
Download
PDF
0.00MB

Read Online
HTML
0.00MB

Download
XML
0.00MB
Abstract
Timeofflight measurements of the photocurrent in thin organic semiconductor layers represent an effective way to extract charge carrier mobility. A common method to interpret the timedependence of the photocurrent in these material systems assumes a positionindependent electric field between two coplanar electrodes. In this letter, we compare timedependence of the photocurrent, measured in the samples comprising thin layers of poly3hexylthiophene, with the Monte Carlo simulations. In the simulations, we have used both, a positionindependent and a positiondependent electric field. We obtained a favorable agreement between the simulations and the measurements only in the case of positiondependent electric field. We demonstrate that the charge carrier mobility may be underestimated by more than one order of magnitude, if a positionindependent electric field is used in the calculations of the mobility.
The work was supported by the European Community’s Seventh Framework Programme (FP7/20072013) Contract No. 212311, ONEP. We acknowledge A. Filipčič and M. Horvat for assistance with numerical simulations, and L. Wang and Y. Olivier for helpful discussions. The work has been supported in part by Slovenian Research Agency under the research program P10055.
Key Topics
 Electric fields
 38.0
 Charge carriers
 37.0
 Electrodes
 36.0
 Carrier mobility
 20.0
 Electric currents
 7.0
Figures
(a) Schematic representation of a polymer layer between two semiinfinite coplanar electrodes on top of a dielectric substrate, with dimensions and parameters used in the calculation of electric field by solving the Poisson equation. (b) Numerical solution of the electric field (E) streamlines (solid lines) and electric potential (dashed lines) in a polymer film between blocking coplanar electrodes on a glass substrate. d is the thickness of the polymer layer, x is the distance from the left electrode, and y is the distance from the substrate. Left electrode is at 0 V and the right electrode is at V bias. L is the separation between the electrodes. d/L ratio is 1:100. The dielectric constant of a polymer layer and the glass substrate are 3 and 10, respectively.
Click to view
(a) Schematic representation of a polymer layer between two semiinfinite coplanar electrodes on top of a dielectric substrate, with dimensions and parameters used in the calculation of electric field by solving the Poisson equation. (b) Numerical solution of the electric field (E) streamlines (solid lines) and electric potential (dashed lines) in a polymer film between blocking coplanar electrodes on a glass substrate. d is the thickness of the polymer layer, x is the distance from the left electrode, and y is the distance from the substrate. Left electrode is at 0 V and the right electrode is at V bias. L is the separation between the electrodes. d/L ratio is 1:100. The dielectric constant of a polymer layer and the glass substrate are 3 and 10, respectively.
A representative pair of timeofflight photocurrent I(t) simulations for positionindependent (dashed line) and positiondependent (solid line) electric field. The bias voltage V bias is 500 V, the distance between electrodes L is 150 μm and the level of energetic disorder is 6.4. Positionindependent electric field E 0 is Vbias/L. Position dependent electric field E x (x) is given by Eq. (1) (see text). The curves are normalized to the number of simulated carriers. The histograms represent the transit time distribution of simulated carriers. The right histogram corresponds to the simulation using E x (x). The left histogram corresponds to the simulation using E 0. The and t0 represent the mean transit times resulting from E x (x) and E 0 simulations, respectively.
Click to view
A representative pair of timeofflight photocurrent I(t) simulations for positionindependent (dashed line) and positiondependent (solid line) electric field. The bias voltage V bias is 500 V, the distance between electrodes L is 150 μm and the level of energetic disorder is 6.4. Positionindependent electric field E 0 is Vbias/L. Position dependent electric field E x (x) is given by Eq. (1) (see text). The curves are normalized to the number of simulated carriers. The histograms represent the transit time distribution of simulated carriers. The right histogram corresponds to the simulation using E x (x). The left histogram corresponds to the simulation using E 0. The and t0 represent the mean transit times resulting from E x (x) and E 0 simulations, respectively.
A series of Monte Carlo simulations of a photocurrent I(t) as a function of energetic disorder . The simulations were performed using the positiondependent electric field E x (x) given by Eq. (1) . at V bias = 500 V and L = 150 μm. Each I(t) curve is normalized to the mean transit time (t0 ) of the corresponding positionindependent electric field (E 0) simulation and to the number of simulated carriers. Arrows represent mean transit times ( ) of E x (x) simulations. Inset: The ratio as a function of . The dashed line is the minimum chisquare fit of a square power law suggesting that is increasing as .
Click to view
A series of Monte Carlo simulations of a photocurrent I(t) as a function of energetic disorder . The simulations were performed using the positiondependent electric field E x (x) given by Eq. (1) . at V bias = 500 V and L = 150 μm. Each I(t) curve is normalized to the mean transit time (t0 ) of the corresponding positionindependent electric field (E 0) simulation and to the number of simulated carriers. Arrows represent mean transit times ( ) of E x (x) simulations. Inset: The ratio as a function of . The dashed line is the minimum chisquare fit of a square power law suggesting that is increasing as .
A 2×2 μm atomic microscope topography scan of a 20 nm thick P3HT layer in the region between coplanar electrodes. The polymer layer was spincoated from a dichlorobenzene solution on a glass substrate. The height range is 15 nm. The polymer structure forms a random network of spherical grains with the diameter of approximately 30 nm. Inset: A closeup topography scan of the P3HT layer.
Click to view
A 2×2 μm atomic microscope topography scan of a 20 nm thick P3HT layer in the region between coplanar electrodes. The polymer layer was spincoated from a dichlorobenzene solution on a glass substrate. The height range is 15 nm. The polymer structure forms a random network of spherical grains with the diameter of approximately 30 nm. Inset: A closeup topography scan of the P3HT layer.
Timeofflight photocurrent I(t) measurement in a P3HT thin film as a function of a bias voltage V bias in a double logarithmic scale. Solid line represents the photocurrent of a simulation using positiondependent electric field E x (x). The simulated photocurrent was used to determine the hole mobility at V bias of 500 V. Inset: Schematic representation of the I(t) measurement. The laser pulse was focused near the biased electrode. I(t) was measured as a voltage drop on a resistor R, connected to the opposite electrode. The separation of coplanar electrodes L was 100 μm.
Click to view
Timeofflight photocurrent I(t) measurement in a P3HT thin film as a function of a bias voltage V bias in a double logarithmic scale. Solid line represents the photocurrent of a simulation using positiondependent electric field E x (x). The simulated photocurrent was used to determine the hole mobility at V bias of 500 V. Inset: Schematic representation of the I(t) measurement. The laser pulse was focused near the biased electrode. I(t) was measured as a voltage drop on a resistor R, connected to the opposite electrode. The separation of coplanar electrodes L was 100 μm.
Article metrics loading...
Abstract
Timeofflight measurements of the photocurrent in thin organic semiconductor layers represent an effective way to extract charge carrier mobility. A common method to interpret the timedependence of the photocurrent in these material systems assumes a positionindependent electric field between two coplanar electrodes. In this letter, we compare timedependence of the photocurrent, measured in the samples comprising thin layers of poly3hexylthiophene, with the Monte Carlo simulations. In the simulations, we have used both, a positionindependent and a positiondependent electric field. We obtained a favorable agreement between the simulations and the measurements only in the case of positiondependent electric field. We demonstrate that the charge carrier mobility may be underestimated by more than one order of magnitude, if a positionindependent electric field is used in the calculations of the mobility.
Full text loading...
Commenting has been disabled for this content