Solution-Processable Crystalline Platinum-Acetylide Oligomers with

Feb 16, 2010 - E-mail: [email protected]. ... Solution-processed bulk heterojuction devices using these oligomers with fullerenes show bicontinuous...
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Chem. Mater. 2010, 22, 2325–2332 2325 DOI:10.1021/cm903329a

Solution-Processable Crystalline Platinum-Acetylide Oligomers with Broadband Absorption for Photovoltaic Cells Xiaoyong Zhao, Claudia Piliego, BongSoo Kim, Daniel A. Poulsen, Biwu Ma, David A. Unruh, and Jean M. J. Frechet* Materials Sciences Division, Lawrence Berkeley National Laboratory and College of Chemistry, University of California, Berkeley, California 94720-1460 Received October 30, 2009. Revised Manuscript Received January 29, 2010

A series of solution-processable and crystalline platinum-acetylide oligomers containing a thienylbenzothiadiazole-thienyl core and oligothiophene alkynyl ligands are synthesized and characterized. X-ray crystallography analysis indicates a two-dimensional arrangement of oligomers through CH-π interactions in single crystals. These oligomers show two intense and broad absorption bands in the visible spectral region, with the short-wavelength absorption band being strongly dependent on the oligothiophene length. In neat films, all the oligomers form large crystalline domains of several hundred nanometers in size upon thermal treatment and exhibit space-charge limited current (SCLC) mobilities on the order of 10-5-10-4 cm2 V-1 s-1. The photovoltaic properties of these oligomers were evaluated by fabricating bulk heterojunction devices with fullerene derivatives (PC61BM and PC71BM) and some of these devices showed high-power conversion efficiencies (PCEs) of up to 3% and a peak external quantum efficiency (EQE) to 50% under AM 1.5 simulated solar illumination. The present work suggests that well-defined platinum oligomers with desirable light-absorbing and self-assembly properties have potential for solution-processed organic photovoltaics. Introduction Progress in organic photovoltaics (OPVs) requires significant advances in the areas of material design and synthesis, film processing, and morphology control, as well as device architecture.1-6 One of the most important breakthroughs in OPVs is the introduction of the bulk heterojunction (BHJ) device structure, in which the photoactive thin film consists of a blend of electron donors and acceptors.7-9 To date, solution-processed BHJ photovoltaic cells have achieved a power conversion efficiency (PCE) of 4%-6%, using various semiconducting polymers as the *Author to whom correspondence should be addressed. E-mail: frechet@ berkeley.edu.

(1) Peet, J.; Senatore, M. L.; Heeger, A. J.; Bazan, G. C. Adv. Mater. 2009, 21, 1521–1527. (2) Dennler, G.; Scharber, M. C.; Brabec, C. J. Adv. Mater. 2009, 21, 1323–1338. (3) Thompson, B. C.; Frechet, J. M. J. Angew. Chem., Int. Ed. 2008, 47, 58–77. (4) Gunes, S.; Neugebauer, H.; Sariciftci, N. S. Chem. Rev. 2007, 107, 1324–1338. (5) Peumans, P.; Yakimov, A.; Forrest, S. R. J. Appl. Phys. 2003, 93, 3693–3723. (6) Chen, L. M.; Hong, Z. R.; Li, G.; Yang, Y. Adv. Mater. 2009, 21, 1434–1449. (7) Yu, G.; Heeger, A. J. J. Appl. Phys. 1995, 78, 4510–4515. (8) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science 1995, 270, 1789–1791. (9) Halls, J. J. M.; Walsh, C. A.; Greenham, N. C.; Marseglia, E. A.; Friend, R. H.; Moratti, S. C.; Holmes, A. B. Nature 1995, 376, 498–500. (10) Ma, W. L.; Yang, C. Y.; Gong, X.; Lee, K.; Heeger, A. J. Adv. Funct. Mater. 2005, 15, 1617–1622. (11) Lee, J. K.; Ma, W. L.; Brabec, C. J.; Yuen, J.; Moon, J. S.; Kim, J. Y.; Lee, K.; Bazan, G. C.; Heeger, A. J. J. Am. Chem. Soc. 2008, 130, 3619. (12) Li, G.; Yao, Y.; Yang, H.; Shrotriya, V.; Yang, G.; Yang, Y. Adv. Funct. Mater. 2007, 17, 1636–1644. r 2010 American Chemical Society

electron donor and fullerene derivatives as the electron acceptor.10-13 In contrast to polymeric systems that intrinsically display large structural variations in molecular weight, polydispersity, and regioregularity, conjugated small molecules and oligomers possess well-defined structures that are relatively easy to modify and purify.14,15 In addition, small molecules and oligomers show a strong tendency to self-assemble into ordered crystalline domains with superior charge transport properties14 and favorable nanoscale phase separation may be achieved via selfassembly of donor and acceptor molecules during solution processing.16 BHJ OPVs based on solution-processable molecular donors, consisting of small molecules, oligomers, or dendrimers with fullerene acceptors, have been described,17-23 (13) Liang, Y.; Feng, D.; Wu, Y.; Tsai, S.-T.; Li, G.; Ray, C.; Yu, L. J. Am. Chem. Soc. 2009, 131, 7792. (14) Lloyd, M. T.; Anthony, J. E.; Malliaras, G. G. Mater. Today 2007, 10, 34–41. (15) Roncali, J. Acc. Chem. Res. 2009, 42 (11), 1719-1730. (16) Schmidt-Mende, L.; Fechtenkotter, A.; Mullen, K.; Moons, E.; Friend, R. H.; MacKenzie, J. D. Science 2001, 293, 1119–1122. (17) Roquet, S.; Cravino, A.; Leriche, P.; Aleveque, O.; Frere, P.; Roncali, J. J. Am. Chem. Soc. 2006, 128, 3459–3466. (18) Lloyd, M. T.; Mayer, A. C.; Subramanian, S.; Mourey, D. A.; Herman, D. J.; Bapat, A. V.; Anthony, J. E.; Malliaras, G. G. J. Am. Chem. Soc. 2007, 129, 9144–9149. (19) Tamayo, A. B.; Walker, B.; Nguyen, T. Q. J. Phys. Chem. C 2008, 112, 11545–11551. (20) Silvestri, F.; Irwin, M. D.; Beverina, L.; Facchetti, A.; Pagani, G. A.; Marks, T. J. J. Am. Chem. Soc. 2008, 130, 17640–17641. (21) He, C.; He, Q. G.; Yi, Y. P.; Wu, G. L.; Bai, F. L.; Shuai, Z. G.; Li, Y. F. J. Mater. Chem. 2008, 18, 4085–4090. (22) Tamayo, A. B.; Dang, X. D.; Walker, B.; Seo, J.; Kent, T.; Nguyen, T. Q. Appl. Phys. Lett. 2009, 94, 103301-103303.

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affording PCEs that have evolved from 90%. Compound 8a: 1H NMR (400 MHz, CD2Cl2, δ): 7.26 (d, J = 4.99 Hz, 1H), 7.29-7.19 (m, 2H), 7.15-7.04 (m, 2H), 3.45 (s, 1H). Compound 8b: 1H NMR (400 MHz, CD2Cl2, δ): 7.28 (d, J = 5.03 Hz, 1H), 7.21 (dd, J = 5.81, 3.92 Hz, 2H), 7.12 (dd, J = 3.75, 3.58 Hz, 2H), 7.06 (dd, J = 4.94, 4.90 Hz, 2H), 3.49 (s, 1H). Compound 8c: 1H NMR (400 MHz, CDCl3, δ): 7.28 (d, J = 5.17 Hz, 1H), 7.23 (d, J = 3.45 Hz, 2H), 7.17-7.10 (m, 4H), 7.07 (dd, J = 6.49, 4.34 Hz, 2H), 3.46 (s, 1H). Compound 5. 4,7-Bis(5-ethynylthiophen-2-yl)benzo[c][1,2,5]thiadiazole (4) (0.4 g, 1.15 mmol) in a mixture of THF/Et2NH (50 mL/50 mL) was degassed for 30 min, then cis-[Pt(PEt3)2Cl2] (1.27 g, 2.53 mmol) was added in one portion. The reaction flask was wrapped in aluminum foil and stirred at room temperature (rt) for 40 h. After evaporation of the solvent, the purple residue was loaded on a silica column and washed first with CH2Cl2/ hexane (1/2), then CH2Cl2/hexane (1/1), and finally CH2Cl2. The purple band was collected affording the title compound as a purple solid (1.1 g, 75%). 1H NMR (500 MHz, CD2Cl2, δ): 7.93-7.86 (d, J = 3.82, 2H), 7.69 (s, 2H), 6.89-6.81 (d, J = 3.81, 2H), 2.02-1.97 (m, 24H), 1.18-1.11 (m, 36H); 13C NMR (125 MHz, CD2Cl2, δ): 152.52, 136.25, 131.11, 128.65, 127.39, 125.20, 124.86, 94.31, 93.07, 14.56, 7.77; 31P NMR (202 MHz, CD2Cl2, δ): 14.17, JPt-P = 2359 Hz. BTD-Pt-T2, BTD-Pt-T3, and BTD-Pt-T4. A mixture of compound 5 and the corresponding oligothiophene alkynyl ligands in THF/Et2NH (2/1, v/v) was degassed for 30 min. A catalytic amount of CuI (10 mol %) was then added and the resulting mixture was stirred at rt for 36 h. After removal of the solvent, the crude product was purified by flash chromatography (silica gel, CH2Cl2/hexane 1:1, 2:1, then CH2Cl2). Further recrystallization of the products by vapor diffusion of hexane into CH2Cl2 solutions yielded crystalline products in good yields (50%81%). BTD-Pt-T2: 1H NMR (500 MHz, CD2Cl2, δ): 7.92 (d, J= 3.85 Hz, 2H), 7.70 (s, 2H), 7.12 (dd, J = 5.10, 0.86 Hz, 2H), 7.05 (d, J = 3.55 Hz, 2H), 6.93 (dd, J = 5.06, 3.63 Hz, 2H), 6.90 (d, J= 3.69 Hz, 2H), 6.87 (d, J= 3.83 Hz, 2H), 6.69 (d, J= 3.68 Hz, 2H), 2.15-2.06 (m, 24H), 1.21-1.13 (m, 36H); 31P NMR (202 MHz, CD2Cl2, δ): 10.62, JPt-P = 2329 Hz; melting point (mp) 234 °C; FAB MS (m/z) [M]þ Calcd for C62H76N2P4Pt2S7, 1587.8; Found 1588.4; Anal. Calcd for C62H76N2P4Pt2S7: C 46.90, H 4.82, N 1.75, S 14.14; Found: C 47.02, H 4.82, N 1.75, S 14.03. BTD-Pt-T3: 1H NMR (400 MHz, CD2Cl2, δ): 7.97 (d, J = 3.75 Hz, 2H), 7.75 (s, 2H), 7.23 (d, J = 4.68 Hz, 2H), 7.17 (d, J = 2.72 Hz, 2H), 7.06 (d, J = 3.69 Hz, 2H), 7.01 (t, J = 4.22 Hz, 4H), 6.96 (d, J = 3.64 Hz, 2H), 6.92 (d, J = 3.73 Hz, 2H), 6.75 (d, J= 3.58 Hz, 2H), 2.82-1.72 (m, 24H), 1.34-1.09 (m, 36H); 31 P NMR (162 MHz, CD2Cl2, δ): 11.94, JPt-P = 2334 Hz; mp 262 °C; FAB MS (m/z) [M]þ Calcd for C62H76N2P4Pt2S7, 1752.0; Found 1752.5; Anal. Calcd for C70H80N2P4Pt2S9: C 47.99, H 4.60, N 1.60, S 16.47; found: C 48.31, H 4.54, N 1.56, S 16.74. BTD-Pt-T4: 1H NMR(500 MHz, CD2Cl2, δ): 7.92 (d, J= 3.74 Hz, 2H), 7.71 (s, 2H), 7.19 (d, J= 5.14 Hz, 2H), 7.14 (d, J= 3.44 Hz, 2H), 7.08-7.00 (m, 6H), 6.97 (t, J= 4.82, 4H), 6.92 (d, J= 3.68 Hz,

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2H), 6.87 (d, J = 3.82 Hz, 2H), 6.70 (d, J = 3.69 Hz, 2H), 2.11 (m, 24H), 1.22-1.11 (m, 36H); 31P NMR (202 MHz, CD2Cl2, δ): 10.65, JPt-P = 2336 Hz; mp 261 °C; FAB MS (m/z) [MþH]þ Calcd for C62H76N2P4Pt2S7, 1916.3; Found 1917.3; Anal. Calcd for C78H80N2P4Pt2S11: C 48.89, H 4.42, N 1.46, S 18.41; Found: C 48.77, H 4.23, N 1.40, S 18.55. X-ray Crystallography. Single crystals of BTD-Pt-T2 were grown via the slow diffusion of hexane into dichloromethane solutions. All measurements were made on a Bruker Model SMART 1000 CCD area detector with graphite monochromated Mo K (λ = 0.71073 A˚) radiation. The structure was solved by direct methods (SHELXS-97). The crystal structure of BTD-Pt-T2 consists of two molecules of the compound packed in a triclinic unit cell with no symmetry relating them. The unitcell parameters are given as follows: triclinic, space group P1, a = 14.106 (1) A˚, b = 15.157 (1) A˚, c = 15.541 (2) A˚, R = 97.079(1)°, β = 96.718(1)°, γ = 90.344(1)°, V = 3274.2(5) A˚3, Z = 2. Device Fabrication. All devices were fabricated on indium tin oxide (ITO)-coated glass substrates (prepatterned, resistivity of R = 20 Ω-1, Thin Film Devices, Inc.). A thin layer (30-40 nm) of PEDOT:PSS (Clevios PH) was spin-coated onto the ITO glass at 4000 rpm for 40 s and then baked at 140 °C for 15 min in air. The photoactive layer containing the oligomer and fullerene in different ratios was spin-cast from chloroform (10 mg/mL) after passing through a 0.45 μm polytetrafluoroethylene filter. The thickness for all devices, measured with a Dektak profilometer, was ∼70 nm. An Al cathode (100 nm) was then thermally evaporated under vacuum (∼10-7 torr) through a shadow mask defining an active device area of ∼0.03 cm2. The current-voltage (J-V) curves were measured using a Keithley Model 236 source-measure unit under AM 1.5 G solar illumination at 100 mW cm-2 (1 sun), using a Thermal-Oriel 300 W solar simulator. A set of 16 devices were tested for each experimental condition (concentration, blend ratio, annealing temperature, and time), and experiments were repeated multiple times, to check the reproducibility of the data. External quantum efficiency (EQE) values were obtained with a monochromator and calibrated with a silicon photodiode. The hole mobility was measured using a SCLC model,42 and the device structure was ITO/PEDOT:PSS/Ptoligomer/Au. The zero-field charge mobility was obtained by fitting the experimental data using the method reported in ref 42 and the values reported are the average of 5-8 devices. Instrumentation. 1H, 13C, and 31P nuclear magnetic resonance (NMR) spectra were recorded in deuterated solvents such as CDCl3 or CD2Cl2, using Bruker Model AVB 400 and Bruker Model DRX 500 spectrometers. UV-vis absorption spectra were recorded at room temperature using a Carey Model 50 Conc UV-visible spectrophotometer. Fluorescence spectra were recorded on a SPEX Model Fluorolog II spectrometer. CV experiments were collected using a Solartron Model 1285 potentiostat under the control of CorrWare II software. A conventional three-electrode cell with gold working electrodes 2 mm in diameter, a silver wire pseudo-reference electrode (calibrated vs Fc/Fcþ), and a platinum wire counterelectrode was purged with nitrogen and maintained under a nitrogen atmosphere during all measurements. CH2Cl2 was distilled over CaH2 prior to use and tetrabutyl ammonium tetrafluoroborate (TBABF4) (0.1 M) was used as the supporting electrolyte. Tapping-mode atomic force microscopy (AFM) was performed on a Veeco Nanoscope V scanning probe microscope. (42) Mihailetchi, V. D.; Xie, H. X.; de Boer, B.; Koster, L. J. A.; Blom, P. W. M. Adv. Funct. Mater. 2006, 16, 699–708.

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Scheme 1. Synthesis of the Platinum-Acetylide Oligomers

Results and Discussion Synthesis and X-ray Crystallography. These symmetrical dinuclear platinum(II) oligomers were synthesized in a convergent way, as outlined in Scheme 1. The core unit (3) based on thienyl-benzothiadiazole-thienyl (BTD) was synthesized following literature procedures.36 Because of the electron-donating property of thiophene and the strong electron-accepting character of benzothiadiazole, this tricyclic push-pull conjugated system shows a large charge transfer from the thiophene rings to the central unit.43 After cleavage of the trimethylsilyl-protecting groups using K2CO3, compound 4 was then reacted with cis-[Pt(PEt3)2Cl2] in a mixture of THF/Et2NH at room temperature for 12 h to afford the chloroplatinum(II) complex (5) in 75% yield. The desired oligomers, BTD-PtT(n), were then obtained in good yields by coupling 5 with different oligothiophene-based alkynyl ligands (8a, 8b, and 8c) in the presence of catalytic amount of CuI. These compounds were characterized by 1H, 31P NMR spectroscopy, fast-atom bombardment-mass spectroscopy (FABMS), and elemental analysis. The 31P NMR spectra of compounds 5 and BTD-Pt-T(n) all show a single signal, which confirms the highly symmetrical structure of these oligomers. The observed platinum satellites with JPt-P ≈ 2300 Hz in the 31P NMR spectra of these compounds are characteristic of a trans-P-Pt-P configuration. To further characterize the structure of these platinum-acetylide oligomers, we obtained the single-crystal (43) Karikomi, M.; Kitamura, C.; Tanaka, S.; Yamashita, Y. J. Am. Chem. Soc. 1995, 117, 6791–6792.

structure of oligomer BTD-Pt-T2. The oligomer crystallizes in a triclinic crystal system, and there are two molecules within the unit cell with no symmetry between them. Figure 1a shows the perspective drawing of BTDPt-T2. The two Pt atoms have an approximately square planar geometry, as typically observed for platinumacetylide oligomers.44 The CC bond lengths are in the range of 1.151(12)-1.243(10) A˚, and the CC bond between the central unit and the Pt (1.205(10)-1.243(10) A˚) is slightly longer than that between the thiophene arms and the Pt center (1.151(12)-1.179(10) A˚), suggesting a more pronounced electronic interaction between the Pt(II) and the central unit. The close S 3 3 3 N contacts (S(1)-N(1) =3.054 A˚, S(3)-N(2) =2.963 A˚), which are significantly shorter than the van der Waals radii of the two interacting atoms, force the central unit to be almost planar, leading to enhanced absorption. The torsion angles between the central benzothiadiazole unit and the two neighboring thiophene rings are -19.3(11)° (S(1)-C(6)-C(7)-C(12)) and -9.0(11)° (S(3)-C(13)C(10)-C(11)). Although the steric requirement of the trialkyl ligand on the metal center makes it difficult to achieve a close packing of the platinum-acetylide oligomers, examination of the crystal packing of oligomer BTDPt-T2 reveals edge-to-face intermolecular interactions, as shown in Figure 1b. The distance is in the range of 2.890-3.287 A˚, which is typical of CH-π interactions.45 In analogy to many oligothiophenes for which edge-to-face (44) Battocchio, C.; D’Acapito, F.; Fratoddi, I.; Groia, A.; Polzonetti, G.; Roviello, G.; Russo, M. V. Chem. Phys. 2006, 328, 269–274. (45) Nishio, M. CrystEngComm 2004, 6, 130–158.

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Figure 1. (a) Oak Ridge Thermal Ellipsoidal plot (ORTEP) diagram of BTD-Pt-T2, with thermal ellipsoids drawn at 50% probability level. One of the two molecules in the unit cell is shown, and H atoms have been omitted for clarity. (b) Intermolecular interactions in the crystals. (c) View down the long axis of the linear molecule.

interactions are favored,46 oligomer BTD-Pt-T2 packs in a herringbone geometry, as shown in the packing diagram in Figure 1c. This particular two-dimensional arrangement of BTD-Pt-T2, likely similar for the other two oligomers, could be potentially beneficial for photovoltaic applications compared to strong π-π interactions, which usually favor onedimensional in-plane charge transport.47 Optical and Electrochemical Properties. Figure 2 shows the absorption spectra for the thin films of BTD-Pt-T(n) spin-coated on quartz substrates from chloroform solutions (10 mg/mL) and then annealed at 70 °C for 30 min. All oligomers exhibit a broad and structureless absorption band centered at 570 nm with a molar absorbance of ca.104 cm-1, characteristic of the intramolecular chargetransfer (ICT) transition. The absorption band at short wavelength corresponding to a πfπ* transition shows the expected dependence on the length of the oligothiophene “arms”. Increasing the oligothiophene length (2T, 3T to 4T) leads to a significant red-shift of the short-wavelength absorption from 385 nm for BTD-PtT2, to 415 nm for BTD-Pt-T3, and 430 nm for BTD-PtT4, while leaving the long-wavelength absorption peak unchanged. A similar trend was observed in the absorption spectra for the series of oligomers in solution and in preannealed thin films. Comparing the solution absorption spectra (see Figure S1 in the Supporting Information), there is a pronounced red-shift of ∼30 nm for the long-wavelength absorption on thin films of all the oligomers, (46) Murphy, A. R.; Frechet, J. M. J. Chem. Rev. 2007, 107, 1066–1096. (47) Sirringhaus, H. Adv. Mater. 2005, 17, 2411–2425.

Figure 2. Absorption spectra of BTD-Pt-T(n) thin films after annealing at 70 °C for 30 min.

indicating strong intermolecular interactions. A gentle thermal treatment led to a slight enhancement of longwavelength absorbance and the evolution of a weak shoulder in the absorption band. To understand the effect of oligothiophene length on the optical properties, we performed time-dependent density function theory (TDDFT) calculations for BTD-Pt-T2. Based on the calculation results, the origins of the two distinct absorption bands at long wavelengths and short wavelengths were assigned to the HOMO f LUMO and HOMO f LUMO þ 1 transitions, respectively. As illustrated from the contour surface of molecular orbitals (see Figure S2 in the Supporting Information), the HOMO of oligomer BTD-PtT2 is mainly delocalized over the central core with some contribution from the d orbitals of Pt and the LUMO is highly localized on the benzothiadiazole unit. In contrast, the LUMO þ 1 orbital is delocalized over the

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Table 1. Summary of Physical Properties of BTD-Pt-T(n) a

Eopt (eV)

Eox onset

b

(V)

red Eonset (V)b

HOMO (eV)c

LUMO (eV)d

Eg (eV)e

μhole (cm2 V-1 s-1) f

BTD-Pt-T2 BTD-Pt-T2

1.9 1.9

0.19 0.66

-1.75 -1.75

-5.29 -5.29

-3.35 -3.35

1.94 1.94

2.7  10-4 1.3  10-4 g

BTD-Pt-T3 BTD-Pt-T3

1.9 1.9

0.18 0.51

-1.72 -1.72

-5.28 -5.28

-3.38 -3.38

1.9 1.9

2.5  10-4 1.5  10-4 g

BTD-Pt-T4 BTD-Pt-T4

1.9 1.9

0.19 0.48

-1.72 -1.72

-5.29 -5.29

-3.38 -3.38

1.91 1.91

9  10-5 4.3  10-5 g

a Estimated from the absorption edge of BTD-Pt-T(n) thin films. b Determined by cyclic voltammetry and calculated with reference to ferrocene ox red þ 5.1) eV. d EHOMO = -(Eonset þ 5.1) eV. e Eg = -(EHOMO - ELUMO). f Zero-field charge mobility calculated (5.1 eV vs vacuum). c EHOMO = -(Eonset according to the SCLC model.42 g Annealed at 70 °C for 30 min.

oligothiophene alkynyl ligands with only minor contributions from the central unit, which explains the dependence of the short-wavelength absorption band on the oligothiophene length. In addition to the red-shift of the high-energy absorption band, increasing the oligothiophene length also leads to a continuous increment of absorbance for the longer-wavelength absorption, which is likely due to the addition of red-edge absorption of the short wavelength transition to the ICT transition. Both absorption spectroscopy and TD-DFT calculation have suggested that, in the current system, increasing the length of oligothiophene does not shift the long-wavelength ICT transition, but instead leads to considerable broadening of the absorption by reducing the energy gap for the HOMO f LUMOþ1 transition. Because the optical bandgap (Eopt) of conjugated systems is usually determined from the onset of the long-wavelength absorption, all oligomers exhibit a band gap of 1.9 eV, based on their film absorption. To further study the effect of “arm” length on the electrochemical properties of these oligomers, cyclic voltammetry (CV) was performed in anhydrous CH2Cl2 solution with 0.1 M TBABF4 as the supporting electrolyte. The potentials are reported relative to an internal Fcþ/Fc standard and summarized in Table 1. The cyclic voltammograms of BTD-Pt-T(n) are shown in Figure S3 in the Supporting Information. All oligomers exhibited a reversible cathodic wave with an onset at approximately -1.70 V, corresponding to one electron reduction of the core unit, which is consistent with the data reported for oligomeric and polymeric systems containing a similar core unit.48 In contrast, all oligomers showed a similar irreversible anodic wave with an onset at ∼0.20 V, corresponding to the oxidation of the core unit and a second anodic wave with the onset decreasing with the oligothiophene length. Subsequent scans induced a gradual increase in current response for the second oxidation (see Figure S3 (insets) in the Supporting Information) and the formation of a dark film on the gold electrode for all oligomers resulting from electropolymerization of the platinum-acetylide oligomers at higher voltage. These results further confirm that varying the length of oligothiophene “arms” does not affect the HOMO and LUMO levels, which are determined from the first reduction and (48) Kitamura, C.; Tanaka, S.; Yamashita, Y. Chem. Mater. 1996, 8, 570–578.

Figure 3. Tapping-mode AFM images of thin films before annealing ((a) BTD-Pt-T2, rms = 0.4 nm; (c) BTD-Pt-T3; rms = 0.5 nm; (e) BTD-PtT4, rms = 1.4 nm) and after annealing ((b) BTD-Pt-T2, rms = 7.6 nm; (d) BTD-Pt-T3, rms = 5.6 nm; (f) BTD-Pt-T4, rms = 14.3 nm). Image size is 2 μm  2 μm.

oxidation to be -5.3 eV and -3.4 eV, respectively, for all oligomers (see Table 1). Self-Assembly and Charge Mobility. One important feature of these platinum-acetylide oligomers is the incorporation of alkynyl ligands based on oligothiophenes, which have been well-known for their self-assembling properties.49 Because the self-assembly of electroactive molecules has a strong effect on their optical and electronic properties,49 we have examined the morphology of thin films cast from all oligomers using atomic force microscopy (AFM). Figure 3 shows the AFM topographic images of thin films of BTD-Pt-T2 with and without thermal treatment. The surface of the pristine films of BTD-Pt-T2 is quite smooth, showing an (49) Mishra, A.; Ma, C. Q.; Bauerle, P. Chem. Rev. 2009, 109, 1141– 1276.

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Table 2. Summary of Device Performance for BTD-Pt-T(n) active layer (wt:wt = 1:4)

Voc (V)

short-circuit current density, Jsc (mA cm-2)

fill factor, FF

photoconversion efficiency, PCE (%)

BTD-Pt-T2:PC61BM BTD-Pt-T2:PC71BM BTD-Pt-T3:PC61BM BTD-Pt-T3:PC71BM BTD-Pt-T4:PC61BM BTD-Pt-T4:PC71BM

0.73 0.71 0.78 0.82 0.72 0.73

-4.3 -7.91 -5.4 -8.45 -5.3 -7.66

0.39 0.42 0.39 0.43 0.39 0.40

1.4 2.3 1.7 3.0 1.5 2.2

average root-mean-square (rms) roughness of 0.4 nm, 0.5 nm for BTD-Pt-T3, and 1.4 nm for BTD-Pt-T4. Large crystalline domains of BTD-Pt-T2, on the order of ∼200 nm in size, were formed upon annealing at 70 °C, presumably through CH-π interactions, as observed in the single-crystal structure. Accordingly, the rms roughness of the films increased significantly from 0.4 nm to 7.6 nm. Similarly, particulate-shaped crystalline domains formed for other complexes upon annealing, though with smaller size, ∼70 nm for BTD-Pt-T3 and ∼130 nm for BTD-PtT4. Compared with thin films without thermal treatment, a considerable increase of the rms roughness after annealing was also observed for both BTD-Pt-T3 and BTD-PtT4. The crystalline order in the neat films was also probed by two-dimensional grazing-incidence X-ray scattering (2D-GIXS) measurements (see Figure S4 in the Supporting Information). In agreement with AFM studies, the pristine films of all the oligomers showed no observable feature in the GIXS patterns, hence supporting the amorphous nature of these films. Upon thermal treatment at 70 °C for 30 min, concentric Debye rings with crescent peaks in the z-direction appeared in the GIXS patterns, suggesting that thermal annealing induced the formation of highly ordered crystalline domains in the films. In addition, the slightly stronger intensity of the peaks in the z-direction indicates that there is some degree of long-range ordering, with respect to the substrate in the annealed films. Because charge mobility is strongly dependent on the molecular packing and film morphology, we have measured the hole mobilities of these platinum-acetylide oligomers using the hole-only devices of the following structure: ITO/PEDOT:PSS/BTD-Pt-T(n)/Au. The mobilities calculated using the SCLC model42 are summarized in Table 1. These values are not affected significantly by the oligothiophene length, likely because the holes are preferably localized on the core unit, as suggested by CV experiments. Hole mobility values on the order of 10-5-10-4 cm2 V-1 s-1 were obtained for preannealed films of BTD-Pt-T(n), which are higher than those of corresponding platinum-containing polymeric systems.36 The general trend of decreased hole mobility upon annealing observed for all oligomers suggests that grain boundaries are detrimental to hole transport, regardless of molecular packing within the crystalline domains. Photovoltaic Devices. The broad light absorption, selfassembly into ordered domains, and good hole mobility of these platinum-acetylide oligomers support their use as electron donors in OPVs. Bulk heterojunction (BHJ) devices were fabricated with PC61BM or PC71BM as the acceptor in a simple device structure of ITO/PEDOT:PSS (30 nm)/blend (∼70 nm)/Al. Since the blend ratio is key to

device performance, we have carefully examined the effect of blend ratio of BTD-Pt-T3 with PC61BM/ PC71BM in a very broad range from 2:1 (wt:wt) to 1:9 (wt:wt). The best performance was obtained for devices with a 1:4 weight ratio of BTD-Pt-T3:PCBM. We also investigated different thermal annealing conditions10 to optimize the device performance, and the optimal results were obtained by performing the thermal treatment at 70 °C for 30 min after electrode evaporation. All oligomers were evaluated under standard AM 1.5G solar illumination in devices with a 1:4 weight ratio, using the optimized annealing conditions. Table 2 summarizes the various device characteristics (short-circuit current density (Jsc), open circuit voltage Voc, fill factor (FF), and photoconversion efficiency (PCE)). These photovoltaic (PV) cells containing different platinum-acetylide oligomers gave a similar Voc of 0.71-0.82 V, because of the fact that the HOMO remains on the core unit, regardless of the oligothiophene length. Among these new compounds, BTD-Pt-T3 showed the best device performance. Figure 4a shows the current density versus voltage (J-V) characteristics of devices of BTD-Pt-T3/PC61BM and BTDPt-T3/PC71BM. Short-circuit current densities of 5.4 mA cm-2 and 8.45 mA cm-2 have been achieved for BTD-PtT3/PC61BM and BTD-Pt-T3/PC71BM, respectively. Combined with high fill factors (FF = 0.39 for PC61BM device and 0.43 for PC71BM device), these devices gave power conversion efficiencies of 1.7% with PC61BM and 3.0% with PC71BM. Figure 4b shows the external quantum efficiencies (EQEs) for these devices across the absorption range of the photoactive materials. High EQEs of 40% and 50% at 440 nm have been achieved for the PC61BM and PC71BM devices, respectively. Importantly, for the device with PC61BM, which has a very low molar absorbance and, therefore, does not contribute to the light absorption in the range of 350-700 nm, the EQE spectrum correlates very well with the absorption spectrum of BTD-Pt-T3 film. This indicates that the oligothiophene “arms” also contribute largely to the photocurrent, despite the fact that the HOMO and LUMO orbitals are localized on the core unit, as shown by TDDFT calculations and CV experiments. Compared to the PC61BM device, the improved performance for PC71BM device is mainly due to the large increment of the shortcircuit current, which could be attributed to the better absorption properties of PC71BM in the visible range, as evident from the EQE spectrum.50 Although BTD-Pt-T4 (50) Wienk, M. M.; Kroon, J. M.; Verhees, W. J. H.; Knol, J.; Hummelen, J. C.; van Hal, P. A.; Janssen, R. A. J. Angew. Chem., Int. Ed. 2003, 42, 3371–3375.

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Figure 4. (a) Current density-voltage (J-V) of PV devices of BTD-PtT3 under AM 1.5 G, 100 mW cm-2. (b) EQE spectra.

exhibits the broadest absorption and highest molar absorbance, its poor solubility in chloroform has limited the photovoltaic performance. We have also investigated the surface morphology of devices processed from 1:4 blend of different oligomers with PC61BM. As shown in the AFM topographical images (see Figures S5a-S5c in the Supporting Information), despite the tendency of these oligomers to form crystalline domains, the surface of the blends remains smooth, with an average rms roughness of ∼0.3 nm. In the phase images (see Figures S5d-S5f in the Supporting Information), two distinct domains (∼20 nm) with fiberlike structures are observed, likely corresponding to oligomers and PC61BM. It is believed that these bicontinuous domains provide percolated pathways for holes and electrons to be efficiently transported through the blend and subsequently collected at the opposite electrodes, thus ensuring the large photocurrent and high overall PCE efficiency. Conclusions In summary, we have designed and synthesized a series of crystalline platinum-acetylide oligomers containing a

Zhao et al.

thienyl-benzothiadiazole-thienyl core and oligothiophene alkynyl ligands. The absorption spectra of these oligomers are significantly broadened by tuning the length of the oligothiophene alkynyl ligands, thus enhancing the overlap of the absorption of these materials with the solar spectrum. In addition to the light-absorbing properties, incorporation of the oligothiophene “arms” enables the solid state intermolecular packing of these platinum-acetylide oligomers via edge-to-face interactions. The use of these molecules as electron donor was demonstrated in bulk heterojunction (BHJ) devices with either PC61BM or PC71BM as electron acceptors in a simple ITO/PEDOT:PSS/blend/Al device structure. Solution-processed photovoltaic cells show bicontinuous morphology, which is optimal for charge separation and high power conversion efficiencies (PCEs). Some of these devices show high PCEs of up to 3% and a peak external quantum efficiency to 50% under AM 1.5 simulated solar illumination. Our work suggests that solution-processable platinum-acetylide oligomers with broad lightabsorbing and self-assembly properties can lead to highly efficient organic photovoltaic (OPV) devices. It is expected that further improvement in device efficiency can be achieved by fine-tuning the optical absorption, redox, and solid-state packing properties of this class of materials. Acknowledgment. This work was supported by the Director, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division, of the U.S. Department of Energy (under Contract No. DE-AC0205CH11231). GIXS measurements were performed at the Stanford Synchrotron Radiation Laboratory, a national user facility operated by Stanford University on behalf of the U.S. Department of Energy, Office of Basic Energy Sciences. We also acknowledge the use of the Molecular Graphics and Computing Facility at UC, Berkeley which was supported by a National Science Foundation Grant (No. CHE-0233882). Supporting Information Available: X-ray crystallography file (CIF), and figures showing absorption spectra of all oligomers in solution and thin films, time-dependent density function theory (TD-DFT) calculation of BTD-Pt-T2, cyclic voltammetry (CV) of all the oligomers, two-dimensional grazing-incidence X-ray scattering (2D-GIXS) patterns of neat films, and AFM images of platinum-oligomers/PC61BM blends (PDF). This material is available free of charge via the Internet at http:// pubs.acs.org.