| Since the 21st century,the non-renewable problem and environmental pollution of fossil energy have become increasingly prominent,which cannot meet the concept of sustainable development in nowadays human society.However,as a clean and environmentally friendly renewable energy,solar energy has attracted extensive attention.Recently,due to its excellent photoelectrical conversion efficiency(PCE)and simple preparation process,organic-inorganic hybrid perovskite solar cell(PSC)has becomes one of the most powerful photovoltaic power generation technologies in a short period of time,which is expected to have great application prospects.Now the photoelectric conversion efficiency of PSC has exceeded 25%.However,the stability problem has become one of the major restrictions to limit its widely commercial application and further development.While,the environmental factors,such as light,temperature,water and oxygen,affect greatly on the stability of the PSCs.Beside,during long-term light illumination,temperature raising(often reach up to 65?)is always a very important problem,which would severely influence the performance of the PSCs.So it is urgently needed to study the thermal stability of perovskite solar cells at high temperatures.Perovskite solar cells always have five layers.The thermal stability of perovskite solar cells is not only determined by each layer itself,but also restricts by the interface problem and interface interaction between each layer.At present,lots of studies focused on the thermal stability of single layer in perovskite solar cell,such as the studies on perovskite layer and hole transport.However,few researches have been reported on the disclosing of failure mechanism in the whole device at a high temperature.In this thesis,the degradation process of perovskite thin films and devices at high temperature is characterized by advanced spherical aberration correction transmission electron microscopy(AC-TEM)at micro-nano or even atomic scale.The results are presented as follows:(1)The perovskite solar cell device is annealed in a glove box with light-proof and oxygen-free environment in order to eliminate the influence of other factors.The performance evolution of the perovskite solar cell is measured by a solar simulator.It is found that the open circuit voltage does not drop while the photoelectric conversion efficiency of the perovskite solar cell drops at high temperature,meanwhile the short circuit current and the efficiency drop simultaneously.So,the failure mechanism of the perovskite solar cell at high temperature may not be induced by decomposition of perovskite materials,but mainly caused by blocking of the transmission of charge carriers.(2)The samples are sliced by FIB and then observed directly by SEM.It is found that the samples are most damaged at the MAPb I3/Sn O2 interface.According to the TEM observation results,the thermal decomposition in the perovskite film does not process synchronously,but has a gradient difference taking along the perpendicular direction to Sn O2-ETL,the perovskite film at the MAPb I3/Sn O2 interface took a serious and complete decomposition.(3)In order to understand the effect of ITO and Sn O2 on the degradation pathway of perovskite film,the degradation rates of perovskite films with and without the ITO and Sn O2 layers are compared.Experiment results show that ITO will accelerate the thermal decomposition of MAPb I3 as comparing with its intrinsic decomposition,while Sn O2-ITO will further accelerate this decomposition process.So,the deposited substrates will affect the decomposition pathway of perovskite films.(4)By using the first-principle calculation,the bond energy of Pb-O and Sn-I at the interface between MAPb I3 and Sn O2 are calculated,it can be found that oxygen at the Sn O2/MAPb I3 interface has a tendency to break away from Sn O2 and enter into the interior of MAPb I3 lattice.In addition,the calculation results of oxygen vacancy formation energy and oxygen diffusion barrier in Sn O2 show that oxygen atoms in the oxygen-deficient Sn O2 lattice can easily migrate to the interface of Sn O2/MAPb I3.After the oxygen atoms in Sn O2 enter into MAPb I3,the deprotonation of MAPb I3 will be accelerated and the decomposition will continue.(5)After plasma treatment of Sn O2-ETL in the oxygen gas environment,a transformation from anoxic state Sn O2 to oxygen-enriched state Sn O2 can be realized.Meanwhile,the oxygen vacancy formation energy and diffusion barrier in the oxygen-enriched state Sn O2 are extremely high,the diffusion of oxygen from Sn O2 to MAPb I3can be effectively inhibited.So,it will be great helpful for the improvement of the thermal-stability of the Sn O2/MAPb I3 interface in perovskite solar cell. |
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