. . . "As the solvent used for the hole transport layer coating solution, usable are ketones such as acetone and methyl ethyl ketone, esters such as methyl acetate and ethyl acetate, aromatic hydrocarbons such as toluene and xylene, ethers such as 1,4-dioxane and tetrahydrofuran, and hydrocarbons substituted with a halogen atom, such as chlorobenzene, chloroform and carbon tetrachloride." . .