Multiple Temporal‐Scale Photocarrier Dynamics Induced by Synergistic Effects of Fluorination and Chlorination in Highly Efficient Nonfullerene Organic Solar Cells

Abstract

Fluorination and chlorination have yielded a novel class of materials and achieved tremendous progress in enhancing photovoltaic efficiency in organic solar cells (OSCs). However, their effects on photocarrier dynamics remain elusive in these organic photovoltaic systems. Herein, a comprehensive study on the underlying mechanisms is conducted based on a 2 × 2 photovoltaic matrix, consisting of PBDB‐T, PBDB‐T‐2Cl, ITIC, and IT4F. Chlorination of donors enhances exciton migration and relaxation rates and promotes the extraction of polarons. The more efficient charge transfer and a larger proportion of long‐lived polarons are observed in fluorine‐containing acceptor‐based systems, which are in favor of charge generation in the actual devices. According to the enlarged dielectric constant in the PBDB‐T‐2Cl:IT4F blend, the improved exciton delocalization, the decreased exciton binding energy, and Coulomb capture radius are obtained relative to other three binary systems, which can increase charge separation efficiency and reduce the probability of bimolecular recombination. The simultaneous fluorination and chlorination can optimize molecular packing and nanoscale phase separation, facilitating effective exciton diffusion, exciton dissociation, and charge transport. These results highlight the important role of fluorination and chlorination on these fundamental mechanisms, possibly resulting in some new molecular design principles toward high‐performance OSCs.

Suppressing Kinetic Aggregation of Non‐Fullerene Acceptor via Versatile Alloy States Enables High‐Efficiency and Stable Ternary Polymer Solar Cells

“…[ 60 ] In addition, the crystalline coherence lengths (CCL) were calculated to estimate the crystallinity of blend films in accordance with the Scherrer equation: CCL = 2 πk /FWHM, where k and FWHM are Scherrer constant of 0.9 and the full width at half the maximum of the π–π stacking peak in the OOP direction severally. [ 52,61 ] The optimized ternary blends deliver the increased CCL values relative to that of the binary system (Table S6, Supporting Information), which implies the enhanced crystallinity brought forth by two types of the alloy states. It is noteworthy that the optimized packing arrangement and intermolecular interaction are vitally crucial to boosting charge transport and reinforcing the stability of BHJ morphologies to increase the lifetime of OPV devices.…”

“…However, these relevant ESA bands will become quite complicated and render some long‐lived features down to nanosecond time scale in binary and ternary blends, which can be regarded as the superposition of ESA signals of singlet excitons in BTP‐4Cl acceptor and hole polarons on charge‐separated (CS) state in PM6 donor arising from hole transfer. [ 51,52 ] According to the aforementioned analysis, we further extracted the kinetic decay curves of these blend films at 1000 nm to identify the relevant multiple temporal‐scale photocarrier dynamics, as shown in Figure 2h. The dynamics of these TA spectra display two interesting phenomenon: one is that the kinetic decay processes of two ternary blends are faster than that of binary system in the shorter timescale part (<1 ps), which reflects chiefly the hole injection information and can be in good consistence with the results of transient GSB features.…”

Antioxidant Additive with a High Dielectric Constant for High Photo‐Oxidative Stabilization of Organic Solar Cells without Almost Sacrificing Initial High Efficiencies

“…The measurement was performed in the low‐frequency range of 10 3 –10 6 Hz in the dark, and the active layer was fully depleted. The relative dielectric constants of the blend films with DIO, BHT–ref, BHT–CN, and BHT–PF as functions of frequency were calculated using ε r = Cd / ε 0 A , where C is the capacitance, ε 0 is the permittivity of free space, A is the device area (0.6 cm 2 ), and d is the thickness of the films [9c] . The ε r values are in the sequence of DIO (≈3.47) < BHT–ref (≈3.82) < BHT–CN (≈4.71) ≤ BHT–PF (≈4.82) in the low‐frequency regime, indicating relatively enhanced dielectric properties within the active layer due to the polar CN and PF alkyl chains.…”

“…The ε r values are in the sequence of DIO (≈3.47) < BHT–ref (≈3.82) < BHT–CN (≈4.71) ≤ BHT–PF (≈4.82) in the low‐frequency regime, indicating relatively enhanced dielectric properties within the active layer due to the polar CN and PF alkyl chains. This can lead to improved dipolar polarization of the excited state at the donor/acceptor interface and charge transfer state delocalization in the active layer; therefore, improved charge transport and recombination dynamics in the BHT–CN and BHT–PF‐processed devices can be expected [9c,14] …”

“…Meanwhile, numerous researchers have proposed enhancement of the dielectric constant of the active‐layer matrix as a way of improving the OSCs’ performance by modulating various factors linked to the Coulombic interactions in the films. [ 9 ] Recently, we proved that the use of appropriately designed nonvolatile additives (e.g., polymer additives with a high dielectric constant and ferroelectric properties) can create some advantageous properties and ultimately enhance the PCE values. [ 10 ]…”