Catching the Killer: Dynamic Disorder Design Rules for Small‐Molecule Organic Semiconductors

Abstract

Small‐molecule organic semiconductors are promising materials for applications ranging from solar cells to medical sensors. Their nearly infinite design space means that, in theory, it is possible to tailor the material to the exact specifications of a particular application. In reality; however, design rules to improve mobility (μ) remain elusive because of the complex calculations required to understand its limiters. Transient localization theory posits that charge carriers are slowed down by the collective phonon motions, called dynamic disorder, which localize charge carriers temporarily. Traditionally, a mode‐by‐mode analysis is performed to try and identify “killer” modes. Herein, the flaws are demonstrated with this analysis and present a streamlined simulation workflow that simplifies simulation of dynamic disorder and enables engineering of new molecular structures based on phonon dynamics. This workflow is combined with a novel visualization technique that enables per‐atom insights into how μ is limited. This workflow is applied and analyzed to a series of ‐acenes and a series of BTBT‐based molecules. Then design rules are identified in each series that identify possible ways to improve the μ beyond current experimental limits using phonon engineering.

Assessing alkyl side chain effects on electron transport properties of Y6-derived non-fullerene acceptors

“…This modulation results in localisation of charge carriers and thus hinders transport. Yet, it is a difficult feature to limit due to its nature, although some strategies based on identifying ''killer modes'', electrostatic interactions, and reduced overlap between nearest neighbours have been suggested, 36,[72][73][74] despite this latter inevitably leads to lower transfer integrals. To the best of our knowledge, there are essentially two ways of estimating dynamic disorder: (i) through the evaluation of transfer integrals gradients with respect to vibrations, 50,[72][73][74][75][76][77] (ii) computing transfer integrals along an MD trajectory.…”

Untangling the Fundamental Electronic Origins of Non‐Local Electron–Phonon Coupling in Organic Semiconductors

“…[9][10][11] While the exact mechanism of charge transport in organic semiconductors remains a topic of debate, designing new materials with improved charge-carrier mobilities is a critical and highly-active area of study. [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30] A variety of factors play a role on dictating charge-carrier mobility, including the electronic structure of the material, molecular packing, and detrimental effects such as electron-phonon coupling, to name a few. [9,10] The latter is particularly relevant for organic semiconductors, as these weakly-bonded van der Waals solids are well-known to exhibit a rich set of low-frequency vibrational dynamics (1-100 cm −1 , 0.03-3 THz, 0.12-12.4 meV) that are highly-excited at ambient temperatures -precisely the modes responsible for well-known thermal energetic disorder in organic semiconductors.…”

“…[ 9–11 ] While the exact mechanism of charge transport in organic semiconductors remains a topic of debate, designing new materials with improved charge‐carrier mobilities is a critical and highly‐active area of study. [ 12–30 ]…”

Towards a fast machine-learning-assisted prediction of the mechanoelectric response in organic crystals

“…Some indications that the variation of transfer integrals may have a stronger impact than phonon modes on the performances was recently obtained, 16 but its magnitude may strongly depend on the molecule of interest. On the other side, several studies have highlighted the importance of dynamic disorder 17,18 and how a single vibrational “killer mode” can be identified as responsible for limiting transport, 19,20 with mechanical perturbations as a tool to limit such vibrations. 11…”

“…Some indications that the variation of transfer integrals may have a stronger impact than phonon modes on the performances was recently obtained, 16 but its magnitude may strongly depend on the molecule of interest. On the other side, several studies have highlighted the importance of dynamic disorder 17,18 and how a single vibrational ''killer mode'' can be identified as responsible for limiting transport, 19,20 with mechanical perturbations as a tool to limit such vibrations. 11 Seminal studies investigating mechanoelectric properties of organic compounds focused on polycrystalline semiconductors, [21][22][23] which, however, do not allow to investigate the intrinsic behaviour of the material, since electrical properties are mainly dominated by extrinsic effects like grain size and defects.…”