The Study Of The Morphology Control And Interface Engineering For Low-bandgap Perovskite Solar Cells

In recent years,organic and inorganic hybrid perovskite materials have become a new generation of photovoltaic device materials due to their excellent photoelectric properties.The efficiency of single-junction perovskite solar cells has developed to 24.2%of the certified efficiency and is close to its theoretical limit.Therefore,in order to break through the bottleneck of efficiency,a new device structure-tandem perovskite solar cell is needed.Tandem perovskite solar cell is generally consisted of a wide-bandgap top cell and a low-bandgap bottom cell.The development of wide-bandgap perovskite solar cells has progressed well,but the efficiency of low-bandgap perovskite solar cell needs to be further improved.This thesis focuses on the morphology and interfacial engineering of low-bandgap perovskite solar cell.The morphology is a vital factor which affects the performance of perovskite solar cell.However,the morphology of low-bandgap perovskite solar cell is difficult to control due to its rapid crystallization as it contents Sn.In terms of interface,because the energy level of low-bandgap perovskite solar cell is not completely consistent with that of lead-based perovskite solar cell,we have done several research on interface in order to improve the device performance of low-bandgap perovskite solar cell.The main work of this thesis is divided into the following three parts:In chapter 3,we mainly studied the vacuum-assisted thermal annealing process to improve the device performance of perovskite solar cell.After comparing CH₃NH₃Sn_0.5Pb_0.5I_xCl_3-x films annealed in a vacuum and in a nitrogen environment,we find that vacuum-assisted thermal annealing can result in CH₃NH₃Sn_0.5Pb_0.5I_xCl_3-x films with better film coverage and crystallinity.This process can also accelerate the sublimation of methylammonium chloride and reduce the trap density in the CH₃NH₃Sn_0.5Pb_0.5I_xCl_3-x film and reduce the leakage current of the device.Combined with film and device characterization,X-ray diffraction,scanning enectron microscope,energy-dispersive spectrometry and photo-induced thermal diffraction spectroscopy,the effects of vacuum-assisted thermal annealing on morphology and crystallization were discussed.With this process,we successfully fabricated an efficient low-bandgap perovskite solar cell with a power conversion efficiency of more than 12%and good device reproducibility as well as long-term stability.We attributed this improvement to the fact that good film morphology and high crystallinity can inhibit charge recombination in the device and improve the charge extraction of the device.This kind of highly efficient low-bandgap perovskite solar cell can promote the development of tandem perovskite solar cell.In chapter 4,we successfully fabricated an efficient low-bandgap perovskite solar cell based on the conventional structure by modifying the electron transport layer with the PCE of 13.8%,and this structure provides the possibility of an efficient conventional tandem perovskite solar cell.We consider the extremely large V_oc loss to be one of the main causes of the inferior performance of low-bandgap perovskite solar cell based on the conventional structure,while our findings prove that the utilization of a cascade-type electron transport layer?ETL?(i.e.ZnO/SnO₂/C₆₀-SAM)is an efficient strategy to reduce the V_oc loss.First,in the context of energy level alignment,the C₆₀-SAM up-shifted the energy level of SnO₂ from-4.5 eV to-4.1eV,which minimized the offset of conduction bands between SnO₂ and perovskite,and hence reduced the energy loss.In addition,the larger energy offset between ITO and SnO₂/C₆₀-SAM was also reduced by the insertion of ZnO.Such a cascade-type ETL provided a smooth pathway for the electron,facilitating a better electron extraction and a low V_oc loss in the device.Second,in the context of recombination suppression,the C₆₀-SAM passivated the surface traps of SnO₂and consequently reduced the trap-assisted recombination at the interface.Moreover,the fast electron transfer from perovskite to the additional ZnO layer spatially separated the photo-excited electron and hole,leading to a further suppression of interfacial recombination,which also facilitated a reduced V_oc loss in the device.In chapter 5,we mainly studied the problem of silver diffusion to perovskite layer in inverted structure perovskite solar cell.To solve this problem,we inserted a thin ZnO layer between the PC₆₁BM layer and Ag electrode to suppress the Ag diffusion and hence minimize the negative effect caused by the Ag contamination.With the help of this ZnO layer,eventually,we demonstrated a low-bandgap perovskite solar cell with high PCE of 18.1%and V_oc loss of 0.40V,which becomes one of the state-of-the-art results for low-bandgap perovskite solar cell to date.Through the study,we found that the diffused Ag presented in the form of metal nanoparticle and contaminated the perovskite layer with two main changes of film properties:First,as the work function of Ag metal is deeper than the conduction band of perovskite,there was a possible electron transfer from the conduction band of perovskite to Ag nanoparticles,which made Ag nanoparticles act as trap states and hence increased the overall trap density of the whole perovskite layer.Second,both the conduction band and valence band of perovskite layer downed shift after being contaminated by Ag nanoparticles.The down shift of valence band of perovskite layer could enlarge the energy offset between the perovskite and PEDOT:PSS layer,which made the device suffer from larger energy loss at this interface.Both of these two aspects could cause a large V_oc loss for the whole device and thus the device with bare PC₆₁BM ETL suffered from an inferior PCE of 11.0%and V_oc loss as high as 0.6 V.Interestingly,an inserted layer of ZnO between the PC₆₁BM layer and Ag electrode efficiently suppressed the Ag diffusion towards the perovskite layer,and hence suppressed the negative impacts caused by the Ag contamination.