An efficient nonfullerene acceptor for all-small-molecule solar cells with versatile processability in environmentally benign solvents
Introduction
In the past decade, bulk-heterojunction (BHJ) organic solar cell research has made great leap forward with the development of efficient organic photoactive materials, currently exhibiting power conversion efficiencies (PCEs) exceeding 10% [1]. Until now, such material developments have focused mostly on the high-performance donor materials because fullerene-based [6,6]-phenyl-(C61 or C71)-butyric acid methyl ester (PC61BM or PC71BM) molecules which have outstanding electron accepting and transporting abilities have been exclusively utilized as the acceptor material in the BHJ devices. While these PCBM-based devices have shown successful results, some drawbacks of the PCBM, such as weak visible light absorption and difficulty of energy level control, limit the further improvement of the device efficiency. In this respect, nonfullerene electron acceptors are considered potential alternatives to the conventional fullerene-based acceptor in recent years [2], [3]. With the diverse and versatile molecular design, however, the nonfullerene acceptors could have enhanced light harvesting abilities and finely tuned frontier molecular orbital energy levels, providing possibilities of enhanced device efficiencies, especially in increased open-circuit voltages (Voc). Very recently, a variety of nonfullerene small molecule/polymeric acceptors have been reported in combination with several high-performance donor materials to have PCEs of 6.0–7.7% [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15].
Attaining favorable donor–acceptor blend morphologies to facilitate efficient charge separation and transport is an important goal for high-performance organic solar cells. Because the film characteristics are affected mainly by processing solvent properties like boiling point, viscosity, and so on, selection of a suitable solvent system is the most critical in the device optimization process. Thus far, halogenated and/or aromatic solvents such as chloroform (CF), chlorobenzene (CB), and o-dichlorobenzene (o-DCB) have been used commonly in a large number of devices due to their high solubility for active materials [16], [17]. However, such halogenated ones have to be avoided in application for practical large-scale manufacturing due to their environmentally harmful nature [18], [19]. Therefore, the need for using environmentally benign solvents, such as n-butanol (n-BuOH) [20], 2-methyltetrahydrofuran (2-MeTHF) [21], N-methyl-2-pyrrolidone (NMP) [22], and non-halogenated aromatic ones [23], [24], [25], has been growing in recent years. Nonetheless, low solubility of the PCBM acceptor as well as the donor materials in non-halogenated solvents has limited their extensive applications in various processing conditions. In this regard, nonfullerene acceptors could be promising materials well compatible with various organic solvents because their solubility characteristics could be controlled easily by modifying molecular structures. In spite of potential applications of the nonfullerene acceptors including large area roll-to-roll processing [26], [27], there have been only few reports of high-performance nonfullerene solar cells with unconventional environmentally benign processing solvents [15], [28], [29].
Recently, we developed dicyanodistyrylbenzene-naphthalimide (DCS-NI) type molecular acceptors, which show outstanding electron accepting and transporting properties with a balanced self-assembly behavior that suppress undesirable aggregations in films. The resulting DCS-NI acceptor-based solar cells showed maximum 2.7% and 5.4% PCEs with a polymer poly(3-hexylthiophene) (P3HT) and a small molecule 7,7′-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene-2,6-diyl)bis(6-fluoro-4-(5′-hexyl-[2,2′-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole) (p-DTS(FBTTh2)2) as the donor, respectively [30], [31]. Unfortunately, although these combinations yielded successful results for solar cells, these devices had to fabricated using only CF as a processing solvent due to unfavorable film formation features in other aromatic solvents. This difficulty for favorable film formation in non-halogenated solvents may limit the broad applications in nonfullerene solar cells, so improving processability of DCS-NI acceptors in various processing conditions remained challenging.
Here we present a new DCS-NI type acceptor, (2E,2′E)-3,3′-(1,5-bis(hexyloxy)naphthalene-2,6-diyl)bis(2-(5-(4-(N-(2-ethylhexyl)-1,8-naphthalimide)yl)thiophen-2-yl)acrylonitrile) (NIDCSN), with remarkable processability in various organic solvents. This compound was designed by replacing a phenyl unit on the core of our previously published high-performance DCS-NI compounds [30], [31] with a naphthalene unit, showing highly enhanced solubility in various solvents with uniform film formation characteristics. In addition, as for a donor material selection of the solar cell devices, the high-performance small molecular donor p-DTS(FBTTh2)2 [31], [32] (Fig. S1 in Supplementary material) was also adopted in this work because small molecular systems have an advantage of superior solubility in most of solvents over polymeric systems as well as circumvent batch-to-batch variation issues caused by molecular weight distribution of synthetic polymers. The resulting all-small-molecule solar cells fabricated using p-DTS(FBTTh2)2 as the donor and NIDCSN as the acceptor in five different solvents, such as CF, CB, tetrahydrofuran (THF), toluene, and o-xylene, all exhibited similar device performances at a 110 °C annealing condition with a best PCE of 3.45% and remarkable Voc values of 1.04–1.08 V. The detailed blend characteristics in all of the processing conditions were examined by optical, morphological, and electrical analyses.
Section snippets
(2E,2′E)-3,3′-(1,5-bis(hexyloxy)naphthalene-2,6-diyl)bis(2-(5-(4-(N-(2-ethylhexyl)-1,8-naphthalimide)yl)thiophen-2-yl)acrylonitrile) (NIDCSN)
Compound (1) [33] and (2) [30] were synthesized using the procedure reported elsewhere. Compound (1) (0.10 g, 0.26 mmol) and (2) (0.24 g, 0.56 mmol) were dissolved in MeOH (30 mL) and stirred at 50 °C. Potassium tert-butoxide (t-BuOK, 20 wt% solution in THF) (0.41 mL, 0.65 mmol) was then injected into the mixture followed by stirring for 6 h. After cooling to room temperature, the orange precipitate was collected by filtration and washed with MeOH. Flash silica gel and alumina column
Results and discussion
NIDCSN was synthesized via the Knoevenagel condensation reaction between compound 1 [33] and 2 [30], and purified by flash column chromatography and recrystallization (Fig. 1a). The compound exhibited considerable solubility in common organic solvents like CF, THF, and CB. Thermogravimetric analysis revealed an excellent thermal stability (5% weight loss at 367 °C, Fig. S2 in Supplementary material), and melting and crystallization temperatures measured by differential scanning calorimetry were
Conclusions
In summary, we synthesized a novel DCS-NI acceptor NIDCSN with the outstanding electron accepting property and superior processability in various solvents. NIDCSN showed uniform blend film formation with favorable nanoscale phase separation in the five different solvent conditions such as CF, CB, THF, toluene, and o-xylene. The resulting solar cells using p-DTS(FBTTh2)2 as the donor and NIDCSN as the acceptor exhibited a best PCE of 3.45% with a Voc of 1.04 V, a Jsc of 5.94 mA cm−2, and a FF of
Acknowledgments
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea Government (MSIP) (No. 2009-0081571), and in parts by the Global Frontier R&D Program on Center for Multiscale Energy System funded by the NRF under the MSIP, Korea (2012M3A6A7055540).
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