Silicon　heterojunction　solar　cells　are　expected　to　increase　their　market　share　in　the　near　future.　Qu　et　al.　identify　an　embedded　nanotwin　structure　at　the　crystalline　silicon/hydrogenated　amorphous　silicon　interface　of　silicon　heterojunction　cells　that　limits　the　device　performance　and　devise　an　approach　to　suppress　its　formation.　The　interface　of　high-quality　crystalline　silicon/hydrogenated　amorphous　silicon　(c-Si/a-Si:H)　is　indispensable　for　achieving　the　ideal　conversion　efficiency　of　Si　heterojunction　solar　cells.　Therefore,　it　is　extremely　desirable　to　characterize　and　control　the　interface　at　the　atomic　scale.　Here,　we　employ　spherical　aberration-corrected　transmission　electron　microscopy　to　investigate　the　atomic　structure　of　the　c-Si/a-Si:H　interface　in　high-efficiency　Si　heterojunction　solar　cells.　Their　structural　evolution　during　in　situ　annealing　is　visualized　at　the　atomic　scale.　High-density　embedded　nanotwins,　detrimental　to　the　device　performance,　are　identified　in　the　thin　epitaxial　layer　between　c-Si　and　a-Si:H.　The　nucleation　and　formation　of　these　nanotwins　are　revealed　via　ex　situ　and　in　situ　high-resolution　transmission　electron　microscopy.　Si　heterojunction　solar　cells　with　low-density　nanotwins　are　fabricated　by　introducing　an　ultra-thin　intrinsic　a-Si:H　buffer　layer　and　show　better　performance,　indicating　that　the　strategy　to　restrain　embedded　nanotwins　can　further　enhance　the　conversion　efficiency　of　Si　heterojunction　solar　cells.