  Two-dimensional (2D) noncentrosymmetric systems offer potential opportunities
for exploiting the valley degrees of freedom for advanced information
processing, owing to non-zero Berry curvature. However, such valley
polarization in 2D materials is crucially governed by the intervalley excitonic
scattering in momentum space due to reduced electronic degrees of freedom and
consequent enhanced electronic correlation. Here, we study the valley excitonic
properties of two 2D noncentrosymmetric complementary structures, namely,
BC$_6$N and B$_3$C$_2$N$_3$ using first principles-based GW calculations
combined with the Bethe-Salpeter equation (BSE), that brings the many-body
interactions among the quasiparticles. The \textbf{k}-resolved oscillator
strength of their first bright exciton indicates their ability to exhibit
valley polarization under the irradiation of circularly polarized light of
different chiralities. Both the systems show significant singlet excitonic
binding energies of 0.74 eV and 1.31 eV, respectively. Higher stability of dark
triplet excitons as compared to the singlet one can lead to higher quantum
efficiency in both the systems. The combination of large excitonic binding
energies and the valley polarization ability with minimal intervalley
scattering make them promising candidates for applications in advanced optical
devices and information storage technologies.
