Insight on charge-transfer regimes in electron-phonon coupled molecular systems via numerically exact simulations

Abstract Various simulation approaches exist to describe charge transport in organic solids, offering significantly different descriptions of the physics of electron-phonon coupling. This variety introduces method-dependent biases, which inevitably result in difficulties to interpret charge transport processes in a unified picture. Here, we combine numerical and analytical quantum approaches to investigate the charge-transfer dynamics in an unbiased framework. We unveil the fading of transient localisation and the formation of polarons in a broad range of vibrational frequencies and temperatures. By studying the joint electron-phonon dynamics from femtoseconds to nanoseconds, we identify three distinct charge-transport regimes: transient localisation, Soft Gating, and polaron transport. The dynamic transitions between such regimes are ruled by a buildup of the correlations between electronic motion and nuclei, which lead to the crossover between transient localisation and polaron transport. This transition is seamless at all temperatures and adiabaticities, even in the limit of low-frequency vibrational modes

Data related to manuscript [‘Short Excited State Lifetimes Mediate Charge Recombination Losses in Organic Solar Cell Blends with Low Charge Transfer Driving Force', Advanced Materials (2021), https://doi.org/10.1002/adma.202101784]

We investigate a blend of a low optical-gap diketopyrrolopyrrole polymer and a fullerene derivative, with near-zero driving force of 50 meV for interfacial electron transfer. Using femtosecond transient absorption and electro-absorption spectroscopy, we quantify the charge transfer (CT) and recombination dynamics as well as the transport at early timescales. Electron transfer is found to be ultrafast, which is consistent with a semiclassical Marcus-Levich-Jortner description at low driving force and low reorganization energy. However, we observe significant geminate recombination and unusually short S1 and CT state lifetimes in the investigated system (13-14 ps). At low S1-CT offset, a short excited state lifetime mediates charge recombination because i) back-transfer from the CT to the S1 state followed by S1 recombination can occur and ii) additional S1-CT hybridization can decrease the CT lifetime. Both effects are confirmed by density functional theory calculations. In addition, we observe relatively slow (tens of picoseconds) dissociation of charges from the interfacial CT state, in contrast to polymer:fullerene blends with high CT driving force. We identify low local charge carrier mobility as a primary reason for the slow rise of free charge population. Simulations using a four-state kinetic model entailing the effects of energetic disorder reveal that the free charge yield could be increased from the observed 12% to 60% by increasing the S1 and CT lifetimes to 150 ps. Alternatively, decreasing interfacial CT state disorder while increasing bulk disorder of free charges enhances the yield to 65% in spite of the short lifetimes

Influence of synthetic pathway, molecular weight and side chains on properties of indacenodithiophene-benzothiadiazole copolymers made by direct arylation polycondensation

Atom-economic protocols for the synthesis of poly(indacenodithiophene-alt-benzothiadiazole) (PIDTBT) are presented in which all C-C coupling steps are achieved by direct arylation. Using two different synthetic pathways, PIDTBT copolymers with different side chains (hexylphenyl, octylphenyl, dodecyl, methyl/2-octyldodecylphenyl, 2-octyldodecylphenyl/2-octyldodecylphenyl) and molecular weight (MW) are prepared. Route A makes use of direct arylation polycondensation (DAP) of indacenodithiophene (IDT) and 4,7-dibromo-2,1,3-benzothiadiazole (BTBr2) leading to PIDTBT in high yields, with adjustable MW and without indications for structural defects. Route B starts from a polyketone precursor also prepared by DAP following cyclization. While route B allows introduction of asymmetric side chains at the IDT unit, polymer analogous cyclization gives rise to defect formation. The absorption coefficient of PIDTBT with alkylphenyl side chains made by route A increases with MW. Field-effect hole mobilities around similar to 10(-2) cm(2) V-1 s(-1) are molecular weight-independent, which is ascribed to a largely amorphous thin film morphology. PIDTBT with linear dodecyl side (C12) chains exhibits a bathochromic shift (20 nm), in agreement with theory, and more pronounced vibronic contributions to absorption spectra. In comparison to alkylphenyl side chains, C12 side chains allow for increased order in thin films, a weak melting endotherm and lower energetic disorder, which altogether explain substantially higher field-effect hole mobilities of similar to 10(-1) cm(2) V-1 s(-1)