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(a) Schematic band diagram at of a typical bipolar device, ignoring band bending. Gray areas indicate metallic contacts; in the used devices 1 is PEDOT:PSS and 2 is either Al or . (b) At the same bias as under (a), band bending due to carrier diffusion from the contacts results in a nonzero field in the bulk. In order to achieve flatband in the bulk again, the bias must be reduced from to (c). The thick dashed lines indicate the electron and hole quasi-Fermi levels, see text.
(Color online) Calculated potential (left axis) and density (right axis) vs for different temperatures. The metallic contact is located at . Used parameters: , , and .
(Color online) vs for all studied devices. Symbols indicate experimental data, lines are model calculations, using the parameters given in Table I, and (Ref. 6). ∎: ; ●: blend, no LiF; ▴: ; ▾: ; ◆: PCBM, no LiF ; ◀: MDMO-; ▶: MDMO-PPV, no LiF. The inset shows typical (in )vs (in volts) curves of a blend sample for (top) to (bottom). Arrows indicate the position of .
Parameters used in the model calculations. is calculated as with given by Eq. (4), the crossover depth (see text), and the boundary condition for the electron/hole contact from Eq. (5). The uncertainty in all energies is around . In the calculations, the values of are reduced by with the broadening of the density of states (DOS) to account for the fact that the transport level in a Gaussian DOS lays below its center (Ref. 6). For the same reason, the obtained values for have to be regarded as lower limits. For MDMO-PPV and for PCBM (Ref. 6).
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