Surfaces and interfaces in sensors and photovoltaic devices

Surfaces and interfaces control much of the behaviour of electronic and optoelectronic devices, yet they are often ignored, or considered with the most simplistic models when devices are designed and analysed in the laboratory. Typically, the Schottky-Mott and Anderson approximations, which assume vacuum level alignment at metal/semiconductor and semiconductor/semiconductor interfaces, respectively, are assumed in the literature. Despite their wide use, these approximations have been known to be incorrect since 1947 when John Bardeen noted that interfacial alignment would be governed by high densities of interfacial electronic states within the semiconductor gap(s), due to chemical interactions between the two materials, as well as inherent surface states, due to reduced coordination and resulting changes in bonding geometries of atoms at the surface of a material, compared to those in the bulk. These states tend to restrict the movement of the Fermi level within the semiconductor(s) gap(s), and the resulting charges localized near the interface form "interface dipoles", abruptly shifting the vacuum levels. The complexities of the surface caused Wolfgang Pauli to quip, "God made the bulk; surfaces were invented by the devil". "Interface Engineering" is the intentional reaction of a surface with chemical species, or the inclusion of thin interlayers - sometimes one molecule thick - at interfaces in electronic devices in order to modify their properties in advantageous ways. For instance, layers of species with oriented dipole moments can be used to offset the energy levels between two semiconductors, or a semiconductor and a metal, in a way that aids charge injection or extraction. Another application of interface engineering is to affect a change in the way one material deposits on another. An everyday example of this is a freshly-waxed car, where water deposited on the paint no longer spreads out and forms a film, rather, it "beads up". Wax has been used to "engineer" the paint/water interface. Similarly, layers placed on one material (metal, dielectric, substrate), can affect the morphology of a semiconductor film grown on top. In this proposal, we have broadened the typical definition of interface engineering to include interlayers that change the propagation and coupling of light from one material to another. Here we aim to apply our experience with these techniques to three areas of research: 1) using optical interface engineering to enable a new type of sensor that can be use to detect exposure to chemical, biological and radiological threats, 2) studying and modifying the interfaces in printed electronic sensors, and 3) studying and modifying the interfaces in lead-free perovskite solar cells to improve their efficiency through the elimination of charge extraction barriers.