Detection of Ultralow Concentrations of Non-emissive Conjugated Polymer Aggregates via Fluorescence Correlation Spectroscopy

Abstract

The aggregation of conjugated polymers in common organic solvents is investigated using fluorescence correlation spectroscopy (FCS), burst analysis, and microscopy. Poly(3-hexylthiophene) and poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] are both shown to form weakly bonded non-emissive aggregates in toluene that persist even at picomolar concentrations. These aggregates decrease the bulk emission intensity in solution but do not affect the fluorescence spectra or lifetimes, consistent with a static quenching mechanism. Passing the solutions through a syringe filter causes an increase in the number of emitters as measured by FCS, indicating that this process dissociates the aggregates. Films cast from solutions that have been filtered are more uniform and significantly more emissive than those made from unfiltered solutions. These results show that FCS is a highly sensitive probe of non-emissive aggregates in solution that have a deleterious effect on the emission properties and overall quality of spin-cast thin films, even at sub-nanomolar concentrations.

Role of Triplet-State Shelving in Organic Photovoltaics: Single-Chain Aggregates of Poly(3-hexylthiophene) versus Mesoscopic Multichain Aggregates

“…We conclude that it is highly unlikely that a multi-chain aggregate would be missed in this analysis due to transient quenching and charging effects. 1 Therefore, the number of fluorescent spots before and after solvent vapor annealing is a good estimate for the average number of chains incorporated in a multi-chain aggregate, which yields ~20 chains per aggregate and is slightly higher compared to the minimum number extracted from the average PL intensity (see Figure S1). , incorporated in each aggregate can be extracted, as stated in the panels.…”

“…SVA was performed for 30 minutes on the microscope setup using a gas-flow chamber with a mixture of 95 % acetone and 5 % chloroform vapor to induce aggregation. 11 for a more detailed reasoning of this estimate 12 ). The average spectrum of 50 aggregates and a histogram of the 0-0 peak positions of the individual spectra is given in Figure 1d (red).…”

“…Triplet excitons play a major part in degradation processes and loss mechanisms in the bulk heterojunctions of organic photovoltaics. 1 For instance, triplet excitons can efficiently be quenched by ground-state triplet oxygen, which subsequently forms highly reactive singlet oxygen and irreversibly destroys the functionality of the active material. 2 In addition, singlet-triplet annihilation can limit the lifetime of singlet excitons in the organic material, which potentially also reduces the efficiency of organic solar cells.…”

Molecular weight dependence of carrier mobility and recombination rate in neat P3HT films

“…At very low molecular weights P3HT forms a one‐phase, paraffinic structure comprised of unconnected, chain‐extended crystallites. As the chain length increases the polymer chains become entangled in solution, and these morphological structures are transferred to the film during the casting process, leading to amorphous interlamallar regions that interconnect the crystalline domains (see Fig. ) .…”

Unraveling the Contribution of Residual Monomer to the Emission Spectra of Poly(3-hexylthiophene) Aggregates: Implications for Identifying H- and J-type Coupling

“…In the optoelectronics field, regioregular poly­(3-hexylthiophene) (rr-P3HT, Chart ) is a benchmark polymer that has been incorporated into devices such as transistors − and photovoltaics. − As is the case for most conjugated polymers, rr-P3HT will aggregate in a highly concentrated solution or in the solid state if nothing preemptive is done such as filtering the solution or diluting the polymer into a host matrix. , The standard means of determining the degree of aggregation and the type of chain packing is via analysis of absorption/emission spectra derived from the well-known model of small-molecule aggregation of Kasha …”