Monitoring degradation and carrier collection losses of narrow bandgap perovskite solar cells with different organic hole transport layers by spectroscopic ellipsometry and external quantum efficiency

Narrow bandgap organic–inorganic lead halide-based perovskites have attracted tremendous attention in photovoltaics due to their advantages of low cost, easy synthesis and high efficiency. Selection of suitable charge transport layers and evaluation of device stability and optimization is necessary for commercialization. Degradation of encapsulated narrow bandgap tin–lead perovskite solar cells made with poly(3,4-ethylenedioxythiophene): polystyrenesulfonate (PEDOT:PSS), poly[3-(6-carboxyhexyl)thiophene-2,5-diyl] (P3CT), and poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) hole transport layers (HTLs) in ambient air is investigated using spectroscopic ellipsometry measurements. Optical and structural properties of the perovskite absorber layer remain relatively stable after 10 days of aging in ambient air. External quantum efficiency (EQE) simulations based on spectroscopic ellipsometry determined models identify carrier collection losses when compared with experimental EQE. A fresh device with P3CT HTL has 90 ± 1 % collection of photogenerated carriers in the perovskite absorber near the front contact interface. 3-, 5-, and 10-days aged devices with P3CT have 88 ± 1 % collection probability near the front contact interface. Fresh and 3-days aged devices with PEDOT:PSS HTL have 90 ± 1 % collection of photogenerated carriers in the perovskite absorber near the front contact interface. 5- and 10-days aged devices have 88 ± 1 % near the front contact interface. Fresh, 3-, 5-, and 10-days aged devices with PTAA HTL have 82 ± 1 % collection near the front contact interface. Devices with P3CT and PEDOT:PSS HTLs have 2.6 to 4.8 % higher power conversion efficiency and reduced electronic losses compared to a device with a PTAA HTL. Understanding how carrier collection losses, particularly near the front and back contacts, varies with different HTLs is necessary for optimizing perovskite solar cell performance.