Study On The Front Surface Energy Level Gradient Of The Absorber In Perovskite Photovoltaic Devices

Organic-inorganic metal halide perovskite solar cells have become a hot spot in scientific research,photovoltaic industry and national science and technology strategies.Although the power conversion efficiency of single-junction perovskite solar cells has reached 25.7%,continuing to improve this index is still one of the key tasks to promote the industrialization of technology.In recent years,most of the methods to significantly improve the power conversion efficiency of perovskite photovoltaic devices can be summarized as reducing the non-radiative recombination of carriers by passivating defects in the bulk and surface of perovskite absorber.Despite the remarkable results achieved by defect passivation,these strategies generally ignore the spatial distribution of minority carriers and pay insufficient attention to the front surface of the absorber,limiting the development direction of the technology.This thesis aims to achieve a significant performance improvement for perovskite photovoltaic devices,and conducts systematic study in the aspects of the first successful implementation of the strategy,principle demonstration and application expansion,which fully proves that construct the front surface energy level gradient of the absorber is a new strategy and strikingly different with defect passivation principle.Meanwhile,it is proved that this strategy is generally effective for mainstream perovskite materials and film formation processes.The details are as follows:(1)Pioneeringly,the front surface energy level gradient of the absorber was successfully constructed in the perovskite photovoltaic device,and the open-circuit voltage gain was realized.In the classical normal-type MAPbI3 device,CsPbBr3 quantum with deep valence band edge energy level was used as the front surface layer of main absorber-MAPbI3,and the front surface valence band energy level gradient(F=▽EV)pushes holes(minority carriers)away from the device the front surface.It is proved that a sufficient ▽EV between QDs and MAPbI3 is a necessary condition for the formation of open-circuit voltage gain.After the CsPbBr3 quantum dots are coated with sulfate,they can be prevented from being damaged by polar solvents during the preparation process.The manipulation of the spatial distribution of holes by the front surface level gradient reduces nonradiative recombination,resulting in an open-circuit voltage gain as high as 90 mV.The normal-type MAPbI3 device exhibits an open-circuit voltage of 1.17 V and a power conversion efficiency of 20.53%,which is comparable to the state-of-the-art devices using a defect passivation strategy.(2)It is proved that the construction of the front surface energy level gradient is a universal strategy to improve the performance of mainstream perovskite devices,and its working principle is completely different from the traditional strategy.To realize the front surface energy level gradient strategy in other perovskite photovoltaic material systems,a front surface energy level gradient based on CsPbBr3 quantum dots was constructed in normal-type ternary cation perovskite(CsFAMA)photovoltaic devices.This strategy achieves an open-circuit voltage gain of 50 mV in the CsFAMA system,enabling the device to exhibit an open-circuit voltage of 1.18 V(corresponding to an S-Q limit of 93%).The power conversion efficiency of 22.36%is competitive with similar devices.In addition,by exploring the effect of ligand materials on device performance,the valence band energy level gradient is fully demonstrated through one-dimensional diffusion-drift model simulation,UV photoelectron spectroscopy analysis,built-in electric field characterization,and carrier recombination kinetics characterization,it is demonstrated that the valence band energy level gradient can formed a front surface electric field that directed from the surface to the body of the absorber.In this case,the non-radiative recombination of the device can be fully reduced by taking advantage of the characteristic that the recombination rate in the body is much lower than the surface recombination rate,and the open circuit voltage and power conversion efficiency of the device can be significantly improved without affecting the photocurrent.The strategy works independently of defect passivation.(3)Combining the front surface energy level gradient strategy with the twostep film formation process to improve the performance of perovskite photovoltaics.The previous two parts of perovskite photovoltaic devices both adopted the experimental process of one-step film formation method,and the two-step film formation method is currently a common preparation process for high-performance perovskite photovoltaics.In order to overcome the impact of multiple spin coating on the front surface layer in the two-step method,a front surface material based on CdSeZnS quantum dots was developed.The valence band-edge level gradient formed by CdSe-ZnS quantum dots and FAPbI3 effectively reduces the nonradiative recombination on the front surface of the absorber,enabling the device to achieve an open-circuit voltage gain of 60 mV and an open-circuit voltage of 1.17 V.The larger thickness of the absorber formed by the two-step method achieved short-circuit currents as high as 25.12 mA cm-2.The lattice constant matching of CdSe-ZnS QDs to FAPbI3 increases the fill factor to 0.81.Finally,the power conversion efficiency of the champion device reaches 23.66%.This efficiency value is highly competitive among similar divices.