Meeting the increasing demand for cleaner and cheaper energy sources has recently lead to unprecedented research efforts on photovoltaic (PV) solar energy conversion. In this field, the quaternary semiconductors Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe) are emerging as serious contenders to replace Cu(In,Ga)Se2 (CIGSe), reaching up to 11.1% by using Cu2ZSn(S,Se)4 (CZTSSe) photoactive alloys. This encouraging result has prompted a new effort for a better understanding of the properties of CZTS, CZTSe and CZTSSe, both through experimental and theoretical means.
CZTSe is a direct p-type semiconductor with a reported optical band gap of 1.0  1.44 eV and an absorption coefficient that is larger than 104 cm-1, which match the prerequisites for a solar absorber material, and it contains no rare or expensive elements. In fact, many researches have been done in CZTSe thin film under vacuum and non-vacuum processes to enhance the solar energy conversion efficiency and to reduce the materials cost. However, there are only limited reports related to bulk materials, which have been synthesized at high temperatures above 800 oC in a sealed tube under vacuum after considering the easy vaporization of selenium-related components. In this study, CZTSe is synthesized using a simple and costeffective liquid-phase reactive sintering method for the purpose of preparing dense CZTSe bulk materials from the reactions of selenide powders of Cu2Se, ZnSe, and SnSe2.
This work has included three parts. The first part deals about the densification and its mechanism, composition control, and characterizations of the un-doped Cu1.75ZnSnSe4 pellets sintered at 600 oC for 2 h. We have employed two different sintering aids of Sb2S3 and Te. With low melting point and excellent wettability, Sb2S3 and Te act as a liquid-phase sintering flux to increase atomic diffusion rate during the sintering process, and furthermore the two raw materials provide chemical driving force for the CZTSe phase formation. By the intentional introduction of Sb2S3 and Te adding into CZTSe, the well-crystallized ceramic samples have been perfectly sintered at 600 oC. In addition, serving as a liquid phase to promote the densification, it is important to recognize that the additive components also played an important role in enhancing the grain growth of the sintered pellets. We have also used SnSe2 and Se together to form a compensation disc for compensating Se loss upon sintering. SnSe2 is used to supply Se vapor by the decomposition into SnSe and 1/2Se2 vapor. The composition changes at different sintering approaches and different heating stages are also discussed. Another central point that has mentioned for this part is the effects of conventional and planetary ballmilled powders on sintering and densification of the pellets.
The second part gives an insight on how the doping elements affect the densification, the structure, and the electronic properties of the CZTSe-based bulks. The intrinsic and extrinsic dopants are investigated. The study on intrinsic dopant is performed by changing the Zn/Sn ratio in the format of Cu1.75(Zn1+xSn1-x)Se4, and Cu2.2(Zn1+xSn1-x)Se4. With changing the Zn or Sn content of the pellets, a comprehensive characterization on the structure, microstructure, lattice parameters, and electrical properties have been detailed. The Cu variation in the format of CuxZnSnSe4 has been also investigated. In this investigation, the effects of Cu-content are studied. It is the intention to prepare pellets which have their compostions changing from Cu deficiency to Cu surplus. The effects of increased Cu content at x= 1.82.0 during sintering were clear: significantly larger grains were observed. For the study of extrinsic dopant, it involves IIIA elements in the format of Cu1.75Zn(Sn1-xMx) Se4 with M= Al, Ga, and In and x= 00.6. The basic studies such as surface morphology, lattice constants (a and c), and electrical properties are comparatively explored among the dopants. At x= 0.4, Al-doped CZTSe (Al-CZTSe) pellets showed the highest hole mobility of 32.5 cm2 V-1 s-1, and large grains of 34 m. The high mobility is mainly attributed to the low scattering factor of Al.
In the third part, the effects of sulfurization to different degrees on the composition, morphology, structure, and electrical property of the Cu1.75ZnSn(Se1-xSx)4 solid solutions are investigated. Two types of sintering aids of Sb2S3 and Te, and two types of compensation discs of SnSe2 for selenization and CuS and SnSe2 for sulfo-selenization and CuS for sulfurization have been used. The compensation disc of CuS is used to supply S vapor by the decomposition into Cu2S and 1/2S2 vapor. The highest mobility of 3.5 cm2V-1s-1 was obtained for CZTSSe at x= S/(S +Se) = 0.5, which can be an important factor in selecting the Se- and S-coexisting absorber materials.

Meeting the increasing demand for cleaner and cheaper energy sources has recently lead to unprecedented research efforts on photovoltaic (PV) solar energy conversion. In this field, the quaternary semiconductors Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe) are emerging as serious contenders to replace Cu(In,Ga)Se2 (CIGSe), reaching up to 11.1% by using Cu2ZSn(S,Se)4 (CZTSSe) photoactive alloys. This encouraging result has prompted a new effort for a better understanding of the properties of CZTS, CZTSe and CZTSSe, both through experimental and theoretical means.
CZTSe is a direct p-type semiconductor with a reported optical band gap of 1.0  1.44 eV and an absorption coefficient that is larger than 104 cm-1, which match the prerequisites for a solar absorber material, and it contains no rare or expensive elements. In fact, many researches have been done in CZTSe thin film under vacuum and non-vacuum processes to enhance the solar energy conversion efficiency and to reduce the materials cost. However, there are only limited reports related to bulk materials, which have been synthesized at high temperatures above 800 oC in a sealed tube under vacuum after considering the easy vaporization of selenium-related components. In this study, CZTSe is synthesized using a simple and costeffective liquid-phase reactive sintering method for the purpose of preparing dense CZTSe bulk materials from the reactions of selenide powders of Cu2Se, ZnSe, and SnSe2.
This work has included three parts. The first part deals about the densification and its mechanism, composition control, and characterizations of the un-doped Cu1.75ZnSnSe4 pellets sintered at 600 oC for 2 h. We have employed two different sintering aids of Sb2S3 and Te. With low melting point and excellent wettability, Sb2S3 and Te act as a liquid-phase sintering flux to increase atomic diffusion rate during the sintering process, and furthermore the two raw materials provide chemical driving force for the CZTSe phase formation. By the intentional introduction of Sb2S3 and Te adding into CZTSe, the well-crystallized ceramic samples have been perfectly sintered at 600 oC. In addition, serving as a liquid phase to promote the densification, it is important to recognize that the additive components also played an important role in enhancing the grain growth of the sintered pellets. We have also used SnSe2 and Se together to form a compensation disc for compensating Se loss upon sintering. SnSe2 is used to supply Se vapor by the decomposition into SnSe and 1/2Se2 vapor. The composition changes at different sintering approaches and different heating stages are also discussed. Another central point that has mentioned for this part is the effects of conventional and planetary ballmilled powders on sintering and densification of the pellets.
The second part gives an insight on how the doping elements affect the densification, the structure, and the electronic properties of the CZTSe-based bulks. The intrinsic and extrinsic dopants are investigated. The study on intrinsic dopant is performed by changing the Zn/Sn ratio in the format of Cu1.75(Zn1+xSn1-x)Se4, and Cu2.2(Zn1+xSn1-x)Se4. With changing the Zn or Sn content of the pellets, a comprehensive characterization on the structure, microstructure, lattice parameters, and electrical properties have been detailed. The Cu variation in the format of CuxZnSnSe4 has been also investigated. In this investigation, the effects of Cu-content are studied. It is the intention to prepare pellets which have their compostions changing from Cu deficiency to Cu surplus. The effects of increased Cu content at x= 1.82.0 during sintering were clear: significantly larger grains were observed. For the study of extrinsic dopant, it involves IIIA elements in the format of Cu1.75Zn(Sn1-xMx) Se4 with M= Al, Ga, and In and x= 00.6. The basic studies such as surface morphology, lattice constants (a and c), and electrical properties are comparatively explored among the dopants. At x= 0.4, Al-doped CZTSe (Al-CZTSe) pellets showed the highest hole mobility of 32.5 cm2 V-1 s-1, and large grains of 34 m. The high mobility is mainly attributed to the low scattering factor of Al.
In the third part, the effects of sulfurization to different degrees on the composition, morphology, structure, and electrical property of the Cu1.75ZnSn(Se1-xSx)4 solid solutions are investigated. Two types of sintering aids of Sb2S3 and Te, and two types of compensation discs of SnSe2 for selenization and CuS and SnSe2 for sulfo-selenization and CuS for sulfurization have been used. The compensation disc of CuS is used to supply S vapor by the decomposition into Cu2S and 1/2S2 vapor. The highest mobility of 3.5 cm2V-1s-1 was obtained for CZTSSe at x= S/(S +Se) = 0.5, which can be an important factor in selecting the Se- and S-coexisting absorber materials.

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