Prof Laura Herz FRS

Abstract:

Fully conjugated three-dimensional covalent organic frameworks (COFs) are a newly emerged class of materials that expands reticular chemistry to extended electron delocalization for optoelectronic applications. To overcome the limitations of sp3-connected 3D frameworks, the pseudo-tetrahedral motif cyclooctatetrathiophene (COTh) has gained attention for forming fully conjugated 3D COFs. We report on a novel COTh building block, featuring functional formyl groups directly attached to the core's conjugated thiophenes. The modulation synthesis approach with mono-functionalized inhibitors enables the formation of COTh-1P COF, which exhibited remarkable crystallinity and permanent porosity. By following this approach and by optimizing the synthesis conditions to the solvothermal growth of thin films, we fabricated the first preferentially oriented conjugated 3D COF films on various substrates without pre-functionalization. With these thin films, optical pump terahertz probe studies allowed us, for the first time with 3D-fully conjugated COFs, to provide insights into the excited state and charge-carrier dynamics of these unique organic frameworks. Low effective masses are discovered for valence and conduction bands by density functional theory simulations. The ability to create crystalline and oriented films of fully π-conjugated 3D COTh-based COFs on non-modified substrates is expected to open the way for integration of such frameworks into diverse optoelectronic applications.

Abstract:

The perovskite semiconductor, CsPbI₃, holds excellent promise for solar cell applications due to its suitable bandgap. However, achieving phase-stable CsPbI₃ solar cells with high power conversion efficiency remains a major challenge. Ruddlesden-Popper (RP) defects have been identified in a range of perovskite semiconductors, including CsPbI₃. However, there is limited understanding as to why they form or their impact on stability and photophysical properties. Here, the prevalence of RP defects is increased with increased Cs-excess in vapor-deposited CsPbI₃ thin films while  superior structural stability but inferior photophysical properties are observed. Significantly, using electron microscopy, it is found that the atomic positions at the planar defect are comparable to those of a free surface, revealing their role in phase stabilization. Density functional theory (DFT) calculations reveal the RP planes are electronically benign, however, antisites observed at RP turning points are likely to be malign. Therefore it is proposed that increasing RP planes while reducing RP turning points offers a breakthrough for improving both phase stability and photophysical performance. The formation mechanism revealed here can apply more generally to RP structures in other perovskite systems.

Abstract:

Two-dimensional (2D) halide perovskites are a versatile material class, exhibiting a layered crystal structure, consisting of inorganic metal–halide sheets separated by organic spacer cations. Unlike their 3D counterparts, 2D perovskites have less strict geometric requirements, allowing for a wider range of molecules to be incorporated. This potentially offers a way to engineer the properties of a 2D perovskite through adequate selection of the organic spacer cations. Our study systematically analyzes the effect of spacer cation length on the electronic and optical properties of Ruddlesden–Popper lead-iodide-based 2D perovskites, using alkylammonium cations of varying chain lengths. Intriguingly, no linear correlation between interlayer distance and the optical gap or valence band position is observed in our measurements. Rather it matters whether the spacer cation contains an odd or even number of carbon atoms in the chain. Notably, these odd-even effects manifest in variations of ionization energy, optical gap as well as charge carrier mobility. Density functional theory calculations reproduce the changes in optical properties, allowing us to identify the underlying mechanism: while even-numbered carbon chains pack efficiently within the organic spacer layer, the shorter odd-numbered chains increase distortions. These distortions lead to variations in the Pb–I–Pb bond angle within the inorganic sheets, resulting in the observed odd-even effect in the (opto-)electronic properties. This understanding will be helpful to make more informed choices regarding the incorporated spacer molecules which can potentially help to enhance performance when integrating such 2D perovskite interlayers into devices.

Abstract:

Two-dimensional perovskites show intriguing optoelectronic properties due to their anisotropic structure and multiple quantum well structure. Here, we report the first three gold-based Ruddlesden–Popper type two-dimensional double perovskites with a general formula (NOP)4AuIBIIII8 (B = Au, Bi, Sb) employing naphthalene-O-propylammonium (NOP) as an organic cation. They were found to form highly crystalline thin films on various substrates, predominantly oriented in the [001] direction featuring continuous, crack-free film areas on the μm2 scale. The thin films show strong optical absorption in the visible region, with band gap energies between 1.48 and 2.32 eV. Density functional theory calculations support the experimentally obtained band gap energies and predict high charge-carrier mobilities and effective charge separation. A comprehensive study with time-resolved microwave conductivity (TRMC) and optical-pump-THz-probe (OPTP) spectroscopy revealed high charge-carrier mobilities for lead-free two-dimensional perovskites of 4.0 ± 0.2 cm2(V s)−1 and charge-carrier lifetimes in the range of μs. Photoconductivity measurements under 1 sun illumination demonstrated the material’s application as a photodetector, showing a 2-fold increase in conductivity when exposed to light.