Femtosecond Time-Resolved Fluorescence Study of P3HT/PCBM Blend Films

Abstract

In order to understand the dependence of photoinduced initial processes on thermal annealing, the femtosecond time-resolved fluorescence dynamics of regioregular poly(3-hexylthiophene) (P3HT) in (thermally) annealed P3HT/[6,6]-phenyl-C61 butyric acid methyl ester (PCBM) blend films has been studied by using the fluorescence up-conversion technique. For comparison, a P3HT solution, pristine P3HT, and unannealed P3HT/PCBM blend films have been investigated as well. The fluorescence dynamics of the P3HT solution showed wavelength dependence. Excitation energy transfer between the segments and torsional relaxation possibly occurred in a time scale of several ps in the solution. Observed rise times at longer wavelength emission suggested the formation of these relatively lower emission states (at 650 and 700 nm). Charge transfer (or excitonic quenching) was the dominant process in the fs time scale with emission at 650 nm in the unannealed blend film. In the annealed blend film, the charge transfer (334 fs) and downhill relaxation (942 fs) of self-trapped (dynamic localized) excitons were competitive processes due to the well aligned nanodomains in the P3HT/PCBM blend films. There were different charge transfer rates at different excited states (650 and 700 nm) in the annealed film. The charge transfer process occurred faster at a lower excited state, and a stronger electronic and vibrational coupling in the annealed P3HT/PCBM films was revealed within these measurements as well. The ultrafast anisotropy decays suggested that a strong and ultrafast reorientation of the molecular dipole moments occurred at excited states. The anisotropy decay was mainly determined by the ultrafast process, whereas the energy could continuously migrate along or between P3HT chains in a time scale of ∼100 ps. The ultrafast process suggested that there was an excitation delocalization associated with vibrational modes, as was consistent with the observation from steady-state measurements. On the basis of the understanding of the mechanisms above, the optimized cell performance has been established.

Excited-State Photophysics in a Low Band Gap Polymer with High Photovoltaic Efficiency

“…It should be noted that electron transfer on this time scale has been observed in related systems. 50,61,66,81,86,87,89,90,95 Fifth, Figures 8 and 9 show that both fluorescence quenching and TG spectroscopy observe an increase in charge separation efficiency with increasing PCBM concentration. This increase in charge separation is attributed to a reduction in the average distance between PCBM domains, which in turn suppresses the radiative deactivation of photoexcitations in the polymer domains.…”

“…Because the 120 fs component is too fast to represent incoherent energy transfer, we assign it to self-trapping of the electronic excitation (i.e., contraction of the exciton size) caused by nuclear relaxation. 39, [60][61][62][63][64] In addition, we suggest that the 0.98 ps process represents a mechanism of energy transfer distinct from that corresponding to the ∼7.2 ps time constant. For example, the origin of the two time constants may be geometric in nature.…”

“…This finding is consistent with several investigations of BHJ materials possessing fullerene acceptors, where it has been concluded that electron transfer occurs on the subpicosecond time scale when photoexcitations in the polymer are initiated near the interface with the PCBM domain. 22,61,66,81,82,[86][87][88][89][90] It is difficult to conclusively interpret the dynamics on longer time scales because charge separation becomes convoluted with the diffusion of electronic excitations in the polymer. Therefore, based on the present measurements we propose only two strong conclusions: (i) the increase in TG signal amplitude at long delay times is a signature of charge separation and (ii) a significant amount of separated charge is accumulated in less than 500 fs.…”

Ultrafast Relaxation of the Poly(3-hexylthiophene) Emission Spectrum

“…This is in qualitative agreement with previous FU investigations of P3HT in chloroform solution (using lower molecular weight polymer and other excitation wavelengths). 16,37 Unlike those studies, many more emission wavelengths covering the entire spectrum were recorded here, and we chose to analyze the dynamics globally using the sum of four exponential functions. Time constants of τ 1 = 0.7 ps, τ 2 = 6.0 ps, τ 3 = 41 ps, and τ 4 = 530 ps were found, and the spectra of the pre-exponential amplitude factors are shown in Figure 3A.…”

Connecting physical properties of spin-casting solvents with morphology, nanoscale charge transport, and device performance of poly(3-hexylthiophene):phenyl-C₆₁-butyric acid methyl ester bulk heterojunction solar cells

“…[17][18][19][20][21][22][23][24][25][26] In addition, the morphology of the active layer was also found to be dependent on the donor polymer material which can be controlled using choice of solvent and additives. [27][28][29] Among these methods, an effective control of blend morphology can be achieved through the use of appropriate spin-casting solvents for DA components.…”