Charge Photogeneration Dynamics In The Photovoltaic Blends Of P3HT And PFDTBT With Fullerenes

The recent development of polymer solar cell （PSC） has achieved a powerconversion efficiency （PCE） exceeding9%, yet PSCs still have significantmargins for further improvement, which is needed for practical applications.Accordingly, it is important to deepen the understandings in the light conversionmechanisms, which may be helpful for revealing the critical PCE-limitingfactors. This thesis has investigated, by the use of visible-to-near infraredtransient absorption （TA） spectroscopy, the influence of the solvent vaporannealing （SVA） treatment, the LUMO level offset and the macroscopicelectrical field strength on the processes of primary charge photogeneration inthe photovoltaic blends of P3HT and PFDTBT with fullerenes. The results mayshed light on the design of new polymeric photovoltaic materials, and on theoptimization of device configurations.First, we investigated the primary dynamics of exciton and chargephotogeneration in the neat P3HT and the blend P3HT/PC₆₁BM films, andexamined the effects of solvent vapor annealing treatment, excitationwavelength and photon fluence on the ultrafast charge photogenerationprocesses. Spectroelectrochemistry and time-resolved spectroscopy revealedtwo different types of polarons, the delocalized and the localized ones,inhabiting the crystalline and the disordered P3HT phases, respectively. Thefingerprint absorption spectrum of anionic P3HT polaron was revealed for thefirst time to the best of our knowledge. By studying the dynamics of exciton andpolarons in P3HT/PC₆₁BM films, we found that polaron photogenerationprocesses can be categorized into the prompt and the delayed ones, andproposed the detailed mechanisms of polaron photogeneration from the P3HTcrystallites in the P3HT/PC₆₁BM blend films. Moreover, we found that thedimension of disordered intermediate between the P3HT crystallite and thePC₆₁BM aggregate of the SVA films was less than that for thermally annealedfilms, and thereby to be advantageous in minimizing the traps and/or blocks ofcharge carriers.Second, we investigated the charge recombination （CR） dynamics ofP3HT/PC₆₁BM blend films in the initial ultrafast timescale. Quantitativeanalyses of the influence of film morphology on the trap density and hole mobility in ultrafast timescale were achieved. The CR dynamics were analyzedby the use of trap filling and Langevin’s hole-limited models. We demonstratedthat, with reference to the CS2-cast, the SVA treatment improved the holemobility2～3folds and reduced the density of polymer-localized traps5foldsfor the P3HT/PC₆₁BM blend. The results are consistent with the deviceperformance including the photocurrent, the filling factor, the external quantumefficiency and the PCE, implying that the optimization of film morphology canimprove the device performance.For the narrow bandgap polymers, we examined the influence of Lowestunoccupied molecular orbital （LUMO） level offset on the chargephotogeneration processes by using PFDTBT blended with four different kindsof fullerene derivatives, tuning systematically the LUMO level offset over110～320meV. The species-associated kinetics were derived via decomposingthe time-resolved data matrices. For neat PFDTBT films, the photoexcitaitoncan generate the interchain charge transfer state and charge seperated state.However, the neutral excitons rather than the charge states dominated theprimary photoexcitation （>50%）. For PFDTBT/PC₆₁BM blend film, weestablished the mechanisms of paralleling interfacial charge transfer state （ICT）and the charge separated （CS） state formation, and found the critical role of ICT inyielding CS, redressing the sequential mechanism for this specificpolymer/fullerene. It is found that the exciton-to-CS and the ICT-to-CSconversion, respectively, obey Marcus’ nonadiabetic electron transfermechanism and Braun-Onsager’s escape probability for a coupled e-h+pair, andthat the optimized LUMO level offset for polymer/fullerene solar cells is0.33eV. The role of LUMO level offset in yielding charge species may shed light onthe development of new photovoltaic materials.Finally, we examined the influence of macroscopic electric field on thesubnanosecond charge photogeneration dynamics in PFDTBT/PC₆₁BM solar cell.The results suggested that the macroscopic electric field of thePFDTBT/PC₆₁BM solar cell was ineffective in the subnanosecond chargephotogeneration/recombination dynamics under working condition. However,under the reverse biases of2V, significant enhancement of chargephotogeneration and apparent suppression of ICT recombination were observed.