Intrachain Delocalization Effect of Charge Carriers on the Charge-Transfer State Dynamics in Organic Solar Cells

Abstract

We studied the charge-generation mechanism in low-bandgap polymer (P4TNTz-2F)-fullerene bulk heterojunction (BHJ) organic solar cells (OSCs) using transient absorption (TA) spectroscopy. The highly crystalline nanowire structure of P4TNTz-2F in a blend film prepared with chlorobenzene (CB) and 1,8-diiodooctane (DIO) induced more long-lived charge carriers than those in a blend film prepared with CB only. Pump-wavelength-dependent TA data revealed that the increased charge-delocalization by the intrachain ordering of P4TNTz-2F in the blend film prepared with CB/DIO is the key factor to increasing the OSC efficiency. The intrachain charge-delocalization increased the charge-transfer (CT) state lifetime and suppressed geminate recombination losses, resulting in the efficient dissociation of CT states into free carriers. Our findings provide new insights into the excited-state dynamics study of BHJ blends, which can serve as a good guide for the development of novel OSC materials.

Ultrafast Intermolecular Interaction Dynamics between NIR-Absorbing Unsymmetrical Squaraines and PCBM: Effects of Halogen Substitution

“…The species-associated spectra of ABSQ derivatives upon addition of PCBM are provided in Figures S25–S28 in the Supporting Information. The rate constant of a particular process can be obtained from the fraction of population conversion from one state to another divided by the associated time constant of the state. − The relative fraction of population is tested by multiplying the amplitude of each of the kinetic traces with the corresponding species-associated spectrum and summing the result, leading to the observed transient absorption spectra within the experimental noise. − Thus, based on the target analysis, the charge transfer proposed to occur from the solvent-stabilized local excited state with the relative population of ∼89% and the time constants of charge transfer from ABSQ-H , ABSQ-F , ABSQ-Cl , and ABSQ-Br to PCBM were found to be 86.43, 93.72, 96.52, and 102.56 ps, respectively.…”

Long-Lived Charge-Separated State in Naphthalimide–Phenothiazine Compact Electron Donor–Acceptor Dyads: Effect of Molecular Conformation Restriction and Solvent Polarity

“…Formation of long-lived charge separation (CS) states in organic electron donor–acceptor dyads/triads is crucial for fundamental photochemistry studies, as well as for artificial photosynthesis, − photocatalysis, − and photovoltaics. − Marcus theory can be used for describing the relation between the kinetics and thermodynamics of electron transfer (ET; eq ) k ET = true( 4 π 3 h 2 λ k normalB T true) 1 / 2 V 2 0.25em exp [ − ( Δ G ET 0 + λ ) 2 4 λ k B T ] where h and k B represent the Planck constant and Boltzmann constant, respectively. Δ G ET 0 is the Gibbs free energy change between the equilibrium reactant and final states of the electron transfer, λ is the total reorganization energy, and the magnitude of the electronic coupling matrix element V indicates the coupling strength between the initial and the final states of the electron transfer.…”

Fluorination-Induced Conversion of the Quasi- to the Normal Interchain Packing in Push–Pull Conjugated Polymer Aggregates

“…Push–pull conjugated polymers (CPs) have emerged as alternative electron donor materials, replacing first-generation CPs like poly­(3-hexylthiophene), and have become the mainstream materials in the field of bulk heterojunction (BHJ) organic solar cells (OSCs). − Extensive research has been conducted on exciton dynamics in various push–pull CP systems, where the chain backbone consists of alternating electron-rich and electron-deficient units. − In the organic CP aggregate systems with low dielectric constants, singlet (S 1 ) excitons with excess energy can give rise to excited-state species such as polaron pairs (loosely bound charge carriers) and separated polarons (free carriers), referred to as quasi-charge-transfer (CT) states with lower binding energy compared to Frenkel excitons. ,− The formation mechanisms of polaron states, i.e., exciton dissociation, are influenced by chain rigidity, length, and aggregation pattern. , Furthermore, CP chains at the interfaces with electron acceptors exhibit more disorder than the chains in pure CP domains away from the interfaces within the BHJ blends . Therefore, unraveling the exciton dynamics dependent on these factors within the push–pull CP aggregates is crucial for understanding the exciton dissociation and carrier diffusion dynamics in high-efficiency BHJ OSCs.…”