Accurate and simultaneous determination of carrier density and mobility in organic semiconducting materials
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Accurate and simultaneous determination of carrier density and mobility in organic semiconducting materials Kai Shum1, (a), Zhuo Chen1, C. M. Xue2, and Shi Jin2, (b) 1
Department of Physics, Brooklyn College of CUNY, Brooklyn, NY 11020, U.S.A.
2
Department of Chemistry, College of Staten Island, CUNY, Staten Island, NY 10314, U.S.A.
(a) [email protected] (b) [email protected]
ABSTRACT How to accurately determine carrier mobility and density in organic semiconducting materials is a very important subject for their optoelectronic applications including light-emitting diodes, solar cells, and thin film field-effect transistors. In this work, we report on a unique data analysis procedure for space-charge limited currents to simultaneously obtain the carrier density and mobility in semiconducting organic-materials. This procedure has been used for a few newly synthesized perylene tetracarboxylic diimide (PDI) derivatives with tunable π-stack structures without altering the electronic characteristic of individual molecules. How π-stack structural variation and residual carrier density affect electron transport performance will be discussed. INTRODUCTION Carrier mobility and density in organic materials are very important parameters for various electronic applications including organic light-emitting diodes (OLEDs), solar cells, and thin film field-effect transistors. Therefore, accurate determinations of these two parameters in unintentionally doped pristine organic materials are crucial to understand the carrier transport physics and to fully control the chemical synthesis in terms of molecular designs. There are two main methods to determine the carrier mobility in organic materials: 1) time-of-flight method, [1] and 2) steady-state space-charge-limited current (SCLC) method. [2] In the first method, both electrons and holes are generated by short light pulses in the organic materials sandwiched by two conducting electrodes. For electron (hole) mobility measurements, the light excitation is set to be near the negative (positive) electrode. Transit time, Δtransit for either electrons or holes optically generated near one electrode drifted across the device to another electrode with the thickness of L is then measured from time-resolved photocurrent data. Carrier mobility, μc, is then determined by the following simple equation: μc E = L/Δtransit.
(1)
The electric field, E, in the above equation is related the applied voltage, V, by E = V/L. This method assumes uniform electric field within the device. Its validation depends on the density level of photogenerated carriers. In the second method, the conventional steady-state current is measured as a function of applied bias on the same device as in the first method except that one of electrodes does not have
to be transparent necessary for photo-excitation. For small applied voltages, the measured current follows Ohm’s law: J = σE, where σ is related carrier charge, q, carrier density, nc, and carrier mobility by σ = qncμc. Electrodes act to hold
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