Cyclometalated Platinum-Containing Diketopyrrolopyrrole Complexes

Sep 11, 2017 - (7, 19) However, the fill factors of the devices reported to date with both cyclometalated platinum and platinum acetylide chromophores...
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Cyclometalated Platinum-Containing Diketopyrrolopyrrole Complexes and Polymers: Photophysics and Photovoltaic Applications Subhadip Goswami,† Jeff L. Hernandez,‡ Melissa K. Gish,§ Jiliang Wang,† Bethy Kim,† Amrit P. Laudari,∥ Suchismita Guha,∥ John M. Papanikolas,§ John R. Reynolds,‡ and Kirk S. Schanze*,†,⊥ †

Department of Chemistry and Center for Macromolecular Science and Engineering, University of Florida, P.O. Box 117200, Gainesville, Florida 32611-7200, United States ⊥ Department of Chemistry, University of Texas at San Antonio, San Antonio, Texas 78023, United States ‡ School of Chemistry and Biochemistry, School of Materials Science and Engineering, Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States § Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599,United States ∥ Department of Physics and Astronomy, University of Missouri, Columbia, Missouri 65211, United States S Supporting Information *

ABSTRACT: A series of organometallic complexes and polymers has been synthesized with an objective of studying their fundamental photophysical properties together with their organic photovoltaic and organic field-effect transistor properties. The metal chromophores consist of a diketopyrrolopyrrole (DPP) core, end functionalized with cyclometalated platinum “auxochrome”. The photophysical properties of the metal complex and polymers are compared with the unmetalated chromophore DPP-C8-Th-Py. The polymers Poly-DPP-Th-Pt and Poly-DPP-Ph-Pt differ structurally in their cyclometallating ligands, where they consist of 2-thienylpyridine and 2-phenylpyridine, respectively. Efficient solar spectrum coverage was observed for all chromophores; specifically, the polymer Poly-DPP-Th-Pt has an onset of absorption at ∼900 nm with an optical band gap of 1.4 eV. The triplet excited state was detected for all chromophores and probed by both nanosecond and picosecond transient absorption spectroscopy. Both polymers were employed as donors in bulk-heterojunction solar cells with a polymer:PC71BM ratio of 1:7. The thiophene-containing polymer Poly-DPP-Th-Pt shows a respectable power conversion efficiency (PCE) of 1.66% with a high fill factor (FF) of ∼66%. Higher charge carrier mobility was observed for Poly-DPP-Th-Pt when used in field-effect transistors compared to Poly-DPP-Ph-Pt.

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INTRODUCTION Organic π-conjugated materials have gained significant attention over the past few years for application in organic photovoltaic devices (OPVs) due to their light weight, cost effectiveness, and ease of solution processability.1−4 Among several types of OPVs, bulk-heterojunction solar cells are promising where an interpenetrating network of donor and acceptor materials is present to minimize the effects of low exciton diffusion length (5−10 nm) and low thickness of the active layer in bilayer devices. Heavy atom-containing organometallic π-conjugated chromophores have recently drawn © 2017 American Chemical Society

attention as donor materials in bulk heterojunction solar cells because of their enhanced generation of triplet excited states.5−9 Heavy metals such as platinum and iridium significantly enhance spin−orbit coupling in π-conjugated electronic systems, giving rise to rapid and efficient singlet− triplet intersystem crossing.5,10−12 There are several fundamental reasons that the triplet excited state might be important Received: July 18, 2017 Revised: September 10, 2017 Published: September 11, 2017 8449

DOI: 10.1021/acs.chemmater.7b03018 Chem. Mater. 2017, 29, 8449−8461

Article

Chemistry of Materials in organic photovoltaic devices. Specifically, it has been shown that the triplet state can improve photocurrent by increasing the exciton diffusion length (due to its longer lifetime) and also by prohibiting charge recombination of the geminate ion− radical pair.5,7,13,14 A number of investigations have explored the role of heavy metals in the mechanism and efficiency of charge generation in organic solar cells. Most of this work has focused on polymers featuring platinum acetylide functionality, e.g., (−ArCC PtL2CC−).8,15,16 Several recent studies explored the role of cyclometalated metal complexes in the optical and excited state properties of π-conjugated polymers. In particular, Fréchet and co-workers incorporated the cyclometalated platinum motif within a thiophene- and fluorene-containing polymer backbone as an alternative to platinum acetylide-containing conjugated materials. Among two synthesized polymers, in organic solar cell application, the thiophene-containing cyclometalated polymer gave a power conversion efficiency (PCE) of 1.3% with PCBM as an acceptor.17 Cheng and co-workers reported a series of polymers containing indacenodithiophene and cyclometalated platinum achieving PCE of 2.9%.18 Cyclometalated iridium chromophores have also been explored as candidates for use in bulk-heterojunction solar cell devices.7,19 However, the fill factors of the devices reported to date with both cyclometalated platinum and platinum acetylide chromophores are low, likely due to weak π−π interaction among the polymer chains. Additionally, the triplet excited state properties of the cyclometalated polymers have not been explored in detail, and therefore, it is not possible to assess whether the triplet state is involved in the mechanism for response of the OPVs. Diketopyrrolopyrrole (DPP)-containing small molecules and polymers are among the versatile class of donor−acceptor chromophores that have been widely used for both bulkheterojunction (BHJ) solar cells and organic field-effect transistors (OFET).20−24 Fréchet and co-workers have shown that by incorporating planar end groups to DPP chromophores it is possible to enhance the OPV performance by promoting enhanced intermolecular π−π interactions.25 Several reports of photovoltaic devices and OFETs take advantage of the selfassembly property of DPP chromophores to improve device performance.26−28 Despite extensive prior research on DPPcontaining polymers and molecules for optoelectronic devices, there is no prior report on organometallic DPP chromophores for the same objective. We have recently shown an enhanced efficiency for intersystem crossing in DPP-containing cyclometalated donor−acceptor chromophores compared to the analogous platinum acetylide systems.29 In another line of study we have shown that these materials exhibit fast photoinduced electron transfer and slow charge recombination to various acceptors, including phenyl-C61-butyric acid methyl ester (PC61BM). This previous insight encouraged us to study this metal complex for photovoltaic applications.30,31 On the basis of the previous reports on DPP as well as cyclometalated platinum-containing polymers, we speculated that by incorporating planar cyclometalated platinum “auxochromes” to a conjugated DPP chromophore the fill factor of the photovoltaic devices can be improved. Unfortunately, devices fabricated using DPP-Pt(acac) (Chart 1) as donor and [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) as an acceptor gave very low efficiency (∼0.1%). Careful inspection of the thin film absorption spectra suggested that this is due to the formation of aggregates in the deposition solution which results in separation of the metal complex during filtration. These aggregates are

Chart 1. Chemical Structures of the Metal Complex and DPP Precursor Studied Previously29

likely formed due to the relatively poor solubility of DPPPt(acac) in the solvent used for film casting. To test the idea if the triplet excited state is responsible for photocurrent generation and the effect of the cyclometalated C∧N ligand on photophysical and bulk-heterojunction solar cell properties, in the present contribution we detail the synthesis and photophysical and optoelectronic device properties of a set of organometallic−DPP polymers and model complex that feature an improved solubilizing acetylacetonate ancillary ligand (octyloxy-phenyl-acac). Two DPP complexes were synthesized, one with ethylhexyl and the other with octyldodecyl side chains on the DPP nitrogens in order to optimize the solubility properties. These complexes are abbreviated DPP-C8-Pt and DPP-C18-Pt, respectively, and their structures are shown in Chart 2. The unmetalated chromophore DPP-C8-Th-Py was synthesized to compare the photophysical properties with the metal complexes. The polymers Poly-DPP-Th-Pt and PolyDPP-Ph-Pt (Chart 2) were synthesized with 2-thienylpyridine and 2-phenylpyridine as cyclometalated ligands, respectively (both of the polymers feature the C18 solubilizing groups). All of the organometallic chromophores exhibit efficient population of triplet excited state via S1 → T1 intersystem crossing (ISC) that is enhanced by the heavy-atom effect. Ultrafast transient absorption spectroscopy suggests an enhanced rate of ISC for Poly-DPP-Th-Pt in comparison to Poly-DPP-Ph-Pt. The thiophene-containing polymer Poly-DPP-Th-Pt gave a power conversion efficiency (PCE) of η = 1.66% with a relatively low weight fraction of polymer in the active layer (polymer:PC71BM ≈ 1:7), whereas the highest PCE observed for Poly-DPP-Ph-Pt was η = 0.52% with a polymer:PC71BM ratio of 1:3. The fill factor (FF) for the Poly-DPP-Th-Pt was ∼66%, which is the highest reported value to date for an organometallic polymer. Both polymers were tested in fieldeffect transistor architectures to evaluate their charge carrier mobility. The thiophene-containing polymer Poly-DPP-Th-Pt featured a higher charge carrier mobility (μh ≈ 10−5 cm2/(V s)) compared to Poly-DPP-Ph-Pt (μh ≈ 10−6 cm2/(V s)) with SiO2 as the dielectric layer, consistent with the photovoltaic results. The charge carrier mobility was enhanced up to 10−3 cm2/(V s) for Poly-DPP-Th-Pt when cross-linked polymer PVP was used as the dielectric layer. The results presented herein offer an important way of tuning the photophysical and OPV/OFET performances by tailoring of cyclometallating ligands in organometallic DPP chromophores.

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EXPERIMENTAL SECTION

Synthesis. Synthetic schemes and details of procedures along with compound and polymer characterization are provided in the Supporting Information. 8450

DOI: 10.1021/acs.chemmater.7b03018 Chem. Mater. 2017, 29, 8449−8461

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Chemistry of Materials Chart 2. Chemical Structures of the Materials Reported Herein

Instrumentation and Methods. NMR spectra were recorded on a Varian Mercury-300 FT-NMR spectrometer, which operated at 300 MHz for 1H NMR and 75.4 MHz for 13C NMR. 1H NMR were also collected on a Varian Inova-500 FT-NMR instrument, operating at 500 MHz. The mass spectral analysis of the newly synthesized molecules and complexes was performed by the mass spectrometry services located in house at the University of Florida. Polymer molecular weights were analyzed by gel permeation chromatography (GPC) on a system consisting of a Shimadzu SPD-20A photodiode array (PDA) detector and THF as eluent with a flow rate of 1 mL· min−1. The system was calibrated using linear polystyrene standards in THF solution. All photophysical studies were performed using dry HPLC-grade THF as solvent in a 1 × 1 cm2 quartz cuvette unless noted otherwise. The solutions used for the study were diluted from a stock solution according to the experiment. UV−vis absorption spectra were collected on a Shimadzu UV-1800 dual-beam spectrophotometer. The calculations of the concentration for polymers were performed by using the polymer repeat unit (PRU) as the molecular weight. Steadystate photoluminescence spectroscopy was obtained on a Photon Technology International QuantaMaster fluorimeter and collected at 90° with respect to the excitation beam. All measurements were performed with a solution OD ≤ 0.1 to avoid chromphore selfquenching and spectral and intensity distortions due to high solution absorption. The fluorescence quantum yields were calculated by taking into account the refractive index correction of THF as well as the standard solvent toluene. Fluorescence lifetime measurements were performed with a PicoQuant FluoTime 100 Compact Fluorescence Lifetime Spectrophotometer by time-correlated single-photon counting (TCSPC) and by exciting the solutions by a PDL-800B Picosecond Pulsed Diode Laser (375 nm). Nanosecond−microsecond transient absorption spectroscopy was performed using the third harmonic of a Continuum Surelite series Nd:YAG laser (λ = 355 nm, 10 ns fwhm, 7 mJ per pulse). Probe light was produced by a xenon flash lamp, and the transient absorption signal was detected with a gated-intensified CCD mounted on a 0.18 m spectrograph (Princeton PiMax/Acton Pro 180). During the measurement the samples were continuously circulated in the pump− probe region and contained in a cuvette of 1 cm path length with a

total volume of 10 mL. The optical density of the samples was maintained at 0.7 at 355 nm, and solutions were deoxygenated by bubbling argon for at least 45 min prior to the measurement. Femtosecond transient absorption data were obtained using a pump−probe configuration with a 1 kHz Ti:sapphire chirped pulse amplifier (Clark-MXR CPA-2001). The 388 nm (600 nJ) pump pulse was generated via frequency doubling a portion of the 775 nm regenerative amplifier beam, while the 665 nm (100 nJ) pump pulse was produced by frequency doubling the signal from a home-built Optical Parametric Amplifier (OPA). A portion of the 775 nm fundamental was focused in a translating CaF2 window to generate a white light continuum, which was used as the probe pulse. A mechanical chopper synchronized to the laser chopped the pump at 500 Hz. Pump and probe polarizations were set to the magic angle. The two beams were focused to a 150 μm spot size and spatially overlapped at the sample. A fiber optic-coupled multichannel spectrometer with a CMOS sensor collects the probe pulse, where pump-induced changes in the white light continuum were measured on a pulse-to-pulse basis with a sensitivity of 0.1 mOD. Time-resolved transient absorption spectra were collected with an approximate time resolution of 250 fs with a spectral window of 320−800 nm. The time delay between pump and probe are regulated using a computercontrolled delay stage. Transient absorption measurements from picosecond to microsecond were collected with the same pump pulse and CMOS sensor as the femtosecond setup, while the white light probe was obtained from continuum generation in a diode-laser-pumped photonic crystal fiber electronically delayed from the pump pulse. Electrochemical measurements were performed on a BAS CV-50W voltammetric analyzer (Bioanalytical Systems, Inc., www.bioanalytical. com) using dry dichloromethane solvent with 0.1 M tetra-nbutylammonium hexafluorophosphate (TBAH) as an electrolyte. The experiments were performed with three electrodes: a platinum microdisk (2 mm2) as the working electrode, a platinum wire as the auxiliary electrode, and a silver wire as the reference electrode. The concentration of the sample solutions were ∼1 mM, and a scan rate of 100 mV/s was used during the measurement. The sample solutions were deoxygenated prior to the measurement and maintained under a constant pressure of argon during the measurement. All electro8451

DOI: 10.1021/acs.chemmater.7b03018 Chem. Mater. 2017, 29, 8449−8461

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Chemistry of Materials

Figure 1. Steady-state absorption spectra of (A) DPP-C8-Th-Py and DPP-C18-Pt, (C) Poly-DPP-Th-Pt and Poly-DPP-Ph-Pt in THF. Steadystate photoluminescence spectra of (B) DPP-C8-Th-Py and DPP-C18-Pt and (D) Poly-DPP-Th-Pt and Poly-DPP-Ph-Pt in THF. chemical potentials were calibrated with respect to a ferrocene internal standard (E(Fc/Fc+) = 0.43 V vs SCE in CH2Cl2). The potentials are reported here vs SCE using the above value for conversion. Atomic force microscopy (AFM) was carried out using a Veeco Innova Microscope in tapping mode with a silicon tip (radius ≈ 8 nm) at 325 kHz with a force constant of approximately 40 N/m (Mikromasch USA). Device Fabrication and Characterization for OPVs. Patterned InSnO2 (ITO) on glass was ultrasonicated in sodium dodecyl sulfate in water followed by acetone and then isopropanol for 15 min in each cleaning solution. ITO was dried with Ar gas and then subjected to UV ozone treatment for 10 min. Poly(ethylenedioxy)thiophene/polystyrenesulfonate (PEDOT:PSS,Al 4083) was spin coated on clean ITO at 5000 rpm for 60 s and then annealed at 120 °C for 15 min. Solutions of Poly-DPP-Th-Pt:[6,6]-phenyl C71 butyric acid methyl ester (PC71BM, purchased from American Dye Source) or Poly-DPP-PhPt:PC71BM, 1:7 by weight, in chloroform, 9 mg mL−1, were spin coated in an Ar-filled glovebox onto PEDOT:PSS substrates at 600 rpm for 50 s. Then 10 nm of Ca followed by 80 nm of Al were evaporated at 10−6 mbar as the top contacts. Devices were tested using a Newport ABB solar simulator with a Keithley SMU for measuring device efficiency under standard AM 1.5 conditions calibrated to 95 mW/cm2. The active area was 0.07 cm2, defined by the overlap between the top and the bottom electrodes. Statistics were obtained from five devices. Device Fabrication and Characterization for FETs. Organic field-effect transistors (FETs) were fabricated using a top-contact bottom gate structure with gold source and drain electrodes (40 nm) thermally evaporated. Heavily p-doped silicon (100) wafers, with resistivity of 0.001−0.005 Ω·cm, were used as the gate electrode with 200 nm bare SiO2 layer as the gate dielectric. The substrates were cleaved into ∼1 in. × 1 in. pieces, ultrasonicated in acetone, rinsed with isopropyl alcohol, and submerged in Piranha solution (7:3 H2SO4/H2O2) at 115 °C for 15 min. OTS (octadecyltrichlorosilane) from Sigma-Aldrich was used to form self-assembled monolayers on the SiO2 surface.

Solutions of Poly-DPP-Th-Pt and Poly-DPP-Ph-Pt were prepared in chloroform (10 mg/mL) and heated at 60 °C for 0.5 h. The solutions were filtered with 0.45 μm PTFE filters. About 0.25 mL of each solution was spun cast onto the substrates at 1500 rpm. The films were annealed at 100 °C for 15 min. Top contacts of 40 nm gold were deposited for the source (S)−drain (D) contacts via thermal evaporation at a base pressure of 10−6 mbar through a shadow mask. The typical S−D channel length and width for the FETs were 50 μm and 1 mm, respectively. Furthermore, a polymer dielectric was also used to improve the dielectric−semiconductor interface. Poly(4vinylphenol) (PVP, 90 nm) cross-linked with 5 wt % PMMF was spin coated on Al-coated glass substrate at 5000 rpm for 60 s. The film was annealed at 180 °C for 15 min. This was followed by spin casting the Poly-DPP-Th-Pt layer, followed by gold electrode deposition. Although Poly-DPP-Ph-Pt FETs were also prepared using crossliked PVP, these devices did not perform most likely due to the film morphology on the dielectric layer. Room-temperature dc electrical characterizations were performed using two source meters, Keithley 2400 and Keithley 236, using a customized LabVIEW program.

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RESULTS AND DISCUSSION UV−vis Absorption Spectroscopy. The ground-state absorption spectra of the ligand, metal complex, and polymers were recorded in THF solution, and they are presented for selected chromophores in Figure 1A and 1C. Due to their similar optical properties, only the spectra of DPP-C18-Pt are shown here (see Supporting Information for the optical characterization of DPP-C8-Pt). The absorption maxima (λmax) and molar extinction coefficient (ε) values are listed in Table 1. In general, the spectra are dominated by an intense absorption at 500−900 nm and a weak to moderate absorption in the near UV region (300−500 nm). According to previous studies on DPP chromophores, the low-nergy transition can be attributed to donor−acceptor charge transfer transition, while 8452

DOI: 10.1021/acs.chemmater.7b03018 Chem. Mater. 2017, 29, 8449−8461

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Chemistry of Materials Table 1. Summary of Photophysical Propertiesa compound

λmax (nm)

ε × 104 (M−1 cm−1)

λem (nm)

ϕFlb

τFl (ns)

τTA (μs)

DPP-C8-Th-Py DPP-C18-Pt Poly-DPP-Th-Pt Poly-DPP-Ph-Pt

639 672 790 702

2.2 4.1 2.5 1.7

676 710 872 850

0.27 0.01