Construction Of Solid-State Luminescent Doped Carbon Dots And Their Application In Optoelectronic Devices

Currently,energy and environmental issues have become one of the most important key factors restricting the sustainable development of human society.Reducing energy consumption and developing new energy sources have also become the focus of global concern.In recent years,light-emitting diodes?LEDs?with the advantages of small size,low energy consumption,high brightness,long life,high reliability,and organic solar cells?OSCs?with the advantages of low cost,light weight,flexibility,and large-area preparation have received extensive attention from research and industry.Carbon dots?CDs?,a novel kind of nanoparticles with size below 10 nm and rich in various functional groups,have gradually attracted extensive interest owing to their unique photoelectric properties,excellent photothermal stability,low cost and non-toxicity.In order to solve the problems of CDs for LEDs?e.g.easy solid-state fluorescence quenching and monochrome for CDs based LEDs?and OSCs?harsh preparation method and light-soaking issue?,this dissertation aims to construct solid-state luminescent CDs phosphors for multicolor LEDs and CDs modifier for light-soaking-free OSCs with simple preparation and excellent performance.On the basis of heteroatom doping effect,citric acid?CA?,L-ascorbic acid?AA?and trimethylolpropane-based five-membered cyclic carbonates?TBCC?,which are rich in oxygen and can react with amino groups,were used as carbon sources,and ethanediamine?EDA?,L-cysteine?L-cys?and ammonium persulphate?APS?with different amino activities were served as dopants.The reaction conditions?molar ratio of raw materials,reaction time and temperature?were optimized for microwave or hydrothermal?solvothermal?method.The morphology,chemical structure,photoelectric characteristics and stability of CDs were investigated to realize solid-state luminescence of CDs and their application in multicolor LEDs and light-soaking-free OSCs.The main work includes the following four parts:?1?To obtain high quantum yield?QY?and solid-state luminescent CDs phosphors for LEDs,nitrogen-doped carbon dots?N-CDs?with QY up to 75.96%were synthesized by one-step microwave method using CA as carbon source,EDA as nitrogen source and deionized water as solvent.The effects of molar ratio of raw materials,reaction time and temperature on the fluorescence properties of N-CDs were investigated by three-factor and three-level orthogonal experiments.The morphology,chemical structure,optical properties and stability of N-CDs obtained under the optimum reaction parameters were characterized and analyzed.It was proposed to prepare N-CDs/silicone composites by mixing N-CDs with silicone to realize solid-state luminescence,because the distance between adjacent N-CDs is increased by the steric-hinerance effect of polymer chains in silicone.As a result,yellow LEDs with color coordinates of?0.42,0.40?and correlated color temperature of 3416 K were fabricated by combining UV-LED chip with N-CDs/silicone composite.?2?To obtain high-efficiency and low-cost white LEDs,and explore a maneuverable dispersing medium to avoid the quenching of solid-state fluorescence,nitrogen and sulfur co-doped carbon dots?N,S-CDs?with QY as high as 67.41%were synthesized by one-step hydrothermal method using CA as carbon source,L-cys as additive and deionized water as solvent.The effects of molar ratio and reaction temperature on the fluorescence properties were discussed.The morphology,chemical structure and optical characteristics of N,S-CDs obtained under the optimum parameters were characterized and analyzed.It was proposed to prepare N,S-CDs/polyvinyl alcohol?PVA?fluorescent film by mixing N,S-CDs and PVA.Because of rich hydroxyl groups and long chains in PVA,the solid N,S-CDs can be effectively dispersed to increase the distance between adjacent N,S-CDs particles,thus realizing N,S-CDs solid-state luminescence with red-shifted optimal emission,broadened emission spectra and increased red-green-blue spectral composition?from 31.60%to 61.30%?.As a result,warm white LEDs with color coordinates of?0.40,0.38?,correlated color temperature of 3980 K and brightness of 3629 cd m^-2 were obtained by combining UV-LED chip with N,S-CDs/PVA fluorescent film.?3?To obtain direct solid-state luminescent CDs and apply them to prepare high-efficiency and low-cost multicolor LEDs,N-CDs were synthesized by one-step solvothermal treatment using TBCC as carbon source,EDA as additive and ethanol as solvent.The morphology,structure and optical characteristics of the products obtained at different temperatures were investigated.It was proposed that TBCC and EDA are polymerized by polyaddition and cross-linking,and then dehydrated and carbonized to form N-CDs with carbamate polymer chains on their surface.Owing to the steric-hindrance effect of the polymer chains and their luminescent properties,direct solid-state luminescence of N-CDs was realized.By comparing the optical properties of solution and solid-state N-CDs,it was found that solid-state luminescent N-CDs can also red shift the optimal emission in the PL spectra,broaden the emission spectra and greatly increase the red-green-blue spectral composition.On one hand,white,yellow,orange and red LEDs were obtained by changing the emission wavelength of the chip.On the other hand,under the excitation of 420 nm blue chip,the emitting color of LEDs gradually moved from white light to yellow light region with increasing content of solid-state light-emitting N-CDs.Therefore,a new strategy for fabricating multicolor LEDs by regulating the emission wavelength of the chip and the content of solid-state luminescence N-CDs has been developed,thus promoting the application process of carbon nanomaterials in high-efficiency,low-cost and multicolor LEDs field.?4?To solve the problems of light-soaking caused by the mismatch of interface work functions and harsh preparation process of CDs for OSCs,N,S-CDs were synthesized by hydrothermal method using AA as carbon source,APS as additive and deionized water as solvent.The morphology,chemical structure,optical properties and stability of N,S-CDs were investigated and analyzed.The N,S-CDs with obvious lattice fringes and various polar functional groups on the surface were used as surface modifier for ZnO electron transport layer in OSCs,which effectively reduced the work function,surface roughness and surface energy of ZnO thin films,and passivated ZnO surface defects by coordination.The light-soaking effect of the inverted PTB7-Th:PC₇₁BM based OSCs was eliminated and the device performance was improved with a power conversion efficiency of 9.31%.This work opens up new applications of CDs in the field of photovoltaics.