The Working Principle of Integrated Perovskite-Organic Solar Cells

In recent years multi-junction devices comprising perovskite and organic solar cells gained increasing interest, as a potentially more stable alternative to all-perovskite devices.[1,2] In parallel with the rise of perovskite-organic tandem solar cells, so called “integrated” perovskite-organic solar cells have garnered interest in the community with reported efficiencies exceeding 24%.[3] In this type of devices, the organic bulk-heterojunction (BHJ) is directly processed on top of the perovskite wide-gap cell without any charge transport or interconnecting layers in between. While the principle of operation for tandem solar cells is well-known and widely accepted, there is still a lack of sound fundamental understanding of how these integrated perovskite-organic devices work. Although the J-V characteristics often exhibit a strong s-shape, numerous studies have reported an extension of the external quantum efficiency into the near infrared by adding a BHJ on top of the perovskite absorber. Indeed, these integrated devices seem to operate inversely to a serially connected tandem solar cell, as instead of the voltage, the current is added, which somewhat contradicts the intuitive understanding of how a tandem solar cell with sub-cells connected in series would behave and seemingly violates some very basic rules of thermodynamics. For a direct comparison we fabricated perovskite-organic tandems as well as integrated cells with the similar choice of perovskite and BHJ and similarly reproduced the fundamentally different behaviours of both concepts.<br/><br/>In this work, we provide for the first time a conclusive description of the working mechanism of integrated perovskite-organic solar cells. In stark contrast to real tandem solar cells, where the sub-cells are connected in series, we reveal that integrated devices comprise two solar cells connected in parallel. We introduce a simple equivalent circuit model that includes the parallel connection and an extraction barrier at the interface between the narrow-gap absorber and the wide-gap perovskite, to account for the offset between the narrow- and wide-gap absorber energy gap. Our model is able to not only reproduce the characteristics of devices built in our lab almost ideally but it can also explain multiple reports from the literature. After validation through subcell-selective characterization, photoelectron spectroscopy, and complementary drift-diffusion simulations, we use our model to evaluate the general prospects of integrated devices.<br/>We demonstrate that the extraction barrier present in the organic subcell is crucial for the functionality of the device. Reducing this barrier, aside from being thermodynamically questionable, would deteriorate the performance of the integrated device. Finally, we conclude that integrated devices most likely do not share the prospects of tandem devices to overcome the detailed-balance limit. Instead, integrated devices should be considered single-junction devices that may benefit from enhanced passivation, charge extraction, and transport through the bulk-heterojunction processed on top.<br/><br/>1. Brinkmann et al. Nature 604, 280 (2022)<br/>2. Brinkmann et al. Nature Reviews Materials 9, 202 (2024)<br/>3. Zhou et al. Adv. Mater. 34, 2205809 (2022)