Quantification of Temperature‐Dependent Charge Separation and Recombination Dynamics in Non‐Fullerene Organic Photovoltaics

Abstract

Transient optical spectroscopy is used to quantify the temperature-dependence of charge separation and recombination dynamics in P3TEA:SF-PDI 2 and PM6:Y6, two non-fullerene organic photovoltaic (OPV) systems with a negligible driving force and high photocurrent quantum yields. By tracking the intensity of the transient electroabsorption response that arises upon interfacial charge separation in P3TEA:SF-PDI 2 , a free charge generation rate constant of ≈2.4 × 10 10 s −1 is observed at room temperature, with an average energy of ≈230 meV stored between the interfacial charge pairs. Thermally activated charge separation is also observed in PM6:Y6, and a faster charge separation rate of ≈5.5 × 10 10 s −1 is estimated at room temperature, which is consistent with the higher device efficiency. When both blends are cooled down to cryogenic temperature, the reduced charge separation rate leads to increasing charge recombination either directly at the donor-acceptor interface or via the emissive singlet exciton state. A kinetic model is used to rationalize the results, showing that although photogenerated charges have to overcome a significant Coulomb potential to generate free carriers, OPV blends can achieve high photocurrent generation yields given that the thermal dissociation rate of charges outcompetes the recombination rate.

Slow Hole Transfer Kinetics Lead to High Blend Photoluminescence of Unfused A–D–A′–D–A‐Type Acceptors with Unfavorable Highest Occupied Molecular Orbitals Offset

“…Previous studies have established a model with the S 1 ÀCTÀFC equilibrium, where the singlet excitons are proposed to be in dynamic equilibrium with FCs. [17,22,24] Furthermore, with the S 1 ÀCTÀFC equilibrium, when the FCs are effectively extracted under SC, the PL should be quenched because the equilibrium will be shifted toward the FC generation direction (to the right-hand side of the energy scheme in Figure 5). However, for highly efficient OPV systems with high PLQ, the continuous extraction of FCs does not appreciably decrease the PL intensity.…”

“…[ 35 ] This EA response can thus be considered as the signature of charge separation and polaron formation. [ 24 ] Due to the sufficient driving force for electron transfer, we expect to see this EA response in the early time scale.…”

“…The charge separation of longlived CT states is found to be thermally activated, and takes a considerably longer time to complete than conventional fullerene devices. [22][23][24] The charge dissociation process thus needs to outcompete detrimental charge recombination for efficient charge generation, resulting in a trade-off between the energy offset and the efficient charge generation efficiency. [24][25][26] Consequently, the systems with small offsets exhibit relatively low charge generation efficiency and FFs.…”

“…[22][23][24] The charge dissociation process thus needs to outcompete detrimental charge recombination for efficient charge generation, resulting in a trade-off between the energy offset and the efficient charge generation efficiency. [24][25][26] Consequently, the systems with small offsets exhibit relatively low charge generation efficiency and FFs. Therefore, understanding the mechanism of energetically unfavorable systems and breaking the trade-off to achieve high V OC , J SC , and FF simultaneously is a key issue in further improving the performance of OPVs.…”

In Situ Visualization and Quantification of Electrical Self‐Heating in Conjugated Polymer Diodes Using Raman Spectroscopy

“…[25,26] One important, yet often neglected, factor contributing to performance loss in organic electronics is self-heating, whereby electrical energy is converted to thermal energy. Self-heating may originate from three main phenomena: 1) Joule heating [27][28][29][30][31] occurs when electrical energy is dissipated as heat because of the flow of current, 2) recombination heating results when electrical energy that is lost during recombination processes is absorbed by lattice phonons, [30,32,33] and 3) Peltier heating and cooling can take place due to current flow at material hetero-junctions. [34] These processes have been studied in the context of thermal management for operational inorganic devices, [34][35][36] but have so far received less attention in the field of organic electronics.…”