Fabrication Of High-efficiency Cu₂ZnSn（S,Se）₄ Solar Cells Using Metal-salts/thiourea Solution Processes

Cu₂Zn Sn（S,Se）₄（CZTSSe） semiconductor compounds is considered to be the high efficiency thin film solar cell absorber layer material, owing to the reduced toxicity and greater abundance of their constituent elements, an adjustable band gap of 1.0-1.5 e V and the high absorption coefficient of 104 cm-1, becoming one of the hot research topic in the field of solar cells in recent years. Theoretically, photoelectric conversion efficiency（PCE） of CZTSSe solar cells can reach 33%, but reported the highest photoelectric conversion efficiency of 12.7% only. Thereby, increasing the PCE of solar cells is the most important issue in CZTSSe researching. Researches indicate that the key scientific issues of affecting efficiency of CZTSSe solar cells include crystal quality, electrical properties of CZTSSe, lattice and the energy band matching of Cd S/CZTSSe heterojunction interface, as well as electrical properties of CZTSSe/Mo back electrode contact. To solve these key scientific issues, in this dissertation, metal-salt/thiourea solution method combined with the sulfurization/selenization heat treatment was used. Combining with first-principles calculation, we carried out research work on preparation, characterization, and physical mechanisms of CZTSSe thin film and solar cells by optimizing the heat treatment process, doping and optimizing solution system, etc. The contents and results are as follows:（1） CZTSSe films and solar cells were prepared using metal-salt/thiourea solution and spin coating technique, combined with selenization process. Studied changes in structure and crystal quality of CZTSSe films, thickness of Mo（S,Se）2 layer between CZTSSe/Mo with selenization temperature and time, and the laws and mechanisms for these changes in PCE of CZTSSe solar cells. It was found that, at selenization temperature of 530℃, PCE of the CZTSSe solar cell increases with increasing selenization time in 5-15 min, and decreases with increasing selenization time in 15-30 min. This efficiency is not determined by the change in series resistance of CZTSSe solar cells, but its parallel conductance, diode ideality factors and reverse saturation current. And the series resistance is mainly affected by crystal quality of CZTSSe film, rather than thickness of Mo（S,Se）2. Since the parallel conductance, diode ideality factors and reverse saturation current are relevant to the reconmbination of photo-generated carriers in interface of Cd S/CZTSSe heterojunction and CZTSSe/Mo, therefore, the selenization temperature and selenization time mainly affect the crystal quality of CZTSSe film and thickness of Mo（S,Se）2, and determine the PCE of CZTSSe solar cells. Through optimization, in selenization temperature of 530℃ and selenization time of 15 min, the highest PCE of 7.48% of CZTSSe solar cells was obtained.（2） Lattice matching and the bandgap arrangement between Cd S and Cu2 Zn Sn S4（CZTS） heterojunction interface can significantly influence PCE of CZTS solar cell. To find more match absorbent layer material with buffer layer（Cd S）, we launched a research about changes of crystal structure, bandgap and efficiency in Cd alloying CZTS and the impact of rules and mechanisms. Through experiments and first-principles calculations: Cd alloying CZTS, Cd substituting Zn formed Cu2 Cdx Zn1-x Sn S4（CCZTS） alloy with the same structure of CZTS; lattice constants of CCZTS increased with increasing Cd content; optical bandgap decreased with increasing Cd content, which is due to energy level of Cd-d electrons higher than Zn-d electron, produced a stronger repulsion effect on top of valence band（p-state）, caused the valence band shift. To study the effect of efficiency in Cd alloying solar cells, the traditional structure of the solar cells prepared with CZTS and CCZTS with Cd content of 0.47 absorption layers, and found Cd alloy make short-circuit current and fill factor increased, oc（Voc=Eg/q-Voc, Eg is the band gap, q is the charge of the electron） reduced, so that the conversion efficiency is improved. This is mainly due to the Cd alloy make lattice mismatch ratio of CCZTS and Cd S smaller than CZTS and Cd S, resulting in reduction of interface state, resulting in enhanced the ability to transport photo-generated electrons, decreased probability of electron-hole recombination.（3） It is reported that open circuit voltage and PCE can be improved by In diffusing to CZTSSe and Cd S from In2S3/Cd S buffer layer. But states of In doping in the CZTSSe and its mechanism of improving the PCE is still under debate. We conducted a research on the states of In doping in CZTSSe and its impact on performance of CZTSSe film and CZTSSe solar cell. The structure, chemical valence of elements and electrical performance were tested in In-doped CZTSSe, combined with first-principles calculations, it was found that, In substituting Cu and Sn to form In Cu + In Sn donor-like complex in CZTSSe, rather than the reported In Cu or In Sn. Such donor-like complex reduce the hole concentration. PCE of In doping CZTSSe solar cell was lower than the one with In-free, indicating high open-circuit voltage and PCE cannot be attributed to the In-doping effect leading to the higherconductivity of CZTSSe film through In2S3/Cd S hybrid buffer layer.（4） For low-cost, safety and environmental-friendly preparing CZTSSe solar cell, an ideal route is to use non-toxic, cheap water as a solvent is. However, we found that the traditional solution, which using thiourea, metal-salt, aqueous solution and a small amount of ethanol to prepare an CZTS precursor solution, was unstable and prone to precipitation, the precipitate concludes mainly copper ion coordination polymers obtained by reaction of thiourea, copper deficient in precursor solution, resulting in the film contains multiphase, and there are a lot of pinholes. In order to improve the stability of the CZTS precursor solution, we added a small amount of mercaptopropionic acid（MPA） in CZTS precursor solution to inhibiting the precipitation, and successfully prepared stable CZTS precursor solution. Using spin coating the stable CZTS precursor solution and combining with selenization, a high-quality and single phase CZTSSe film was prepared and the PCE of CZTSSe solar cell reaches 7.25%, which is the highest efficient in using of water-based molecular precursor solution method to obtain CZTSSe solar cells currently.