Heavy　doping　critically　minimizes　depletion　region　widths　for　efficient　charge　transport　in　organic　solar　cells　(OSCs),　yet　systematic　studies　elucidating　its　underlying　mechanisms　remain　scarce.　To　address　this,　two　polydopamine-polyoxometalate　composites　(PDA-PMA　and　PDA-PMA(N))　are　designed　via　innovative　mutual　doping　pathways.　PDA-PMA　achieved　ultrahigh　doping　density　(1.17　x　1023　cm-3)　through　H3[P(Mo3O10)4]　(PMA)　initiated　oxidative　polymerization　of　dopamine,　where　electron　transfer　simultaneously　induced　p-doped　PDA　and　n-doped　PMA.　Remarkably,　neutralization　with　ammonia　yielded　PDA-PMA(N),　which　retained　even　higher　doping　density　(3.74　x　1023　cm-3)　via　structural　rearrangement-driven　organic　doping.　XPS/ESR　studies　revealed　distinct　pathways:　dual　organic/inorganic　doping　in　PDA-PMA　versus　organic-dominated　doping　in　PDA-PMA(N).　This　deep　doping　compressed　depletion　region　widths　from　44.42　nm　in　undoped　controls　(the　blend　of　PDA　and　PMA)　to　0.052　nm　(PDA-PMA(N)),　surpassing　PEDOT:PSS　(0.238　nm)　and　enabling　barrier-free　hole　transport.　Consequently,　PBDB-TF:BTP-eC9-based　OSCs　with　PDA-PMA　and　PDA-PMA(N)　achieved　exceptional　power　conversion　efficiencies　(PCEs)　of　20.02%　and　20.29%,　respectively.　Furthermore,　the　neutralized　PDA-PMA(N)　demonstrated　superior　stability　(86.2%　PCE　retention　after　1800　h　illumination)　by　suppressing　interfacial　corrosion.　This　work　elucidates　structure-dependent　doping　mechanisms　and　provides　a　universal　strategy　for　developing　high-performance　hole　transport　layers　through　tailored　doping,　advancing　OSC　commercialization.