Superconductivity is one of the most fascinating phase transition phenomena, offering deep insights into fundamental physics. In organic charge-transfer complexes, superconductivity is believed to arise from unconventional mechanisms beyond the conventional BCS theory. To explore the mechanisms underpinning unconventional superconductivity in organics, we investigated the quasiparticle spectra in the superconducting state through thermodynamic measurements. For the κ-type organic system, comprehensive heat capacity measurements revealed detailed quasiparticle excitation spectra. These results demonstrated that the superconducting state exhibits either dx2-y2-wave or dxy-wave symmetry, depending on the dimer lattice structure. This finding suggests that the superconductivity in κ-type systems originates from two types of antiferromagnetic spin fluctuations. In contrast, the β′′-type organic system, which lacks antiferromagnetic spin fluctuations, was found to exhibit a quasiparticle excitation spectrum different from that of the κ-type system. In this case, charge fluctuations driven by intersite Coulomb repulsions were identified as the origin of the primary mechanism for Cooper pairing. These results highlight that the determination of quasiparticle excitation spectra using thermodynamic quantities is a powerful approach to uncovering the detailed origins of superconductivity in organic systems.