The Role of Hydrogen from ALD‐Al₂O₃ in Kesterite Cu₂ZnSnS₄ Solar Cells: Grain Surface Passivation

Abstract

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.

Room‐Temperature Surface Sulfurization for High‐Performance Kesterite CZTSe Solar Cells

“…After sulfurization, sulfur passivates the defects at the absorber's surface . XPS results indicate that the charged vacant Se sites‐related defects can be replaced by negatively charged S atoms (Figure ) . S and Se are electrically related, and S Se is thus considered to be an electrically neutral defect.…”

Doping and alloying of kesterites

“…Recently, Hao et al reported the potential passivation role of hydrogen in pure sulfide kesterite [22]. Hydrogen was introduced into the surface region of kesterite through the atomic layer deposition (ALD) of figure 3).…”

Mutual Insight on Ferroelectrics and Hybrid Halide Perovskites: A Platform for Future Multifunctional Energy Conversion

“…This will result in the creation of internal junctions at ferroelectric domains and help in the separation of photoexcited charge carriers. At the same time, the role of grain boundaries is also worth considering for enhanced ion transport . Therefore, an improved knowledge of the fundamental ferroelectric nature of potential materials could significantly help in designing smart materials with better performance .…”