Air-Stable Transparent Silver Iodide–Copper Iodide Heterojunction Diode

Transparent AgI–CuI heterojunctions with high rectifying diode behavior were prepared via vapor-phase iodization of metal thin films on transparent conducting oxide substrates. At room temperature, Ag and Cu metal thin films were quickly transformed into the transparent and well-crystallized β-phase of AgI and the γ-phase of CuI, respectively. The AgI and CuI films exhibited n-type and p-type semiconductor properties, respectively, with wide band gaps. The heterojunctions were obtained by applying the CuI film to the AgI film in a sequential iodization process. AgI compounds generally have poor air-stability under light, making them suboptimal for use in electronic applications. Here, we used a CuI top layer to inhibit the photodecomposition of the AgI bottom layer, resulting in an air-stable and smooth AgI–CuI film. We also propose a simple patterning method for the AgI–CuI layer using selective decomposition of AgI without the need for lithography equipment or toxic chemicals. Although there is metal ion exchange between the two layers, each layer has a different chemical composition and crystal structure; therefore, the AgI–CuI heterojunction exhibits pn-diode behavior with a rectifying ratio of 9.4 × 104, which is comparable to that of other transparent pn-diodes. These findings open a new path for electronic application of AgI materials.