Charge-carrier photogeneration in single-component organic carbazole-based semiconductors via low excitation power triplet-triplet annihilation

Abstract

It is generally believed that intrinsic charge generation via an autoionization mechanism in pristine single-component organic semiconductors is impossible upon photoexcitation within the lowest excited singlet state due to the large exciton binding energy. However, we present measurements of thermally stimulated luminescence, light-induced ESR, and photocurrent in the carbazole-based molecule 3 ,5di(9H -carbazol-9-yl)-[1,1 -biphenyl]-3-carbonitrile (mCBP-CN) films, revealing that charge-carrier pairs are efficiently produced upon excitation near their absorption edges. The photocurrent measurements show a superlinear dependence on the cw-photoexcitation intensity even at very low excitation power (below 1 mW/cm 2 ), suggesting a bimolecular nature of the charge photogeneration process. The photocurrent measured over a broad temperature range of 5-300 K exhibits a prominent maximum at moderately low temperature around 170 K and rolls off significantly at higher temperatures. This correlates remarkably with the maximum of delayed fluorescence induced by bimolecular triplet-triplet annihilation (TTA), i.e., triplet fusion, in this material. This behavior implies that the photocurrent is governed mainly by the TTAinduced production of geminate pairs and only a little by their subsequent dissociation. Moreover, we find that the field-assisted dissociation probability of photogenerated charge pairs becomes almost temperatureindependent at temperatures below 100 K. This can be quantitatively described using a charge dissociation model accounting for the energy disorder and the distribution of geminate-pair radii. The key conclusion of this study is that triplet fusion can promote the energy up-conversion (to 5.42 eV), thereby enabling the autoionization of a high-energy neutral excited state. This serves as the predominant mechanism of intrinsic photogeneration in this single-component heavy-atom-free system. We attribute the effect to efficient intersystem crossing in mCBP-CN, a high triplet energy level (2.71 eV), and very long-lived triplet excitations. A broader implication of this finding is that the so far neglected mechanism of TTA-facilitated charge-carrier generation might be relevant for organic long-persistent luminescence materials, and even for organic photovoltaics and potentially for photocatalytic water splitting processes.

Unexpected Performance of a Bifunctional Sensitizer/Activator Component for Photon Energy Management via Upconversion

“…It was shown recently how a single-component photoactive layer of (2, 3, 7, 8, 12, 13, 17, 18-octaethyl-porphyrinato) Pt II (PtOEP), capable of TTA-driven photocurrent generation, offers the competitive advantage of serving simultaneously as photon absorber, triplet annihilator, activator for charge generation and charge extraction agent when embedded in an organic photodetector (OPD) device . Additional results corroborating the generic character of TTA-induced charge photogeneration were recently demonstrated for a carbazole-based molecular system that is typically used in organic light emitting diode (OLED) device platforms. In this regard, interlayers developed by single-component TTA-UC materials can be interfaced with active layers of light-harvesting systems within a device structure, by maintaining a cascade energy level alignment and without introducing charge extraction barriers.…”