Claude Guet

Limitations of Cs3Bi2I9 as Lead-Free Photovoltaic Absorber Materials

Lead (Pb) halide perovskites have attracted tremendous attention in recent years because of their rich optoelectronic properties, which have resulted in more than 22% power conversion efficient photovoltaics (PVs). Nevertheless, Pb-metal toxicity remains a huge hurdle for extensive applications of these compounds. Thus, alternative compounds with similar optoelectronic properties need to be developed. Bismuth possesses electronic structure similar to that of lead with the presence of ns2 electrons that exhibit rich structural variety as well as interesting optical and electronic properties. Herein, we critically assess Cs3Bi2I9 as a candidate for thin-film solar cell absorber. Despite a reasonable optical band gap (∼2 eV) and absorption coefficient, the power conversion efficiency of the Cs3Bi2I9 mesoscopic solar cells was found to be severely lacking, limited by the poor photocurrent density. The efficiency of the Cs3Bi2I9 solar cell can be slightly improved by changing the stoichiometry of the precursor solutions, which is most probably due to the reduction in nonradiative defects as evident from our single-crystal photoluminescence spectroscopy. However, detailed investigations on pristine Cs3Bi2I9 reveal that zero-dimensional molecular crystal structure remains one of the main bottlenecks in achieving high performance. On the basis of our comprehensive studies, we have proposed that a continuous network of three-dimensional crystal structure should be another major criterion in addition to proper band gap and suitable optical properties of the future PV compounds.

Nonlinear Photoabsorption in Metallic Clusters

The optical response of simple metal clusters to strong laser fields should bear evidence of possible anharmonicities of dipole surface plasmon collective excitations. We present a theoretical method that allows one to calculate the excitation spectrum of sodium clusters beyond the linear response theory, including highly excited states with more than one plasmon. We make use of a coordinate transformation to separate center of mass (c.m.) and intrinsic motion of delocalized electrons. The coupling between c.m. motion and intrinsic excitations leads to some anharmonicity of the giant dipole oscillations. We calculate the photoabsorption spectrum of sodium clusters for increasing laser intensities and find sizeable anharmonic effects.

Plasmon Resonance in Photoabsorption of Colloidal Highly Doped ZnO Nanocrystals

A new type of dipole plasmon excitations in colloidal highly doped ZnO nanocrystals has been studied by means of many-body quantum mechanical approach. We demonstrate that in photodoped ZnO nanocrystals, the conduction band electrons are localized close to the surface and the plasmon oscillations are induced by their angular motion. The transition of this plasmon mode from classical to quantum regime is defined by the nanocrystal size. The size dependence of the resonance frequency which results from quantum effects is in remarkable agreement with experimental observations.