Research On Core-shell ZnO/C Quantum Dots As Electron Transport Layer In Organic Solar Cell

Organic solar cells（OSCs）have attracted the extensive attention of researchers due to their light weight,roll-to-roll flexible printing and promising outer space applications,but their disadvantages such as low power conversion efficiency（PCE）and poor stability limit their commercialization.As multi-layered structural devices,the interface engineering of OSCs is crucial in enhancing their PCE and stability.Zinc oxide（ZnO）nanoparticles,with the advantages of high conductivity,easy preparation and matchable work function,are the most common electron transport layer（ETL）materials in OSCs.However,their high surface defect density,easy oxygen adsorption,easy agglomeration and photocatalytic properties seriously affect the PCE and stability of OSCs.In recent years,due to the excellent photovoltaic properties,energy level tunability and solution processability of carbon nanomaterials,such as graphene and carbon dots（CDs）,they have shown great potential for the application in OSCs as interfacial modification layers.In view of this,this thesis creatively combines ZnO and carbon nanomaterials in situ to prepare two types of core-shell quantum dot materials:carbon-coated ZnO（CD@ZnO）and ZnO-coated CDs（ZnO@C）,which effectively passivate the surface defects of ZnO nanoparticles,improve their dispersion and suppress their photocatalytic properties,and finally effectively enhance the PCE and stability of OSCs.These researches provide a new way to enhance the performance of OSCs.The specific research contents and results include the following two aspects:（1）Preparation of CD@ZnO core-shell quantum dots and their application in OSCs as ETL.Firstly,CDs were synthesized by a solvothermal method using citric acid and ethylenediamine,which were then added into the ZnO generation reaction as nucleating agents,and CD@ZnO was synthesized in situ by particle precipitation.Compared with ZnO,the particle size of CD@ZnO increase from 3-5 nm to 5-8 nm,accompanying with improved monodispersity,increased band gap and effective passivation of defects.Thanks to the stronger charge separation and extraction ability of the CD@ZnO and lower charge recombination at the interface of active layer/ETL,when PM6:IT-4F was used as the active layer to fabricate non-fullerene-based OSCs,compared with ZnO-based devices,the open circuit voltage(V_OC)increase from 0.80 V to 0.83 V and fill factor（FF）increase from 0.68 to 0.71 of CD@ZnO-based devices,and the PCE increase from 11.26%to 12.23%.When P3HT:bis-PCBM was used as the active layer to prepare a fullerene-based device,the V_OC increase from 0.70 V to 0.73 V and FF increase from 0.62 to 0.66 of CD@ZnO-based device,and the PCE increases from 4.13%to 4.64%.（2）Preparation of ZnO@C core-shell quantum dots and their application in OSCs as ETL.ZnO@C was first synthesized by particle precipitation using potassium hydroxide and zinc acetate,and then ZnO was used as a nucleating agent and o-phenylenediamine as a small molecule carbon source,ZnO@C quantum dots were successfully synthesized by solvothermal method.Compared to ZnO（3-5 nm）,the particle size of ZnO@C（5-8 nm）increase significantly,and the band gap is reduced,the hydrophobicity is enhanced,and the surface defect is effectively passivated.Ultimately,the PCE and stability of device,with ZnO@C as ETL and PM6:Y6 as active layer,are significantly improved.The PCE of ZnO@C-based devices（15.48%）are higher than that of ZnO-based devices（15.04%）,which is attributed to the better matching work function of ZnO@C and better contact with the active layer,resulting in higher V_OC（0.84 V）and FF（0.74）.ZnO@C-based devices exhibit higher air stability,with36%degradation in ZnO@C devices and 64%degradation in ZnO-based devices after 100 h of aging in air at 60%humidity.ZnO@C-based devices exhibit higher UV irradiation resistance,with 25%degradation in ZnO@C-based devices and 70%degradation in ZnO-based devices after 3 h of strong UV irradiation.This is due to the fact that,on the one hand,the carbon shell on ZnO@C surface is hydrophobic and effectively passivates ZnO surface defects;on the other hand,the carbon shell layer absorbs most of the UV light and effectively blocks the contact between ZnO and the active layer,which finally inhibits both the oxygen adsorption on ZnO surface and the photocatalytic reaction with the acceptor material,ultimately improving the device stability.In summary,two types of core-shell structure quantum dots（CD@ZnO and ZnO@C）were synthesized,both of which can passivate ZnO defects.Especially,the defect passivation effect of ZnO@C is better.They can be used as the ETL,which effectively improve the PCE of OSCs.Moreover,ZnO@C can also play a significant role in improving the air and UV stability of OSCs.This thesis provides a new idea for passivating metal oxide defects and improving the PCE and stability of OSCs.