Atomic Layer Deposition for CZTS Solar Cells

Abstract

Kesterite Cu2ZnSnS4 (CZTS) solar cells have recently emerged as a promising candidate for scalable thin-film solar cell development, mainly due to a generic similarity to commercialized CuInGaSe2 (CIGS) and CdTe solar cell technologies while consisting of only earth-abundant and non-toxic constituents. However, the photovoltaic performance of CZTS solar cells is still hampered by a relatively large open-circuit voltage (VOC) deficit which is correlated to bulk defects in CZTS absorber and recombination at the interfaces. To achieve high-performance CZTS solar cells, nanoscale layer coating and interface modification by atomic layer deposition (ALD) is an effective strategy to improve the device performance. This thesis starts with the synthesis of ALD Zn1-xSnxO films for application as a buffer layer in CZTS solar cells. A favorable band alignment is achieved using a 10 nm Zn0.77Sn0.23O buffer layer which enabled an impressive 10 % increase in VOC and a 9.3% efficient CZTS solar cell, a world-record at the time of fabrication. The decreased interfacial defects stemming from the minor lattice mismatch at the CZTS/Zn(S,O)/ZTO hetero-interface in combination with the passivation provided by a higher sodium concentration throughout the CZTS/ZTO device explains the significant increase in VOC. Another strategy is the application of ALD Zn1-xMgxO films as a window layer, which is essential to prevent shunt paths in CZTS thin-film solar cells. The wider bandgap of Zn0.8Mg0.2O layers minimizes the optical loss from the window layer and leads to an enhanced JSC. A more favorable conduction band alignment is believed to contribute to the improvement in VOC. A 9.2% efficient Cd-free CZTS device with a Zn1-xMgxO window layer was fabricated without anti-reflection coating, thanks to the significantly enhanced Jsc and VOC. Lastly, I demonstrate the first Cd-free CZTS solar cell with an efficiency beyond 10 %, bringing this low-cost and green absorber one step closer to commercial practicality. This efficiency improvement is obtained by using an ALD Al2O3 passivation layer to reduce interface recombination at the heterojunction, thus significantly reducing the VOC deficit. It demonstrates the effective application of ALD Al2O3 layers in Cd-free CZTS solar cells and provides fresh insights into the mechanism of Al2O3 passivation on the nanoscale.