Highly conductive charge transport layers impair charge extraction selectivity in thin-film solar cells

TL;DR

This study investigates how highly conductive charge transport layers in thin-film solar cells can impair charge extraction selectivity, revealing that high conductivity layers fail to block minority carriers at operating voltages, affecting device efficiency.

Contribution

The paper provides new insights into the design of charge transporting layers, showing that high conductivity layers do not effectively block minority carriers at typical operating voltages.

Findings

High conductivity layers do not block minority carriers at operating voltages.

Direct bi-molecular recombination occurs across interfaces with highly conductive layers.

Design principles for selective charge extraction are proposed.

Abstract

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.