Highly Efficient Perovskite Light-Emitting Devices Containing A Cuprous Thiocyanate Hole Injection

Organic-inorganic hybrid perovskite materials have excellent properties such as high carrier mobility,long carrier lifetime,simple preparation process and low cost,which are widely used in perovskite solar cells and perovskite light-emitting diodes etc.The structure of the multilayer perovskite light-emitting diode device consists of many functional layers including anode,hole injection/transport layer,light-emitting layer,electron injection/transport layer,and cathode,arranged in order from the bottom to top.The hole injection/transport layer commonly used in perovskite light-emitting devices is poly（3,4-ethylenedioxythiophene）-polystyrenesulfonic acid（PEDOT:PSS）.PEDOT:PSS has a higher work function,which can improve the injection of holes.However,the strong acidity and hygroscopicity of PEDOT:PSS will affect the performance of the device.Because of the strong acidity,PEDOT:PSS can corrode ITO electrode and The PSS component of PEDOT:PSS is very easy to absorb water,which would lead to the rapid decline of perovskite material performance,thereby affecting the stability and efficiency of the device.In addition,PEDOT:PSS is expensive and increases the cost of device preparation.The superior properties of inorganic hole transport layer materials such as stable chemical properties,high work function and low price can solve the abovementioned problems of PEDOT:PSS.However,the inorganic hole transport materials are mainly used in perovskite solar cells,yet are rarely used in perovskite light-emitting devices.The thesis studies perovskite light-emitting diodes with the inorganic material cuprous thiocyanate（CuSCN）as the hole injection layer.The results indicate that CuSCN has a significant quenching effect on the perovskite luminescence,but such quenching of luminescence in perovskite light emitting devices are harmful,which must be avoided.Adding an organic interlayer poly（9-vinylcarbazole）（PVK）between the CuSCN hole injection layer and the perovskite emission layer can prevent the occurrence of luminescence quenching.In addition,in order to further improve the external quantum efficiency of light-emitting devices,the effect of using butylamine bromide（BABr）as a passivating agent on perovskite light-emitting devices was studied.We further explore the use of a molybdenum phosphorous molybdic acid（PMA）doped PVK hole transport layerinstead of ultraviolet ozone（UV-Ozone,UVO）treated PVK hole transport layer.The main content includes the following aspects:（1）Since CuSCN has a large quenching effect on perovskite luminescence,we added a PVK layer between the hole injection layer and the perovskite luminescence layer to prevent this luminescence quenching.However,due to the surface energy difference between PVK and MAPbBr₃,it is difficult to prepare MAPbBr₃ film with a high substrate coverage on PVK.In order to solve this problem,the PVK film was treated with UVO.The results indicate that as the UVO treatment time increases,the water contact angle of the PVK layer decreases,and the surface energy of PVK layer increases.The X-ray photoelectron spectroscopy measurements show that O-C=O,O-C,C=O and other groups are present on the surface of PVK layer after UVO treatment.Ultraviolet photoelectron spectroscopy measurements showed that the PVK layer after UVO treatment had increased work function（3.80 eV）and ionization potential（6.08 eV）.Scanning electron microscope measurements reveal that the perovskite films prepared on the UVO treated PVK layer show improved substrate coverage and reduced grain size.Light-emitting device with the structure of ITO/CuSCN/PVK/MAPbBr₃/TmPyPB/CsF/Al.Containing a PVK layer with 15 s UVO treatment show the maximum luminous efficiency of the device was 14.5 cd A^-1,and the power efficiency was10.4 lm W^-1,which are more than one order of magnitude and 4-5 times higher than those of the counterparts with an intrinsic PVK layer and PEDOT:PSS hole transport layer.（2）In order to further improve the external quantum efficiency of light-emitting device,MAPbBr₃:BABr film samples were prepared by using BABr ether:isopropyl alcohol solutions as the anti-solvent during the spin-coating process.X-ray diffraction measurement results show that MAPbBr₃:BABr film possesses a diffraction peak from BABr at 17.2°（JCPDS No.10-0814）.Compared with MAPbBr₃ film,the intensity of the（100）diffraction peak of MAPbBr₃:BABr films is significantly reduced,indicating that the crystallinity of the sample is reduced upon the addition of BABr.Scanning electron microscope measurements confirm that the grain size of the MAPbBr₃:BABr film is much smaller than that of the MAPbBr₃ film.With the addition of BABr,the PL spectrum of the sample is blue shifted.When the concentration of BABr is 0.15 mg ml^-1,light-emitting device show the maximum luminous efficiency of 39.3 cd A^-1,the external quantum efficiency of 10.2%and the power efficiency of 29.3 lm W^-1,which is nearly doubled compared with the device without BABr.（3）The cross-linked PVK:PMA layer was used as the hole transport layer in perovskite light-emitting devices,and the morphological,crystal structural and photophysical properties of perovskite films ontop were studied.As the PMA doping concentration increases,the PVK:PMA film and the perovskite film form good contact,and fewer holes and cracks are present on the surface of perovskite films.Meanwhile,the substrate coverage of the perovskite films prepared onto PVK:20%PMA film increases with the increase of annealing temperature.Due to PVK:PMA film has good solvent resistance,various anti-solvents can be used to prepare high-quality perovskite layers.At the same time,the addition of PMA promotes hole transport and improves the luminous efficiency of devices.Compared with light-emitting devices with a PVK hole transport layer,light-emitting devices using PVK:PMA hole transport layer show an improved maximum luminous efficiency of 22.1 cd A^-1,and the power efficiency of 18.2 lm W^-1.In addition,the performance of perovskite light-emitting device with a PVK:PMA hole-transport layer is almost the same regardless of the presence of a hole injection layer,which illustrates the widely applicability of PVK:PMA hole-transport layers in perovskite light-emitting devices.