Impact of lithium dopants in hole-transporting layers on perovskite solar cell stability under day–night cycling

Abstract

Lithium cation dopants enhance hole-transport efficiency and optimize interfacial charge extraction in the hole-transporting layers of perovskite solar cells. Although the migration of lithium cations is known to induce phase transition from α-phase to δ-phase in perovskites, reports of long-term device stability present apparent contradictions. Here we show that, under dark/light alternating conditions, lithium migration induces a rapid degradation of the α-phase perovskite. This degradation is not observed under continuous light-only or dark-only conditions commonly used within the field to test the devices. To address the instability under dark/light cycling, we replace the lithium dopant with a methylammonium dopant. Importantly, we show no unreacted methylammonium dopant in the hole-transport layer film different to the lithium dopant, hinting at a better device stability. We achieve an efficiency of 26.1% (25.6% certified) and T95 lifetimes (that is, time for the device efficiency to decay to 95% of its initial value) of over 1,200 h of continuous light–dark cycling (ISOS-LC-1 certified) and 3,000 voltage-on/off cycles, conditions that are relevant to real-world operation. Lithium doping in spiro-OMeTAD negatively affects the long-term performance of perovskite solar cells. Qin and team map the degradation mechanism under repeated voltage cycling and how lithium-free dopants improve stability.