What sort of molecules or polymers are used in OLED displays? OLED displays have been emerging as competitors for LCD displays for some time and are now common in mobile phones (see comparison of a Google Nexus 1 and an iPhone here)

The displays use small molecules or polymers that emit light when a current is passed through them (like a semiconductor LED). But what is the structure of the emitters?
A typical organic light emitting diode (OLED) structure consisting of a glass substrate, a indium titanium oxide (ITO) anode, and an organic bilayer diffused into a metallic cathode, is given below:



The organic bilayer consists of:


a hole transport layer (HTL), typically of N, N'-diphenyl-N, N'-bis(3-methylphenyl) 1, 1'-biphenyl-4, 4' diamine (TPD) or N,N′-di(naphthalene-1-yl)-N,N′- diphenylbenzidine (NPB)
an emitting/electron transport layer (EML/ETL), typically of tris(8- hydroxyquinoline) aluminum (Alq3)


Source: http://espace.library.uq.edu.au/eserv.php?pid=UQ:8598&amp;dsID=SMS.pdf