Designing　a　highly　efficient　and　stable　photoelectrochemical　(PEC)　tandem　cell　for　unassisted　solar　water　splitting　is　considered　a　promising　approach　for　large-scale　solar　energy　storage.　To　date,　various　tandem　device　configurations　have　been　reported.　However,　the　achievement　of　a　solar-to-hydrogen　conversion　efficiency　(eta(STH))　of　10%　is　still　full　of　challenges,　due　to　the　incompatibility　between　the　two　photoelectrode　materials.　Here,　we　report　a　highly　efficient　and　stable　photovoltaic　(PV)-PEC　tandem　device　composed　of　a　dual　Si　photoelectrode　and　two　series-connected　ordinary　Si　PV　cells.　The　dual-photoelectrode　device　consists　of　an　n(+)np(+)-Si　photocathode　and　p(+)pn(+)-Si　photoanode,　which　allow　back　illumination　from　the　Si　substrate,　spatially　and　functionally　decoupling　the　optical　absorption　and　the　catalytic　activity.　By　further　employing　a　Ni　protective　layer　together　with　a　bifunctional　Ni-Mo　catalyst　for　both　the　Si　photocathode　and　photoanode,　the　PV-PEC　tandem　cell　can　perform　spontaneous　water　splitting　without　any　applied　bias.　A　high　eta(STH)　of　9.8%　with　a　stability　of　over　100　h　was　achieved　in　alkaline　solution　under　parallel　AM　1.5G　1　sun　illumination.　This　work　essentially　allows　a　modular　independent　optimization　of　each　component　which　can　enhance　the　efficiency　and　stability　of　the　PV-PEC　cells.