Dopant-free carrier-selective contacts are emerging in the field of crystalline silicon (c-Si) photovoltaic solar
cells, which are potential to further improve the power conversion efficiency (PCE) and lower the cost of c-Si
solar cells. Here, we demonstrate tens of microns thin c-Si heterojunction solar cells with substochiometric MoOx
and LiFx as dopant-free hole- and electron-selective contacts, respectively. Chemical thinning of 200-μm thick c-
Si wafers enables the production of proof of concept devices with good flexibility and strong performance. When
the wafer thickness is reduced to 49.4 μm (24.7% of the initial thickness), the power conversion efficiency (PCE)
of the solar cell still maintains 88.2% of the initial value for the 200-μm thick cell. When the wafer thickness
becomes less than 10% (or even 3.4%) of the initial value, 61.2% and 39.2% of the initial PCEs are still achieved
for the 14.8- and 6.8-μm thick cells, respectively. Passivating and carrier-selective effects of the MoOx and LiFx
films allow for the maintenance of performance. An oxide interlayer at the MoOx/c-Si interface passivates the
dangling bonds of the c-Si surface and improves the minority carrier lifetime. Field-effect passivation and carrierselective
effects induced by the band bending near the MoOx/c-Si interface and the Al/LiFx/c-Si interface play an
important role in maintaining high open-circuit voltage and high fill factor. To the best of our knowledge, this is
the first time that<100-μm thin c-Si heterojunction solar cells are reported with undoped contacts. Our solar
cells have been fabricated on thin c-Si wafers with low-temperature processes and without additional doping,
and thus our work provides a promising cost-effective means in the field of thin and flexible c-Si solar cells.