Strongly Correlated Insulators for Photovoltaics

Strongly correlated insulators have relatively narrow bandgaps (of the order of 1 eV) and they have been predicted to have fast impact ionization rates. In fact these rates are faster than other pathways for relaxation. Thus when a photon with energy larger than twice the energy gap excites an electron to the conduction band. Rather than giving up that excess energy to heating the lattice (phonons) the electron can use it to promote another electron to the conduction band, i.e., more than one elecrton-hole pair can be generated from one photon. Experimentally we have shown proof of principle that this is indeed happening in VO2sub> thin films. Pump-probe optical spectroscopy was used to investigate proposed charge-carrier multiplication via impact ionization. By comparing the transient reflectivities of the film when pumped at less than and then more than twice the band-gap energy, we observed a larger ultrafast response with the higher energy pump color while the film was still transiently in the insulating phase. For more details see Holleman et al., Phys. Rev. B (2016)

As presented in "Zhang et al. Phys. Rev. Materials (2021)", LaVO3 (LVO) also has been predicted to have fast impact ionization rates. In this work, we intentionally grow off-stoichiometric LVO films by changing the growth conditions such as laser fluence. Our aim is to study how deviating La:V stoichiometries affect the electronic properties of LVO thin films. We find that the off-stoichiometry clearly alters the physical properties of the films. Structural characterization shows that both La-rich and V-rich films have different levels of structural distortion, with La-rich (V-rich) films showing a larger (smaller) out-of-plane lattice parameter compared to what one would expect from epitaxial strain effects alone. In transport measurements, La-rich films display clear signatures of electronic phase separation accompanying a temperature induced metal-insulator transition, while V-rich films behave as Mott insulators. The out-of-plane lattice parameter plays a crucial role in determining the transport properties, as the crossover from Mott-insulating to disorder-induced phase-separated behavior occurs around a lattice parameter value of 3.96 Å, quite different from what has been previously reported.