Investigation of the effect of cation substitution in Cu2CdSnS4 thin film solar cells

Kesterite-related materials, especially Cu2ZnSnS4 (CZTS), have been touted as one of the alternative technologies to replace the already available commercial solar panels on the market. However, its commercialization effort is hampered by the stagnant CZTS device efficiency owing to its large Voc deficit. One of the reasons for this deficit is due to the disordered structure caused by similar sizes of its cations. Substitution using cations with dissimilar ionic sizes to either Cu, Zn, or Sn has been explored recently to address the issue. One of the compounds, Cu2CdSnS4 (CCTS), can be formed in a stannite structure by replacing Zn with the larger ionic Cd. The disorder of Cu and Cd in this compound is relatively suppressed compared to its Zn counterpart. CCTS has a bandgap of 1.41 eV, suitable for photovoltaic devices, and has a high absorption coefficient. CCTS solar cells hold the highest efficiency among the other kesterite-inspired materials. Despite the excellent progress of the material, concerns about the defects remain, especially the Sn-related as these defects are highly detrimental to the Voc of the solar cells. Nevertheless, the good properties of CCTS warrant further study to understand the material thoroughly. Previous thin film solar cells benefited greatly from cation substitution for fundamental understanding and device improvement. This approach can also be beneficial for developing CCTS as solar cell material. This thesis investigates the cation substitution on CCTS to understand the interaction between each element involved and the impact after the substitutions. The first study is on Ag and K doping on CZTS using spin-coating of the doped ACZTS at the bottom and the doped KCZTS at the top. The improved Voc and Jsc lead to a higher efficiency device compared to the CZTS reference and the single precursor co-doped AKCZTS. This study highlights the effect of Ag doping and the possibility of forming a good film using two different spin-coating precursors at different positions. The second study in this thesis covers the systematic substitution of Ag on CCTS. The precursor solutions for CCTS, the partial and the full substitution of Ag, could be obtained and utilized for the spin-coating deposition. The full Ag substitution resulted in a doping type inversion from p-type CCTS to n-type Ag2CdSnS4. The low doping amount of 5% Ag improved the device performance to 7.72%, with Jsc as the main factor contributed by good film formation and better interface properties. Finally, the third study is Ge incorporation using a similar spin-coating deposition process. This study uses two methods of absorber preparation. Both methods exhibited improved device efficiency. Interestingly, the direct mixing method successfully increased the Voc to 599 mV, one of the highest Voc for Ge-based emerging kesterite-inspired solar cells. In summary, this thesis explores the effect of cation substitutions on CCTS thin film solar cells. Cu and Sn site substitutions were investigated thoroughly using Ag or Ge for the cation substitutions. Multiple strategies were also utilized to obtain a good film with the correct phase. Finally, this thesis demonstrated the versatility of the solution process for thin film solar cell fabrication with different compositions.