The Investigation Of Physical Process Of Organometal Halide Perovskite Solar Cells

In recent years,lead halide perovskites solar cells（PeSCs）have made significant progress,because of the advantages of perovskite materials,such as high absorption coefficients,low exciton binding energy,superb charge carrier mobility and long-range balanced charge transport lengths.In parallel with the rise in PeSC performance,the increasing interests on PeSCs have been focused on unraveling the basic physical processes which limit the performance of device.In PeSC devices,not only perovskite layer itself,but also the ETL/HTL that extracts and transports charges plays crucial role for the performance of PeSCs.For PeSCs,only the electrons and holes originate from the photons absorbed by the perovskite layer contribute to the photocurrent of PeSCs.As usual,the internal absorption（Q）can represent the amount of incident photons absorbed by the perovskite layer in a device.So,the analysis of the light absorption ability of the perovskite material is essential for optimizing Q of the perovskite layer and maximising the generated photocurrent of PeSCs,and further accounting for the light losses of absorption by non-perovskite and reflection at incident interface of device.In addition,the charge recombination and collection process is one of the important factors affecting the internal quantum efficiency（IQE）and limiting the energy conversion efficiency of PeSCs.IQE can be influenced by some processes:（1）charge generation process after photons absorbed in the perovskite layer,（2）charge transport to counter electrodes and（3）charge collection through electrodes.The perovskite layer is expected to function as thus:generating charges by photoexcitation under the internal absorption.Then photogenerated charges must transport through the bulk perovskite layer to be extracted by the electron and hole transport layer without recombination.In addition,the photo-generated electrons should migrate through the electron transport layer（ETL）to the electrode,while holes should migrate through the hole transport layer（HTL）to the counter electrode for effective charge extraction.In this paper,we mainly aimed to deep investigate the internal absorption,exciton generation and charge transport processes of PeSCs based on ZnO ETL by varying perovskite layer thickness,changing the material and annealing temperature of perovskite layer,and selecting different hole transport layer materials.So four parts work were carried out as follows.1.The planar CH₃NH₃PbI₃（MAPbI₃）perovskite solar cells（PeSCs）with ZnO as electron transporting layer（ETL）were fabricated.By varying the wide range of MAPbI₃active layer thickness,we estimate their device parameters and external quantum efficiencies（EQE）in addition to internal absorption spectra（Q）by means of transfer matrix method（TMM）.Furthermore,the spectrally and spatially resolved internal quantum efficiencies（IQE）as a function of the active layer thickness within PeSCs were calculated and the relationship between IQE and device parameters extracted from the current–voltage（J–V）behaviors was discussed.It was found that the PeSC with MAPbI₃film thickness around 303 nm has a relatively high IQE and PCE,indicating that there is more power loss of PeSCs when the thickness of MAPbI₃ layer is either thinner or thicker than the thickness of about 300 nm.Furthermore,time-resolved photoluminescence together with the thickness dependent morphology and crystallinity of MAPbI₃ film demonstrate that there are two power loss processes in the fabricated PeSCs:one at the ZnO/MAPbI₃ interface and the other in bulk.2.We systematically study the internal absorption,charge recombination and internal quantum efficiency in MAPbI₃ and FAPbI₃ PeSCs.The TMM was used to model the internal absorption Q of perovskite layer of PeSCs based on MAPbI₃ and FAPbI₃perovskite,and the results revel that FAPbI₃ perovskite with lower bandgap has a wider Q spectrum than MAPbI₃ perovskite.The photoluminescence（PL）dynamics show that FAPbI₃ perovskite has superior charge transport property than MAPbI₃ perovskite.The analysis of charge carrier recombination by using photo induced charge extraction by linearly increasing voltage（photo-CELIV）technique and the investigation of charge collection process by photocurrent density(J_ph)versus effective voltage(V_eff)measurement indicate that PeSCs based on FAPbI₃ perovskite possess a significant improvement in IQE,due to lower charge recombination and higher charge collection than PeSCs based on MAPbI₃.In addition,the results of Jsc from Spectroscopic Limited Maximum Efficiency（SLME）mathematical models,TMM and J-V characteristics show that the limited factors of photocurrent in PeSCs mainly comes from the combined effects of（1）light loss of non-perovskite absorption,（2）light loss of reflection at incident interface of device and（3）photogenerated charge recombination.3.The effects of annealing temperature in the range from 100 to 150°C on the internal absorption,charge recombination and internal quantum efficiency of FAPbI₃based PeSCs were studied.These optoelectronic properties of PeSCs are greatly dependent on the film quality,which in turn is dependent on the thermal annealing treatment.The high-quality FAPbI₃ perovskite films were observed with the annealing temperature at 145°C.In addition,the optical simulation by TMM shows that the PeSC with FAPbI₃ film annealed at 145°C exhibits better internal absorption.The analysis of photo-CELIV and photocurrent density(J_ph)versus effective voltage(V_eff)characteristics suggests that the PeSCs with FAPbI₃ film annealed at 145°C possess lower charge recombination and higher charge collection because of the superior film property.Furthermore,higher IQE of photocurrent for PeSC with FAPbI₃ film annealed at 145°C was obtained,due to the better internal absorption,suppressed charge recombination and higher charge collection.4.A comprehensive study of the internal absorption,charge recombination and collection in FAPbI₃ PeSCs with P3HT and spiro-OMeTAD as the hole transport layers is presented.The optical simulation shows that the optical-field distribution and internal absorption of the perovskite layer are different in the cavity region for wavelength>500nm in PeSCs with HTLs of P3HT and spiro-OMeTAD.The transient photo-current（TPC）and photo-CELIV investigations reveal that the charge recombination in PeSCs is much better restrained by using spiro-OMeTAD as the HTL.The light intensity dependent J–V performance indicates that spiro-OMeTAD is more favorable for efficient charge collection due to its suppressed charge recombination.