Inhomogeneous Doping of Perovskite Materials by Dopants from Hole-Transport Layer

Dopants, which are often indispensable in the organic hole-transport layer (HTL) in perovskite solar cells (PSCs), are used to enhance HTL conductivity and tune the energy level of HTLs. Here, we show that the lithium (Li) dopant from HTLs can significantly dope the perovskite absorber layer through diffusion. Kelvin probe force microscopy mapping shows that Li diffusion increases the surface potential of the perovskite film. The potential at grain boundaries showed more increase than in grain interiors, indicating inhomogeneous distribution of Li dopant in the perovskite layer, which was further confirmed by lateral elemental mapping using time-of-flight secondary ion mass spectrometry. For both p-i-n and n-i-p device structures, the diffusion and nonuniform distribution of Li⁺ ions are shown to influence the carrier-transport properties of perovskites and device characteristics. This work reveals the hidden doping effects of the HTL dopants on perovskite films and their impact on PSC performance. Dopants are often indispensable in the organic hole-transport layer (HTL) in perovskite solar cells (PSCs). Lithium (Li) ions, the common dopants used in HTL, are known to enhance the conductivity and tune the energy level of HTL. However, Li⁺ ions can diffuse from HTL to the perovskite layer, and the corresponding effect had often been overlooked. Here, we report that Li⁺ ions from HTL can significantly dope the perovskite layer. The accumulation of Li⁺ ions at the grain boundaries and their impact on perovskite were investigated by multiple characterization techniques. For different device structures, the diffusion and nonuniform distribution of Li⁺ ions are shown to influence perovskite charge transport and device characteristics. This work reveals the hidden doping effects of the HTL dopants on perovskite films and PSC performance. Property changes of the perovskite layer in an actual device from its pristine state should be considered for future PSC development. Li⁺ ions from the hole-transport layer (HTL) dope the perovskite absorber layer through diffusion. Kelvin probe force microscopy reveals potential change at perovskite grain boundaries before and after Li⁺ diffusion. This work reveals the hidden doping effect of Li⁺ ions from HTL on perovskite films and device characteristics.