Electroabsorption investigations of advanced polymer light-emitting diodes
In this thesis we employ electroabsorption (EA) spectroscopy in the study of encapsulated blue polymer light-emitting diodes (PLEDs), introduced generally in Chapter 1, that also incorporate a hole injection layer poly(3,4-ethylene dioxythio- phene) doped with poly(styrene sulfonate) (PEDOT:PSS). In addition to providing valuable information about the polymer film, EA. described in Chapter 2, allows the probing of the built-in voltage, Vbi, generated through the equilibration of the chemical potential across the PLED heterostructure. Typically, VBI is measured by applying a voltage of the form V = Vdc + Vacsm(ujt) across the diode, and finding Vdc (or Vnuii) at which the EA signal vanishes. In Chapter 3, apart from describing our EA experimental set-up, we measure the EA response of a simple one-layer PLED (without PEDOT:PSS), and find full agreement between the experimental results and the Stark theory. In devices with PEDOT:PSS (Chapters 4-6), the Stark elec- troabsorption signal is mixed with a smaller intensity signal, not predicted in the Stark effect. In some devices, this causes deviation from the expected behaviour, for example by introducing Vnuu dependence on the photon energy and on ac frequency. Although this poses a potential problem for accurate Vbi measurements, the effect is minimal at energies near the Stark response peak and high frequencies, which makes the measurement of Vbi possible. We also consider the origin of the 'other' signal, and present evidence which strongly suggests that it is generated by trapped charge at the PEDOT:PSS/emitting-polymer interface. We use Vbi measurements to probe energy level alignment across the PLED heterostructure, in a series of devices which vary only in the composition of the emitting polymer layer. Our results, which show that Vbi is polymer dependent, in full account with the theory of alignment of the chemical potential across the PLED heterostructure, suggest Fermi level pinning to the polymer bipolaron levels. Finally, we investigate the effects of electrical driving on these devices, and find strong evidence for degradation of PEDOT:PSS (particularly near the interface) and its work function, in full agreement with the available literature.