Structural Design and Stability of Perovskite Solar Cells

Author:Wei Dong

Supervisor:li mei cheng


Degree Year:2019





Perovskite has such outstanding characteristics as long carrier diffusion length,high electron/hole mobility,high light-absorption coefficient,and simple preparation process,which make the power conversion efficiency(PCE)records of perovskite solar cells(PSCs)have been constantly broken in recent years.In the pursuit of high PCE,the problem of PSCs stability has become a prominent problem.This paper focuses on the improvement of the stability and performance of perovskite materials and devices,explores the key mechanisms leading to the degradation of perovskite solar cells,and improves the performance and stability of perovskite materials and devices through the modifications of perovskite materials and the new-structure design of perovskite solar cells.The main research works are as follows:(1)Study on the new-type TiO2 embedded structure of perovskite solar cells.Combining the advantages of mesoporous and planar structure of perovskite solar cells,a novel structure for highly-efficient and stable PSCs is proposed,i.e.,the embedded structure.The embedded structure utilizes TiO2 nanoparticles embedded perovskite(CH3NH3PbI3)film as the absorption layer.The presence of TiO2 nanoparticles in perovskite film could improve the electron extraction,and promote the formation of a compact perovskite layer with large grains,which enables the photo-generated carriers in the devices to be separated and extracted efficiently and reduces the recombination loss of carriers.Consequently,the performance of the PSCs is significantly improved with the efficiency increasing from 16.6%for planar structure to 19.2%for the embedded structure.Furthermore,the TiO2 embedded perovskite films present better long-term stability than the pristine perovskite films,and the corresponding PSCs,which have no any other chemical modifications,also show excellent stability with efficiency keeping approach 80%(for average)after being exposed in air for 28 days without encapsulation.This TiO2 embedded structure of PSC processes higher carrier transfer efficiency,and realizes the all-low temperature preparation with high efficiency and long-term stability,which provides a reference for the structure design of stable PSCs(2)Study on the influences of the organic cation-π interaction on the stability of perovskite materials and solar cells.For inhibiting the migration of ions in perovskite solar cells(PSCs),the supramolecular cation-π interaction between aromatic rubrene and organic cations had been introduced in OIPs.The energy of cation-π interaction between rubrene and perovskite has been found as strong as 1.5 eV,which is enough to immobilize the organic cations in OIPs,thus will lead to the obvious reduce of defects in perovskite films and outstanding stability in devices.By employing the cation-immobilized OIPs to fabricate perovskite solar cells(PSCs),the certified efficiency of 20.80%with negligible hysteresis are acquired.In addition,the long-term stability of cation-immobilized PSCs has been improved definitely(98%of the initial efficiency after 720h operation),which is assigned to the inhibition of ionic diffusions in cation-immobilized OIPs.This cation-π interaction between cations and supramolecular πsystem enhances the stability and the performance of PSCs efficiently and would be a potential universal approach to get the more stable perovskite devices.(3)Study on the effects of the doped moisture-resistant material on the long-term stability of perovskite solar cells in humid air.Long-term stability of perovskite materials,especially in humidity,is one of the major limitations to promote the development of perovskite devices.Here,to eliminate the degradation of perovskite solar cells(PSCs)in humid air,a water-resistant perovskite absorption layer was proposed by introducing a macrocycle-type cyclodextrin material(β-CD)into the films.The P-CD was proved to be capable of facilitating crystallization of grains and enhancing stability of perovskite by forming a supramolecular interaction with organic cations through the hydrogen bonds in the perovskite films.Consequently,the average efficiency of the PSCs remarkably increased from 16.19%to 19.98%.The PSCs withβ-CD exhibit superior long-term stability in ambient air without encapsulation,which remains 90%of initial efficiency after continuous AM 1.5 illumination in ambient air with the 80%relative humidity for 300 h.(4)Study on the mechanisms of degradation of the MAPbI3-based perovskite solar cells.One side:study on the photo-induced degradation of perovskite solar cells.Lead halide perovskite solar cells(PSCs)suffer from poor long-term stability,especially due to photo-induced degradation.Herein,the photo-induced degradation of PSCs with mesoporous and planar architectures are investigated,respectively,and the main origin is proved to be correlated with the hole transport material(HTM)/metal(Au)electrode interface.The solar irradiation of PSCs causes significant deterioration of device performance.Electrical analysis of the PSCs and XPS measurements show that the deteriorated performance is induced by retarded carrier extraction from the HTM to the Au electrode,due to a broken interface binding.Accordingly,in situ renewal of the Au electrode was found to cause notable recovery of the device performance of both mesoporous and planar PSCs.In comparison,the material degradation of perovskite and the electron transport layer/perovskite(ETL/perovskite)interface were also studied;however,these showed minor effects on the photo-induced degradation of PSCs.The other side:study on the degradation of perovskite solar cells in air environment.The degradation of PSCs is observed to be significant,with the decrease in efficiency from 18.2%to 11.5%in ambient air for 7 days.The degradation of PSCs derives from deteriorated photocurrent and fill factor,which are proven to be induced by increased trap states for enlarged carrier recombination in degraded PSCs.The increased trap states in PSCs over storage time are probably induced by the increased defects at the surface of perovskite.The trap states induced degradation provides a physical insight into the degradation mechanisms of PSCs.Moreover,as the investigations were performed on real PSCs instead of individual perovskite films,the findings here present one of their actual degradation mechanisms and provide theoretical and experimental supports for the design and preparation of more stable and efficient PSCs in the future.