Opto-electronic devices that make use of organic materials are becoming increasingly desirable for a number of reasons.  Many of the materials used to make such devices are relatively inexpensive, so organic opto-electronic devices have the potential for cost advantages over inorganic devices.  In addition, the inherent properties of organic materials, such as their flexibility, may make them well suited for particular applications such as fabrication on a flexible substrate.  Examples of organic opto-electronic devices include organic light emitting devices (OLEDs), organic phototransistors, organic photovoltaic cells, and organic photodetectors.  For OLEDs, the organic materials may have performance advantages over conventional materials.  For example, the wavelength at which an organic emissive layer emits light may generally be readily tuned with appropriate dopants. 
OLEDs make use of thin organic films that emit light when voltage is applied across the device.  OLEDs are becoming an increasingly interesting technology for use in applications such as flat panel displays, illumination, and backlighting.  Several OLED materials and configurations are described in U.S. Pat.  Nos.  5,844,363, 6,303,238, and 5,707,745, which are incorporated herein by reference in their entirety. 
One application for phosphorescent emissive molecules is a full color display.  Industry standards for such a display call for pixels adapted to emit particular colors, referred to as “saturated” colors.  In particular, these standards call for saturated red, green, and blue pixels.  Color may be measured using CIE coordinates, which are well known to the art. 
One example of a green emissive molecule is tris(2-phenylpyridine) iridium, denoted Ir(ppy)3, which has the following structure:

In this, and later figures herein, we depict the dative bond from nitrogen to metal (here, Ir) as a straight line. 
As used herein, the term “organic” includes polymeric materials as well as small molecule organic materials that may be used to fabricate organic opto-electronic devices. “ Small molecule” refers to any organic material that is not a polymer, and “small molecules” may actually be quite large.  Small molecules may include repeat units in some circumstances.  For example, using a long chain alkyl group as a substituent does not remove a molecule from the “small molecule” class.  Small molecules may also be incorporated into polymers, for example as a pendent group on a polymer backbone or as a part of the backbone.  Small molecules may also serve as the core moiety of a dendrimer, which consists of a series of chemical shells built on the core moiety.  The core moiety of a dendrimer may be a fluorescent or phosphorescent small molecule emitter.  A dendrimer may be a “small molecule,” and it is believed that all dendrimers currently used in the field of OLEDs are small molecules. 
As used herein, “top” means furthest away from the substrate, while “bottom” means closest to the substrate.  Where a first layer is described as “disposed over” a second layer, the first layer is disposed further away from substrate.  There may be other layers between the first and second layer, unless it is specified that the first layer is “in contact with” the second layer.  For example, a cathode may be described as “disposed over” an anode, even though there are various organic layers in between. 
As used herein, “solution processible” means capable of being dissolved, dispersed, or transported in and/or deposited from a liquid medium, either in solution or suspension form. 
A ligand may be referred to as “photoactive” when it is believed that the ligand directly contributes to the photoactive properties of an emissive material.  A ligand may be referred to as “ancillary” when it is believed that the ligand does not contribute to the photoactive properties of an emissive material, although an ancillary ligand may alter the properties of a photoactive ligand. 
As used herein, and as would be generally understood by one skilled in the art, a first “Highest Occupied Molecular Orbital” (HOMO) or “Lowest Unoccupied Molecular Orbital” (LUMO) energy level is “greater than” or “higher than” a second HOMO or LUMO energy level if the first energy level is closer to the vacuum energy level.  Since ionization potentials (IP) are measured as a negative energy relative to a vacuum level, a higher HOMO energy level corresponds to an IP having a smaller absolute value (an IP that is less negative).  Similarly, a higher LUMO energy level corresponds to an electron affinity (EA) having a smaller absolute value (an EA that is less negative).  On a conventional energy level diagram, with the vacuum level at the top, the LUMO energy level of a material is higher than the HOMO energy level of the same material.  A “higher” HOMO or LUMO energy level appears closer to the top of such a diagram than a “lower” HOMO or LUMO energy level. 
As used herein, and as would be generally understood by one skilled in the art, a first work function is “greater than” or “higher than” a second work function if the first work function has a higher absolute value.  Because work functions are generally measured as negative numbers relative to vacuum level, this means that a “higher” work function is more negative.  On a conventional energy level diagram, with the vacuum level at the top, a “higher” work function is illustrated as further away from the vacuum level in the downward direction.  Thus, the definitions of HOMO and LUMO energy levels follow a different convention than work functions. 
More details on OLEDs, and the definitions described above, can be found in U.S. Pat.  No.  7,279,704, which is incorporated herein by reference in its entirety.