We demonstrate that, through formation of Zn1-xMgxO nanocrystal solid solution (NCSS) coupled with lead sulfide (PbS) nanocrystal (NC) of different size, optoelectronic properties of a donor-acceptor energy harvesting assembly can be probed and tuned. In the ITO/PEDOT:PSS/PbS/Zn1-xMgxO/Al arrangement as a photovoltaic test probe, simultaneous tuning of Zn1-xMgxO NC solid solution (NCSS) and PbS NC in a narrow range in the energy levels is achieved altering the open circuit voltage and the short circuit current as measures of donor/acceptor interfacial electronic properties. With the Mg composition increasing, the energy band gap of the Zn1-xMgxO NCSS (acceptor) increases with its conduction band edge becoming closer to the lowest occupied molecular orbital level of the PbS NC (donor), increasing splitting of quasi-Fermi energy levels in the electron donor/acceptor assembly under illumination. More amount of Mg in the NCSS leads to transition from an energy level limited regime to a trap-induced charge transport limited regime, evidenced by a simultaneous decrease in the open circuit voltage and the short circuit current. This new finding provides insights of trade-off between the energy level off-set and the trap-induced charge transport degradation in engineering interfacial charge transfer and transport via formation of NCSS in integrating modern electronic devices.