Device Fabrication Of Perovskite Solar Cells Based On CH₃NH₃PbI₃ And Theoretical Study Of The Charge Transporting Mechanism

The perovskite solar cells（PSCs）exhibit rapid development.Now the power conversion efficiency of PSCs in laboratory has reached to a certified 22.1%.It is the research focus to the function of PSCs key component materials and interface properties on photovoltaic performance.In this thesis,both the PSCs performance and device stability were increased as the carrier dynamics behavior among the PSCs interfaces was controlled by optimizing crystal morphology of perovskite,introducing new organic hole-transporting materials and modified PSCs interfaces with reduced-oxide graphene.（1）Under the investigation of perovskite crystal morphology,MAPb I₃ was choosed as the perovskite precursor,and a perovskite layer with hollow tube structure was attained by a simple“one-step”spin-coating method onto the mesoporous TiO₂ scaffold.For comparison,a planar perovskite layer was also fabricated using chlorobenzene as anti-solvent process.XRD results imply that both the perovskite tube and slab display the same tetragonal CH₃NH₃Pb I₃structure.The tubular structure enlarges the interface area between perovskite layer and spiro-OMeTAD,which favors to the extraction and injection of holes.According to the photovoltaic results,tubular PSCs reduce interfacial recombination and accelerate the efficiency of charge collection.Accordingly,the V_oc of tubular cells reaches to 841 mV,which is 42 mV higher than the planar ones,and the PCE of tubular cell based on carbon electrode reaches 7.4%.（2）Herein the PSCs based on mesoporous TiO₂ are fabricated for scrutinizing the dynamics of current evolution in response to the perturbation of bias voltage,time and intensity of lighting soaking.The relaxation of photocurrent as well as dark current is found to be well evaluated by an empirical function with power-law form.Fitting results show the half-times for the evolution of dark current and photocurrent are in the milliseconds and seconds scales,respectively.Combining the results of charge quantity of PSCs,we speculate that capacitive effect dominates the current response in the dark,while the sluggish evolution of photocurrent is primarily due to the modification of built-in electric field via ion migration.Moreover,j-V characteristics indicate that the dependence of hysteresis behaviors on the thickness of CH₃NH₃PbI₃ layer.（3）Here we designed and synthesized four kinds of new organic hole-transporting materials（HTMs）,and investigated the function of molecule structure and dopants in HTM on the device photovoltaic performance.Firstly,a couple of isomers Q216 and Q219 were synthesizedwithcarbazole-diphenylaminemoietiesasendgroupsand1,1′-bi-2-naphthylamine as central core.Q216 has bis（4-methoxyphenyl）amine unit（DMPA）at“2”position in carbazole and Q219 at“3”position.Both HTMs exhibit excellent thermochemical stability and their energy match with the perovskite and carbon electrode.The hole mobility and conductivity of Q219 is higher than that of Q216,approaching to that of spiro-OMeTAD.Accordingly,Q219-based cell yields a best efficiency of 9.31%with a V_occ of 949 mV which is higher 99 mV than that of Q216.The PCE of spiroOMeTAD-based device is 10.01%under the same conditions.By introducing the“3-DMPA”structure,a novel hole-transporting material（Q221）was synthesized by introducing benzyl groups onto the 1,10-bi-2-naphthol central core as edge chains and bis（4-methoxyphenyl）amine-substituted 9H-carbazole as donor groups.A reference molecule（Q222）was prepared with hexyl edge chains.While Q221 with large rigid benzyl groups onto the 1,10-bi-2-naphthol central core as edge chains,exhibits 3.5 folds higher of hole mobility and 4.5 times higher of conductivity than that of Q222,which favors to enhance hole mobility and render charge recombination.At a low doping level of Li-TFSI/TBP（15 mM/100 m M）,the highest power conversion efficiency of Q221-based CH₃NH₃PbI₃ perovskite solar cells is increased to 10.37%,which is 1.5%higher than that of Q222.The PCE of device based on spiro-OMeTAD is 8.84%.Non-capsulated devices with a dopants of 15 mM Li-TFSI and 100 mM TBP were aged in a dark condition of 28℃,RH 35%for 192 h.The consequence of device stability is Q221>spiro-OMeTAD>Q222,which is the same rank with hydrophobicity of hole-transporting materials.（4）It is investigated that the function of reduced oxide-graphene for photovoltaic performance and stability in perovskite solar cells.Firstly,in the research of electron transporting layers（ETLs）,reduced oxide-graphene was doped into ETLs to improve the conductive of mesoporous TiO₂.Under forward scanning,PCE is improved to 16.13%from 13.76%;under reverse scanning,PCE is improve to 17.51%from 15.78%.These results imply that the conductivity could be enhanced by doping reduced-oxide graphene into TiO₂ layer.In addition,it is also discussed the modification of reduced oxide-graphene on the interface between spiro-OMeTAD and Au electrode.Capsulated-devices were aged at 28℃,RH 40%for 1400 h under continuous sun illumination and maximum power point tracking.The decay time is improved to 540 h from 100 h when the PCE is decreased 50%,which implies that device light-stability was enhanced double times.