Defect Control for High‐Efficiency Cu₂ZnSn(S,Se)₄ Solar Cells by Atomic Layer Deposition of Al₂O₃ on Precursor Film

Abstract

Cu2ZnSn(S,Se)4 are emerging as promising photovoltaic materials due to their outstanding photoelectrical performances, benign grain boundaries, and Earth‐abundant constituent elements. However, there are largely distributed cation‐disordering defects and defect clusters, which lead to an increase in recombination and a large open‐circuit voltage deficit and thus deteriorate device performance. Herein, defect control for a high‐efficiency Cu2ZnSn(S,Se)4 solar cell by atomic layer deposition of aluminum oxide (ALD‐Al2O3) on the precursor film is reporter. CuZn defects and Sn‐related deep defects are largely suppressed because of the decrease in Sn2+ and the increase in Sn4+ in the film by ALD‐Al2O3 on the precursor are found, and the crystallinity of absorber layer is improved from a double‐layer structure to a completely single‐layer structure. Furthermore, the carrier lifetime and recombination in the bulk and interface are significantly improved for devices with ultrathin Al2O3. Using this approach, the conversion efficiency increases from 8.8% to 11.0% and the open‐circuit voltage deficit decreases from 0.621 to 0.577 V. Herein, a deep understanding of the relationship between Al2O3 incorporation and high‐efficiency Cu2ZnSn(S,Se)4 devices and a new direction for controlling defects to further improve the performance of kesterite solar cells are provided.

Defect Passivation for Kesterite CZTSSe Solar Cells via In Situ Al2O3 Incorporation into the Bulk CZTSSe Absorber

“…In a previous report by Sun et al, Al incorporation enhanced crystallization of spin-coated CZTSe films during the postselenization process. [15] However, this crystallization enhancement was not observed for the sprayed films in this study. The contradictory results regarding the Al incorporation effect may be attributed to the different deposition methods.…”

“…Sun et al reported that Al 2 O 3 incorporation resulted in the conversion of Sn 2þ to Sn 4þ . [15] In our results, no noticeable difference is observed in the Sn 3d 5/2 spectra. Figure 4f shows the XPS spectra of Al 2p; the peak center is located at %74.6 eV, which is close to the Al 2p of Al 2 O 3 .…”

“…The suppression of the Raman E-mode for Alincorporated CZTSSe is in agreement with a previous report, and it is attributed to the decrease in the defect density of the Zn Cu , Zn Sn , and (2Cu Zn þ Sn Zn ) cluster. [15,20] When these defects are suppressed, the solar cell performance is expected to improve.…”

“…[ 14 ] However, regarding the effectiveness of Al 2 O 3 , Sun et al recently reported that the ALD Al 2 O 3 layer deposited on CZTSe before selenization enabled passivation of both the bulk and interface, thereby increasing the solar cell performance. [ 15 ] The aforementioned studies all focused on the role of a thin Al 2 O 3 layer on the front or rear surface as a passivation layer. In contrast, Septina et al reported that Al 2 O 3 incorporation into the whole CIGS absorber (CIGS:Al 2 O 3 ) increased both the open‐circuit voltage ( V oc ) and the FF, consequently increasing the PCE, which was attributed to passivation of both bulk and interface defects.…”

Highly efficient Cu2ZnSn(S,Se)4 bifacial solar cell via a composition gradient strategy through the molecular ink

“…In our previous work, the effects of annealing conditions were studied systematically for the devices on the transparent F:SnO 2 (FTO) substrate, which suggests the combination of high temperature, short time, and higher amount of chalcogen through a rapid thermal annealing process is crucial to get high-quality film as well as preserve a good back interface characteristic [15]. Besides, using different elemental compositions, the performance of CZTSSe devices on Mo substrate has been investigated, suggesting that Zn rich, Cu, and Sn poor composition is beneficial to suppressing the formation of deep level defects [16][17][18][19][20]. However, Zn rich and Cu, Sn poor composition leads to Btype defect clusters, such as 2Zn Cu , Zn Sn , and Zn(S,Se) secondary phases in the CZTSSe film [18].…”