Hybrid inorganic-organic tandem solar cells for broad absorption of the solar spectrum.

We report the first hybrid tandem solar cell with solution processable active layers using colloidal PbS quantum dots (QDs) as the front subcell in combination with a polymer-fullerene rear subcell. Al/WO3 is introduced as an interlayer, yielding an open circuit voltage (VOC) equal to about 92% of the sum of the VOC of the subcells. The device exhibits a power conversion efficiency of 1.8%. Optical simulations of various tandem configurations show that combining PbS QDs with small-bandgap polymers is a promising strategy to obtain tandem solar cells with a very broad absorption range and a high short circuit current.

Temperature dependence of exciton diffusion in conjugated polymers.

The temperature dependence of the exciton dynamics in a conjugated polymer is studied using time-resolved spectroscopy. Photoluminescence decays were measured in heterostructured samples containing a sharp polymer-fullerene interface, which acts as an exciton quenching wall. Using a 1D diffusion model, the exciton diffusion length and diffusion coefficient were extracted in the temperature range of 4-293 K. The exciton dynamics reveal two temperature regimes: in the range of 4-150 K, the exciton diffusion length (coefficient) of approximately 3 nm (approximately 1.5 x 10 (-4) cm2/s) is nearly temperature independent. Increasing the temperature up to 293 K leads to a gradual growth up to 4.5 nm (approximately 3.2 x 10 (-4) cm2/ s). This demonstrates that exciton diffusion in conjugated polymers is governed by two processes: an initial downhill migration toward lower energy states in the inhomogenously broadened density of states, followed by temperature activated hopping. The latter process is switched off below 150 K.

Ultrafast formation of nonemissive species via intermolecular interaction in single crystals of conjugated molecules.

We investigate the influence of interchain interactions on the photoluminescence processes in a sexithiophene single crystal by applying hydrostatic pressure. We perform transient photoluminescence spectroscopy in the time domain of 100 fs for pressures up to 60 kbar. The combined use of steady-state and time-resolved optical spectroscopies allows us to show that the pressure-induced quenching of the photoluminescence is caused by an ultrafast (approximately 100 fs) formation of intermolecular species.