Development of hybrid organic-inorganic light emitting diodes using conducting polymers deposited by oxidative chemical vapor deposition process

Abstract

Difficulties with traditional methods of synthesis and film formation for conducting polymers, many of which are insoluble, motivate the development of CVD methods. Indeed, conjugated polymers with rigid linear backbones typically crystallize readily and overcoming the resultant heat of crystallization makes them difficult to dissolve. Poly(3,4-ethylenedioxythiophene) (PEDOT) thin films were obtained through oxidative chemical vapor deposition (oCVD) by using a new oxidant- bromine. The use of bromine eliminates any post processing rinsing step required with other oxidants like iron chloride and hence makes the process completely dry. Accelerated aging experiments show longer retention of electrical conductivity for the PEDOT films obtained using bromine as the oxidant. Conductivities as high as 380 S/cm were obtained for PEDOT films deposited using bromine as the oxidant at 80 'C, which is significantly higher than that for PEDOT films deposited using iron chloride as the oxidant at the same temperature. Cross-sectional SEM of the PEDOT films deposited using bromine on silicon trench wafers demonstrates high conformal deposition of the films. All the results show the possibility of depositing highly conducting, conformal PEDOT films on any substrate including silicon, glass, paper, plastic. One of the many applications of conducting polymer is as hole-transport layer in light emitting diode. To be competitive in the LED market, improvements in hybrid-LED quantum efficiencies as well as demonstrations of long-lived HLED structures are necessary. In this work, we consider improvement in the stability of the HLED. The device fabricated can be configured as ITO/ Poly (EDOT-co-TAA)/CdSe (ZnS)/ Au. All the materials used in the device synthesis are stable in ambient conditions and all the synthesis steps on ITO substrate are done either in air or in very moderate pressure conditions. This significantly reduces the cost of the device fabrication by obviating the need of packaging layers and ultrahigh vacuum tools. The operating voltage as low as 4.3 V have been obtained for red-LEDs. We believe that with optimization of various layers in the device, further improvements can be made. For green LEDs we obtained the characteristic IV curve of a diode, but we still need to work on getting a functioning green LED.