Highly Conductive Charge Transport Layers Impair Charge Extraction Selectivity in Thin-Film Solar Cells

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.