Nanoscale materials offer key advantages for earth-abundant chalcogenide solar cells (CdTe, CuInGaSe2 (CIGS) and Cu2ZnSnS4 (CZTS) solar cells etc.), such as the ability to achieve high transparency layers together with well-controlled interface layers. Atomic layer deposition (ALD), a vapor phase deposition method, provides precise modification at the interface due to its self-terminating process, which enables atomic-level control over the coating thicknesses and conformal coverage over large area substrates. With regard to the application of ALD ZnSnO on CZTS solar cell, we discuss how to replace traditional toxic Cd element in the device while maintaining the solar cell performance in high level. By tuning the composition ratio of ZnSnO, we obtain a suitable component which enables a favorable band alignment at the CZTS heterojunction interface and reduces optical losses as well (see Figure 1 and 2). We demonstrated a record efficiency of 9.3% for Cd-free CZTS solar cells, with an increase of open circuit voltage (Voc) by up to 10 % as shown in Figure 1(b). The nanoscale microstructure and chemistry of the champion device was carefully studied and the physical mechanism of the improvement was proposed. This work reveals the benefits from a higher transparency buffer layer and the importance of the management at absorber/buffer hetero-interface, providing an effective pathway to relieve interface recombination for the fabrication of high-efficiency, earth-abundant solar cells. As earth-abundant chalcogenide solar cells continue to be developed, ALD will continue to play a key role in modifying interfaces for improved solar cell performance in nanoscale level.