We used a variety of optoelectronic techniques such as broadband fs transient and cw photomodulation spectroscopies, electroabsorption, and short-circuit photocurrent in bulk heterojunctions organic solar cells for studying the photophysics in pi-conjugated polymer-fullerene blends with below-gap excitation. In contrast to the traditional view, we found that below-gap excitation, which is incapable of generating intrachain excitons, nevertheless efficiently generates polarons on the polymer chains and fullerene molecules. The polaron action spectrum extends deep inside the gap as a result of a charge-transfer complex state formed between the polymer chain and fullerene molecule. With appropriate design engineering the long-lived polarons might be harvested in solar cell devices.

We investigate trap-state passivation by addition of ultra-low amounts of n-dopants in organic field-effect transistors (OFET) made of as-received and purified fullerene C60. We find a strong dependence of the OFET threshold voltage (VT) on the density of traps present in the layer. In the case of the unpurified material, VT is reduced from 17.9 V to 4.7 V upon trap passivation by a dopant:C60 ratio of ∼10−3, while the Ion/off current ratio remains high. This suggests that ultra-low doping can be used to effectively compensate impurity and defect-related traps.

The development of efficient organic light-emitting diodes (OLED) and organic photovoltaic cells requires control over the dynamics of spin sensitive excitations. Embedding heavy metal atoms in π-conjugated polymer chains enhances the spin-orbit coupling (SOC), and thus facilitates intersystem crossing (ISC) from the singlet to triplet manifolds. Here we use various nonlinear optical spectroscopies such as two-photon absorption and electroabsorption in conjunction with electronic structure calculations, for studying the energies, emission bands and ultrafast dynamics of spin photoexcitations in two newly synthesized π-conjugated polymers that contain intrachain platinum (Pt) atoms separated by one (Pt-1) or three (Pt-3) organic spacer units. The controllable SOC in these polymers leads to a record ISC time of <~1 ps in Pt-1 and ~6 ps in Pt-3. The tunable ultrafast ISC rate modulates the intensity ratio of the phosphorescence and fluorescence emission bands, with potential applications for white OLEDs.

We demonstrate a long-sought reliable method for determining the important branching ratio between photogenerated charged polarons and neutral excitons in -conjugated polymer films and solutions, using femtosecond transient photomodulation spectroscopy with broad spectral range from 0.14 to 2.7 eV. We found that both excitons and polarons are instantaneously photogenerated, but critically depends on the film nanomorphology, which ultimately controls the interchain coupling strength. We also found that a correlation exists within each polymer family between the obtained value, photoluminescence quantum efficiency, and the transient polarization memory lifetime; where the interchain coupling strength in the film determines them all. We show that varies from less than 1% in solutions and glassy films of poly͑p-phenylene-vinylene͒ derivatives, where the polymer chains are relatively isolated; to more than 30% in ordered films that contain lamellae, such as regio-regular poly͑3-hexyl-thiophene͒. Our results may serve for matching polymers to specific device applications, where polymers with large values are good candidates for photodetector and photovoltaic applications, whereas those with small values are more suitable for active layers in organic light emitting devices.

We measured the picoseconds (ps) transient dynamics of photoexcitations in blends of poly (3-hexyl-thiophene) [P3HT] (donors-D) and fullerene [PCBM] (acceptor-A) using the transient pump/probe photomodulation technique in an unprecedented broad spectral range from 0.25 to 2.5 eV; and compared the results with organic solar cell performance based on the same blends.In D-A blends with maximum domain separation such as regio-regular P3HT/PCBM with (1.2:1) weight ratio having solar cell power conversion efficiency of ~4%, we found that although the photogenerated intrachain excitons in the polymer nano-domains decay within ~10 ps, no charge polarons are generated on their expense up to ~ 2 ns. Instead, there is a build-up of chargetransfer (CT) excitons at the D-A interfaces having the same kinetics as the exciton decay, which dissociate into separate polarons in the D and A domains at a much later time (>>1 ns). This 'two-step' charge photogeneration process may be typical in organic bulk heterojunction cells.Although the CT excitons are photogenerated on the exciton expense much faster in D-A blends having smaller domain size such as in regio-random P3HT/PCBM, their dissociation is less efficient because of larger binding energy. This explains the poor solar cell power conversion efficiency (<0.1%) based on this blend. Our results support the 'two-step' charge photogeneration mechanism in polymer/fullerene blends, and emphasize the important role of the CT binding energy in generating free charge polarons in organic solar cells. For the polarized transient photomodulation spectroscopy we used the femtoseconds (fs) twocolor pump/probe correlation technique with two different pulsed laser systems based on a Ti:Sapphire oscillator [26]. These are: a low-power (energy/pulse ~0.1 nJ) high repetition rate (~80 MHz) pulsed laser system for the mid-IR spectral range; and a high power (energy/pulse ~10 µJ) low repetition rate (~1 kHz) pulsed laser system for the near-IR/visible spectral range. The pump excitation photon energy, ω(pump) was set at 3.1 eV for above-gap and 1.55 eV for below-gap excitation, respectively. The pulse energy flux on the film was adjusted so that the initial was modulated at frequency, f=40 kHz and the changes, ΔT in the probe transmission, T were measured using a phase-sensitive technique. For the transient near-IR/visible spectroscopy measurements we used white light super-continuum as probe, spanning the spectral range from 1.15 to 2.5 eV; the pump modulation frequency here was synchronized with the laser rep. rate.The transient photomodulation signal, ΔT/T is positive for photoinduced absorption (PA) and negative for photo-bleaching and stimulated emission. ΔT(t)/T was measured using a computer controlled translation stage up to 2 ns, with time resolution of ~150 fs set by the pump/probe cross-correlation. ΔT(0)/T spectra from the two laser systems were normalized to each other at several wavelengths in the near-IR/visible spectral range [26].Since some photoexcitations may live longer than...