Antimony Selenide Thin Film Solar Cells:the Characteristic One-dimensional Crystal Structure Properties And Its Influence On Device Performance

Seeking earth-abundant,low-toxicity,low-cost materials for efficient solar cells is a long-lasting goal for solar cell researchers.Antimony selenide?Sb₂Se₃?is a promising absorber material for high efficiency,low-cost thin film solar cells due to its suitable bandgap of¹.10 eV,high absorption coefficient of 10⁵ cm^-1 at short wavelength,low-cost,low-toxic and single phase.The Shockley-Queisser efficiency limit of single Sb₂Se₃ solar cell can approach above 30%.However,unlike conventional photovoltaics absorber materials such as Si,GaAs,CdTe and Cu?In,Ga?Se₂,which have an isotropic three-dimensional?3D?crystal structure,Sb₂Se₃ has a strongly anisotropic one-dimensional?1D?crystal structure.Antimony and selenium atoms are bonded by covalent bonds in the[001]direction to form the[Sb₄Se₆]_n ribbons,and the ribbons were stacked along the[100]and[010]directions via van der Waals force.This thesis mainly forces on the influence of Sb₂Se₃ one-dimensional crystal structure on carriers transport,grain boundary defects and interfacial diffusion in Sb₂Se₃ solar cells,and the main content includes:At the beginning of the research,Sb₂Se₃ has not been used as an absorber material in thin film solar cells.Many basic properties of Sb₂Se₃ that are directly related to photovoltaic application as well as its potential as absorber layer for solar cells are not yet clear.Therefore,in this paper,a high-purity,dense Sb₂Se₃ thin film with grains size of micrometer was prepared by hydrazine solution process firstly.The Hall effect measurement and absorption spectrum showed that Sb₂Se₃ is a P-type semiconductor with a direct bandgap of 1.06 eV,and the hole mobility is 5.1 cm²/Vs.Sb₂Se₃ thin film solar cells with an efficiency of 2.26%and excellent stability were fabricated using TiO₂ as the buffer layer,demonstrating the potential of Sb₂Se₃ thin film solar cells for the first time.Subsequently,the effect of the strongly anisotropic one-dimensional crystal structure on devices performance was investigated.The rapid thermal evaporation process was carried out to deposite Sb₂Se₃ thin film and fabricate CdS/Sb₂Se₃ devices.The devices efficiency was highly correlated with the orientation of Sb₂Se₃ film:devices efficiency monotonically decreased when the texture coefficient of[120]orientation increased.The essential cause of the orientation-dependent device performance is that the[Sb₄Se₆]_n ribbons in the[221]-oriented Sb₂Se₃ grains stacked vertically on the substrate,and carriers efficiently travel within the[Sb₄Se₆]_n ribbons;the[Sb₄Se₆]_n ribbons in the[120]-oriented Sb₂Se₃ grains stacked in parallel with the substrate,and carries are required to hop between ribbons held via van der Waals forces in this case,corresponding to reduced carriers transport efficiency.Meanwhile,the KPFM and EBIC results of the high efficiency CdS/Sb₂Se₃ device revealed that the grain boundaries in Sb₂Se₃ film exhibited similar surface potential and carriers collection efficiency to that within the crystal,indicating that grain boundaries in Sb₂Se₃ is benign,in accordance with the results of first-principle calculation.Finally,CdS/Sb₂Se₃ device with a certified efficiency of 5.6%was fabricated and the device showed good stability.Sb₂Se₃ has attracted great research interest as a promising absorber material for high efficiency thin film photovoltaics due to its special materials properties and increasing device efficiency.High efficiency Sb₂Se₃ thin film solar cells,either in superstrate or substrate configuration,employed CdS as buffer layer.The perculiar one-dimensional crystal structure of Sb₂Se₃ results in that impurities can easily diffuse into Sb₂Se₃ layer along the gap between the[Sb₄Se₆]_n ribbons.In this thesis,we systematically studied the interfacial diffusion in CdS/Sb₂Se₃ devices from a combined material and device physics characterizations.Results indicated that a buried homojunction located deeply inside Sb₂Se₃ layer due to Cd diffusion,replacing the apparent CdS/Sb₂Se₃ heterojunction to dictate carriers separation in CdS/Sb₂Se₃ devices.Diffused Cd converted p-type Sb₂Se₃ to n-type by introducing a donor level with an activation energy of 0.22 eV.This work clarified the different carrier transport,grain boundary defects and interfacial diffusion properties in Sb₂Se₃ photovoltaics devices,which was caused by the peculiar one-dimensional crystal structure of Sb₂Se₃,promoting the development of Sb₂Se₃ thin film solar cells.We hope this thesis also provides reference for the construction of thin film solar cells for other materials with low dimensional crystal structure.