High-Throughput Exploration of Defect Passivation Strategies for Kesterite Photovoltaics

Thesis (Ph.D.)--University of Washington, 2015-12The need for low-cost, sustainable energy generation has driven materials research towards the development of thin film solar cells based on earth-abundant materials. One of the leading candidates is Cu2ZnSn(S,Se)4, (CZTSSe), a material which bears many resemblances to its parent compound Cu(In,Ga)Se2, but is comprised entirely of earth-abundant primary metals (metals mined for directly). CZTSSe devices have reached 12.6% efficiency,1 but will need to improve further to be competitive to current technologies and reach grid parity. Compared to the theoretical limits, the world record CZTSSe device collects about 81% of the available current but only produces 58% of the maximum voltage. The low open-circuit voltage is believed to result from a large quantity of intrinsic defects,2, 3 a problem faced by almost all solar cell materials. To rapidly advance the efficiency and understanding of defects in CZTSSe, we have developed a high-throughput screening process based on a combinatorial spray deposition system and a confocal photoluminescence mapping instrument calibrated to determine the absolute photon flux. By fitting the absolute intensity photoluminescence (AIPL) spectra, we are able to extract quantitative metrics about the quantity of charged defects and determine the quasi-Fermi level splitting (the maximum possible Voc at a given illumination intensity) without making completed devices. This process allows us to test thousands of samples in the time it formerly took to test one, and reduces error due to additional processing steps. By combining over 40,000 AIPL spectra, I have been able to map out how the optoelectronic properties depend on the relative quantities of Cu, Zn, and Sn. Further, I have investigated the impact of over 25 different extrinsic species in varying concentrations, including the effect of germanium alloying to produce the larger band gap material Cu2Zn(Sn,Ge)(S,Se)4, and an in-depth study of group-I dopants. This work has contributed to the understanding of lithium as an effective dopant for CZTSSe and greatly improved the communities understanding of both intrinsic and extrinsic defects, enabling efficiencies from our lab as high as 11.8%. Additionally through Germanium alloying we have demonstrated an 11% CZTGSSe solar cell with the lowest reported voltage deficit of any kesterite device