We have used a femtosecond-resolved spectroscopic technique based on the Stark effect (electromodulated
differential absorption) in order to investigate free charge generation and charge drift in solar cell devices of
neat conjugated polymer pBTTT and in its 1:1 (by weight) blend with PCBM. In the latter, the fullerene
molecules intercalate between the polymer side-chains, yielding a co-crystal phase. Our results show that free
charge generation in both materials is ultrafast and strongly dependent on the applied reverse bias. Charge
drift to the electrodes (under strong reverse bias) occurs with comparable dynamics on the 1.2 ns time scale
for neat pBTTT and the blend, and is probably dominated by hole transport within/between polymer chains.
In this work, we study the nature of long-lived photoexcitations in intercalated, partially and
predominantly non-intercalated semicrystalline poly(2,5-bis(3-tetradecyl-thiophen-2-yl)thieno
[3,2,-b]thiophene) (pBTTT):phenyl-C61 -butyric acid methyl ester (PC61BM) blend films by
quasi-steady-state photoinduced absorption (PIA) spectroscopy. We find that polarons are generated in
these microstructures. However, the polarons generated in partially and predominantly non-intercalated
films (1.7 eV) are at higher energy than in intercalated film (1.4 eV). After comparing with the polaron
generation in neat pBTTT polymer film, we propose that the polarons generated in partially and
predominantly non-intercalated film are delocalized charges, and the polarons generated in intercalated
film are localized charges. Furthermore, we also find that the polarons generated in the partially
non-intercalated film have the longest lifetime.
Polymeric semiconductors such has regioregular poly(3-hexylthiophene) have electronic proprieties that can be tuned by proper control of the solid-state microstructure. We process thin films of P3HT of different molecular weight ranging from 2 kg/mol to 341 kg/mol. The polymer undergo a transition from a paraffinic, non-entangled microstructure to a two-phase microstructure defined by entangled chains embedded in amorphous regions at around 50 kg/mol. We observe an abrupt decrease in the intermolecular coupling from an average of ~20 meV for molecular weight below 50 kg/mol to ~5 meV above 50 kg/mol. We assign this decrease in the interchain coupling and associated free-exciton bandwidth at higher molecular weight to a transition from a one-phase morphology to a two-phase morphology defined above. In steady-state photoluminescence, we associate the lower Huang-Rhys factors at higher molecular weight to more planar backbone.
The formation of a well-defined, reproducible ZnO nanorod scaffold for hybrid photovoltaic applications has been investigated. A standard hydrothermal growth method was used and the influence of chemical additions in controlling length, width, density, and orientation was studied. The nanostructures prepared have been characterized by scanning electron microscopy, x-ray diffraction, UV-visible spectroscopy in addition to measurement of the wetting behavior. A standard procedure for the production of vertically orientated nanorods with a narrow size distribution, high areal density, and good wettability in aqueous solutions is presented.