SPACE-CHARGE LIMITED INJECTION INTO TRAP-FREE POLYMERS

The existence of trap-free unipolar charge transport polymers designed for application in electrophotographic receptors is exploited in a test for distinguishing ohmic from emission limited contacts. The test involves comparison of dark injection transients created by the application of a voltage step with time-of-flight transients obtained by UV photoexcitation, and comparison of steady-state dark-injection currents with the current values predicted for space charge limited currents in these materials. Contacts are characterized between electrodes such as carbon-black filled polymer, gold, indium-tin oxide and aluminum, and trap-free hole transport media, N,N'-diphenyl-N,N'-bis(3-methylphenyl)-[1,1'-biphenyl]-4,4' -diamine (TPD) and a condensation polymer containing TPD transport-active units (PTPB). Evidence is provided that many of these contacts are truly ohmic.