Sulfide Treatment of ZnO Single Crystals and Nanorods and the Effect on P3HT−ZnO Photovoltaic Device Properties

Surface chemical changes or passivation can be used to enhance the electrical performance of device structures. The solution-based (NH 4 ) 2 S x treatment of ZnO single crystals of (0001), (101̅0), and (0001̅) orientations as well as ZnO nanorods was studied by using X-ray and ultraviolet photoelectron spectroscopies (XPS and UPS). The (101̅0) and (0001̅)-oriented single crystals and ZnO nanorods each showed evidence that the ZnO was consumed, forming a ZnS layer at the surface. In contrast, the ZnO (0001) crystal did not substantially react with the sulfide solution. However, the surface hydroxyl species were displaced by a sulfide passivation layer of 0.21 ML. This process resulted in an apparent increase in upward band bending by ∼0.1−0.3 eV. P3HT−ZnO photovoltaic devices fabricated on O-polar single crystals showed electrical properties which can be correlated with ZnS formation due to the sulfide treatment.