Study On The Preparation And Properties Of N-doped Carbon Dots Based Luminescent Solar Concentrator

Luminescent solar concentrator（LSC）is a new type of concentrating device without tracking devices and cooling systems,which can reduce the use of expensive solar cells with a large area of cheap materials.Therefore,it can reduce the cost of photovoltaic power generation.Moreover,LSC can be used for building-integrated photovoltaics（BIPV）.Organic dyes were often used as luminescent materials for LSC fabrication in the past,but their inherent problems such as self-absorption,instability,and high cost have hindered the improvement of LSC’s efficiency.In recent years,semiconductor quantum dots with good stability,less self-absorption and high photoluminescence quantum yield have attracted more and more attention.In this paper,we propose that N-doped carbon-dots（N-CDs）can be used as luminophores for LSC in order to improve the cost-efficacy of LSC.Low-toxic N-CDs with a mean size of 2.8 nm were prepared through one-step microwave method.Bulk-type LSCs doped with N-CDs were prepared with polymethyl methacrylate（PMMA）as the waveguide.Meanwhile,thin-film LSCs doped with N-CDs were prepared by spin-coating.The effect of different doping concentrations of N-CDs on the optical properties of waveguide and the photoelectric properties of LSCs were investigated.The main results are as follows:（1）The N-CDs prepared by the one-step microwave method have good dispersion and narrow size distribution,and the average size is around 2.8±0.2 nm.There are hydrophilic groups on the surface,such as hydroxyl group,carboxyl group,so they have good water solubility.Owing to strong electron donor（such as hydroxyl group,amino group,etc.）,the fluorescence emission intensity of N-CDs can greatly increase.In addition,N-CDs had a broad absorption band in the range of 300-600 nm and a maximum intensity at around 350 nm.N-CDs showed excellent excitation wavelength dependence through fluorescence emission spectroscopy.（2）For bulk LSCs doped with diffierent concentrations（0 wt%,0.02 wt%,0.04wt%,0.08 wt%）of N-CDs:The intensity of absorption increased with the increase of N-CDs concentration,accompanied by the red shift（from 344 nm to 359 nm）of absorption peaks.The intensity of emission spectra increased with the increase of N-CDs concentration,and a slight red shift of the emission peak.When the N-CDs doping concentration was 0.08 wt%,the photocurrent density J_sc,open circuit voltage V_oc,optical efficiencyη_opt,fill factor FF,and power conversion efficiencyηwere 9.29mA/cm²,0.47 V,12.23%,60.72%and 2.63%,respectively.（3）For thin-film LSCs doped with diffierent concentration（0 wt%,0.1 wt%,0.3wt%,0.5 wt%）of N-CDs and various film thicknesses:The intensity of absorption increased with the increase of N-CDs concentration and N-CDs/PMMA thin films thickness.The fluorescence intensity of N-CDs/PMMA composite films and the efficiency of thin-film LSCs increased with the increase of film thickness for low N-CDs doping concentration（0.1 wt%）.As for higher N-CDs doping concentrations（0.3 wt%,0.5 wt%）,with the increase of film thickness,the fluorescence intensity of the N-CDs/PMMA composite film and the efficiency of thin-film LSCs increased at first,and then decreased.When N-CDs doping concentration was 0.3 wt%,the N-CDs/PMMA composite film with a thickness of 6.67μm can obtain the maximum power conversion efficiency 3.94%.Finally,the best performance sample was selected for stability testing.It showed that the fluorescence intensity just decreased by 29%,and the optical efficiency only decreased by 22%after being kept outside for four months.