Charge recombination studies in polyfluorene:[6, 6]-phenyl c~ 6~ 1-butyric acid methyl ester blend photovoltaic cells [5215-47] (original) (raw)
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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.
Organic Electronics, 2006
We investigate charge formation in bulk-heterojunction solar cells based on conjugated polymers in the form of alternating polyfluorene copolymers and the methanofullerene PCBM. Using transient absorption spectroscopy we show that optimal charge formation is obtained with 20-50 wt% PCBM. This is in contrast to the maximum short circuit current density obtained at $80 wt% PCBM as determined by steady state current density-voltage characterization. Hence, we show explicitly that the solar cell performance of these interpenetrating polymer networks containing PCBM is limited by charge transport rather than by formation of charges.
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.