Ionic Junctions Exhibit Photovoltage and Electroluminescence

Solid-state devices such as LEDs and photovoltaic cells primarily employ electronic charge carriers (electrons and holes) that, when diffused across a PN junction, create a built-in potential that allows for the preferential flow of current in one direction. Now scientists at Cornell University in Ithaca, N.Y., have found analogous results using ionic charge carriers, where the same built-in potential arises from the diffusion of anions and cations between two organic layers.

Although the number of materials with semiconducting behavior and ionic conductivity is small, there are several organic semiconductors with these intrinsic properties, and the researchers note that almost any organic semiconductor can be modified to satisfy the conditions for ionic conductivity. These materials can be classified as N-type and P-type, where anions and cations are the predominant current carriers, respectively, and can be fabricated to resemble electronic junctions.

As reported in the Sept. 8 issue of Science, the team used soft-contact lamination, in which two substrates with patterned electrodes were coated with ionic films and brought together to form a PN junction. The diffusion of ions across the junction resulted in a built-in potential and rectification, which were especially pronounced under forward bias compared with reverse bias. Under forward bias, the PN junction displayed light emission from the N-type material with a luminance of 500 cd/m² at 5 V. Conversely, it exhibited a photovoltage of 0.45 V under illumination from a halogen lamp.

Based on the observed rectification, electroluminescence and photovoltage at PN junctions, the researchers suggest that mobile ionic charge can be used to control the flow of electronic current in many solid-state devices. These junctions might also help to decrease recombination and increase the efficiency in organic solar cells. The coupling of electronic and ionic carriers may improve the performance of existing solid-state devices and enable new functionalities.