Energy Band Regulation And Performence Enhancement Of Panchromatic Quaternary Cu₂ZnSn（S,Se）₄ Quantum Dot Sensitized Solar Cells

Multicomponent Cu₂ZnSn（S,Se）₄（CZTSSe）semiconductor material is one of the ideal candidates for quantum dots due to its advantages of rich constituent elements,high optical absorption coefficient,and adjustable band gap.At present,CZTS quantum dots have been successfully introduced into QDSSCs by hydrolysis or cation exchange,but their device performance is not ideal.By consulting the literature,we find that the main reasons for the low efficiency of device are the following two points: First,the spectral response range of quantum dot is narrow;Second,the energy band between CZTS quantum dot and TiO₂ film electrode does not match.In addition,the particle size of quantum dots is also one of the key factors affecting device performance.It is well known that the particle size of quantum dots needs to be controlled within a certain range（usually around 5 nm）to ensure random dispersion in porous TiO₂ films.However,the nucleation and growth conditions of small-sized CZTS quantum dots are quite strict,and commonly used capping ligands such as oleylamine and oleic acid cannot meet these conditions.Therefore,in order to solve the above-mentioned problems,this article will start from exploring the synthesis of high-quality wide-spectrum CZTSSe quantum dots,and further optimize the energy band structure and broaden the light absorption range of quantum dots through element ratio control and cation substitution.Systematic study on the performance and mechanism of high efficiency QDSC based on multiple CZTSSe quantum dots.The specific content can be divided into the following sections:（1）Performance enhancement and controllable synthesis of CZTSSe quantum dots synthesized by hot injection: First,through the high temperature hot injection method,we synthesized CZTS quantum dots with adjustable size and narrow size distribution.We comprehensively utilized the advantages of fast nucleation process of high temperature hot injection method and the strong reduction characteristics of Na BH4,and optimized the quantum dot growth temperature and time to finally prepare quantum dots of around 5.5 nm,meetting the size requirements of high-load quantum dots.In addition,in order to solve the problem of narrow absorption range for the quantum dots,without changing the particle size,we effectively broadened the optical response range of CZTS quantum dots from 850 nm to 950 nm by introducing Se element of 20 % into the system,and its mechanism of action was analyzed by UV-visible absorption spectrum and UV photoelectron spectroscopy.Benefiting from the broadened light absorption range,the short circuit current Jsc（16.80 Vs 14.13 m A/cm2）of the device had been significantly improved,and finally the photoelectric conversion efficiency had been increased from 3.17 % to 3.54 %.The above results showed that this DDT-free ligand synthesis method solved the main obstacles in the application of CZTSSe quantum dots in QDSSCs,and provided a simpler method for ligand exchange,light absorption improvement and particle size control.（2）Energy band regulation of broad spectrum CZTSe quantum dots synthesized by one-step method: Although in the previous work,synthesis of materials and ligand exchange process are very convenient,it is difficult to improve the performance of device.It is mainly caused by the following two reasons: first,the amount of Se introduced is relatively low,and the light absorption range is still narrow,while the introduction of high Se will increase the size of the quantum dots;second,the shorter growth temperature is not conducive to the improvement of crystallinity.Therefore,based on this consideration,in this section,we will optimize the scheme for preparing small-sized CZTSe quantum dots based on DDT ligand synthesis and cation exchange.By optimizing the process,we first prepared small-sized wide-spectrum CZTSe quantum dot.Then,for the energy band mismatch problem caused by Se,we can optimize the energy level structure by the replace of Sn with Ge.The experimental results prove that the introduction of Ge can make the conduction band move up and further enhance the driving force of photo-generated electron injection.Benefiting from the wider light collection range and fast photoelectron injection rate,when the Ge substitution amount is 30 %,we increased the performance of device from 3.83 % to 4.42 %.