Evidence of the Ambipolar Behavior of Mo 6 Cluster Iodides in All-Inorganic Solar Cells: A New Example of Nanoarchitectonic Concept.

Ambipolar materials such as carbon nanotubes, graphene, or 2D transition metal chalcogenides are very attractive for a large range of applications, namely, light-emitting transistors, logic circuits, gas sensors, flash memories, and solar cells. In this work, it is shown that the nanoarchitectonics of inorganic Mo 6 cluster-based iodides enable to form thin films exhibiting photophysical properties that enable their classification as new members of the restricted family of ambipolar materials. Thus, the electronic properties of the ternary iodide Cs 2 [{Mo 6 I 8 i }I 6 a ] and those of thin films of the aqua-complex-based compound [{Mo 6 I 8 i }I 4 a (H 2 O) 2 a ]· x H 2 O were investigated through an in-depth photoelectrochemical study. Once hole/electron pairs are created, the holes and electrons turn to be transported simultaneously in opposite directions, and their lifetimes exhibit similar values. The ambipolar properties were demonstrated via the integration of [{Mo 6 I 8 i }I 4 a (H 2 O) 2 a ]· x H 2 O as light harvesters in an all-solid solar cell. A significant photoresponse with a typical diode characteristic clearly provides evidence of the simultaneous transfer and transport of holes and electrons within the [{Mo 6 I 8 i }I 4 a (H 2 O) 2 a ]· x H 2 O layer. The ambipolar behavior results, on the one hand, from the confinement of electrons imposed by the nanometric size of the molecular metal clusters and, on the other hand, from the poor electronic interactions between clusters in the solid state. Such molecular structure-based layers lead naturally to an intrinsic semiconducting behavior.