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)^1,2,3,4. However, WBG pero-SCs usually exhibit higher voltage losses than regular pero-SCs, which limits the performance of TSCs^5,6. One of the major 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 behaviors. The cis-CyDAI₂ passivation treatment reduces the Quasi–Fermi level splitting (QFLS)–open circuit voltage (V_oc) mismatch of the WBG pero-SCs with a bandgap of 1.88 eV and enhanced its V_oc to 1.36 V. Combining the cis-CyDAI₂ treated perovskite and the organic active layer with a narrow-bandgap of 1.24 eV, the constructed monolithic perovskite/organic TSC demonstrates a PCE of 26.4% (certified as 25.7%).