Graphitic carbon nitride (g-C3N4), a 2D-organic semiconductor, has rapidly emerged as a potential alternative to the 2D-inorganic semiconductors in photocatalysis, but rare studies have been made hitherto about its applicability in optoelectronic devices. Considering the specific requirements of light-emitting diodes with efficient recombination of injected-carriers and photodetector devices with better charge separation, this work deals with synthesizing two variants of g-C3N4 samples with exclusively modified optical/electronic properties while keeping its basic structural framework. One sample is two-coordinated nitrogen deficient g-C3N4 (Nd-gCN) having very high photoluminescence (PL) and the other is hydrogen substituted g-C3N4 (H-gCN) exhibiting vanishingly low PL and ≈0.66 eV smaller bandgap than Nd-gCN. Role of nitrogen-vacancy and hydrogen substitution towards modulating optical/electronic properties of g-C3N4 are studied by combining experiments and density functional theory. Following strong luminescence, Nd-gCN sample manifests visibly blue emission in light-emitting devices; contrarily H-gCN sample shows potential in demonstrating efficient broadband photodetection. Besides moderate self-powered feature, photodetectors perform best at –5.0 V, corresponding to the highest responsivity Rλ=0.34A/W, EQEλ=59% and response time (0.18/0.29 sec). Efficient broadband photodetection performance of the heterojunction-devices is ascribed to the conjunct effects of drastic reduction in photogenerated carrier recombinations (PL quenching) and broadening of absorption regime facilitated by reduced bandgap and Si self-absorption.