Transient and steady state lateral charge transport in polymeric semiconductors (original) (raw)
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Materials Science and Engineering B-advanced Functional Solid-state Materials, 2006
Photogenerated charge carrier profiles in poly(2-methoxy-5-(2 -ethyl-hexyloxy)-1,4-phenylene vinylene) (MEH-PPV) light-emitting diodes were determined from steady-state photocurrent spectra and transient photocurrent decay measurements. The observation that the photocurrent spectra behavior is strongly dependent on the bias polarity and amplitude suggests the existence of an intrinsic electric field, determined by the difference in the work function of the metallic electrodes, as well as a field dependence on the free charge carrier generation rates. The obtained results reveal built-in voltages of +0.2 and −0.6 V for Au and Al electrodes, respectively. The photocurrent spectra can be semi-quantitatively explained using a simple model which takes into account the internal built-in electric field of the device structure, the band gap energy and the migration/diffusion of the photogenerated charge carriers through the polymeric film. The drift mobility of carriers was investigated by the time-of-flight technique (TOF). The value for the mobility of holes was obtained from the change of slopes in the double logarithmic plot of the transient current and was independent of the applied field. The TOF transients for electrons showed no change of slopes in the double logarithmic plot and the drift mobility of electrons could not be calculated.
Photoinduced charge transport in polymer field effect transistors
Synthetic Metals, 2004
Organic/polymer field effect transistors (FETs) based on poly(3-alkylthiophenes) (P3ATs) demonstrate substantial changes upon photoexcitation. The photogeneration of charge carriers and carrier transport in these FETs were studied using the transient and modulation techniques, along with the spectral responses. It is observed that the photoinduced charge carrier generation depends on the gate voltage, light intensity and temperature, while the transport characteristics are controlled by the carrier mobility and factors influencing the local built-in electric field parameters. A prominent feature in the photoinduced effect is the slow relaxation of photocurrent upon switching off the photoexcitation.
Photoinduced reversible switching of charge carrier mobility in conjugated polymers
The European Physical Journal Applied Physics, 2009
Photoinduced reversible switching of charge carrier mobility in conjugated polymers was studied by theoretical and experimental methods. The quantum chemical calculations showed that the presence of dipolar species in the vicinity of a polymer chain modifies the on-chain site energies and consequently increases the width of the distribution of hopping transport states. The influence of photoswitchable charge carrier traps on charge transport was evaluated by current-voltage measurement and by impedance spectroscopy method. It was found that deep traps switchable by photochromic reaction may significantly control the transport of charge carriers, which is exemplified as a significant decrease of the current and increase of parallel resistance measured by impedance spectroscopy.
Journal of Physics: Conference Series, 2019
Modified modulation photocurrent (MPC) measurements were carried out to investigate the carrier transport process in organic photovoltaics based on polymer and fullerene derivatives. In conventional MPC measurements, a photocurrent component modulating at the same frequency as the modulated incident light is recorded; however, in this study, a component modulating at twice the modulation frequency was detected. A linear process does not contribute to the detected signals; thus, this technique allows for focus on a process that converts incident photons into photocurrent in a nonlinear manner. An analysis using rate equations suggested that the nonlinearity resulted from the carrier density dependence of the internal electric field, which is the driving force of the carriers under the short-circuit condition. Furthermore, features due to both electron and hole transports were more clearly observed in the modified MPC measurements than in the conventional MPC measurements.
Light‐Induced Change of Charge Carrier Mobility in Semiconducting Polymers
Macromolecular Symposia, 2008
Summary: Light‐driven devices based on reversible change of carrier mobility in semiconducting polymers were investigated. The mobility was altered using a photochromic spiropyran capable of a reversible change of permanent dipole moment and ionization potential. While the latter attribute may result in formation of chemical traps and is more important for matrices with similar ionization potential such as PVK, the former phenomenon results in formation of polar traps and is more pronounced in the case of lower‐band‐gap materials.
Organic semiconductors with bipolar (electron and hole) transport capability play a crucial role in electronic and optoelectronic devices such as organic light-emitting diodes (OLEDs), bipolar transistors and photovoltaic cells. Recently, a considerable amount of work has been devoted to the characterization of ambipolar transport in organic materials, allowing for a better understanding of their properties as well as the physical processes, which take place in materials and devices [1-4]. The experimental methods used to obtain information about charge transport in organic semiconductors – time-of-flight (TOF) transient photoconductivity [5], charge extraction by linearly increasing voltage (CELIV) [6], current-voltage measurements in space charge limited current regime [7], and field effect transistor (FET) measurements [8, 9] are mostly focused on determination of charge carrier mobility. On the other hand, for many devices (e.g. organic photovoltaic solar cells or light emitting diodes) the knowledge of the transport and recombination characteristics of both carriers (electron and hole), and specifically their diffusion D L Dτ = (here D is the diffusion coefficient and τ is the photocarriers lifetime) and drift lengths 0 0 L E μτ = (here μ is the carrier's mobility and 0 E is the electric dc field) is important.
Internal electric field in organic-semiconductor-based photovoltaic devices
Applied Physics Letters, 2006
The authors performed transient photocurrent measurements under applied bias and electroabsorption spectroscopy on devices based on a pristine poly͑phenylene vinylene͒ derivative as well as its mixture with 1% of a methanofullerene electron acceptor. Combining both techniques allows us to directly determine the internal electric field and to conclude on its implication on the photovoltaic performance of the devices. The electric field is identified as the driving force of the photocurrent, hence indicating the maximum obtainable photovoltage. Acceptor concentrations as low as 1% shift the energetic alignment of the top electrode to the reduction potential of the acceptor, reducing the internal electric field.
Spectroscopic studies of photoinduced transport in polymer field effect transistors
Synthetic Metals, 2005
Polymer field effect transistors (FETs) exhibit several interesting features upon photo-excitation. We review recent results from our laboratory of these polymer photo-FETs and present spectroscopic evidence to verify proposed mechanisms of charge-carrier generation and transport. We present results, which suggest that charge generation and recombination processes depend on the operating mode of FET. The location of photoinduced charge generation is probed using the intensity modulated photocurrent spectrum as a function of gate voltage with light incident from either side of the indium tin oxide (ITO)-gated FET structure.