Modulation doping of absorbent cotton derived carbon dots for quantum dot-sensitized solar cells
In order to improve the power conversion efficiency (PCE) of quantum dot-sensitizer solar cells (QDSC), a series of absorbent cotton derived carbon quantum dots (CQD) with different dopants (namely Carbamide, Thiourea, and 1,3-Diaminopropane) have been successfully synthesized by one-pot hydrothermal method. The average particle sizes of the three doped CQDs are 1.7 nm, 5.6 nm, and 1.4 nm respectively, smaller than that of the undoped one (24.2 nm). The morphological and structural characteristics of the four CQDs have been detailed studied. In addition, the three doped CQDs exhibit better optical properties compared with the undoped one in the UV-Vis and PL spectra. Then CQD-based QDSC are experimentally fabricated, showing that short current density (Jsc) and open circuit voltage (Voc) of QDSC are distinctly improved owing to the dopants. Especially the QDSC with 1,3-Diaminopropane doped CQD achieves the highest PCE (0.527%), 299% larger than that without dopant (0.176%). In order to highlight the reasonable mechanism, UV-Vis diffuse reflectance spectrum of CQD sensitized TiO2 and calculated energy band structures of various CQDs are investigated. It’s found from the above analysis that the addition of Carbamide, Thiourea, and 1,3-Diaminopropane are beneficial to obtain CQDs of smaller size, smaller band gap, and more nitrogenous or sulphureous functional groups, which enhance the light absorption performance and photo-excitation property. The above factors are helpful to improve Jsc of QDSC. Nitrogen, acting as a donor to the CQDs, will assist the sensitized photoanode with higher Fermi level, resulting in larger Voc of QSDC. Finally this study builds the relation among microstructure of CQD, three characteristics of CQD (namely spectra, energy band structure and functional groups) and photoelectric properties of QDSC, which will provide guidance for the modulation doping of CQDs to improve the PCE of QDSC.