Role of Disorder in the Extent of Interchain Delocalization and Polaron Generation in Polythiophene Crystalline Domains

Abstract

To understand how disorder within conjugated polymer aggregates influences the polaron generation process, we investigated poly(3-hexylthiophene) (P3HT) and a congeneric random copolymer incorporating 33 mol % substituent-free thiophene units (RP33). Steady-state absorption and fluorescence spectra showed that increasing the intrachain torsional disorder in aggregates increases the energy and breadth of the density of states (DOS). By extracting polaron dynamics in the transient absorption spectra, we found that an activation energy barrier of 0.05 eV is imposed on the charge separation process in P3HT, whereas that in RP33 is essentially barrierless. We also found that a significant amount of excitons in P3HT are deactivated by traps, while no trapped excitons are generated in RP33. This efficient polaron generation in RP33 was attributed to the excess energy and enhanced interchain delocalization of precursor states provided by the intrachain torsional disorder and the close-packing structure in the absence of hexyl substituents.

The Importance of Microstructure in Determining Polaron Generation Yield in Poly(9,9-dioctylfluorene)

“…Increased charge carrier mobility has also been measured in better-connected two-phase systems of P3HT, − further highlighting the profound influence molecular arrangement has on optoelectronic properties. Reports have also highlighted the importance of electronic delocalization for enhanced polaron generation. − The width of the energetic density of states (DOS, also termed energetic disorder), resulting from conformational variation within populations of polymer chains, together with local effects such as proximity to polar groups and surfaces, has been identified as another critical factor controlling charge generation in neat materials . Recent theoretical simulations and modeling of D–A heterojunctions further suggest that charge separation is facilitated by or as a result of energetic disorder, with a broader density of states (higher degree of energetic disorder) decreasing the activation energy barrier to charge separation and hence increasing the separation probability. − …”

Influence of Intramolecular Charge-Transfer Characteristics of Excitons on Polaron Generation at the Donor/Acceptor Interface in Polymer Solar Cells

“…On the other hand, the generation of free charge carrier from the relaxed and/or thermalized CT state demands thermal activation because of its large binding energy and thus the charge separation rate of the relaxed and/or thermalized CT state exhibits distinct temperature dependency. That is, at low temperatures, the lack of thermal energy leads to a slower charge separation rate than at high temperatures. , Thus, the charge separation pathway according to the CT state energy can be distinguished through the analysis of the temperature dependence of charge generation dynamics. To focus our discussion on the effect of hot and delocalized state on charge separation in BHJs, temperature-dependent TA measurements were conducted using PTB7:NIDCSEO3 and P3HT:NIDCSEO3 blend films at 77 and 294 K. Figure shows the TA spectra and donor polaron dynamics of PTB7:NIDCSEO3 and P3HT:NIDCSEO3 blend films upon selective photoexcitation of the donor polymers at 670 and 610 nm, respectively.…”

“…In polymer solar cells (PSCs), photoinduced charge separation is a very important step among the whole operating processes, and understanding the mechanism of charge separation is crucial for obtaining high-performance PSCs . It has been proposed that charge separation is affected by several factors, such as the energetics and disorder at the donor and acceptor (D/A) interface, charge delocalization, and blend morphology of the bulk heterojunction (BHJ). − In particular, the role of the interfacial charge-transfer (CT) state in charge generation has drawn substantial interest in understanding the mechanism of converting excitons into free charge carriers. The CT state, consisting of Coulombically bound electron and hole pair, is the result of photoinduced CT at the interface of donor and acceptor materials. , Recently, it has been proposed that charge separation is greatly influenced by the energetics of CT states, leading to different charge separation pathways through hot or relaxed (cold) CT states.…”

“…Several researchers have reported efficient charge separation through the electronically and/or vibrationally excited and delocalized CT state. − The hot CT state refers to a loosely bound and delocalized electron–hole pair, facilitating the charge separation at the D/A interface. Numerous experiments have suggested that the sufficient energy of the hot CT state is beneficial for overcoming the Coulomb barrier and avoiding geminate recombination. ,,− For instance, Grancini et al reported that efficient charge separation occurs before the relaxation within CT states in the PCPDTBT/PC 60 BM blend with the aid of a hot and delocalized CT state, and Bakulin et al also offered clear evidence of exciton dissociation through hot CT states, enabling long-range charge separation. , The hot charge separation process was also supported by the temperature-independent polaron generation dynamics. , However, another scenario for charge separation in PSCs is also proposed stating that the lowest (cold) CT state acts as a precursor for the charge separation because of the fast internal conversion/relaxation process, where the hot CT state is no longer important. Excitation energy-independent internal quantum efficiency (IQE) of the photovoltaic device supports the hypothesis that excess energy is not necessary for efficient charge separation and emphasizes the importance of cold CT state dissociation. − Thus, the role of the hot CT state in charge separation is still unclear and a subject of the ongoing debate.…”

“…9,12 The hot charge separation process was also supported by the temperature-independent polaron generation dynamics. 4,10 However, another scenario for charge separation in PSCs is also proposed stating that the lowest (cold) CT state acts as a precursor for the charge separation because of the fast internal conversion/relaxation process, where the hot CT state is no longer important. Excitation energy-independent internal quantum efficiency (IQE) of the photovoltaic device supports the hypothesis that excess energy is not necessary for efficient charge separation and emphasizes the importance of cold CT state dissociation.…”