Real and Apparent Photoconduction in Single Crystals of Methyltriphenylphosphonium (TCNQ)₂⁻

Abstract

Photocurrents in single crystals and compressed discs of methyltriphenylphosphonium (TCNQ)z can be observed at low temperatures where the dark current is greatly reduced, or by the use of a chopped light source and a narrow band amplifier or oscilloscope display. By chopping a t appropriate frequencies the true photocurrent can be observed and bolometric effects eliminated. The true photocurrent, which is ohmic and linear with light intensity, shows a threshold a t (2.7 f-0.1) pm. The carrier lifetime is 140 ms above and below the phase change a t 310 K, but a t the phase change more than one lifetime is observed due to trapping.Fotostrome in Einkristallen und komprimierten Scheiben von Methyl-Triphenyl-Phosphonium (TCNQ)z werden bei niedrigen Temperaturen beobachtet, wo der Dunkelstrom stark reduziert ist, oder mit Hilfe zerhackten Lichts und eines Schmalbandverstarkers oder Oszillografen. Mit entsprechenden Zerhackerfrequenzen wird der wahre Fotostrom beobachtet, und bolometrische Effekte werden eliminiert. Der wahre Fotostrom, der ohmsch und linear zur Lichtstirke ist, zeigt eine Schwelle bei (2,7 f 0,l) pm. Die Ladungstriigerlebensdauer ist 140 ms oberund unterhalb der PhasenLnderung bei 310 K, aber am Phasengang wird auf Grund von Anhaftprozessen mehr als eine Lebensdauer beobachtet.

Lifetime of Photogenerated Carriers in TCNQ Single Crystals

“…1) yields an activation energy Eph = 0.21 to 0.25 eV. This value is markedly lower than the value for dark conduction, suggesting that thermal effects are small, in contrast to some TCNQ complexes where thermal effects are dominant [9]. By chopping the light beam the thermal processes are shown to be insignificant, and the equivalence of the photocurrent under continuous illumination and the AdsttLt value derived from the Ryvkin equation displayed.…”

“…It has been clearly demonstrated that in contrast to certain proteins [2] and TCNQ complexes [9], the increase in conduction of TCNQ crystals on continuous illumination is a true photocurrent, with insignificant contribution from the bolometric effect.…”