Bernd Sturdza

Abstract:

The ability to characterise perovskite phases non-destructively is key on the route to ensuring their long-term stability in operando. Raman spectroscopy holds the promise to play an important role in this task. Among all perovskites, formamidinium lead iodide (FAPbI₃) has emerged as one of the most promising candidates for single-junction photovoltaic cells. However, Raman spectroscopy of FAPbI₃ remains challenging as is evidenced by conflicting reports in the literature. Here, we demonstrate that due to the vulnerability of FAPbI₃ to laser-induced degradation, the detected Raman spectrum depends strongly on the experimental conditions. This can lead to conflicting results and is revealed as the origin of discrepancies in the literature. We overcome this issue by deploying defocused Raman spectroscopy, preventing laser-induced damage to the sample and simultaneously improving the signal-to-noise ratio, allowing us to furthermore resolve much weaker Raman modes than was previously possible. We offer step-by-step instructions on how to apply this technique to a given spectrometer. Non-destructive characterisation of the FAPbI₃ phases further enables us to quantify the phase stability of pristine FAPbI₃ crystals and FAPbI₃ grown with the high-performance additive methylenediammonium chloride (MDACl₂). This shows that the neat crystals fully degrade within two weeks, whereas in samples grown with the additive only about 2% of the crystal bulk is in the δ-phase after 400 days. This establishes defocused Raman spectroscopy as a powerful tool for the characterisation of FAPbI₃ and other perovskite materials.

Abstract:

Formamidinium lead triiodide (FAPbI₃) is the leading candidate for single-junction metal-halide perovskite photovoltaics, despite the metastability of this phase. To enhance its ambient-phase stability and produce world-record photovoltaic efficiencies, methylenediammonium dichloride (MDACl₂) has been used as an additive in FAPbI₃. MDA²⁺ has been reported as incorporated into the perovskite lattice alongside Cl^-. However, the precise function and role of MDA²⁺ remain uncertain. Here, we grow FAPbI₃ single crystals from a solution containing MDACl₂ (FAPbI₃-M). We demonstrate that FAPbI₃-M crystals are stable against transformation to the photoinactive δ-phase for more than one year under ambient conditions. Critically, we reveal that MDA²⁺ is not the direct cause of the enhanced material stability. Instead, MDA²⁺ degrades rapidly to produce ammonium and methaniminium, which subsequently oligomerizes to yield hexamethylenetetramine (HMTA). FAPbI₃ crystals grown from a solution containing HMTA (FAPbI₃-H) replicate the enhanced α-phase stability of FAPbI₃-M. However, we further determine that HMTA is unstable in the perovskite precursor solution, where reaction with FA⁺ is possible, leading instead to the formation of tetrahydrotriazinium (THTZ-H⁺). By a combination of liquid- and solid-state NMR techniques, we show that THTZ-H⁺ is selectively incorporated into the bulk of both FAPbI₃-M and FAPbI₃-H at ∼0.5 mol % and infer that this addition is responsible for the improved α-phase stability.

Abstract:

All data used in the publication are deposited both as Origin Pro and csv files.

Abstract:

We report process improvements for the fabrication of single-walled carbon nanotube ethylene-vinyl acetate transparent conductive films. CNT:EVA films demonstrate high resilience against folding and can replace the external dopant in a spiro-OMeTAD based hole selective contact of n-i-p perovskite solar cells achieving a steady-state efficiency of 16.3%. The adapted process is fully scalable, and compared to previous reports (Mazzotta et al., 2018) lowers the material cost dramatically and improves DC to optical conductivity ratio by two orders of magnitude to σdc/σop = 3.6 for pristine and σdc/σop = 15 for chemically doped films. We analyse the microstructure of our films via small angle neutron scattering and find a positive correlation between the long range packing density of the CNT:EVA films and the σdc/σop performance. Increasing monomer ratio and chain length of the EVA polymer improves resilience against bending strain, whereas no significant effect on the CNT wrapping and electrical conductivity of resulting films is found.

Abstract:

Carbon nanomaterials are a promising class of materials for optoelectronics with low environmental impact. However, dark states limit their photoluminescence (PL) efficiency and thus applicability. In this thesis, hitherto-unrecognised twilight states in graphene nanoribbons are identified, where inter-valley mixing produced by periodic edge modulation and strong electron-phonon coupling are key mechanisms that make conventionally-dark states at least four times brighter, resulting in a 7.7% PL quantum yield. Strong vibron-electron coupling is revealed by the high spectral definition, with the bandgap modulated by coupling to the radial breathing-like mode. Unexpectedly, the predominant phonon modes affect absorption and emission differently, due to the simultaneous presence of Herzberg-Teller and Franck-Condon couplings. The achievement of efficient light emission from carbon nanostructures, both in solutions and thin films, opens the path to their integration into electro-optical circuits, and to the optical investigation and manipulation of topological states in graphenoids. Further, a detailed study of carbon nanotube-ethylene-vinyl acetate (CNT-EVA) transparent conductive films (TCFs) and the connection between processing and optoelectronic film properties is presented, leading to a 100-fold improvement of the electrical conductivity at the same transmittance. The dispersion technique and polymer removal steps are established as crucial steps for improving TCF performance. Changing the composition of the EVA copolymer allows tuning the mechanical and chemical resilience of CNT-EVA films and the films perform well in flexible applications. CNT-EVA films are chemically p-doped with halogenated metals and low-cost dopants identified. The improved CNT-EVA films perform well in transparent touch-sensitive devices and as transport layer in pervoskite photovoltaic (PV) devices, highlighting the potential of hybrid transport layers and opening a pathway for the large-scale application of low-cost CNT-EVA conductive films. Finding ways to stabilise perovskites under ambient conditions is the main challenge to overcome before they can be applied on a large scale. Substantial stability improvement is demonstrated for the pervoskite formamidinium lead iodide (FAPbI3) in single crystals by the addition of methylenediammonium dichloride (MDACl2). This is quantified by Raman spectroscopy which is established as a powerful tool for the characterisation of FAPbI3 by devising the defocused Raman spectroscopy method. The MDACl2 additive reduced the trap density and increased the excited carrier lifetime significantly, whereas the band gap at room temperature appears unaffected by the addition of MDACl2.