An organic light-emitting device comprising: a first electrode; a second electrode facing the first electrode; and an emission layer disposed between the first electrode and the second electrode, wherein the emission layer comprises a host and a dopant, the emission layer emits a phosphorescent light, the dopant is an organometallic compound, a photoluminescence quantum yield (PLQY) of the dopant is 0.8 or greater and 1.0 or less, a decay time of the dopant is 0.1 microseconds (μs) or greater and 2.9 μs or less, 0.1 eV ≤ HOMO (dopant) - HOMO (host) ≤ 0.4 eV, wherein the HOMO (dopant) represents a highest occupied molecular orbital (HOMO) energy level (expressed in electron volts) of the dopant, and the HOMO (host) represents, in a case where the host comprised in the emission layer comprises one type of host, a HOMO energy level (expressed in electron volts) of the one type of host; or in a case where the host comprised in the emission layer is a mixture of two or more different types of host, a highest HOMO energy level from among HOMO energy levels (expressed in electron volts) of the two or more different types of host, the PLQY of the dopant is a PLQY of Film 1, the decay time of the dopant is calculated from a time-resolved photoluminescence (TRPL) spectrum with respect to Film 1, Film 1 has a thickness of 40 nanometers (nm) obtained by vacuum-deposition of the host and the dopant comprised in the emission layer in a weight ratio of 90:10 on a quartz substrate at a vacuum degree of 10 -7 torr, the HOMO (dopant) is a negative value measured by using a photoelectron spectrometer in an ambient atmosphere with respect to a film having a thickness of 40 nm obtained by vacuum-deposition of 1,4-bis(triphenylsilyl)benzene and the dopant comprised in the emission layer in a weight ratio of 85:15 on an ITO substrate at a vacuum degree of 10 -7 torr, and the HOMO (host) is, i) in a case where the host comprises one type of host, a negative value measured by using a photoelectron spectrometer in an ambient atmosphere with respect to a film having a thickness of 40 nm obtained by vacuum-deposition of the one type of host on an ITO substrate at a vacuum degree of 10 -7 torr; or ii) in a case where the host is a mixture of two or more different types of host, a largest negative value from among negative values measured by using a photoelectron spectrometer in an ambient atmosphere with respect to films having a thickness of 40 nm obtained by vacuum-deposition of each of the two or more different types of host on an ITO substrate at a vacuum degree of 10 -7 torr. a first electrode; a second electrode facing the first electrode; and an emission layer disposed between the first electrode and the second electrode, wherein the emission layer comprises a host and a dopant, the emission layer emits a phosphorescent light, the dopant is an organometallic compound, a photoluminescence quantum yield (PLQY) of the dopant is 0.8 or greater and 1.0 or less, a decay time of the dopant is 0.1 microseconds (μs) or greater and 2.9 μs or less, 0.1 eV ≤ HOMO (dopant) - HOMO (host) ≤ 0.4 eV, wherein the HOMO (dopant) represents a highest occupied molecular orbital (HOMO) energy level (expressed in electron volts) of the dopant, and the HOMO (host) represents, in a case where the host comprised in the emission layer comprises one type of host, a HOMO energy level (expressed in electron volts) of the one type of host; or in a case where the host comprised in the emission layer is a mixture of two or more different types of host, a highest HOMO energy level from among HOMO energy levels (expressed in electron volts) of the two or more different types of host, the PLQY of the dopant is a PLQY of Film 1, the decay time of the dopant is calculated from a time-resolved photoluminescence (TRPL) spectrum with respect to Film 1, Film 1 has a thickness of 40 nanometers (nm) obtained by vacuum-deposition of the host and the dopant comprised in the emission layer in a weight ratio of 90:10 on a quartz substrate at a vacuum degree of 10 -7 torr, the HOMO (dopant) is a negative value measured by using a photoelectron spectrometer in an ambient atmosphere with respect to a film having a thickness of 40 nm obtained by vacuum-deposition of 1,4-bis(triphenylsilyl)benzene and the dopant comprised in the emission layer in a weight ratio of 85:15 on an ITO substrate at a vacuum degree of 10 -7 torr, and the HOMO (host) is, i) in a case where the host comprises one type of host, a negative value measured by using a photoelectron spectrometer in an ambient atmosphere with respect to a film having a thickness of 40 nm obtained by vacuum-deposition of the one type of host on an ITO substrate at a vacuum degree of 10 -7 torr; or ii) in a case where the host is a mixture of two or more different types of host, a largest negative value from among negative values measured by using a photoelectron spectrometer in an ambient atmosphere with respect to films having a thickness of 40 nm obtained by vacuum-deposition of each of the two or more different types of host on an ITO substrate at a vacuum degree of 10 -7 torr.