Anticorrelated Photoluminescence and Free Charge Generation Proves Field-Assisted Exciton Dissociation in Low-Offset PM6:Y5 Organic Solar Cells

Understanding the origin of inefficient photocurrent generation in organic solar cells with low energy offset remains key to realizing high performance donor-acceptor systems. Here, we probe the origin of field-dependent free charge generation and photoluminescence in non-fullerene acceptor (NFA) based organic solar cells using the polymer PM6 and NFA Y5 - a non-halogenated sibling to Y6, with a smaller energetic offset to PM6. By performing time-delayed collection field (TDCF) measurements on a variety of samples with different electron transport layers and active layer thickness, we show that the fill factor and photocurrent are limited by field-dependent free charge generation in the bulk of the blend. We also introduce a new method of TDCF called m-TDCF to prove the absence of artefacts from non-geminate recombination of photogenerated- and dark charge carriers near the electrodes. We then correlate free charge generation with steady state photoluminescence intensity, and find perfect anticorrelation between these two properties. Through this, we conclude that photocurrent generation in this low offset system is entirely controlled by the field dependent exciton dissociation into charge transfer states

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