Investigation On The Cu₂ZnSnSe₄ Photovoltaic Material And Two Dimensional Photoelectrical Material Based On In Se/MoS₂ Heterostructure

In recent years, Cu₂ZnSnSe₄（CZTSe） photovoltaic material has been studied as an ideal material to substitute Cu（In,Ga）Se2（CIGS） material. The highest conversion efficiency based on CZTSe solar cell reached to 11.6% nowadays, which is very closed the world record for Cu2ZnSn（S,Se）4（CZTSSe） solar cell efficiency at 12.6%. However, these values are much lower than the best efficiency in CIGS solar cells, which is as high as 23.3% very recently. One of the main reason for limitation of the efficiency in CZTSe solar cell is that the binary or ternary compounds always exist after preparing the CZTSe thin films, which is very hard to eliminate using the previous preparation methods. Therefore, we developed two kinds of non-vacuum methods here to prepare high quality and almost pure CZTSe thin films easily, which are CZTSe thin films prepared by selenization of co-electroplated Cu-Zn-Sn precursor and selenization of sol-gel made Cu-Zn-Sn precursor.For the first time, we prepared CZTSe thin films by selenization of co-electroplated Cu-Zn-Sn precursor process. Relying on the suitable complexing agent and electrolyte solution, the Cu-Zn-Sn precursors are firstly co-electroplated on Mo substrate, and the high crystalline CZTSe thin films are formed after precisely selenization under high temperature. The stoichiometry ratio of the elements in the prepared films is consistence with the single phase of CZTSe. However, there are few Mo9Se11 impurities in the interface of CZTSe and Mo substrate, which might result from the high selenization temperature of the substrate.For the CZTSe thin films prepared by selenization of sol-gel made Cu-Zn-Sn precursor, the crystalline quality and the stoichiometry ratio of the CZTSe films are impacted a lot by the substrate temperature during the selenization process. The results indicate that the optimum temperature for Mo substrate and glass substrate are 500℃ and 550℃ respectively when CZTSe thin films are prepared on them. The CZTSe thin films have ideal stoichiometry ratio of elements and more importantly, there is no evidence of impurities after verifying by many characterization tools simultaneously. In addition, we estimate the band gap of CZTSe based on transmission and reflection spectra, and the band gap is located at 1.0 eV, which is very in accordance with the theory calculation. Finally, by using the same method, we prepared the CZTSSe thin films with two different Se/（S+Se） ratios, however the samples have rich Zn and poor Sn that we have to improve this in the future work.Two dimensional（2D） semi-heterostructures are attractive recently because of the possibility of combining specific properties of different materials in a device which maintains a 2D character. For example, various 2D semi-heterostructures based on transition metal dichalcogenide（TMDC/TMDC） are fabricated and have successfully applied into photo-detection and light emission diodes（LEDs）. However, there is no clear mechanism explanation about the interaction between the incident light and 2D semi-heterostructures, especially the light absorption inside which is very important for the device application. In this thesis, we provide a simulation model to explain the light distribution and absorption in the 2D semi-heterostructure, and more importantly, this model is successfully used to explain the Raman enhancement or attenuation in our 2D InSe/MoS₂ heterostructure. In addition, we fabricated a new photo-detector based on 2D InSe/MoS₂ heterostructure, and found some novel properties in the device.The high quality 2D semi-heterostructures such as InSe/MoS₂ are fabricated by our random transfer technique. This simple and high yield technique is very suitable for the amount of property study based on these heterostructures. From the Raman scattering experiment, we find the Raman intensity are enhanced or attenuated as function of the thickness of each layer, which could be explained and simulated by the multiple reflection model very well. Moreover, we simulate the light distribution and absorption with the same model, and the results indicate that the interface and thickness of each material impact a lot. The light distribution is very similar when the thickness of one layer is larger than 400 nm. In addition, we can obtain the optimum thickness of each materials when the heterostructure absorbs the maximum light. According to the simulation, we can further improve the performance of devices based on the optimum thicknesses.At last, the photo-detector based on 2D InSe/MoS₂ heterostructure is fabricated and the performances are characterized. The device has some novel properties such as current rectification and photovoltaic effect which is in contrast to the traditional photo-detectors based on 2D materials. The photo-responsivity and external quantum efficiency are 0.17 A/W and 39.6% respectively without any bias voltage. This is very comparable to other photo-detectors based on TMDC/semiconductor. However, the photocurrent distribution is not uniform inside and a new device based on graphene/InSe/MoS₂ heterostructure is then fabricated to improve it. The results indicate that the photocurrent distribution is almost uniform by adding the charge transport layer graphene, which means the structure of this device is very reasonable. However the photoresponsivity and EQE did not increase mainly because of the large size of this device.