Interfaces between nanoscale and bulk electroactive materials are important for the design of electronic devices using solution-processed nanoparticles. We report that thin films of hexanethiolate-capped gold nanoparticles with a core diameter of 2.1+/-0.4 nm deposited onto n-InP wafers form Schottky contacts whose barrier height can be actively tuned from 0.27+/-0.03 to 1.11+/-0.09 eV by electrochemically adjusting the nanoparticle Fermi level. This result is remarkable because interfacial barriers at conventional metal-semiconductor contacts are largely insensitive to the initial Fermi level of the metal. Furthermore, it highlights two general features of solution-processed nanoparticle assemblies in comparison with traditional bulk electronic materials: (1) the ability of ions to permeate the nanoparticle assembly enables the electrochemical injection of charges and hence active control of the Fermi level, and (2) ligand passivation of nanoparticle surfaces prevents interfacial reactions with the semiconductor that could otherwise lead to strong Fermi-level pinning.