Abstract
Because of the increasing demand for clean energy, there has been growing interest in fabrication of photovoltaic devices which can harvest and directly convert solar energy into electricity. The most efficient thin film solar cells are based on Cu(In,Ga)(S,Se)2 (CIGSSe) and CdTe compounds, known as second generation polycrystalline thin films. The challenge of these materials is to reduce the cost per watt of solar energy conversion, but they are actually formed by expensive and scarce elements in the earth’s crust such as In, Ga, Te and others that present toxicity issues like Cd. The wide spread applications of solar cells will require dramatic decrease in cost through the use of non-toxic, inexpensive, and earth-abundant materials. Semiconductor compounds with kesterite structure (Cu2ZnSn(SxSe1-x)4, Cu2ZnSnS4, Cu2ZnSnSe4) and others like Cu2SnS3 (CTS), Cu2SnSe3 (CTSe) and their alloys Cu2Sn(S,Se)3 (CTSSe), copper-based sulfides CuSbS2 (mineral name Chalcostibite) and CuBiS2 (Emplectite), all of them Cadmium-free have been proposed as new candidates for thin film solar cells due to their similarity in material properties with CIGS and the relative abundance of raw materials. However, reported solar cell efficiencies for these compounds have not yet reached the expected values. In this work, we have investigated the microstructure, structural and electrical properties of the earth abundant copper-based materials such as Cu2ZnSnSe4, Cu2SnSe3 and CuSbS2 so as to identify the different mechanisms that limit cell performance.
This investigation has four parts. The first part deals about the effect of magnesium doping in Cu2ZnSnSe4. For the study of Mg-doped CZTSe bulk material, (Cu2-xMgx)ZnSnSe4 formula at x = 0, 0.1, 0.2, 0.3, and 0.4 were designed. The soluble sintering aids of Sb2S3 and Te were used for densification. Defect chemistry was studied by measuring structural and electrical properties of Mg-doped CZTSe as a function of dopant concentration. From the measured values, except at x = 0, all Mg-doped CZTSe pellets showed a n-type behavior. n-type Mg-CZTSe pellets at x = 0.1 showed the highest electrical conductivity of 24.6 S cm-1 and hole mobility of 120 cm2 V-1 s-1, as compared to 11.8 S cm-1 and 36.5 cm2 V-1 s-1 for the undoped p-type CZTSe. Mg dopant was a strong promoter of electrical mobility. Mg dopant behaves as a donor defect in CZTSe at 5% doping content, but is also used as an acceptor at a high content above 5%. From these results it was predicted that Mg doping will further developed CZTSe into a promising semiconductor.
The second part deals about the improvements in electrical properties for the Sn-rich Cu2ZnSnSe4 bulks. Effects of the Cu variation in Cu2-xZn0.9Sn1.1Se4 (Sn-rich CZTSe) bulks with x = 0–0.3 on the morphological, structural, and electrical properties have been investigated. Cu2-x(Zn0.9Sn1.1)Se4 pellets show as the p type at x = 0 and 0.1 and the n type at x = 0.2 and 0.3. Sn-rich CZTSe at x = 0 and 0.1 has high mobilities of 87.1 and 58.4 cm2/Vs and favorable hole concentrations of 7.52×1017 and 4.88×1017 cm-3, respectively. SEM surface images have shown that the grains are less densely packed as the copper content decreases. The non-stoichiometric compositions of Sn-rich CZTSe under various Cu contents led to the intrinsic defects, with which the changes in the structural and electrical properties of the bulks with the Cu ratio can be explained. This work provides the promising results for Sn-rich CZTSe with the Sn excess to control the hole concentration and the Cu content to keep high electrical mobility.
The third part deals about the effect of Mg doping in Cu2SnSe3. Mg-doped Cu2SnSe3 bulk materials with the (Cu2-xMgx)SnSe3 (Mg-x-CTSe) formula at x = 0, 0.05, 0.1, 0.15, and 0.2 were prepared at 550 ˚C for 2 h with soluble sintering aids of Sb2S3 and Te. Defect chemistry was studied by measuring structural and electrical properties of Mg-doped Cu2SnSe3 as a function of dopant concentration. Mg-x-CTSe pellets show p-type at x = 0, 0.05 and 0.1 and n-type at x = 0.15 and 0.2. The low hole concentration of 3.2×1017 cm-3 and high mobility of 387 cm2 V-1 s-1 were obtained for (Cu2-xMgx)SnSe3 bulks at x = 0.1 (5% Mg) as compared to 2.2×1018 cm-3 and 91 cm2 V-1 s-1 for the undoped one. The explanation based upon the Mg-to-Cu antisite donor defect for the changes in electrical property was declared. A high Mg content for Mg-x-CTSe at x  0.1 can lead to the formation of second phases. The study in bulk Mg-x-CTSe has been based upon defect states and is consistent and supported by the data of structural and electrical properties.
The fourth part deals about process limitation for p-type CuSbS2 semiconductor. CuSbS2 bulks were prepared by reactive sintering the mixture of CuS and Sb2S3 at 350, 375, 400, 450, and 500 oC for 2 h and at the sintering temperature of 400 oC for 0.5, 1, 2, and 3 h under a compensation disc of CuS for atmospheric control. Composition, structure, morphology, and electrical properties of the sintered bulks were analyzed. The compositions of Cu, Sb and S did not change until the temperature reached at and above 450 oC. The highest electrical conductivity of 15 S cm-1 and the highest mobility of 20 cm2 V-1 s-1 were obtained for CuSbS2 sintered at 400 oC for 2 h. 5% deviations in the Cu/Sb and S/(Cu+Sb) rations caused a serious problem in the degradation of electrical properties, though the CuSbS2 remained as a single phase. From these results, it has been concluded that CuSbS2 is the semiconductor which needs to have a controlled composition.

Abstract
Because of the increasing demand for clean energy, there has been growing interest in fabrication of photovoltaic devices which can harvest and directly convert solar energy into electricity. The most efficient thin film solar cells are based on Cu(In,Ga)(S,Se)2 (CIGSSe) and CdTe compounds, known as second generation polycrystalline thin films. The challenge of these materials is to reduce the cost per watt of solar energy conversion, but they are actually formed by expensive and scarce elements in the earth’s crust such as In, Ga, Te and others that present toxicity issues like Cd. The wide spread applications of solar cells will require dramatic decrease in cost through the use of non-toxic, inexpensive, and earth-abundant materials. Semiconductor compounds with kesterite structure (Cu2ZnSn(SxSe1-x)4, Cu2ZnSnS4, Cu2ZnSnSe4) and others like Cu2SnS3 (CTS), Cu2SnSe3 (CTSe) and their alloys Cu2Sn(S,Se)3 (CTSSe), copper-based sulfides CuSbS2 (mineral name Chalcostibite) and CuBiS2 (Emplectite), all of them Cadmium-free have been proposed as new candidates for thin film solar cells due to their similarity in material properties with CIGS and the relative abundance of raw materials. However, reported solar cell efficiencies for these compounds have not yet reached the expected values. In this work, we have investigated the microstructure, structural and electrical properties of the earth abundant copper-based materials such as Cu2ZnSnSe4, Cu2SnSe3 and CuSbS2 so as to identify the different mechanisms that limit cell performance.
This investigation has four parts. The first part deals about the effect of magnesium doping in Cu2ZnSnSe4. For the study of Mg-doped CZTSe bulk material, (Cu2-xMgx)ZnSnSe4 formula at x = 0, 0.1, 0.2, 0.3, and 0.4 were designed. The soluble sintering aids of Sb2S3 and Te were used for densification. Defect chemistry was studied by measuring structural and electrical properties of Mg-doped CZTSe as a function of dopant concentration. From the measured values, except at x = 0, all Mg-doped CZTSe pellets showed a n-type behavior. n-type Mg-CZTSe pellets at x = 0.1 showed the highest electrical conductivity of 24.6 S cm-1 and hole mobility of 120 cm2 V-1 s-1, as compared to 11.8 S cm-1 and 36.5 cm2 V-1 s-1 for the undoped p-type CZTSe. Mg dopant was a strong promoter of electrical mobility. Mg dopant behaves as a donor defect in CZTSe at 5% doping content, but is also used as an acceptor at a high content above 5%. From these results it was predicted that Mg doping will further developed CZTSe into a promising semiconductor.
The second part deals about the improvements in electrical properties for the Sn-rich Cu2ZnSnSe4 bulks. Effects of the Cu variation in Cu2-xZn0.9Sn1.1Se4 (Sn-rich CZTSe) bulks with x = 0–0.3 on the morphological, structural, and electrical properties have been investigated. Cu2-x(Zn0.9Sn1.1)Se4 pellets show as the p type at x = 0 and 0.1 and the n type at x = 0.2 and 0.3. Sn-rich CZTSe at x = 0 and 0.1 has high mobilities of 87.1 and 58.4 cm2/Vs and favorable hole concentrations of 7.52×1017 and 4.88×1017 cm-3, respectively. SEM surface images have shown that the grains are less densely packed as the copper content decreases. The non-stoichiometric compositions of Sn-rich CZTSe under various Cu contents led to the intrinsic defects, with which the changes in the structural and electrical properties of the bulks with the Cu ratio can be explained. This work provides the promising results for Sn-rich CZTSe with the Sn excess to control the hole concentration and the Cu content to keep high electrical mobility.
The third part deals about the effect of Mg doping in Cu2SnSe3. Mg-doped Cu2SnSe3 bulk materials with the (Cu2-xMgx)SnSe3 (Mg-x-CTSe) formula at x = 0, 0.05, 0.1, 0.15, and 0.2 were prepared at 550 ˚C for 2 h with soluble sintering aids of Sb2S3 and Te. Defect chemistry was studied by measuring structural and electrical properties of Mg-doped Cu2SnSe3 as a function of dopant concentration. Mg-x-CTSe pellets show p-type at x = 0, 0.05 and 0.1 and n-type at x = 0.15 and 0.2. The low hole concentration of 3.2×1017 cm-3 and high mobility of 387 cm2 V-1 s-1 were obtained for (Cu2-xMgx)SnSe3 bulks at x = 0.1 (5% Mg) as compared to 2.2×1018 cm-3 and 91 cm2 V-1 s-1 for the undoped one. The explanation based upon the Mg-to-Cu antisite donor defect for the changes in electrical property was declared. A high Mg content for Mg-x-CTSe at x  0.1 can lead to the formation of second phases. The study in bulk Mg-x-CTSe has been based upon defect states and is consistent and supported by the data of structural and electrical properties.
The fourth part deals about process limitation for p-type CuSbS2 semiconductor. CuSbS2 bulks were prepared by reactive sintering the mixture of CuS and Sb2S3 at 350, 375, 400, 450, and 500 oC for 2 h and at the sintering temperature of 400 oC for 0.5, 1, 2, and 3 h under a compensation disc of CuS for atmospheric control. Composition, structure, morphology, and electrical properties of the sintered bulks were analyzed. The compositions of Cu, Sb and S did not change until the temperature reached at and above 450 oC. The highest electrical conductivity of 15 S cm-1 and the highest mobility of 20 cm2 V-1 s-1 were obtained for CuSbS2 sintered at 400 oC for 2 h. 5% deviations in the Cu/Sb and S/(Cu+Sb) rations caused a serious problem in the degradation of electrical properties, though the CuSbS2 remained as a single phase. From these results, it has been concluded that CuSbS2 is the semiconductor which needs to have a controlled composition.

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