The　efficiency　improvement　of　organic　light-emitting　diodes　(OLEDs)　is　important　but　challenging.　Here,　we　introduce　a　unique　OLED　with　a　hole　modulation　layer　(HML)　in　the　middle　of　its　emission　layer　(EML).　The　external　quantum　efficiency　and　power　efficiency　can　be　improved　by　approximately　58%　when　an　HML　with　optimized　thickness　is　inserted.　HML　insertion　can　efficiently　retard　hole　flow,　thus　improving　(i)　exciton　distribution　uniformity　and　(ii)　local　electron–hole　charge　balance.　A　systematic　study　of　the　individual　contributions　of　two　EMLs　separated　by　the　HML　shows　that　the　former　factor　dominantly　works　at　low　current　densities　(＜　10　mA/cm2),　whereas　the　latter　factor　functions　over　the　entire　current　density　range　of　the　OLED.　Therefore,　the　efficiency　improvement　is　greatest　at　low　current　densities,　which　aligns　with　the　typical　operating　range　in　display　applications.　The　results　provide　a　deeper　understanding　of　the　OLED　emission　mechanism,　and　the　proposed　OLED　structure　can　significantly　benefit　high-performance　OLED　displays.　Graphical　Abstract:　[Figure　not　available:　see　fulltext.]　©　2023,　The　Author(s)　under　exclusive　licence　to　The　Korean　Institute　of　Metals　and　Materials.