Transporting holes stably under iodide invasion in efficient perovskite solar cells

Highly efficient halide perovskite solar cells generally rely on lithium-doped organic hole transporting layers that are thermally and chemically unstable, in part because of migration of iodide anions from the perovskite layer. We report a solution strategy to stabilize the hole transport in organic layers by ionic coupling positive polymer radicals and molecular anions through an ion-exchange process. The target layer exhibited a hole conductivity that was 80 times higher than that of the conventional lithium-doped layer. Moreover, after extreme iodide invasion caused by light-soaking at 85°C for 200 hours, the target layer maintained high hole conductivity and well-matched band alignment. This ion-exchange strategy enabled fabrication of perovskite solar cells with a certified power conversion efficiency of 23.9% that maintained 92% under standard illumination at 85°C after 1000 hours. Although lithium-doped organic hole transport layers (HTLs) enable efficient charge extraction in perovskite solar cells, they also promote degradation, because lithium ions can adsorb water and positive radicals that form promote migration of iodide anions from the perovskite layer. Wang et al . developed an organic HTL that coupled positive polymer radicals and molecular anions through an ion-exchange process. The resulting highly conductive HTL has improved energy-level alignment with the perovskite compared with commonly used lithium-doped HTLs. High thermal and chemically stability with respect to iodide migration resulted in stable perovskite solar cells that maintained 92% power conversion efficiency of 23.9% for 1000 hours at 85°C. —PDS Lithium-free hole-transporting layers that couple positive polymer radicals and molecular anions inhibit iodide migration.