record 9.5% efficiency pure sulfide CZTS solar cell, [4] but it is still much lower than the S-Q limit and its CuInGaSe 2 (CIGS) counterpart. It has been identified that the main reason for the efficiency limitation is the open-circuit voltage (V oc ) deficit (E g /q −V oc , where q is the elemental charge) as a result of band fluctuation and nonradiative recombination. [5] In addition, unfavorable conduction band alignment, i.e., cliff-like conduction band offset, at the interface of CZTS absorber/ CdS has been reported to lead to high recombination, contributing to the loss of V oc . [6] The majority of electron-hole pairs are generated near the CZTS/CdS interface, [7] so the passivation of the CZTS/ CdS interface is of crucial importance in achieving high-performance CZTS solar cells. In CIGS solar cells, Al 2 O 3 was employed for better surface passivation. [8] In these papers, both field effect passivation by fixed negative charge and chemical passivation play a role in passivating the CIGS surface, and decrease surface recombination velocity. The thickness of Al 2 O 3 used in these studies is few tens of nanometers which are not suitable for the heterojunction passivation as such a thick layer would impede carrier flow at the junction area.We employed an ultrathin atomic layer deposited (ALD)-Al 2 O 3 layer, which is believed to effectively reduce interfacial recombination by reacting with the surface defects. [8a] It is known that, in ALD process, the precursor molecule and the reactant which are trimethylaluminum (TMA, Al(CH 4 ) 3 ) and H 2 O respectively, [9,10] are the hydrogen sources needed for chemical passivation. The passivation mechanism of ALD-Al 2 O 3 in the silicon solar cells can be divided into two categories: i) field effect passivation induced by negative fixed charges [11] and ii) chemical passivation by hydrogen. [12] For field effect passivation, it has been reported that a critical thickness is required to ensure this field effect. [13] Based on the benefits of ALD-Al 2 O 3 , the kesterite group at IBM has recently reported the passivation of CZTSSe/CdS interface by a thin ALD-Al 2 O 3 layer (less than 1 nm) showing enhanced performance due to surface passivation effects. [14] However, the detailed passivation route is unknown. The effect of ultrathin (less than 1 nm) Al 2 O 3 on interface passivation is unclear as negative fixed charges may not be involved in the passivation effect.

Three kesterite thin-film solar cells, Cu2ZnSnSe4 (CZTSe), Cu2ZnSn(S,Se)4 (CZTSSe), and Cu2ZnSnS4 (CZTS), and based on low light intensity measurements, examined the possibility of using kesterite devices for indoor applications.

Recent efficiency advancements in kesterites have reinforced the use of Cu2ZnSn(S,Se)4 (CZTSSe) in indoor photovoltaic applications. However, the performance of kesterites under low light intensity conditions is mainly hindered by deep‐level defects. In this study, a strategic approach of silver (Ag) and germanium (Ge) cation substitution to cure these defects are employed. The Ag‐doped CZTSSe (CZTSSe:Ag) and Ge‐doped (CZTSSe:Ge) samples experimentally demonstrated a significant improvement in kesterite device performance under all intensities of LED and white fluorescent lamp conditions are prepared. Interestingly, the CZTSSe:Ag device exhibited the highest performance levels, i.e., 1.2–1.5 and 2.5–3 times better than those of Ge‐doped CZTSSe:Ge and undoped CZTSSe, respectively. This improved device performance is mainly attributed to the reduced energy level of deep‐level defects in CZTSSe:Ag. Moreover, these defects assisted in the generation of a larger potential difference between the grain boundary and grain interior in the CZTSSe:Ag sample, attracting minority carriers near the grain boundary. Consequently, the improved carrier separation process reduced the carrier recombination losses and enhanced the power output under low light intensity conditions. This Ag and Ge cation substitution in kesterite is found to be an effective approach to improve the device performance under low light intensity conditions.