title: Ion and Defect Migration Control and its Effect in Organo Lead Halide Perovskites
Creators: name: Zhong, Yu
Publisher: name: University of Bayreuth
Year: 2021
Subjects: subject: perovskite
subject: solar cell
subject: ion
subject: defect
subject: migration
subject: crystallization
subject: 530
subject: 540

description: Organolead halide perovskites have emerged as the pioneer of new photovoltaic materials in the last decade, because their extraordinary optoelectronic properties lead to corresponding highly efficient photovoltaic devices. Although a lot of researchers worked on these materials in very recent time, many fundamental questions still remain unanswered to date. One of them is the material instability which can be induced by illumination and external electric field. As these two factors unavoidably play a role under operational conditions of perovskite solar cells (PSCs), it is significant to find out their influence factors and to minimize their impacts. Two peculiarities related to the photo- and electric-induced instability, which impede the stable output of PSCs, are studied in this thesis. One is that the power conversion efficiency (PCE) of PSCs declines quickly under continuous illumination. The second one is that the current density-voltage (J-V) curve of PSCs shows hysteresis, depending on the respective measurement conditions. In order to clarify the first problem, a wide-field photoluminescence (PL) microscope was employed to study the local changes of perovskite films under illumination. The investigation in Chapter 4 presents that perovskite individual grains (nanocrystals) exhibit PL intermittency, indicating the spacially confined transitions between bimolecular recombination and Auger non-radiative recombination. This reflects that a large number of ions/defects exists in polycrystalline perovskite films and those in most cases can result in poor emission properties. Chapter 5 discusses the observed decrease of local PL, while at the macroscale the PL enhances. The decrease in PL intensity happens at a similar timescale as the PCE decay of PSCs. Hence, we propose that this local PL quenching is related to the photo-induced PSC deterioration. Combined with the observations regarding the PL blinking in Chapter 4, this behavior is attributed to the accumulation of ions/defects by light soaking. In a working PSC, the ion redistribution under illumination leads to the decrease of external luminescence, which will directly influence the open circuit voltage (Voc) and the PCE. The J-V hysteresis problem has attracted attention in the field and studies manifest that phenyl-C61-butyric acid methyl ester (PCBM) can suppress hysteresis. Besides the positive effects of trap state elimination by PCBM, Chapter 6 demonstrates that PCBM hinders the migration of iodine related defects under working bias, which is supposed to be responsible for hysteresis. The retarded ionic migration is proven by PL microscopy investigations and by the increase of its activation energy estimated from temperature-dependent chronoamperometric measurements. Using a PCBM-grafted polymer as an interfacial layer in PSCs, hysteresis is still pronounced compared to the device with a PCBM layer. It indicates that the diffusion or incorporation of PCBM molecules in the perovskite material is crucial, as the diffusion of PCBM molecules that are grafted on polymer backbones is strongly impeded. Thus, PCBM molecules in the bulk of the perovskite suppress iodine ions or vacancies migration, moderating the built-in field shift in PSC caused by external bias. Chapter 4 and Chapter 5 highlight that the PL intermittency and local PL intensity decay phenomena are weakened by the PCBM layer, while in Chapter 6 the passivation of mobile ions or vacancies by PCBM is emphasized. These investigations suggest a way to eliminate the photo- and electric-induced negative effects by the use of additives or interfacial layer. Apart from this method, the preparation of high-quality perovskite films is another important way. The high-quality film typically exhibits less defect states and mobile ions in the material and can improve the PSC stability. Perovskite films with less grain boundary area, i.e. with large grain sizes, also have the similar effect. Hence, Chapter 7 and Chapter 8 focus on the control of perovskite film formation by looking into the perovskite film evolution during spin coating and annealing. In Chapter 7, absorption and PL spectra were in-situ recorded when methylammonium iodide (MAI) dissolved in dimethylformamide (DMF) was spin coated on a PbI2 layer. The results reveal that the crystal formation occurs in the first 20 seconds. We firstly explicitly analyzed the optical properties on this time scale. In brief, the formation of MAPbI3 capping layer and its dissolution-recrystallization were the major changes detected during spin coating. As the dissolution-recrystallization process strongly influences the final film quality, it is important to control the speed of this process. Furthermore, developing an in-depth understanding of the measured optical spectra allows us to explore the change of the film thickness during the spin coating and the respective phases of perovskite formation. In Chapter 8, with the assistance of a solvent vapor assisted annealing method, we could grow perovskite films with large grains and concomitantly minimize grain boundary area and defect densities. From in-situ grazing incidence wide-angle X-ray scattering (GIWAXS) measurements, it is found that DMF vapor treatment during annealing slows down the crystallization. By systematically increasing the vapor concentration, it is possible to enlarge the crystal size. However, too high DMF vapor concentrations lead to the formation of pinholes, as DMF can liquefy and partially dissolve perovskite crystals. This work indicates that the precise control of solvent vapor assisted annealing is necessary to obtain high-quality perovskite films.