  Behaving like atomically-precise two-dimensional quantum wells with
non-negligible dielectric contrast, the layered HOIPs have strong electronic
interactions leading to tightly bound excitons with binding energies on the
order of 500 meV. These strong interactions suggest the possibility of larger
excitonic complexes like trions and biexcitons, which are hard to study
numerically due to the complexity of the layered HOIPs. Here, we propose and
parameterize a model Hamiltonian for excitonic complexes in layered HOIPs and
we study the correlated eigenfunctions of trions and biexcitons using a
combination of diffusion Monte Carlo and very large variational calculations
with explicitly correlated Gaussian basis functions. Binding energies and
spatial structures of these complexes are presented as a function of the layer
thickness. The trion and biexciton of the thinnest layered HOIP have binding
energies of 35 meV and 44 meV, respectively, whereas a single exfoliated layer
is predicted to have trions and biexcitons with equal binding enegies of 48
meV. We compare our findings to available experimental data and to that of
other quasi-two-dimensional materials.
