Effect Of Chloride Precursors And Additives On The Properties Of Halide Perovskite Films And Solar Cells

Halide perovskites(HPs)are promising materials for efficient photovoltaic(PV)electricity production owing to their exceptional photon absorption and chargetransporting properties easily tunable via chemical modification as well as their economically affordable preparation methods in thin films.Because of these excellent PV properties,perovskite solar cells(PSCs)have demonstrated very impressive progress in power conversion efficiency(PCE)and still have potential to achieve higher efficiencies.It is frequently observed that the use of additives leads to efficient and stable PSCs,but deeper understanding of the underlying effects behind properties improvement is still needed.Hence,in this dissertation,the multi role of some chloride precursors or additives used in the preparation of perovskite films and PSCs are investigated.The second chapter of this thesis examines and compares the effect of organic methylammonium chloride(MAC1)and inorganic cadmium chloride(CdCl2)on the properties of lead acetate(PbAc2)-based MAPbI3 PSCs.PbAc2 holds great potential as Pb2+ source for homogenous and smooth perovskite films over large areas without antisolvent.But PbAc2-based perovskite has low activation energy for nucleation,leading to smaller crystals(～250 nm),many grain boundaries and defects.We found that MACl and CdCl2 help tackle those issues.Our experimental and first-principle calculations analysis indicated that some Cl ions remain in the final films at grain boundaries,decreasing the grain boundary trap states.Also,the analysis of the device J-V performance showed that low fill factors(FF)is the main limiting factor for unoptimized PbAc2-based PSCs,and the parameter is correlated to the charge transport in perovskite layers.After the modification with MAC1,our best PSCs showed improved FFs,up to 82%.It was also noted that the additives increase the ambient stability of the perovskite phase and solar cells.In the third chapter of this work,a precursor solution engineering approach was developed to produce wide-bandgap CsxMA1-xPb(I0.6Br0.4)3 perovskites films(Eg～1.8 eV)with reduced grain boundaries,improved phase crystallinity and low defect density,which usually hinder the charge transport in films and affect their PV properties.The proposed approach is based on non-stoichiometric precursor solutions prepared by adding cesium chloride(CsCl)to MAPb(I0.6Br0.4)3 solutions,and was compared to the conventional method based on cesium iodide(CsI).Our CsCl-based method led to films exhibiting longer carrier lifetime and higher mobility than CsI-derived films.Additionally,CsCl also slows down the perovskite phase degradation of CsxMA1xPb(I0.6Br0.4)3 against moisture and thermal stress.As result the device performance was considerably higher compared to CsI case.In the fourth chapter,we compared the effect of three organic chloride additives,namely butylammonium chloride(BACl),propylammonium chloride(PACl)and ethylammonium chloride(EACl)on methylammonium(MA)-free Cs0.1FA0.9PbI3 perovskite films and solar cells.We found that the additives tune the crystal towards large and favorably oriented grains along the(100)crystallographic direction.EACl with two-carbon alkyl chain gave larger grain sizes and reduced grain boundaries films,but evaporates during thermal annealing.Whereas longer alkyl chain BACl and PACl influence moderately the crystal growth while passivating the grain boundaries with residual BA and PA cations.We subsequently developed a double BACl/EACl additive approach,which significantly improved the film quality,and the mechanism for the crystal growth was proposed.Also,conversely to EACl,it was observed that EAPbCl3 perovskite increases the bandgap of Cs0.1FA0.9Pb(I1-yBry)3,thus appearing as suitable way to limit the Br content commonly used to increase the bandgap of perovskites for tandem solar cell application.Moreover,the engineered Cs0.1FA0.9PbI3 films showed improved electronic properties,fewer defects and consequently improved performance Notably,the optimized BACl/EACl PSC could achieve PCE over 20%,whereas unengineered Cs0.1FA0.9PbI3 PSCs had a maximum PCE below 18%.In the fifth chapter of this thesis,high-quality Ruddlesden-Popper perovskite(RPP)BA2FA2Pb3I8Cl2 thin films prepared with BACl was demonstrated and compared to the control films of BA2FA2Pb3I10 and BA2FA2Pb3I8Br2 fabricated with BAI and BABr.The use of BACl 1ed to less-defective RPP films with longer carrier lifetime,most grains oriented perpendicularly to the substrate and optimal carrier transport.Consequently,BACl-based RRP device achieved higher PCE of 13.31%compared to BAI-based PSCs(7.68%)and BABr PSCs(4.87%).Overall,this dissertation demonstrates a range of additives to positively influence the perovskite device photoelectric performance while at the same time increase the device stability.