Research On Defects Passivation In Improving The Performance Of Perovskite Solar Cells

The photovoltaic research was revolutionized by the perovskite solar cells（PSCs）because of the impressive improvement in power conversion efficiency（PCE）from 3.8%to 25.5%in the last decade.It has emerged as one of the most promising solar cell materials owing to their unique combination of properties,such as outstanding absorption coefficient,superb carrier mobility and long charge carrier diffusion length.Despite the rapid development of PSCs,the deep level defects of perovskite thin films in PSCs is one of the main factors which retard the PCEs of PSC.The predominant trap sources for non-radiative recombination losses in PSCs are deep level defects,which have a thermal activation energy higher than k _BT.In comparison,shallow level defects have a negligible impact on non-radiative decays and Voc.Moreover,all these deep level defects in perovskite resulting in a decrease in the performance of the perovskite device.In this thesis,a variety of defect passivation methods are used to improve the photovoltaic performance of PSCs.The results are as followings:（1）Aiming at the undercoordinated lead ion defects at the perovskite grain boundaries,we designed carbon quantum dots（CQDs）rich in hydroxyl and carbonyl groups on the surface as additives to passivate this defect.Grain boundaries passivation occurred as a result of interactions between uncoordinated lead ions and hydroxyl and carbonyl functional groups on CQDs,which effectively reduced the non-radiative recombination in perovskite layer.Above all,the incorporation of CQDs in perovskite solar cell improves the PCE from 15.67%to 18.24%.In addition,the incorporation of CQDs improves the stability of PSCs.（2）Based on the research of carbon quantum dots,we designed a small molecule dicyandiamide（DICY）as a passivation additive for deep level defects in perovskite films.The Lewis basic imine（-C=N）and cyano（-C≡N）functional groups of DICY could passivate the Lewis acidic undercoordinated Pb²⁺defects.On the other hand,the C-N bond（C-NH-C）of DICY was found to be Lewis basic due to the inductive effect of imine and cyano functional groups of DICY.So,the Lewis basic C-N bond of DICY could passivate the Lewis acidic Pb-I antisite defects.The density functional theory（DFT）calculations theoretically revealed the interaction between the DICY and deep level defects in the perovskite,indicating its defects passivation effect.We achieved PCE value higher than 20%and prolonged stability of DICY passivated PSCs without any encapsulation.（3）On the basis of carbon quantum dots and DICY additives,we have employed GACl and PEACl as the mixed bulky organic cations additive for MAPb I₃perovskite thin films.Highly efficient and stable PSCs were achieved by addition of mixed bulky organic cations.It is found that the trap-state densities,Urbach energy and carrier transport of mixed cation based film is better than the mono cation based film,which is attributed to the mixed bulky organic cations.Above all,the incorporation of mixed cations in perovskite solar cell improves the PCE from 18.92%to 20.64%.Device stability is also enhanced dramatically through the addition of mixed cations.The mixed cations based device sustained 61.7%of its initial PCE after aging under 60-80%humidity in the dark at room temperature for 720 hours,while only 39.3%of its initial PCE remained for the control device.（4）In addition to the study of defect passivation in the perovskite layer,we also studied the defect passivation of the interface between the perovskite and the carrier transport layer.We use narrow band gap Cu O_xQDs as the interface layer of the hole transport layer to improve the carrier transfer efficiency of the hole transport layer and passivate the interface defects between the hole transport layer and the perovskite layer to inhibit carrier recombination.The narrow band gap Cu O_xQDs interface layer can form a dipole layer with the wide band gap Ni O_xlayer.When working in PSCs,it can provide favorable electric field force for the charge transport at the interface between the hole transport layer and the perovskite layer.In addition,the interface defects between the perovskite layer and the hole transport layer are passivated by the Cu O_xinterface layer to inhibit the recombination of carriers.Compared with the control PSC,the PCE of the PSC based on the improved hole transport layer increased from 17.34%to 19.91%.