Study On The Anisotropic Mechanical Properties Of GeSe And Its Applications In Solar Cells

The development and utilization of sustainable and clean energy such as solar energy and heat energy is an inevitable choice for the future development of human society.However,it is difficult to directly produce and utilize these energy sources.The development of efficient energy conversion materials is the key to solving the above problems.As a new type of two-dimensional semiconductor material,GeSe displays great potential in the field of thermoelectric,photovoltaics and photodetectors,etc.GeSe exhibits significant physical and chemical anisotropy behavior along different in-plane crystal directions due to its low crystal lattice symmetry.The unique orientation-dependent properties of in-plane anisotropic two-dimensional(2D)materials allow for designing novel electronic and optoelectronic devices that are hardly realized in isotropic materials.All such anisotropic studies and relevant applications require identification of the crystal orientation in advance.On the other hand,GeSe has been proposed as a promising alternative absorber material for high-efficiency thin film solar cells due to its suitable bandgap,high absorption coefficient,earth abundancy and non-toxicity.However,the study on GeSe thin-film solar cells is still in its infancy,and there is still an inadequate exploration of fabrication technology and performance optimization.Hence,further understanding of the fabrication technology is needed to broaden its application in the field of photovoltaics.For this consideration,in this thesis,we mainly investigated the following two parts:(1)Based on experiment and first principle calculation,we investigate the fracture behavior of GeSe.We first apply density functional theory(DFT)calculations to investigate the anisotropic mechanical properties of GeSe including elastic energy,Young’s modulus,and breaking stress.Guided by the theoretical results,we then propose that exfoliated GeSe flakes may show long-straight cleaved edges along the zigzag direction perpendicular to the armchair direction with the lowest Young’s modulus.Finally,polarization-resolved transmission spectroscopy(PRTS)experimentally confirms the above proposed cleavage tendency of GeSe,providing a direct in-situ nondestructive method for the crystal orientation identification just through the exfoliated morphology.(2)The purpose of this part is to optimize fabrication technology of GeSe thin films,thereby improving the energy conversion efficiency of GeSe thin-film solar cells.A novel post-annealing process was introduced to deposit high quality GeSe thin films.The crystal structure characterization of the GeSe films was identified by the X-ray diffraction(XRD).The morphology of the films was observed using top-view scanning electron microscopy(SEM)images.SEM and XRD characterizations show that the GeSe films fabricated by post-annealing methods have better crystallinity and larger grain size.Then we applied UV-vis-NIR absorption spectroscopy to investigate the optical properties.The results show that the GeSe films fabricated by different methods share similar optical properties and absorption onset.Furthermore,a significant efficiency improvement to 1.28% of GeSe solar cells is obtained by introducing postannealing process,and it is proved that post-annealing devices exhibit substantially improved stability for months in ambient conditions.