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Deterministically achieving on-chip photon storage and retrieval is a fundamental challenge for integrated photonics. Moreover, this requirement is increasingly urgent as photon storage and retrieval is crucial to realize truly scalable room-temperature quantum computing. However, most of existing quantum memories integrated on chips must either work at cryogenic temperature or else are strongly coupled with the environment, which hugely reduces the efficiency. Here, we propose an on-chip room-temperature quantum memory comprising three coupled microcavities, which presents an ideal dark state decoupled by a waveguide, thereby allowing on-demand photon storage and retrieval with high efficiency and high fidelity simultaneously. Furthermore, we demonstrate that the single-photon temporal duration can be increased or decreased by a factor of 103, thereby enabling many crucial quantum applications. Our error-robust approach highlights the potential for a solid-state photonic molecule for use as on-chip quantum memory and for optical quantum computing.