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Device structure. Devices A, B, and C correspond to the layer structure shown here, with the thickness of the nanocrystal layer varying from 160 down to 80 nm. In device D there was no P3OT layer.
External quantum efficiency as a function of wavelength for all four devices. At 1260 nm the external quantum efficiency decreases as the thickness of the nanocrystal layer is reduced. As the wavelength of the incident light is decreased, the external quantum efficiency of bilayer device B increases and overtakes that of bilayer device A (thickest) and the pure nanocrystal device D. The inset shows the absorption spectrum of P3OT. P3OT does not absorb in the wavelength region shown in the main figure.
Energy band diagram. On the left is the published (see Ref. 8) lowest unoccupied molecular orbital and HOMO levels for P3OT. On the right, using the same vertical vacuum reference, the first and second quantum-confined energy levels of the nanocrystals, calculated from the nanocrystals’ absorption spectrum and in the approximation that the hole and electron effective masses are similar.
Measured performance of devices A–D at 1260 nm (center of first exciton peak) illumination. The thickness of the P3OT layer was kept fixed in devices A–C. The incident optical power at 1260 nm was .
Measured external (EQE) and calculated internal (IQE) quantum efficiency at the first (1260 nm) and the onset of the third exciton peaks (720 nm). The internal quantum efficiency was calculated by dividing the external quantum efficiency by the single-pass absorbance of the active layer at that wavelength.
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