  Van der Waals heterojunctions between two-dimensional (2D) layered materials
and nanomaterials of different dimensions present unique opportunities for
gate-tunable optoelectronic devices. Mixed dimensional p-n heterojunction
diodes, such as p-type pentacene (0D) and n-type monolayer MoS2 (2D), are
especially interesting for photovoltaic applications where the absorption
cross-section and charge transfer processes can be tailored by rational
selection from the vast library of organic molecules and 2D materials. Here, we
study the kinetics of excited carriers in pentacene-MoS2 p-n type-II
heterojunctions by transient absorption spectroscopy. These measurements show
that the dissociation of MoS2 excitons occurs by hole transfer to pentacene on
the time scale of 6.7 ps. In addition, the charge-separated state lives for 5.1
ns, up to an order of magnitude longer than the recombination lifetimes from
previously reported 2D material heterojunctions. By studying the fractional
amplitudes of the MoS2 decay processes, the hole transfer yield from MoS2 to
pentacene is found to be approximately 50 percent, with the remaining holes
undergoing trapping due to surface defects. Overall, the ultrafast charge
transfer and long-lived charge-separated state in pentacene-MoS2 p-n
heterojunctions suggest significant promise for mixed-dimensional van der Waals
heterostructures in photovoltaics, photodetectors, and related optoelectronic
technologies.
