Snaith group

Abstract:

The severely insufficient operational lifetime of perovskite light-emitting diodes (LEDs) is incompatible with the rapidly increasing external quantum efficiency, even as it approaches the theoretical limit, thereby significantly impeding the commercialization of perovskite LEDs. In addition, Joule heating induces ion migration and surface defects, degrades the photoluminescence quantum yield and other optoelectronic properties of perovskite films, and induces the crystallization of charge transport layers with low glass transition temperatures, resulting in LED degradation under continuous operation. Here, a novel thermally crosslinked hole transport material, poly(FCA₆₀ -co-BFCA₂₀ -co-VFCA₂₀ ) (poly-FBV), with temperature-dependent hole mobility is designed, which is advantageous for balancing the charge injection of the LEDs and limiting the generation of Joule heating. The optimised CsPbI₃ perovskite nanocrystal LEDs with poly-FBV realise approximately a 2-fold external quantum efficiency increase over the LED with commercial hole transport layer poly(4-butyl-phenyl-diphenyl-amine) (poly-TPD), owing to the balanced carrier injection and suppressed exciton quenching. Moreover, because of the Joule heating control provided by the novel crosslinked hole transport material, the LED utilising crosslinked poly-FBV has a 150-fold longer operating lifetime (490 min) than that utilizing poly-TPD (3.3 min). The study opens a new avenue for the use of PNC LEDs in commercial semiconductor optoelectronic devices.

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 power conversion efficiency of tin-based halide perovskite solar cells is limited by large photovoltage losses arising from the significant energy-level offset between the perovskite and the conventional electron transport material, fullerene C₆₀. The fullerene derivative indene-C₆₀ bisadduct (ICBA) is a promising alternative to mitigate this drawback, owing to its superior energy level matching with most tin-based perovskites. However, the less finely controlled energy disorder of the ICBA films leads to the extension of its band tails that limits the photovoltage of the resultant devices and reduces the power conversion efficiency. Herein, we fabricate ICBA films with improved morphology and electrical properties by optimizing the choice of solvent and the annealing temperature. Energy disorder in the ICBA films is substantially reduced, as evidenced by the 22 meV smaller width of the electronic density of states. The resulting solar cells show open-circuit voltages of up to 1.01 V, one of the highest values reported so far for tin-based devices. Combined with surface passivation, this strategy enabled solar cells with efficiencies of up to 11.57%. Our work highlights the importance of controlling the properties of the electron transport material toward the development of efficient lead-free perovskite solar cells and demonstrates the potential of solvent engineering for efficient device processing.