Synthesis Of Highly Luminescent Carbon Nanodots And Application In Solar Cells As Luminescent Down-shifting Materials

Carbon dots series materials, including the nanodiamond, graphene quantum dots(GQDs) and carbon dots(carbon quantum dots(CQDs) and carbon nanodots(C-dots)), have always been a focus of study for its environment friendly property. Based on their good biocompatibility and photoluminescent(PL) property, they have been widely applied in bioimaging, chemical ion detection, photocatalysis. In addition, GQDs have been applied in solar cells as electron separation and transfer layer due to their unique band structure and electron transport properties. Currently, there are many other luminescent materials, such as semiconductor quantum dots, rear earth ions and molecular fluorescence materials, serve as luminescent down-shifting(LDS) materials on the surface of solar cells, while the relevant researches about the carbon nanoparticles are not too much. In this paper, we mainly discuss the synthesis, property characterization, luminescence mechanism and application in silicon nanowire(Si NW) solar cells of the highly luminescent C-dots.Firstly, we choose citric acid(CA) and ethylenediamine(EDA) as carbon source and nitrogen source, respectively. The C-dots with a PLQY of 76.7% have been yielded by hydrothermally treating CA and EDA. The obtained C-dots have both the sp2 graphitic carbon and sp3 amorphous carbon, and its average size is around 3.7 nm. There are two absorption peaks located at 239 and 348 nm respectively, and one emission peak at 439 nm with the excitation-independent property. The large Stokes shift minimizes the reabsorption loss, demonstrating the feasibility of the C-dots as LDS materials.Secondly, we have further investigated the pH effect on the luminescence property of the C-dots. The absorption and emission properties can be affected by the pH environment, due to the protonation-deprotonation of the C-dots surface functional groups, demonstrating the surface states should be related to the luminescence of C-dots. In addition, the excitation-independent property may be caused by the unitary surface states of C-dots, which is benefit for the highly luminescence of C-dots.Thirdly, we for the first time add ammonia water(AW) into the precursors, and make the PLQY of C-dots as high as 84.8%, a 10.56% enhancement is achieved comparing to the case without AW. To explore the relationship between the PLQY and the various structure of C-dots induced by the different AW concentration, we have employed the X-ray photoelectron spectroscopy(XPS), the Fourier transform infrared spectroscopy(FTIR) and fluorescence lifetime techniques to reveal the chemical bonds, compositions in C-dots and the recombination channel. It is found that the PLQY of C-dots increases with the decreasing O-state, or with the increasing ratio of N-state to O-state. Moderate use of AW can effectively enhance the luminescent states by bonding more amide groups on the surface and decrease the nonradiative or low efficient radiative recombination channels by the removal of O-state, yielding C-dots with high PLQY.Finally, we have successfully fabricated the C-dots based LDS layers on the surface of SiNW solar cells. It is worth mentioning that the PLQY of the close packed C-dots can be dramatically decreased owing to the aggregation-caused fluorescence quenching. To solve this issue, we have employed polyvinyl alcohol(PVA) as the host material for the LDS layer due to its high transmittance and good compatibility with C-dots. The blue emission(under 365 nm illumination) LDS layer has been fabricated by spin-coating the C-dots embedded PVA solution on the surface of SiNW solar cells, which enhances the short current density of the device by 0.30 mA/cm2. Through the analysis of external quantum efficiency spectra of the solar cells, we find that the underlying mechanism of the enhancement is attributed to the competing result of the deterioration of surface reflectance and the gains from the optical absorption redistribution as well as the down-shifting. The down-shifting contribution(43.3%) is accurately extracted by our proposed theoretical model.