Sb₂S₃-Based Heterojunction Films With Tailored Structures For Efficient Solar Cells

Photovoltaic technology for the sustainable energy production on a large scale is mainly limited by lacking the solar cells with efficient,stable and low-cost characteristics.Antimony trisulfide（Sb₂S₃）has recently emerged as a promising photovoltaic material for application in solar cells,due to its intrinsic thermodynamic stability,appropriate band-gap,high charge carrier mobility,and low exciton binding energies.This dissertation aims to study the preparation approaches for solution-processed quality Sb₂S₃-based heterojunction films,tailor the structure and photovoltaic function of the heterojunction films in solar cells,reveal the heterojunction growth principles and the intrinsic correlation between material structure and device performance,and provide the theoretical basis for the research and development of efficient Sb₂S₃ solar cells.The main activities and results in this dissertation are summarized as follows:（1）Sb₂S₃ single-crystalline nanorod array is grown on a large scale over polycrystalline TiO₂ film by a solution-processing strategy under the assistance of tiny Sb₂S₃ seeds and a novel material concept of nanoarray heterojunction（NHJ）is proposed,creating an Sb₂S₃/TiO₂ nanoarray heterojunction（Sb₂S₃/TiO₂-NHJ）.It is found that the Sb₂S₃ nanorod growth on the TiO₂ film follows a tiny-seed-governed orientation-competing-epitaxial nucleation/growth mechanism.Using a conjugated polymer as hole transporting material and the Sb₂S₃/TiO₂-NHJ as photoactive layer,a novel Sb₂S₃/TiO₂-NHJ solar cell without negative impact of photogenerated electric field on device performance is obtained and a local architecture-dependent charge distribution model is proposed to understand the unique photovoltaic behaviors in the solar cell,resulting in an efficiency of 5.70%in such device with a good long-term stability.In contrast,with a small organic molecule Spiro-OMeTAD as hole transporting material and an insulating organic interfacial layer,a novel Sb₂S₃/TiO₂-NHJ solar cell with strong negative impact of photogenerated electric field is fabricated,in which a hole-selective-tunneling interfacial engineering method is developed to improve the device performance.（2）A tiny-Sb₂Se₃-seed assisted solution-processing method for growing Sb₂S₃-NA on different substrates is developed,and the Sb₂S₃ nanorod growth is found to follow an orientation-competing-epitaxial nucleation/growth mechanism based on the heteroepitaxial nucleation effects of randomly formed tiny heterogeneous seeds.In combination with the growth features of Sb₂S₃-NA induced by tiny Sb₂S₃ seeds on ploycrystalline TiO₂ film,the correlation between the Sb₂S₃-NA growth and the surficial characteristics（non-epitaxial-for-seed-growth or epitaxial-for-seed-growth）of beneath substrate and the related principle get elucidated,offering the new insights into the growth of semiconductor single-crystals on polycrystalline substrates.Post-selenization of tiny-Sb₂Se₃-seed-derived Sb₂S₃/TiO₂-NHJ makes the transformation of single-crystalline Sb₂S₃ nanorods into single-crystalline Sb₂（S,Se）₃ quasi-nanorods,creating a novel material system of Sb₂（S,Se）₃/TiO₂ quasi-nanoarray heterojunction（Sb₂（S,Se）₃/TiO₂-quasi-NHJ）;additionally,with a conjugated polymer as hole transporting material,the Sb₂（S,Se）₃/TiO₂-quasi-NHJ solar cell without negative impact of photogenerated electric field on device performance is fabricated with a best efficiency of 7.18%,and the physical principles for the effects of S/Se atomic ratio on the device performance.（3）With TiO₂ nanorod array as a template,a TiO₂/CdS-core/shell nanorod array（TiO₂-CdS-NA）is prepared to offer a composite electron transporter for Sb₂S₃ solar cells,in which TiO₂ nanorod acts as core and polycrystalline CdS nanostructured film on the TiO₂ nanorod serves as shell;afterwards,the in-situ growth of Sb₂S₃ nanoparticles in TiO₂-CdS-NA renders an Sb₂S₃-based bulk heterojunction（BHJ）film（TiO₂-CdS-NA/Sb₂S₃）,featuring an Sb₂S₃ nanoparticle film interdigitated by the TiO₂-CdS-NA.Using TiO₂-CdS-NA/Sb₂S₃ as photoactive layer and a small organic molecule Spiro-OMeTAD as hole transporting material,we achieve the Sb₂S₃-based BHJ solar cell with an efficiency of 6.14%,in particular,with an open-circuit voltage（Voc）up to 0.76 V,which is,up to date,the highest Voc value among the Sb₂S₃ solar cells;With respect to the TiO₂-NA/Sb₂S₃ control device,the introduction of the CdS interfacial layer enhances,respectively,the Voc and efficiency by 31%and 73%.On one hand,it is found that there in TiO₂-CdS-NA/Sb₂S₃ layer exist the band-bending at interfaces,photo-induced space-charging in CdS interfacial layer,and strong interactions at TiO₂/CdS 和 CdS/Sb₂S₃ interfaces;moreover,a photo-induced dipole electric field at CdS/Sb₂S₃ interface increases the apparent built-in electric field of the solar cell for a rather high device Voc,for which a model on the Voc generation correlated to photo-induced dipole electric field is proposed.On the other hand,it is elucidated that the CdS interfacial layer boosts the charge collection efficiency and short-circuit current mainly by improving the band continuity for charge transportation,passivating the oxygen vacancy defects on TiO₂ nanorods and rendering an effective spatial spacer with an energy barrier to reduce the interfacial charge recombination.