the pyrylium salt and its derivatives used in the present invention have the following general structure : in the above formula ( i ), r 1 , r 2 and r 3 represent an alkyl , aryl , or heteroaryl having up to 50 carbon atoms ; z represents an anionic function - including ion , such as cl − , br − , f − , tetrafluoroborate , perchlorate , methanesulfonate , or phosphohexafluoride ; and x is oxygen , sulfur , or selenium . the compound expressed according to formula ( i ) exhibits characteristics of high electron affinity and excellent reversibility . representative charge transport mechanisms are shown below in accordance with schemes 1 and 2 , which show examples of the charge injection process for 2 , 4 , 6 - triphenylpyrylium salt and 4 , 4 ′- bipyryllium salt , respectively . some preferred pyrylium salts or pyrylium derivatives represented by formula ( i ) include the following compounds : in the above preferred pyrylium salts or pyrylium derivatives , x is either o or s ; and z is an anionic function - including ion , such as cl − , br − , f − , tetrafluoroborate , perchlorate , methanesulfonate , or phosphohexafluoride . the compound expressed according to formula ( i ) can be used directly as the charge transport layer , or can be incorporated into a polymer as a unit or as a pendant side group . in addition , such a pyrylium salt or pyrylium derivative can be doped into a matrix medium when the charge transport layer comprises a charge transport functional material plus a dopant . further , the pyrylium salt or pyrylium derivative can be used as a host to enhance charge injection and transport property wherein a dopant emitter may be doped for controlling emission color . in one typical application , the compound expressed according to the above formula ( i ) can be used directly as the transport layer in multi - layer devices , such as a two layer device , shown in fig1 or a three layer device , shown in fig2 as described more fully below . in fig1 a two layer device comprises an emissive layer 103 and an electron transport layer 105 sandwiched between a cathode 106 and an anode 101 . in fig2 a three layer device comprises an emissive layer 203 sanwiched between an electron transport layer 205 and a hole transport layer 202 . additionally , the emissive layer 203 , electron transport layer 205 and hole transport layer 202 are sandwiched between a cathode 206 and an anode 201 . general procedures for an fabrication of an oled are as follows : to contruct a three layer device , as in fig2 a clean substrate coated with a patterned layer of indium tin oxide ( ito ) is first obtained . next , the substrate is treated with o 2 plasma for 1 - 5 minutes . afterwards , the substrate is placed in a thermal evaporator and the pressure is lowered . then , organic and metallic layers are evaporated onto the substrate at a rate approximately between 1 - 3 å / s . these organic and metallic layers may vary depending upon the desired oled . a hole transport layer is usually evaporated with a thickness of ˜ 200 å . next , an emissive layer is evaporated with a host and dopant . normally , 100 - 400 å of the emissive layer is deposited . then , an electron transport material is evaporated to form a layer that is usually 200 - 400 å thick . after the evaporation of the preferred organic and metallic layers , a mask is placed adjacent to the layer to define where metal areas corresponding to cathodes are to be evaporated . then , about 120 å of a li — al alloy is evaporated to improve electron injection into the device . finally , after about 1500 å of al is deposited , the evaporator is allowed to cool . fabrication of a suitable charge transport layer using a pyrylium salt or its derivatives according to the above formula ( i ) can be accomplished through use of thermal deposition in a vacuum , or by spin coating of a solution thereof . in addition , high - density pixeled displays can be fabricated through use of suitable masking procedures , or by use of thermal or piezoelectric ink jet printing techniques . in another typical application , the compound expressed according to the above formula ( i ) can be incorporated into an emissive layer as a host to enhance charge transport property in a single layer device , as shown in fig3 in which the pyrylium salt or pyrylium derivative is doped into the emissive layer . in fig3 a single layer device comprises a combined layer 303 , comprising an emissive layer and an electron transport layer , sandwiched between a cathode 306 and an anode 301 . cyclic voltametric ( cv ) analysis of known electron transport material , tris ( 8 - hyroxyquinoline ) aluminum ( alq3 ), was carried out using tetrabutylammonium tetrafluoroborate ( tbabf 4 , 0 . 1 m in acetonitrile ) as an electrolyte , pt as a work electrode and ag / ag + ( 0 . 1 m ) as a reference electrode at a scan speed of 100 mv / s . fig4 shows the cv curve , indicating an onset reduction , or surge of reduction at the electrode surface , of − 1 . 68 v . the lowest unoccupied molecular orbital ( lumo ) was estimated according to a general formula that gives lumo ( in ev ) as the difference between − 4 . 8 and the ered ( onset reduction potential in volts ) of − 1 . 68 . according to this formula , the lumo was approximately − 3 . 12 ev . a similar cv measurement for a pyrylium salt , 2 , 4 , 6 - triphenyl pyrylium tetrafluoroborate ( tppfb 4 ), was performed according to the same conditions as above , in the measurement of alq3 . fig5 shows the cv curve of the pyrylium salt , which indicates a lower onset reduction of − 0 . 44 v and − 1 . 53 v , demonstrating that reduction ( or electron injection ) is easier for pyrylium salt than alq3 . the lumo for the pyrylium salt was − 3 . 27 ev ( using the second reduction onset for the estimation ), being lower than alq3 . a cv comparison also indicates that pyrylium salt not only has lower lumo level ( easier electron injection ), but also has more reversible charge injection character ( more stable electrochemical property ) than alq3 , as revealed by the features of both cv curves . an oled device was fabricated with the device structure of ito / α - npd 30 nm / alq3 + dcm2 ( 2 %) 20 nm / tppfb 4 30 nm / al 100 nm , in which ito refers to indium tin oxide coated glass substrate , α - npd refers to a hole transport layer with n , n ′- di ( naphthalen - 1 - yl )- n , n ′ diphenyl - benzidine , dcm2 refers to a red dopant emitter , 4 -( dicyanomethylene )- 2 - methyl - 6 -( p - dimethylaminostyryl )- 4h - pyran , and tppfb 4 refers to 2 , 4 , 6 - triphenyl pyrylium tetrafluoroborate as the electron transport layer . the device was fabricated according to procedures known in the art . the device emitted red light with a brightness of 315 cd / m 2 when a forward bias voltage of 5 . 2 v was applied . the brightness changed to 250 cd / m 2 at 5 . 2 v ( retained 80 % original brightness ) after continuously working for 6 hours at 5 . 2 v . an oled device was fabricated in a similar manner as described in example 2 , with the exception of using alq3 as the electron transport layer . the oled had a device structure of ito / α - npd 30 nm / alq3 + dcm2 ( 2 %) 20 nm / alq3 30 nm / al100 nm . the device emitted red light with a brightness of 315 cd / m 2 when a forward bias voltage of 5 . 8 v was applied . the brightness changed to 180 cd / m 2 at 5 . 8 v ( retained 58 % original brightness ) after continuously working for 6 hours at 5 . 8 v . an oled device was fabricated in a similar manner as described in example 2 , with the exception of using tppbf 4 as a host material . the oled had a device structure of ito / α - npd 30 nm / tppbf 4 + dcm2 ( 2 %) 20 nm / tppbf 4 30 nm / al 100 nm . the device emitted red light with a brightness of 315 cd / m 2 when a forward bias voltage of 4 . 8 v was applied . the brightness changed to 300 cd / m 2 at 4 . 8 v ( retained 95 % original brightness ) after continuously working for 6 hours .