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Abstract
The newly emerged kesterite Cu2ZnSnS4 (CZTS), derived from CuInxGa(1-x)Se2(CIGS), has been considered as one of the most promising absorber materials for thin film solar cells, due to its merits like earth abundant and low-toxic constituents, and good optical-electrical properties (high absorption coefficient of above 104 cm-1 and adjustable bandgap). Improving CZTS thin film quality and optimising the device structure are two important aspects of the development of CZTS based solar cells.
In terms of the improvement of CZTS thin film quality, the sulfurization process is critical for “two-stage” process, i.e., precursor deposition followed by sulfurization/selenisation treatment. We investigated the impact of rapid thermal sulfurization processing conditions on dimethylformamide (DMF)-based precursors. The precursors are prepared by spin-coating of solution from dissolving metal salts into DMF on molybdenum (Mo) -coated soda lime glass (SLG) substrates. Precursors are then converted to CZTS by a sulfurization process in a rapid thermal processing system (RTP). The effects of heating rate, total pressure during sulfurization and two-step annealing conditions on CZTS thin film properties and device performance are investigated. It is found that higher heating rate leads to a bi-layer structure of CZTS absorber and high series resistance of CZTS device, whilst the lower heating rate results in better crystallinity, single-layer structure of CZTS absorber, but thicker MoS2 layer. High total pressure during sulfurization is beneficial to the formation of large CZTS grain and compact morphology. Reducing total sulfurization time can suppress the decomposition of CZTS in the two-step annealing process but it does not bring a significant improvement in film quality. Through the optimization of the RTP processing conditions, the fabricated best CZTS cell yields efficiency of 5.84% with Voc of 593mV, Jsc of 15.95 mA/cm2, and FF of 61.7%.
Regarding device structure optimisation, this thesis focuses on the improvement of current transportation at the back contact region by introducing an ultra-thin carbon intermediate layer at back contact Mo/ absorber CZTS interface. The effectiveness of carbon intermediate layer is double confirmed in both non-vacuum (sol-gel) and vacuum (sputtering) processed CZTS solar cells.
The morphological, structural and compositional analysis demonstrates that introduction of this carbon intermediate layer has no negative effects on the quality of upper CZTS absorbers. Both the current density-voltage (J-V) characterization and external quantum efficiency (EQE) measurement show that the introduced carbon intermediate layer does not lead to any deterioration of either open circuit voltage (Voc) or fill factor (FF) but improves the short circuit current (Jsc) by ~25% in sol-gel processed device (efficiency increase from 4.47% to 5.52%) and ~17% by sputter-processed device (efficiency increase from 4.10% to 5.20%). The boost in Jsc and thereby efficiency is due to the fact that the carbon intermediate layer reconnecting the CZTS absorber and back contact (carbon aggregation on the wall of the voids (at the back contact region) which block current transportation reduces the contact resistance) reduces the series resistance.