Nonplanar Perylene Diimides as Potential Alternatives to Fullerenes in

Letter pubs.acs.org/JPCL

Nonplanar Perylene Diimides as Potential Alternatives to Fullerenes in Organic Solar Cells Sridhar Rajaram,*,† Ravichandran Shivanna,‡ Sunil Kumar Kandappa,† and K. S. Narayan*,‡ †

International Centre for Materials Science and ‡Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore, India S Supporting Information *

ABSTRACT: Perylene diimides (PDIs) are attractive alternatives to fullerenes as electron transporters because of their optoelectronic properties, durability, and ease of synthesis. Belying this promise, devices that utilize PDIs as electron acceptors have low efficiencies. The primary deficiency in such cells is the low short circuit current density (JSC), which is traceable to the crystallinity of PDIs. Therefore, disrupting the crystallinity without adversely impacting the charge-transfer properties of PDIs is proposed as an important design principle. This has been achieved using a nonplanar perylene. In combination with a hole transporting polymer, a device efficiency of 2.77% has been achieved. A 10-fold increase in JSC is observed in comparison with a planar PDI, resulting in one of the highest JSC values for a solution processed device featuring a PDI. Indeed, this is one of the highest efficiencies for devices featuring a nonfullerene as the electron transporter. SECTION: Energy Conversion and Storage; Energy and Charge Transport

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Fréchet groups correlated the short-circuit current density (JSC) to the separation distance between donor and acceptor molecules at the interface.16 It was shown that small separation distances facilitated the formation of a stable charge-separated state leading to inefficient charge carrier separation. A combination of planar donor and planar acceptor molecules results in small separation distances at the interface. Conversely, a marginal increase in the separation distance between the donor and acceptor can lead to a more loosely bound CT state, resulting in better charge separation and higher short circuit current density. A common theme emerging from these studies is that the micrometer-scale crystallization of perylenes in blends may have an unfavorable impact on the performance of PDI-based solar cells. This suggests that reducing the cofacial stacking of perylene molecules without adversely impacting their charge-transport properties may be an important design principle. Keeping this in mind, the previously reported PDI 1 (Chart 1) was chosen for study.17−20 In this molecule, two PDI units are brought together using hydrazine as a linker. The central N−N single bond is surrounded by four carbonyl groups, wherein the oxygens carry a partial negative charge. To minimize the electronic repulsion between the oxygen atoms, the imide planes and hence the perylene units are oriented perpendicular to each other. The resulting loss of planarity was posited to lower phase segregation, reduce the formation of intermolecular states, and expedite charge separation at the interface. In addition, the molecule has a higher extinction

ulk heterojunction polymer solar cells (BHJ-PSCs) are being extensively researched as a clean energy source, and efficiencies around 8% have been recently reported.1−4 A common constituent in high-efficiency BHJ-PSCs is PCBMC70, which is used as an acceptor in the active layer. Alternatives to PCBM-C70 would open up more avenues, including enhanced device stability and efficiency. In this regard, perylene diimides (PDIs) offer a promising choice.5−8 The perylene scaffold exhibits good light absorption and high electron mobilities. Apart from this, the procedures for synthesis of perylenes are scalable and are relatively inexpensive. In general, perylene systems are chemically robust, resistant to photo degradation,9 relatively easy to manipulate synthetically, and have tunable energy levels. Despite such promising properties, BHJ solar cells with PDIs as acceptors often exhibit very low efficiencies. Tuning the molecular architecture of perylene provides a possible solution to this problem.10 In this report, we address issues related to low efficiencies and demonstrate a specific PDI-based system, which exhibits good efficiency. This lays down a route to achieve highefficiency C70-free BHJ-PSCs. Several studies have explored the reasons for the low efficiencies of PDI-based solar cells. PDIs have been shown to crystallize from blends with polymers leading to large-scale phase separation.11 To reduce the formation of micrometersized crystals, Fréchet and coworkers reported the use of a diblock copolymer as a compatibilizer.12 Howard and coworkers have reported that the formation of intermolecular states is an important loss mechanism for excitons generated in PDIs.13 Additionally, charge transport in perylene crystals is expected to be anisotropic with high mobility in the π-stacking direction.14,15 A recent report from the Brédas, Salleo, and © 2012 American Chemical Society

Received: July 27, 2012 Accepted: August 13, 2012 Published: August 17, 2012 2405

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Chart 1. Structures of Perylenes and Donor−Acceptor Polymer Used in This Study

Figure 1. (a) Device architecture used in the study and (b) energy level of materials used in this study.

coefficient in comparison with PCBM-C70, which should increase the generation of excitons in the acceptor. This would be particularly useful when the hole transporting material is a donor−acceptor polymer. Typically these polymers have a bimodal absorption spectrum with very little absorption in the 500 nm to 600 nm range. The intense optical absorption of the nonplanar perylenes in this region makes them well-suited for use as electron transporters in blends with donor−acceptor polymers. We synthesized this molecule using minor modifications to a previously reported procedure.17 The synthetic route is modular in nature and allows the possibility of synthesizing analogs with different substitution patterns on each of the perylene units. However, our studies were carried out with only nonplanar perylene 1. To evaluate the effect of the loss of planarity, comparative studies were made with a planar perylene 2.21 Bottom-gate FET device structures with PDI 1 as the semiconductor layer and benzocylcobutene (BCB) as the dielectric layer exhibited electron mobilities (μe) of ∼8 × 10−3 cm2/V sec. This value is about an order of magnitude lower than mobilities for various perylenes, which have a larger degree of crystallinity.22 However, the μe magnitude is comparable to that of PCBM-C70.23 To assess the utility of this perylene as an acceptor component in solar cells, we studied BHJ films using the donor polymer PBDTTT-C-T (3).24 Organic solar cells using this polymer have exhibited device efficiencies exceeding 7%. PBDTTT-C-T exhibits two absorption bands with minimal absorption between 400 and 550 nm.24 Perylene 1 exhibits an absorption maximum at 545 nm along with a lower intensity band at 506 nm.25 Therefore, use of perylene 1 should give spectral coverage over a broader wavelength range and lead to good device efficiencies. The HOMO and LUMO levels of perylene were determined by a combination of optical absorption and cyclic voltammetry.25 Corresponding values for the polymer have been reported in literature. A comparison of these values (Figure 1) shows that favorable band offsets exist for transfer of electrons from the polymer to the perylene and for transfer of holes in the reverse direction. Using a blend of these materials, inverted-device structures (Figure 1) were fabricated, as described in the Supporting Information. Active layer composition and annealing conditions were optimized to yield a set of devices with VOC ≈ 0.76 V, JSC ≈ 9.5 mA/cm2, and fill factor ≈ 0.46 for AM 1.5 G, 1.2 sun units illumination (Figure 2). The measurements were carried

Figure 2. J−V characteristics measured under the illumination of AM 1.5 G, 1.2 sun units for devices fabricated with polymer 3 and perylenes 1 and 2. Active layer was spun cast from chlorobenzene solution containing a 1:1 ratio by weight of perylene and polymer. Blend concentration was 12 mg/mL.

out for a large number of devices, and an average power conversion efficiency (PCE) of 2.77% was observed for devices of area ≈ 14 mm2. It must be noted that this JSC of ∼7.9 mA/ cm2 (for 1 Sun) is one of the highest reported to date for solution-processed cells containing PDIs, and the efficiency is one of the highest for a nonfullerene electron transporter. For comparison, devices were fabricated with perylene 2 under identical conditions. This resulted in devices with a VOC of 0.65 V, JSC of 0.85 mA/cm2, and fill factor of 0.3 leading to a PCE of 0.13%. Across all parameters, devices fabricated with PDI 1 outperformed the ones fabricated with PDI 2. However, the largest difference was seen in the JSC, wherein the devices with PDI 1 are better by an order of magnitude. This supports the hypothesis that use of a nonplanar perylene will lead to better short circuit current density. To understand further the features that lead to an enhanced device performance, the incident photon to current conversion efficiencies (IPCEs) of devices with PDIs 1 and 2 were measured in the wavelength range of 400 to 850 nm (Figure 3). Across all wavelengths, the device with PDI 1 gave higher efficiencies with a maximum close to 40%. A comparison of the absorption spectra of the individual components of the active layer reveals (see the Supporting Information) an IPCE of ∼35% near the absorption maximum of the nonplanar perylene. The hole-transporting polymer has very little optical absorption in this region. This makes a clear case for the use of these perylenes as electron acceptors in solar cells. 2406

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Fréchet groups has shown that JSC can be improved by increasing the separation distance between the donor and acceptor at the interface. This was accomplished using a pendant aryl group that would be oriented perpendicular to the polymer backbone. Perylene 1 has a similar molecular architecture, and this also potentially contributes toward the observed increase in the JSC. Lastly, by making our perylene nonplanar, we have increased the dimensionality of the perylene from one to two. Recent work from the Gregg26 and Durrant27 groups suggests that this can lead to increased charge separation at the donor−acceptor interface. Overall, a combination of these three effects could account for the order of magnitude increase in the JSC. In conclusion, we have evaluated a nonplanar perylene as an electron transporter in organic solar cells. The modular nature of this perylene should facilitate the synthesis of PDIs with two distinct perylene units. This could potentially lead to increased spectral coverage and enhanced device performance. Effort toward the synthesis of such analogs is currently under progress in our laboratories. The disruption of planarity enhances the short circuit current density leading to better device performance in comparison with planar perylenes. IPCE studies indicate good spectral coverage of the blends and enhanced charge generation for PDI 1. Large scale phase separation, a significant problem in PDI based solar cells, has been controlled to a great extent with this perylene. Spectroscopic and microscopy studies are currently being pursued to understand the structure at the interface and its role in charge generation. Another important area of further research is the identification of a suitable hole transporting material that can perform more effectively with nonplanar perylenes. This may include polymers and small molecules that do not perform well in combination with fullerenes. These activities are currently under progress in our laboratories.

Figure 3. IPCE spectra of inverted solar cells containing perylenes 1 and 2.

The morphologies of the active layer blends were studied using optical microscopy and near-field scanning optical microscopy (NSOM). Optical microscopy images of PDI 2based blends show that the perylene phase separates from the polymer, giving rise to micrometer-sized crystals (Figure 4, panel a), which are similar to microstructural characteristics found in these systems.11,12 Similar optical images of blends with perylene 1 are featureless (Figure 4, panel b). The uniformity and the homogeneity of the perylene 1 blends were observed even at a higher resolution of 150 nm length scales, as indicated by NSOM images that are relatively featureless (Figure 4, panel c). This indicated the blends with PDI 1 are more intimately mixed and have a larger interfacial area. The disruption of stacking is likely to prevent the crystallization of this perylene. Therefore, large-scale phase separation is not observed. The more intimate mixing enhances the interfacial area. In turn, this should lead to a greater percentage of the excitons generated in either domain reaching the donor− acceptor interface. Our studies along with previous work indicate three plausible sources for the increase in the short circuit current density. The first is the reduction in the formation of micrometer-size crystals of the acceptor in the active layer blend. Fréchet and coworkers have studied the role of a compatibilizer in reducing the phase segregation of PDIs from blends with poly(3-hexyl thiophene) (P3HT).12 In blends with the compatibilizer, Fréchet and coworkers observed a decrease in the formation of micrometer-sized crystals along with an ∼50% increase in the JSC. On the basis of these trends, it appears unlikely that the change in morphology alone can completely account for the sizable (order of magnitude) increase in the JSC seen with perylene 1. Second, recent work from the Brédas, Salleo, and

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ASSOCIATED CONTENT

S Supporting Information *

Includes synthetic procedures, characterization of perylene 1, CV data, absorption, details of device fabrication, and NSOM data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (S.R.); [email protected] (K.S.N.).

Figure 4. (a,b) 100 μm × 100 μm optical images of blend films (1:1 ratio by wt)) of 2 and 1 with PBDTTT-CT, respectively. (c) 5 μm × 5 μm NSOM-transmission contrast image of blend films with 1, obtained from 150 nm glass-aperture tip using a 543 nm light source and scan step-size of 20 nm. The image represents a typical region of the blend film. 2407

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Notes

(17) Langhals, H.; Jona, W. Intense Dyes through ChromophoreChromophore Interactions: Bi- and Trichromophoric Perylene3,4:9,10-bis(dicarboximide)s. Angew. Chem., Int. Ed. 1998, 37, 952− 955. (18) Langhals, H.; Saulich, S. Bichromophoric Perylene Derivatives: Energy Transfer from Non-Fluorescent Chromophores. Chem.Eur. J. 2002, 8, 5630−5643. (19) Holman, M. W.; Yan, P.; Adams, D. M.; Westenhoff, S.; Silva, C. Ultrafast Spectroscopy of the Solvent Dependence of Electron Transfer in a Perylenebisimide Dimer. J. Phys. Chem. A 2005, 109, 8548−8552. (20) Wilson, T. M.; Tauber, M. J.; Wasielewsky, M. R. Toward an nType Molecular Wire: Electron Hopping within Linearly Linked Perylenediimide Oligomers. J. Am. Chem. Soc. 2009, 131, 8952−8957. (21) This molecule was chosen because it was used in ref 16 along with different polythiophenes and gave a good JSC. (22) See ref 6 for the mobilities of various perylenes. (23) An electron mobility of 1 × 10−3 cm2/Vs has been measured for C70 PCBM. See: Anthopoulos, T. D.; de Leeuw, D. M.; Cantatore, E.; van’t Hof, P.; Alma, J.; Hummelen, J. C. Solution Processible Organic Transistors and Circuits Based on a C70 methanofullerene. J. Appl. Phys. 2005, 98, 054503-1−054503-6. (24) Huo, L.; Zhang, S.; Guo, X.; Xu, F.; Li, Y.; Hou, J. Replacing Alkoxy Groups with Alkylthienyl Groups: A Feasible Approach To Improve the Properties of Photovoltaic Polymers. Angew. Chem., Int. Ed. 2011, 50, 9697−9702. (25) See the Supporting Information. (26) Gregg, B. A. Entropy of Charge Separation in Organic Photovoltaic Cells: The Benefit of Higher Dimensionality. J. Phys. Chem. Lett. 2011, 2, 3013−3015. (27) Clarke, T. M.; Durrant, J. R. Charge Photogeneration in Organic Solar Cells. Chem. Rev. 2010, 110, 6736−6767.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge support from the Department of Science and Technology, India through the Indo-UK Apex Program.

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REFERENCES

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