Abstract
In this paper we demonstrate the use of low-temperature-solution- processable bismuth iodide (BiI <sub>3</sub> ) nanosheets as hole transport layers in organic photovoltaics with an active layer comprising poly(3-hexylthiophene) (P3HT) mixed with a fullerene derivative. The performance of the resulting devices was comparable with that of corresponding conventionally used systems incorporating polyethylenedioxythiophene:polystyrenesulfonate (PEDOT:PSS). UV-vis spectroscopy revealed that the transparency of a BiI <sub>3</sub> layer in the visible (>620 nm) and near-infrared range is greater than that of a PEDOT:PSS layer. X-ray photoemission spectroscopy of a BiI <sub>3</sub> film revealed signals at 158.8, 164, 618.6, and 630 eV - characteristic of Bi 4f <sub>7/2</sub> , Bi 4f <sub>5/2</sub> , I 3d <sub>5/2</sub> , and I 3d <sub>3/2</sub> , respectively - that indicated a stoichiometric BiI <sub>3</sub> film. Wet milling of BiI <sub>3</sub> crystals resulted in the formation of nanosheets, the presence of which we confirmed using scanning electron microscopy. The resultant power conversion efficiency of the device was approximately 3.5%, with an open-circuit voltage of 0.56 V, a short-circuit current density of 10.4 mA cm <sup>-2</sup> , and a fill factor of 60.1% under AM1.5G irradiation (100 mW cm <sup>-2</sup> ). © 2013 Published by Elsevier B.V.