Isomeric diammonium passivation for perovskite-organic tandem solar cells

In recent years, perovskite has been widely adopted in series-connected monolithic tandem solar cells (TSCs) to overcome the Shockley-Queisser limit of single-junction solar cells. Perovskite-organic TSCs, comprising a wide-bandgap (WBG) perovskite solar cell (pero-SC) as the front cell and a narrow-bandgap organic solar cell (OSC) as the rear cell, have recently drawn attention owing to the good stability and potential high power conversion efficiency (PCE)1-4. However, WBG pero-SCs usually exhibit higher voltage losses than regular pero-SCs, which limits the performance of TSCs5,6. One of the main obstacles comes from interfacial recombination at the perovskite-C60 interface, and it is important to develop effective surface passivation strategies to pursue higher PCE of perovskite-organic TSCs7. Here we exploit a new surface passivator cyclohexane 1,4-diammonium diiodide (CyDAI2), which naturally contains two isomeric structures with ammonium groups on the same or opposite sides of the hexane ring (denoted as cis-CyDAI2 and trans-CyDAI2, respectively), and the two isomers demonstrate completely different surface interaction behaviours. The cis-CyDAI2 passivation treatment reduces the quasi-Fermi-level splitting-open circuit voltage (Voc) mismatch of the WBG pero-SCs with a bandgap of 1.88 eV and enhanced its Voc to 1.36 V. Combining the cis-CyDAI2-treated perovskite and the organic active layer with a narrow bandgap of 1.27 eV, the constructed monolithic perovskite-organic TSC demonstrates a PCE of 26.4% (certified as 25.7%). A new surface passivator cyclohexane 1,4-diammonium diiodide naturally contains two isomeric structures with ammonium groups on the same or opposite sides of the hexane ring, and the two isomers demonstrate completely different surface interaction behaviours.