Enhancing carrier injection balance in near-infrared (NIR) to visible upconversion devices (UCDs) is crucial for improving efficiency and stability. This study presents the incorporation of an insulating polymer (polymethyl methacrylate (PMMA)) between an aluminum cathode and an electron transport layer to reduce excess electrons in a light-emitting layer, thus balancing electrode-injected electrons and NIR-generated holes. The optimized device achieved a fivefold increase in maximum luminance and upconversion efficiency compared to the control. Additionally, it demonstrated a fast optical response, a broad optical modulation range, and significant potential for bioimaging applications, achieving a maximum resolution of 1693 dpi. This approach provides an effective solution for enhancing carrier injection balance in UCDs.