Article pubs.acs.org/JPCC
Diarylfluorene-Modified Fulleropyrrolidine Acceptors to Tune Aggregate Morphology for Solution-Processable Polymer/Fullerene Bulk-Heterojunction Solar Cells Bao-Yi Ren,†,‡ Chang-Jin Ou,†,‡,∥ Chao Zhang,⊥ Yong-Zheng Chang,† Ming-Dong Yi,† Ju-Qing Liu,† Ling-Hai Xie,*,† Guang-Wei Zhang,† Xian-Yu Deng,*,§ Sheng-Biao Li,⊥ Wei Wei,† and Wei Huang*,† †
Key Laboratory for Organic Electronics & Information Displays (KLOEID) and Institute of Advanced Materials (IAM), Nanjing University of Posts & Telecommunications, Nanjing 210046, China ‡ College of Applied Chemistry, Shenyang University of Chemical Technology, No. 11 Street, Shenyang Economic and Technological Development Area, Shenyang 110142, China § Research Center for Advanced Functional Materials and Devices, Department of Materials Science and Engineering, Harbin Institute of Technology Shenzhen Graduate School, Shenzhen 518055, China ∥ School of Chemistry and Chemical Engineering, Inner Mogolia University, Huhhot 010021, China ⊥ College of Chemistry, Central China Normal University, Wuhan, 430079, P. R. China S Supporting Information *
ABSTRACT: A series of n-type fulleropyrrolidine derivatives as the acceptors, including Th-C60, PFTh-C60, and OPFTh-C60, have been synthesized via the key step of the typical Prato reaction to investigate the steric hindrance effect of various phenylfluorenyl moieties on the electronic structures, aggregate morphologies, and device performances of solar cells. Conjugation-interrupted linkage obviously does not change the energy bandgaps and lowest unoccupied molecular orbital (LUMO) energy levels in PFTh-C60 and OPFTh-C60 models with respect to that of precursor Th-C60 according to UV−vis spectra and cyclic voltammetry. In contrast, dramatically different phase separation behaviors in the bulk heterojunction (BHJ) film blending with poly(3-hexylthiophene) (P3HT) were observed by atomic force microscopy. A prototype OPFTh-C60-based BHJ polymer solar cell (PSC) with the configuration of ITO/PEDOT:PSS/P3HT:OPFTh-C60 (1:1) (200 nm)/Ca/Al has the performance with the short-circuit current (Isc) of 8.68 mA/cm2, open-circuit voltage (Voc) of 0.63 V, fill factor of 0.51, and power conversion efficiency of 2.80%, better than that in PFTh-C60 or Th-C60-based counterpart devices. Our results indicate that high-performance solar cells can be achieved by the morphology control of active thin films. Diarylfluorene-modified C60 derivatives are promising n-type organic semiconductors for their applications in BHJ PSCs.
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INTRODUCTION Polymer solar cells (PSCs) have attracted intensively scientific and industrial attention to address the energy crisis since the discovery of bulk heterojunction (BHJ) polymer/fullerene solar cells in 1995.1 Normally, high power conversion efficiency (PCE) can be obtained by the device based on the BHJ structures, which © 2012 American Chemical Society
have obvious advantages such as effective exciton dissociation and charge carrier transport over conventional double-layered Received: December 20, 2011 Revised: March 5, 2012 Published: March 14, 2012 8881
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systems.2−5 However, the interpenetrating polymer network and nanoscale phase separation of BHJ films are strongly dependent on the p-type low-bandgap π-conjugated polymers, n-type organic semicondutors, and their solution-processed blending procedures. In this framework, numerous donor−acceptor (p-n) π-conjugated copolymers with the wide absorption and low bandgap have been synthesized to tune energy levels matched with acceptors, resulting in the great improvement of the light-electrical conversion efficiency.6−10 In contrast, less attention has been paid to developing n-type acceptors. Currently, the most common used acceptor materials are fullerene (C60) derivatives owing to their suitable energy levels and weak charge trapping ability in photovoltaic effect, such as [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM). The excellent acceptors of n-type C60 derivatives include several requirements, such as the suitable solubility, energy levels well-matched with donors, and wide absorption bands. However, PC61BM has the drawbacks of weak light harvesting in the visible region, relatively low lowest unoccupied molecular orbital (LUMO) energy, and instable three-member cycle, resulting in low open-circuit voltages11 and poor device stability under long-term sunlight irradiations.12 Nevertheless, to understand the structure−property relationships and design the new n-type acceptors is one of the most effective approaches to optimize the device performances.13,14 Li et al. investigated carbon chain length of PCBM derivatives on photovoltaic properties, followed by a new indene-C60 bisadduct to obtain a high PCE up to 6.5%.15−17 Indene-C60 bisadduct has a LUMO energy level of 170 meV higher than that of PC61BM, which benefits the larger open-circuit voltage of 0.84 V in poly(3-hexylthiophene) (P3HT)-based BHJ polymer/fullerene solar cells. To explore new C60 derivatives will be one effective way to improve the performance and stability. Among the C60 derivatives, fulleropyrrolidine derivatives with excellent stability of five-member cycles also exhibit high LUMO energy levels, easy preparation, and tunable modification, showing a promising n-type acceptor candidate for BHJ PSCs.12,18−20 Morphology-directed design will open another way to further improve the performance of organic devices. In this aspect, it is important to construct the model of substituent effect and to investigate the influence of morphology driving from phase separation behavior. Phenylfluorenyl moieties (PFMs) give rise to the unique conjugation-interrupted and rigid linkages through the sp3-hybridized carbon atoms, consequently becoming a unique tool to adjust conformation and morphology in polymer thin film. We have demonstrated the PFM modified-π-stacked polymers for the wide bandgap electrophosphorescent host materials and electrically memorable materials in our previous works.21−23 In this Article, three proof-of-concept modeling compounds, including Th-C60, PFTh-C60, OPFTh-C60, have been synthesized. In our design, bulky diarylfluorene building blocks21,24−26 with steric hindrance effect can alter the intermolecular interaction and aggregates without obviously changing the molecular electronic structures, avoiding closely intermolecular stacks of fullerenes with conjugated polymers. Furthermore, various other groups, alkyloxy side chains, can also be flexibly introduced to fine the morphology and nanoscale phase separation, resulting in the improvements of performances. As a result, the alkyloxy-substituted hindrance-functionalized OPFTh-C60 exhibits a nanoscale phase separation blending with P3HT, close to that of PCBM/P3HT bulk heterojunction film, and gives the best performance among them with the short-circuit current (Isc) of 8.68 mA/cm2, open-circuit
voltage (Voc) of 0.63 V, fill factor (FF) of 0.51, and PCE of 2.80%.
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EXPERIMENTAL SECTION Chemicals. Bromobenzene and BF3·Et2O were obtained from Lingfeng (China). Sarcosine, 9-fluorenone, p-bromophenol, and n-octyl bromide were purchased from Jingchun (China). Thiophene was purchased from Alfa Aesar. n-BuLi was purchased J&K Chemical. 2-Bromothiophene was purchased from Shouerfu (China). C60 was purchased from Yongxin (China). All of the chemicals were used without further purification. 9-(4-(Octyloxy)phenyl)-fluorene-9-ol (1b) and diarylfluorene were prepared according to the previous reports.24 Characterization. 1H NMR in CDCl3 was recorded at 400 MHz using a Varian Mercury 400 plus spectrometer. Absorption spectra were measured with a Shimadzu UV-3150 spectrometer. Cyclic voltammetry (CV) studies were conducted using a CHI600C in a typical three-electrode cell with a glassy carbon electrode as working electrode, a platinum wire counter electrode, a silver/silver nitrate (Ag/Ag+) reference electrode, and the potential of the ferrocene/ferrocenium (Fc/Fc+) utilized as an external standard. The AFM images of P3HT/fulleropyrrolidines blend films were obtained by using a Dimension 3100 (Veeco, CA) in tapping model with a Si tip (resonance frequency: 320 kHz; spring constant: 42 N m−1) at a scanning rate of 1 Hz. Synthesis of N-Methyl-2-(2-(5-(9-phenyl-9-fluorenyl))thiophenyl) fulleropyrrolidines. C60 (1 g 0.0014 mol), 3a (0.49 g 0.0014 mol), and sarcosine (0.25 g 0.0028 mol) were dissolved in toluene (300 mL) and refluxed for 18 h under nitrogen atmosphere. The reaction mixture was evaporated and purified by column chromatography to obtain brown powder (0.28 g, 19.4%). 1H NMR (400 MHz, CDCl3): δ 7.76−7.74 (d, J = 7.6 Hz, 2 H), 7.53−7.52 (d, J = 7.6 Hz, 1 H), 7.40−7.35(m, 3 H), 7.23−7.09 (m, 4 H), 7.03−7.01 (d, J = 6.8 Hz, 1 H), 6.67−6.66 (d, J = 3.6 Hz, 2 H), 5.12 (s, 1 H), 4.95−4.93 (d, J = 9.6 Hz, 1 H), 4.23−4.21 (t, J = 9.6 Hz, 1 H), 2.90 (s, 3 H). Synthesis of N-Methyl-2-(2-(5-(9-(4-octyloxyphenyl)9-fluorenyl))thiophenyl) fulleropyrrolidine. The procedures were similar to OPFTh-C60. C60 (1 g 0.0014 mol), 3b (0.67 g 0.0014 mol), and sarcosine (0.25 g 0.0028 mol) were used, and brown powder was obtained (0.18 g, 10.7%). 1H NMR (400 MHz, CDCl3): δ 7.74−7.72 (d, J = 7.6 Hz, 2 H), 7.52−7.51 (d, J = 7.2 Hz, 1H), 7.38−7.36 (d, J = 7.6 Hz, 2 H), 7.38−7.36 (d, J = 4.0 Hz, 1 H), 7.25−7.21 (t, J = 7.6 Hz, 2 H), 7.10−7.09 (d, J = 3.6 Hz, 1 H), 6.94−6.91 (d, J = 8.8 Hz, 2 H), 6.63−6.62 (t, J = 6.4 Hz, 2 H), 6.60 (s, 1 H), 5.16 (s, 1 H), 4.95−4.93 (d, J = 9.6 Hz, 1 H), 4.23−4.21 (d, J = 9.6 Hz, 1 H), 3.83−3.80 (t, J = 6.4 Hz, 2 H), 2.89 (s, 3 H), 1.70−1.66 (m, 2 H), 1.28−1.22 (m, 10 H), 0.89−0.85 (m, 3 H). Synthesis of N-Methyl-2-(2-thiophenyl) Fulleropyrrolidine. The procedures were similar to PFTh-C60. C60 (1 g 0.0014 mol), thiophene-2-carbaldehyde (0.15 g 0.0014 mol), and sarcosine (0.25 g 0.0028 mol) were used, and brown powder was obtained (0.30, 25.1%). 1H NMR(400 MHz, CDCl3): δ 7.41− 7.40 (d, J = 7.2 Hz, 1 H), 7.19−7.17 (d, = 7.2 Hz, 1 H), 7.07− 7.05 (t, J = 4.4 Hz, 1 H), 5.29 (s, 2 H), 5.01−5.99 (d, J = 9.6 Hz, 1 H), 4.27−4.24 (d, J = 9.6 Hz, 1 H), 2.91 (s, 3 H). Device Fabrication and Characterization. The PSC devices were fabricated onto prepatterned indium−tin oxide (ITO) with a sheet resistance of 10 Ω/square. The substrates were ultrasonic cleaned sequentially with detergent, 2-propanol, acetone, and deionized water, then dried in an oven, and finally treated in an ultraviolet-ozone chamber. P3HT and 8882
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of three fulleropyrrolidine derivatives were confirmed by H NMR spectroscopy. (See the Supporting Information for more details.) These three models exhibit different solubility. PFTh-C60 has a very limited solubility in toluene and o-DCB and a good solubility in CS2 solvent. In contrast, Th-C60 and OPFTh-C60 possess better solubility in common organic solvents such as THF, chloroform, toluene, and o-DCB. Therefore, OPFTh-C60 is more soluble than PFTh-C60, probably owing to the existence of alkyloxy groups. Optical and Electrochemical Properties of Fulleropyrrolidine Derivatives. The UV−vis absorption spectra of fulleropyrrolidine derivatives Th-C60, PFTh-C60, and OPFThC60 in dilute toluene with a concentration of 10−5 mol/L, together with C60 and PFTh, are shown in Figure 1. The absorption spectra of fulleropyrrolidines closely correspond to a superposition of the individual spectra of C60 and PFTh. Two absorption peaks with high absorption coefficient at 270 and 350 nm and a week featuring peak at 420 nm of C60 are observed, which is consistent with the value in the literature.28 With respect to PFTh with two absorption peaks at 283 and 305 nm, a strong absorption peak of OPFTh-C60 at c.a. 280 nm
fulleropyrrolidines (1:1 wt ratio, 20 mg/mL for each) were dissolved in dichlorobenzene (DCB) separately to make a blend solution. The solution was then spin-coated on the ITO/glass substrate with a precoated poly(ethylenedioxythiophene)/polystyrene sulfonate (PEDOT:PSS). The polymer/acceptor blend films were then put into glass Petri dishes while still wet to undergo the solvent annealing process. The thickness of the photoactive layer is 200 nm. Finally, 10-nm-calcium and 100-nm-thick Al as cathode were deposited onto the active polymer layer. The active device area is 0.12 mm2 for all devices. Device characterization was done in air under simulated AM1.5G irradiation (100 mW cm−2) using a xenon-lamp-based solar simulator. The EQE measurements were measured by a Stanford Research Systems model SR830 DSP lock-in amplifier coupled to a Omni-λ 300 monochromator and 150W xenon lamp. The light intensity at each wavelength was calibrated with a standard single-crystal Si photovoltaic cell. All measurements were carried out in the ambient environment without encapsulation.
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RESULTS AND DISCUSSION Synthesis of Fulleropyrrolidine Derivatives. Syntheses of fulleropyrrolidine derivatives, including Th-C60 serving as a reference sample, PFTh-C60 as hindrance-functionalized model, and OPFTh-C60 as alkyloxy-substituted hindrancefunctionalized model, are outlined in Scheme 1. Diarylfluorenes Scheme 1. Synthetic Routes of Fulleropyrrolidine Derivatives of Th-C60, PFTh-C60 and OPFTh-C60a
Figure 1. UV−vis absorption spectra of PFTh, C60, Th-C60, PFThC60, and OPFTh-C60 in diluted toluene solution (10−5 mol/L) and OPFTh-C60 in thin film.
Reaction condition: (i) 2-bromothiophene, BF3·Et2O, 0 °C, 1 h; (ii) n-BuLi, −78 °C, 1 h; anhydrous DMF, −78 °C, 1 h; nitrogen atmosphere, r. t., 18 h, HCl; (iii) C60, sarcosine, ultrasonic, 5 min; nitrogen atmosphere, 115 °C, 18 h.
Figure 2. Cyclic voltammograms of Th-C60, PFTh-C60,and OPFThC60 on glassy carbon electrode in acetonitrile solution with 0.1 M TBAPF6 at the scanning of 0.1 V/s at room temperature (EFc/Fc+ is ∼.003 V for Th-C60, −0.005 V for PFTh-C60 and OPFTh-C60).
have been synthesized via a BF3·Et2O-mediated Friedel−Crafts reaction of 9-(4-(octyloxy)phenyl)-fluorene-9-ol and unsubstituted 9-phenylfluorene-9-ol, respectively, according to our previous work.24 Bromide-substituted diarylfluorenes 2a and 2b were easily converted into CHO-substituted diarylfluorenes under the n-BuLi and anhydrous DMF, followed by 1,3-dipolar cycloaddition (Prato reaction) with C60 and sarcosine to give corresponding diarylfluorene-modified C60 derivatives (PFThC60 and OPFTh-C60) in a yield of 19.4 and 10.7% according to the literature.27 Th-C60 has been also synthesized under the same condition with a yield of 25.1%. Their chemical structures
Table 1. Electrochemical Data of Th-C60, PFTh-C60,and OPFTh-C60a
a
entry
Eox (V)
Ered (V)
HOMO (eV)
LUMO (eV)
Eg (eV)
ref
Th-C60 PFTh-C60 OPFTh-C60 PC61BM
1.27 1.28 1.29 1.22
−1.03 −0.94 −1.08 −0.88
−6.07 −6.09 −6.10 −5.93
−3.77 −3.86 −3.72 −3.91
2.30 2.23 2.38 2.02
this work this work this work 11, 16
a EFc/Fc+ is ∼.003 V for Th-C60 and −0.005 V for PFTh-C60 and OPFTh-C60. (Th-C60 was not measured at same time with OPFThC60 and PFTh-C60, so it has different reference potential EFc/Fc+.)
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Figure 3. AFM topographic and phase images of the thermal annealed films: P3HT/Th-C60 (a,b), P3HT/PFTh-C60 (c,d), and P3HT/OPFThC60 (e,f).
PFTh-C60 and OPFTh-C60 is at the 250−300 regions owing to the lost feature peak of fluorene. The absorbance of PFThC60 and OPFTh-C60 is slightly stronger than that of C60 in the UV−visible region from 250 to 700 nm. Broad absorption spectra of n-type components in the visible region are favorable for the improvement of photovoltaic performance. These
and one shoulder at c.a. 307 nm are ascribed to the bulky PFTh group. As a result, 329 nm and weak absorption peak at c.a. 431 nm originate from the fulleropyrrolidine group. All of the Th-C60, PFTh-C60, and OPFTh-C60 have a spectral profile and exhibit the feature weak peak of fulleropyrrolidine derivatives at 431 nm.29 The only difference of Th-C60 from 8884
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same experimental condition using 1,2-DCB solvent. The morphology of the blend films has been investigated by atomic force microscopy (AFM), as shown in Figure 3. There is obviously different phase separation from phase images of all the blend films. It can be seen that Th-C60 aggregates form heterogeneous films from topology image and the unique separation, different from that of P3HT/PCBM films from phase images. However, PFTh-C60 forms large particles without obvious nanostructures forming, indicating poor miscibility between PFTh-C60 and P3HT. Fortunately, the AFM morphology images of P3HT/OPFTh-C60 blend films exhibit uniform and fine structures with the domain about 25 nm, showing good phase separation and bicontinuous interpenetrating networks, which are similar to P3HT/PCBM systems, probably a benefit for the exciton dissociation, and form excellent gathering channels of charge carriers. The results indicate that the steric hindrance can dramatically change the morphology through the suitable solubility, and modified steric hindrance can optimize the film morphology, resulting in the performance enhancement of PSCs. Hindrance functionalization is a flexible model to understand the relation between molecules and device performances and a new concept to design effective acceptors of PSCs. OPFTh-C60 probably is the promising n-type organic semiconductors for the application of polymer BHJ solar cells. Photovoltaic Properties of Fulleropyrrolidine Derivatives. To investigate the photovoltaic properties, we used Th-C60, PFTh-C60, and OPFTh-C60 as acceptors to fabricate PSCs with commercial P3HT as donor, and the weight ratio was 1:1. (See the Supporting Information for details.) The current density−voltage (J−V) curves of the devices with the configuration of ITO/PEDOT:PSS/photoactive layer (200 nm)/ Ca/Al under the illumination of AM1.5G, 100mW/cm2 are shown in Figure 4a, and the open-circuit voltage (Voc), shortcircuit current (Isc), FF, and PCE of the devices are summarized in Table 2. The electronic energy levels of P3HT, PC61BM, and OPFTh-C60 have been shown in Figure 4a, in which the HOMO and LUMO of P3HT are from the data reported by
results indicate that fulleropyrrolidine derivatives as the acceptor materials in PSCs have better light-harvesting ability than that of C60. Furthermore, the absorption of OPFTh-C60 in thin film state becomes much stronger than that in solution over the range from 350 to 700 nm. The redox properties of fulleropyrrolidines have been studied by cyclic voltammetry (Figure 2). Three reversible reduction waves were observed in the negative region, with only one irreversible oxidation wave in the positive region. Table 1 listed the measured electrochemical data, estimated HOMO, LUMO, and bandgaps of fulleropyrrolidines, together with that of PC61BM according to the literature.11,16,30,31 The HOMO energy levels of Th-C60, PFTh-C60, and OPFTh-C60 are −6.07, −6.09, and −6.10 eV according to the on-set oxidation potential (Eox) of 1.29, 1.28, and 1.27 V, respectively. LUMO energy levels of Th-C60, PFTh-C60, and OPFTh-C60 are −3.77, −3.86, and −3.72 eV according to their first reduction potential (Ered) of −1.03, −0.94, and −1.08 V, respectively. It can be seen that the LUMO energy levels of fulleropyrrolidines are higher than that of PCBM and are comparable with that of Th-C60; the electronic structure of PFTh-C60 and OPFThC60 does not change obviously. Band gap (Eg = LUMO− HOMO) of the fulleropyrrolidines can be estimated to be 2.30 eV for Th-C60, 2.23 eV for PFTh-C60, and 2.38 eV for OPFTh-C60. In general, the open-circuit voltage (Voc) is proportional to the difference between the HOMO energy level of donors and the LUMO energy level of fullerene derivatives, and thus the higher LUMO level of fullerene derivatives will give the higher values of open-circuit voltage (Voc) in the fabricated solar cells. The slight alternation of HOMO and LUMO energy level of
