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

In thin-film photovoltaics, such as organic and perovskite solar cells, charge extraction selectivity is crucial. In order to improve selectivity, charge transporting layers (doped and undoped) are frequently used; 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 study clarifies how well charge transporting layers with varying majority carrier conductivities block minority carriers. The charge extraction by a linearly increasing voltage technique is used to determine the surface recombination velocity of minority carriers in model system devices with varying majority carrier conductivity in the transporting layer. The 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 bimolecular recombination across the transporting layer-absorber layer interface. Design principles are furthermore discussed and proposed to achieve selective charge extraction in thin-film solar cells using charge transporting layers.

Automated summary

This study determines the surface recombination velocity of minority carriers in model system devices with charge transporting layers of varying majority carrier conductivities. It clarifies that high conductivity in the transporting layer does not effectively block minority carriers. Design principles for achieving selective charge extraction in thin-film solar cells using charge transporting layers are proposed.