We analyze the photocurrent spectra for bulk heterojunction organic solar cells having a range of active layer thicknesses. Normalized by the number of incident photons, the photocurrent peak red shifts with respect to the absorption maximum as the sample thickness increases. The shift in photocurrent peak can be understood by comparing the carrier extraction for bulk versus surface generated carriers. Fitting to the spectra provides a measure of the electron collection length and the surface enhanced recombination. The variation in the electron collection length with contacting parameters is used as a guide for optimization of the device structure. Furthermore, a photocurrent analysis has been done for different active layer thicknesses with different interfacial layers. Again the model fit for this comparison provides an estimate for carrier recombination distance that fits well with experimental data. For this study a common conjugated polymer regioregular poly(3-hexylthiophene) (P3HT) is used while poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT-PSS), P3HT and Al are used as interfacial layers.
Photo-induced charge transfer from an Indium Tin Oxide (ITO) contact into [6,6]-phenyl-C61-butyric acid methyl
ester (PC60BM) and [6,6]-phenyl-C71-butyric acid methyl ester (PC70BM) is measured. Charge transfer peaks
are observed for a series of excitation energies below the PCBM absorption edge. If charge transfer is blocked
using a tunnel barrier or an applied electric field, the peaks disappear. The observed transitions are similar to
those predicted by theoretical calculations of the absorption spectra for negatively charged C60 and C70 chains. This observation suggests that charge transfer occurs preferentially at the polaronic transition energies in the
PCBM, providing a means for polaronic state spectroscopy.
The carbon nanotube photoexcitation spectrum is dominated by excitonic transitions, rather than interband transitions
between continuum states. There are eight distinct excitonic transitions (four singlet and four triplet), each with two-fold
degeneracy. Because the triplet excitons are spin polarized with electron and hole spins both pointing in the same
direction, they are optically inactive, and optical spectroscopy has revealed no evidence for their existence. Here, we
show that by the interaction with a spin filter ferromagnetic semiconductor, photoexcitation of the carbon nanotube
triplet exciton is possible, and its contribution to the photocurrent can be detected. The perturbation provided by the spin
filter allows for inter-system mixing between the singlet and triplet excitonic states, and relaxes the spin selection rules.
This supplies the first evidence for the existence of the triplet exciton, and provides an avenue for the optical excitation
of spin polarized carriers in carbon nanotubes.