A Study Of High-performance Air-processed Perovskite Solar Cells And Its Degradation Mechanisms

Perovskite solar cells?PSCs?have drawn tremendous interests in the past ten years,due to its high efficiency,low-cost and easy fabrication.The power conversion efficiency?PCE?has promoted significantly:soar from 3.8%to 25.2%.However,the air-processed PSCs and degradation mechanism still need many efforts to improve device performance and stability.In this master thesis,we systematically studied the performance improvement of air-processed PSCs and degradation mechanism.In addition,a physical understanding about the enhancement of coupling between grain boundary relaxation and dipole relaxation of C₆₀ was also studied.Firstly,we added the Cesium-containing aqueous solution into perovskite precursor to improve the PCE of air-processed inverted PSCs.The individual study of water and cesium demonstrate that the water can enhance the crystallinity and result in larger grain size?from250 nm for pristine to 550 nm for optimization?by retarding the onset time of crystallization,while the cesium can tuning the band gap and the energy level alignment to improve the charge transport.By the synergistic benefits of Cesium-containing aqueous solution,a PCE as high as 16.6%for air-processed inverted PSCs without BCP buffer layer and prepared at relative humidity of 50%has been achieved.Due to the poor physical contact and energy level alignment mismatch between P3HT and perovskite,the PCE of P3HT based PSCs still locate at lower level,especially air-processed P3HT based perovskite solar cells.Herein,a gradient mixed-cation perovskite designed by spin coating FAI upon the perovskite intermediate phase film containing excess lead.The gradient mixed-cation PSCs based on P3HT hole transport layer prepared at relative humidity of 50%achieved a PCE up to 16.26%.To our knowledge,it's the highest value for air-processed P3HT based perovskite solar cells.The further study indicates that the gradient mixed-cation perovskite can lower the trap density significantly.The tuned valence band maximum results in an improved energy level alignment between P3HT and perovskite.In addition,the long term stability was also improved.This strategy is prone to fabricating high performance air-processed perovskite solar cells with P3HT.To understand the mechanism of the water and oxygen co-induced perovskite degradation on the initial stage,dielectric spectrum and ¹³C NMR has been performed.The Deby-type relaxation observed by dielectric spectrum was used as“probe”to study the physical image of degradation.The results suggest that the responsible dipoles are generated from CH₃NH₃⁺,when absorbed water break their bonds with Pb-I cages,then paired with O₂^-,when electrons transfer from I to absorbed oxygen.These dipoles act as charges recombination centers,thus seriously impede photovoltaic performance.According to the reorientation properties of dipole under the electric field,the migration energy of MA⁺has been estimated by fitting the Deby-type relaxation,which is 630 meV.This study revealed that the dynamic behavior of water and oxygen absorbed on the perovskite,and also provided an insight into understanding the perovskite degradation.Lastly,we studied the coupling effect between grain boundary relaxation and dipole relaxation,which lead to an increased energy barrier for molecular reorientation.In addition,the investigation proved the existence of grain boundary relaxation in solid C₆₀ as well.