Direct observation of phase transitions between delta- and alpha-phase FAPbI 3 via defocused Raman spectroscopy
Journal of Materials Chemistry A Royal Society of Chemistry (RSC) (2024)
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.Understanding the Degradation of Methylenediammonium and Its Role in Phase-Stabilizing Formamidinium Lead Triiodide.
Journal of the American Chemical Society American Chemical Society (ACS) 145:18 (2023) 10275-10284
Abstract:
Formamidinium lead triiodide (FAPbI<sub>3</sub>) 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 (MDACl<sub>2</sub>) has been used as an additive in FAPbI<sub>3</sub>. MDA<sup>2+</sup> has been reported as incorporated into the perovskite lattice alongside Cl<sup>-</sup>. However, the precise function and role of MDA<sup>2+</sup> remain uncertain. Here, we grow FAPbI<sub>3</sub> single crystals from a solution containing MDACl<sub>2</sub> (FAPbI<sub>3</sub>-M). We demonstrate that FAPbI<sub>3</sub>-M crystals are stable against transformation to the photoinactive δ-phase for more than one year under ambient conditions. Critically, we reveal that MDA<sup>2+</sup> is not the direct cause of the enhanced material stability. Instead, MDA<sup>2+</sup> degrades rapidly to produce ammonium and methaniminium, which subsequently oligomerizes to yield hexamethylenetetramine (HMTA). FAPbI<sub>3</sub> crystals grown from a solution containing HMTA (FAPbI<sub>3</sub>-H) replicate the enhanced α-phase stability of FAPbI<sub>3</sub>-M. However, we further determine that HMTA is unstable in the perovskite precursor solution, where reaction with FA<sup>+</sup> is possible, leading instead to the formation of tetrahydrotriazinium (THTZ-H<sup>+</sup>). By a combination of liquid- and solid-state NMR techniques, we show that THTZ-H<sup>+</sup> is selectively incorporated into the bulk of both FAPbI<sub>3</sub>-M and FAPbI<sub>3</sub>-H at ∼0.5 mol % and infer that this addition is responsible for the improved α-phase stability.Improving performance of fully scalable, flexible transparent conductive films made from carbon nanotubes and ethylene-vinyl acetate
Energy Reports Elsevier 8:S11 (2022) 48-60
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.Chemical Interaction at the MoO3/CH3NH3PbI3-xClx Interface.
ACS applied materials & interfaces 13:14 (2021) 17085-17092
Abstract:
The limited long-term stability of metal halide perovskite-based solar cells is a bottleneck in their drive toward widespread commercial adaptation. The organic hole-transport materials (HTMs) have been implicated in the degradation, and metal oxide layers are proposed as alternatives. One of the most prominent metal oxide HTM in organic photovoltaics is MoO3. However, the use of MoO3 as HTM in metal halide perovskite-based devices causes a severe solar cell deterioration. Thus, the formation of the MoO3/CH3NH3PbI3-xClx (MAPbI3-xClx) heterojunction is systematically studied by synchrotron-based hard X-ray photoelectron spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and Raman spectroscopy. Upon MoO3 deposition, significant chemical interaction is induced at the MoO3/MAPbI3-xClx interface: substoichiometric molybdenum oxide is present, and the perovskite decomposes in the proximity of the interface, leading to accumulation of PbI2 on the MoO3 cover layer. Furthermore, we find evidence for the formation of new compounds such as PbMoO4, PbN2O2, and PbO as a result of the MAPbI3-xClx decomposition and suggest chemical reaction pathways to describe the underlying mechanism. These findings suggest that the (direct) MoO3/MAPbI3-xClx interface may be inherently unstable. It provides an explanation for the low power conversion efficiencies of metal halide perovskite solar cells that use MoO3 as a hole-transport material and in which there is a direct contact between MoO3 and perovskite.Filamentary high-resolution electrical probes for nanoengineering
Nano Letters American Chemical Society 20:2 (2020) 1067-1073