Enhancing carrier transport in flexible CZTSSe solar cells via doping Li strategy

The passivation of non-radiative states and inhibition of band tailings are desirable for improving the open-circuit voltage (Voc) of CZTSSe thin-film solar cells. Recently, alkali metal doping has been investigated to passivate defects in CZTSSe films. Herein, we investigate Li doping effects by applying LiOH into CZTSSe precursor solutions, and verify that carrier transport is enhanced in the CZTSSe solar cells. Systematic characterizations demonstrate that Li doping can effectively passivate non-radiative recombination centers and reduce band tailings of the CZTSSe films, leading to the decrease in total defect density and the increase in separation distance between donor and acceptor. Fewer free carriers are trapped in the band tail states, which speeds up carrier transport and reduces the probability of deep-level defects capturing carriers. The charge recombination lifetime is about twice as long as that of the undoped CZTSSe device, implying the heterojunction interface recombination is also inhibited. Besides, Li doping can increase carrier concentration and enhance build-in voltage, leading to a better carrier collection. By adjusting the Li/(Li + Cu) ratio to 18%, the solar cell efficiency is increased significantly to 9.68% with the fill factor (FF) of 65.94%, which is the highest FF reported so far for the flexible CZTSSe solar cells. The increased efficiency is mainly attributed to the reduction of Voc deficit and the improved CZTSSe/CdS junction quality. These results open up a simple route to passivate non-radiative states and reduce the band tailings of the CZTSSe films and improve the efficiency of the flexible CZTSSe solar cells.