Study Of Stability Of Novel Photovoltaic Materials And Devices Based On Metal Oxide Substrates

The increasing global demand for energy,along with the continuous dwindling of fossil fuels and the environmental problems caused by their long-term use have left human beings in an energy crisis and forced politicians,academics and industry to invest in and develop sustainable alternative energy sources.The inexhaustible solar energy is one of the hopes that can be foreseen to solve the energy crisis.Although traditional silicon-based solar cell dominate the solar PV market today,its high manufacturing costs have led scientists to look for more cost-effective materials and device designs.In recent years,the emerging new solar cells mainly include perovskite solar cells?PSCs?,organic solar cells?OPVs?,dye-sensitized solar cells?DSSCs?and quantum dot sensitized solar cells?QDSSCs?.These new PV tecnologies are generally low-cost,easy to prepare and can be made of flexible devices,which overwhelm crystalline silicon and CIGS thin-film solar cells in these cases.As technology continues to improve in recent years,the efficiency of these emerging devices has also been rapidly increased.For example,quantum dot sensitized cells have exceeded the 13%conversion efficiency so far,and what is more gratifying is that the perovskite solar cell efficiency has reached 22.1%under laboratory conditions,which makes PSCs become the most dazzling solar cells in PV field today and probable to lead the new trend of the entire industry.Especially when the nanostrcuture of oxide substrate?ZnO,TiO₂,SnO₂ and Ni_xO,etc.?is improving,new solar cell devices have reached a higher level of performance.However,researches have found that these new generation solar cells do not have the same long-term stability,which mainly include chemical stability,photo stability,electrical stability and thermal stability,as the crystalline silicon cells in the practical working environment.For example,semiconductor materials in the quantum dot sensitized solar cells structure may become an excited state due to light absorption,and thus photochemical instability such as photo-corrosion is prone to occur under certain conditions.Moreover,the instability of perovskite solar cells,as we know,is the most important factor hindering its development.How to improve the stability of perovskite and PSCs in practical application is an inevitable problem to be solved in the development of this device.This paper mainly focuses on the light stability of quantum dot sensitized cells and the thermal stability of perovskite cells.The contents and achievements are as follows:We have innovatively proposed the in-situ photo-assisted chemical deposition?PACD?method to successfully coat CdSe QDs on the nanostructured substrates,and then improve device efficiency and photochemical stability of the material and devices based on the mechanisms study of the proposed modulation mixed light-assisted deposition method.At present,compound semiconductors have been widely used in light-collecting materials,conductive substrates and other functional components in the field of energy.However,in order to efficiently coat them on the target substrates in various forms,restrictions are often immerged in terms of growth efficiency,simplicity and controllability.In this work,we first have applied the PACD method to uniformly coat the CdSe quantum dots on ZnO/CdS nanorod arrays.Experiments by modulating the optical parameters?power desity and wavelength?of the injected light can significantly affect the morphology and quality of the CdSe quantum dot layer.Detailed mechanistic studies have found that the incident light plays an important role in the key reaction between the excited substrate material and reaction solution in the growth process.At the same time,the obvious photocorrosion of the substrates have been found when AM1.5 simulated light source and UV xenon lamp were used as the injection light source.It is found that ZnO substrates and CdX?X=S,Se?are easily to be excited by the incident light and these materials in the excited state are prone to occur photo-instability in a specific chemical environment.According to further experiments by introducing the modified AM1.5 light source filtered which the UV and IR components that are prone to induced photo-instability,the stability and quality of the synthesized materials are greatly improved.Finally,the unmodified ZnO/CdS/CdSe quantum dot cells efficiency reached 3.59%,significantly higher than CdSe sensitized solar cells cabricated by the conventional CBD method?2.88%?,and the growth rate increased by an order of magnitude.At the same time,the results may play a decisive role in exploring the potential applications and the stability of other composites and optoelectronic devices.Furthermore,we initiatively proved that the temperature gradient-induced ion transport plays a decisive role in the stability of the perovskite materials?MAPbI₃,FAPbI₃,etc.?.Further experiments showed that the thermal stability of materials and devices can be effectively improved by simply using a thermal reflector.In this paper,we compared the effects of uniform heating and non-uniform heating in the ZnO/MAPbI₃ system.It is found that the perovskite is tends to occur obvious instability or even complete decomposition in the presence of thermal inhomogeneity in the heating system.After excluding the influence of functional groups on the oxide surface,a detailed mechanism study revealed that ionic groups can be transported into the perovskite layers under the temperature gradient,eventually triggering an the unstable decomposition reaction chain,demonstrating the decisive role of the temperature gradient in this phenomenon.Further experiments have found that unstability phenomenon also happens on TiO₂?SnO₂,PCBM?/MAPbI₃ and ZnO/FAPbI₃ systems,which proves the universality of this phenomenon.In addition,the detail survey found that perovskite instability caused by ion migration also appeared in the cases discovered by the other researchers,and this may also be the key point of stability study of perovskites in the future.Finally,the thermal stability of perovskite materials and perovskite cells has been greatly improved by modifying the structure of the annealing device to reduce the temperature gradient.This study is expected to promote the stability of perovskite materials and devices,and make its synthesis mechanism a more in-depth understanding.Moreover,our group also proposed a two-step electrochemical synthesis method.The successful implementation of all-electrochemical method for synthesis perovskites on TiO₂substrates facilitated the conversion efficiency of perovskite cells to 15.65%.Long-term?about 1000h?atmospheric stability study on perovskite solar cells show that electro-chemically synthesized perovskites have excellent stability.A further comparison with the traditional two-step synthesis found that the perovskite films prepared by electrochemical method have higher quality lattice structure and denser film density,which is better for binding ion movement inside the film and preventing the ivasion from external harmful molecules.These results offering a new way to improve the stability of perovskite films and devices.