In order to reduce the power-generating cost of silicon solar cells, it is necessary to achieve a high conversion efficiency using a thinner crystalline silicon (c-Si) substrate. The HIT (Heterojunction with Intrinsic Thin-layer) solar cell is an amorphous silicon (a-Si) / c-Si heterojunction solar cell that exhibits the potential to make this possible. Our recent R&D activities have achieved the world’s highest conversion efficiency of 23.0% with a practical sized (100.4 cm2) HIT solar cell, by improving the quality of the surface passivation, reducing the optical absorption loss and reducing the resistance loss. We have also developed a HIT solar cell with a thickness of only 98 mm, which has a very high conversion efficiency of 22.8%. This value is comparable to that of the conventional HIT solar cell, which has a thickness of more than 200 mm. Moreover, we have fabricated HIT solar cells using thinner c-Si substrates (96 to 58 μm), and found that the Voc increased with decreases in the substrate thickness, and reached an extremely high value of 0.745 V with a thickness of only 58 μm. This indicates that the surface recombination velocity of the HIT structure is extremely low due to the excellent passivation of the c-Si surface.