Photovoltaic & Optoelectronic Device Group

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:

Narrow-bandgap tin and mixed tin-lead halide perovskites are attracting growing interest for optoelectronic applications, yet the difficult-to-control crystallization process has hindered their development. Although additive engineering has effectively improved film formation, the fundamental origins of their distinct crystallization behavior remain less explored. Here, through direct comparison with Pb counterparts, we investigate the pre-crystallization stages of Sn-based perovskite precursor solutions through complementary structural characterizations. We show that Sn precursors are intrinsically more reactive and sensitive to their chemical environment, exhibiting poorer colloidal stability compared to Pb and a strong inherent tendency to agglomerate. These findings explain their narrower processing window, where small variations in solution chemistry strongly affect nucleation and crystallization dynamics. To fabricate high-quality tin-based perovskite through solution methods, we highlight the importance of controlling the often-overlooked pre-crystallization stages, though, for example, rational solvent and additive designs. Overall, we provide fundamental insights into precursor solution chemistry and establish pre-crystallization engineering as a key strategy for overcoming long-standing limitations in thin-film fabrication, particularly in light of the field's rapid progression toward large-scale, sustainable, and solvent-conscious manufacturing.

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:

Narrow‐bandgap tin and mixed tin–lead halide perovskites are attracting growing interest for optoelectronic applications, yet the difficult‐to‐control crystallization process has hindered their development. Although additive engineering has effectively improved film formation, the fundamental origins of their distinct crystallization behavior remain less explored. Here, through direct comparison with Pb counterparts, we investigate the pre‐crystallization stages of Sn‐based perovskite precursor solutions through complementary structural characterizations. We show that Sn precursors are intrinsically more reactive and sensitive to their chemical environment, exhibiting poorer colloidal stability compared to Pb and a strong inherent tendency to agglomerate. These findings explain their narrower processing window, where small variations in solution chemistry strongly affect nucleation and crystallization dynamics. To fabricate high‐quality tin‐based perovskite through solution methods, we highlight the importance of controlling the often‐overlooked pre‐crystallization stages, though, for example, rational solvent and additive designs. Overall, we provide fundamental insights into precursor solution chemistry and establish pre‐crystallization engineering as a key strategy for overcoming long‐standing limitations in thin‐film fabrication, particularly in light of the field's rapid progression toward large‐scale, sustainable, and solvent‐conscious manufacturing. A comparative NMR/SAXS study of lead‐ and tin‐based perovskite precursor solutions reveals fundamental differences in reactivity, environmental sensitivity, and colloidal stability. Tin systems show weaker colloidal stability, enhanced agglomeration, and strong solvent dependence. We provide the first detailed description of the pre‐crystallization stages governing tin halide precursor chemistry.

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.