  Charge selective interlayers are crucial in thin-film photovoltaics, such as
organic and Perovskite solar cells. Charge transporting layers (doped and
undoped) constitute perhaps the most important class of charge selective
interlayers; however, it is not well understood how a charge transporting layer
should be designed in order to ensure efficient extraction of majority carriers
while blocking minority carriers. This work clarifies how well
charge-transporting layers with varying majority carrier conductivities block
minority carriers. We use the Charge Extraction by a Linearly Increasing
Voltage technique to determine the surface recombination velocity of minority
carriers in model system devices with varying majority carrier conductivity in
the transporting layer. Our results show that transporting layers with high
conductivity for majority carriers do not block minority carriers - at least
not at operating voltages close to or above the built-in voltage, due to direct
bi-molecular recombination across the transporting layer-absorber layer
interface. We furthermore discuss and propose design principles to achieve
selective charge extraction in thin film solar cells using charge transporting
layers.
