Lead-halide perovskites have emerged as highly-promising contenders for optoelectronic applications, including light-emitting diodes (LEDs) for ultrahigh definition displays, photovoltaics that can operate close to the radiative limit, and photoelectrochemical cells (PECs) for green hydrogen production. A key enabling property is the ability of halide perovskites to ‘tolerate’ point defects, such that low rates of non-radiative recombination can be achieved in materials processed using low-temperature solution processing.
However, the thermodynamic model predicting defect tolerance in the bulk breaks down at the surface of perovskites, particularly in nanocrystals (NCs). In this talk, I will discuss our recent work showing how the surface chemistry of CsPbBrxI3-x is influenced by the polarity of the antisolvent used during purification, and the proposed mechanism for this based on nuclear magnetic resonance measurements .
Another important limiting factor is the presence of toxic lead in a soluble form. I will also discuss our recent work on discovering nontoxic alternatives based on electronic structure models for defect tolerance, especially our work on bismuth oxyiodide (BiOI) and sodium bismuth sulphide (NaBiS2). I show that BiOI has the ideal bandgap for indoor light harvesting, and has potential for sustainably powering Internet of Things devices , as well as for acting as a photocathode for solar water splitting . In the latter work, we develop a device structure that improves the operational stability of BiOI photocathodes from minutes to months, and combine these devices with BiVO4. With NaBiS2, we find the material to exhibits extremely high absorption coefficients reaching >105 cm-1 at its optical bandgap of 1.4 eV, which is well-suited for ultra-thin solar cells. I will discuss experimental and computational insights into how this high absorption strength arises, and what the wider implications are on charge-carrier transport . Finally, I will discuss some of the broader challenges towards achieving defect tolerance in wide-bandgap semiconductors, and some promising pathways to overcoming these .
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