Towards High Efficiency All-Perovskite Tandem Solar Cell by Preventing Performance Loss Arising from Physically Mixed Interfacial Layers

Physically mixed layers are major sources of optical and electronic losses in inorganic-organic metal halide perovskite-based photovoltaics (PV). Physical mixing of adjacent component layers at interfaces within a two-terminal all-perovskite tandem PV is identified by through-the-glass spectroscopic ellipsometry. Using measured device structures and optical properties of component layers including these physical mixtures, external quantum efficiency (EQE) spectra are simulated and compared to experimental EQE. Deviations between experimental and simulated EQE help to identify physically mixed layers as sources of optical and electronic losses and to quantify these losses. A physically mixed layer formed between the silver (Ag) electrical contact and electron transport fullerene (C60) / hole blocking bathocuproine (BCP) layers shows parasitic absorption losses of 2.3 mA/cm2 in the terms of short circuit current density (JSC). Similarly, electronic loss of 1.5 mA/cm2 is observed in the bottom subcell due to only 66% collection of photogenerated carriers stemming from poor passivation in the narrow-Eg perovskite nucleation layer formed near the hole transport layer. Optimized combinations of wide-Eg (1.74 eV) and narrow-Eg (1.25 eV) perovskite absorber thicknesses to maximize the tandem JSC are determined. Preventing physical mixing of component materials at interfaces and using optimized absorber layer thicknesses will help in fabricating higher efficiency all-perovskite tandem PV.