posted on 2021-04-23, 19:03authored byRitabrata Sarkar, Moumita Kar, Md Habib, Guoqing Zhou, Thomas Frauenheim, Pranab Sarkar, Sougata Pal, Oleg V. Prezhdo
Carbon
nanotubes (CNTs) are appealing candidates for solar and
optoelectronic applications. Traditionally used as electron sinks,
CNTs can also perform as electron donors, as exemplified by coupling
with perylenediimide (PDI). To achieve high efficiencies, electron
transfer (ET) should be fast, while subsequent charge recombination
should be slow. Typically, defects are considered detrimental to material
performance because they accelerate charge and energy losses. We demonstrate
that, surprisingly, common CNT defects improve rather than deteriorate
the performance. CNTs and other low dimensional materials accommodate
moderate defects without creating deep traps. At the same time, charge
redistribution caused by CNT defects creates an additional electrostatic
potential that increases the CNT work function and lowers CNT energy
levels relative to those of the acceptor species. Hence, the energy
gap for the ET is decreased, while the gap for the charge recombination
is increased. The effect is particularly important because charge
acceptors tend to bind near defects due to enhanced chemical interactions.
The time-domain simulation of the excited-state dynamics provides
an atomistic picture of the observed phenomenon and characterizes
in detail the electronic states, vibrational motions, inelastic and
elastic electron–phonon interactions, and time scales of the
charge separation and recombination processes. The findings should
apply generally to low-dimensional materials, because they dissipate
defect strain better than bulk semiconductors. Our calculations reveal
that CNT performance is robust to common defects and that moderate
defects are essential rather than detrimental for CNT application
in energy, electronics, and related fields.