Exciton interaction with a spatially defined charge accumulation layer in the organic semiconductor diindenoperylene

Abstract

Abstract:We present an investigation of the microscopic interplay between excitons and charge carriers by means of combined photoluminescence (PL) and charge carrier transport measurements on organic thin film transistors (OTFT). For this purpose, the prototypical organic semiconductor Diindenoperylene (DIP) was utilized as active material. The OTFT accumulation layer provides a spatially defined interaction zone for charges and photo-generated excitons leading to a PL intensity reduction of up to 4.5%. This effect correlates with the accumulated hole carrier density and provides a lower estimate of about 1.310 -10 cm 3 /s for the cross-section of non-radiative exciton-hole processes. It is rationalized that these processes are preferentially mediated by trapped holes. 7,8,9 Despite their importance, studies on the exciton loss mechanisms in neat organic single layers together with an estimation of the pivotal cross-sections are rather the exception, due to difficulties in precisely controlling the underlying boundary-conditions and in achieving a response of suited size to quantify the occurring loss processes.In this paper we focus on microscopic exciton quenching mechanisms induced by charge carriers and their investigation by photoluminescence (PL) quenching measurements on organic thin film transistor (OTFT) devices. This novel approach offers two essential advantages compared to studies on real opto-electronic devices. At first, material inherent mechanisms can be identified without superimposed effects at non-ideal heterojunction interfaces, such as locally inhomogeneous electric field distributions. Moreover, the choice of contact metal, in principle, enables discrimination of recombination effects induced by electrons from those related to holes.At second, the tunable charge density in the TFT accumulation layer provides an additional degree of freedom to systematically influence charge carrier mediated exciton annihilation processes.Complementary studies of the PL quenching at different operational regimes of the transistor might unveil the origin of the participating charges, in particular if they are free or spatially immobilized, e.g. by traps. 3As organic semiconducting material the polyaromatic hydrocarbon Diindenoperylene (DIP) was chosen. Besides its exceptional exciton diffusion properties in correlation with a long-range ordered thin film structure, which are necessary for the excitons to reach the accumulated charges, this compound has already proven successfully in OTFT applications as well as in photovoltaic cells. 10,11,12,13 Furthermore, the low optical absorption of DIP proves beneficial for the intended studies on exciton-charge carrier interaction since it avoids pronounced quenching contributions by bimolecular recombination.Thin film transistors were prepared via a five-step shadow mask process in bottom-gate topcontact geometry as described by Klauk et al.14 Illustrated in Fig. 1 a), a 60 nm thick aluminium film comply with the condition d DIP < /4 and therefore, interference eff...

Effect of polaron diffusion on exciton-polaron quenching in disordered organic semiconductors

“…The full lines connect points which are obtained from a theoretical fit using Eqs. (6) and (10), with a capture radius R c = 0.92 nm (as used for systems without disorder in Fig. 3) and a NN immediate quenching probability p NN ∼ = 6n h /N t .…”

“…Experimental methods for studying EPQ include transient absorption spectroscopy [2,5] of thin-film materials, within which the polarons are produced as a result of exciton dissociation, combined photoluminescence (PL) and charge carrier transport measurements on organic thin-film transistors [6], and transient and steady-state photoluminescence studies of unipolar (single-carrier) metal/semiconductor/metal sandwich-type devices [7][8][9]. In the latter devices, the application of a voltage difference between the electrode layers * r.coehoorn@tue.nl gives rise to an injected polaron density.…”

Nonthermally activated exciton transport in crystalline organic semiconductor thin films

“…As will be shown, this method reveals singlet exciton motion in crystalline DIP layers to be nonactivated at low temperatures, and thus offers insight into the material inherent transport mechanism. In comparison to other techniques facilitating the estimation of the exciton diffusion length, such as complementary analyses of the spectrally resolved photocurrent and absorption [7,9], PL measurements feature the advantage of investigating the intrinsic exciton transport properties without external influences by metal penetration, field-induced exciton dissociation, or recombination of charge carriers as, e.g., in OPV devices under operation [4,[10][11][12]. Moreover, theoretical modeling of our experimental data benefits from the lack of approximations on the exciton diffusion and absorption lengths, which would cause constraints in the present case where L D approaches α −1 .…”

“…For the PL studies step-wedged DIP samples covering a thickness range between 10 and 300 nm are half-side capped [5,17]. By choosing an interface to CuPc and therefore supporting exciton annihilation rather than dissociation, we minimize undesired effects by the interaction of excitons with free or trapped charges in the DIP transport layer [12]. Furthermore, CuPc absorption is negligible at the 532 nm excitation wavelength of the cw Nd:YAG laser used in this study, thereby avoiding the unintended effects on the exciton distribution in DIP.…”

Reducing exciton-polaron annihilation in organic planar heterojunction solar cells

“…This rolloff can be associated with EPA, triplet-triplet annihilation, electric field induced dissociation of excitons, and imbalanced charge injection [7][8][9][10]. Electron-polaron annihilation has also been studied in organic field effect transistors [11]. When the effect was studied in bulk heterojunction solar cells, it was found to be important at high light intensities [12].…”