| Description |
In this work, we used the pump-probe photomodulation (PM) spectroscopy technique to measure the transient PM spectrum and decay kinetics in various π - conjugated polymers (PCPs) films and blends with appropriate molecular acceptors. Using two ultrafast laser systems, we covered a broad spectral range from 0.25 - 2.5 eV in the time domain from 100 fs to 1 ns with 150 fs time resolution. We also used continuous wave (CW) photomodulation spectroscopy, photoluminescence, electro-absorption, doping-induced absorption, and x-ray diffraction to study the excitations and other optical properties of PCPs and polymer donor-fullerene acceptor blends. In P3HT/PCBM blend with maximum domain separation we found that although the intrachain excitons in the polymer domains decay within ~ 10 ps, no charge polarons are generated at their expense. Instead, there is a build-up of charge-transfer (CT) excitons at the donor (D) - acceptor (A) interfaces that may dissociate into separated polarons in the D and A domains at a later time. Our results elucidate the charge photogeneration mechanism in polymer/fullerene blends, and unravel the important role of the binding energy in generating free charge polarons. We also studied the photophysics of a low band gap polymer, namely poly-thienophene-benzodithiophene seven (PTB7) film and its blend with acceptor [6,6] phenyl C71 butyric acid methyl ester [PC71BM]. In the CW PM spectrum of PTB7/PC71BM blend, clear signatures of polarons are observed. Whereas PA bands of triplet excitons and trapped polarons are observed in pristine PTB7 film. In the transient ultrafast PA spectrum of PTB7/PC71BM blend, surprisingly, we found singlet exciton, charge transfer exciton and a polaron band that are generated simultaneously; this is different from the transient PM spectrum of P3HT/PCBM blend. We also focused on the photophysics of DOO-PPV with different isotopes and their blends with PCBM. The transient ultrafast PA spectra of all isotope films are dominated by a singlet exciton. Similar results were observed in the isotope/PCBM blends. As in P3HT/PCBM blend, we do not observe the charge transfer exciton and polaron band immediately after photoexcitation, but they show up in the PM spectrum at a later time. |
