Molecular Design for Tuning Work Functions of Transparent Conducting Electrodes

Abstract

In this Perspective, we provide a brief background on the use of aromatic phosphonic acid modifiers for tuning work functions of transparent conducting oxides, for example, zinc oxide (ZnO) and indium tin oxide (ITO). We then introduce our preliminary results in this area using conjugated phosphonic acid molecules, having a substantially larger range of dipole moments than their unconjugated analogues, leading to the tuning of ZnO and ITO electrodes over a 2 eV range as derived from Kelvin probe measurements. We have found that these work function changes are directly correlated to the magnitude and the direction of the computationally derived molecular dipole of the conjugated phosphonic acids, leading to the predictive power of computation to drive the synthesis of new and improved phosphonic acid ligands.

Tuning colloidal quantum dot band edge positions through solution-phase surface chemistry modification

“…Our analysis of the experimental band edge shifting data suggests that ligands containing fluorinated functional groups behave differently from those that do not—a phenomenon that has been previously reported for solid-state surface modification of ITO and ZnO with functionalized phosphonate molecules5. In particular, the 4-CN-CA − /QD complex has the most negative (electron withdrawing) projected ligand dipole, but its measured band edge is not as deep as either the 4-CF 3 -CA − /QD or 3,5-F-CA − /QD complexes (Figs 3c and 4a).…”

“…The study of QDs has long been focused on the inorganic core; specifically, on quantum-confinement (for example, size-dependent band gaps1 and enhanced Auger-type processes2) or increased surface-to-volume ratio effects (for example, size-dependent phase transitions3). However, it has become increasingly clear that post-synthetic surface chemistry modification, or ligand exchange, can critically influence QD optoelectronic properties, as well4567. Ligand exchanges are often performed in the solid state, where films of QDs with long, aliphatic surface ligands are exposed to solutions of shorter alkyl-chain or atomic ligands for exchange.…”

Stable and tunable phosphonic acid dipole layer for band edge engineering of photoelectrochemical and photovoltaic heterojunction devices

“…[15][16][17] In traditional PEC, the dipole layer is added at the semiconductor/electrolyte interface, which usually leads to very limited stability and only a small voltage increase due to shielding of the dipole by the electrolyte. [18][19][20][21] In order to compete with commercially well-established PV coupled with electrolyzers, PEC systems have to be stable for years, likely in extreme pH electrolytes. 22 Thus, corrosion protection layers have been investigated, using chemically stable materials such as TiO 2 .…”

Improved photovoltaic performance of phosphonic acid‐based sensitized solar cells via an electron‐withdrawing moiety: A density of functional theory study

“…[19] Several studies have evaluated phosphonic acids and described their binding pattern on different metal oxides. [20][21][22][23][24][25] Both ester (PO 3 R 2 ) and acid (PO 3 H 2 ) demonstrate a high affinity with other organic functional groups and are good candidates for surface modifications in many organic solvents and water. [20] The semiconductors mainly used as a photoanode for DSSC are MnO (CB = −3.4 eV), SnO (CB = −3.6 eV), TiO 2 (CB = −4 eV), and FeO (CB = −4.4 eV).…”