Electrical Properties and Microstructure of Undoped and Doped Cu(In,Ga)Se2 Bulk Materials

• Main Author: MEHRDAD MONSEFI
• Other Authors: Dong-Hau Kuo
• Format: Others
• Language: en_US
• Published: 2013
• Online Access: http://ndltd.ncl.edu.tw/handle/29ak93

博士 === 國立臺灣科技大學 === 材料科學與工程系 === 102 === Cu(In,Ga)Se2 (CIGSe) semiconductor compound showing record photovoltaic conversion efficiencies near 20% has become a leading material for thin film solar cell applications. Investigations in AIBIIISe2 materials have been focused on device performance in order to enhance solar cell efficiency by maximizing the open-circuit voltage and short-circuit current, whereas controlling the electrical properties of the absorber material such as mobility, crystal defect formation, and charge carrier concentration are very important issues to achieve highly efficient CIGSe-base solar cells. In the processing of CIGSe thin film deposition, the ratio of Cu:In:Ga changes undesirably and it influences the electrical properties of the absorber layer and the solar cell performance consequently. Without accurate control in composition, the induced defects may be attributed to many factors and it would be so complex and filled with assumptions to vindicate an explanation. However, investigations on electrical properties of CIGSe material are feasible by systematic study of its bulk form with the favorable composition design. In the first part of this work, the Cu-poor, Cu-rich, and In-rich CIGSe bulk materials have been sintered in order to study the roles of Cu and In vacancies and antisite defect formation in electrical properties and microstructure of CIGSe material. The reactive liquid-phase sintering technique has been used to fabricate CIGSe dense bulks at a low temperature in order to maintain the atomic composition of the compounds very close to our favored design. Sintering of CIGSe bulk material has been carried out in the presence of Sb2S3 and Te sintering aids to assist densification at 650 ˚C. Electrical properties of the CIGSe bulk materials showed that carrier concentration and mobility enhanced with increasing Cu content. The larger grains of CIGSe material have been achieved with high Cu content. The maximum amount of mobility was 5.38 cm2/V.s for the Cu-rich sample with Cu1.1(In0.7Ga0.3)Se2 formula. The change in carrier concentration and transition of conductivity type from p-type to n-type material in In-rich sample confirmed our explanation about the In and Cu vacancy and interstitial defects. We could achieve CIGSe material with a lower carrier concentration by introducing the excess amount of In to A site of AIBIIISeVI2 structure. In the second part of this work, some elements like Sn, Mg, and Al have been doped into CIGSe bulks to investigate the influence of these dopants on electrical properties and microstructure of CIGSe material. Increasing the Sn-dopant content led to the increase in hole concentration but at a doping content of 15.6% the conductivity type of CIGSe transformed from p-type to n-type regardless of the sintering temperature. More Sn doping led to larger CIGSe grains and lattice shrinkage. Carrier mobility above 12 cm2/V责s could be achieved for the n-type Sn-doped CIGSe with a higher Cu content. The favored low concentration of holes in the order of 1016 cm–3 and mobility above 4 cm2/V责s was achieved for CIGSe material doped with 10% of Mg. The n-type Cu0.7[(In0.6Al0.1)Ga0.3]Se2 material was obtained in Al-doped CIGSe bulks and the p-type one was achieved at higher Cu contents. The data of lattice parameters have been used to confirm the change in electrical properties of CIGSe bulk materials and explanation of defect formation mechanisms.