ABSTRACT

The characterization of blue and white organic light-emitting diodes (WOLEDs) containing phosphorescent emitters are examined with a focus on the methods used for the characterization of new materials and device architectures. Early blue phosphorescent OLEDs had limited efficiency due to the lack of host materials with large triplet energies that could exothermically transfer energy to high-energy phosphorescent guests. Unfavorable efficiency was overcome with wide bandgap host materials that enabled internal quantum efficiencies near unity. One path to saturated deep-blue emitters favored the use of hosts that were electrically inert and required the use of phosphorescent guests capable of transporting charge. WOLED characterization is also discussed. White devices have been studied due to their potential for high-efficiency solid-state lighting; hence, their optical and electrical characterizations have been standardized for display and illumination engineers. Early

reports presented basic information on device performance, but more rigorous characterization methods have been developed and codified to foster the successful growth of the technology. Several tools for studying white device parameters and determining material properties are summarized in this chapter. 14.1 Introduction

Organic thin-film semiconductors are fascinating and promising materials for the development of new electronic and optoelectronic devices.1 Traditional inorganic semiconductor analytical techniques are often not sufficient to fully characterize organic semiconductors, but significant progress has been made in modifying tools to accommodate the unique features of organic materials. In parallel, organic device characterization has steadily matured, and new international standards have been delineated for the reporting of device properties. This chapter focuses on the discussion of blue and white phosphorescent organic light-emitting devices (PHOLEDs). Both devices contain phosphorescent emissive molecules, which imbue them with the ability to convert near 100% of injected current into light; however, devices need significant improvement in operational stability and high-yield-mass production methods to successfully compete with incumbent display and lighting technologies, so improvements in characterization and analyses of PHOLEDs are necessary to achieve the requisite device features. Section 14.2 of this chapter describes the development of blue PHOLEDs and the characterization methods deployed in understanding the fundamental properties of these devices. Section 14.3 provides insight into white OLED (WOLED) optical and materials characterization methods. 14.2 Blue ElectrophosphorescenceThe development of PHOLEDs emitting blue wavelengths progressed at a slower pace than counterparts emitting in the red and green. Blue PHOLED emission efficiency and operating lifetime are two example characteristics that have had a longer development time than

comparable green and red device characteristics. However, advances over the past decade continue to indicate that blue PHOLEDs remain the most promising path to high emission efficiency in displays and solid-state lighting. This section discusses some of the improvements made during the early development of blue PHOLEDs. 14.2.1 Device Architecture and Energy Transfer

In a typical PHOLED, a common approach to realizing high efficiency uses Förster energy from a fluorescent host material to excite a phosphorescent guest material. To ensure a suitable energetic alignment for this transfer (i.e., exothermic energy transfer from host to guest), the exciton energies of the host should exceed those of the guest. This ensures that energy transfer occurs from both the singlet and the triplet exciton state of the host and that the lowest excited energy state for the system is the guest triplet (i.e., the excited state is ultimately confined to the guest). This favorable energy alignment was difficult to realize during the initial development of blue PHOLEDs. Phosphorescent dopants were synthesized with high triplet energy levels, but the singlet energies of the dopants were also necessarily high. As a result, to ensure efficient energy transfer and exciton confinement, both the singlet and triplet levels of a suitable host had to be even higher in energy than hosts previously employed for other emitters. The lack of a suitable high-triplet-energy host was not a deterrent to the first demonstration of blue electrophosphorescence, which relied on endothermic energy transfer.2 In an Adachi et al. paper2 demonstrating the first blue electrophosphorescence, the host and guest materials were N,N¢-dicarbazolyl-4-4¢-biphenyl (CBP) and iridium(III)bis[(4,6-difluorophenyl)-pyridinato-N,C2¢]picolinate (FIrpic), respectively. In that paper, the singlet energy level of CBP was shown to be larger than that of FIrpic; however, the CBP triplet level is below that of FIrpic. The result of the energy difference is an excited state that resides at the CBP triplet level prior to recombination on FIrpic. The endothermic energy transfer is not the most efficient method to produce emission, so new hosts with appropriate energies and transport materials were developed and new analytical tests were adapted to select guest-host combinations for device fabrication.