The Effect of Ketone Defects on the Charge Transport and Charge Recombination in Polyfluorenes (original) (raw)
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Charge-carrier trapping in polyfluorene-type conjugated polymers
Journal of Applied Physics, 2005
The trap spectrum of a fluorene-based conjugated polymer poly͓9,9-bis͑2-ethylhexyl͒ fluorene-2,7-diyl͔ ͑PF2/6͒ and poly͓9,9-bis͑2-ethylhexyl͒fluorene-2,7-diyl͔ end capped with hole-transporting moieties N, N-bis͑4-methylphenyl͒-N-phenylamine ͑PF2/6am10͒ is investigated by means of thermally stimulated current ͑TSC͒ and thermally stimulated luminescence ͑TSL͒ techniques. A high-temperature TSC peak observed at 240 K in PF2/6 is strongly affected by photooxidation and could be identified as electron trap with a depth of about 0.6 eV. It is ascribed to on-chain keto defects in the polymer. In contrast, end capping of PF2/6 led to the appearance of a moderately deep trap for holes ͑E a = 0.24 eV͒ responsible for a TSC peak at 120 K. On the other hand, TSC data of this polymer reveal no keto-related traps for electrons implying much higher stability of PF2/6am10 against oxidation. Besides the deep electron trap in PF2/6, a very low-temperature TSC peak at about 60 K appears in PF2/6 samples and correlates perfectly with thermally stimulated luminescence data. It is ascribed to shallow hole trapping most probably related to the tail states of the intrinsic density-of-state distribution. In general, it was found that the TSC in PF2/6 samples is detectable only when a sufficiently high load voltage is applied during optical trap filling. The TSC spectra exhibit a striking difference with the polarity of load voltage for conjugated polymers, implying an important role of electrodes on the charge-carrier photogeneration in these polymers.
Polyfluorene as a model system for space-charge-limited conduction
Physical Review B, 2007
Ethyl-hexyl substituted polyfluorene (PF) with its high level of molecular disorder can be described very well by one-carrier space-charge-limited conduction for a discrete set of trap levels with energy ∼ 0.5 eV above the valence band edge. Sweeping the bias above the trap-filling limit in the as-is polymer generates a new set of exponential traps, which is clearly seen in the density of states calculations. The trapped charges in the new set of traps have very long lifetimes and can be detrapped by photoexcitation. Thermal cycling the PF film to a crystalline phase prevents creation of additional traps at higher voltages. PACS numbers: 72.80.Le,73.61.Ph,85.60.Jb Polyfluorenes (PF) have emerged as an especially attractive conjugated polymer (CP) due to their strong blue emission and excellent electronic properties, and thus great prospects for device applications. 1,2 Such applications rely on charge carrier injection and transport. 3 Recently it was shown the parallel electron mobilities in field-effect transistors based on PF copolymers are one of the highest: 10 −3 -10 −2 cm 2 /Vs. 4 Charge carrier transport in CPs mainly occurs by variable-range hopping, 5 where polarons are the actual carriers, 6 and is strongly affected by the presence of traps at the metal-organic interface and in the bulk. Such trap states (shallow or deep) are favorable energy states; below the conduction band edge these states can capture an electron, and above the valence band edge can capture a hole. Impurities and structural defects typically result in discrete trap states within the energy gap. 7 Thermally stimulated currents, 8 photoinduced absorption, 9 impedance spectroscopy, 10 and current-voltage (I-V ) characteristics in the space-charge-limited current (SCLC) regime 11 are some of the experimental methods for the detection of trap states. The latter yields information on the energy and density of traps as well as charge carrier mobilities. A recent work on polyacenes shows that a percolative fluctuation process gives a better insight into the electronic conditions determining the crossover from Ohmic to SCLC regime. 12 SCLC-based models have been used to study trap states in organic molecules and polymers over the last ∼40 years. Trapping of carriers in multi trap levels have been observed in thiophene-based films. 13 Shallow trapping with a single trap level in naphthalene and anthracene 14 was clearly observed in the I-V characteristics, which is distinguished by four different regions. These include (a) the Ohmic region supported by thermal carrier generation, (b) Child's Law in the presence of shallow trapping, (c) trap-filled limit, and (d)
Current injection and transport in polyfluorene
Applied Physics Letters, 2007
A comprehensive numerical model is established for the electrical processes in a sandwich organic semiconductor device with high carrier injection barrier. The charge injection at the anode interface with 0.8eV energy barrier is dominated by the hopping among the gap states of the semiconductor caused by disorders. The Ohmic behavior at low voltage is demonstrated to be not due to the background doping but the filaments formed by conductive clusters. In bipolar devices with low work function cathode it is shown that near the anode the electron traps significantly enhance hole injection through Fowler-Nordheim tunneling, resulting in rapid increases of the hole carrier and current in comparison with the hole-only devices.
Excitation energy transfer from polyfluorene to fluorenone defects
Synthetic Metals, 2004
Poly(9,9-dioctylfluorene-co-fluorenone) with 1% fluorenone, (PFO-F(1%)), was synthesized as a model compound to investigate the optical and electrical effects of fluorenone defects in poly(9,9-dioctylfluorenyl-2,7-diyl), PFO. Photoluminescence (PL) studies indicate efficient Förster energy transfer from PFO to fluorenone "impurities" which are responsible for the pronounced green emission from PFO-F(1%). In electroluminescence (EL), the more pronounced green emission from PFO-F(1%) results from a combination of Förster energy transfer, charge carrier trapping, and relatively easy injection (from the electrodes) of carriers into the fluorenone traps.
Journal of Applied Physics, 2010
The trap parameters in hole-only diodes using a blue emitting polyspirobifluorene copolymer were explored by using two complementary techniques such as: charge based deep level transient spectroscopy and thermally stimulated current. The trap analyses have been further completed by the measurements of the current-voltage characteristics as a function of the temperature. The use of unipolar structures allowed us to determine hole traps in the devices and further to identify electron traps in bipolar structures. Two hole traps which are located at 0.19 and 0.37 eV from the highest occupied molecular orbital, have been identified in blue polyspirobifluorene based devices. In bipolar diodes, four electron traps are determined and are located at 0.32, 0.58, 0.74, and 0.83 eV from the lowest unoccupied molecular orbital. We also showed that the use of a palladium electrode introduced new trap states of mean activation energy of 0.4 eV. As the nature of the cathode changes from Ba/Al to Pd, these traps are inferred to be related to the interface of polymer/Pd contact.
The role of keto defect sites for the emission properties of polyfluorene-type materials
Synthetic Metals, 2003
In this paper we report on the identification of a dominant degradation mechanism of blue light emitting polyfluorene (PFs) type polymers, which turns the desired blue emission color of these polymers into an undesired blue-green emission. As shown the low energy emission band at 2.2-2.3 eV can be identified as the emission from exciton and/or charge trapping keto defect sites (9-fluorenone sites), which in fact can be regarded as emission of a fluorescing guest effect incorporated into the -conjugated PF backbone. We present spectroscopic evidence for the formation of keto defect sites in 9-monoalkylated polyfluorenes (PF-a) which are identified as the source of the strong low energy emission band. Furthermore, we show that the keto defects can be generated directly during polymer synthesis as shown for the PF-a or can be formed as main channel of an (photo-, thermal-or electro-)oxidative degradation process as shown for the 9-bisalkylated PF.
Donor and Acceptor Behavior in a Polyfluorene for Photovoltaics
Journal of Physical Chemistry C, 2007
We investigate photovoltaic devices based on a red-absorbing conjugated polymer poly(2,7-(9,9-dioctylfluorene)-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)) (APFO-3). We show that the polymer acts as an electron donor when blended with ZnO nanoparticles, giving a short-circuit quantum efficiency of 28%. When blended with poly(3-hexylthiophene) (P3HT), however, the APFO-3 acts as an electron acceptor, giving a short-circuit quantum efficiency of 12%. We study this charge-transfer process by comparing photoinduced absorption spectra of the hybrid blends with the absorption spectra of chemically doped APFO-3, which allows us to distinguish features due to positive and negative polarons. We also present dark current measurements of single-carrier devices which demonstrate that APFO-3 has similar mobilities for electrons and holes, consistent with ambipolar behavior in photovoltaic devices.
Investigation of hole-mobility in a polyfluorene copolymer by admittance spectroscopy
Applied Physics Letters, 2010
Transport of holes in a low band gap polyfluorene, APFO-Green6, was investigated by means of admittance spectroscopy in the modulation frequency range 1 -10 6 Hz. At room temperature, hole mobility of APFO-Green6 is dependent on the applied electric field, as commonly observed in disordered organic materials. The excess capacitance toward low frequencies provides evidence for charge relaxation in trap levels. A dispersion parameter of 0.4 was achieved from the trend of hole transit times with the electric field.
Study of trap states in polyfluorene based devices by using TSC technique
Thin Solid Films, 2008
The trap states in poly(9,9-dihexylfluorene-coN ,N-di(9,9-dihexyl-2-fluorenyl)-N-phenylamine) (PF-N-Ph) based light emitting diodes have been investigated by using the thermally stimulated current (TSC) technique in the temperature range of 90-320 K. The studied structure consisted of indium-tin-oxide/polyethylene-dioxythiophene: polystyrene-sulfonate/PF-N-Ph/Al. Four traps centers denoted as A, B, C, and D trap types have been identified with densities in the range of 10 16-10 17 cm − 3. Study of the dependence of TSC characteristics on the device polarity suggested that the A, C and D type traps are electron traps while the B type traps are hole traps. They can be described by Gaussian distributions centered on mean trap levels.
Organic Electronics, 2009
The effect of the molecular weight of poly[9,9-didecanefluorene-alt-(bis-thienylene) benzothiadiazole] (PF10TBT) on the photovoltaic performance of fullerene-based bulk heterojunction solar cells is investigated. An increase in molecular weight of two orders of magnitude results in a 30% increase of the short-circuit current and a rise of the fill factor from 0.45 to 0.63. Electron and hole transport are found to be virtually unaffected by changing molecular weight, which means that space-charge effects do not play a role in low molecular weight devices. Using optical modeling and numerical device simulations, we demonstrate that at low molecular weight the efficiency is mainly limited by a short lifetime of bound electron-hole pairs. This short lifetime prohibits efficient dissociation and is attributed to a deficiency in phase separation for low molecular weights.