Effective Negative Diffusion of Singlet Excitons in Organic Semiconductors

Abstract

Using diffraction-limited ultrafast imaging techniques, we investigate the propagation of singlet and triplet excitons in single-crystal tetracene. Instead of an expected broadening, the distribution of singlet excitons narrows on a nanosecond time scale after photoexcitation. This narrowing results in an effective negative diffusion in which singlet excitons migrate toward the high-density region, eventually leading to a singlet exciton distribution that is smaller than the laser excitation spot. Modeling the excited-state dynamics demonstrates that the origin of the anomalous diffusion is rooted in nonlinear triplet–triplet annihilation (TTA). We anticipate that this is a general phenomenon that can be used to study exciton diffusion and nonlinear TTA rates in semiconductors relevant for organic optoelectronics.

Non-Markovian diffusion of excitons in layered perovskites and transition metal dichalcogenides

“…They are formed when light interacts with a wide variety of systems. This includes inorganic and organic molecular semiconductors, [1][2][3] photosynthetic complexes, 4,5 polymers, [6][7][8] molecular aggregates, [9][10][11] semiconductor quantum wells and quantum wires, [12][13][14][15] colloidal quantum dots and nanoplatelets, [16][17][18][19][20][21][22][23][24][25] transition metal dichalcogenides, [26][27][28][29][30] and perovskites. [31][32][33][34][35][36][37] The dynamical properties of these systems are crucial for the advancement of optoelectronic applications including novel lasers, 16 photodetectors, 38 light-emitting diodes [39][40][41] and solar cells.…”

Unveiling the mixed nature of polaritonic transport: From enhanced diffusion to ballistic motion approaching the speed of light

“…This clearly shows that, even though the transport within this parameter range is strongly influenced by the underlying disorder in the system, which makes it non-ballistic, the coherent coupling with the photons and the long-range correlations induced by it play a crucial role in the transport dynamics. In fact, recalling that the typical diffusion coefficient for excitons in semiconductors (either organic or inorganic) is normally within the range of 10 −3 − 10 −7 µm 2 /ps 4,31,32,35 , our results signify an enhancement of more than six orders of magnitude in the diffusion coefficient.…”

“…This kind of cavity-enhanced transport has been drawing increasing interest over the past few years, with both experimental [5][6][7][8][9][10][11][12][13][19][20][21] and theoretical [22][23][24][25][26][27][28][29] efforts devoted to understanding how strong coupling affects transport phenomena. Although the long-range propagation of polaritons has been directly visualized and studied in steady-state experiments 6,8,10 , constructing a complete picture of their transport dynamics necessitates access to the kinetics of the polaritonic motion, in a similar manner to the various time-resolved 2/22 microscopy techniques used for studying normal organic semiconductors [30][31][32][33][34][35] . Indeed, by employing ultrafast microscopy, a few recent experiments successfully resolved the spatiotemporal evolution of polaritons 7,13 .…”

Visualizing Hot‐Carrier Expansion and Cascaded Transport in WS2 by Ultrafast Transient Absorption Microscopy

“…In contrast to the “cold‐state” excitons, the hot carriers have a fast velocity, as visualized in silicon, [ 8 ] leading to transient superdiffusion in the spatial domain. Similar diffusive behaviors have been examined in organic semiconductors and metallic gold, [ 38,39 ] which may induce anomalous carrier mobility and thus impact the photoresponse and heat dissipation of the devices.…”