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Simply Complex: the efficient synthesis of an intricate molecular acceptor for high-performance air-processed and air-tested fullerene-free organic solar cells Seth M McAfee, Sergey Dayneko, Pierre Josse, Philippe Blanchard, Clément Cabanetos, and Gregory C Welch Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b04862 • Publication Date (Web): 06 Jan 2017 Downloaded from http://pubs.acs.org on January 7, 2017
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Chemistry of Materials
Simply Complex: the efficient synthesis of an intricate molecular acceptor for high-performance air-processed and air-tested fullerenefree organic solar cells Seth M. McAfee,[a] Sergey Dayneko,[a] Pierre Josse,[b] Philippe Blanchard,[b] Clément Cabanetos,[b] and Gregory C. Welch*[a] 1
Department of Chemistry, University of Calgary, 2500 University Drive N.W., Calgary, AB, Canada, T2N 1N4. CNRS UMR 6200, MOLTECH-Anjou, University of Angers, 2 Bd Lavoisier, Angers, France, 49045.
2
ABSTRACT: A perylene diimide (PDI) flanked diketopyrrolopyrrole (DPP) -conjugated small molecule has been synthesized through an efficient and sustainable direct heteroarylation protocol. When paired with the donor polymer PTB7-Th, air processed and tested bulk-heterojunction (BHJ) organic solar cells (OSCs) achieved a high power conversion efficiency (PCE) of 5.6 %. The new acceptor showed favourable morphological changes upon solvent vapour annealing leading to a near 3-fold increase in PCE. This result is among the best reported utilizing DPP based acceptors in air processed and tested OSCs. All solar cells exhibited good air and light stability over a 35-day evaluation period.
Introduction Solution-processable organic -conjugated small molecules represent an important class of materials for organic electronics.1–4 In particular, their use in organic photovoltaics offer the potential to deliver low-cost and scalable technologies for solar energy conversion.5–8 Traditionally, fullerene derivatives have been used as the acceptor component in the donor-acceptor bulk heterojunction (BHJ) solar cell; however, their absorption and energy level tuning deficiencies, coupled with their high cost have stimulated the rapid development of alternative electron accepting materials.9–11 While there exist several varieties of high performance nonfullerene acceptors,11–18 perylene diimide-based materials (PDI) are the most studied. PDI has proven to be an ideal material for electron transport, it offers a low-lying electron affinity with the added benefit of strong light absorption in the visible region. Coupled with the widespread availability of its precursors, and the potential for a low-cost, versatile, and scalable material synthesis, PDI-based materials have gained significant attention in the development of non-fullerene acceptors.19 A popular strategy in the design of high-efficiency PDIbased non-fullerene acceptors has been to link two or more PDI monomers together, forming non-planar multichromophoric molecular structures to tailor self-assembly.17,18,20–22 In surveying the many high-performance PDI-based non-fullerene acceptors of this design, it was noticed that very few respond favourably to post-deposition solvent vapour annealing (SVA).23–25 SVA has been a powerful tool for increasing device efficiencies through induced morphological changes in the active layer blend.26,27 With this in mind, we sought to design a PDI-based molecular acceptor that would respond to SVA as post-deposition strategy to increase device efficiency and compatibility with various donor materials. Utilizing the PDI-X-PDI design strategy, we selected the organic dye DPP28–33 as the link between two PDI units. DPP has
been known to exhibit favourable response to SVA conditions,34–37 which we envisioned could be imparted to our final molecule. Incorporating this electron-deficient dye as the central unit creates a unique acceptor-based push-pull system with an A-A’-A framework. Rarely have PDI units been linked via electron deficient units; however, utilizing this strategy ensures the final material will have suitable energy levels for use as an electron acceptor in BHJ solar cells. Herein we discuss how rational materials design and efficient synthetic chemistry can be exploited for straightforward access to a complex molecular acceptor with impressive photovoltaic performance using SVA as a post-deposition processing method.
Material Design and Synthesis
Figure 1. Direct heteroarylation synthesis of PDI-DPP-PDI with a silica-supported catalyst.
The non-fullerene acceptor PDI-DPP-PDI was designed as an A-A’-A type framework with a DPP core flanked by two PDI terminal units (Figure 1). Functionalization to the bay-position of PDI in the form of an N-annulation has been recently reported by our research group,38 and is a low-cost and scalable method to synthesize PDI-based materials. N-annulation offers synthetic versatility through the inclusion of an additional alkyl side chain, which promotes solubility and influences self-assembly tendencies.39–41 Furthermore, by annulating one side of
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Chemistry of Materials
Table 1. Optical and electrochemical data for PDI-DPP-PDI. Soln max (nm)a 538 a 1.0
Soln onset (nm)a 685
Film max (nm)b 538
Film onset (nm)b 760
e(M-1cm-1)c
Oxonset (V)d 0.50
121,352
Redonset (V)d -1.14
Oxidation Potentials (V)d 0.50, 0.85, 1.26
Reduction Potentials (V)d -1.14, -1.49, -1.85
IP (eV)e
EA (eV)f
5.3
3.7
w/v% solution in CHCl3 b 1.0 w/v% solution in CHCl3 cast at 1500 rpm c 1.0 w/v% solution in CHCl3 measured at 538 nm potentials from solution cyclic voltammetry e IP= (Oxonset + 4.80) f EA= (Redonset + 4.80)
d Estimated
a)
1.5
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a)
0.3
140,000
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80,000 60,000 0.1
40,000
1.5 1.0
0.5
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IP= 5.3 eV OxOnset = 0.50 V
0.0 -0.5 RedOnset = -1.16 V EA= 3.7 eV
-1.0
0
PDI
-2
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Figure 3. a) Solution and thin-film absorption spectra of PDIDPP-PDI. b) As-cast (black), post-deposition thermal annealing (TA, red), and CHCl3 solvent vapour annealing (SVA, blue) of thin-films.
0.0 -0.5
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0 300
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-1.5
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DPP
1.0
b)
120,000
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Optoelectronic characterization was completed by cyclic voltammetry (CV) and UV-visible spectroscopy (UV-vis), for full experimental details see the Supporting Information. A summary of optical and electrochemical data can be found in Table 1.
Solution e (M-1cm-1)
Optoelectronic Characterization
The estimated EA is consistent with other PDI-based nonfullerene acceptors and only 0.1 eV lower than the bay-linked N-annulated PDI (ca. 3.8 eV).38 Compared to DPP-based nonfullerene acceptors, the IP is similar to that of those substituted with phthalimide and naphthalimide terminal units,46 as well as a couple recent multichromophoric DPP-based acceptors.59,60 By directly comparing the CVs of the individual components (DPP and PDI) and PDI-DPP-PDI (Figure 2b) it appears that the EA is dictated by PDI (3.5 eV versus 3.7 eV) and the IP is dictated by DPP (5.2 eV versus 5.3 eV). Rather than a fully delocalized -system it is proposed that the dihedral angle between the DPP core and the PDI terminal units is too large for efficient -orbital overlap and leads to localized highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO). The proposed localization of the HOMO and LUMO was supported by TD-DFT calculations completed at the B3LYP/6-31G(d,p) level of theory,61 which depicts a localized HOMO (DPP) and LUMO (PDI) and predicts no HOMO to LUMO transitions (SI, Figure S9), all predicted transitions occur from PDI to PDI or DPP to DPP. Such features may prove favourable for single-component OSCs.62
Film Absorbance
the PDI chromophore, the mono-bromination of PDI is straightforward, allowing for a facile synthesis of PDI materials for use as the terminal acceptor in molecular frameworks. Using direct heteroarylation,42–46 and employing a silicasupported catalyst, an approach that has been previously optimized by our research group,47–54 PDI-DPP-PDI was successfully synthesized in 70 % yield (0.46 g). The final material was initially purified by a standard column chromatography procedure; however, recognizing the importance of purity for -conjugated materials to be tested in electronic devices a more rigorous purification was also conducted. Using recycling HPLC we were able to remove an impurity (ca. 0.03 %) identified as residual mono-substitution (SI, Figure S27-S31). The synthesis of PDI-DPP-PDI from two low-cost building blocks through a simple and sustainable synthetic route is a significant achievement in the development of non-fullerene acceptors, and could foreseeably meet the demands of large scale production for commercial applications.55,56
Normalized Current
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-1.5 -1 -0.5 0 0.5 1 Potential v. Ferrocene (V)
1.5
2
Figure 2. a) Solution cyclic voltammograms of PDI-DPP-PDI. b) PDI-DPP-PDI (solid, black) with the individual components DPP (dashed, blue) and PDI (dashed, red).
The CV of PDI-DPP-PDI exhibits triply reversible oxidation and reduction waves (Figure 2a). Reduction potentials were observed at -1.14, -1.49, and -1.85 V, with the first two attributed to the reduction of PDI and the latter of DPP. Oxidation potentials were observed at 0.50, 0.85, and 1.26 V with the highest attributed to PDI and the lower two to DPP. The assignment of the oxidation and reduction potentials are supported by the two-fold increase in intensity observed for the waves attributed to PDI, consistent with the molecular structure. The onsets of oxidation and reduction were observed at 0.50 V and 1.16 V respectively, referenced to ferrocene. Using a conversion factor of 4.80,57 the electron affinity (EA) and ionization potential (IP) were estimated to 3.7 and 5.3 eV, respectively.58
Solution and thin-film UV-vis spectra of PDI-DPP-PDI were obtained from 1.0 w/v% solutions in CHCl3 (Figure 3a), with solubility measured at > 50 mg/mL. The molar absorptivity reaches a high value of 121,300 M-1cm-1 at max (538 nm), which is greater than the individual components DPP (31,700 M-1cm1 at 549 nm) and PDI (83,800 M-1cm-1 at 534 nm). Transitioning from solution to thin-film is accompanied by a 75 nm red-shift in the onset (760 nm), a broadening of the max (538 nm) and an increase in prominence of a broad low-energy shoulder (656 nm); offering an absorption profile that is ideal for BHJ pairing with low band-gap materials. Based on the UV-vis profiles of the individual components (SI, Figure S4) it is proposed that the two peaks at 512 and 538 nm arise from the PDI terminal units and the low energy shoulder is a DPP contribution. Thin-films were subject to post-deposition thermal and solvent vapour annealing in an attempt to influence the solid-state
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Analysis of the influence of post-deposition SVA was first investigated by space charge limited current (SCLC) measurements in the following device configuration: ITO/DPP-PDIDPP/LiF/Al (SI, Figure S25). Upon CHCl3 SVA a 7-fold increase in electron mobility was observed compared to the ascast device. This increase in mobility suggests a preferential reordering of the material in the solid state for rapid charge transport, which would also prove to be crucial for improving as-cast photovoltaic device efficiencies. Powder X-ray diffraction (PXRD) measurements of the thin-films (SI, Figure S26) show no diffraction peaks upon SVA of the as-cast thin-film, indicating that any changes in crystalline nature of thin-film nanostructure are minor and not observable by our instrument. The photovoltaic performance of PDI-DPP-PDI was obtained from the following inverted device architecture: ITO/ZnO/BHJ/MoOx/Ag (Figure 4a), full details on the device fabrication procedure including current-voltage curves and data tables for device optimization and statistics are included in the Supporting Information. In the assessment of the photovoltaic performance of new non-fullerene acceptors the donor selection continues to be a complicated issue. Currently, there exists a lack of discussion on the importance of donor selection and this has led to a wide range of donor materials being used, complicating the comparison of non-fullerene acceptors and their design principles. One common theme does exist, with P3HT64 being the most popular wide band-gap polymeric donor, and PTB7-Th65 the most popular narrow band-gap polymeric donor. Considering the absorption of PDI-DPP-PDI in the higher energy region of the visible spectrum PTB7-Th was selected as the donor component in the BHJ. The PTB7-Th:PDI-DPP-PDI active layer blend offers a suitable energy level offset (Figure 4a) as determined by solution CV measured in our laboratory (SI, Figure S8), with complementary absorption profiles (SI, Figure S11). PTB7-Th contributes an absorption profile from 300 to 800 nm with a max at 720 nm, while PDI-DPP-PDI is able to fill in the regions of low PTB7-Th absorption from 450 to 600 nm, leading to a panchromatic absorption in the entire visible region (400 to 800 nm, Figure 4c-d). PDI-DPP-PDI also exhibited emission quenching of PTB7-Th (SI, Figure S12), further highlighting the acceptable pairing of the two active layer materials. The active layer was processed under simple and repeatable conditions. Using CHCl3 as the processing solvent, the active layer materials were dissolved with a total concentration of 10
-4 -6
-8 As-cast
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-12
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45 Quantum Efficiency (%)
Device Performance
mg/mL and processed in air at room temperature. We found spin-casting the films at 1500 rpm gave best results (SI, Table S2). The best donor-acceptor ratio was found to be 40:60, yet acceptor heavy devices 30:70 and 20:80 still gave appreciable device performance (Table 2, SI, Figure S10), an important feature considering the associated costs of PTB7-Th. The devices with greater PDI-DPP-PDI content maintained high open-circuit voltages (VOC) but led to lower fill factors (FF) and shortcircuit currents (JSC). These losses in performance can be correlated to the limited PTB7-Th content, which is supported by the UV-vis spectra where limited contribution from PTB7-Th is observed (SI, Figure S11). The CHCl3 SVA of our devices was found to reach their best efficiencies after 2-5 minutes, where longer times (10 minutes) were met performance metrics similar to the as-cast devices (Table 2, Figure 4b). Clearly it is detrimental to device performance for the CHCl3 SVA to reach 10 minutes; however, it is difficult to pinpoint the optimal time in the much narrower 2-5 minute window (SI, Table S5). Recognizing this, and the disparity in the SVA procedure from lab to lab, we needed to identify a more diagnostic method to routinely reach the best device efficiencies.
PDI-DPP-PDI
self-assembly of the material (Figure 3b). Unlike other high performance DPP-based materials,28,59,63 thermal annealing at a range of temperatures from 100 to 200 °C was shown to have no observable effect on the thin-film morphology by UV-vis (SI, Figure S5). SVA was carried out using a range of solvents (SI, Figure S6) with CHCl3 appearing to induce the most significant change in the UV-vis profile. CHCl3 SVA leads to a 75 nm blue-shift in the onset with a loss of the broad low-energy shoulder (686 nm) and the appearance of a sharper one (586 nm), likely a result of re-organization dictated by the DPP chromophore. The emergence of this low-energy peak upon SVA has been previously observed in DPP based small molecules.36 Thin-film photoluminescence was also influenced by CHCl3 SVA (SI, Figure S12), where a doubling in the intensity was observed in comparison to that of the as-cast thin-film.
PTB7-Th
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Chemistry of Materials
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Figure 4. a) Device architecture and active layer electron affinities and ionization potentials. b) Current-voltage curves for as-cast (black) and CHCl3 SVA, 2-5 minutes (blue) and 10 minutes (red). c) Active layer blend UV-vis profiles: as-cast (black), 2-5 minutes (blue) and 10 minutes (red) CHCl3 SVA. d) External quantum efficiency profiles for as-cast (black), 2-5 minutes (blue), and 10 minutes (red) CHCl3 SVA. Table 2. Organic solar cell performance metrics for the optimization of donor:acceptor ratios and CHCl3 vapour annealing. Ratio
Processing
VOC (V)
JSC (mAcm-2)
FF (%)
PCE (%)
40:60a
as-cast
0.98 (0.97)
6.13 (6.41)
34.6 (35.2)
2.1 (2.2)
40:60b
CHCl3 vapour (2-5 min)
0.97 (0.98)
10.42 (11.32)
49.3 (50.1)
5.0 (5.6)
40:60c
CHCl3 vapour (10 min)
1.00 (1.00)
6.15 (6.23)
34.2 (34.7)
2.1 (2.2)
30:70c
as-cast
1.02 (1.01)
5.50 (5.79)
34.0 (34.6)
1.9 (2.0)
30:70c
CHCl3 vapour (2-5 min)
0.98 (0.98)
9.32 (9.58)
49.9 (50.4)
4.5 (4.7)
20:80c
as-cast
1.00 (0.99)
4.17 (4.51)
35.0 (35.1)
1.5 (1.6)
20:80c
CHCl3 vapour (2-5 min)
1.00 (1.00)
7.38 (7.44)
50.6 (51.6)
3.7 (3.9)
a Average
b Average
c Average
of 8 devices (Best); of 11 devices (Best); of 4 devices (Best). One device per substrate. Device size = 9 mm2. Substrate size = 15 x 15 x 0.7 mm.
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Chemistry of Materials Analysis of the UV-vis profile of the active layer blend under these different CHCl3 SVA times has highlighted a change in the profile at 586 nm (Figure 4c). The emergence of the new peak in this region is linked to the CHCl3 SVA conditions that reached the highest device efficiencies. Comparing the external quantum efficiency (EQE) profiles (Figure 4d) of the as-cast and solvent vapour annealed devices has provided further evidence to support the importance of this distinct change in the active layer absorption profile. Accompanying the emergence of the peak at 586 nm is a two-fold increase in the photocurrent generation where PDI-DPP-PDI absorbs (450-600 nm) compared to that of PTB7-Th (650-750 nm). This indicates that the peak observed at 586 nm upon SVA can be directly correlated to a more favourable BHJ where PDI-DPP-PDI is significantly contributing to photocurrent generation. Further characterization of the active layer morphology under the SVA conditions was investigated by atomic force microscopy (AFM). AFM images (Figure 5) show a smooth ascast thin-film (RMS= 1.15 nm) and after 2-5 minutes the formation of many small aggregates are observed (RMS= 2.36 nm). After 10 minutes several large aggregates are formed (RMS= 8.66 nm) and phase separation is observed. Comparison of the AFM images correlates the formation of small aggregates in the active layer morphology to the best device efficiencies and supports the noted importance of the peak that emerges at 586 nm in the UV-vis profile. Utilizing our optimized processing conditions (Table 2), 25 minute post-deposition CHCl3 SVA was accompanied by a significant increase in both JSC (6.4 v. 11.3 mAcm-2) and FF (35 v. 50 %) (Figure 4b) to reach a best device efficiency of 5.6 % in comparison to 2.2 % for the as-cast device. Our efficiencies proved to be remarkably consistent over 11 devices, with one device per substrate (SI, Table S5, Figure S17) and are comparable to the best reported air solution-processed and tested small molecule non-fullerene devices.66 Additionally this is among the highest PCEs reported for an OSC using a DPP based acceptor.67 0.35
Film Absorbance
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was found that no significant difference in performance was observed with recycling HPLC (5.0 % PCE), suggesting that the residual mono-substitution impurity is not detrimental to device performance. Preliminary device stability was probed by leaving devices exposed to air and ambient light for a period of 35 days (SI, Figure S23-24). For the first 7 days there was no changes PCE. After 14 days a ~10% decrease in PCE was observed, while after 35 days only a ~30% decrease in PCE was observed. Overall the performance decreased from ~5.0 % to 3.5 % after 35 days. This loss in efficiency has been attributed to degradation of the Ag contacts considering the UV-vis and EQE spectra remain consistent, suggesting that the active layer is maintaining its integrity. All considering, these organic solar cells proved to be quite stable and are suitable for further stability and lifetime measurements.
Conclusion In summary, the incorporation of a DPP dye between two PDI terminal units through a simple and sustainable synthetic pathway proved to be an effective method to synthesize a complex A-A’-A type non-fullerene acceptor. This material exhibited favorable optical, electronic and self-assembly properties for use as an active layer material in OSCs. OSC devices were fabricated in a straightforward method, processed in air at room temperature and reached PCEs of 5.6%. This performance is among the highest reported to date for DPP-based acceptor OCSs and is among the best reported for fullerene-free OSCs fabricated and tested in air, a feature that is important for the simplified industrial manufacturing of the OSC technology.
ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Synthetic details on materials and methods, identity and purity confirmation by 1H, 1H-1H COSY, 13C NMR spectra and MALDITOF. Complete data for optoelectronic properties, TD-DFT calculations, electron mobility measurements and organic solar cell device data (statics and stability).
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* G. C. Welch, e-mail:
[email protected] Author Contributions All authors have given approval to the final version of the manuscript.
Funding Sources
Figure 5. UV-vis profiles and AFM images of the PTB7-Th:PDIDPP-PDI active layer blend under different processing conditions: as-cast (black), 2-5 minutes CHCl3 SVA (blue) and 10 minutes CHCl3 SVA (red).
To assess the influence of material purification on solar cell efficiencies, devices were fabricated to compare the standard column chromatography with recycling HPLC purification. It
GCW acknowledges NSERC Discovery Grants Program (4357152013), CFI John Evans Leadership Fund (34102), Canada Research Chairs Program, and the University of Calgary. SMM is grateful for an NSERC post-graduate scholarship. CC thanks the “Ministère de la Recherche” for the Ph-D grants of P. Josse. The authors acknowledge Compute Canada and WestGrid for computational resources. REFERENCES
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