Close look at charge carrier injection in polymer field-effect transistors

Abstract

Parasitic contact resistance effects are becoming a major issue in organic transistors in that they can severely limit or even dominate their overall transistor performance. We present a systematic study of the contact resistance in bottom-contact polymer field-effect transistors made from poly(3-hexylthiophene) (P3HT) as well as poly-9,9′dioctyl-fluorene-co-bithiophene (F8T2). A microscopic approach based on noncontact scanning-probe potentiometry was used to directly separate the transport properties of the transistor channel and the electrode/polymer contacts, giving very accurate experimental access to both the source and drain contact resistance. The influence of the relevant parameters (temperature, electrode work function, ionization potential of the polymer, charge carrier mobility) on the source/drain contact resistance is investigated. We find that for “good” source/drain contacts that give rise to relatively small overall contact resistances (⩽50 kΩ cm), e.g., P3HT with chromium–gold electrodes, the source and the drain contact resistances are almost identical and are governed by bulk transport through the conjugated polymer. However, for “bad” contacts with a Schottky barrier for hole injection φb⩾0.3 eV, e.g., F8T2 with gold electrodes, the source contact resistance is considerably larger than the drain contact resistance and is dominated by charge-carrier injection at the source. Surprisingly small activation energies of 60–140 meV have been found for the source contact resistance, which are smaller than both φb and the activation energy of the mobility. From this we conclude that the commonly assumed (diffusion-limited) thermionic-emission models do not adequately describe the charge injection process in bottom-contact polymer transistors. On the basis of our results we propose a simple model, in which the source contact resistance is assumed to be the sum of resistance arising from the injection process and resistance due to bulk transport through a depletion region, whereas only the latter contributes to the drain contact resistance.

Molecular-weight dependence of interchain polaron delocalization and exciton bandwidth in high-mobility conjugated polymers

“…In P3HT TFTs with pure gold contacts fabricated in this way voltage drops across the source-drain contacts are negligible. 17 The channel lengths of devices are varied from 20 m to 2 m, and the channel widths are fixed at 1 cm. The patterned substrates were treated with hexamethyldisilazane ͑HMDS͒ to produce a hydrophobic surface.…”

Influence of film structure and light on charge trapping and dissipation dynamics in spun-cast organic thin-film transistors measured by scanning Kelvin probe microscopy

“…[11][12][13][14] Proximal probe techniques, such as scanning Kelvin probe microscopy (SKPM), allow us to fully explore the structure-property relationships in working OTFT structures. [14][15][16][17][18][19][20][21][22][23][24][25] SKPM provides the ability to monitor changes in charge transport phenomena in both space and time, a capability not afforded by traditional electrical performance measurements a Author to whom correspondence should be addressed. Electronic mail: lucile.teague@srnl.doe.gov alone.…”

Probing stress effects in single crystal organic transistors by scanning Kelvin probe microscopy

“…© 2010 American Institute of Physics Scanning Kelvin probe microscopy (SKPM) has demonstrated utility in correlating the relationship between film structure and charge transport in organic thin-film transistor (OTFT) devices as well as providing a detailed view of charge injection at the source and drain contacts. [1][2][3][4][5][6][7] While traditional field effect transistor (FET) I-V (performance) measurements yield the average mobility of the device, SKPM offers the advantage of probing the local mobility of the device. Previous studies of rubrene single crystal (SC) OTFTs utilized SKPM data, specifically the slope of the potential profile in the device channel (V/µm), to directly calculate the intrinsic charge mobilities of individual devices (µ).…”