Dr. Sam Teale

Abstract:

Perovskite solar cells (PSCs) consisting of interfacial two- and three-dimensional heterostructures that incorporate ammonium ligand intercalation have enabled rapid progress toward the goal of uniting performance with stability. However, as the field continues to seek ever-higher durability, additional tools that avoid progressive ligand intercalation are needed to minimize degradation at high temperatures. We used ammonium ligands that are nonreactive with the bulk of perovskites and investigated a library that varies ligand molecular structure systematically. We found that fluorinated aniliniums offer interfacial passivation and simultaneously minimize reactivity with perovskites. Using this approach, we report a certified quasi-steady-state power-conversion efficiency of 24.09% for inverted-structure PSCs. In an encapsulated device operating at 85°C and 50% relative humidity, we document a 1560-hour T₈₅ at maximum power point under 1-sun illumination.

Abstract:

Band gap tuning in mixed-halide perovskites enables efficient multijunction solar cells and LEDs. However, these wide band gap perovskites, which contain a mixture of iodide and bromide ions, are known to phase segregate under illumination, introducing voltage losses that limit stability. Previous studies have employed inorganic perovskites, halide alloys, and grain/interface passivation to minimize halide segregation, yet photostability can be further advanced. By focusing on the role of halide vacancies in anion migration, one expects to be able to erect local barriers to ion migration. To achieve this, we employ a 3D "hollow" perovskite structure, wherein a molecule that is otherwise too large for the perovskite lattice is incorporated. The amount of hollowing agent, ethane-1,2-diammonium dihydroiodide (EDA), varies the density of the hollow sites. Photoluminescence measurements reveal that 1% EDA in the perovskite bulk can stabilize a 40% bromine mixed-halide perovskite at 1 sun illumination intensity. These, along with capacitance-frequency measurements, suggest that hollow sites limit the mobility of the halide vacancies.

Abstract:

The tunable bandgaps and facile fabrication of perovskites make them attractive for multi-junction photovoltaics^1,2. However, light-induced phase segregation limits their efficiency and stability^3-5: this occurs in wide-bandgap (>1.65 electron volts) iodide/bromide mixed perovskite absorbers, and becomes even more acute in the top cells of triple-junction solar photovoltaics that require a fully 2.0-electron-volt bandgap absorber^2,6. Here we report that lattice distortion in iodide/bromide mixed perovskites is correlated with the suppression of phase segregation, generating an increased ion-migration energy barrier arising from the decreased average interatomic distance between the A-site cation and iodide. Using an approximately 2.0-electron-volt rubidium/caesium mixed-cation inorganic perovskite with large lattice distortion in the top subcell, we fabricated all-perovskite triple-junction solar cells and achieved an efficiency of 24.3 per cent (23.3 per cent certified quasi-steady-state efficiency) with an open-circuit voltage of 3.21 volts. This is, to our knowledge, the first reported certified efficiency for perovskite-based triple-junction solar cells. The triple-junction devices retain 80 per cent of their initial efficiency following 420 hours of operation at the maximum power point.