Photovoltaic & Optoelectronic Device Group

Abstract:

Evaporation-based manufacturing of halide perovskite light-emitting diodes has garnered increasing attention as a promising alternative for addressing issues with conventional solvent-based processing methodologies. Similar to existing organic light-emitting diode processing infrastructures, evaporation-based processing uses vapour-phase precursor transport and deposition, enabling solvent-free synthesis, precise nanoscale thickness control and enhanced patterning resolution. The physicochemical mechanism of such vacuum-based deposition and growth processes, which is radically different from that of solution-based processes, involves complex thermodynamic and kinetic factors regarding solid-vapour-solid transitions. This imposes much more stringent requirements for deposition environment, mandating concurrent advances in the fundamental understanding of evaporation and growth phenomena, as well as deposition equipment design. In this Perspective, we present a chemistry-driven framework for incorporating fundamental physicochemical principles into evaporation-based processing, with the aim of guiding reproducible and scalable perovskite light-emitting diode deposition system.

Abstract:

Strongly-confined perovskite nanoplatelets (PeNPLs) offer opportunities not found in conventional isotropic nanocubes, especially in producing linearly polarized light, as well as enhancing outcoupling through control over the transition dipole moment. But this requires ultrathin nanoplatelets with three or fewer monolayers of PbI₆ octahedra across the thickness, which are challenging to synthesise uniformly, and their luminescence is strongly affected by surface defects. Together, these limit the performance of ultrathin PeNPLs in light-emitting diodes (LEDs). Here, we address these challenges with an ancillary ligand engineering strategy. We demonstrate that ligands with phosphoryl functional groups strongly bind to the perovskite surface, while having an organic backbone that is not sterically bulky ensures high ligand density. By modulating nucleation and growth, these ancillary ligands lead to monodisperse PeNPLs that stack more uniformly when self-assembled into superlattices, with suppressed agglomeration. As a result, from edge-up PeNPL superlattices, we achieve an enhanced degree of polarization, while from face-down PeNPL superlattices, we achieve enhanced outcoupling that results in LEDs with 13.1% external quantum efficiency, the highest reported for ultrathin PeNPL LEDs. This work establishes ancillary ligand-induced synthesis as a decisive route to achieve uniform nanoplatelets with robust orientation control, enabling full utilization of the multifunctionality of anisotropic PeNPLs.

Abstract:

Monolithic all-perovskite tandem solar cells (TSCs) offer a route beyond single-junction efficiency limits through band-gap engineering. However, stability is hampered by hygroscopic degradation and phase segregation of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), the most common hole-transport material for narrow band-gap subcells. Here, we investigate the interface-mediated crystallization dynamics in mixed tin-lead (Sn-Pb) perovskites through in situ studies. We find that solvent-underlayer synergetic interactions with PEDOT:PSS induce metastable phase segregation during crystallization. Replacing PEDOT:PSS with a phenothiazine-functionalized interface facilitates direct phase transition and achieves preferential (100) orientation, yielding high-quality perovskite films. This enables a single-junction narrow band-gap subcell with 23.2% efficiency. Furthermore, we apply a hybrid interlayer integrating thiol and phosphonic acid anchoring groups on SnO2/Au, achieving a dense interconnecting layer for monolithic all-perovskite TSCs with 29.1% efficiency. The device retains 90% of the initial efficiency over 800 h of maximum power point tracking under simulated 1-sun illumination at 40°C, demonstrating robust operational stability.

Abstract:

Robust interconnecting layers are critical for all-perovskite tandems but are typically limited by acidic poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) and interdiffusive gold, compromising stability. Here, we substitute PEDOT:PSS with a conjugated polyelectrolyte-modified bithiophene-triphenylamine small molecule as the hole transport layer for tin-lead perovskite subcells, ensuring superior hole extraction and interfacial robustness. The gold recombination junction is replaced with a 5-nm-thick sputtercoated indium-doped zinc oxide to eliminate metal diffusion and minimize parasitic absorption. This design yields certified efficiencies of 29.80% and 30.19% for double-junction tandems employing methylammonium (MA)-free and MA-containing narrow-band-gap subcells, respectively. Furthermore, 1.0-cm² devices and 11.3-cm² mini-modules show efficiencies up to 28.7% and 25.0%, respectively. Encouragingly, encapsulated tandems maintain 90% of their initial efficiency after over 770, 530, and 220 h of maximum power point tracking under illumination at 45^◦C, 65^◦C, and 85^◦C, respectively. This work presents a viable route beyond PEDOT:PSS for narrow-band-gap perovskites toward reliable all-perovskite tandem photovoltaics.