Prof Henry Snaith FRS

Abstract:

Performance losses in positive–intrinsic–negative architecture perovskite solar cells are dominated by nonradiative recombination at the perovskite/organic electron transport layer interface, which is particularly problematic for wider bandgap perovskites. Large endeavours have been dedicated to the replacement of fullerenes, which are the most commonly used class of electron transport layers, with limited success thus far. In this work, we demonstrate blending the fullerene derivatives [6,6]-phenyl C61 butyric acid methyl ester (PCBM) and indene-C60 bis-adduct (ICBA) as a thin interlayer between 1.77 eV bandgap perovskite and an evaporated C60 layer. By tuning the fullerene blend to a trace 2% by mass of PCBM in ICBA, we remarkably form an interlayer which features improved energetic alignment with the perovskite and the PCBM : ICBA fullerene mixture, together with a stronger molecular ordering and an order of magnitude higher electron mobility than either neat PCBM or ICBA. Additional molecular surface passivation approaches are found to be beneficial in conjunction with this approach, resulting in devices with 19.5% steady state efficiency, a fill factor of 0.85 and an open-circuit voltage of 1.33 V, which is within 10% of the radiative limit of the latter two device parameters for this bandgap. This work highlights the complex nonlinear energetic behaviour with fullerene mixing, and how control of the energetics and crystallinity of these materials is crucial in overcoming the detrimental recombination losses that have historically limited perovskite solar cells.

Abstract:

The copper–silver–bismuth–iodide compound Cu2AgBiI6 has emerged as a promising lead-free and environmentally friendly alternative to wide-bandgap lead-halide perovskites for applications in multijunction solar cells. Despite its promising optoelectronic properties, the efficiency of Cu2AgBiI6 is still severely limited by poor charge collection. Here, we investigate the impact of commonly used charge transport layers (CTLs), including poly­[bis­(4-phenyl)­(2,4,6-trimethylphenyl)­amine] (PTAA), CuI, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), and SnO2, on the structural and optoelectronic properties of coevaporated Cu2AgBiI6 thin films. We reveal that while organic transport layers, such as PTAA and PCBM, form a relatively benign interface, inorganic transport layers, such as CuI and SnO2, induce the formation of unintended impurity phases within the CuI–AgI–BiI3 solid solution space, significantly influencing structural and optoelectronic properties. We demonstrate that identification of these impurity phases requires careful cross-validation combining absorption, X-ray diffraction and THz photoconductivity spectroscopy because their structural and optoelectronic properties are very similar to those of Cu2AgBiI6. Our findings highlight the critical role of CTLs in determining the structural and optoelectronic properties of coevaporated copper–silver–bismuth–iodide thin films and underscore the need for advanced interface engineering to optimize device efficiency and reproducibility.

Abstract:

Improved understanding of heterojunction interfaces has enabled multijunction photovoltaic devices to achieve power conversion efficiencies that exceed the detailed-balance limit for single-junctions. For wide-bandgap perovskites, however, the pronounced energy loss across the heterojunctions of the active and charge transport layers impedes multijunction devices from reaching their full efficiency potential. Here we find that for polycrystalline perovskite films with mixed-halide compositions, the crystal termination—a factor influencing the reactivity and density of surface sites—plays a crucial role in interfacial passivation for wide-bandgap perovskites. We demonstrate that by templating the growth of polycrystalline perovskite films toward a preferred (100) facet, we can reduce the density of deep-level trap states and enhance the binding of modification ligands. This leads to a much-improved heterojunction interface, resulting in open-circuit voltages of 1.38 V for 1.77-eV single-junction perovskite solar cells. In addition, monolithic all-perovskite double-junction solar cells achieve open-circuit voltage values of up to 2.22 V, with maximum power point tracking efficiencies reaching 28.6% and 27.7% at 0.25 and 1.0 cm2 cell areas, respectively, along with improved operational and thermal stability at 85 °C. This work provides universally applicable insights into the crystalline facet-favourable surface modification of perovskite films, advancing their performance in optoelectronic applications.

Abstract:

Narrow bandgap lead-tin perovskites are essential components of next-generation all-perovskite multi-junction solar cells. However, their poor stability under operating conditions hinders successful implementation. In this work, we systematically investigate the underlying mechanisms of this instability under combined heat and light stress (ISOS L-2 conditions) by measuring changes in phase, conductivity, recombination and current-voltage characteristics. We find an increased impact of the redistribution of mobile ions during device operation to be the primary driver of performance loss during stressing, with further losses caused by a slower increase in non-radiative recombination and background hole density. Crucially, the dominant degradation mode changes with different hole transport materials, which we attribute to variations in iodine vacancy generation rates. By quantifying the impact of these mechanisms on device performance, we provide critical insights for improving the operational stability of lead-tin perovskite solar cells.

Abstract:

Perovskite solar cells (PSCs) for space applications have garnered significant attention due to their high tolerance to proton radiation. While the self-healing mechanism of PSCs is largely attributed to mobile inorganic halide ions, the effects of radiation on organic A-site cations remain underexplored. In this study, wide-band-gap Cs/formamidinium (FA) PSCs, which are promising for tandem applications in space environments, were subjected to harsh proton radiation testing. Photovoltaic (PV) device parameters of the PSCs measured pre- and post-irradiation demonstrated that propane-1,3-diammonium iodide (PDAI2) treatment effectively mitigates radiation-induced damage to the perovskite layer. Advanced characterization techniques, including X-ray photoelectron spectroscopy (XPS) depth profiling using femtosecond laser ablation (fs-LA) and time-of-flight elastic recoil detection analysis (ToF-ERDA), were employed to analyze the impact of proton radiation on A-site organic cations. Additionally, time-resolved Kelvin probe force microscopy (tr-KPFM) was utilized to elucidate the mechanism by which PDAI2 treatment mitigates proton-induced damage to the organic cations.