Quantum　dots　(QDs)-based　light-emitting　metal-insulator-semiconductor　junction　(LE-MISJ)　devices　currently　face　challenges　of　low　brightness　and　short　lifetime.　The　degradation　mechanisms　of　the　device　remain　poorly　understood.　In　this　work,　to　explore　carrier　dynamics　and　QDs　degradation　mechanism　in　the　QDs　layer,　we　construct　a　LE-MISJ　device　consisting　of　electrode/insulator/monolayer　QDs/electrode　to　investigate　its　photoelectric　properties.　It　is　found　that　continuous　carrier　injection　and　carrier　transport　between　QDs　lead　to　electric　field-induced　dielectric　breakdown　between　QDs,　accelerating　the　formation　of　defects　on　the　surface　of　QDs　and　leading　to　QD　degradation.　Experimental　and　theoretical　analyses　demonstrate　that　filling　the　QDs　gap　with　poly(methyl　methacrylate)　(PMMA)　effectively　protects　and　supports　surface　ligands　and　reduces　carrier-induced　damage.　The　device　with　2%　PMMA　shows　180%　enhanced　peak　electroluminescence　intensity　and　spectral　narrowing　(full　width　at　half　maximum　from　38.8　nm　to　31.2　nm)　compared　to　PMMA-free　controls.　Moreover,　the　device　with　2%　PMMA　decays　to　80%　of　the　original　intensity　in　4.5　times　the　time　of　the　device　without　PMMA.　This　study　can　provide　some　research　value　to　understand　the　working　principle　and　optimization　of　LE-MISJ.　©　2025　Optica　Publishing　Group　under　the　terms　of　the　Optica　Open　Access　Publishing　Agreement.