The $\mathrm{Pt}/\mathrm{Ti}/{\mathrm{Si}\mathrm{O}}_2/\mathrm{Si}$ stack is the most widely adopted substrate for integrating piezoelectric oxide thin films with silicon-based devices. In this work, we demonstrate that lead-free potassium sodium niobate (KNN) piezoelectric thin films fabricated by chemical solution deposition (CSD) exhibit markedly different structural and functional responses when subjected to varying $\mathrm{Ti}$ concentrations, arising from both unintentional $\mathrm{Ti}$ diffusion and intentional $\mathrm{Ti}$ doping. It was found that ${\mathrm{Ti}}^{4+}$ ions incorporated into the KNN lattice substitute ${\mathrm{Nb}}^{5+}$ at B site, acting as acceptor defects to compensate donor-type oxygen vacancies, forming the defect complexes $[\mathrm{T}{{\mathrm{i}}^{\mathrm{\prime }}}_{\mathrm{Nb}}-V_{\mathrm{O}}^{..}{]}^{.}$ and $[\mathrm{T}{{\mathrm{i}}^{\mathrm{\prime }}}_{\mathrm{Nb}}-V_{\mathrm{O}}^{..}-\mathrm{T}{{\mathrm{i}}^{\mathrm{\prime }}}_{\mathrm{Nb}}]$. These defect complexes reduced the dielectric constant, suppressed dielectric loss, and enhanced electric-field induced strain. At 1 mol % $\mathrm{Ti}$ doping, multiphase coexistence was observed, enabling polarization rotation. The coexistence of defects and multiphases yield a remarkably large effective piezoelectric coefficient $d_{33}^{e\mathrm{ff}}$ equivalent to 272 pm/V at 1.5 kHz. These findings establish a pathway for defect and phase engineering in perovskite oxides and highlight the significance of $\mathrm{Ti}$ control for reliable integration of high-performance, lead-free piezoelectric films with silicon-based devices.

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• Defects
• Dielectric properties
• Doping effects
• Phase transitions
• Piezoelectricity
• Solid-state chemistry
• Vacancies
• Thin films