Charge recombination studies in polyfluorene:[6, 6]-phenyl c~ 6~ 1-butyric acid methyl ester blend photovoltaic cells [5215-47] (original) (raw)
Recombination studies in a polyfluorene copolymer for photovoltaic applications
Synthetic Metals, 2005
We present detailed continuous wave (cw) and transient photoinduced absorption (PA) measurements in thin films of a novel alternating polyfluorene copolymer, poly[2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4 ,7 -di-2-thienyl-2 ,1 ,3-benzo-thiadiazole)] (DiO-PFDTBT), and its blends with the soluble fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) in weight ratios of 1:0, 4:1 and 1:4. We measure the frequency, intensity and temperature dependence of the PA signal in the frequency domain, and compare with the results obtained from the transient PA decay measurements in the time domain. In all blends, the PA spectrum shows a broad high energy PA band ranging from ∼1 eV to 2 eV as well as a low energy band peaking at ∼0.35 eV. We attribute the low energy band to the P 1 transition of polarons and part of the high energy band to the correlated P 2 transition of polarons. Both frequency and time domain measurements show that the high energy band has two decay components, a faster component in the microsecond time regime and a slower component in the millisecond time regime. The slow component is strongly dispersive, whereas the fast component is practically non-dispersive.
Macromolecules, 2001
We report the composition profile in blends of hole-accepting and electron-accepting polyfluorenes. During solvent evaporation to form thin (∼150 nm thickness) solid films, demixing of the polymers produces micrometer scale lateral phase separation. We show here that demixing is more complete in regions closer to the interface between the two phases. We propose that carriers transport is easier in these regions than in the bulk. We show how this affects the photovoltaic efficiency of diodes made with these blends, which show a linear dependence of efficiency on the length of the interface between the phases.
The Effect of Ketone Defects on the Charge Transport and Charge Recombination in Polyfluorenes
Advanced Functional Materials, 2011
The effect of on-chain ketone defects on the charge transport of the polyfl uorene derivative poly(9,9-dioctylfl uorene) (PFO) is investigated. Using MoO 3 as ohmic hole contact, the hole transport in a pristine PFO diode is observed to be limited by space-charge, whereas fl uorenone contaminated PFO (PFO-F) is shown to be trap limited by the occurrence of an exponential trap distribution with a trap depth of 0.18 eV. The electron transport in PFO is also observed to be trap limited, but in order to describe the electron transport of PFO-F, an additional trap level with a depth of 0.46 eV must be introduced. The obtained energy levels of the fl uorenone trapping sites are in close agreement with cyclic voltammetry (CV) measurements reported in literature. As a result, the fl uorenone defects are shown to simultaneously act as hole-and electron trap. Moreover, through ideality factor measurements, the green emission associated with these defects is observed to originate from trapassisted recombination.
Macromolecules, 2006
We report the composition profile in blends of hole-accepting and electron-accepting polyfluorenes. During solvent evaporation to form thin (∼150 nm thickness) solid films, demixing of the polymers produces micrometer scale lateral phase separation. We show here that demixing is more complete in regions closer to the interface between the two phases. We propose that carriers transport is easier in these regions than in the bulk. We show how this affects the photovoltaic efficiency of diodes made with these blends, which show a linear dependence of efficiency on the length of the interface between the phases.
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.
Optical and electrochemical properties of polyfluorenes with pyridine-triphenylamine bipolar unit
Polymer International, 2012
We report optical and electrochemical properties of polyether derivatives of perylenediimides (PDIs) thin films formed in various materials (semiconductor, insulator, amorphous and self-assembly). Perylenediimides adsorbed on nanocrystalline TiO 2 (NT) nanocrystalline alumina (NA), amorphous silicon (PS) and neat self-assemblied (SA) films were prepared and characterized based on spectroscopic, electrochemical, spectro-electrochemical techniques. The absorption and fluorescence spectra of PDIs in chloroform exhibit vibronic features. The fluorescence quantum yields (U f) of PDIs with end amino substituents in chloroform solutions are over 0.95, while the quantum yield of triethoxyphenyl substituted PDI U f value is 0.024 in solution. Optical spectroscopy proves that PDIs in metal oxide thin films form aggregated type complexes. An electrochromism, a color change from red to blue/violet, is observed on metal oxide films, that indicates existence of mono and dianion forms of PDIs. Reversibility of electrochemical reductions in NT film depends on the scanning rate. However, electrochromism in NA films is stable and reversibility is independent from scanning rate. Stable mono and diaionic species are formed on NA films. SA films show broad absorption peaks during the voltammetric scan. On the other hand, the first reduction onset potentials of PDIs are almost equal to the onset potential of capacitive current of TiO 2 which lead to low efficiency in dye-sensitized solar cells.
New low band gap alternating polyfluorene copolymer-based photovoltaic cells
Solar Energy Materials and Solar Cells, 2007
New low band gap alternating polyfluorene copolymers were synthesized for use in plastic solar cells and their optical, electrochemical, and photovoltaic characteristics were determined. These polymers incorporated fluorene units alternating with groups including electronwithdrawing (A) and electron-donating (D) groups in donor-acceptor-donor (DAD) sequence to achieve the lowering of band gaps. The HOMO-LUMO values were estimated from electrochemical studies. By varying the donor and acceptor strength and position of the solubilizing substituents, similar HOMO values were obtained. These values were also found to correlate well with the open circuit voltage (V OC ) values determined from photovoltaic data of the polymers blended with the acceptor PCBM. Despite similar HOMO values, the absorption spectra of the polymers differ significantly. This prompted the preparation of photovoltaic devices consisting of blends of two polymers with complementary absorptions in combination with PCBM to harvest more photons in the polymer solar cells.
Efficient organic solar cells with polyfluorene derivatives as a cathode interfacial layer
Organic Electronics, 2009
Use of a polyfluorene derivative (WPF-oxy-F) as the cathode interfacial layer was investigated for low-cost and high-efficiency organic solar cells (OSCs) based on poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C 61 (PCBM). Insertion of the WPF-oxy-F interfacial layer between the P3HT/PCBM active layer and the metal cathode increased overall power conversion efficiency from 2.95% to 3.77% primarily due to the improved open circuit voltage and enhanced fill factor, resulting from a reduction of the metal work-function through the introduction of WPF-oxy-F.
ACS Applied Materials & Interfaces, 2014
In this work, we synthesized water-soluble polyfluorene derivatives (WPFs) with anionic and/or cationic side chains, which were used as an indium tin oxide (ITO) cathode interfacial layer in inverted polymer solar cells. Three WPFs (WPFN+, WPFZW, and WPFS-) were obtained via Suzuki coupling reactions. Their solubility in polar solvents allowed the WPFs to be used as interfacial layers in inverted polymer solar cells (I-PSCs). Among the WPF-modified ITO electrodes, WPFN+ (with ammonium side chains)-modified ITO can be used as a cathode for electron extraction, while WPFS-(with sulfonate side chains)-modified ITO cannot extract electrons in I-PSCs based on poly(3-hexylthiophene): [6,6]-phenyl-C 61-butyric acid methyl ester (P3HT:PC 61 BM). The electron extraction of WPF-modified ITO can mainly be attributed to the different dipole formations at the WPF/ITO interfaces, based on the types of ionic groups on the side chains of the polyfluorene. In addition, we observed that the extent of ITO work-function modification was not always exactly correlated with the device performance based on the results obtained using a WPFZW (with ammonium and sulfonate side chains)modified ITO electrode.