Kinetics Of Charge Separation And Photo-stability Of Perovskite Solar Cells

Perovskite solar cell(PSC)has become a hot topic in the field of thin film solar cell for its low cost,high efficiency,easy processing and adjustable band gap.To date,power conversion efficiency of perovskite solar cell has boosted to 25.7%.However,the commercialization of PSCs is still limited by the stability of PSCs under light soaking.Due to the low dissociation energy of perovskite crystal,persisting irradiation is believed to engender partial dissociation of perovskite crystal and hence give birth to more defect ions and vacancies.As a result,non-radiative recombination in the device becomes severe,which leads to considerable photovoltage loss.In addition,many works reported the segregation of iodide ions and bromide ions in mixed-halide perovskite under light soaking.The halide segregation is shown to affect the energy band alignment and light harvesting performance of the perovskite films.Interestingly,photoinduced phase segregation and photovoltage loss of perovskite devices is found to recover partially once the radiation is shut sown.Therefore,it is important to explore the mechanism of photovoltage loss in depth and analyze the dynamics of photoinduced phase segregation,which would help to develop effective strategy in the fabrication of perovskite devices with long term stability under light soaking.In this thesis,two perovskite films with different grain sizes are studied by using comprehensive photoelectrochemical measurements.The photovoltage evolution is correlated to interface charge accumulation in the perovskite devices.According to the photovoltage evolution curves,the rise of photovoltage is at the time scale of second.The photovoltage decay further indicates that interfacial charge accumulation is associated with the suppression of non-radiative recombination.The photovoltage stability measurement demonstrates that the size of perovskite crystal has a remarkable influence on device stability.In addition,capacitance relaxation measurement is combined with the photovoltage decay for elucidating the performance of potassium chloride(KCl)modified tin oxide layer when the PSC is under persisting radiation.Interface modification with KCl is found to delay the accumulation of interface charge and hence inhibit interface recombination.However,KCl modification seems to be less helpful for light stability of the device.The dynamics of photoinduced phase separation and lattice expansion in mixed halide perovskite films are analyzed also.The perovskite films with various Br/I ratio were fabricated for monitoring fluorescence spectra and XRD responses under light soaking.It is found that photo-induced phase separation gives rise to the decrease of perovskite crystallization and the broadening of diffraction peak.Such phenomenon could be used to distinguish the photo-induced phase segregation in mixed halide perovskite films.The XRD results further show more evident lattice expansion in perovskite phase rather than lead iodide phase.Furthermore,the recovery of lattice expansion in the dark is faster than that of phase segregation.And they are at different time scales.In addition,the analysis on transient fluorescence decay dynamics shows that light soaking helps to inhibit non radiative recombination and improve carrier lifetime even though phase segregation occurs in the perovskite devices.