A method of preparing an organic light-emitting device, the method comprising: forming an organic emission unit on a substrate; and forming a thin film encapsulation layer that contacts an environmental element, and that comprises at least one inorganic layer (23") comprising a low temperature viscosity transition inorganic material comprising a tin oxide, wherein the inorganic layer is formed using a process comprising: forming a pre-inorganic layer comprising the LVT inorganic material by providing the LVT inorganic material onto the organic emission unit on which the environmental element is located; performing a first healing process on the pre-inorganic layer at a temperature greater than a viscosity transition temperature of the LVT inorganic material; performing a second healing process on the pre-inorganic layer having undergone the first healing process to increase a binding force between the environmental element and the LVT inorganic material, and to increase a binding force among the LVT inorganic material, wherein the second healing process comprises the use of chemical treatment; plasma treatment; or hot chamber treatment, wherein the chamber contains oxygen having an oxygen partial pressure in a range of 2% to 100% of a total pressure in the chamber, and a temperature in a range of 25°C to 150°C; or hot chamber treatment, wherein the chamber has an oxygen partial pressure in a range of 2% to 100% of a total pressure in the chamber, a relative humidity in a range of 10% to 100%, and a temperature in a range of 25°C to 150°C; and wherein the thin film encapsulation layer further comprises at least one organic layer comprising a polymer. forming an organic emission unit on a substrate; and forming a thin film encapsulation layer that contacts an environmental element, and that comprises at least one inorganic layer (23") comprising a low temperature viscosity transition inorganic material comprising a tin oxide, wherein the inorganic layer is formed using a process comprising: forming a pre-inorganic layer comprising the LVT inorganic material by providing the LVT inorganic material onto the organic emission unit on which the environmental element is located; performing a first healing process on the pre-inorganic layer at a temperature greater than a viscosity transition temperature of the LVT inorganic material; performing a second healing process on the pre-inorganic layer having undergone the first healing process to increase a binding force between the environmental element and the LVT inorganic material, and to increase a binding force among the LVT inorganic material, wherein the second healing process comprises the use of chemical treatment; plasma treatment; or hot chamber treatment, wherein the chamber contains oxygen having an oxygen partial pressure in a range of 2% to 100% of a total pressure in the chamber, and a temperature in a range of 25°C to 150°C; forming a pre-inorganic layer comprising the LVT inorganic material by providing the LVT inorganic material onto the organic emission unit on which the environmental element is located; performing a first healing process on the pre-inorganic layer at a temperature greater than a viscosity transition temperature of the LVT inorganic material; performing a second healing process on the pre-inorganic layer having undergone the first healing process to increase a binding force between the environmental element and the LVT inorganic material, and to increase a binding force among the LVT inorganic material, wherein the second healing process comprises the use of chemical treatment; plasma treatment; or hot chamber treatment, wherein the chamber contains oxygen having an oxygen partial pressure in a range of 2% to 100% of a total pressure in the chamber, and a temperature in a range of 25°C to 150°C; or hot chamber treatment, wherein the chamber has an oxygen partial pressure in a range of 2% to 100% of a total pressure in the chamber, a relative humidity in a range of 10% to 100%, and a temperature in a range of 25°C to 150°C; and wherein the thin film encapsulation layer further comprises at least one organic layer comprising a polymer.