Prof Henry Snaith FRS

Abstract:

Lead-halide perovskite nanocrystals (NCs) have gained attention for optoelectronics, but careful selection of the antisolvent used for purification is essential to achieve high monodispersity and yield while minimizing surface damage. Current understanding indicates that this requires lowering the relative polarity of the antisolvent, yet high-polarity antisolvents are widely used for purification, as we confirm through data mining. We show that polarity alone is insufficient for antisolvent selection by comparing ethyl acetate and acetonitrile for CsPbI3 NC purification. Despite its higher polarity, acetonitrile yields improved colloidal stability compared to ethyl acetate. Using 1H NMR, FTIR, and XPS measurements, alongside DFT calculations, we demonstrate that acetonitrile acts as a stronger Lewis base, binding to and passivating the NC surface. Coordination of acetonitrile to the perovskite NC surface enhances stability and improves their performance in light-emitting diodes. These findings establish a mechanistic framework for antisolvent selection to realize bright and stable halide perovskite NCs.

Abstract:

We explore silane-coated formamidinium lead bromide (FAPbBr₃) quantum dots (QDs) as single photon emitters and compare them to FAPbBr₃ QDs passivated with a phosphoethylammonium derivative (PEAC₈C₁₂), which represents current state-of-the-art ligand passivation. We compare properties including single-photon purity (g⁽²⁾(τ)), line width, blinking, and photostability. We find that at room temperature, these silane-coated dots perform comparably to PEAC₈C₁₂-passivated dots, while exhibiting improvements in photostability. However, we find that at 4 K, silane-coated FAPbBr₃ QDs perform worse than the PEAC₈C₁₂-passivated samples, exhibiting faster blue-shifting and photobleaching under illumination. Analysis of fluorescence lifetime intensity distributions from the photon-counting data indicates increased efficiency of fast nonradiative processes in the silane-coated QDs at 4 K. We propose a trion-related degradation pathway at low temperatures that is consistent with the observed kinetics and estimate that at 4 K with 6.1 μJ/cm², 472 nm excitation the silane-coated QDs build up double the trion population of their PEAC₈C₁₂-passivated counterparts.

Abstract:

Carrier mobility, recombination rates and diffusion length directly govern the efficiency of hybrid lead-halide perovskites. Yet, their behavior across different carrier concentrations and the effects of microstructure remain poorly understood. Using time-resolved photoluminescence and optical pump-THz probe spectroscopy, we quantify mobility, carrier recombination rates and diffusion length for polycrystalline films of methylammonium (MA)- and formamidinium (FA)-rich lead-halide perovskites, across carrier concentrations ranging from ∼10¹⁵ to ∼10¹⁹ cm^-3. For example, at a carrier concentration of ∼10¹⁸ cm^-3, FA_0.95MA_0.05Pb(I_0.95Br_0.05)₃ exhibits a mobility of 127 ± 9 cm² V^-1 s^-1 and a diffusion length of 392 ± 85 nm, compared to 69 ± 1 cm² V^-1 s^-1 and 139 ± 1 nm for MAPbI₃. These differences in mobility and diffusion length persist across different fluences, and are captured by a fluence-dependent rate model that accounts for both carrier generation and recombination at different material depths. From scanning electron microscopy and THz time-domain spectroscopy measurements, we attribute the increased mobility and diffusion length for the FA-rich perovskite mainly to a larger average grain size, after considering possible Fröhlich-type interactions between carriers and THz-active phonon modes. Our work establishes a mechanistic link between material microstructure and ultrafast carrier dynamics, informing crucial design principles for perovskite-based photovoltaic and optoelectronic applications.

Abstract:

Mixed-halide perovskites offer ideal bandgaps for tandem solar cells, but they suffer from light-induced halide segregation, which compromises their operational stability. Here, we directly probe the impact of halide segregation on charge-carrier dynamics at the interface between a mixed-halide perovskite and charge transport layers by using a free-space synchronous multimodal spectroscopy approach, combining time-resolved microwave conductivity, time-resolved photoluminescence (PL) and steady-state PL. We present a method to distinguish directly between charge-carrier dynamics dominated by either majority or minority carriers, enabling us to isolate effects arising from charge-selective extraction from the perovskite to commonly used hole- or electron transport layers, i.e. poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) and SnO2, respectively. We show that halide segregation creates iodide-rich phases that capture charge carriers within sub-nanoseconds, which slightly reduces their mobilities at microwave frequencies. We reveal that charge extraction from such iodide-rich domains is still surprisingly feasible, but competes with enhanced radiative recombination resulting from higher charge concentrations caused by funnelling into these minority phases. We demonstrate that together such effects reduce charge diffusion lengths and can account for the widely observed reduction in open-circuit voltages and short-circuit currents in solar cells under operational conditions. Our findings unravel the causes underpinning the adverse impact of halide segregation and provide guidelines to improve device performance.

Abstract:

Vacuum-based deposition is a scalable, solvent-free industrial method ideal for uniform coatings on complex substrates. However, all vacuum-deposited perovskite solar cells fabricated by thermal evaporation trail solution-processed counterparts in efficiency and stability due to film quality challenges, necessitating advancement and improved understanding. Here, we report a co-evaporation route for 1.67-eV wide-bandgap perovskites by introducing a PbCl2 co-source to optimize film quality. We promote perovskite formation with pronounced (100) “face-up” orientation and deliver a certified all vacuum-deposited solar cell with 18.35% efficiency (19.3% in the lab) for 0.25-cm2 devices (18.5% for 1-cm2 cells). These cells retain 80% of peak efficiency after 1,080 hours under the ISOS-L-2 protocol. Leveraging operando hyperspectral imaging, we provide spatiotemporal spectral insight into halide segregation and trap-mediated recombination, correlating microscopic luminescence features with macroscopic device performance while distinguishing radiative from non-ideal recombination channels. We further demonstrate 27.2%-efficient 1-cm2 evaporated perovskite-on-silicon tandems and outdoor stability of all vacuum-deposited tandems in Italy, retaining ~80% initial performance after 8 months.