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%).