We measure the transient photocurrent of APFO3:PCBM bulk heterojunction solar cells illuminated with ns-laser and sub-ms LED light sources. The ratio of the number of collective charges to the number of excited photon (external quantum efficiency, EQE) and the transient photocurrent fall times have been carried out with difference pulse durations and fluences. The EQEs characterized by ns-laser source are shown to obey the bimolecular recombination at high excitation fluences. The increasing of transient photocurrent fall times suggests that the fall times of free charge carriers are effected by deep trap density of state (DoS) and thus the free charge carriers have a sufficient time for bimolecular recombination at short circuit condition. At the same fluences, however, the EQEs characterized by sub-ms LED sources exhibit an excitation fluences independence of EQE. The transient photocurrent fall times with sub-ms LED sources are rather constant when the excitation fluences increases indicating that the deep trap DoS has less effect at short circuit condition for longer pulse duration.
Charge separation dynamics in a Ladder-Type Methyl substituted Poly(Para-Phenylene) (m-LPPP) was investgiated by means of electric filed modulated femtosecond pump-probe absorption spectroscopy. The Stark shift of the absorption band modified by created excitons and electron-hole pairs provided information about the average intrapair charge separation distance. The electron and hole initially separated by ca. 7 Å moved away from each other to more than 30 Å during 600 ps and later the separation rate decreased. Neither the initial separation distance, nor that obtained at 600 ps were found to depend on the applied electric field, only the separation rate at stronger field was slightly faster.
Investigation of the photoinduced processes in pure C60 films was performed by means of sub-100 fs resolution spectrometer. Excitation by 100 fs laser pulses with photon energy above and below the mobility edge revealed that both charged and neutral components appear during the excitation pulse. At both excitation wavelengths electrons and holes are generated by direct optical excitation and not due to singlet-singlet annihilation. Anions are created with a delay of 10-13 - 10-11s in a result of electrons capture by C60 molecules.
Investigation of the photoinduced processes in pure C60 films and in C60 films doped by Ti or Sn was performed by means of sub-100 fs resolution spectrometer. Excitation by 100 fs laser pulses with photon energy above and below the mobility edge revealed that both charged and neutral components appear during the excitation pulse. At both excitation wavelengths electrons and holes are generated by direct optical excitation and not due to singlet-singlet annihilation. Anions are created with a delay of 10-13 - 10-11 s in a result of electrons capture by C60 molecules. Compared to the pure fullerene this process is faster for Sn-doped and slower for Ti-doped sample. Oxygen uptake in the film is mainly responsible for the observed characteristic features of the relaxation in Ti-doped C60. For Sn-doped C60 the yield of charge carriers photogeneration is noticeably larger than for pure C60. Non-monotonic relaxation, observed in the former case, is explained by the charge exchange between C60 anion and metal.
We have used approximately 40 fs pulses from a Ti:sapphire laser to investigate the ultrafast energy transfer and relaxation dynamics in the light-harvesting pigment-proteins of photosynthetic purple bacteria. Our results reveal two new features, not earlier observed in light-harvesting pigments: (1) ultrafast relaxation characterized by a time constant of approximately 20 fs, which we interpret as interexciton state relaxation; (2) coherent nuclear motions. These new results are challenging the applicability of the `standard' Forster theory to the excitation transfer in photosynthetic antennae which motivated us to examine a more detailed description of the dynamics.
With infrared transient absorption spectroscopy we have studied how energy migrates through the light-harvesting antenna of photosynthetic purple bacteria, and how the energy is trapped by the reaction center. In Bchl a-containing purple bacteria the light-harvesting (LH) antenna is highly heterogeneous, consisting of several spectroscopically distinct pigments. Energy transfer among these pigments occurs on several different time-scales. The overall trapping time at room temperature is about 60 ps, while energy transfer from the high-energy to the low-energy pigments of the lightharvesting antenna only takes 10 ps (measured at 77 K). This implies that trapping is relatively slow, 35 ps at 77 K and probably not much faster at room temperature. Energy transfer among neighboring Bchl molecules within a pigment-protein complex occurs on the 1 ps to subpicosecond time scale. For the Bchl b-containing purple bacterium Rps.viridis the results show that the antenna is homogeneous, and trapping can approximately be described by a randomwalk model, provided that the light-harvesting antenna and reaction center are more tightly coupled than in the Bchl a containingpurple bacteria.