Effects of ultraviolet soaking on surface electronic structures of solution processed ZnO nanoparticle films in polymer solar cells

Abstract

Abstract:We systematically show the effect of UV-light soaking on surface electronic structure and chemical states of solution processed ZnO nanoparticle (ZnONP) films in UHV, dry air and UV-ozone. UV exposure in UHV induces a slight decrease in work function and surfacedesorption of chemisorbed oxygen, whereas UV exposure in presence of oxygen cause an increase in work function due to oxygen atom vacancy filling in the ZnO matrix. We demonstrate that UV-light soaking in combination with vacuum or oxygen can tune the work function of the ZnONP films over a range exceeding 1 eV. Based on photovoltaic performance and diode measurements, we conclude that the oxygen atom vacancy filling occurs mainly at the surface of the ZnONP films and that the films consequently retain their n-type behavior despite significant increase in measured work function.

UV-Induced Oxygen Removal for Photostable, High-Efficiency PTB7-Th:PC71BM Photovoltaic Cells

“…[26][27][28] In the event that such UV-treated ZnO layers are placed again in ambient air the reverse process occurs within minutes. We completed a cell with such UVN treated ZnO layer but placed in air, and observed a performance no different than the reference cell.…”

Surface passivation effect by fluorine plasma treatment on ZnO for efficiency and lifetime improvement of inverted polymer solar cells

“…In this regard, many studies have focused on engineering the surface of ZnO films through improving the interfacial electrical properties, better aligning energy-level and controlling the surface energy. Among the most efficient methods to modify the ZnO surface are the ultraviolet (UV) or UV-ozone treatment, [32][33][34][35][36][37] and the incorporation of a surface modification interlayer, such as a coordination polymer 38,39 or a self-assembled monolayer (SAM). [40][41][42][43][44] However, UV treatment may cause severe degradation of the device lifetime whereas the insulating properties of coordination polymers results in a trade-off between effective passivation and charge transport.…”

Simultaneous Effect of Ultraviolet Radiation and Surface Modification on the Work Function and Hole Injection Properties of ZnO Thin Films

“…It has been shown that PA molecules, as used in this study undergo tridentate binding to the ZnO surface, meaning that each PA molecule occupies three surface sites. The fact that the presence of the SAM does not reduce the strength of the light‐induced WF shift suggests that this phenomenon is not entirely caused by changes of the ZnO surface chemistry, but may also involve internal surfaces such as grain boundaries . As for the bare ZnO, the WF increases again when these UV‐soaked ZnO samples are stored in the dark (triangles in Figure ).…”

“…Self‐assembled monolayers (SAMs) based on phosphonic acids (PAs) are frequently employed as interface modifiers to tune the WF of ZnO, enabling efficient injection of electrons and even holes into a wide range of semiconducting molecules and polymers . In contrast, ultraviolet (UV) illumination (light‐soaking) was shown to lead to transient and persistent changes of the WF of metal oxides, and particularly of ZnO . Several studies deal with the influence of the sample environment during UV light soaking on the strength and direction of the WF shift .…”