Study On The Performance Of Quaternary Cu₂ZnSnS₄ Quantum Dots Sensitized Solar Cells

As promising sensitizers in quantum dots sensitized solar cells?QDSSCs?, inorganic semiconductor quantum dots?QDs? have attracted increasing attention due to their low production cost, tunable band gap, high molar extinction coefficient and multiple exciton generation. The earth-abundant multicomponent alloy Cu₂ZnSnS₄?CZTS? QDs show higher chemical stability and narrower band gap because of "optical bowing". Furthermore, the band gap of CZTS QDs can be tuned by varying the composition without changing the particle size. However, the applications of CZTS QDs in QDSSCs have never been reported. The key feature of this problem is that the high fairly small-sized CZTS QDs require the utilization of 1-dodecanethiol?DDT? to suppress the growth of QDs, which can not be replaced by bifunctional molecular linker. This paper contains the following content:?1? Application of Cu₂ZnSnS₄ QDs in QDSSCs based on the hydrolysis approach: We developed a new synthesis and functionalization strategy for CZTS QDs by using mercapto-acetic acid octyl ester as capping ligand. Unlike the common ligand exchange approach, the water-soluble QDs were obtained by the hydrolysis of capping ligand at pH > 7 insteading of the replacement of weak bonded capping ligand by the strong ones. Benefitting from the broad absorption spectral range, well loading ability and the improvement of electron transport process after ligand hydrolysis, the constructed “green” CZTS QDSSCs finally achieved an impressive conversion efficiency of 3.29%. This efficiency demonstrates CZTS QDs can be utilized in QDSSCs as sensitizers and the ligand hydrolysis approach is an efficient surface functionalization strategy.?2? Application of Cu₂ZnSnS₄ QDs in QDSSCs based on the cation exchange approach: We utilized DDT as S source and capping ligand insteading of mercapto-acetic acid octyl ester due to the higher decomposition temperature of DDT. However, the DDT capping ligand can not be exchange by bifunctional molecular linker because of the existence of-SH. Herein, we utilized Cd-oleate as cation units to exchange the nature Cu/Zn/Sn-DDT units on the surface of QDs and the type-? core/shell CZTS/CdSe QDs capping oleic acid?OA? ligand was obtained simultaneously. The MPA capped CZTS/CdSe QDs were obtained randomly via ligand exchange due to the weak coordination ability of OA. The constructed water soluble CZTS/CdSe QDSSCs achieved a conversion efficiency of 4.70% because of the removal of long alkyl chain and the existence of the Cd Se shell, suggesting that cation exchange approach is more efficient and convenient for surface functionalization.?3? The stability improvement of Cu₂ZnSnS₄ quantum dots sensitized solar cells: The stability of QDSSCs is limited by the corrosion of brass substrate by polysulfide electrolyte. In order to improve the stability, we combine the superior catalytic activity of Cu₂ S and the inert character of fluorine doped tin oxide?FTO? glass to polysulfide electrolyte. The FTO-Cu₂ S counter electrodes possess similar catalytic activity and higher stability comparing with common Cu₂ S counter electrodes.