Enhanced Hetero‐Junction Quality and Performance of Kesterite Solar Cells by Aluminum Hydroxide Nanolayers and Efficiency Limitation Revealed by Atomic‐resolution Scanning Transmission Electron Microscopy

Abstract

A strategy for interface engineering of hetero‐junctions in kesterite solar cells by using Al(OH)3 is demonstrated. The hydroxide nanolayers are prepared via a facile and fast wet chemical route, based on an aqueous solution of aluminum chlorides and thioacetamide. Considerable enhancement of open circuit voltage (Voc) (30–60 mV) and fill factor (FF) (10–20%) after this chemical treatment are observed, achieving a champion conversion efficiency of 9.1% and a champion FF of 70% (among the best FF in kesterite solar cells). The functional mechanism is systematically studied by current‐voltage, capacitance‐voltage, temperature dependence of current–voltage and photoluminescence measurements, which reveal that Al(OH)3 nanolayers can effectively reduce the interface recombination and largely improve the shunt resistance. Furthermore, atomic resolution high angle annular dark field scanning transmission electron microscopy (HAADF‐STEM) evidences the epitaxial relationship of Al(OH)3 with kesterite and CdS, indicating the benign and effective interface passivation achieved by this chemical treatment. Finally, based on HAADF‐STEM and electron energy loss spectroscopy mappings, insights into the efficiency limiting and beneficial factors for CZTSSe solar cells, as well as suggestions to further improve both the bulk and related interfaces are presented.

Interface engineering of p-n heterojunction for kesterite photovoltaics: A progress review

“…To date, metal oxide (e.g. Al 2 O 3 , TiO 2 and Al(OH) 3 ) as a passivation layer has been introduced and studied in the case of kesterite [41,84,85]. Among them, Al 2 O 3 has been found to passivate the interface region dramatically, especially through the ALD process [41,42,82,83].…”

Partial substitution of the CdS buffer layer with interplay of fullerenes in kesterite solar cells

“…In this case, the surface passivation of the absorber is of significant interest, as the interface between CZTS and CdS presents several issues due to the valence band offset and secondary phases which induces interface recombination. Usually, an etching treatment 31,32 or an oxide interlayer 33,34 is required for a passivation of interface defects. Figure 4 shows the J-V curves of the best device for each condition under AM1.5 illumination condition, and the average values are showed in the Table 2.…”

Insights into interface and bulk defects in a high efficiency kesterite-based device

“…To mention some: i. At the front interface: non-optimum band alignment with the CdS buffer layer, [11][12][13] Fermi level pinning due to CuZn antisite point defects, 14 incomplete CdS coverage, 15,16 high density of interface defects, 17,18 and ZnSe secondary phase formation. 19 ii.…”

“…19,27 Front interface recombination can also be reduced through surface passivation treatments 10,36 or the use of passivating nanolayers. 15,18,37,38 Regarding the back interface, the use of a thin ZnO layer on the Mo back contact has been demonstrated to reduce chemical instability 33 and the thickness of the MoSe2 layer can be controlled through multilayer Mo configurations and the use of selenization barriers. 34 Finally, in the bulk of the absorber, partial substitution of Cu by Ag or of Zn by Cd has been proposed to reduce Cu-Zn disordering 14 and Na-doping is commonly regarded as a means of grain boundary passivation.…”