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Strong Coulomb interactions in single-layer transition metal dichalcogenides (TMDs) result in the emergence of strongly bound excitons, trions and biexcitons. These excitonic complexes possess the valley degree of freedom, which can be exploited for quantum optoelectronics. However, in contrast to the good understanding of the exciton and trion properties, the binding energy of the biexciton remains elusive, with theoretical calculations and experimental studies reporting discrepant results. In this work, we present the high-quality low-temperature photoluminescence (PL) spectra of BN encapsulated monolayer WSe₂, which directly reveals the biexciton state only exists in charge neutral WSe₂ and one free electron binds to a biexciton and forms the trion-exciton complex, the binding energy is ~17 meV and ~49 meV for biexciton and trion-exciton complex, respectively. The magneto-PL also reveals unambiguous evidence of the dark exciton. The improved understanding of the biexciton, trion-exciton complexes and other excitonic complexes improve our understanding of the many-body interaction in TMDs, promising novel application in low-dimensional quantum optoelectronics.