A Study On Charge Transport, Carbon Electrodes Tother With Their Interfaces In Printable Mesoscopic Perovskite Solar Cells

Halide perovskites are a kind of potential materials for solar cells due to their excellent optoelectronic properties,such as a suitable optical bandgap,high absorption coefficient,low exciton binding energy,high charge mobility of both electrons and holes,and long charge carrier diffusion lengths.In the past decade,perovskite solar cells(PSCs)have an unprecedented development and their power conversion efficiency(PCE)has been boosted to 25.5%from 3.8%.Therefore,PSCs are recognized to be the great candidate for the future photovoltaic technology.Carbon-electrode-based hole-conductor-free printable mesoscopic perovskite solar cells(p-MPSCs)with mesoscopic TiO₂/mesoscopic ZrO₂/mesoscopic carbon electrodes,in which the cheap carbon electrode replaced the expensive noble metal electrode is replaced with cheap carbon electrode and HTM is removed,are regarded as the most potential for upscaling and commercializing due to their low cost and easy preparation properties.However,since hole transport materials are omitted and the devices'thickness is much thicker than that of conventional perovskite solar cells,the efficiency of p-MPSCs are highly restricted by the inherent inefficient hole extraction and transportation and more serious non-radiative recombination.Hence,to suppress the carrier recombination,improve the carrier extraction and transportation,this thesis made attempts on modifying the interface between perovskite/carbon electrode,developing novel materials for carbon electrode and novel additives for perovskites.1.Modifying the interface between perovskite and carbon electrodePolydopamine is developed to coat carbon electrode(D@CE).The coating of polydopamine leads to a decrease in the conductivity of the carbon electrode,which restricts the performance of the device.This result indicates that the chemically inert material of carbon electrode can be modified by a proper process,and the interface modification have an impact on the device performance of p-MPSC.On the other hands,[2,2?,7,7?-tetrakis(N,N-di-p-methoxyphenylamine)9,9?-spirobifluorene](Spiro-OMe TAD)and 2-phenylethanamine iodide(PEAI)are used for p-MPSC by post-treatment.Spiro-OMe TAD is immersed into the perovskite/carbon electrode interface,while PEAI reacts with perovskite and forms a broad-band gap low-dimension perovskite.Both post-treatment methods reduce the recombination of carriers at the perovskite/carbon electrode interface.By using(5-AVA)_0.03MA_0.97Pb I₃ as the active material,the device with Spiro-OMe TAD and PEAI post-treatment shows the PCE of 13.63%and 14.59%,respectively,which are higher than that of control device(13.26%)without post-treatment.2.Novel materials for carbon electrodesCellulose,a most abundant biomass resource in nature with high carbon and oxygen content,was carbonized and oxidized by an in-suite approach to synthesize the oxygen-rich cellulose-based activated carbon(CAC)for carbon electrodes(CAC-E).After CAC was introduced,the CAC-E's work function is increased and the contact between CAC-E and the perovskite is enhanced.By using(5-AVA)_0.03MA_0.97Pb I₃ as the active material,the optimized device with CAC-E shows the best performance of 15.5%with fill factor and voltage enhancements,which is significantly higher than that of control device(13.8%)with the carbon electrode without CAC,due to enhanced charge extraction and better energy level alignment3.Novel additives for halide perovskitesA novel additive of N,1-fluoroformamidinium iodide(F-FAI),whose cation replaces one amino group in guanidinium(GA⁺)with electronwithdrawing fluorine group,is synthesized and applied as the additive for p-MPSC.The electron-withdrawing effect of fluorine promotes the molecular polarity of N,1-fluoroformamidine(F-FA),enhancing the interaction of N,1-Fluoroformamidinium(F-FA⁺)with MAPbI₃.Compared with the nonpolar GA⁺,F-FA⁺improves the crystallinity,passivates the defect and downshifts the Fermi level of MAPbI₃ more significantly.The charge transfer and built-in field in printable triple mesoscopic PSCs are therefore enhanced.Moreover,charge transport in MAPbI₃ is also promoted by F-FAI.With these benefits,a power conversion efficiency of 17.01%for printable triple mesoscopic PSCs with improved open circuit voltage and fill factor is obtained with the addition of F-FAI,superior to the efficiency of 15.24%for those devices with guanidinium iodide additives.