??r1), r1 is a hydrogen atom, a halogen atom, an aryl group having 6 to 18 carbon atoms, a hetero aryl group having 6 to 18 carbon atoms, or an alkyl group having 1-12 carbon atoms, r2 to r10 are each independently hydrogen, an aryl group having 6 to 30 carbon atoms, or hetero aryl group having 6 to 30 carbon atoms, and at least one of x1 to x4 is a nitrogen atom.. . 05/15/1420140131665Organic ele