Philippe Holzhey

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:

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.

Abstract:

Halide perovskites (HaP), with their exceptional optoelectronic properties and high-power conversion efficiencies in photovoltaic devices, hold promise for photoelectrochemical (PEC) applications in green fuel and chemical production. However, their stability in aqueous environments remains a challenge. This study investigates the stability and degradation mechanisms of the 2D Ruddlesden-Popper phase phenylethyl ammonium lead iodide (PEA⁽⁺⁾ ₂PbI₄) thin films in aqueous electrolytes under dark and illuminated conditions. While PEA⁽⁺⁾ ₂PbI₄ thin films appear to be thermodynamically stable in an aqueous electrolyte with phenylethyl ammonium iodide (PEAI), illumination causes significant photodegradation generating a deprotonated and dehalogenated 2D intercalation product: phenylethylamine-lead iodide, 2PEA⁽⁰⁾-PbI₂. The degradation of the 2D semiconductor leads to substantial reduction in the photovoltage, adversely impacting the material performance in photoelectrochemical (PEC) devices. To intercept photo-excited charge carriers in the 2D semiconductor, the I₃ ^-/I^- redox is added, which reduced photodegradation. The findings underscore that while catalytic reactions at halide perovskite electrodes in aqueous electrolytes are feasible, reversible and irreversible photodegradation remains a critical limitation that must be addressed in the design of PEC devices employing metal halide semiconductor layers for direct electrochemical energy conversion.

Abstract:

Perovskite solar cells achieve high power conversion efficiencies but usually rely on vacuum-deposited metallic contacts, leading to high material costs for noble metals and stability issues for more reactive metals. Carbon-based materials offer a cost-effective and potentially more stable alternative. The vast majority of carbon-electrode PSCs use the negative-intrinsic-positive (n-i-p) or “hole-transport-layer-free” architectures. Here, we present a systematic study to assess the compatibility of “inverted”, p-i-n configuration PSC contact layers with carbon top electrodes. We identify incompatibilities between common electron transport layers and the carbon electrode deposition process and previously unobserved semiconducting properties in carbon electrodes with unique implications for charge extraction and electronic behavior. To overcome these issues, we introduce a double-layer atomic layer deposited tin oxide (SnO2) and Poly­(2,3-dihydrothieno-1,4-dioxin)-poly­(styrenesulfonate) (PEDOT:PSS), yielding up to 16.1% PCE and a retained 94% performance after 500 h of outdoor aging. The study is a crucial step forward for printable, metal-electrode-free, and evaporation-free perovskite PV technologies.

Abstract:

Perovskite solar cells (PSCs) offer an efficient, inexpensive alternative to current photovoltaic technologies, with the potential for manufacture via high-throughput coating methods. However, challenges for commercial-scale solution-processing of metal-halide perovskites include the use of harmful solvents, the expense of maintaining controlled atmospheric conditions, and the inherent instabilities of PSCs under operation. Here, we address these challenges by introducing a high volatility, low toxicity, biorenewable solvent system to fabricate a range of 2D perovskites, which we use as highly effective precursor phases for subsequent transformation to α-formamidinium lead triiodide (α-FAPbI3), fully processed under ambient conditions. PSCs utilising our α-FAPbI3 reproducibly show remarkable stability under illumination and elevated temperature (ISOS-L-2) and “damp heat” (ISOS-D-3) stressing, surpassing other state-of-the-art perovskite compositions. We determine that this enhancement is a consequence of the 2D precursor phase crystallisation route, which simultaneously avoids retention of residual low-volatility solvents (such as DMF and DMSO) and reduces the rate of degradation of FA+ in the material. Our findings highlight both the critical role of the initial crystallisation process in determining the operational stability of perovskite materials, and that neat FA+-based perovskites can be competitively stable despite the inherent metastability of the α-phase.