Impacts Of Defects On Organic-Inorganic Hybrid Perovskites Based Optoelectronic Devices

This dissertation focuses on the defects in perovskites,and researches on the impact of defects on the performance of perovskite optoelectronic devices.The main contents can be divided into three parts:(1)CH3NH3Pbl3-xClx based inverted planar solar cells were fabricated with the power conversion efficiency(PCE)of 15.1%.The illumination intensity dependence of current density-voltage(J-V)has revealed the presence of trap-assist recombination at low fluences.The capacitance-voltage(C-V)spectra exhibit some distinct variations at a wide range of ac-modulation frequencies with and without photo-excitations.Since the frequency dependent chemical capacitance(C?)has concern with surface and bulk related density of states(DOS)in CH3NH3Pbl3-xClx,we have verified this by fitting the corresponding DOS by a Gaussian distribution function.We ascertain the electronic sub-gap trap states present in the solution processed CH3NH3PbI3-xClx and their distribution differs from the surface to bulk.In fact,we demonstrate that both surfaces which are adjacent to the electron and hole transport layers feature analogous DOS.Despite this,photo-and bias-induced giant dielectric responses(i.e.,both real and imaginary parts)have been detected.A remarkable reduction of C? at higher frequencies(i.e.,more than 100 kHz)is ascribed to the effect of dielectric loss in CH3NH3PbI3-xClx.(2)We introduce non-fullerene acceptor ITIC molecules into perovskite films with both uniform and gradient distributions to passivate the grain boundary defects and improve the electron mobility based on the inverted perovskite-ITIC bulk heterojunction solar cells.The thermal Admittance Spectroscopy confirms that dispersing ITIC molecules can decrease the densities of both shallow and deep traps.In particular,we found that the uniform and gradient ITIC distributions can dominantly decrease the shallow and deep traps,respectively.The time-resolved photoluminescence indicates that ITIC can largely increase the PL lifetime,suggesting the decrease on the non-radiative recombination.On the other hand,by using space-charge limited current model,dispersing ITIC can increase electron mobility by a factor of 2 or 3,towards balancing the bipolar mobilities.The UV-Vis and FTIR spectral studies indicate a bounding formation between the Pb and N and O,showing a direct interaction between the hybrid perovskite MAPbI3-xClx and dispersed ITIC molecules.Dispersing ITIC molecules can also surprisingly enhance the spin-orbital coupling with the consequence of increasing the conversion from photogenerated spin-singlets to spin-triplets,boosting the spin states available for photovoltaic actions.Specifically,we find that dispersing ITIC molecules can largely decrease the ?Jsc(photocurrent difference)generated by switching the photoexcitation from linear to circular polarization with same intensity.This phenomenon provides a direct evidence that the dispersed ITIC molecules can increase the SOC in organic-inorganic hybrid perovskites through the interaction between introduced electrical polarization and SOC.Clearly,introducing optically polarizable non-fullerene electron acceptor molecules presents a new opportunity to enhance photovoltaic actions in perovskite solar cells through SOC and grain boundary passivation effects.(3)A sizable(>5 mm)and high quality CH3NH3PbI3 single crystal has been synthesized by the inverse temperature crystallization method,and a white-light photodetector of the structure glass/ITO/Ga/CH3NH3PbI3/Au was fabricated.Over broad photo-excitation intensities ranging from 0.1 mW/cm2 to 100 mW/cm2 using a sun-light simulator,the on-off ratio is tunable in a wide-range from 65 to 2250 at zero bias voltage.The responsivity(R)and detectivity(D*)are 36.2 mA/W and 2.68 x 1011 Jones respectively at a weak white-light intensity such as 0.1 mW/cm2.Both the photodetective parameters decrease with the increase of the illumination intensity.Based on impedance spectra obtained at working condition and light intensity dependent Jsc measurements,the surface trap-assist recombination may play dominating role.The corresponding lifetime(?surf)and resistance(Rsurf-trap)exhibit fast decays at higher illumination intensities.This fundamental study may pave the way for exploring the contribution of the surface trap-assist recombination in the CH3NH3PbI3 single crystal based photodetector.