mater.
(2016) - et al.
11.4% Efficiency non-fullerene polymer solar cells with trialkylsilyl substituted 2D-conjugated polymer as donor
Nat. Commun.
(2016) - et al.
High-performance electron acceptor with thienyl side chains for organic photovoltaics
J. Am. Chem. Soc.
(2016)