Research On Charge Transport Layers And Interface Control Of Perovskite Photovoltaic Cells

The performance of the perovskite photovoltaic cells(PPCs)has been improved rapidly in recent years.The lastest certified highest power conversion efficiency(PCE)reached 25.5%which is very close to the highest reported PCE of silicon photovoltaic cells.PPCs have become the highest-PCE photovoltaic cells among the third-generation photovoltaic cells.PPCs consist of charge transport layers,electrodes and light harvester layers.Charge transport layers play an important role in transporting carriers and protecting light harvest layers.The charge transport layers include electron transport layers(ETLs)and hole transport layers(HTLs).Besides the charge transport layers,the interfaces between perovskite and charge transport layers also influence the photovoltaic performance and stability of PPCs dramatically.Therefore,we have done some researches on the charge transport layers and interfaces in this thesis.The researches include selecting different metal oxide materials to reduce the preparation temperature,modifying interfaces to improve the photovoltaic performance,studying the dopant of the HTL to enhance the performance stability.We also prepared the flexible perovskite solar cells ultimately.The details of the research are shown as following paragraphs.1.We used Lithium-contained salt to modify the compact Ti O₂ films at a high temperature.The defect state density in Ti O₂ films was reduced after the modification,which contributed to the electrical properity improvement of Ti O₂.The modified Ti O₂based PPCs yielded a much higher PCE of 17.1%compared with the unmodified Ti O₂based PPCs.2.To enhance the operation stability of PPCs,the hydrophilic Lithium ditrifluoromethane sulfimide(Li-TFSI)was replaced with Non-hydrophilic dopants to dope HTLs,which was improve the hole conduction ability of HTLs and enhanced the water-block capacity of HTLs at the same time.The photovoltaic performance of PPCs based on non-hydrophilic HTLs is slightly higher than that based on Li-TFSI doped HTLs.PPCs based on non-hydrophilic HTLs also had relatively better performance stability of the device in air condition.3.To decrease the preparation temperation of ETLs,we introduced the zinc oxide as ETLs materials of PPCs.A self-assembled molecular(SAM)layer was grown on the surface of the zinc oxide(Zn O)film using 3-(Aminopropyl)trimethoxy silane molecule.The SAM layer reduced the contact between the perovskite film and zinc oxide,which decreased the neutralization reaction between the perovskite film and Zn O during the annealing process.The inhibited neutralization reaction improved the chemical composition and morphology of perovskite films.The self-formed solvent annealing was introduced into the second annealing step to promote the formation of the perovskite with a larger average grain size.The perovskite with a larger average grain size contained the lower defect density.Ultimately,the PCE of Zn O based perovskite photovoltaic cells achieved 18.34%.The stability of the modified Zn O based perovskite photovoltaic cells was also improved.4.To overcome the decomposition of the perovskite on low-temperation ETLs,the tin oxide(Sn O₂)was employed as the ETLs material of PPCs.We used trimethoxy silane with different functional groups to modify the surface of Sn O₂ films.These two different trimethoxy silanes contain long-chain alkane and ammonia propyl,respectively.We tailored the surface affinity of ETLs to control the grain size of perovskite films via adjusting the mole ratio between these two different trimethoxy silane molecules.We found that the perovskite on ETLs had the largest average grain size and the smoothest surface when the mole ratio between the long-chain alkane and ammonia in silane solution was 3:7.The PPCs based on the perovskite with the largest average grain size yielded the highest PCE of 20.3%.5.Due to the complexity of the trimethoxy silane modification on Sn O₂,we used 1-butyl 3-methylimidazolium tetrafluoroborate([BMIM]BF₄)to modify the interface between Sn O₂ and perovskite.[BMIM]BF₄ can form the interfacial dipole layer at the interface between perovskite and Sn O₂.The interfacial dipole layer improved the energy level alignment and passivated the defects.Hence,the interfacial dipole layer enhanced the efficiency of the electron transfer at the interface.The defect density caused by anionic vacancies was reduced through modifying perovskite surface with triphenyl phosphorus oxide.Based on the above two strategies,the highest-performance PPCs achieved a PCE of over 21%.6.Based on the research on the low-temperature-prepared ETLs,we started the preparation of PPCs on flexible substrates.To improve the photovoltaic performance of flexible PPCs,firstly,we optimized the thickness and annealing temperature of Sn O₂films prepared with nano crystals.Then,the ionic liquid and zwitterionic materials were used to form an interface dipolar layer and reduce the density of interface defects,which improved the carrier transport capacity inside the PPCs.After the optimization process,a flexible PPCs with a PCE of 18.1%was finally obtained.